KINKEN
Research Highlights
2020

Institute for Materials Research, Tohoku University

KINKEN Research Highlights 2020
Contents
Preface���������������������������������������������������������������������������������������������������������������������������������������������������������� 4
Research Highlights
1. Infrastructural Materials
Microstructure Design of Structural Metallic Materials Research Laboratory���������������������������������������������� 8
Environmentally Robust Materials Research Laboratory������������������������������������������������������������������������������ 9
Structure-Controlled Functional Materials Research Laboratory��������������������������������������������������������������� 10
Deformation Processing Research Laboratory�������������������������������������������������������������������������������������������11
Analytical Science Research Laboratory��������������������������������������������������������������������������������������������������� 12
Creation of Life Innovation Materials for Interdisciplinary and International Researcher Development������ 13
Professional development Consortium for Computational Materials Scientists (PCoMS)�������������������������� 14

2. Energy-Related Materials
Crystal Physics Research Laboratory�������������������������������������������������������������������������������������������������������� 16
Magnetism Research Laboratory��������������������������������������������������������������������������������������������������������������� 17
Quantum Functional Materials Physics Research Laboratory�������������������������������������������������������������������� 18
Materials Design by Computer Simulation Research Laboratory��������������������������������������������������������������� 19
Nuclear Materials Engineering Research Laboratory�������������������������������������������������������������������������������� 20
Crystal Chemistry Research Laboratory���������������������������������������������������������������������������������������������������� 21
Hydrogen Functional Materials Research Laboratory�������������������������������������������������������������������������������� 22

3. Electronic Materials
Theory of Solid State Physics Research Laboratory���������������������������������������������������������������������������������� 24
Surface and Interface Research Laboratory���������������������������������������������������������������������������������������������� 25
Low Temperature Physics Research Laboratory���������������������������������������������������������������������������������������� 26
Low Temperature Condensed State Physics Research Laboratory����������������������������������������������������������� 27
Quantum Beam Materials Physics Research Laboratory��������������������������������������������������������������������������� 28
Physics of Electronic Materials Research Laboratory ������������������������������������������������������������������������������� 29
Chemical Physics of Non-Crystalline Materials Research Laboratory������������������������������������������������������� 30
Solid-State Metal-Complex Chemistry Research Laboratory��������������������������������������������������������������������� 31
Non-Equilibrium Materials Research Laboratory���������������������������������������������������������������������������������������� 32

Magnetic Materials Research Laboratory�������������������������������������������������������������������������������������������������� 33
Actinide Materials Science Research Laboratory�������������������������������������������������������������������������������������� 34
Materials Science of Non-Stoichiometric Compounds Research Laboratory�������������������������������������������� 35
Exploratory Research Laboratory�������������������������������������������������������������������������������������������������������������� 36

Research Centers
International Research Center for Nuclear Materials Science�������������������������������������������������������������������� 38
Cooperative Research and Development Center for Advanced Materials������������������������������������������������ 39
High Field Laboratory for Superconducting Materials�������������������������������������������������������������������������������� 40
Trans-Regional Corporation Center for Industrial Materials Research������������������������������������������������������� 41
Center for Computational Materials Science���������������������������������������������������������������������������������������������� 42
Collaborative Research Center on Energy Materials���������������������������������������������������������������������������������� 43
International Collaboration Center (ICC-IMR)��������������������������������������������������������������������������������������������� 44
Center of Neutron Science for Advanced Materials����������������������������������������������������������������������������������� 45
Laboratory of Low Temperature Materials Science������������������������������������������������������������������������������������ 46
Laboratory of Alpha-Ray Emitters�������������������������������������������������������������������������������������������������������������� 47
Analytical Research Core for Advanced Materials������������������������������������������������������������������������������������� 48

International Collaborative Research
Crystal Physics Research Laboratory�������������������������������������������������������������������������������������������������������� 50
Magnetism Research Laboratory��������������������������������������������������������������������������������������������������������������� 51
Materials Design by Computer Simulation Research Laboratory��������������������������������������������������������������� 52
Irradiation Effects in Nuclear and Their Related Materials Research Laboratory��������������������������������������� 53
Solid-State Metal-Complex Chemistry Research Laboratory��������������������������������������������������������������������� 54
Magnetic Materials Research Laboratory�������������������������������������������������������������������������������������������������� 55
Crystal Chemistry Research Laboratory���������������������������������������������������������������������������������������������������� 56
International Research Center for Nuclear Materials Science�������������������������������������������������������������������� 57
Cooperative Research and Development Center for Advanced Materials������������������������������������������������ 58
High Field Laboratory for Superconducting Materials�������������������������������������������������������������������������������� 59
Center for Computational Materials Science���������������������������������������������������������������������������������������������� 60
Center of Neutron Science for Advanced Materials����������������������������������������������������������������������������������� 61

Author Index����������������������������������������������������������������������������������������������������������������������������������������������� 62
Keyword Index������������������������������������������������������������������������������������������������������������������������������������������� 63

Preface

Dear Colleagues,

We are pleased to present KINKEN Research Highlights 2020, which is the annual report that
includes a collection of research outputs of the past year from the Institute for Materials Research (IMR),
Tohoku University. KINKEN is the abbreviation for “Kinzoku Zairyo Kenkyujo,” the Japanese name for
IMR, which is well known in the materials science community.

IMR was the first Tohoku University research institute, of which there are currently six, to beestablished in 1916. Aiming to contribute to the well-being of the human race and the development ofcivilization through the creation of new materials that are useful to society, it carries out research on boththe fundamentals and applications of a wide range of substances and materials. After its founding, IMR
first focused on steel but quickly expanded its research area to the other metals and alloys. Then, in
response to changing times and economic growth, it came to cover semiconductors, ceramics, and awide range of other materials. Ever since our first director, Professor Kotaro Honda, invented the first
artificial permanent magnet called “KS steel,” we have developed many new materials for practical use,
including Sendust alloy, SiC fibers, and soft magnetic amorphous alloys. We have also focused on
fundamental research, which is important for the development of new materials, as well as carried out
cutting-edge research on magnetism, optical properties, superconductivity, and microstructure ofmaterials. While doing so, IMR has become a leading research institute in the field of materials science.

IMR is notable for its fusion of fundamental and applied research, as well as of science andengineering. We offer some of the best large research facilities in the world — covering high-energy

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IMR KINKEN Research Highlights 2020

Director Prof. Tadashi Furuhara

irradiation, strong magnetic fields, and supercomputing — enabling researchers from Japan and abroad
to engage in a variety of collaborative projects. To develop superior or new materials that are useful, we
believe it is necessary to organically connect three “pillars”: (1) the “exploration” of physical phenomenathat govern materials’ functions, (2) the “creation” of materials via various methods, and (3) the“measurement” of materials’ properties from various perspectives. IMR has for many years been
contributing to the materials research community via our joint usage/research programs. IMR celebrated
its 100-year anniversary in 2016. Having been designated in 2018 as an International Joint Usage/
Research Center in Materials Science, known as Global Institute for Materials Research Tohoku (GIMRT),
we have now further heightened our efforts to enhance such collaborations and cultivate new talent.

In the 21st century, our society faces global-scale environmental issues, such as global warming,
resource depletion, acquiring a stable energy supply, and new kinds of environmental pollution. IMR willcontinue to work towards a sustainable society both by engaging in important research to solve various
problems as well as by creating new areas of research that can bring about paradigm shifts in the future.

We hope that the KINKEN Research Highlights will enable you to better understand our recent
research activities and will aid the promotion of worldwide collaboration with IMR. We ask for yourcontinued support and welcome any suggestions.

IMR KINKEN Research Highlights 20205

Infrastructural Materials

IMR KINKEN Research Highlights 2020

Research

Infrastructural Materials

Atom Probe Compositional Analysis of Interphase Precipitated NanoSized Alloy Carbide in Multiple Microalloyed Low-Carbon SteelsThe composition of nano-sized alloy carbide formed by interphase precipitation at the migrating ferrite/
austenite interface during ferrite transformation in vanadium-titanium multiple microalloyed low-carbon steels
was analyzed using a three-dimensional atom probe. As carbide-forming elements, vanadium and titanium
were simultaneously precipitated from the early stage of transformation, while the alloy carbide was morestrongly enriched with titanium than vanadium.
Interphase precipitation of NaCl-structured
nano-sized alloy carbide occurs at the migrating
ferrite/austenite interface during ferrite transformationin steels containing strong carbide-forming elements
such as vanadium (V), niobium (Nb), and titanium(Ti). In our previous studies, the effects of various
factors, including transformation temperature, alloy
composition, etc., on the interphase precipitation
behaviors in V-added low-carbon steels weresystematically investigated [1]. In this study, we
attempted to quantitatively analyze the chemicalcomposition of alloy carbide formed by interphase
precipitation in a low-carbon steel, which is formedby the addition of multiple microalloying elements.
Thus, we aimed to obtain more detailed insights into
the nature of interphase precipitation.
The Fe-0.4C-1.5Mn-0.1Si-0.1V-0.1Ti (at%) alloy
was firstly treated at 1,523 K for 0.6 ks to dissolve all
the added V and Ti, and then held at 923 K for 60 s
to perform isothermal ferrite transformation, followed
by water quenching to freeze the microstructure.
Next, a focused ion beam was used to prepare
needle-shaped samples from the ferrite region withinterphase precipitation in the transformed
specimen, for the subsequent three-dimensional
atom probe (3DAP) measurements.
Figure 1 shows an example of the typical results
obtained from 3DAP analysis. The atom map shows
the inhomogeneous elemental distribution, while theenlarged atom map reveals that V, Ti, and C atoms
were homogeneously distributed in the enriched
regions without any core-shell structure, indicating
the simultaneous precipitation of V and Ti as
complex alloy carbides. This result is also supportedby the one-dimensional concentration profiles
obtained from three mutually perpendicular
directions across the carbide, showing thesynchronized variations in the concentrations ofvarious elements. It is evident that the total amountof V, Ti, and C in the carbide was far below 100%,
which was likely caused by the artifacts in 3DAP. By

Fig. 1 Three-dimensional atom map in the V-Ti multipleadded alloy isothermally transformed at 923 K for 60 s,with one-dimensional concentration profiles across a
specific carbide along three mutually perpendicular
directions [2].

assuming the theoretical stoichiometry of the alloy
carbide, its chemical composition was estimated tobe (V0.33Ti0.67)C. With the same bulk additive
amount in at% of V and Ti, the concentration of Ti inthe carbide was significantly higher than that of V,
possibly due to its relatively stronger affinity to C
and thus a strong tendency to precipitate in steels.
References[1] Y.-J. Zhang, G. Miyamoto, K. Shinbo, and T. Furuhara, Acta
Mater. 128, 166 (2017).
[2] Y.-J. Zhang, G. Miyamoto, and T. Furuhara, Microsc.
Microanal. 25, 447 (2019).

Keywords: atom probe tomography, precipitation, steel
Yongjie Zhang (Microstructure Design of Structural Metallic Materials Research Laboratory)
E-mail: yongjie@imr.tohoku.ac.jp
URL: http://www.st-mat.imr.tohoku.ac.jp/en/index.html

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IMR KINKEN Research Highlights 2020

IMR KINKEN Research Highlights 2020

Detecting Hydrogen Desorption During Deformation-Induced
Martensitic Transformation in Steels

Hydrogen absorption, diffusion, trapping, and desorption are key phenomena to understand hydrogen
embrittlement. In particular, deformation-hydrogen interaction is the origin of the complexity of hydrogen
embrittlement. In this context, we recently succeeded in detecting hydrogen desorption occurring during thedeformation-induced martensitic transformation in steels. The hydrogen desorption behavior showed significant
differences among the fcc-hcp and fcc-bcc transformations, and dislocation glide without transformation.
Deformation-induced martensitic transformation(diffusionless transformation) in metals locally alters thesolubility and diffusivity of hydrogen because of thecrystallographic changes. The changes in solubility anddiffusivity critically affect hydrogen embrittlement
susceptibility when the metals have a significant
hydrogen content [1]. For instance, type 304 austenitic
steel is susceptible to hydrogen embrittlement becauseof the fcc to bcc martensitic transformation. Therefore,monitoring hydrogen motion during deformationinduced martensitic transformation is a key techniquefor clarifying the mechanism of hydrogen embrittlement.
However, the detection of hydrogen during martensitic
transformation is difficult because hydrogen is light/
mobile and martensitic transformation is an
instantaneous phenomenon.
To detect hydrogen desorption during martensitic
transformation, mass spectrometry was utilized [2].
Specifically, a tensile testing machine was installed in a
vacuum chamber equipped with a mass spectrometer.
Then, some austenitic steels were prepared: Fe-19Cr8Ni-0.14C, Fe-15Mn-10Cr-8Ni, and Fe-19Cr-14Ni-0.15C(mass%). The Fe-19Cr-8Ni-0.14C and Fe-15Mn-10Cr8Ni steels showed the fcc-bcc and fcc-hcp martensitic
transformations, respectively, while the Fe-19Cr-14Ni0.15C steel did not show any martensitic transformation
during tensile deformation until fracture. Figure 1 shows
the martensite fraction as a function of strain.
The specimens with different deformation modes
were tensile-deformed at 298 K with an initial strain rateof 5×10 -4 s-1, and the desorbed hydrogen was detected
as shown in Fig. 2. The hydrogen pressure and timecorrespond to the hydrogen desorption rate and nominal
strain, respectively. The most remarkable hydrogen
desorption was detected in the Fe-19Cr-8Ni-0.14C steel
in which the fcc-bcc martensitic transformation
occurred. The hydrogen desorption rate observed in the
fcc-hcp martensitic transformation was much lower thanthat in the fcc-bcc martensitic transformation, andsignificant compared with the case without martensitic
transformation. Interestingly, this result corresponds toFig. 1 Martensite fraction plotted against tensile strain.

Fig. 2 Hydrogen desorption curves.

hydrogen embrittlement susceptibility, i.e., the effect offcc-hcp martensitic transformation on hydrogen
embrittlement susceptibility was smaller than that on
fcc-bcc martensitic transformation. We believe that our
new approach would enable us to understand andpredict the hydrogen embrittlement behavior of metals.
References[1] M. Koyama, D. Yamasaki, A. Ikeda, T. Hojo, E. Akiyama, K. Takai,
and K. Tsuzaki, Int. J. Hydrog. Energy. 44, 26028 (2019).
[2] T. Hojo, M. Koyama, N. Terao, K. Tsuzaki, and E. Akiyama, Int. J.
Hydrog. Energy. 44, 30472 (2019).

Keywords: hydrogen, steel, phase transformation
Motomichi Koyama (Environmentally Robust Materials Research Laboratory)
E-mail: koyama@imr.tohoku.ac.jp
URL: https://akiyamaimr-e.amebaownd.com/

IMR KINKEN Research Highlights 20209

Infrastructural Materials

Novel Mode of Omega(ω)-Transformation in Titanium Alloys

Understanding the mechanism of the omega(ω)-transformation is crucial for the development of beta(β)titanium alloys. We have recently demonstrated that a novel type of ω-transformation, diffusionless isothermal
ω-transformation, can gradually proceed even above the ω-transformation temperature. The transformation
mechanism can be reasonably explained by the quenched-in compositional fluctuations that are inevitablypresent in thermal equilibrium.
Owing to their high strength, corrosion resistance,and biocompatibility, titanium alloys are widely used forstructural and biomedical materials. In the developmentof titanium alloys, the control of the transformation fromthe β-phase (body-centered cubic) to the ω-phase(hexagonal) is crucial because the latter dominates the
microstructure and mechanical properties of the titanium
alloys.
In terms of the transformation kinetics, theω-transformation has been categorized into twotransformation modes: athermal and isothermal
ω-transformations. The former occurs instantaneously
via a diffusionless mechanism, while the latter occurs
gradually via atomic diffusion in conjunction with soluteatom partitioning. We, however, have demonstrated thata diffusionless isothermal ω-transformation can
gradually occur near room temperature by elasticity andinternal friction measurements, X-ray diffraction, atomicresolution transmission electron microscopy, Fig. 1(a),and 3D atom-probe analysis [1].
Such an anomalous “diffusionless isothermal
ω-transformation” was found to occur in locally unstableregions (a few nanometers in size) with less β-stabilizing

elements, which are inevitably present owing to thequenched-in compositional fluctuations, as shown in
Fig. 1(b) [1]. The ω-transformation is generally known toarise from the localized softening of the 2/3[111]βlongitudinal phonon. Therefore, it can be concluded thatthe present localized transformation cannot be
suppressed by the stabilization of the β-phase on
average. The quenched-in compositional fluctuations(in equilibrium originally at a high temperature) determine
the activation energies of the ω-nucleation process andthe dynamic {111}β collapse. Thus, the compositional
fluctuation is key to understanding the thermodynamics
and kinetics of the ω-transformation [1]. This mechanismof the localized transformation is distinct from that ofdisplacive transformations with large transformation
strains, which cooperatively occur in widely spreadregions, accompanied by the softening of longwavelength acoustic phonons at the gamma point.
References[1] M. Tane, H. Nishiyama, A. Umeda, N. L. Okamoto, K. Inoue, M.
Luckabauer, Y. Nagai, T. Sekino, T. Nakano, and T. Ichitsubo, Phys.
Rev. Materials 3, 043604 (2019).

Fig.1 (a) Scanning transmission electron microscopy image of a Ti-21 at% V alloy aged at 323 K for ten days. Electron incidenceis [110]β. The {111}β planes fully collapse at the center of the ω-phase particle. (b) Schematic illustration of the quenched-incompositional fluctuations. As the voxel size (L) decreases, the composition fluctuation becomes more significant.

Keywords: metal, phase transformation, scanning transmission electron microscopy (STEM)
Tetsu Ichitsubo, Norihiko L. Okamoto, Hiroshi Tanimura, and Tomoya Kawaguchi (Structure-Controlled Functional
Materials Research Laboratory)
E-mail: nlokamoto@imr.tohoku.ac.jp (NLO), tichi@imr.tohoku.ac.jp (TI)
URL: http://www.imr.tohoku.ac.jp/en/about/divisions-and-centers/research-division/16.html
http://ilab.imr.tohoku.ac.jp/~ilab/index.html/

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IMR KINKEN Research Highlights 2020

IMR KINKEN Research Highlights 2020

Highly Efficient Process Design for Powder Bed Fusion-Type
Additive Manufacturing Using an Electron Beam

We have developed and demonstrated a novel process design method for powder bed fusion-type additive

manufacturing using an electron beam. This method utilizes the design of experiments and support vector
machine, one of the machine learning methods, to provide the optimum conditions required for obtaining
defect-free, additively manufactured parts without any destructive inspections.

Additive manufacturing (AM) is a technologythat enables near-net shaping of metal parts withcomplex shapes. Powder bed fusion-type AM using
an electron beam (PBF-EB) is an AM technology inwhich the re-coated metal powders are heated andselectively melted in a layer-by-layer manner using
an electron beam. The PBF-EB can provide highquality metal parts as compared to a PBF-type AM
using a laser beam (PBF-LB) because the former
process is conducted at a high temperature under ahigh vacuum, leading to low thermal stress and lessoxidation. Although the defect density in metal parts
built using the PBF-EB is smaller than in those built
using the PBF-LB, the former usually include poresthat degrade the mechanical properties. Although
the mechanism of defect formation and solidification
during the PBF-LB process has been revealed bysynchrotron X-ray imaging, it is still challenging tosuppress defect formation during the PBF-LB and
PBF-EB processes and fabricate defect-free metalparts. Although many efforts have been made to
optimize processes for defect-free AM built parts,
effective methods for process design have not beendeveloped yet.
We have developed a highly efficient process
design method for the PBF-EB process. This method
utilizes the relationship between surface morphology
and defect formation. A non-flat surface provides a
non-uniform powder bed layer that is helpful informing a next layer, leading to defect formation. In
contrast, a flat surface provides a uniform powder
bed layer, leading to defect-free parts. The robust
condition will be one with the largest distance fromthe boundary between the flat and non-flat surfaces.
This method applied the distance from the boundary
between the flat and non-flat surfaces as a process
condition. A support vector machine, which is amachine learning technique, was used to determine
the classification boundary. Although only the signof the decision function is usually used, our method
uses not only the sign but also the value of thedecision function.

Fig. 1 (a) Process map constructed by an SVM. (b) Surfaceand (c) cross-sectional image of the PBF-EB built
cylinder under the optimized condition with the highest
decision function value. (d) Enlarged image of theregion shown by the dotted rectangle in Fig. 1(c). [1]We demonstrated our method for the CoCrMo
alloy, as shown in Fig. 1 [1]. Only eleven data points
were used in this demonstration, and it took only
twelve hours. The PBF-EB built cylinder had noprocess defects. The spherical pores shown in Fig.
1(d) originated from the entrapped gas pores in the
raw material powders. This result indicates that our
method can predict the optimum condition for
defect-free AM built parts with reduced lead time.
Our method can optimize not only two-dimensionalparameters, but also multi-dimensional parameters.
References[1] K. Aoyagi, H. Wang, H. Sudo, and A. Chiba, Addit. Manuf. 27,353 (2019).

Keywords: additive manufacturing, alloy, defects
Kenta Aoyagi (Deformation Processing Research Laboratory)
E-mail: k.aoyagi@imr.tohoku.ac.jp
URL: http://www.chibalab.imr.tohoku.ac.jp/english/index.html

IMR KINKEN Research Highlights 202011

Infrastructural Materials

Statistical Analysis of the Distribution of Alumina Inclusion
Particles in Stainless Steels in Laser-Induced Breakdown
Spectrometry Using 1-kHz Q-Switched Nd:YAG Laser

A two-dimensional distribution of alumina inclusion particles in ferritic stainless steels was obtained fromvariations in the emission intensity of an Al I line, when a 1-kHz Q-switched Nd:YAG laser was scanned on asample surface in laser-induced breakdown spectrometry (LIBS). This method provides a simpler and faster
analytical method to determine the distribution of alumina inclusions than the conventional methods.
Quantitative and morphological assessmentsof non-metallic inclusion particles, which areembedded in the structure of steel materials, is an
important issue in the analysis of steel products.
Especially, ‘hard’ inclusions, such as coarse alumina
particles, may cause any fatigue of structural steel
materials and sometimes leave scratches andcreases on the surface of the final product,
depending on the number density and the size ofthe inclusions.
Laser-induced breakdown spectroscopy (LIBS)
has been employed to analyze various solid samples
directly and rapidly, because little pretreatment of
the samples is required before the measurement.
The scanning LIBS provides the distribution of
constituent elements in a sample area by measuring
their emission intensities. In this study, thequantitative distribution of alumina inclusions inferritic stainless steels was measured when the LIBS
system was operated under the optimal measuringconditions. The objective of this study was todevelop a rapid measuring method suited for suchelemental mapping.
The laser used in the experiment was a
Q-switched Nd:YAG laser, having an oscillation
wavelength of 532 nm. The emission signal from theplasma was measured on a spectrometer,
comprising a grating monochromator and aphotomultiplier, whose effective spectral band-path
was ~0.1 nm at a wavelength of 300 nm [1]. The
plasma chamber could be precisely moved to thelaser beam three-dimensionally, when it was fixedon an X-Y-Z stage. The X-Y-Z stage was adjusted ineach direction until the irradiation position of thelaser had been focused on the target position of thesample surface, and then the laser beam was
moved along a lateral direction at a constantscanning rate [1].
Three kinds of ferritic stainless steels wereprepared as test samples, labeled as samples A, B,

Fig. 1 Two-dimensional maps of the Al I emission intensity
for samples A (a), B (b), and C (c).

and C, which included alumina particles (Al2O3)
precipitated in the iron matrix. From the result ofchemical analysis, the amounts of alumina inclusion
in sample C would be twice larger than those insamples A and B.
A typical result of the two-dimensional map,
based on the emission intensity of the Al I 396.152nm line after the background correction, is illustrated
in Fig. 1 [2]. The number of alumina particles insamples A, B, and C, were 35, 27, and 47,
respectively. There were more alumina particles insample C compared to the samples A and B, which
agreed with the chemical analysis.
The analysis time was ~20 min for the sample
area of 4.5 mm2 to obtain the distribution of alumina
inclusions. This method could provide the map ofinclusions more rapidly and easily, as comparedwith the conventional method.
References[1] T. Matsuda and K. Wagatsuma, ISIJ Int. 59, 1838 (2019).
[2] T. Matsuda, S. Kashiwakura, and K. Wagatsuma, Microchem.
J. 153, 104400 (2020).

Keywords: steel, atomic emission spectrometry, laser plasma
Kazuaki Wagatsuma (Analytical Science Research Laboratory)
E-mail: wagatuma@imr.tohoku.ac.jp
URL: http://www.imr.tohoku.ac.jp/en/about/divisions-and-centers/research-division/27.html

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IMR KINKEN Research Highlights 2020

Development of High-Performance High-Entropy Alloys
Assisted by Thermodynamics and ab initio Calculations

High-entropy alloys have attracted considerable interest, with the stacking fault energy playing an importantrole in regulating their mechanical behaviors. In this study, the regulation principles have been described anda series of high-performance alloys has been designed, assisted by ab initio and thermodynamics calculations
to shed light on the development of such novel materials.

The face-centered cubic (fcc) phase
CoCrFeMnNi high-entropy alloys (HEAs) exhibit
exceptional strength and ductility, both at room andcryogenic temperatures. The stacking fault energy(SFE) is a main factor affecting their plastic
deformation mechanisms, which switches from
dislocation slip to mechanical twinning bydecreasing the fcc phase stability and SFE
consecutively.
We recently demonstrated the principles forregulating SFE using ab initio and thermodynamic
calculations [1-3]. The decreasing Mn, Ni, and Fe
concentrations and increasing Co and Cr
concentrations reduced the stability of the fcc phaseand the SFE of CoCrFeMnNi and/or CoCrFeNi alloys(Fig. 1), and increased the elastic modulus,
anisotropy, and lattice friction stress [1-2]. Further,increasing the Co concentration or minor addition of
Mo promoted the yield strength [3].
Based on these findings, we developed aseries of strong and ductile metastable fcc-phase
HEAs with mechanical properties that were superior
to those of the equiatomic counterparts. The
investigation revealed that the enhancement of
mechanical properties was attributed to the lowSFE-induced restriction of planar behavior of
dislocations, twinning induced plasticity, straininduced martensitic transformation, and/or Moaddition induced strengthening. For instance, Figure
2 shows that mechanical twinning and martensitic
transformation strengthened the HEAs, which
exhibited superior tensile strength and ductility, as
well as very high and stable strain hardening rates[3]. Thus, the present study offers a guideline fordesigning novel alloys.

Fig. 1 (a) Generalized-stacking fault energy of the{111}<112> slip and (b) intrinsic and extrinsic stacking
fault energies of Cox Cr25 (FeNi)75-x (x: 25-65) alloys
calculated by ab initio method [1-2].

Fig. 2 (a) Room temperature engineering tensile stressstrain curves and (b) strain hardening curves of thedesigned Co20 Cr20 Mn20Ni20 Fe20, Co35Cr20 Mn15Ni15Fe15,
and Co35Cr25Mn15Ni15Fe10 high entropy alloys [3].

References[1] D. Wei, X. Li, W. Heng, Y. Koizumi, F. He, W. M Choi, B. J. Lee,
H. S. Kim, H. Kato, and A. Chiba, Mater. Res. Lett. 7 (2), 82(2019).
[2] D. Wei, X. Li, S. Schönecker, J. Jiang, W. M. Choi, B. J. Lee, H.
S. Kim, A. Chiba, and H. Kato, Acta Mater. 181, 318 (2019).
[3] D. Wei, X. Li, J. Jiang, W. Heng, Y. Koizumi, W. M. Choi, B. J.
Lee, H. S. Kim, H. Kato, and A. Chiba, Scr. Mater. 165, 39 (2019).

Keywords: microstructure, mechanical properties, phase transformation
Daixiu Wei (Corresponding Author, Creation of Life Innovation Materials for Interdisciplinary and International Researcher
Development)
E-mail: wei1987xiu@imr.tohoku.ac.jp
Hidemi Kato (Project Leader of Creation of Life Innovation Materials for Interdisciplinary and International Researcher
Development)
E-mail: hikato@imr.tohoku.ac.jp
URL: http://www.imr.tohoku.ac.jp/en/about/divisions-and-centers/research-division/29.html

IMR KINKEN Research Highlights 202013

Infrastructural Materials

Entropy of a High-Entropy AlloyThe Professional development Consortium for Computational Materials Scientists (PCoMS) supports promising
young scholars to engage in joint research at one of the four member institutions. The results obtained by Prof.
T. Q. Nguyen of Osaka University during his extended stay at IMR, in which the entropy of a high-entropy alloy
was calculated from first-principles, are outlined below.
Predicting the stability of alloys is an importanttask for materials design and development. Owing
to the success of ab initio modeling techniquessuch as density functional theory (DFT), calculations
on the ground-state properties of materials havebecome incredibly accurate. Despite such success,however, extending ab initio calculations to multicomponent alloys exhibiting configurational disorderremains a challenge. In addition, obtaining
thermodynamic information at finite temperatures
requires suitable sampling of all the excited states atthermodynamic equilibrium, which would becomputationally prohibitive via brute-force quantum
mechanical calculations alone.
The cluster variation method (CVM) [1]expresses the free energy in terms of a set of cluster
probabilities and incorporates a wide range ofconfigurational freedom. In the present study, firstprinciples CVM formalism was applied to study theentropy and compositions of constituent elements
as a function of temperature for the FCC quaternary
system CrFeMnNi. The equiatomic composition ofthis alloy, Cr25Fe25Mn25Ni25, has been attracting
widespread interest as a high-entropy alloy (HEA).
For the abovementioned quaternary system,
there are ten different pairs. Thus, to find theinteraction energy of each pair, we considered ten
structures with different alloy compositions: the totalenergy calculations for these systems were
performed using the DFT method and pairinteraction energies were extracted based on ourprevious study [2] developed for the Fe-C systemwith point defects. To determine the configurational
entropy, we employed a tetrahedron approximation
within CVM.
Among the various results, the temperaturedependence of entropy was compared between
CVM and a primitive Bragg-Williams (BW)approximation. A separate calculation showed thatthe stable FCC CrFeMnNi alloy was mostly
composed of Mn at low temperature. The
concentration of Mn decreased gradually as thetemperature increased and rapidly dropped toFig. 1 Temperature dependence of entropy calculated by
the Bragg-Williams approximation and cluster variation
method.

~30% at ~850 °C (0.097kBT), while the concentrationof the other three components increased. Beyond
1,235 °C (0.130kBT), the concentration of eachcomponent in the alloy approached the value of
25%. Accordingly, the entropy increased withtemperature and reached a steady-state value. In
the low temperature region, both CVM and BW
provided closed values. However, at a higher
temperature, the BW approximation slightly
overestimated entropy. The difference, however,
was not significant, which may be a characteristicfeature of HEAs.
References[1] R. Kikuchi, Phys. Rev. 81, 988 (1951).
[2] T. Q. Nguyen, M. C. S. Escano, K. Sato, Y. Shibutani, T.
Oguchi, and T. Mohri, ISIJ International 59, 2343 (2019).

Keywords: computational materials science
Yayoi Terada (PCoMS) and Tien Quang Nguyen (Institute for Nano Science Design, Osaka University)
E-mail: terada@imr.tohoku.ac.jp, quang@insd.osaka-u.ac.jp
URL: http://pcoms.imr.tohoku.ac.jp/index_e.html

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IMR KINKEN Research Highlights 2020

Energy-Related Materials

IMR KINKEN Research Highlights 2020

Research

Energy-Related Materials

In Situ Observation of the Twin-Related Dendrite Growth in GaSb

Crystal–melt interfaces were directly observed during the directional solidification of GaSb and the mechanismof faceted dendrite growth was studied. The dendrites propagated in either <110> or <112> direction. It was
found that dendrite growth is related to nucleation at the re-entrant corners formed at twin boundaries.

The GaSb substrate is used in various electronic
devices, e.g., mid-infrared emitting lasers andthermophotovoltaic cells. As the crystal quality ofthe substrate has a significant influence on theelectrical properties of the device, it is important tocontrol the density and distribution of defects suchas grain boundaries and dislocations in the crystal.
It is known that re-entrant corners at twin boundaries
provide favorable nucleation sites and puresemiconductors Si and Ge show twin-related
dendrite growth. Even though twin formation is aserious problem during the solidification of group
III–V compound semiconductors, its effect on
dendrite growth in such semiconductors has notbeen studied extensively. In this study, dendrite
growth during the directional solidification of GaSb
was observed and the growth mechanism wasclarified [1].
The in situ observation system used in thisstudy consisted of a furnace and a digital microscope
equipped with a zoom lens having a long working
distance. The furnace had two carbon heaters thatwere independently controlled to produce a thermal
gradient inside the furnace. GaSb was melted under
an argon atmosphere, and the temperature wassubsequently decreased to promote directional
growth. The digital microscope was used to observe
the GaSb crystal–melt interface and the growthvelocity of the interface (V) was measured by tracing
its position. The crystallographic orientations of the
GaSb dendrites in the areas observed were
determined by electron backscatter diffraction(JSM-6610A, JEOL).
A planar crystal–melt interface was maintained
at low V. The planar shape became instable and afaceted interface was formed at V higher than thecritical growth velocity (Vc). The dendrite growth
was initiated at the faceted interface. Figure 1 shows
the photographic images and schematics of the
GaSb dendrite propagating in the <112> direction.
The dendrite grown along <112> displayed
continuous formation of two triangular crystals witha tip angle of 60°, and the growth direction of thetwo triangular crystals changed alternatingly. The

Fig. 1 In situ observation of the dendrite growth in GaSb.
Schematics of the growth process are shown next tophotographic images.

dendrite grew not only along the <112> direction but
also along the <110> direction. The dendrite grownalong <110> showed that the tip angle was 120°and the growth direction of the tip did not change.
Each type of dendrite exhibited a unique shape andpropagation mechanism, and both growth
processes were similar with those in Si. It wasclarified that dendrites in GaSb grow via nucleation
at the re-entrant corners located at the twinboundaries and more than two twin boundaries are
required for the propagation of the dendrite.
References[1] K. Shiga, M. Kawano, K. Maeda, H. Morito, and K. Fujiwara,
Scr. Mater. 168, 56 (2019).

Keywords: crystal growth, grain boundaries, solar cells
Keiji Shiga (Crystal Physics Research Laboratory)
E-mail: kshiga@imr.tohoku.ac.jp
URL: http://www.imr.tohoku.ac.jp/en/about/divisions-and-centers/research-division/02.html

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IMR KINKEN Research Highlights 2020

IMR KINKEN Research Highlights 2020

Discovery of Magnetic Field-Induced Pyroelectricity above
40 T in Multiferroic LiNiPO4

A new magnetically ordered and pyroelectric phase has been discovered in multiferroic LiNiPO in the course
4of determining its magnetization and a direct measurement of electric polarization in pulsed high magnetic
fields up to 56 T. Pulsed-field Laue neutron diffraction up to 42 T revealed that commensurate canted
antiferromagnetic order was realized in this newly observed phase. A model was proposed to explain how themagnetoelectric effect was driven by the canted antiferromagnetic order.

Multiferroic materials, in which magnetic and electriceffects are simultaneously realized, are believed to be
crucial for achieving cross-control between magnetismand electricity. There are three representative theoreticalmodels explaining multiferroicity in multiferroic materials,
namely, the spin current model based on the inverse
Dzyaloshinskii-Moriya (DM) interaction, spin-dependent
p-d hybridization model, and exchange striction model.
Multiferroic LiNiPO4 is an attractive compound
because it exhibits a rich variety of magnetic phases. In
this material, a linear magnetoelectric (ME) effect isobserved along the a- and c-axes below 20.8 K when amagnetic field is applied along the c- and a-axes,
respectively (Fig. 1(a)). This ME effect is driven by the
non-collinear antiferromagnetic spin structure. Because
this effect cannot be explained by any of the models
mentioned above, it is worth studying LiNiPO4 for a
deeper understanding of multiferroicity in general.
Recently, we discovered the magnetic field-induced
phase transitions in LiNiPO4 at 37.6, 39.4, and 54 T andthe re-entrant ME effects in its phases IV and VII, as
shown in Fig. 1(a) [1,2]. We previously proposed a modelin which the commensurate antiferromagnetic order
generated the linear ME effect in phase IV [1]. On theother hand, the magnetic field dependence of electric
polarization in phase VII was clearly different from that inphases I and IV, in which a constant and a quadratic termin the ME tensor played an important role (i.e., phase VII
was pyroelectric).
To investigate the magnetic structure in these newly
observed phases, neutron diffraction measurements were
performed under the pulsed high magnetic fields up to 42
T on the NOBORU at J-PARC (Ibaraki, Japan) [1,2]. The
combination of the white beam of the spallation neutron
and the pulsed field allowed scanning of a wide area ofreciprocal space in high magnetic fields. Figures 1(b)–1(d)show the results of the neutron diffraction measurements
at 4.2 K, where the magnetic field was applied along thec-axis. The incommensurate K = (0 2/3 0) was observedin phases V and VI, and the commensurate K = (0 1 0)
was observed in phase VII [2]. A model was proposed

Fig. 1 (a) Dependence of magnetization and electric
polarization along the a-axis measured at 4.2 K on themagnetic field. (b)–(d) Integrated intensities of the neutron
Laue diffraction for each phase (V, VI, and VII).

based on the combination of the superexchangeinteraction, DM interaction, and elastic distortion, which
comprehensively explained the ME effect of LiNiPO4.
References[1] R. Toft-Petersen, E. Fogh, T. Kihara, J. Jensen, K. Fritsch, J. Lee, G.
E. Granroth, M. B. Stone, D. Vaknin, H. Nojiri, and N. B. Christensen,
Phys. Rev. B. 95, 064221 (2017).
[2] E. Fogh, T. Kihara, R. Toft-Petersen, M. Bartkowiak, Y. Narumi, O.
Prokhnenko, A. Miyake, M. Tokunaga, K. Oikawa, M. K. Sørensen, J.
C. Dyrnum, H. Grimmer, H. Nojiri, and N. B. Christensen, Phys. Rev. B
101, 024403 (2020).

Keywords: multi-ferroics, neutron scattering, high magnetic field
Takumi Kihara (Magnetism Research Laboratory)
E-mail: t_kihara@imr.tohoku.ac.jp
URL: http://www.hfpm.imr.tohoku.ac.jp/index.html

IMR KINKEN Research Highlights 202017

Energy-Related Materials

Phononic Thermal Hall Effect in Diluted Terbium Oxides

Thermal Hall effect is the deflection of thermal current depending on the sign of the magnetic field. The unconventional
mechanism of the thermal Hall effect in magnetic insulators has generated considerable interest. Herein, the phononic
mechanism of the thermal Hall effect in terbium oxides has been demonstrated by magnetic dilution.
In dielectrics, charge-neutral excitations such as
phonons and magnetic excitations carry the heatcurrent. The Lorentz force, which is the origin of theordinary Hall effect, does not act on these chargeneutral particles. However, similar to the anomalous
Hall effect in itinerant ferromagnets, it is expected thatthe charge-neutral heat current is deflected by the
spin-orbit interaction in magnetic dielectrics. Strohm
et al. first reported the thermal Hall effect in aparamagnetic dielectric, Tb3Ga5O12 [1]. The thermal
Hall effect has subsequently been observed in several
magnetic dielectrics [2,3]. While this mechanism insome dielectrics can be explained by the Berryphase-induced magnon Hall effect due to the
Dzyaloshinskii-Moriya interaction [2], the mechanisms
for other materials are not well understood. In
particular, the thermal Hall effect observed in theparamagnetic state of some frustrated magnets [3]has two possible mechanisms, i.e., the phonon Hall
effect and the Hall effect due to exotic magnetic
excitations, which are quite difficult to differentiate.
Here, we propose that the dilution of the magnetic
moment is useful for clarifying this issue. In the caseof exotic magnetic excitation, the magnetic dilution
should largely suppress the thermal Hall effectbecause the heat current is carried by a magnetic
interaction. On the other hand, the phonon Hall effectis induced by the deflection of the phonon current
originating from the local magnetoelastic coupling
and the effect of magnetic dilution is more gradual.
The dilution effect of the thermal Hall effect in
Tb2Ti2O7 has been studied previously [4]. Hirschberger
et al. observed a large thermal Hall effect in Tb2Ti2O7
below 100 K [3]. Figure 1 shows the magnetic fielddependence of thermal Hall conductivity divided bytemperatureatvarioustemperatures
for(Tb0.3Y0.7)2Ti2O7, in which the magnetic terbium ionsare diluted with nonmagnetic yttrium ions. It is evident
from the plots that there was finite thermal Hall
conductivity for the diluted sample below 80 K.
The magnitude of the thermal Hall effect wascomparable or even larger than that in pristine
Tb2Ti2O7. The dependence of the thermal Hall effecton the magnetic field was quite similar to that ofFig. 1 (a), (b) Dependence of thermal Hall conductivity
divided by temperature on the magnetic field for(Tb0.3Y0.7)2Ti2O7 at various temperatures. The dashedlines indicate the result of fitting with the Brillouin function.

magnetization in a Curie-type paramagnet, for which
the dependence of magnetization on temperatureand magnetic field is described by the Brillouinfunction. In fact, the dependence on magnetic field
can be scaled by paramagnetic magnetization
derived from the Brillouin function (dashed lines). This
indicates that the magnetic scattering of phonons is
the mechanism of the thermal Hall effect.
References[1] C. Strohm, G. L. J. A. Rikken, and P. Wyder, Phys. Rev. Lett. 95,
155901 (2005).
[2] Y. Onose, T. Ideue, H. Katsura, Y. Shiomi, N. Nagaosa, and Y.
Tokura, Science 329, 297 (2010).
[3] M. Hirschberger, J. W. Krizan, R. J. Cava, and N. P. Ong,
Science 348, 106 (2015).
[4] Y. Hirokane, Y. Nii, Y. Tomioka, and Y. Onose, Phys. Rev. B 99,
134419 (2019).

Keywords: magnetism, frustrated spin system, insulator
Yoshinori Onose (Quantum Functional Materials Physics Research Laboratory)
E-mail: onose@imr.tohoku.ac.jp
URL: http://www.imr.tohoku.ac.jp/en/about/divisions-and-centers/research-division/08.html

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IMR KINKEN Research Highlights 2020

Molecular Dynamics Simulation of Wear Process in Diamond-Like
Carbon for Low-Friction and High-Durability Material Design

Low-friction and high-durability material design requires a deep understanding of multi-physics phenomena,such as “chemical reaction” and “friction, impact, and stress.” We investigated the friction of diamond-likecarbon (DLC) asperities using a reactive molecular dynamics simulator developed by us, which can consider
tribochemical reactions. Further, we elucidated the atomic-scale chemical wear of DLC through the generationof hydrocarbon molecules.

Super-low-friction and high-wear-resistanttechnologies are important not only for reducing
energy loss but also for improving the durability ofindustrial machines. Accordingly, diamond-likecarbon (DLC) is a promising solid lubricant due to
its low friction. The friction mechanism of DLC
depends on multi-physics phenomena such as“chemical reaction” and “friction, impact, andstress,” and thus is difficult to elucidate. The tightbinding quantum chemical molecular dynamics(MD) code developed by us has already elucidated
a low-friction mechanism and succeeded indesigning tribology materials. However, thismethodology cannot consider the wear process, as
the model size is limited to a few thousand atoms.
Thus, we developed a reactive MD simulator for
friction, which can consider tribochemical reactions.
Further, we succeeded in elucidating the wear
process of DLC asperities using a model withseveral tens of thousands of atoms [1-3].
Figure 1 shows the wear process of the two
DLC asperities simulated using our MD code. Afterthe contact of the DLC asperities during the sliding,hydrocarbon molecules such as CH4, C2H4, C2H6,
etc. are generated and evaporated from the friction
interface, indicating the atomic-scale chemical wearof DLC (Fig. 1c). In our previous small model, we
could only observe bond generation and dissociation
at the friction interface; we could not observe thegeneration and evaporation of hydrocarbon
molecules. The evaporation of hydrocarbon
molecules by chemical wear was proved using insitu experiments. We employed a quadrupole mass
spectrometer (QMS) in a vacuum chamber, which
can perform the in situ detection of the molecules
generated during friction (Fig. 1d). The QMS
detection also showed the generation of hydrocarbon
molecules such as CH4, C2H4, and C2H6 during thefriction of DLC and the order of the number ofgenerated molecules is CH4 > C2H4 > C2H6. Thus,
the order of the number of evaporated molecules

Fig. 1 Friction snapshots of DLC asperities at (a) 0, (b) 30,
and (c) 100 ps. (d) QMS detection results for each moleculeduring the friction experiment of DLC.

obtained through our simulation is also consistent
with the experiments, indicating that the increase in
the model size enables the direct comparison ofsimulations and experiments.
References[1] Y. Wang, N. Yamada, J. Xu, J. Zhang, Q. Chen, Y. Ootani, Y.
Higuchi, N. Ozawa, M.-I. D. B. Bouchet, J. M. Martin, S. Mori, K.
Adachi, and M. Kubo, Sci. Adv. 5, eaax9301 (2019).
[2] Y. Wang, J. Xu, Y. Ootani, N. Ozawa, K. Adachi, and M. Kubo,
Carbon 154, 7 (2019).
[3] J. Zhang, Y. Wang, Q. Chen, Y. Su, J. Xu, Y. Ootani, N. Ozawa,
K. Adachi, and M. Kubo, J. Comput. Chem. Jpn. 18, 103 (2019).

Keywords: simulation, molecular dynamics, interface
Momoji Kubo (Materials Design by Computer Simulation Research Laboratory)
E-mail: momoji@imr.tohoku.ac.jp
URL: http://www.simulation.imr.tohoku.ac.jp/eng/index.html

IMR KINKEN Research Highlights 202019

Energy-Related Materials

Detection of the Phase Separation of Thermally Aged and NeutronIrradiated Fe-Cr Binary Alloys Using Positron AnnihilationT

he Fe-Cr system is the basis for important engineering materials known as ferritic stainless steels, which areheat-resistant ferritic steels that are suitable as advanced structural materials for fission and fusion reactors. We
detect an atomic-scale phase transition in a thermally aged and neutron-irradiated Fe-Cr system by using
positron annihilation spectroscopy.
Fe-Cr alloys are susceptible to embrittlement at
748 K (475 °C) after thermal aging at approximately
748 K due to a phase transition where the ferrite
phase decomposes into Fe- and Cr-rich ones. As
shown in Fig. 1, increases in hardness after thermal
aging at 748 K can be observed in the Fe1-x Crx
alloys for 0.15 < x < 0.90.
The normalized W-parameters evaluated using
the coincident Doppler broadening (CDB)
measurements in positron annihilation spectroscopy
for the Fe1-x Crx alloys before and after aging areshown in Fig. 2. These experimental results and abinitio calculation results showed a preferential
position annihilation (PPA) of Fe atoms byconsidering the effect of the first- and secondnearest neighbor atoms. After aging at 748 K, theformation of isolated Fe atoms in the Cr-rich phases
enhanced the PPA and increased the W-parameter.
The formation of Fe-rich phases and thechanges in the interface conditions between the Feand Cr-rich phases also influenced the W-parameter.
The progress of phase separation in the Fe-Cr
binary alloys was detected via positron annihilation
CDB measurements.
The phase separation of Fe-Cr alloys after
neutron irradiation at 573 K was also detected via
positron annihilation CDB measurements [2]. The
PPA of the Fe-rich phase can affect the detection ofvacancy clusters in Fe-Cr alloys using positronannihilation lifetime spectrometry.

Fig. 1 Cr dependence of Vickers hardness in Fe-Cr binary
alloys [1]References[1] R. Kasada and K. Sato, Philos. Mag. Lett. 99, 360 (2019).
[2] Y. Noshita, K. Sato, H. Yamashita, R. Kasada, Q. Xu, M.
Hatakeyama, and S. Sunada, Nucl. Mater. Energy 15, 175(2018).

Fig. 2 Cr dependence of the normalized W-parameter of FeCr alloys before and after thermal aging at 475 °C [1]

Keywords: nuclear materials, strength, positron annihilation
Ryuta Kasada (Nuclear Materials Engineering Research Laboratory)
E-mail: r-kasada@imr.tohoku.ac.jp
URL: http://www.imr.tohoku.ac.jp/ja/about/divisions-and-centers/research-division/13.html

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IMR KINKEN Research Highlights 2020

Effect of a Substrate on the Nucleation and Growth of
Colloidal Crystals

S

ubstrate effects on the nucleation and growth of colloidal crystals have been successfully demonstrated.
Interactions between the colloidal particles and substrate were found to play an important role in nucleation,
which is related to interfacial free energy. Further, it was found that lattice mismatch between the substrate andepitaxial layers had a significant effect on nucleation and growth. These findings may help achieve sophisticatedcontrol over the growth of colloidal crystals.
High-quality and large-sized crystals are desired
for the versatile device applications of colloidal
crystals. To this end, control over their nucleation andgrowth is required. Herein, the effects of the substrateon the nucleation and growth of colloidal crystals aredescribed.
The nucleation rates, J, of two-dimensional (2D)
colloidal crystals on uncoated cover glass, Pt-coatedcover glass, and Au-coated cover glass were
measured [1]. Among the three substrates, the J valueof the uncoated cover glass was the largest, while it
was the smallest for the Pt-coated glass under thesame supersaturation conditions. The interfacial freeenergy change, Δσ, was obtained from the nucleation
rate based on the classical nucleation theory. It
revealed that the uncoated and Pt-coated coverglasses possessed the smallest and largest Δσ,respectively. Since Δσ originates from the bond energy
in atomic crystals, we deduced that it is related to theinteractions between the colloidal particles and thesubstrate. The interactions between them aredetermined by surface force measurements using
atomic force microscopy. The results showed that theadhesive force between the particles and uncoatedcover glass was the weakest, while that for the Ptcoated sample was the strongest. A larger attractive
interaction between the colloidal particles and thesubstrate yielded a higher Δσ, and thus a smaller J.
Further. the interaction between the particles andsubstrate had a great influence on the heterogeneous
nucleation rate of the colloidal crystals.
The substrate effect was also recognized in thegrowth of binary colloidal crystals (BCCs). Owing totheir tunable material properties, BCCs are highly
desired for many applications. We have investigatedthe detailed growth process of BCCs in which depletion
forces were induced between the particles by addinga polymer [2]. An AB2 superlattice phase (A: large, B:
small) was grown under various solution conditions,
including various large-to-small particle ratios andpolymer concentrations. In-situ observations revealed

Fig. 1 Particle interaction and nucleation rate for threedifferent substrates.

Fig. 2 One-dimensional heteroepitaxy of binary colloidal
crystals with AB2 structure.

that BCCs typically grew via one-dimensional
heteroepitaxy, which differs from the conventional
colloidal epitaxial growth, demonstrating a novel
technique to control the growth of BCCs.
References[1] S. Guo, J. Nozawa, M. Mizukami, K. Kurihara, A. Toyotama, J.
Yamanaka, H. Niinomi, J. Okada, and S. Uda, Cryst. Growth Des.
19, 3215 (2019).
[2] J. Nozawa, S. Uda, A. Toyotama, J. Yamanaka, J. Okada, and H.
Niinomi, Cryst. Growth Des. 20, 3247 (2020).

Keywords: colloid, nucleation & growth, epitaxial growth
Jun Nozawa (Crystal Chemistry Research Laboratory)
E-mail: nozawa@imr.tohoku.ac.jp
URL: http://www.uda-lab.imr.tohoku.ac.jp/index-e.html

IMR KINKEN Research Highlights 202021

Energy-Related Materials

Complex Hydride Lithium Superionic Conductor for HighEnergy-Density Lithium-Metal-Based All-Solid-State BatteriesT

he complex hydride lithium superionic conductor, 0.7Li(CB9H10)–0.3Li(CB11H12), achieved by designingstructures of hydrogen clusters (complex anions), showed excellent stability against lithium metal and a highconductivity of 6.7 × 10−3 S cm−1 at 25 °C. This material enables lithium-metal-based all-solid-state batteries
with the highest energy to date.
Lithium metal is widely believed to be theultimate anode material for rechargeable batteries
because it has the highest theoretical capacity(3860 mAh g−1) and the lowest potential (–3.04 V vs.
standard hydrogen electrode) among known anodematerials [1]. Complex hydrides have received a lotof attention in making it possible to use the lithium
metal anode because of their outstanding chemicaland electrochemical stability against the lithium
metal. However, because of their low ionic
conductivity, using complex hydrides with the lithium
metal anode has never been attempted in practicalbatteries.
In an effort to overcome the chronic drawbacksof complex hydrides and also realize lithium-metalbased batteries, we developed a new complex
hydride lithium superionic conductor, 0.7Li(CB9H10)–
0.3Li(CB11H12) [2]. The superionic conductivity wasachieved by stabilizing the high temperature (high-T)
phase of the complex hydride containing closo-type(cage-like) complex anions (Fig. 1a). Li(CB9H10) wasused as the host starting material for the stabilizationof the high-T phase because of its low phasetransition temperature (90 °C) and high lithium ion
conductivity, approaching 10−1 S cm−1 for the high-T
phase. Our structural characterizations clarified thatthe high-T phase of Li(CB9H10) was stabilized by thepartial substitution of [CB9H10]− with [CB11H12]−.
0.7Li(CB9H10)–0.3Li(CB11H12) did not show any
phase-transition-derived changes in conductivity
and a linear increase in the logarithmic values (Fig.
1b). At temperatures above 90 °C, the conductivities
of 0.7Li(CB9H10)–0.3Li(CB11H12) matched well withthose of Li(CB9H10). More importantly, the lithium ion
conductivity at 25 °C of 0.7Li (CB9H10)–
0.3Li(CB11H12) was 6.7 × 10−3 S cm−1, which is thehighest value reported to date for complex hydride
lithium ion conductors.
The developed complex hydride has several
advantages as a solid electrolyte in all-solid-statebatteries. In particular, its intrinsic stability with thelithium metal anode enables facile and stable lithium

Fig. 1 (a) Structures of closo−type (cage−like) complex
anions, [CB9H10]− and [CB11H12]−. (b) Arrhenius plots of
the lithium ion conductivities of Li(CB9H10) and
0.7Li(CB9H10)–0.3Li(CB11H12). Inset shows the all-solidstate Li–S battery.

ion transfer at the interface with negligible resistance(<1 Ω cm2). Additionally, the high ionic conductivityof 0.7Li(CB9H10)–0.3Li(CB11H12) coupled with its
stability with the lithium metal results in a stable
cycling of the high-energy-density all-solid-statelithium–sulfur (Li–S) batteries, even near room
temperature.
References[1] D. Lin, Y. Liu, and Y. Cui, Nat. Nanotechnol. 12, 194 (2017).
[2] S. Kim, H. Oguchi, N. Toyama, T. Sato, S. Takagi, T. Otomo, D.
Arunkumar, N. Kuwata, J. Kawamura, and S. Orimo, Nat.
Commun. 10, 1081 (2019).

Keywords: hydride, ionic conductor, energy storage
Sangryun Kim and Shin-ichi Orimo (Hydrogen Functional Materials Research Laboratory)
E-mail: sangryun@imr.tohoku.ac.jp
URL: http://www.hydrogen.imr.tohoku.ac.jp/en/index.html

22

IMR KINKEN Research Highlights 2020

Electronic Materials

IMR KINKEN Research Highlights 2020

ResearchElectronic Materials

Electrical Manipulation of Magnetization in TopologicalSemimetals

We theoretically study the spin-transfer torque induced by an electric field and analyze the dynamics of themagnetic domain walls in magnetic Weyl semimetals. Using several models for magnetic Weyl semimetals, wepredict that the spin-transfer torque can be significantly enhanced. Moreover, due to the suppressed longitudinal
conductivity by thin domain-walls, the dissipation by Joule heating associated with the spin-transfer torquebecomes much smaller than that in bulk metallic ferromagnets. Consequently, domain wall motion can be
efficiently controlled with low energy consumption in Weyl semimetals, as required for spintronic devices.

The electric control of spin magnetization canpotentially be used in next-generation magnetic
devices, allowing information to be written
electronically. For instance, the magnetic racetrack
memory has been proposed as a promising
application of current-induced spin torque to
spintronics devices. However, the sizeable current
density required to operate the device limits its
efficiency because of Joule heating, thus impedingits commercial application. Hence, more efficientmethods of controlling magnetization are
indispensable for practical applications.
To determine whether magnetic Weyl
semimetals may be used in low-energy-consumption
magnetic devices, we theoretically studied the spin
torque acting on inhomogeneous local magnetizationby computing the non-equilibrium spin density
induced by applied electric fields [1,2,3]. We
developed and employed a lattice model of acandidate Weyl semimetal, Co3Sn2S2 [2], andnumerically calculated the induced spin density. We
derived the dynamics of the domain wall driven by
the current-induced spin torque. The estimatedvelocity of a domain wall is one order of magnitudelarger than that in conventional ferromagnetic metals[1,2].
Figure 1 shows the model and the electronic
structures of the system. The conduction andvalence bands are in contact at several points,
around which the energy dispersion is linear. These
results are consistent with those of first-principles
calculations.
To analyze the dynamics of the domain wall, we
employed Thiele’s approach to map the Landau–
Lifshitz–Gilbert equation to the equation of motion
for the center coordinates of the domain wall andcalculated a domain wall velocity that is one order ofmagnitude larger than that of the ferromagnetic
nanowire. When the magnetization texture varies
rapidly, the spin-transfer torque does not depend on

Fig. 1 (a) Crystal lattice structure of the magnetic Weyl
semimetal Co3Sn2S2. (b) Energy band dispersion. (c)Density of state (DOS). (d) Domain wall motion.

the impurity-scattering relaxation time. In the Weyl
semimetal phase, the longitudinal conductivity isvery small because of the vanishing density ofstates; therefore, dissipation due to Joule heating is
suppressed. Consequently, domain wall motion in
the Weyl semimetal can be controlled much more
efficiently than in conventional magnets. Magnetic
Weyl semimetals can overcome the challenge ofenergy consumption for next-generation informationand communication technology and are new
candidates for high-performance magnetic devices.
References[1] D. Kurebayashi and K. Nomura, Sci. Rep. 9, 5365 (2019).
[2] A. Ozawa and K. Nomura, J. Phys. Soc. Jpn. 88, 123703(2019).
[3] S. Kim, D. Kurebayashi, and K. Nomura, J. Phys. Soc. Jpn. 88,
083704 (2019).

Keywords: spintronics, spin transfer, topological insulator
Kentaro Nomura (Theory of Solid State Physics Research Laboratory)
E-mail: nomura@imr.tohoku.ac.jp
URL: http://www.bauer-lab.imr.tohoku.ac.jp/?page=&lang=en

24

IMR KINKEN Research Highlights 2020

IMR KINKEN Research Highlights 2020

The Beginning of Paramagnetic Spintronics

In spintronics, paramagnetic insulators have been believed to be unimportant materials. Here, we show that aparamagnetic Gd3Ga5O12, the most typical substrate for growing magnets, is a very good spin conductor. The
unique advantages of paramagnets are expected to draw interest and open a new era for paramagnetic spintronics.

Current
B

GGG

Pt

150

V (nV)

Exploring new materials for efficient spintransport is important in spintronics. In magnets,
spin waves, the collective motion of magnetization
supported by strong exchange interactions, carryspin current for long distances. However,paramagnets with weak exchange interactions are
considered useless materials in spintronics.
In the present study [1], we report theobservation of long-range spin transport inparamagnetic insulators. We used the paramagnetic
insulator Gd3Ga5O12 (GGG), which exhibits a large
field-induced magnetization M at low temperatures
T under high magnetic fields B. The device consistsof two Pt contacts, which are electrically separated
from each other, on a GGG slab (Fig. 1, top). We
measured the voltage across the right Pt contact
while applying charge current to the left Pt contact.
The applied current drives the spin Hall effect (SHE),a charge to spin conversion phenomenon, andinjects spin current into GGG. The spin current
propagates and generates a voltage signal at theother contact via the inverse process of SHE.
The bottom panel of Fig. 1 shows the B
dependence of the voltage signal (V) at 300 K and
5 K. When B was applied, we found a clear V signal
at 5 K, but no signal appeared at 300 K. V increases
with increasing B. The shapes of V are consistent
with those of M of GGG. From the systematic
measurements, we concluded that the observed
signal is evidence of the long-range spin transportof the paramagnetic GGG.
Considering the results of the experiments andthe model, we found that GGG exhibits a better spin
conductivity than the magnet YIG, which is known
as the best spin conductor. We speculate that longrange dipole interaction is a plausible mechanism of
the spin transport. When GGG is exposed to high B
at low T, it acquires a large M and can support spinwaves mediated by dipole interactions even inparamagnetic insulators.
This result expands the material class of
spintronics to paramagnetic insulators (which haveunique advantages), and develops the new research
field of Paramagnetic Spintronics.

Pt/GGG/Pt
T=5K

100
50
T = 300 K
0
-4

-2

0
B (T)

2

4

Fig. 1 (top) Schematic illustration of the experimental
setup. Spin current in GGG is excited and detected
electrically at the Pt contacts using direct and inverse
spin Hall effects. (bottom) Magnetic field dependenceof the signal. At 5 K, the signal appears when magnetic
field is applied.

References[1] K. Oyanagi, S. Takahashi, L. J. Cornelissen, J. Shan, S.
Daimon, T. Kikkawa, G. E. W. Bauer, B. J. van Wees, and E.
Saitoh, Nat. Commun. 10, 4740 (2019).

Keywords: spintronics, spin current, spin dynamics
Koichi Oyanagi and Eiji Saitoh (Surface and Interface Research Laboratory)
E-mail: k.0yanagi444@gmail.com, eizi@ap.t.u-tokyo.ac.jp
URL: http://saitoh.t.u-tokyo.ac.jp/

IMR KINKEN Research Highlights 202025

Electronic Materials

Make It Flexible! — A Magnetic Alloy for Flexible Hall Sensors

With the rapid technological development toward Internet-of-Things, there has been an increasing demand

Magnetic-field sensors capable of electricallydetecting a magnetic field are widely used inmodern electronics. Their applications include themonitoring of electric current and motion of
micromechanical parts and use as an electricalcompass. One such device is a Hall sensor [1],which employs the ordinary Hall effect insemiconductors such as Si and GaAs. To generatea large output Hall voltage under a given magnetic
field, low-carrier and high-mobility semiconductors
must be used. This inevitably involves hightemperature epitaxial growth of the semiconductor
films, making it difficult to integrate the device
directly on flexible electronics circuits that are
essential in the emerging Internet-of-Things (IoT)
technology.
A similar Hall-sensor function can, in principle,
be obtained using the anomalous Hall effect (AHE).
As this effect is related to magnetization andelectronic band structure, it can be large in a metal[2]. However, magnetic metals with sufficiently large
and linear AHE responses against the magnetic
field have not been discovered as yet. In this study,
we found that thin films of environmentally friendly
Fe-Sn alloy are appealing as a material for the AHEtype Hall sensor [3,4]. The AHE in the Fe-Sn alloy
films is larger than that observed in most metals(Fig. 1) and comparable to that recently reported forthe single crystal of kagome-lattice topologicalferromagnet Fe3Sn2 [5]. The AHE characteristics(Hall voltage vs. magnetic field under a constant
input current) showed thermally stable operation asa Hall sensor over a wide temperature range. The
Fe-Sn alloy films could be grown at room temperature
on various substrates, including a flexible polymer(Fig. 1 inset), using a scalable sputtering method.
These features enabled us to perform a proof-ofconcept experiment for a “flexible Hall sensor” (Fig.
1 inset) [3].
The mechanism explaining the large AHE in theroom temperature-grown, rather disordered Fe-Sn
alloy films, is not clear as yet. On the basis of theresults of impurity doping [6], we proposed that theHall voltage (mV)

for magnetic-field sensors with added functionality. A simple Fe-Sn magnetic alloy was demonstrated to be a
promising material for the development of flexible magnetic-field sensors. The operating principle based on theanomalous Hall effect, which has not been exploited yet in flexible magnetic-field sensors, is now in the limelight.

IMR KINKEN Research Highlights 2020

bent

2.0

flat

1.0

T = 300 K
I = 0.1 mA

0.0
0.0
0.5
1.0
Magnetic field (T)Fe0.60Sn0.40 filmon PEN sheet
Fig. 1 40-nm-thick Fe0.60Sn0.40 alloy thin film sputtered ona flexible polyethylene naphthalate (PEN) sheet (area =15 mm × 15 mm). Inset: AHE characteristics measured
with and without mechanical bending.

intrinsic AHE arising from the topological characterof electronic bands [2] is operative in the Fe-Sn alloy
films. Such a new degree of freedom in engineering
Hall-sensor performance is a lucrative incentive forthe utilization of magnetic materials, especially with
topological features, in the next-generationelectronic devices.
References[1] R. S. Popović, Hall Effect Devices, 2nd ed. (CRC Press, 2003).
[2] N. Nagaosa, J. Sinova, S. Onoda, A. H. MacDonald, and N. P.
Ong, Rev. Mod. Phys. 82, 1539 (2010).
[3] Y. Satake, K. Fujiwara, J. Shiogai, T. Seki, and A. Tsukazaki,
Sci. Rep. 9, 3282 (2019).
[4] J. Shiogai, Z. Jin, Y. Satake, K. Fujiwara, and A. Tsukazaki,
Appl. Phys. Express 12, 123001 (2019).
[5] L. Ye, M. Kang, J. Liu, F. von Cube, C. R. Wicker, T. Suzuki, C.
Jozwiak, A. Bostwick, E. Rotenberg, D. C. Bell, L. Fu, R. Comin,
and J. G. Checkelsky, Nature 555, 638 (2018).
[6] K. Fujiwara, Y. Satake, J. Shiogai, and A. Tsukazaki, APL
Mater. 7, 111103 (2019).

Keywords: alloy, thin films, magnetic properties
Kohei Fujiwara (Low Temperature Physics Research Laboratory)
E-mail: kfujiwara@imr.tohoku.ac.jp
URL: http://mu.imr.tohoku.ac.jp/index_en.html

26

3.0

IMR KINKEN Research Highlights 2020

Lattice Dynamics Coupled to Electronic Ferroelectricity

Inelastic neutron scattering measurements on the molecular dimer-Mott insulator κ-(BEDT-TTF) Cu[N(CN) ]Cl2

2

reveal a phonon anomaly over a wide temperature range. Starting from Tins = 50-60 K, where the charge gap
opens, low-lying optical phonon modes become over-damped upon cooling towards the antiferromagnetic
ordering temperature TN = 27 K, where a ferroelectric ordering at TFE to TN also occurs. Phonon damping
decreases again when spins and charges are ordered below TN, while no change on the lattice symmetry isobserved across TN. We attribute the phonon anomalies to structural fluctuations coupled to the degrees of
freedom of charge and spin in BEDT-TTF molecules.
TINS

Intensity at
E=1.5meV (counts)

TFE
400
300

overdamp

200
100

10000

0.002

χ (emu/mole)

ρ

0.001

localizationof
π-electron

spin/charge
order

8000
6000

itinerant
π-electron

2000

χ

0

0

4000

ρ (Ω cm)

Conventional ferroelectrics (FE), which are
driven by shifts in the atomic positions, are used incommon electronic devices. Electrons and theirinteractions play the key role in electronic FE, whichhave gained popularity due to their ultrafast
response and/or large magnetoelectric effectoriginating from electronic degrees of freedom.
Recently, the simultaneous occurrence of FE andantiferromagnetic spin order in a dimer-Mott system
κ-(BEDT-TTF)2Cu[N(CN)2]Cl (κ-Cl) has been
observed. However, the origin of the electric dipolesin this compound is still disputed because of thelack of direct evidence from optical spectroscopy,
which is sensitive to molecular charges. Given there
is a finite electron-lattice coupling, fluctuations of
the electric dipoles are expected to produce
anomalies in the lattice dynamics that can besensitively probed by neutron scattering. To study
the phonon anomaly associated with charge
disproportionation, we performed inelastic neutron
scattering on deuterated single crystals of κ-Cl
using the triple-axis spectrometers IN8 at ILL and
PUMA at FRM2 [1].
Figure 1 shows the temperature dependence ofthe phonon intensity at Q = (603) and E = 1.5 meV
obtained via the PUMA. The phonon intensity at low
energies are enhanced when TN < T < 60 K. From
the fitting of the constant-Q spectra to the damped
harmonic oscillator function, we found that the lowlying optical phonon mode at 2.6 meV becomes
overdamped over the temperature region. The onset
temperature of phonon damping roughly coincides
with the rapid increase in the electrical resistivity
below Tins = 50–60 K. Furthermore, phonon damping
decreases again when spins and charges areordered below TN = TFE = 27 K, suggesting the close
correlations among the lattice, spin, and charge. In
addition, we found that the phonon anomaly isdifferent from what is expected for conventional FE.
First, the phonon intensity does not diverge at TFE.

20

40

60
T (K)

80

100

0

Fig. 1 (top) Temperature dependence of phonon intensity
at Q = (603) and E = 1.5 meV and (bottom) electricalresistivity and DC magnetic susceptibility.

Second, no change of the lattice symmetry isobserved across TFE. Thus, we attribute the phonon
anomalies to structural fluctuations coupled to thedegrees of freedom of charge and spin in the BEDTTTF molecules. We consider these inelastic neutron
scattering results as an important step that maytrigger further systematic studies on lattice dynamics
and its coupling to the electronic degrees of freedomof organic charge-transfer salts.
References[1] M. Matsuura, T. Sasaki, S. Iguchi, E. Gati, J. Müller, O. Stockert,
A. Piovano, M. Bohm, J. T. Park, S. Biswas, S. M. Winter, R.
Valenti, A. Nakao, and M. Lang, Phys. Rev. Lett. 123, 027601(2019).

Keywords: organic, metal-insulator transition, neutron scattering
Takahiko Sasaki (Low Temperature Condensed State Physics Research Laboratory)
E-mail: takahiko@imr.tohoku.ac.jp
URL: http://cond-phys.imr.tohoku.ac.jp/index_e.html

IMR KINKEN Research Highlights 202027

Electronic Materials

Magnetism and Superconductivity in the T*-type Cuprates
La1-x/2Eu1-x/2Srx CuO4

Change in physical properties such as the magnetism and superconductivity of high-T cuprates in relationcto their local structure is an interesting phenomenon. In this study, T*-type cuprates were synthesized and thesuperconductivity transition in these compounds after annealing was observed. The latter likely gave rise to
additional oxygen atoms without inducing the magnetic transition, suggesting that magnetism played an
important role in the formation of Cooper pairs in T*-type La1-x/2Eu1-x/2Srx CuO4.

The local structure of high-Tc cuprates, inparticular the oxygen sites near the copper sites,
has generated significant research interest. High-Tc
cuprates with “214” structures are known to havethree types of isomers, i.e., the so-called T-. T’-, andT*-type. In these families, physical properties suchas magnetism and superconductivity often change
after annealing, but the origin is unknown as yet.
One of the possibilities is a change in the local
structure associated with the apex oxygen site,which is one of the oxygen sites surrounding acopper site [1]. T*-type cuprates are ideal forinvestigating the relationship between the physicalproperties and local structure because in thesecompounds, a copper atom is surrounded by thefive oxygen sites forming a pyramid-like cage.
Recently, we succeeded in synthesizing T*type cuprates La1-x/2Eu1-x/2Srx CuO4 (LESCO) andinvestigated their magnetism and superconductivity.
Temperature dependencies of the resistivity of assintered (AS) and oxidation annealed (AN) T*-type
LESCO are shown in Fig. 1 [2, 3]. While thesuperconducting transition was observed in the AN
samples, the magnetic transition was absent. In
contrast, the AS samples exhibited evidence for adisordered magnetic state below 10 K (Fig. 2). Thisis the first evidence for the effect of annealing on
magnetism and superconductivity in LESCO.
To investigate the origin of superconductivity in
LESCO, we performed muon spin rotation/relaxation(µSR) measurement on the AS and AN samples.
Figure 2 shows the µSR spectra of AS- and ANLESCO with x = 0.14 against temperature change.
At low temperatures, a relaxation was observed in
all AS-LESCO, whereas this anomaly was absent in
all AN-LESCO. These experimental findings support
the simultaneous development of (quasi-) static
magnetism, and the resistivity at low temperatures
suggests induced magnetism by the suppression ofcarrier mobility.

Fig. 1 Temperature dependence of the electrical resistivityfor (a) AS- and (b) AN-LESCO with x = 0.18, 0.20, and0.22 [3].

Fig. 2 Temperature dependence of µSR spectra in (a) ASand (b) AN-LESCO with x = 0.14 [3].

References[1] J. Akimitsu, S. Suzuki, M. Watanabe, and H. Sawa, J. J. Appl.
Phys. 27, L1859 (1988).
[2] M. Fujita, K. M. Suzuki, S. Asano, A. Koda, H. Okabe, and R.
Kadono, JPS Conf. Proc. 21, 011026 (2018).
[3] S. Asano, K. M. Suzuki, K. Kudo, I. Watanebe, A. Koda, R.
Kadono, T. Noji, Y. Koike, T. Taniguchi, S. Kitagawa, K. Ishida,
and M. Fujita, J. Phys. Soc. Jpn. 88, 084709 (2019).

Keywords: superconducting, magnetism, electronic structure
Masaki Fujita (Quantum Beam Materials Physics Research Laboratory)
E-mail: qblab@imr.tohoku.ac.jp
URL: http://qblab.imr.tohoku.ac.jp/index-e.html

28

IMR KINKEN Research Highlights 2020

IMR KINKEN Research Highlights 2020

Development of Pixilated Scintillator Arrays and Eutectics
for High-Resolution Radiation Imaging

Wafers of Tb-doped GAP/α-Al O eutectic of size 25 × 25 mm were fabricated via the micro-pulling-down
22

3

method using an Ir crucible with a 25 × 25 mm2 die. A prototype of an X-ray phase imaging detector wasdeveloped using a CMOS sensor with a fiber optic plate and eutectic wafer. We also successfully developed aradiation-imaging detector using a 0.1 × 0.1 mm2-pixelated Gd3(Ga,Al)5O12 (GAGG) scintillator combined witha silicon photomultiplier array made of 1 × 1 mm2 channels.
High spatial resolution is desired for a radiationimaging detector for alpha and beta particles, andgamma and X-rays. A pixelated scintillator can
improve the spatial resolution of a scintillation
imaging detector by reducing the light spread of thescintillator. For this type of radiation-imaging
detector, the spatial resolution is sometimes limitedby the pixel size of the scintillators and a scintillator
array with a smaller pixel-size is desired. However,fabricating such a scintillator array manually isdifficult. We reported two fabrication techniques forthis purpose: fine cutting [1] and a rod-like eutectic
structure [2,3].
The side view of the developed GAGG pixelated
plate is shown in Fig. 1(a). The GAGG pixelated
plate had the pixel size of 0.1 × 0.1 × 1 mm3 with
0.05 mm slits between the pixels; thus, the pitch
was 0.15 mm. The number of pixels was 53 × 53
and the total size of the GAGG pixel plate was 8 × 8mm2. The developed imaging detector resolved 0.1
× 0.1 mm2 GAGG-pixels for Am-241 alpha particles.
Figure 2 shows the photographs of the grown
eutectic and its scanning electron microscope(SEM) image. After optimizing the growth conditions
such as seed size, pulling rate, atmosphere, insulator
set-up, and temperature gradient along the growth
direction, a prism-shaped eutectic could be
fabricated (Fig. 2(b)). Figure 2(c) shows a cut andpolished wafer of the Fig. 2(b) eutectic at the crosssection. Optically transparent parts can be
confirmed in more than 80% of the area on the 25 ×
25 mm2 wafer. Figure 2(d) shows a cut and polishedsample of size 3 × 3 × 0.3 mm3. The background of
the surface of the sample shows bundles of opticalfibers.
A prototype of an X-ray phase imaging detector
was developed using a CMOS sensor with a fiber
optic plate and a eutectic wafer. X-ray spots with aperiod of 8.24 µm were observed using the detector.
X-ray phase imaging of the nylon ball was performedin this study. A phase change of approximately 2 μm

Fig. 1 Photo of the developed 0.1 mm pixel size GAGG plate(a) and the developed silicon photomultiplier (Si-PM)based GAGG imaging detector optically coupled to a SiPM array of channel size 1 mm (b)a)b)

The hole
positionc)

The hole
position

25 mm

d)

The hole
positionFig. 2 Photographs of the grown eutectic (a) at the initialstage, (b) after optimization of the growth conditions, (c)cut and polished wafer of the (b) eutectic, (d) sample ofsize 3 × 3 × 0.3 mm3, and (e) the SEM image of theeutectic.

could be confirmed in the air and nylon sphericalinterface. The X-ray phase imaging technique could
be realized in the absence of absorption grating.
References[1] S. Yamamoto, J. Kataoka, K. Kamada, and A. Yoshikawa,Nucl. Instrum. Methods Phys. Res. A 919, 125 (2019).
[2] K. Kamada, N. Yasui, Y. Ohashi, T. Den, H. Yamaguchi, A.
Yamaji, Y. Shoji, M. Yoshisno, K. JinKim, S. Kurosawa, Y.
Ohashi, Y. Yokota, and A. Yoshikawa, IEEE TNS 65, 2036
(2018).
[3] A. Yoshikawa, K. Kamada, S. Kurosawa, Y. Yokota, A. Yamaji,
V. I. Chani, Y. Ohashi, and M. Yoshino, J. Cryst. Growth 498,
170 (2018).

Keywords: scintillator, crystal growth, radiation imaging
Akira Yoshikawa (Physics of Electronic Materials Research Laboratory)
E-mail: yoshikawa@imr.tohoku.ac.jp
URL: http://yoshikawa-lab.imr.tohoku.ac.jp/index-e.html

IMR KINKEN Research Highlights 202029

Electronic Materials

Structure Analysis of MnS2, MnSe2, and MnTe2

T

he structures of the pyrite-type compounds such as MnS2, MnSe2, and MnTe2, were determined by single crystal
X-ray diffraction. The structural features of MnX2 compounds (X = S, Se, and Te) were also discussed [1].
The pyrite-type structure, MX2, is realized as avariety of chalcogen or pnictogen compounds (M =transition metal, X = S, Se, Te, P, As, and Sb). As
depicted in Fig. 1, the pyrite-type structure is a 3D
assembly of MX6 octahedra, in which the M is solely
bound to a single X in the X2 molecule. This interesting
structure may be featured by the variation of covalent
X–X distances and ionic M-X distances. Figure 2 depicts
M–X versus X–X distances in a variety of MX2 pyritetype compounds. As shorter M–X distances comparedto X–X distances indicate diminishing characteristic asan X2 molecule, the compounds located in the upperleft quadrant tend to exhibit a metallic luster and thoselocated in the lower-right quadrant of the plot tend to
exhibit a glass-like luster. Additionally, the variation of
X–X distances realized in the MX2 octahedron also
reflects the MX6 geometry. There are two short and long
X–X edge distances in the MX6 octahedron and theincrease in the electronic polarization of the M cation
leads to an increase in the d(X–X)l/d(X–X)s ratio.
Therefore, precise structural information would aid a
discussion of the possible factors that lower the totalenergy in pyrite-type crystals.
The number of electrons in the d-orbital increases
to d5, d6, d7, and d8 for Mn2+, Fe2+, Co2+, and Ni2+,
respectively. In the low spin state, all electrons in Fe2+
occupy t2g orbitals (S = 0), which is, evidently, a biased
electron arrangement. In the high spin state, electrons in
Mn2+ occupy all of the orbitals evenly (S = 5/2), which
creates a spherical symmetry and its relatively large
ionic size. As depicted in Fig. 2, the extent by which the
M-S distance increases is highest for FeS2, followed by
CoS2, NiS2, and MnS2. This result readily suggests thehigh spin state of Mn2+. Analysis of the d(X–X)l/d(X–X)s
ratio indicates that MnS2 exhibits the smallest distortion
to the octahedral M-site. Considering that FeS6 exhibits
greater distortion than MnS6, the difference in the spinstates of FeS2 and MnS2 is also confirmed.
The d(Mn–X)/d(X–X) ratios corresponding to S, Se,
and Te decrease to 1.000 from S to Te and these results
suggest a corresponding decrease in the molecularcharacteristics of the corresponding X2 groups. Thed(X–X)l/d(X–X)s ratios increase as a function of the X–X
distance, as suggested by the increase in the degree ofoctahedral distortion. However, the d(X–X)l/d(X–X)s ratio

Fig. 1 Crystal structure of hauerite. The S-S bond (indicatedby the yellow arrow) has been indicated in the middle ofthe unit cell.

Fig. 2 M–X versus X–X distances in MX2 pyrite-type
compounds (M = metal ions, X = S, Se, Te, P, As, Sb, Bi).

corresponding to MnTe2 is almost identical to that of
FeS2, and the distortion corresponding to MnTe6 islarge. This result readily suggests that the distortion of
the M-site is not a major factor in the stability of spinstates. This conclusion allows us to propose the ideathat the factors that lower the total energy in MnS2,
MnSe2, and MnTe2 are primarily determined by therespective configurations of anions.
References[1] M. Tokuda, A. Yoshiasa, T Mashimo, H. Arima, H, Hongu, T.
Tobase, A, Nakatsuka, and K. Sugiyama, Z. Kristallogr. Cryst.
Mater. 234, 371 (2019).

Keywords: X-ray diffraction (XRD), crystal structure, MnS2
Makoto Tokuda (Chemical Physics of Non-Crystalline Materials Research Laboratory)
E-mail: tokuda@imr.tohoku.ac.jp
URL: http://www.imr.tohoku.ac.jp/en/about/divisions-and-centers/research-division/15.html

30

IMR KINKEN Research Highlights 2020

IMR KINKEN Research Highlights 2020

Host-Guest Hydrogen Bonding Influences Charge-Orderingin Magnetic Sponges

Electronic state modulation in magnetic frameworks represents an effective strategy for significantly changing theirmagnetism. We demonstrated that the dynamically changeable charge-ordered state in porous magnetic frameworks
is affected by the host-guest hydrogen bonding between the host framework and lattice solvent molecules.

Chemo-responsive magnetism is one of the mostdesired functions in molecular magnets for potential
applications in new types of memories, sensors, andswitches. However, it is challenging to achieve drastic,large, and highly reversible functions in such materials.
We recently proposed electronic state modulation(ESM) as a new mechanism to produce a large magnetic
change in magnets. With ESM, the charge-orderedstate of the magnetic framework is controlled dependingon whether the guest molecules are present or not [1].
Here, we report a pseudo-polymorphic series oflayeredmagnets,[{Ru2(m-F
PhCO2)4}2
TCNQ(OMe)2]·nDCE (1-nDCE;
m-FPhCO2−=
m-fluorobenzoate; TCNQ(OMe)2 = 2,5-dimethoxyl7,7,8,8-tetracyanoquinodimethane), consisting of twoelectron-donor (D) units of [Ru2(m-FPhCO2)4] and one
acceptor (A) unit of TCNQ(OMe)2 [2]. Three types ofcompounds having different D2A charge-orderedstates were distinguished by the amount of
1,2-dichloroethane (DCE) included as the crystallization
solvent, namely, [D+–A2– –D+] ∞ for the fully solvatedcompound (pristine form; 1-4DCE), [D+–A1.5– –D0.5+] ∞for the solvent-free compound (1), and [D+–A – –D] ∞ fora partially solvated intermediate state (1-DCE with n ≤1). Compounds 1-4DCE, 1 and 1-DCE clearly had
modified magnetism as well as their respective Tcs;
they were identified as a paramagnet, ferrimagnet with
Tc = 30 K, and ferrimagnet with Tc = 88 K, respectively(Fig. 1a).
Notably, although 1 and 1-DCE were isostructuralin their framework, their charge-ordered states weremodified. The key factor that allows these differences
is a CH···O-type hydrogen bonding motif between
DCE and the framework only found in 1-DCE (Fig. 1b).
To clarify the effect, density functional theory
calculations were carried out. Consequently, a weakcharge polarization of –Cδ– –H δ+ in the guest DCE
could provide a positive electrostatic field around thehost framework to stabilize the orbital energies, where
the host framework with DCE possesses a lower
highest occupied molecular orbital (HOMO) energy
level than that in a case without DCE (Fig. 1c). Hence,
1-DCE with the CH···O-type hydrogen bonding motif

Fig. 1 (a) Temperature dependence of magnetization at
Hdc = 100 Oe for 1-4DCE (blue), 1-DCE (green), and 1(orange). (b) Crystal structure of 1-DCE. (c) Calculated
molecular orbitals energies around HOMO of thelayered magnet with hydrogen bonding (1-DCE; left)
and without hydrogen bonding (1; right) models.

stabilized the charge state of [D+–A – –D] ∞.
The hydrogen bonding host-guest interactions
could be quite useful as a trigger to control theelectronic state (i.e., charge-ordered state) of theframework materials. Such a delicate systemassociated with host-guest interactions and subsequent
electron transfers resembles the information transfer
system in biological systems. Thus, the design of hostguest interactions in electronically conjugated systems
is a promising strategy for the creation of dynamic
functional electronic/magnetic materials whose
electronic/magnetic, as well as dielectric, properties
can be switched on demand by guest molecules.
References[1] J. Zhang, W. Kosaka, K. Sugimoto, and H. Miyasaka, J. Am.
Chem. Soc. 140, 5644 (2018).
[2] J. Zhang, W. Kosaka, Y. Kitagawa, and H. Miyasaka, Angew.
Chem. Int. Ed. 58, 7351 (2019).

Keywords: computational materials science, magnetic properties, porosity
Jun Zhang and Hitoshi Miyasaka (Solid-State Metal-Complex Chemistry Research Laboratory)
E-mail: zhangjun@imr.tohoku.ac.jp, miyasaka@imr.tohoku.ac.jp
URL: http://www.miyasaka-lab.imr.tohoku.ac.jp/cn5/HOME-Eng.html

IMR KINKEN Research Highlights 202031

Electronic Materials

Overcoming Thermal Coarsening in Nanoporous Materials
via High-Entropy Design

Controlling the feature sizes of three-dimensional bicontinuous nanoporous materials is essential for theiradvanced applications. However, the intrinsic coarsening of nanoporous materials naturally reduces theirsurface energy and leads to the deterioration of their physical properties over time. In this work, a novel 3DNP
material is reported that could overcome thermal coarsening.
Stabilizing the feature sizes of three-dimensionalbicontinuous nanoporous (3DNP) materials ischallenging because their mature ligament size isinversely proportional to the melting point. Although
3DNP materials have outstanding physical properties
because of their unique structure with open porous
networks, the coarsening phenomenon leads to thedegradation of physical properties over time.
The proposition for high-entropy alloys (HEA) is
maximizing the configuration entropy to stabilize amulti-principle solid-solution alloy. Atomic size
differences between constituent elements in HEA
increases the activation energy for diffusion. In that
context, a high-entropy design of nanoporous materials
has the potential for achieving an exceptional stability
against the coarsening.
To synthesize the 3DNP HEA, we utilized liquid
metal dealloying (LMD), a unique technique to fabricate
non-noble porous materials by preventing oxidation ina metallic melt. The high-entropy design of LMD basedon the enthalpy of mixing is depicted in Figure 1a [1].
The composition of the 3DNP HEA was carefully
established among elements immiscible with the Mgmelt, and the selected TiVNbMoTa HEA was thenalloyed with 75 at% miscible Ni. While Ni atoms wereselectively dissolved in the Mg melt, the five immiscible
elements rearranged their atomic configuration into the
3DNP structure.
The 3DNP HEA was able to preserve its nanoscalefeatures under severe conditions. The average
ligament size achieved in a unimodal porous structure
was only 10 nm after 10 min at 837 K (Fig. 1b). The ring
diffraction pattern proved the formation of bcc
ligaments with various crystal orientations. Most of theligaments consist of a few grains, and they form thepolycrystalline 3DNP HEA scaffolds (Fig. 1c).
Compared to pure metals or binary alloys, the
3DNP HEA showed a much slower coarsening rateand loss of desired 3DNP structure; the porous
structure of the 3DNP HEA remains in the nanoscale
regime after the same heat treatment (Fig. 1d).
Suppressed coarsening in the 3DNP HEA likely

Keywords: nanostructure, foam, 3D structure
Hidemi Kato (Non-Equilibrium Materials Research Laboratory)
E-mail: hikato@imr.tohoku.ac.jp
URL: http://www.nem2.imr.tohoku.ac.jp/index-e.html

32

IMR KINKEN Research Highlights 2020

Fig. 1 (a) High-entropy design strategy for the 3DNP HEA
by LMD. (b) Ligament structure after 10 min at 837 K. (c)Inverse pole figure map of the 3DNP HEA. (d) Ligament
size versus homologous temperature.

originated from the high fraction of secure CSL
boundaries and the slow diffusion in multi-principal
elements. These findings provide an impetus toward a
new generation of nanomaterials with excellent longterm performance.
References[1] S.-H. Joo, J. W. Bae, W.-Y. Park, Y. Shimada, T. Wada, H. S. Kim,
A. Takeuchi, T. J. Konno, H. Kato, and I. V. Okulov, Adv. Mater. 32,
1906160 (2020).

IMR KINKEN Research Highlights 2020

Manganese-Based Intermetallic Compound Films with High
Perpendicular Magnetic Anisotropy and Low Magnetizationfor Silicon Technology Application

Materials showing high perpendicular magnetic anisotropy, K , and low saturation magnetization, M , areusessential for future spintronic devices. The (001)-textured Cu2Sb-type manganese-based intermetallic
compound films were fabricated onto silicon substrates with a thermally oxidized amorphous surface layer, andKu was successfully enhanced by tuning the film composition.
Magnetic thin films with high perpendicularmagnetic anisotropy, Ku, and low saturation
magnetization, Ms, are desired for fabricating
spintronic devices, such as magnetoresistive
random access memory, MRAM. A high Ku and low
Ms are important to ensure data retention with asmall writing current and suppressing crosstalkamong the memory cells with a diameter of several
nanometers.
In this study, manganese (Mn)-based
equiatomic intermetallic compounds, MnGaGe and
MnAlGe, were selected as a class of high-Ku andlow-Ms materials. The prototype of the crystal
structure is Cu2Sb (C38 phase), which is depicted inFig. 1(a). We successfully fabricated epitaxiallygrown MnGaGe films onto MgO(100) substrates,
and achieved a Ku value of 8.1 × 105 J/m3 with arelatively small Ms of 2.6 × 105 A/m at room
temperature (RT). The atomically resolved crystal
structure was evident in the transmission electron
microscopy image, which clearly showed the latticespacing between the Ga/Ge layers (Fig. 1(b)) [1].
In addition to the magnetic properties, theCu2Sb-type Mn-intermetallic compounds possess
another merit compared to other Mn-basedmaterials, i.e., the preferred (001) orientation for film
growth, which is necessary for realizing
perpendicular magnetization. The quantitative
magnetic properties were unknown for polycrystalline textured films before our study. Both
MnGaGe and MnAlGe were investigated for thepoly-crystalline film study, and (001)-textured films
were successfully fabricated onto silicon substrates
with an amorphous thermally oxidized layer on thesurface. The maximum values of Ku were 8.6 × 105
J/m3 and 4.8 × 105 J/m3 for MnGaGe and MnAlGe[2] films, respectively, at RT, which are comparablewith those in epitaxially grown films. Further study
was conducted for the enhancement of moderate
Ku of MnAlGe. By substituting Cr for a part of Mn, Ku

(a)

(b)

Ge

Mn
Ga/Ge
(space)Ga/Ge
Mn
Ga or Al

1 nm

Mn
Fig. 1 (a) Schematic crystal structure of the Cu2Sb-type
MnGaGe or MnAlGe. (b) Cross-sectional transmission
electron microscope image of a MnGaGe film, which
demonstrates the lattice spacing between the Ga/Ge
layers as well as the atomic positions.

was successfully enhanced to 7.3 × 105 J/m3 [3],which approached the Ku of MnGaGe films as well
as a target of 10 6 J/m3 for future device applications.
The newly developed Cu2Sb-type Mnintermetallic compounds films are attractive for
future spintronic applications as silicon-substratescompatible high-Ku and low-Ms materials.
References[1] M. Sun, T. Kubota, K. Ito, S. Takahashi, Y. Hirayama, Y. Sonobe,
and K. Takanashi, Appl. Phys. Lett. 116, 062402 (2020).
[2] T. Kubota, Y. Kota, K. Ito, R. Y. Umetsu, M. Sun, M. Mizuguchi,
and K. Takanashi, AIP Adv. 10, 015122 (2020).
[3] T. Kubota, Y. Kota, K. Ito, R. Y. Umetsu, M. Sun, M. Mizuguchi,
and K. Takanashi, Appl. Phys. Express 12, 103002 (2019).

Keywords: spintronics, magnetism
Koki Takanashi (Magnetic Materials Research Laboratory)
E-mail: koki@imr.tohoku.ac.jp
URL: http://magmatelab.imr.tohoku.ac.jp/

IMR KINKEN Research Highlights 202033

Electronic Materials

Novel Spin-Triplet Superconductivity and Huge Upper
Critical Field at 35 T in UTe2

We studied novel spin-triplet superconductivity in the heavy-fermion paramagnet UTe in the proximity of2ferromagnetic order. The superconducting transition temperature is 1.6 K. When the field is applied along thehard-magnetization axis (b-axis), the upper critical field, Hc2, reaches 35 T, which considerably exceeds thePauli limit, indicating the spin-triplet state. The potential applications of UTe2 and the relevant international
collaboration/competition will further intensify in the future.
40
(a)

(c)

(b)

IMR KINKEN Research Highlights 2020

CEP

UTe2
H // b-axis
20

RSC

HR

TCP

10
SC
SC
0
0.1

FM
1

TCurie
10

SC
0.1

T (K)

FM

TCurie

1

10
T (K)

0.1

T max
1

10
T (K)

Fig. 1 Field-reentrant (-reinforced) superconductivity inthe ferromagnets URhGe (a) and UCoGe (b) and in theparamagnet UTe2 (c). SC, RSC, FM, TCP and CEP
denote superconductivity, reentrant superconductivity,
ferromagnetism, tricritical point and critical end point,respectively.

were more than 20 papers on UTe2 in 2019. Our paper
was among the top 1% cited papers in the Web of
Science in 2019 and was the Editors’ Choice in JPSJ
[1].
This work was conducted in collaboration with A.
Nakamura, F. Honda, D. X. Li, Y. Homma, Y. Shimizu,
Y. J. Sato, G. Knebel, J. P. Brison, A. Pourret, D.
Braithwaite, G. Lapertot, Q. Niu, M. Valiska, H. Harima,
W. Knafo, and J. Flouquet.
References[1] D. Aoki, A. Nakamura, F. Honda, D. Li, Y. Homma, Y. Shimizu, Y.
J. Sato, G. Knebel, J.-P. Brison, A. Pourret, D. Braithwaite, G.
Lapertot, Q. Niu, M. Vališka, H. Harima, and J. Flouquet, J. Phys.
Soc. Jpn. 88, 043702 (2019).
[2] S. Ran, C. Eckberg, Q.-P. Ding, Y. Furukawa, T. Metz, S. R. Saha,
I.-L. Liu, M. Zic, H. Kim, J. Paglione, and N. P. Butch, Science 365,684 (2019).
[3] G. Knebel, W. Knafo, A. Pourret, Q. Niu, M. Vališka, D. Braithwaite,
G. Lapertot, J.-P. Brison, S. Mishra, I. Sheikin, G. Seyfarth, D.
Aoki, and J. Flouquet, J. Phys. Soc. Jpn. 88, 063707 (2019).
[4] D. Braithwaite, M. Vališka, G. Knebel, G. Lapertot, J.-P. Brison, A.
Pourret, M. E. Zhitomirsky, J. Flouquet, F. Honda, and D. Aoki,
Commun. Phys. 2, 147 (2019).

Keywords: superconducting, heavy fermion, actinide
Dai Aoki (Actinide Materials Science Research Laboratory)
E-mail: aoki@imr.tohoku.ac.jp
URL: http://www.imr.tohoku.ac.jp/en/about/divisions-and-centers/research-division/25.html

34

Hm

UCoGe
H // b-axis

URhGe
H // b-axis

30

H (T)

The novel heavy fermion superconductor UTe2
attracts considerable attention [1,2], because of therealization of the spin-triplet state. UTe2 is a paramagnet
and crystallizes in an orthorhombic structure with thespace group, Immm. The large Sommerfeld coefficient(~120 mJK-2mol-1) indicates heavy electronic states.
The superconducting transition occurs at 1.6 K with alarge specific heat jump, indicating strong coupling.
When the field is applied along the hard-magnetization
axis (b-axis), the field-reentrant superconductivity
appears [3], as shown in Fig. 1(c). At 35 T, thesuperconductivity is cut off because of the first-ordermetamagnetic transition, where the effective mass ofconduction electrons is enhanced, leading to strong
fluctuations, which are an origin of the field-reentrant
superconductivity. This behavior is similar to those offerromagnetic superconductors. As shown in Fig.
1(a, b) the field-reentrant (-reinforced) superconductivity
is observed in URhGe and UCoGe. In these cases,
the collapse of the ferromagnetic Curie temperatureinduces ferromagnetic fluctuations, leading to thefield-reentrant superconductivity. In contrast, in theparamagnet UTe2, Tχmax, at which the magnetic
susceptibility shows a broad maximum as a crossover,is transformed into the first metamagnetic transition athigh fields.
Another important discovery is the multiple
superconducting states in UTe2 [4]. Under a certain
pressure, the superconducting transition splits into
two, with a maximum of 3 K at 1 GPa for the higher
transition temperature. At 1.5 GPa, the superconducting
state collapses and a new magnetic ordered state isrealized at a higher pressure. Under a magnetic field,
the multiple superconducting states show acomplicated pressure evolution, accompanied by an
abrupt increase in Hc2 in the high-field and lowtemperature region. Different order parameters should
be realized in the multiple superconducting states,
supporting a scenario of the spin-triplet state. Thesestudies are extended from the viewpoints of topologicalsuperconductivity and quantum computing. ThereIMR KINKEN Research Highlights 2020

Formation of Defects in Sintered Polycrystalline 122 Phase
Fe-Based Superconductor

Among the factors influencing electromagnetic properties, grain boundaries are important microstructuralissues for polycrystalline superconducting materials. In this study, based on the observed stacking faults andinter-granular amorphous layers, we have proposed a model for the formation of two types of cracks in thepolycrystalline Ba2(Fe,Co)2 As2 (Ba-122) phase, namely, intra- and inter-granular cracks.

The evolution of intra- and inter-granularstructures in Co-doped BaFe2 As2 (Ba-122) has
been studied to address the key issues associatedwith limited inter-granular current transport in
randomly oriented polycrystalline Fe-based
superconducting materials. In this study, the multiscale structures in Ba-122 bulks fabricated atvarious sintering temperatures were investigatedusing electron microscopy, particularly focusing on
the intra- and inter-grain structures [1].
Co-doped Ba-122 polycrystalline bulk samples
were synthesized from elemental metals. Elemental
Ba, Fe, Co, and As powders were weighed to astoichiometric ratio of Ba(Fe0.92Co0.08)2 As2, andmixed and ground by high-energy ball milling.
Figure 1 shows the SEM back scatteredelectron (BSE) image of the polycrystalline bulk
sintered at 600 °C for 48 h. The grains sintered at
600 °C were connected to each other through dark
contrasted grain boundaries, as seen in the inset
high angle annular dark field scanning transmission
electron microscopy (HAADF-STEM) image. In thebulks sintered over 700 °C, the inter-granular phases
expanded and separated the neighboring Ba-122
grains, while the cracks were generated into the Ba122 grain parallel to each other.
Figure 2 summarizes the microstructuralobservations at a high magnification of the specimen
sintered at 600 °C. The HAADF-STEM image (a)shows dark line contrasts in the intra-grain denotedwith ‘A’ and the dark contrasted regions at grainboundaries denoted with ‘B’. Note that there wereno cracks in the intra-grain. The high magnification
HAADF-STEM image (b) and high resolution TEM
image (c) revealed that there were stacking faults of
Fe-As layers on (001) planes in region ‘A’, and region‘B’ was an amorphous layer with a width of 2 nm.
Based on the observations, we propose that intraand inter- granular cracks were formed by growing
the stacking faults and amorphous layers,respectively.

Fig. 1 SEM-BSE image of the samples heated at 600 °C for
48 h. The insets are HAADF-STEM images.

Fig. 2 (a) HAADF-STEM image of the sample heated at
600 °C for 48 h. (b) Magnified HAADF-STEM image of thedark area distributed in the intra-grain as shown by ‘A’ in(a). (c) High resolution TEM image of the grain boundary
indicated by ‘B’ in (a).

References[1] Y. Shimada, A. Yamamoto, Y. Hayashi, K. Kishio, J. Shimoyama,
S. Hata, and T. J. Konno, Supercond. Sci. Technol. 32, 084003(2019).

Keywords: electron microscopy, nanostructure, superconducting
Yusuke Shimada (Materials Science of Non-Stoichiometric Compounds Research Laboratory)
E-mail: yshimada@imr.tohoku.ac.jp
URL: http://www.imr.tohoku.ac.jp/en/about/divisions-and-centers/research-division/26.html

IMR KINKEN Research Highlights 202035

Electronic Materials

Promoting Rare-Earth Monopnictides as a New Class ofMagnetic Topological Insulator by Thin Film Stabilization

Using molecular beam epitaxy, GdBi thin films were fabricated as a candidate for antiferromagnetic topologicalinsulators. Electrical transport and magnetic torque characterizations suggested lifting of semimetallicity below
eight crystallographic unit cells and robust antiferromagnetism down to the minimum thickness of five
crystallographic unit cells. These results are a promising step toward a bulk insulating antiferromagnetic
topological insulator, which can potentially realize dissipationless spin-polarized currents on its edges in themonolayer limit.
0.10

MetallicTN0.05

ρxx ( mΩ cm )Topological insulators (TIs) constitute a uniqueclass of materials that are characterized by thepresence of protected metallic surface states and abandgap for the bulk part of the material. Combining
TI with magnetism gives rise to exceptional
properties such as dissipationless spin-polarized
currents, which are promising ingredients for energy
and electronic device applications. Magnetic TIs
have been typically realized by magnetic doping
and limiting the onset temperature at which magnetic
effects on the electronic states of the TIs can beobserved in the range of a few-kelvin.
Rare-earth monopnictides (RX) are a series ofcompounds with a variety of magnetic and electronic
properties. Recent prediction of topological states
in RX makes them a promising candidate forrealizing magnetic TIs in intrinsically magnetic
materials [1]. Their robust magnetism and strong
coupling to electronic states can potentially boost
the temperature necessary for the manifestation ofproperties unique to magnetic TIs. Despite theseadvantages, the semimetallic nature of RX inhibits
experimental observations of their topologicalcharacter, which is obscured by the metallic bulk
state.
This study shows that the stabilization of
antiferromagnetic GdBi (R = Gd, X = Bi) in thin film
morphology can lift their semimetallicity while
maintaining their long-range magnetic order [2].
High-quality GdBi (111) epitaxial thin films were
fabricated with thickness ranging from 40 nm down
to five crystallographic unit cells for the first time bymolecular beam epitaxy. The temperaturedependence of resistivity indicated a metallic-toinsulator transition in the thinnest films (see Fig. 1).
Hallmeasurementssuggested
liftingofsemimetallicity in these films by quantum
confinement, supporting the metallic-to-insulator

ElectronEF

0.00 Thickness: 40 nm
1.00 Insula�ng

TN

0.95
0.90

Hole

ElectronEF

Thickness: 5 nm

0

50

100

150

Hole

200

T (K)
Fig. 1 Left panels show the temperature dependence ofresistivity ρxx of GdBi thin films with two different film
thicknesses. Right panels show the micrographs ofsamples and schematic drawings of the band structureof GdBi thin films. Scale bars are 1 mm.

transition in GdBi thin films. Further, it was confirmedthat the long-range antiferromagnetic order with TN= 30 K was nearly unchanged from bulk single
crystals in our thinnest films using torquemagnetometry and transport measurements.
The stabilization of GdBi thin films and ability tocontrol their metallicity by film thickness present apathway to observe the peculiar nature of magnetic
TIs, including high-Chern-number-insulating states,
which were predicted by first-principle calculationsof monolayer GdBi [2].
References[1] M. Zeng, C. Fang, G. Chang, Y.-A. Chen, T. Hsieh, A. Bansil,
H. Lin, and L. Fu, arXiv:1504.03492 (2015).
[2] H. Inoue, M. Han, M. Hu, T. Suzuki, J. Liu, and J. G. Checkelsky,
Phys. Rev. Mater. 3, 101202(R) (2019)

Keywords: topological insulator, thin films, epitaxial growth
Hisashi Inoue (Exploratory Research Laboratory, IMR and Creative Interdisciplinary Research Division, Frontier
Research Institute for Interdisciplinary Sciences)E-mail: hisashi.inoue@fris.tohoku.ac.jp
URL: http://www.imr.tohoku.ac.jp/en/about/divisions-and-centers/research-division/33.html
https://www.fris.tohoku.ac.jp/en/researcher/creative/inouehisashi.html

36

IMR KINKEN Research Highlights 2020

Research Centers

IMR KINKEN Research Highlights 2020

ResearchResearch Centers

Novel Production Method for the Lowest-Energy Nuclear
State of Thorium-229 IsomerInternational Research Center for Nuclear Materials Science

We have developed a novel production method for the lowest-energy state of the thorium-229 isomer (Th-229m).
The method is based on nuclear resonant scattering (NRS) using a strong X-ray source at SPring-8. We successfully
observed the NRS signal and demonstrated the first active pumping from the ground state of Th-229.

Fig. 1 Energy level of Th-229 (ground state and excitedstate) and new scheme for isomer production.

Counts / 1800 sa) Si(440)
280
240
200
160
260
Counts / 3600 s

Thorium-229 (Th-229) is a unique nucleus
having an ultra-low energy level of ~ 8 eV. This
nuclear level (Th-229m) is so low that it can be
excited optically by laser light, and thus, can beutilized as an ultra-precise “nuclear clock.” Despite
numerous attempts for more than 40 years, directlaser excitation has not yet been achieved, and its
basic properties still remain elusive. One of thebottlenecks in this direction is that Th-229m is
produced only through the α decay of U-233, whichis accompanied by a complicated nuclear processwith a high level of radioactivity. We developed anovel method for producing Th-229m via the second
excited state close to 29 keV by using X-ray beams(Fig. 1). The method is based on nuclear resonant
scattering (NRS) using the strong synchrotron X-ray
facility, SPring-8.
To observe the subtle NRS signal against thelarge background from X-ray electron scattering, wemust enhance the signal rate by focusing the X-ray
beam and correspondingly condensing the target
material into a small area. We developed a Th-229
target with a small diameter of φ0.4 mm by the dryup scheme. An ultra-pure sample of Th-229
provided by IRCNMS (Oarai) was used to build thetarget. The sample was chemically separated fromU-233 a long time ago by Prof. Mitsugashira, and it
is no longer contaminated with the radioactive Th228 having a short half-life of 1.9 years.
Using the developed target, we performed thebeam experiment at BL19LXU in SPring-8 2018. We
successfully observed the NRS signal anddetermined the energy level of the second excitedstate of Th-229 (Fig. 2). Th-229m, predominantly
decayed from the second excited state, can now be
produced in a controllable manner. Efforts areunderway to observe deexcitation photon emission
from the isomeric state. This new method is expected
to propel research on Th-229m toward the realizationb) Si(660)

220
180
140
100
0.5

0.6

0.7

0.8

0.9
1
1.1
1.2
Energy - 29189 [eV]IMR KINKEN Research Highlights 2020

1.4

1.5

Fig. 2 Observed resonance peak of Th-229 NRS with (a)Si(440) beam (ΔE = 0.26 eV) and (b) Si(660) beam (ΔE= 0.1 eV) [1].

of a “nuclear clock.”
References[1] T. Masuda et al., Nature 573, 238 (2019).

Keywords: actinide, nuclear materials, X-ray scattering
Koji Yoshimura and Akihiro Yoshimi (Corresponding Authors, Okayama University)
E-mail: yosimura@okayama-u.ac.jp, yoshimi@okayama-u.ac.jp
Yasuyoshi Nagai (Head of International Research Center for Nuclear Materials Science)
E-mail: nagai@imr.tohoku.ac.jp
URL: http://www.imr-oarai.jp/en/

38

1.3

IMR KINKEN Research Highlights 2020

Fabrication, Crystallization, and Properties of Fe-Based
Soft Magnetic Alloys
Cooperative Research and Development Center for Advanced Materials

Researchers at the Cooperative Research and Development Center for Advanced Materials have studied

various fabrication processes for advanced materials and explored the possibility of their application as multifunctional materials for future technologies in collaboration with universities and research institutes around theworld. Herein, the recent research and development activities on soft magnetic material alloys for industrial
applications are highlighted.

The effects of Co addition on the structural,
thermal, and magnetic properties of Fe-Si-B-P-Cu
alloy have been reported previously [1-2]. In thisstudy, we explored the glass formation ability,
thermal stability, and magnetic properties of the
Fe80Cox B14−x Si2P3Cu1 (x = 0, 2, 4, 6) alloy andinvestigated the soft magnetic performances of
Fe80Co4B10Si2P3Cu1 alloy. The addition of small
amounts of Co remarkably affected the glass
formation, crystallization, and magnetic behavior of
the alloy. Co content was limited to 6% in
considerationoffabrication
costs.
This
nanocrystalline alloy presented a high magnetic
induction (Bs) of 1.84 T, low coercivity (Hc) of 5.3
A/m (Fig. 1), high permeability of 12,601 at 1000 Hz,
and low core loss of 62 W/kg at 1000 Hz at Bs of
1.5 T. These results indicated that the addition of asmall amount of Co enhanced the soft magnetic
performance by inducing uniform nanocrystallizationof α-(Fe,Co) and averaging magnetic anisotropy.
It was also found that a high nucleation density
could be obtained in the Fe81.5Si0.5B4.5P11Cu0.5C2
alloy by annealing it at a low heating rate because of
the amounts of new nuclei precipitated during
annealing [3]. The co-growth of pre-nuclei andnewly precipitated nuclei by annealing at the
optimum temperature (Ta) led to a uniform finenanocrystalline structure of α-Fe, thereby resulting
in low coercivity. The nanocrystallization mechanismof the alloy during annealing was different from thatof the Fe-Si-B-P-Cu nanocrystalline alloy (Fig. 2).
The Fe81.5Si0.5B4.5P11Cu0.5C2 alloy showed excellent
magnetic softness. Thus, the developed strategy
suggests that industrial crystallization can be
simplified by a one-step annealing process at a low
heating rate.

Fig. 1 (a) Dependence of Hc on Ta for the Fe80Si2B14P3Cu1
alloy ribbons. (b) Dependence of Bs on Ta for the
Fe80 Co4B10Si2P3Cu1 alloy.

Fig. 2 Schematic illustration for the crystallization of (a)Fe81.5Si0.5B4.5P11Cu0.5C2 and (b) Fe83.3-85Si-B-P-Cu
alloys.

References[1] B. Miao, Q. Luo, C. Chang, T. Liu, Y. Zhang, and J. Shen, J.
Magn. Magn. Mater. 477, 156 (2019).
[2] Y. Zhang, P. Sharma, and A. Makino, J. Alloys Compd. 709,
663 (2017).
[3] L. Jiang, Y. Zhang, X. Tong, T. Suzuki, and A. Makino, J. Magn.
Magn. Mater. 471, 148 (2019).

Keywords: magnetic properties, nanostructure, amorphous
Yan Zhang (Cooperative Research and Development Center for Advanced Materials )E-mail: zy-jp@imr.tohoku.ac.jp
Naoya Masahashi (Head of Cooperative Research and Development Center for Advanced Materials)
E-mail: masahasi@imr.tohoku.ac.jp
URL: http://www.crdam.imr.tohoku.ac.jp/en/index-en.html

IMR KINKEN Research Highlights 202039

Research Centers

Flux Pinning Properties of Practical EuBa2Cu3Oy Tapes with
Artificial Pinning Centers
High Field Laboratory for Superconducting Materials

Flux pinning properties of practical REBa Cu O (REBCO, RE: rare earth and Y) tapes with BaHfO artificial pinning
2

3

y

3

centers (APCs) were studied. A good temperature scaling of the flux pinning force density Fp was found as a functionof the magnetic field in a wide temperature region, in spite of the introduction of APC in REBCO tapes. It is suggested
that the short and sprayed nanorods showed anisotropic random pinning behavior because of the high growth rateof REBCO layers.

It is well known that introducing artificial pinning
centers (APCs) in practical REBa2Cu3Oy (REBCO, RE:
rare earth and Y) tapes can improve their in-fieldcritical current density (Jc). Most APCs are BaMO3 (M:Zr, Sn, Hf, etc.) with a nano-scaled rod structure along
the c-axis of REBCO and they enhance the in-field Jc
for B//c as a c-axis correlated pinning. We successfully
developed homogeneous long EuBCO tapes with highin-field critical currents at low temperatures below 20 K
via hot wall pulsed laser deposition at a high deposition
rate, because of the introduction of APCs [1]. The flux
pinning properties of the APC introduced EuBCO
tapes were investigated in detail under high magnetic
fields up to 24 T and in a wide temperature region
using the 25 T cryogen-free superconducting magnet
at HFLSM, as shown in Fig. 1(a). The flux pinning force(Fp) over 1 TN/m3 was obtained at 4.2 K for B//c, whichcorresponds to ~66% of the world record of the
REBCO short sample. In addition, good temperaturescaling behavior of Fp was observed as a function of
the magnetic field normalized by the Fp peak fields(Fig.1(b)), although the nanorods were introduced in
the EuBCO matrix [2]. Generally, the Fp scaling
behavior is observed for random pinning but not for thec-axis correlated pinning as in the case of well-aligned
nanorods. The segmentation and incline of nanorods
because of the very high growth rate of 20–30 nm/s isa possible reason for the random flux pinning-likebehavior of Fp in Fig. 1(b). The solid lines in Fig. 1(a) are
fitted results using the scaling law of Fp obtained in thisstudy. Excellent agreement was observed between thefitting and experimental results and it was possible toeasily estimate the Jc values at the desired conditions
such as temperature, magnetic field, and field angle
with the scaling function of Fp. This aspect is crucial for
detailed designing of high field superconducting
magnets.

Fig. 1 (a) In-field critical current density Jc and (b) normalized
pinning force densities as a function of normalized
magnetic field for B // c for fast growth EuBCO with APC atvarious temperatures. Solid lines in (a) and broken line in(b) are fitted results using the scaling law.

References[1] S. Fujita , S. Muto , W. Hirata, Y. Adachi, T. Yoshida , M. Igarashi,
K. Kakimoto, Y. Iijima , K. Naoe, T. Kiss, T. Okada, and S. Awaji,
IEEE Trans. Appl. Supercond. 28, 6600604 (2018).
[2] S. Fujita, S. Muto, W. Hirata, T. Yoshida, K. Kakimoto, Y. Iijima, M.
Daibo, T. Kiss, T. Okada, and S. Awaji, IEEE Trans. Appl.
Supercond. 29, 8001505 (2019).

Keywords: high magnetic field, high temperature superconductor
Shinji Fujita (Corresponding Author, Fujikura Ltd.)E-mail: shinji.fujita@jp.fujikura.com
Hiroyuki Nojiri (Head of High Field Laboratory for Superconducting Materials)
E-mail: nojiri@imr.tohoku.ac.jp
URL: http://www.hflsm.imr.tohoku.ac.jp/cgi-bin/index-e.cgi

40

IMR KINKEN Research Highlights 2020

IMR KINKEN Research Highlights 2020

Scientific Breakthrough Toward the Creation of New
Industrial Materials
Trans-Regional Corporation Center for Industrial Materials ResearchThe Institute for Materials Research (IMR) has launched the Trans-Reginal Corporation Center, a comprehensive Institutewide effort that pairs the research groups at IMR across the innovation spectrum to solve industrial challenges of thepresent and address global energy problems for the future. The center was established in 2016 based on an agreement
between the IMR, Osaka Prefecture Government, and enterprises sponsored by the government, taking over Osaka andKansai Center projects.
The Trans-Regional Corporation Center was
established in Osaka as a special unit of the Institute forMaterials Research (IMR), Tohoku University, in April
2016 based on an agreement with the Osaka Prefecture
Government. The Center is sponsored by the Ministry of
Education, Culture, Sports, Science, and Technology,
and takes over the Kansai Center, which carried out its
duties from 2012 to 2016. The Center has three goals.
First, to solve the technical problems that the industries
have struggled to resolve. Second, the Center introduces
academic output to the industries, with the aim ofapplying such output to society. Third, the Center helps
educate the next generation of materials scientists andresearchers. We have organized a bimonthly forum
named “Monodukuri Kisokoza”, which focuses on special
topics regarding materials and processing. Projects are
conducted through wide collaborations between thegovernment, universities, research institutions, and otherorganizations. The Center has four venues to cover theKansai and Tohoku area, namely, the Osaka office at the
Osaka Prefecture University, the Hyogo office at the
University of Hyogo, the Sendai office at the IMR, and the
Monozukuri Business Information Center Osaka at theCreation Core Higashi-Osaka.
The Center has developed various bioactive
materials in collaboration with the industries, universities,
and government [1]. Near-β TiNbSn alloy with low Young’s
modulus was anodized in H2SO4 electrolytes to impartosseointegration to the alloy. The anodized oxide iscomposed of crystallized TiO2 containing a number ofinternal pores and exhibits Hydroxyapatite (HA) formation
after immersion in Hank’s solution. The anodized alloy
after six weeks implantation in rabbit revealed
incorporation of bone ingredients in the TiO2 layer (Fig.
1). A model was proposed to explain the bioactivity of theanodized TiNbSn alloy (Fig. 2). It was concluded that thesulfuric acid electrolyte was favorable for HA formation,and the bioactivity could be attributed to the anodic TiO2
that facilitated the incorporation of bone ingredients.
Similar collaborative research utilizing academic

Fig. 1 TEM-EDX element mapping near the interface between
anodic TiO2 on TiNbSn alloy and rabbit bone six weeks
after implantation: Ti Kα, O Kα, P Kα, Ca Kα.

Fig. 2 Schematic diagrams of (a) micro-sized pore evolution
during anodization, (b, c) bone ingredients migration, and(d) hydroxyapatite formation on the surface of the anodic
oxide subject to immersion in Hank’ solution.

knowledge and industrial technology for practicalapplications is in progress. The Center will expend effort
to be innovative in the metallic material industry throughalliances with the partners and promote materials science
research.
References[1] N. Masahashi, Y. Mori, H. Tanaka, A. Kogure, H. Inoue, K. Ohmura,
Y. Kodama, M. Nishijima, E. Itoi, and S. Hanada, Mater. Sci. Eng. C
98, 753 (2019).

Keywords: biomaterial, osseointegration, photocatalytic
Naoya Masahashi (Head of Trans-Regional Corporation Center for Industrial Materials Research)
E-mail: masahasi@imr.tohoku.ac.jp
URL: http://www.imr.tohoku.ac.jp/en/about/divisions-and-centers/facilities/04.html

IMR KINKEN Research Highlights 202041

Research Centers

Development of an Ultra-Large Scale Micromagnetic
Simulation Code for the Magnetic Process Inside
Permanent Magnets Using a Supercomputer
Center for Computational Materials ScienceMagnetization nucleation inside nanocrystalline magnetic materials was clarified in this study by ultra-large scale
micromagnetic simulations. Initiation of magnetic reversal was affected by the microstructure of the nanocrystalline
magnetic material, such as easy-axis orientation and overlap of grains. Development of the ultra-large scale
micromagnetic simulation code enabled us to improve the performance of magnetic materials.

512 nm

(a)

nm

1.2
1.15

0.5

mz

1.05
1.0

0

0.95
0.9

-0.5

0.85

-1.0

0

0.95

-0.5

1.15

1.1

5

1.0

z

0

0.95
0.9

-5

0.85

Anisotropy field

0.8
1.65 1.7 1.75 1.8 1.85 1.9 1.95 2.0-10

IMR KINKEN Research Highlights 2020

1.0
0.5

1.0

0

0.95

-0.5

0.9

-1.0

Dipole field

-1.5

0.8
1.65 1.7 1.75 1.8 1.85 1.9 1.95 2.0

x (µm)

Fig. 1 (a) Image of the simulation model. (b) Nucleation site at
coercivity. (c) Exchange, (d) anisotropy, and (e) dipole
fields at remanence [3].

The nucleation is related to the orientation of thecrystal axis because the distribution of this orientation
is essential to the strong dipole field.
References[1] R. K. Mishra and R. W. Lee, Appl. Phys. Lett. 48, 733 (1986).
[2] H. Tsukahara, K. Iwano, C. Mitsumata, T. Ishikawa, and K. Ono,
AIP Adv. 7, 056224 (2017).
[3] H. Tsukahara, K. Iwano, C. Mitsumata, T. Ishikawa, and K. Ono,
Phys. Rev. Appl. 11, 014010 (2019).
[4] H. Tsukahara, K. Iwano, C. Mitsumata, T. Ishikawa, and K. Ono,
NPG Asia Mater. 12, 29 (2020).

Keywords: computational materials science, nanostructure, magnetism
Hiroshi Tsukahara and Kanta Ono (Corresponding Authors, High Energy Accelerator Research Organization)
E-mail: thiroshi@post.kek.jp
Momoji Kubo (Head of Center for Computational Materials Science)
E-mail: momoji@imr.tohoku.ac.jp
URL: https://www.sc.imr.tohoku.ac.jp/eng/

42

-1.5

1.5

A

1.05

0.85

x (µm)(e)1.1

Hani (T)

1.05

-1.0

Exchange field
x (µm)

1.2

y (µm)

1.15

0.5

1.0

0.8
1.65 1.7 1.75 1.8 1.85 1.9 1.95 2.0

10

(d)

1.0

1.05

0.85

x (µm)

1.2

1.5

0.9

Magnetization

0.8
1.65 1.7 1.75 1.8 1.85 1.9 1.95 2.0

(c)

1.1

z

1.0

(b)

1.1

y (µm)

48

20

nm

Hexc (T)48

y (µm)

1.2
1.15

20

z

x

Hdip (T)zyy (µm)

Micromagnetic
simulations
canclarifymagnetization dynamics such as domain wall motions
and magnetization reversals in magnetic materials.
Such simulations provide not only magnetization but
also distributions of the magnetic fields and magnetic
energies. As actual magnetic materials have complex
microstructures, visualization of their fields andenergies yields important information to enhance theirperformance.
The simulation size of magnetic materials isdetermined by the exchange length and theirmicrostructure. We had to employ a large number ofgrids inside the exchange length to simulate themagnetization dynamics accurately. Many magnetic
materials consist of micro-meter sized grains [1]. In
addition, the calculation method of the magnetic fields
such as dipole and magnetostrictive fields has highcomputational complexity. Hence, we developed amicromagnetic simulation code for magnetization
dynamics inside bulk magnetic materials using
massively parallel computing [2, 3]. This code enabled
us to perform ultra-large scale simulation of 0.3 billion
cells on MASAMUNE-IMR.
Figure 1(a) shows an example of the simulation
model for magnetic materials [4]. This model is for a
hot-deformed permanent magnet that consists of
tabular grains. The crystal axis was slightly tilted fromthe nominal easy-axis of the permanent magnet. We
considered external, anisotropy, exchange, and dipole
fields in this simulation. Figure 1(b) shows magnetic
nucleation site at coercivity. The magnetization reversal
initiated at a corner of the grain. Figures 1(c)–(e) show
the distributions of the exchange, anisotropy, anddipole fields of remnant magnetization. In contrast withthe anisotropy and exchange fields, the dipole field
was not uniform inside the magnet. A strong dipole
field was applied in the nucleation site.

IMR KINKEN Research Highlights 2020

Ordering Kinetics of Nanoporous FeCo During Liquid Metal
Dealloying and the Development of Nanofacets
Collaborative Research Center on Energy MaterialsT

his study investigates the surface morphology and ordering kinetics of three-dimensional interconnected
nanoporous FeCo. The ordering kinetics were accelerated by the surface-diffusion-driven coarsening in thenanoporous structure. Post-heat treatment led to the development of precise nanofacets via the preferential
growth of surface planes.
(a)

(c)

(b)

200 nm

200 nm

200 nmFig. 1 Surface structures of porous FeCo prepared by
LMD at (a) 600 °C, (b) 700 °C, and (c) 800 °C.
(a)

0.8

(b)

Porous FeCo: 600°C
Solid FeCo: 600°C
Solid FeCo: 500°C

Ordering parameter, S

It is interesting to study nanofacets because thesurface structure has a significant effect on thephysical properties of nanomaterials. However, it isstill a challenge to synthesize well-defined
nanofacets on a large-scale because of thetendency of nanomaterials to agglomerate.
In 2011, liquid metal dealloying (LMD) was
reported and provided a means for overcoming theelemental limitation of chemical dealloying [1]. An
example of a carefully designed LMD system isthree-dimensional interconnected nanoporous(3DNP) FeCo. The order-disorder transition between
the B2 and disordered bcc structures should resultin structural variations. In the present study, theordering behavior of 3DNP FeCo was investigatedduring the LMD process. The findings of orderingand coarsening phenomena were then practicallyapplied for surface reconstruction.
The synthesized porous FeCo exhibited
significant variations in surface morphology
depending on the LMD temperature. At 600 °C,
truncated cubic ligaments had flat surfaces ofparticular facets (Fig. 1a). The ligament surfaces
become roughened at 700 °C (Fig. 1b). Steppedsurfaces were observed at 800 °C (Fig. 1c), which isabove the order-disorder transition temperature (Tc)of FeCo (730 °C).
In chemically ordered materials, the facetedsurfaces depend on the degree of order, the energyof the facet(s), and surface-termination. Interestingly,
the ordering kinetics of 3DNP structures differ fromthose of the bulk materials. When the dealloying
time is increased from 3 min to 36 min at 600 °C, theordered (100) peak is intensified, and the orderingparameter (S) increases drastically at the beginningof the LMD process (Fig. 2a). The bulk FeCo plate
exhibited slower ordering kinetics than thenanoporous FeCo.
The theoretical ordering kinetics based on

0.6

0.4

0.2

0.0

0

500

1000

1500

2000

Time, sec

2500

200 nmFig. 2 (a) Ordering kinetics of porous FeCo and bulk FeCo
samples. (b) Developed nanofacets on porous FeCo
after post-heat treatment.

vacancy concentration agreed well with theexperimental results of the bulk samples. However,there was a large disparity with the ordering kineticsof the 3DNP structure. The surface diffusivity wasusually faster than the bulk diffusivity. In other words,
the surface diffusion accelerated long-rangeordering from the beginning of dealloying andformed a nanocube.
Precise nanofacets were developed on
ligaments with equimolar composition in large-scalebulk-porous FeCo samples at 600 °C after heat
treatment (Fig. 2b). The preferential growth ofdelicate steps and ledges verified the surface
diffusion formation mechanism of the nanofacets[2].
References[1] T. Wada, K. Yubuta, A. Inoue, and H. Kato, Mater. Lett. 65,
1076 (2011).
[2] S.-H. Joo, K. Yubuta, and H. Kato, Scr. Mater. 177, 38 (2020).

Keywords: nanostructure, foam, kinetics
Hidemi Kato (Corresponding Author, Non-Equilibrium Materials Research Laboratory)
E-mail: hikato@imr.tohoku.ac.jp
Tetsu Ichitsubo (Head of Collaborative Research Center on Energy Materials)
E-mail: tichi@imr.tohoku.ac.jp
URL: http://www.imr.tohoku.ac.jp/en/about/divisions-and-centers/facilities/05.html

IMR KINKEN Research Highlights 202043

Research Centers

Spin Transport in Different Oxide Phases of CopperInternational Collaboration CenterIn this study, spin transport in two types of copper oxides are directly compared. The results indicate that antiferromagnetic
CuO exhibits good spin conductivity, whereas diamagnetic Cu2O is a spin insulator. This implies that possessing unpaired
spins should be an important characteristic for good spin conductors. It is also revealed that materials with different
magnetic phases may have the potential to be spin operation materials.
Spin can be transported in an insulator even in the absenceof motion of free electrons, implying that it is not affected by theJoule-heating problem. Electrical spin, rather than electricalcharge, is expected to serve as the carrier of information and/or
energy in next-generation information processing devices [1].
Therefore, one of the most important issues in the field of
spintronics is to understand the mechanism and dominantcharacteristics of spin transports in insulating systems. Spintransport has been actively discussed in the context of various
insulating systems [2-5]. However, the relationship between spintransport and the ambient material's intrinsic features, such asphase, structure, defect, and even its electronic configuration ofions has been rarely investigated.
In this study, the spin transports in two different oxide
phases of the same transition metal copper have been directlycompared. First, cupric oxide (CuO) and cuprous oxide (Cu2O)
were prepared, both of which are stable oxides of copper with
distinct structures and magnetic properties. CuO is
antiferromagnetic, in which Cu2+ has one unpaired spin in the 3dorbital, while Cu2O is diamagnetic with a Cu+ ion having a fully
filled 3d orbital. By comparing the behavior of these two oxides,
it is possible to obtain information about the influence of metallicions with different electronic configurations on spin transport.
During the next step of the process, typical tri-layer
devices were developed, which consisted of a thin CuO or Cu2O
layer sandwiched by a heavy metal Pt layer and an insulating
Yttrium Iron Garnet (YIG) layer Fig. 1(a). Within this structure, theYIG layer acted as a spin generator, in which spin currents weregenerated and from which they were injected into the copperoxides via a spin pumping effect. The Pt layer acted as the spin
detector, in which spin currents conducted through the middle
oxide layer were detected as a voltage signal via the inverse
spin Hall effect (ISHE). CuO and Cu2O were identified based on
X-ray diffraction measurements, as depicted in Fig. 1(b).
Figure 1(c) indicates that antiferromagnetic CuO is a goodspin conductor. The ISHE signal of the YIG/CuO/Pt device wasobserved to be even greater than that of the device combining
YIG and Pt directly. This implies that such an intervening
antiferromagnetic layer enhanced the efficiency of spin transport
at the interface. On the other hand, diamagnetic Cu2O wasFig. 1 Spin transport results in copper oxides

observed to behave as a spin insulator. Spin currents werecompletely blocked by the 5 nm-thick Cu2O layer.
The difference between the degrees of spin transport in
CuO and Cu2O is mainly attributed to the difference in theelectronic configurations of the respective copper ions, which is,
in turn, associated to the different structures and magnetic
phases of the oxides. In CuO, copper loses two electrons toform Cu2+. There is an unpaired spin in the 3d orbital, which
couples with the nearest neighboring unpaired spin in the
opposite direction. Spins can be transported in suchantiferromagnetic systems, as has been reported in several
existing works [2-5]. In Cu2O, Cu- possesses a fully filled 3d
orbital without any unpaired spins. Cu2O is diamagnetic, and its
spin bandgap is equivalent to its electron bandgap. Therefore,
diamagnetic materials act as spin insulators. This work
experimentally establishes that the existence of unpaired spin is
one of the important conditions for a good spin conductor.
References[1] S. Maekawa, Concept in Spin Electronics (Oxford Univ. Press, 2016).
[2] Z. Qiu, J. Li, D. Hou, E. Arenholz, A. T. N'Diaye, A. Tan, K. Uchida, K. Sato, S.
Okamoto, and Y. Tserkovnyak, Nat. Commun. 7, 12670 (2016).
[3] Z. Qiu, D. Hou, J. Barker, K. Yamamoto, O. Gomonay, and E. Saitoh, Nat.
Mater. 17, 577 (2018).
[4] D. Hou, Z. Qiu, and E. Saitoh, NPG Asia Mater. 11, 35 (2019).
[5] Z. Qiu and D. Hou, Chinese Phys. B 28, 088504 (2019).

Keywords: spintronics, spin wave, spin transfer
Zhiyong Qiu (Corresponding Author, Dalian University of Technology)
E-mail: qiuzy@dlut.edu.cn
Gerrit Ernst-Wilhelm Bauer (Head of International Collaboration Center)
E-mail: icc-imr@imr.tohoku.ac.jp
URL: http://www.icc-imr.imr.tohoku.ac.jp/

44

IMR KINKEN Research Highlights 2020

IMR KINKEN Research Highlights 2020

Collaborative Research Programs of CNSAM
Center of Neutron Science for Advanced Materials

T

he Center of Neutron Science for Advanced Materials is a neutron facility with research emphasis on novel materialscience in IMR. It manages three neutron instruments in a research reactor facility, JRR-3, under a general user
program, in addition to a new spectrometer, POLANO, at J-PARC/MLF. We leveraged these instruments and ourendogenous capacity to commence a user program in April 2019. In this report, we present the current status of thisresearch program.
The Center of Neutron Science for Advanced
Materials (CNSAM) was established in 2010 to promote
novel material science research that exploits the use ofneutrons. CNSAM operates two neutron spectrometers
and one diffractometer at the JRR-3 research reactorof the Japan Atomic Energy Agency. In addition, thecenter is in the process of constructing a polarized
neutron spectrometer, POLANO, at the Materials andLife Science Experimental Facility (MLF) in the Japan
Proton Accelerator Research Complex (J-PARC).
POLANO is a newly developed spectrometer that was
designed to realize polarization analysis in higher
energy regions, beyond the measurements that are
facilitated using a conventional triple-axis spectrometer[1]. The construction of POLANO began in 2012 undercollaboration with the High Energy Accelerator
Research Organization. After several years ofdesigning, manufacturing, installing devices, and otherrelated matters, we finally commenced beamcommissioning in June 2017 and initiated a generaluser program in April 2019 at J-PARC MLF that exploit
unpolarized neutron beams.
Additionally, we initiated a collaborative research
program in IMR in last April to provide opportunities
not only for experimental research using neutron
beams, but also for the development of neutron
devices and analysis methods, and theoretical work
based on our neutron data. During FY 2019, we
received 12 and 7 proposals from domestic andoverseas researchers, respectively. The proposals
were classified into several categories including“neutron experiment at MLF,” “instrumental
development,” “development in analysis method,”“structural analysis and data handling,” and “others”.
For the category of “others,” proposals based on thecomplemental use of neutron and other quantum
beams, such as X-rays and muon beams were deemed
to be acceptable.
Figure 1 represents an image of the user support
in the POLANO cabin and the development of asolenoid coil for polarized measurement. Four staff

Fig. 1. Image of the user support in the POLANO cabin at
MLF and researchers engaged in the development of asolenoid coil for polarized measurements.

members of the center are primarily responsible forsupporting the users at both IMR and MLF.
Our strategic use of the four neutron instruments
includes structural research for new materials andadvanced research on magnetism using polarized
neutrons to improve our understanding of fundamental
concepts in materials science. We are now preparing
for full-scale operation of POLANO using polarized
neutrons and the re-operation of JRR-3, which isscheduled for the end of FY2020.
References[1] T. Yokoo, N. Kaneko, Y. Ikeda, S. Itoh, M. Fujita, M. Ohkawara, T.
Ino, and K. Hayashi, MLF Annual Report 2018 (2019).

Keywords: neutron diffraction, neutron scattering, instrumentation
Masaki Fujita (Head of Center of Neutron Science for Advanced Materials)
E-mail: nc-imr@imr.tohoku.ac.jp
URL: http://nc-imr.imr.tohoku.ac.jp/index-e.html

IMR KINKEN Research Highlights 202045

Research Centers

Development of an Ultrasound Measurement System Available in High Field
and Low Temperature Regions and its Application to Multipole Physics
Laboratory of Low Temperature Materials ScienceT

his study reports the successful measurement of changes in ultrasound velocities of CeTe, demonstrating the field-induced
antiferroquadrupole orders with high accuracy at high fields of up to 28 T at 50 mK. This is achieved using a cryogen-free hybrid
magnet and a dilution refrigerator with a plastic mixing chamber.

Fig. 1 CHM and DR constructed for High Field Laboratory for
Superconducting Materials.
1.66

CeTe

I (AFM)

II (AFQ)
IV (AFQ)

1.60

9

1.58
1.56
1.54

1.55
5.5

IMR KINKEN Research Highlights 2020

6.0

6.5

7.0

B (T)

0

5

10

15

20

25

30

B (T)

Fig. 2 Field dependence of the elastic constant C44 of CeTe
at 50 mK (B || [110]) [2]. The letters k and u denote thewave vector and polarization of ultrasound, respectively.

References[1] K. Watanabe, G. Nishijima, S. Awaji, K. Takahashi, K. Koyama, N. Kobayashi,
M. Ishizuka, T. Itou, T. Tsurudome, and J. Sakuraba, IEEE Trans. Appl. Super.
16, 934 (2006).
[2] S. Nakamura, S. Awaji, T. Yanagisawa, T. Saito, T. Matsumura, and A. Ochiai,
JPS Conf. Proc. 30, 011155 (2020).
[3] H. Takaguchi, Y. Hayashi, T. Matsumura, K. Umeo, M. Sera, and A. Ochiai, J.
Phy. Soc. Jpn. 84, 044708 (2015).

Keywords: high magnetic field, ultrasound, multipole
Shintaro Nakamura (Corresponding Author, Laboratory of Low Temperature Materials Science)
E-mail: nakamura@imr.tohoku.ac.jp
Takahiko Sasaki (Head of Laboratory of Low Temperature Materials Science)
E-mail: takahiko@imr.tohoku.ac.jp
URL: http://ltsd.imr.tohoku.ac.jp/index-e.html

46

V (IF)

3

1.60

50mK, B || [110]k || [100], u || [010]

C44 (10 J/m )

3

1.62

9

1.64

C44 (10 J/m )Multipole physics is currently one of the leading research
topics in the field of magnetism. Due to the limited availability ofexperimental techniques to access multipole phenomena,
progress in their methods of measurement is highly desired. In
this context, ultrasounds are capable of modulating crystal
electric fields via lattice strains and inducing quadrupoleelectric fields. Such fields couple with electric quadrupole
moments on local electrons in magnetic ions, and the responses
to this interaction appear as changes in the velocities v ofultrasounds. Multipoles of high rank do not couple with magnetic
fields directly, and interactions among multipoles are usuallymuch weaker than dipole-dipole interactions. Studies on
multipole physics, therefore, require high field and low
temperature conditions. Based on this perspective, ultrasoundserves as one of the most optimal probes, as it can be generated
by low electric power translated via conductors.
In this study, we succeeded in developing an ultrasonicmeasurement system available at fields of up to 28 T and attemperatures as low as 50 mK, using a cryogen-free hybrid
magnet (CHM) and a dilution refrigerator (DR) with a plastic
mixing chamber (MC). The CHM was composed of an outer
9T-superconducting magnet and an inner 19T-resistive magnet[1]. The plastic MC suppressed the eddy currents arising fromthe high-speed sweep induced by the resistive magnet andenabled measurements at 50 mK. Figure 1 depicts the DR set
in the CHM. Figure 2 presents an example of the observation of
the elastic constant C44 (proportional to v2) at 50 mK in CeTe,
which is a heavy fermion system exhibiting antiferroquadrupole(AFQ) orders. As illustrated in Fig. 2, CeTe (TN=2 K) was
observed to undergo successive transitions from
antiferromagnetic phase I to an AFQ phase II at 0.7 T, and,eventually, into another AFQ phase IV at around 6.3 T. The
vanishing field of phase IV was determined to be 18.5 T, which
agrees well with the theoretical prediction [3]. We confirmed thequenching of the AFQ phase by the field in CeTe andexperimentally identified the transition field at the lowtemperature limit for the first time. Oscillations in C44 above 20
T were observed to have originated from the acoustic de Haasvan Alphen effect in the field-induced ferromagnetic phase byheavy fermions.

IMR KINKEN Research Highlights 2020

Research Facility for Physics and Chemistry of Radioactive
and Nuclear Materials
Laboratory of Alpha-Ray Emitters

More than 170 species of radioisotopes and nuclear materials are available for study at the Laboratory ofAlpha-ray Emitters. This laboratory is an established international center for the study of the physical andchemical properties of radioactive materials such as actinide compounds. Researchers from many of theleading universities and institutes all over Japan visit this facility every year to prepare a variety of materials andperform chemical and physical measurements.

The Laboratory of Alpha-ray Emitters provides a
research environment for the study of 170 radionuclides
and elements, especially alpha-ray emitters such as
actinide. This lab functions as a source for the
preparation of pure crystals of actinide compounds
and supplies them to other universities for application
to synchrotron orbit-radiation. In the past decade,actinium-225 (Ac-225), an actinide element, has been
produced as part of an inter-university collaboration.
The radiation-controlled area of this laboratory includes
three chemistry rooms that are equipped with local
exhaust ventilation systems to facilitate the handling ofvarious kinds of material. Several spectrometers,
including units for gamma rays and alpha rays, areavailable.
A recent achievement is the determination of theenergy of an optical nuclear transition in Th-229 usingalpha-ray emitters. The low-energy nuclear transitionin Th-229 is in the optical region and thus provides
unique opportunities for high-resolution laser
spectroscopy of the atomic nuclei. A promising
application is an optical nuclear clock: an atomic clockthat references a nuclear transition. Extensive
experimental efforts have been devoted to the accurate
measurement of this nuclear transition energy.
However, the reported energy value ranges from 3.5
eV to 8.28 eV, and further measurements based on
different experimental techniques are necessary to
improve our confidence in the transition energy.
By using a transition edge sensor (TES)
microcalorimeter, we have determined the absoluteenergy of the 29.2-keV gamma rays emitted fromU-233 [1] (Fig. 1). Combining this result with that for thecase wherein the 29.2-keV nuclear state in Th-229 wasresonantly excited using narrow-band synchrotron
radiation [2], we determined the optical nuclear
transition energy in Th-229 to be 8.30(92) eV [1]. Thisvalue agrees with the two latest values measured using
different experimental techniques.
We have recently succeeded in improving the

Fig. 1 (a) Developed TES microcalorimeter chip. (b) Spectrumof the 29-keV gamma ray emitted from U-233 wasobserved with an energy-resolution of 36 eV (FWHM), andused to determine the energy of the optical nuclear
transition in Th-229.

energy-resolution of our TES chip to achieve better
accuracy. The enhanced energy resolution wasobserved to be 15 eV at 29 keV, an improvement by afactor of 2.5 compared to previous sensors. A series ofmeasurement tests using this new sensor is in progressat the Laboratory of Alpha-ray Emitters.
References[1] A. Yamaguchi, H. Muramatsu, T. Hayashi, N. Yuasa, K. Nakamura,
M. Takimoto, H. Haba, K. Konashi, M. Watanabe, H. Kikunaga, K.
Maehata, N. Y. Yamasaki, and K. Mitsuda, Phys. Rev. Lett. 123,
222501 (2019).
[2] T. Masuda et al., Nature 573, 238 (2019).

Keywords: actinide, atomic clock, transition edge sensor
Kenji Shirasaki (Head of Laboratory of Alpha-Ray Emitters)
E-mail: kshira@imr.tohoku.ac.jp
URL: http://alpha.imr.tohoku.ac.jp/en.html

IMR KINKEN Research Highlights 202047

Research Centers

Microwave Digestion Using a Quaternary Acid Mixture forInductively-Coupled Plasma Atomic Emission
Spectrometric Analysis of Steel Samples
Analytical Research Core for Advanced Materials

We hereby introduce a microwave digestion method that is routinely used to prepare analytical samples. Thisdigestion process is suitable for the quantification of volatile elements as well as alloyed elements in metallic
samples. Reliable analytical results can be obtained by optimizing the digestion conditions.

Chemical analysis is an essential aspect in thecharacterization of advanced materials. To quantify
alloyed elements in steel using an inductivelycoupled plasma optical emission spectrometer (Fig.
1), the sample material is decomposed using
different types of acid mixtures (e.g., an acid mixtureof hydrochloric and nitric acid (mixed acid A), acidmixture of sulfuric and phosphoric acid (mixed acidB), and acid mixture of hydrofluoric and nitric acid(mixed acid C)). For example, low-alloyed steel canbe digested using mixed acid A, B, or C. However,mixed acid B is often used for several high-alloyedsteels as well (e.g., tool steel containing tungsten).
However, silicon, phosphorous, and sulfur cannot
be quantified using this mixed acid because it
contains phosphorous and sulfur. Additionally,silicon precipitates in the acid solution.
For the preparation of steel samples, aquaternary acid mixture of hydrochloric, hydrofluoric,
nitric, and phosphoric acids [1] was tested. A
microwave oven (Fig. 2) was used for the digestion
experiment. This quaternary acid mixture (HCl, HF,
HNO3, and H3PO4) was superior to the tertiary acidmixtures (HCl, HF, and HNO3; HCl, HNO3, andH3PO4; HCl, HF, and H3PO4; and HF, HNO3, andH3PO4). The working parameters of the microwave
digestion were optimized to quantify alloyedelements of vanadium, chromium, manganese,
cobalt, nickel, molybdenum, and tungsten, in
addition to a volatile element of sulfur in various steel
samples including stainless steel and tool steel.
References

Fig. 1 Inductively-coupled plasma optical emission
spectrometer model Arcos® MV130 (SPECTRO
Analytical Instruments GmbH, Germany)

Fig. 2 Microwave oven model SpeedWave® MWS-3 usedas the microwave pressure digestion unit (Berghof
Products + Instruments GmbH, Germany)[1] K. Nakayama and K. Wagatsuma, ISIJ Int. 60, 193 (2020).

Keywords: atomic emission spectrometry, steel
Kenichi Nakayama (Corresponding Author, Analytical Research Core for Advanced Materials)
E-mail: ken1naka@imr.tohoku.ac.jp
Kazuaki Wagatsuma (Analytical Science Research Laboratory)
E-mail: wagatuma@imr.tohoku.ac.jp
Toyohiko J. Konno (Head of Analytical Research Core for Advanced Materials)
E-mail: tjkonno@imr.tohoku.ac.jp
URL: http://bunseki-core.imr.tohoku.ac.jp/top_eca_eng.html

48

IMR KINKEN Research Highlights 2020

International Collaborative Research

IMR KINKEN Research Highlights 2020

Research

International Collaborative ResearchElectronic Materials

In Situ Observation of Crystal Growth and Re-Melting
Process of Si1-xGex Alloy Semiconductor

R

apid cooling of a liquid phase is a well-known method for obtaining materials with fine-grain structure.
However, it was found that the grain sizes of the rapidly cooled and slowly cooled Si1-xGex did not differ. We
directly observed the solidification process of this material and found that the initially grown fine dendrites remelted during the rapid cooling process, and finally formed a coarse-grain structure.
Rapid cooling of an alloy typically results in apolycrystalline material with a fine-grain structure
because a large number of nucleation sites areformed instantaneously. Therefore, it is expectedthat a sample of rapidly cooled Si1-xGex would havea fine-grain structure with relatively low variations ofGe composition and improved thermoelectricperformance. However, such rapid crystallization
experiments of the Si1-xGex alloy have not beenreported as yet.
Therefore, in the present study, we investigated
the growth process of Si-rich Si1-xGex (x = 0.1, 0.2,
and 0.3) under rapid cooling to achieve the finegrain structures for high thermoelectric performance[1]. The crystallized samples were analyzed by EDX
and EBSD to study the compositional variation andgrain structures.
Si1-xGex samples were solidified under rapid(~330 K/min) and slow (1 K/min) cooling processes.
Unexpectedly, the grain size and structures for both
crystals were almost the same, even though thecooling rate in the rapid cooling experiment wasmore than 300 times faster than that in the slow
cooling experiment.
To understand the formation of similar grain
structures, the crystal growth process under rapid
cooling was observed in situ using a high-speed
microscopic camera with a recorder [1]. Figure 1
shows the snapshots recorded during the in situ
observation of the crystal growth process of (a)Si0.7Ge0.3 and (b) pure Si at different temperaturesduring rapid cooling. Fine dendrites started to growin Si0.7Ge0.3 after the heater was switched off. The
dendrites grew rapidly for a short period but started
to re-melt even as the temperature decreased. The
re-melting was continued and ultimately completely.
Finally, the recrystallization was observed as shownin the final picture of Fig. 1(a). In the case of pure Si(Fig. 1(b)), two dendrites grew under the observed
area and the size of the dendrites increased withtime without re-melting. From the in situ observations,
it was obvious that the coarse-grain structure wasFig. 1 Crystallization processes of (a) Si0.7Ge0.3 and (b)pure Si as a function of time during rapid cooling [1].

formed in Si1-xGex even during the rapid coolingprocess. The deviation from the equilibrium liquidus
and solidus composition was relatively larger when
the initial Ge composition was higher in Si1-xGex andthereby the re-melting occurred immediately.
References[1] M. Arrivanandhan, G. Takakura, D. Sidharth, K. Maeda, K.
Shiga, H. Morito, and K. Fujiwara, J. Alloys Compd. 798, 493(2019).

Keywords: crystal growth, interface, crystal structure
Kozo Fujiwara (Crystal Physics Research Laboratory)
E-mail: kozo@imr.tohoku.ac.jp
URL: http://www.imr.tohoku.ac.jp/en/about/divisions-and-centers/research-division/02.html

50

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IMR KINKEN Research Highlights 2020

Electronic Materials

Random Hopping of Majorana Fermion Found in Diluted
α-Ru0.8Ir0.2Cl3

In this study, we have investigated the thermodynamics of the well-known dilute dα-Ru Ir Cl -a model substance
0.8 0.2

3of Kitaev honeycomb lattice in which quantum spin liquids and fractional Majorana fermions are expected to appear.
The observation of low-power behavior of heat capacity has been explained as a consequence of random hoppingof Majorana fermions.

The investigation of spin liquids has been a longstanding research topic in condensed matter physics.
Exotic properties are expected in spin liquids in which
no-magnetic orders appear despite substantial
correlation. One of the most important predictions inthis context is the fractionalization of the quantum
number and the appearance of Majorana Fermion statein Kitaev honeycomb lattice systems [1], which is aunique system with an exactly solvable ground state.
α-RuCl3 has been extensively investigated as themodel substance of the Kitaev model, in which 4d5 Ru3+ ions form strongly anisotropic spin-arrays on thehoneycomb lattice. Several physical properties
demonstrate evidence of spin liquid-like excitations athigh temperatures above the magnetic order at around
7 K. Its phase transition to zig-zag antiferromagnetic
state prevents the investigation of its spin liquid groundstate with lesser thermal disturbances. One strategy toaccomplish this is a dilution with non-magnetic Ir3+ ionsto suppress the magnetic order. In fact, the original zigzag magnetic order disappears by about 10% Ir
substitution and another magnetic order appears andpersists at up to 20% substitution. In the present study,
we have examined certain thermodynamic properties,
such as magnetization, susceptibility, and specificheat, to examine the thermodynamic properties of thematerial at the quantum critical regime between theordered and the disordered states.
The experiments have been conducted using a
micro-calorimeter installed in a superconducting
magnet generating magnetic fields of up to 16 T [2].
The specific type of the calorimeter is crucial tominimize the stress on the sample, which is very
sensitive to pressure. The temperature dependence ofheat capacity of the sample demonstrates the vastdifference between its behavior and that of the nonsubstituted sample. Specifically, the clear peak of thezig-zag magnetic order was observed to disappear
and a sizable residual entropy was detected at theultra-low temperature regime. At above 2.5 K, the C/T
was observed to be nearly constant, exhibiting slightdecrement towards higher temperatures. A rapid

Fig. 1 Scaling relation of heat capacity in α-Ru0.8Ir0.2Cl3 at
various magnetic fields. The relation, H0.81C/T-T/H, is found.

decrease in the value of C/T was observed
corresponding to lower temperatures with a clear
power-law dependence.
To elucidate the origin of the power-law
dependence, we have tried to use different types ofscaling functions. One of the most successful ones has
been depicted in Fig. 1, for which the relation H0.81C/TT/H could be applied. The reduction in the value of C/Tfrom T-linear behavior found during the Raman
scattering demonstrates the hierarchy of the spin
excitations. Specifically, the natures of the excitations
vary over higher energy and lower energy regimes.
Such reduction is expected in the bond-disorder
Kitaev model. Therefore, we inferred that the observed
thermodynamic properties were induced by thequantum spin liquid with bond disorder. In terms of
Fermions, this can be related to the random hopping of
the Majorana Fermion in the pinned random flux
background, which appears at the quantum criticalregime due to the suppression of magnetic orderings.
References[1] A. KItaev, Ann. Phys. (Berl.) 321, 2 (2006).
[2] S. H. Do, C. H. Lee, T. Kihara, Y. S. Choi, S. Yoon, K. Kim, H.
Cheong, W. T. Chen, F. Chou, H. Nojiri, and K. Y. Choi, Phys. Rev.
Lett. 124, 047204 (2020).

Keywords: high magnetic field, spin liquid, heat capacity
Hiroyuki Nojiri (Magnetism Research Laboratory)
E-mail: nojiri@imr.tohoku.ac.jp
URL: http://www.hfpm.imr.tohoku.ac.jp/index.html

IMR KINKEN Research Highlights 202051

International Collaborative Research

Energy-related Materials

Prediction of Macroscopic Mechanical Properties of
Diamond-Like Carbon from Atomic-Scale Simulation

Understanding the relation between the atomic-scale structure and mechanical properties of diamond-likecarbon (DLC) is necessary and helpful for its improvement. Herein, we proposed a parameter to predict themechanical properties of DLC by molecular dynamics simulation.

The amorphous carbon material called diamondlike carbon (DLC), mainly containing sp3 - and sp2hybridized carbon and hydrogen atoms, shows
tremendous potential in various fields and applications
because of its excellent mechanical properties.
Generally, the mechanical properties of materials are
determined by the atomic-scale structure. However,the detailed relation between the atomic-scale
structure and the mechanical properties of DLC is still
unclear. In this study, molecular dynamics (MD)approach was used to clarify the correlation between
the atomic-scale structure and the macroscale Young's
modulus of DLC.
To investigate the relation between the atomicscale structure and the Young's modulus of DLC,
tensile MD simulations were conducted on DLC withvarious atomic-scale structures.
First, we examined the coordination number (CN),which is known to be related to mechanical properties,
as a parameter to predict the Young's modulus of DLC.
The CN is given as follows:CN =ଶே(ୱ୮) ା ଷே(ୱ୮మ ) ା ସே(ୱ୮య ),Fig. 1 (a) Relation between CN and density, (b) Young’s
modulus and CN, (c) CNeff and density, and (d) Young's
modulus and CNeff.

(1)

ே(ୱ୮) ା ே(ୱ୮మ ) ା ே(ୱ୮య )
where N(sp), N(sp2), and N(sp3) are the numbers of

sp-, sp2- and sp3-hybridized carbon atoms, respectively.
However, we found that the Young’s modulus (E) anddensity (ρ) of DLC do not show a monotonic correlation
with CN (Fig. 1(a) and (b)), and thus CN fails as aparameter to predict the Young’s modulus of DLC.
We considered that the failure of CN might be
attributed to the treatment of the hydrogen atoms. In
the case of hydrogenated DLC, hydrogen atoms do
not participate in constructing the carbon network.
Therefore, we examined effective coordination number(CNeff) as a parameter, that indicated the averagecoordination number of carbon atoms without thecontributions from hydrogen atoms (N(H)).
ଶே(ୱ୮) ା ଷே(ୱ୮మ ) ା ସே(ୱ୮య ) – ே(ு)

,

(2)

The relation between CNeff, ρ, and E is shown in
Figure 1(c) and (d). It was observed that CNeff waslinearly proportional to ρ, CNeff = 0.87ρ + 1. The
monotonic positive correlation between CNeff and E

CNୣ୤୤ =

ே(ୱ୮) ା ே(ୱ୮మ ) ା ே(ୱ୮య )Fig. 2 Relation between the density and Young’s modulus.

was also observed, E = 180(CNeff − 2.0)1.5. Thus, CNeffshould be used rather than CN to predict the Young’s
modulus of DLC.
Finally, the applicability of the above obtainedcorrelation between ρ and CNeff was verified bycomparison with the existing experimental results (Fig.
2). The simulation and experimental results showed thesame monotonically increasing correlation between
the Young’s modulus, verifying that our obtainedcorrelations among density, CNeff, and Young’s
modulus were correct [1].
References[1] J. Xu, Y. Wang, Y. Cen, B. Xing, X. Zheng, Y. Ootani, and M. Kubo,
J. Phys. Chem. C 123, 24609 (2019).

Keywords: amorphous, mechanical properties, computational materials science
Momoji Kubo (Materials Design by Computer Simulation Research Laboratory)
E-mail: momoji@imr.tohoku.ac.jp
URL: http://www.simulation.imr.tohoku.ac.jp/eng/index.html

52

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IMR KINKEN Research Highlights 2020

Infrastructural Materials

Two Types of Precipitates in High-Entropy Alloy
FeCoNiCrTi0.2 Analyzed by Atom Probe Tomography

A precipitation-strengthened high-entropy alloy, FeCoNiCrTi , was analyzed by atom probe tomography.
0.2

Two types of precipitates, nano-sized spherical precipitates within grains and lamellar precipitates near grainboundaries, were observed. Atom probe analysis revealed that although the morphology of the precipitates in
the inner grains was different from the one near the grain boundaries, the chemical partitioning in these tworegions was almost the same.

High-entropy alloys (HEAs), a new class ofstructural materials, have attracted considerable
attention in recent years because of their excellent
properties and potential industrial applications. The
face-centered cubic (FCC) HEAs are one of themost extensively investigated type of HEAs because
they exhibit outstanding ductility, exceptional
toughness, as well as excellent corrosion resistance.
However, the FCC HEAs are not very strong, which
limits their applications. The most promising method
to strengthen FCC HEAs involves introducing hard
precipitates into the FCC matrix.
Atom probe tomography (APT) is used to obtain
three-dimensional atom maps of materials withnearly atomic-scale resolution [1-3]. This technique
has high mass resolving power to identify isotopes,
and therefore may be used to characterize thecomposition of different kinds of precipitates in
HEAs, which include various elements [4].
Figure 1 shows the atom maps in the inner
region of a grain in the precipitate-strengthened
FeCoNiCrTi0.2. The nano-sized spherical precipitatesin the grain were clearly observed using APT. The
average radius of these precipitates was 4.1 ± 1.6
nm. Figure 2 shows the atom maps near the grainboundaries, which clearly showed lamellar
precipitates. It was found that Fe and Cr were almost
depleted from the lamellar precipitates. APT analysis
revealed that although the morphology of the
precipitates near the grain boundaries was different
from the one in the inner grains, the chemical
partitioning in these two regions was almost thesame. Besides, nano-sized spherical precipitates
formed from the homogeneous nucleation within thegrains, the lamellar precipitates were also formed
because of the heterogeneous nucleation neargrain boundaries. These precipitates contributed tothe strength enhancing of FCC HEAs.

Fig. 1 Atom maps inside the grain with spherical precipitates.

Fig. 2 Atom maps near the grain boundary with lamellarshape precipitates.

References[1] M. Tane, H. Nishiyama, A. Umeda, N. L. Okamoto, K. Inoue,
M. Luckabauer, Y. Nagai, T. Sekino, T. Nakano, and T. Ichitsubo,
Phys. Rev. Mater. 3, 043604 (2019).
[2] Y. Ono, Y. Funakawa, K. Okuda, K. Seto, N. Ebisawa, K. Inoue,and Y. Nagai, Tetsu To Hagane 105, 452 (2019).
[3] E. Tsuchiya, Y. Matsumura, Y. Hosoya, Y. Miyamoto, T.
Kobayashi, K. Seto, K. Tomura, K. Inoue, and Y. Nagai, Tetsu To
Hagane 105, 76 (2019).
[4] Y. Tong, D. Chen, B. Han, J. Wang, R. Feng, T. Yang, C. Zhao,
Y. L. Zhao, W. Guo, Y. Shimizu, C. T. Liu, P. K. Liaw, K. Inoue, Y.
Nagai, A. Hu, and J. J. Kai, Acta Mater. 165, 228 (2019).

Keywords: atom probe tomography, high entropy alloy, microstructure
Yasuo Shimizu, Koji Inoue, and Yasuyoshi Nagai (Irradiation Effects in Nuclear and Their Related Materials ResearchLaboratory)
E-mail: nagai@imr.tohoku.ac.jp
URL: http://www.imr.tohoku.ac.jp/en/about/divisions-and-centers/research-division/11.html

IMR KINKEN Research Highlights 202053

International Collaborative ResearchElectronic Materials

Water Vapor Sensitive Spin-State Switching in an Iron (III)Complex with Nucleobase Pendants Making Flexible
Hydrogen-Bonded Networks

Sensitive chemo-switchable magnetic materials that can operate at room temperature are potential candidates

for use in novel chemical devices. Here, we report a discrete iron (III) complex that exhibits high-spin/low-spin
switching and is sensitive to water vapor above room temperature. The transformation of the hydrogen-bonded
network triggered by water is closely associated with spin-state changing.
Porous magnetic materials with properties
controllable by chemical stimuli (such as guest
molecules) have potential applications in chemical
switches. We focused on a structure-flexiblehydrogen-bonded framework with multiple hydrogenbonding sites from nucleobase moieties attached toa discrete iron (III) spin-crossover (SCO) core,[FeIII(Tsal2trien)]ClO4·H2O (1-H2O) [1]. Here, wereport low-spin/high-spin (LS/HS) state switching
using water adsorption/desorption above room
temperature (RT) between the solvated compound
1-H2O and its solvent-free compound 1.
When lattice water was removed from 1-H2O toobtain 1, the color of the solid changed from dark
blue to dark red, corresponding to the LS and HS
states of the FeIII ion, respectively (Fig. 1a). When 1
was exposed to the atmosphere, the color quickly
changed to dark blue, indicating the recovery of the
1-H2O phase, as shown in the powder X-ray
diffraction (PXRD) pattern (Fig. 1a). The spin-state
assignment and its reversibility were also confirmedby the Mössbauer spectra (Fig. 1b).
In the χT-T plots of 1-H2O under heating and areduced pressure (10 Torr), χT abruptly increased at
323 K (Fig. 2a), indicating that the spin statechanged from LS (1-H2O) to HS (1) by removing thewater. In the cooling process, the value of χT
remained constant (Fig. 2a), indicating that 1
remained in the HS state. Meanwhile, in the χT-T
plots under atmospheric pressure (760 Torr), thetransition temperature (Tc) increased to 343 K (Fig.
2b). Thus, it is clear that Tc is pressure dependent.
The plot of γHS vs. time at 330 K shows that as soon
as water was introduced, γHS abruptly decreased,
and the electronic state completely switched fromHS to LS within 30 s (Fig. 2c). The reversible
switching of the spin state could be tuned by
reducing the pressure and adding saturated water
vapor at 340 K (Fig. 2d). This means that thebidirectional spin-state switching above RT can becontrolled with only water vapor.

Fig. 1 Structure and spin-state changes during water
elimination/adsorption. (a) PXRD patterns at RT andimages of micro-crystals. (b) Mössbauer spectra at RT.

Fig. 2 (a) χT −T plots under 10 Torr of pressure of humidity.
(b) Humidity dependence of χT −T plots. (c) Timedependence of HS fraction (γHS) at 330 K, where water
vapor (760 Torr) was introduced to 1 after 4 min. (d)Variation of γHS at 340 K by repeating introducing water
vapor (760 Torr) and reducing pressure (10 Torr). H = 2
kOe.

References[1] K. Kagesawa, Y. Ichikawa, H. Iguchi, B. K. Breedlove, Z. Li, M.
Yamashita, A. Okazawa, W. Kosaka, and H. Miyasaka, Chem.
Lett. 49, 1221 (2019).

Keywords: crystal structure, magnetic properties, porosity
Wataru Kosaka and Hitoshi Miyasaka (Solid-State Metal-Complex Chemistry Research Laboratory)
E-mail: w-kosaka@imr.tohoku.ac.jp; miyasaka@imr.tohoku.ac.jp
URL: http://www.miyasaka-lab.imr.tohoku.ac.jp/cn5/HOME-Eng.html

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IMR KINKEN Research Highlights 2020

Electronic Materials

Non-Destructive Imaging for the Improvement of
Fabrication Yields of Magnetic Tunnel Junctions

We have developed a new non-destructive imaging technique for a buried interface. In this technique, thepenetration depth of the incident electrons and the corresponding depth of the generated backscatteredelectrons were controlled precisely. Therefore, this technique is highly useful as a quality assurance tool for
nanoelectronic devices.

Current nanoelectronic devices typically use overthirty layers, where each interface needs to be
optimized to maintain their smoothness without any
contaminations and defects. To date, such interfacial
structures have been evaluated by cross-sectionaltransmission electron microscopy (TEM) imaging withatomic resolution. However, TEM imaging requires adevice to be sliced into approximately 100 nm thickpieces with electron transparency, which does notallow observing correlations between the interfacial
structures and their transport properties. In this study,
we have developed a new imaging method with acontrolled electron-beam in scanning electron
microscopy (SEM) that enabled us to observe buriedinterfaces in a non-destructive manner [1].
A magnetic tunnel junction (MTJ), consisting of Cr(80)/Pd (5)/Co2Fe0.4Mn0.6Si (5)/MgO (2)/Co0.5Fe0.5 (5)/
IrMn3 (10)/Ru (7) (thickness in nm), was deposited
onto an MgO(001) substrate using ultrahigh vacuum
sputtering ][2]. This structure was then etched into a
series of MTJ pillars with dimensions between 10×10
and 50×50 µm2 by photolithography and Ar-ion milling.
The milled regions were filled with Al–O and finally a
top electrode of Au (80 nm) was sputtered. The
fabricated MTJs were measured using a four-terminal
method, categorizing into large (>80%) and small(<20%) tunneling magneto-resistance (TMR) ratios.
Electron flight simulations [3] was used to estimate
the representative electron acceleration for SEM
imaging. By taking over 5% changes in the number ofbackscattered electrons, two voltages were selected
to highlight any defects at the MgO interfaces. SEM
images taken at 10.5 keV exhibited some redepositedmaterials at the edges of the low TMR device only, as
shown in Figs. 1(a) and (d). Further energy dispersive
X-ray spectroscopy (EDX) mapping confirmed theredeposition of aluminum carbides, which were likely
formed by the reaction between the residual resist
from photolithography and Al during Al-O sputtering[see Figs. 1(b), (c), (e), and (f)].
Accordingly, the Al-O sputtering power was
reduced, which increased the yield to 15%. This

Fig. 1 (a) and (d) Scanning electron images of large andsmall TMR devices, respectively. EDX mapping is shownfor (b) Al and (c) C for the large TMR device and for (e)Al and (f) C for the small TMR device. After Ref. [4].

demonstrated that our non-destructive imaging
technique is useful to identify production faults andimprove the yield [4].
This work was done in collaboration with Prof.
Atsufumi Hirohata from University of York, UK.
References[1] A. Hirohata, Y. Yamamoto, B. A. Murphy, and A. J. Vick, Nat.
Commun. 7, 12701 (2016).
[2] E. Jackson, M. Sun, T. Kubota, K. Takanashi, and A. Hirohata,
Sci. Rep. 8, 7585 (2018).
[3] P. Hovington, D. Drouin, R. Gauvin, D. C. Joy, and N. Evans,
Scanning 19, 29 (1997).
[4] E. Jackson, Y. Wu, W. Frost, J.-Y. Kim, M. Samiepour, K. Elphick,M. Sun, T. Kubota, K. Takanashi, T. Ichinose, S. Mizukami, and A.
Hirohata, J. Phys. D: Appl. Phys. 53, 014004 (2019).

Keywords: spintronics, interface, magnetic properties
Koki Takanashi (Magnetic Materials Research Laboratory)
E-mail: koki@imr.tohoku.ac.jp
URL: http://magmatelab.imr.tohoku.ac.jp/

IMR KINKEN Research Highlights 202055

International Collaborative ResearchElectronic Materials

Nonclassical Crystal Growth Dynamics Visualized ThroughInterfacial Potential Modification by Electrical Field of
Optical Trapping Laser

Although the laser trapping of crystalline clusters in an aqueous solution can induce forced crystallization(even from an unsaturated solution), its crystal growth mechanism remains unclear. Herein, we revealed bydirect in-situ observation on the crystal surface that crystal growth dynamics was triggered by the interfacialpotential modification, which was caused by the electrical field of the incident laser.
The optical trapping of crystalline clusters with alaser tightly focused on the air/solution interface can
induce forced crystallization, even on an unsaturated
solution [1]. The laser trapping induced crystallization(LTIC) technique enables high controllability of
crystallization, involving spatiotemporal control of
nucleation, polymorph control depending on laser
polarization, and switchable crystal growth, among
others. However, the detailed crystal growthmechanism of LTIC remains unclear because thedirect investigation on the crystal surface, where
crystal growth actually progresses through molecularadsorption and desorption from an ambient phase,
has not been carried out. Here, we performed an insitu optical microscopic observation on the surface
of an ionic crystal grown via LTIC to investigate theactual crystal growth mechanism.
A sodium chlorate metastable single crystal
floating on the air/saturated solution interface was
optically trapped with a laser tweezers systemconstructed by tightly focusing a continuous-wave
laser (λ= 532 nm, 1 W) with an objective lens (60X,
NA = 0.9) equipped on an inverted polarized-lightmicroscope (Fig. 1A). The dynamics on the crystal
surface was visualized using an optical microscope.
The irradiation of the trapping laser on the
crystal triggered the wetting transition of the solution
thin layer covering over the crystal surface. The
solution thin layer changed from complete wetting
state to partial wetting state by the laser irradiation.
(Fig. 1B). The wetting transition suggests that theelectrical field from the laser propagating though the
crystal modified the interfacial potential by imposing
an additional electrostatic potential. We found thatthe liquid droplets formed through the wetting
transition, the size of which was hundreds of nm,
fused into a crystal while keeping their volume until
they reached the crystal step ledge through diffusionon the crystal terrace [2] (Fig. 1C and 1D). This
growth process, through the collective incorporationof building molecules to form the crystal, deviates
Keywords: crystal growth, interface, thin films
Hiromasa Niinomi (Crystal Chemistry Research Laboratory)
E-mail: h.niinomi@imr.tohoku.ac.jp
URL: http://www.uda-lab.imr.tohoku.ac.jp/index-e.html

56

IMR KINKEN Research Highlights 2020

Fig. 1 (A) Experimental schematic. (B) Wetting transitionof solution thin film. (C) Cluster assimilation like
nonclassical crystal growth dynamics. (D) Fusion
process of the liquid droplet illustrated by the Kossel
crystal model.

from the assumption of the classical crystal growththeory, in which crystal growth progresses throughthe sequential incorporation of each building unit.
Our result not only provides a novel viewpoint thatelucidates the mechanism of LTIC, but also is thefirst direct in-situ optical visualization of thenonclassical cluster assimilation crystal growth
dynamics from diffusion to fusion of clusters [3].
References[1] T. Sugiyama, K. Yuyama, and H. Masuhara, Acc. Chem. Res.
45, 1946 (2012).
[2] T. Niinomi, T. Sugiyama, T. Ujihara, S. Guo, J. Nozawa, J.
Okada, T. Omatsu, and S. Uda, Cryst. Growth Des. 19, 4138
(2019).
[3] M. Sleutel and A. E. S. Van Driessche, Proc. Natl. Acad. Sci. U.
S. A. 111, E546 (2014).

IMR KINKEN Research Highlights 2020

Catch Atoms in Action Using In-Situ Annealing TEM
International Research Center for Nuclear Materials ScienceI

n-situ high-resolution TEM imaging is useful for understanding how thermal energy and partial pressure affectthe crystal growth and surface atomic structures of metals, alloys, and compounds. Our in-situ annealing
spherical aberration-corrected TEM/STEM is now ready for atomic-scale relaxations of platinum nanoparticles
for a fuel-cell reactor or post-irradiation-annealing of nuclear reactor pressure vessel steels.
The international Japan-UK collaborationachieved direct atomic-scale visualization of surface
structures during a chemical reaction in the fuel-cell
reactor. The partial-pressure dependence of thesurface atomic structure of platinum nanoparticles(NPs) on amorphous carbon films was studied inreactive gas atmospheres under both the e-beam
and controlled heating conditions.
Spherical aberration-corrected TEM images
indicated that the Pt atoms on the topmost surface
layer formed a hexagonal lattice without theadsorbed atoms. Few individual Pt atoms wereobserved on the carbon film in 10−2 Pa H2 and N2
atmospheres (Fig. 1(a)). Such faceted Pt NPs with
distinct edges are a key feature for clarifying thestabilizing effect of reducing or non-reactive gas
molecules. As expected, the Pt NPs in CO and O2
atmospheres had an markedly round (less faceted)shape, as shown in Fig. 1(b). This is critical evidence
that the gas molecules directly contributed to thestabilization of the surface structure.
The transformation of NPs into disorderedclusters was systematically quantified undercontrolled in-situ annealing experiments at 423,
573, and 723 K using our specially designed in-situ
annealing TEM [1]. The in-situ annealing TEM wasthen applied to various nuclear materials such asreactor pressure vessel (RPV) and oxide dispersion
strengthened (ODS) steels for understanding theirembrittlement mechanisms. Fig. 2 shows the WBSTEM images obtained from RPV steels before andafter post-irradiation-annealing at 673 K. During
annealing, two ½ <111> dislocation loops coalesced
each other and formed a <100> dislocation loop.
Such growth and lattice relaxation processes of the½ <111> loops were consistent with the molecular
dynamics at the atomic scale. This is the first direct
visualization of relaxation processes of neutroninduced dislocation loops during post-irradiation
annealing.

Fig. 1 SAC images of the Pt/C catalysts under H2
atmosphere of 1 × 10 −2 Pa (a), CO atmosphere of 1 ×
10 −2 Pa (b); the fast Fourier transform patterns areshown as insets. The reconstructed surfaces of Pt NPs
are enlarged.

Fig. 2 Weak-beam STEM images of the neutron-irradiatedreactor pressure vessel steel acquired at R.T. (a) and
400 °C under isothermal annealing (b). The arrows in (a)show the dislocations and dislocation-loops.

References[1] K. Yoshida, X. Zhang, Y. Shimada, Y. Nagai, T. Hiroyama, N.
Tanaka, L. Lari, M. R. Ward, E. D. Boyes, and P. L. Gai, Nanotech.
30, 175701 (2019).

Keywords: electron microscopy, nanocluster, nuclear materials
Kenta Yoshida (Corresponding Author, International Research Center for Nuclear Materials Science)
E-mail: k_yoshida@imr.tohoku.ac.jp
Yasuyoshi Nagai (Head of International Research Center for Nuclear Materials Science)
E-mail: nagai@imr.tohoku.ac.jp
URL: http://www.imr-oarai.jp/en/

IMR KINKEN Research Highlights 202057

International Collaborative Research

Structure and Magnetic Properties of Nanoporous FePt
Fabricated by Dealloying Melt-Spun Fe-Pt-B Alloy
Cooperative Research and Development Center for Advanced Materials

Soft magnetic nanoporous fcc-FePt with an average pore size of 12 nm was fabricated by dealloying a meltspun Fe60Pt20B20 alloy in H2SO4 aqueous solution. The fcc-FePt transformed to L10 -FePt by annealing thenanoporous alloy, which significantly increased the coercivity up to 8.42 kOe.

Ordered face-centered-tetragonal FePt (L10 FePt) has attracted considerable attention for the
fabrication of high-performance permanent magnets
and ultrahigh density magnetic data storage media
owing to their extremely high magnetocrystalline
anisotropy [1]. Although nanostructured L10 -FePt
alloys, including thin films, nanoparticles, andnanocomposites have been synthesized by various
techniques and studied extensively, there havebeen only a few reports on the development of
nanoporous structures [2]. Here, we report themagnetic properties of a nanoporous L10 -FePt
magnet, which was fabricated by dealloying [2] of amelt-spun Fe60Pt20B20 alloy and subsequentannealing.
Pure Fe, Pt, and B were arc-melted to obtain analloy ingot. Ribbon samples were prepared by meltspinning. The alloy ribbons were dealloyed in 0.1
mol/L H2SO4 solution for 1800 s. Subsequently,
annealing was carried out in vacuum at 803–943 K
for 600 s. The structure of the samples was examined
by XRD and TEM. SEM was used to observe themorphology and to measure the chemicalcomposition. The magnetic properties of thesamples were measured with a VSM at the CRDAM
in IMR, Tohoku University.
A nanoporous alloy with an Fe/Pt atomic ratio of~55.3/44.7 was fabricated by dealloying the
Fe60Pt20B20 alloy composed of nanoscale
amorphous and fcc-FePt phase. The average
ligament and pore sizes were ~27 nm and 12 nm,
respectively (Fig. 1(a)). The TEM image (Fig. 1(b))
confirms the formation of a nanoporous architecture.
The corresponding SAED pattern reveals thepolycrystalline fcc-FePt diffraction rings without theamorphous phase. The nanoporous fcc-FePt alloy
showed soft magnetic characteristics. A phasetransformation from fcc-FePt to L10 -FePt was
realized in the nanoporous alloy after annealing(Fig. 2(a)). The VSM results showed that theFig. 1 SEM image (a) and bright-field TEM image of meltspun Fe60Pt20B20 alloy dealloyed in 0.1 mol/L H2SO4
solution. Inset in (b) is the corresponding SAED pattern [3].

Fig. 2 XRD patterns (a) and hysteresis loops (b) of thenanoporous fcc-FePt alloys annealed at 823–943 K for
600 s [3].

coercivity of the nanoporous alloy increased from
0.14 kOe to 8.42 kOe with the increase in annealing
temperature from 823 K to 943 K (Fig. 2(b)), which
could be attributed to the increased volume fractionof the L10 -FePt phase. The SEM observation
revealed that the ligament size increased from 31
nm to 60 nm, whereas the average pore size
remained nearly constant at ~16 nm with theincreasing annealing temperature.
References[1] K. Watanabe and H. Masumoto, Trans. Jpn. Inst. Met. 24, 627(1983).
[2] J. Erlebacher and R. Seshadri, MRS Bull. 34, 561 (2009).
[3] D. Ma, Y. Wang, Y. Li, R.Y. Umetsu, S. Ou, K. Yubuta, and W.
Zhang, J. Mater. Sci. Tech. 36, 128 (2020).

Keywords: nanoporous alloy, dealloying, magnetic properties
Wei Zhang (Dalian University of Technology)
E-mail: wzhang@dlut.edu.cn
Naoya Masahashi (Head of Cooperative Research and Development Center for Advanced Materials)
E-mail: masahasi@imr.tohoku.ac.jp
URL: http://www.crdam.imr.tohoku.ac.jp/en/index-en.html

58

IMR KINKEN Research Highlights 2020

IMR KINKEN Research Highlights 2020

Enhancement of the Critical Current Density in Cu/Ag
Composite Sheathed (Ba, K)Fe2As2 Tapes by Pre-Annealing
Process
High Field Laboratory for Superconducting Materials

Iron-based superconductors are promising candidates for high-field applications. Here, a pre-annealing
method was used to promote the transport critical current density (Jc) in Cu/Ag composite sheathed (Ba,K)
Fe2 As2 tapes. It was found that such tapes showed a transport Jc up to 4.9×104A/cm2 and 6.5×104A/cm2 (4.2
K, 10 T) under ambient pressure and using a hot-press technique, respectively.

For 122-type iron-based superconductor (IBS)
tapes produced by pure silver sheath, the transport
critical current density (Jc) is 1.5×105 A/cm2 (4.2 K,
10 T). To reduce the usage of pure silver in IBS tapes,
a Cu/Ag composite sheath was tested. However,owing to the voids and residual cracks formed duringcold deformation, the Cu/Ag composite sheathed
IBS tapes had low Jc from being sintered at low
temperatures. This suggests that high-temperature
pre-annealing may be an effective method toenhance the transport current of the Cu/Ag composite(Ba,K)Fe2As2 (Ba-122) tapes [1].
For the tape prepared without any pre-annealing
process (Tape 1), a transport Jc of ~2.0×104 A/ cm2(4.2 K, 10 T) was obtained, as shown in Fig. 1.
However, by a high-temperature pre-annealing
process and final ambient pressure sintering process,the Jc increased to 4.9×104 A/ cm2 (4.2 K, 10 T, Tape2). This result indicates that the transport Jc of the Cu/
Ag composite sheathed Ba-122 tapes fabricated by
the ambient pressure sintering process could besignificantly improved by the high-temperature preannealing process. Moreover, the Jc of the preannealed tape could be further improved by a hotpress process.
When the tapes were sintered at a low sintering
temperature, the grains were not well connected in
the Cu/Ag composite sheathed IBS tapes. Hightemperature pre-annealing process combined withcold rolling improved the grain alignment andenhanced the grain connections. As shown in Fig. 2,
the Vickers hardness of the superconducting core in
the pre-annealed tapes (Tape 2) reached an averagevalue of 112.7, but the Vickers hardness of the nonpre-annealed sample (Tape 1) only had an averagevalue of 77.6. The reason for the high hardness ofTape 2 is that there were more large grains and fewer

Fig. 1 Magnetic field dependence of transport Jc in
different kinds of tapes [1].

Fig. 2 Vickers hardness of superconducting core of fourtypes of tapes [1].

cracks in the samples pre-annealed at a relatively
high temperature (880 °C). The well-constructed
crystallization and grain connectivity obtained in the
pre-annealed tapes were considered as the major
causes of the high Jc.
References[1] L. Li, X. Zhang, C. Yao, S. Liu, H. Huang, C. Dong, Z. Cheng, S.
Awaji, and Y. Ma, Mater. Res. Express 6, 096003 (2019).

Keywords: iron-based superconductor, high magnetic field, critical current density
Yanwei Ma (Corresponding Author, Institute of Electrical Engineering, Chinese Academy of Sciences)E-mail: ywma@mail.iee.ac.cn
Hiroyuki Nojiri (Head of High Field Laboratory for Superconducting Materials)
E-mail: nojiri@imr.tohoku.ac.jp
URL: http://www.hflsm.imr.tohoku.ac.jp/cgi-bin/index-e.cgi

IMR KINKEN Research Highlights 202059

International Collaborative Research

Negative Thermal Expansion and Negative Linear Compressibilityof Black Phosphorene: A First-Principles Prediction
Center for Computational Materials ScienceT

he Center for Computational Materials Science supports various advanced studies on computational
materials science by providing users with an efficiently tuned high-performance computer as an international
collaborative research institute. Here, we introduce an activity selected from ~200 users. Dr. Chen investigatedthe properties of black phosphorene.
While most materials expand on heating andcontract on pressure, some abnormal negative
response, such as the negative thermal expansion(NTE) and negative linear compressibility (NLC),
exists in very few materials. For the first time, wehave found using first-principles calculations that
NTE and NLC can coexist in low-dimensional black
phosphorene [1]. Black phosphorene is a graphene
analog with a strongly puckered structure (Fig. 1). In
the temperature-field, the volume-NTE in black
phosphorene was observed below 170 K, and three
axial-NTEs were observed in the whole studied
temperature range, illustrating its anisotropic feature(Fig. 2(a)). In the hydrostatic pressure-field, the NLC
behavior occurred along the zigzag or armchair
direction in different pressure ranges (Fig. 2(b))
while the volume maintained positive compression.
The lattice dynamics analysis revealed that thereentrant honeycomb network was formed from avery flexible hinge and a widely variable angle of thelocal structure unit in black phosphorene facilitatedthe NTE and NLC behaviors. The calculated phonon
spectra and Gruneisen parameters in the Brillouinzone showed that the out-of-plane transverse
acoustic phonon mode TAz corresponded to thenegative modes of Gruneisen parameter. As thenegative values of Gruneisen parameter implied anincrease in the frequencies of the vibrational modes
with increasing volume, this TAz phonon was likely
conducive to the occurrence of abnormal physicalproperties in black phosphorene. This study
recognized that black phosphorene is a promising
2D functional material with rich physics, and also
provided some foundation and clues for exploring
the abnormal properties in other 2D materials.

Fig. 1 Structure of black phosphorene with a strongly
puckered feature and the supercell used in calculations.
The resultant structural parameters agree well with theexperimental values.

Fig. 2 Negative thermal expansion (NTE) and negative
linear compressibility (NLC) in black phosphorene
detected in the calculations.
(a) Temperature dependence of the structuralparameters along three directions.
(b) Pressure dependence of the structural parameters
along zigzag and armchair directions.

References[1] L. Wang, C. Wang, and Y. Chen, J. Phys.: Condens. Matter 31,
465003 (2019).

Keywords: 2D materials, unusual response properties, first-principles calculations
Ying Chen (Corresponding Author, School of Engineering, Tohoku University)
E-mail: ying@rift.mech.tohoku.ac.jp
Momoji Kubo (Head of Center for Computational Materials Science)
E-mail: momoji@imr.tohoku.ac.jp
URL: https://www.sc.imr.tohoku.ac.jp/eng/

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IMR KINKEN Research Highlights 2020

IMR KINKEN Research Highlights 2020

Observation of Magnetic Excitation in a Quantum Spin
System Using POLANO
Center of Neutron Science for Advanced MaterialsT

he neutron spectrometer POLANO at J-PARC is a powerful instrument that can achieve polarization analysis
in higher energy regions up to 100 meV to elucidate detailed spin and lattice dynamics. Using POLANO, we
measured the spin excitations and phonons in the quantum spin system of CuGeO3 and observed the thermal
evolution of both dynamics.
The study of low-dimensional spin systems isimportant for understanding the fundamental
natures of high-temperature superconductors [1].
CuGeO3 is a typical one-dimensional material. The
magnetism of the edge-sharing CuO2 chains of
CuGeO3 can be described by the S = 1/2 Heisenberg
model, and the two spinon continuum excitation,
which are associated with the quantum nature of
spins, were indeed observed by the neutron
scattering measurement [2]. Below 14 K (Tsp), a spin
gap opens due to the formation of a spin-singlet pairwith the spontaneous lattice distortion. This behavior
is recognized as the spin-Peierls transition, andCuGeO3 was thought to be the first inorganic spinPeierls compound. However, it remains unclear
whether CuGeO3 is an archetypal spin-Peierls
system, because phonon softening accompaniedby spin-Perierls transition has not yet been observedon CuGeO3. To clarify the mechanism of theanomalous phase transition in CuGeO3, theidentification of the dynamical behaviors, meaning
both spin fluctuations and phonons at and aroundthe transition temperature (not in the ground state ata base temperature), is essential.
We performed inelastic neutron scatteringexperiments to elucidate the thermal evolution of thespin and lattice dynamics using the chopper
spectrometer POLANO, which was recently
supplied for a user program at MLF.
Figure 1 shows the spin excitation spectra
observed in a wide energy and momentum spaces
at 5 K (below Tsp) and 50 K (above Tsp) At 5 K, desCloizeaux-Pearson-like modes emerge at Qc =±0.5, which corresponds to the magnetic zone
center, and they vanish above Tsp. Theseobservations are consistent with previously reported
results [2], showing the potential of POLANO in
observing the magnetic excitation of quantum spin
systems. Furthermore, we found an anomalous
temperature dependence in a high-energy opticalFig. 1 Spin excitation spectra observed at 5 K and 50 K for
CuGeO3.

phonon, which could be evidence of spin-phonon
coupling. Since POLANO can carry out polarization
analyses in high energy regions, details of the spinphonon coupling in CuGeO3 could be clarified infuture polarized neutron scattering measurements
taken with POLANO.
References[1] S. Asano, K. M. Suzuki, K. Kudo, I. Watanabe, A. Koda, R.
Kadono, T. Noji, Y. Koike, T. Taniguchi, S. Kitagawa, K. Ishida,
and M. Fujita, Phys. Soc. Jpn. 88, 084709 (2019).
[2] M. Arai, M. Fujita, M. Motokawa, J. Akimitsu, and S. M.
Bennington, Phys. Rev. Lett. 77, 3649 (1996).

Keywords: neutron scattering, quantum spin system, spin excitation
Masaki Fujita (Head of Center of Neutron Science for Advanced Materials)
E-mail: nc-imr@imr.tohoku.ac.jp
URL: http://nc-imr.imr.tohoku.ac.jp/index-e.html

IMR KINKEN Research Highlights 2020

61

Author Index
NamepageNamepageA
Aoki, DaiAoyagi, Kenta

34
11

B
Bauer, Gerrit E. W.

44

O
Okamoto, Norihiko L.
Ono, Kanta
Onose, Yoshinori
Orimo, Shin-ichi
Oyanagi, Koichi

10
42
18
22
25

C
Chen, Ying

60

Q
Qiu, Zhiyong

44

F
Fujita, Masaki
Fujita, Shinji
Fujiwara, Kohei
Fujiwara, Kozo

28, 45, 61
40
26
50

I
Ichitsubo, Tetsu
Inoue, Hisashi
Inoue, Koji

10, 4336
53

K
Kasada, RyutaKato, HidemiKawaguchi, Tomoya
Kihara, Takumi
Kim, Sangryun
Konno, Toyohiko J.
Kosaka, WataruKoyama, Motomichi
Kubo, Momoji
M
Ma, Yanwei
Masahashi, Naoya
Miyasaka, Hitoshi
N
Nagai, Yasuyoshi
Nakamura, Shintaro
Nakayama, Kenichi
Niinomi, Hiromasa
Nojiri, Hiroyuki
Nomura, Kentaro
Nozawa, Jun
Nguyen, Tien Quang

62

IMR KINKEN Research Highlights 2020

20
13, 32, 43
10
17
22
48
54
9
19, 42, 52, 60

59
39, 41, 58
31, 5438, 53, 57
46
48
56
40, 51, 59
24
21
14

S
Saitoh, Eiji
Sasaki, Takahiko
Shiga, Keiji
Shimada, Yusuke
Shimizu, Yasuo
Shirasaki, Kenji

25
27, 46
16
35
53
47

T
Takanashi, Koki
Tanimura, Hiroshi
Terada, Yayoi
Tokuda, Makoto
Tsukahara, Hiroshi

33, 55
10
14
30
42

W
Wagatsuma, Kazuaki
Wei, Daixiu

12, 48
13

Y
Yoshida, Kenta
Yoshikawa, AkiraYoshimi, Akihiro
Yoshimura, Koji

57
29
38
38

Z
Zhang, Jun
Zhang, Wei
Zhang, Yan
Zhang, Yongjie

31
58
39
8

Keyword Index
Keywordpage

Keywordpage

A
actinide
additive manufacturing
alloy
amorphous
atomic emission spectrometry
atomic clock
atom probe tomography

I
instrumentation45
insulator18
interface
19, 50, 55, 56
ionic conductor
22
iron-based superconductor
59

34, 38, 47
11
11, 2639, 52
12, 4847
8, 53

B
biomaterial41
Ccolloid21
computational materials science
14, 31, 42, 52
critical current density
59
crystal growth
16, 29, 50, 56
crystal structure
30, 50, 54
Ddealloying58
defects11
E
electron microscopy
electronic structureenergy storageepitaxial growth35, 5728
22
21, 36F
first-principles calculationsfoam
frustrated spin system

60
32, 4318

G
grain boundaries

16

H
heat capacity
51
heavy fermion
34high entropy alloy
53high magnetic field
17, 40, 46, 51, 59
high temperature superconductor
40
hydride22hydrogen9

K
kinetics43
L
laser plasma

12M
magnetic properties 26, 31, 39, 54, 55, 58
magnetism
18, 28, 33, 42
mechanical properties
13, 52
metal10
metal-insulator transition
27
microstructure
13, 53
MnS230
molecular dynamics
19
multi-ferroics17
multipole46
Nnanocluster57
nanoporous alloy
58
nanostructure
32, 35, 39, 42, 43
neutron diffraction
45
neutron scattering17, 27, 45, 61
nuclear materials
20, 38, 57
nucleation & growth
21
Oorganic27
osseointegration41
Pphase transformation
9, 10, 13
photocatalytic41
porosity
31, 54
positron annihilation
20
precipitation8

IMR KINKEN Research Highlights 2020

63

Keywordpage
Q
quantum spin system

61

R
radiation imaging

29

Sscanning transmission
electron microscopy (STEM)
10
scintillator29
simulation19
solar cells
16spin current
25
spin dynamics
25
spin excitation
61
spin liquid
51
spin transfer
24, 44spin wave
44
spintronics
24, 25, 33, 44, 55
steel
8, 9, 12, 48
strength20
superconducting28, 34, 35
Tthin films
3D structure
topological insulator
transition edge sensor
2D materials

26, 36, 56
32
24, 3647
60

U
ultrasound46
unusual response properties
60
X
X-ray diffraction (XRD)
X-ray scattering64

IMR KINKEN Research Highlights 2020

30
38

Editors
 yuta Kasada, Yoshinori Onose, Rie Umetsu, R
Shigeyuki Takagi, Haruhiko Morito, and
Motomichi Koyama

Editorial Staff
Misa Y. Tomimatsu, Aki Oikawa, and Yuko Godart

Printing
HOKUTO Corporation

Organization
Institute for Materials Research
Tohoku University
2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
Tel. +81-(0)22-215-2144 Fax. +81-(0)22-215-2482
URL: http://www.imr.tohoku.ac.jp/
E-mail: pro-adm@imr.tohoku.ac.jp© IMR July 2020