KINKEN
Research Highlights
2021

Institute for Materials Research, Tohoku University

KINKEN Research Highlights 2021
Contents
Preface���������������������������������������������������������������������������������������������������������������������������������������������������������� 4
Research Highlights
1. Infrastructural Materials
Microstructure Design of Structural Metallic Materials Research Laboratory���������������������������������������������� 8
Environmentally Robust Materials Research Laboratory������������������������������������������������������������������������������ 9
Chemical Physics of Non-Crystalline Materials Research Laboratory������������������������������������������������������� 10
Deformation Processing Research Laboratory�������������������������������������������������������������������������������������������11
Creation of Life Innovation Materials for Interdisciplinary and International Researcher Development������ 12
Professional development Consortium for Computational Materials Scientists (PCoMS)�������������������������� 13

2. Energy-Related Materials
Crystal Physics Research Laboratory�������������������������������������������������������������������������������������������������������� 16
Materials Design by Computer Simulation Research Laboratory��������������������������������������������������������������� 17
Irradiation Effects in Nuclear and Their Related Materials Research Laboratory��������������������������������������� 18
Nuclear Materials Engineering Research Laboratory�������������������������������������������������������������������������������� 19
Non-Equilibrium Materials Research Laboratory���������������������������������������������������������������������������������������� 20
Hydrogen Functional Materials Research Laboratory�������������������������������������������������������������������������������� 21
Analytical Science Research Laboratory��������������������������������������������������������������������������������������������������� 22
Exploratory Research Laboratory�������������������������������������������������������������������������������������������������������������� 23

3. Electronic Materials
Theory of Solid State Physics Research Laboratory���������������������������������������������������������������������������������� 26
Magnetism Research Laboratory��������������������������������������������������������������������������������������������������������������� 27
Low Temperature Physics Research Laboratory���������������������������������������������������������������������������������������� 28
Low Temperature Condensed State Physics Research Laboratory����������������������������������������������������������� 29
Quantum Beam Materials Physics Research Laboratory��������������������������������������������������������������������������� 30
Quantum Functional Materials Physics Research Laboratory�������������������������������������������������������������������� 31
Advanced Crystal Engineering Research Laboratory�������������������������������������������������������������������������������� 32
Structure-Controlled Functional Materials Research Laboratory��������������������������������������������������������������� 33
Solid-State Metal-Complex Chemistry Research Laboratory��������������������������������������������������������������������� 34

Magnetic Materials Research Laboratory�������������������������������������������������������������������������������������������������� 35
Crystal Chemistry Research Laboratory���������������������������������������������������������������������������������������������������� 36
Actinide Materials Science Research Laboratory�������������������������������������������������������������������������������������� 37
Materials Science of Non-Stoichiometric Compounds Research Laboratory�������������������������������������������� 38

Research Centers
International Research Center for Nuclear Materials Science�������������������������������������������������������������������� 40
Cooperative Research and Development Center for Advanced Materials������������������������������������������������ 41
High Field Laboratory for Superconducting Materials�������������������������������������������������������������������������������� 42
Trans-Regional Corporation Center for Industrial Materials Research������������������������������������������������������� 43
Collaborative Research Center on Energy Materials���������������������������������������������������������������������������������� 44
Center for Computational Materials Science���������������������������������������������������������������������������������������������� 45
Center of Neutron Science for Advanced Materials����������������������������������������������������������������������������������� 46
International Collaboration Center (ICC-IMR)��������������������������������������������������������������������������������������������� 47
Laboratory of Low Temperature Materials Science������������������������������������������������������������������������������������ 48
Laboratory of Alpha-Ray Emitters�������������������������������������������������������������������������������������������������������������� 49
Analytical Research Core for Advanced Materials������������������������������������������������������������������������������������� 50

International Collaborative Research
Magnetism Research Laboratory��������������������������������������������������������������������������������������������������������������� 52
Advanced Crystal Engineering Research Laboratory�������������������������������������������������������������������������������� 53
Structure-Controlled Functional Materials Research Laboratory��������������������������������������������������������������� 54
Solid-State Metal-Complex Chemistry Research Laboratory��������������������������������������������������������������������� 55
Non-Equilibrium Materials Research Laboratory���������������������������������������������������������������������������������������� 56
International Research Center for Nuclear Materials Science�������������������������������������������������������������������� 57
High Field Laboratory for Superconducting Materials�������������������������������������������������������������������������������� 58
Center for Computational Materials Science���������������������������������������������������������������������������������������������� 59
Center of Neutron Science for Advanced Materials����������������������������������������������������������������������������������� 60

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

Preface

Dear Colleagues,

We are pleased to present KINKEN Research Highlights 2021, 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 2021

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 2021

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Infrastructural Materials

IMR KINKEN Research Highlights 2021

Research

Infrastructural Materials

Interaction of Alloying Element with Migrating Ferrite/Austenite
Interface in Phase TransformationA

lloying elements significantly influence the phase transformation kinetics in steel. A quantitative understanding of alloying
effects is key to tailoring the mechanical properties of modern high-strength steels. Therefore, in this study, alloying effects
were quantitatively investigated, with special attention to energy dissipation during interface migration in phase transformation.
A variety of phase transformations have been utilized
in microstructure design to tailor the mechanical propertiesof steel products. In the development of modern highstrength steel, sophisticated control of the transformation
microstructure is required for improved strength andductility-toughness balance. Therefore, a quantitative
understanding of alloying effects on phase transformation
kinetics in a basic Fe-C system has become increasingly
important.
An alloying element can influence phasetransformation kinetics by (1) changing the phase stabilityor (2) dynamic interaction with the migrating interface. The
development of the CALPHAD method has made it
possible to calculate alloying effects on phase stability.
However, the effects of the latter have yet to be elucidated
in detail.
Our group developed a multi-aspect characterization
method to clarify the interaction between the alloying
element and the interface quantitatively [1]. The method
includes energy dissipation measurement based on thedeviation of the interfacial carbon composition from theequilibrium using a field-emission electron probe microanalyzer; the direct observation of alloying element
distribution at the interface using a three-dimensional
atom probe (3DAP); and interface crystallography
characterization using electron backscatter diffraction.
Figure 1(a) shows an incoherent interface separating
ferrite with a low C content (left) and austenite with a highC content (right) [1]. Mo segregation at the ferrite/austenite
interface is clearly seen in Mo atom map in Fig. 1(a). The
amount of Mo segregation was quantified from thecomposition profile across the interface shown in Fig.
1(b). Such Mo segregation resulted from the strong
interaction between Mo and the interface, which dissipatedenergy during migration leading to transformation delay.
These results are in good agreement with the theoreticalmodel developed in this study, as shown in Fig. 1(c).
It was clarified that the interaction between thealloying element and semi-coherent interface wassignificantly weaker than that at the incoherent interface,which induced remarkably different alloying effects on
displacive transformation, e.g., bainite transformation [2].
It was also found that transformation strain caused energy

Fig. 1 (a) C and Mo atom map at the α/γ interface measured
using 3DAP in the Fe-0.5Mo-0.4C alloy transformed at 973 K
for 600 s. (b) C and Mo composition profile obtained
perpendicular to the α/γ interface in (a). (c) Relation between
the amount of Mo segregation (interfacial excess) and energy
dissipation compared with the model calculation [1].

dissipation during migration of the semi-coherent ferrite/
austenite interface even in a Fe-C binary alloy [3].
References[1] G. Miyamoto, K. Yokoyama, and T. Furuhara, Acta Mater. 177, 187(2019).
[2] G. Miyamoto and T. Furuhara, ISIJ Int. 60, 2942 (2020).
[3] H. Dong, Y. Zhang, G, Miyamoto, Z. Yang, and T. Furuhara, Scr.
Mater. 188, 59 (2020).

Keywords: steel, microstructure, phase transformation
Goro Miyamoto (Microstructure Design of Structural Metallic Materials Research Laboratory)
E-mail: miyamoto@imr.tohoku.ac.jp
URL: http://www.st-mat.imr.tohoku.ac.jp/en/index.html

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IMR KINKEN Research Highlights 2021

IMR KINKEN Research Highlights 2021

Dislocation Motion After Hydrogen Charging Characterized by In Situ
Electron Channeling Contrast Imaging

H

ydrogen contributes to the plasticity-induced cracking of metallic materials. In particular, the interactions between hydrogen,dislocations, and grain boundaries have been studied to understand the hydrogen-assisted cracking mechanism. Therefore, avisualization technique for dislocation motion interaction with hydrogen near grain boundaries is required. We successfully
applied in situ scanning electron microscopy to observe the dislocation motion in a hydrogen-charged metallic bulk specimen.
Hydrogen-dislocation interactions in metals have beenrecognized as the most crucial factor that triggers hydrogen
embrittlement. Therefore, the visualization of dislocation
motion under the influence of hydrogen may be the key tounderstanding the underlying mechanism of hydrogen
embrittlement. However, there are two challenges inobserving the dislocation motion with the hydrogen effect:(1) hydrogen can be desorbed and diffused easily even atroom temperature; (2) the hydrogen-dislocation interaction
suffers from the thin film effect. Therefore, to characterize thehydrogen-related phenomenon, a dislocation-resolved andkinetically resolved method must be used with a bulk
specimen. In this context, we applied in situ electron
channeling contrast imaging (ECCI) to characterize thedislocation motion in hydrogen-charged steel. ECCI is ascanning electron microscopy (SEM)-based method.
Through ECCI, we successfully observed dislocation motion
after hydrogen charging in an Fe-based bulk specimen.
A schematic of the experiment is shown in Fig. 1 [1].
The mechanically polished specimen with a thickness of 1
mm was hydrogen-charged for 3 h in an aqueous solution of
5% H2SO4 with 3 g L-1 NH4SCN (Fig. 1A, 1B). The specimen
surface was then repolished to remove the contaminants
that formed during hydrogen charging (Fig. 1C). The
polished surface was observed by ECCI (Fig. 1D), which
visualized the dislocation motion at different times (Fig. 1E).
The ECC images with different vacuum exposure times
in the SEM are shown in Fig. 2. The dislocations, denoted asa to g, moved from the grain boundary to the grain interior. In
particular, the dislocation motion became distinct after
exposure for 2 h. These results indicate the following: (1) ahydrogen-related phenomenon occurred, which became
significant after 2 h; and (2) there is a factor that facilitates
dislocation motion from the grain boundary. According toatomistic simulations [1], the phenomenon corresponding tofact (1) is hydrogen diffusion to the grain boundary. The
hydrogen localization at the grain boundary generated
sufficient shear stress to trigger dislocation motion.
The hydrogen-assisted dislocation motion from thegrain boundary is important for understanding plasticityinduced cracking because grain boundary cracking withhydrogen has been reported to result from dislocation motion

Fig. 1 Schematic illustrations of the in situ ECCI observations afterhydrogen charging.

Fig. 2 Dislocation motion with different exposure times afterhydrogen charging. The exposure times are indicated on the topof the images.

near the grain boundaries. The in situ ECCI technique canbe performed under tensile loading. Further in situ ECCI
experiments with the hydrogen effect will be carried out
under tension after hydrogen charging.
References[1] M. Koyama, S.M. Teheri-Mousavi, H. Yan, J. Kim, B.C. Cameron, S.S.
Moeini-Ardakani, J. Li, and C.C. Tasan, Sci. Adv. 6, eaaz1187 (2020).

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

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Infrastructural Materials

Surface Characterization of Functional Iron-Gallium Alloys
Annealed Under Different Conditions

F

e-Ga alloys are functional magnetostrictive materials that are promising for application in actuators andsensors. In this study, the surface state of Fe80Ga20 alloys annealed under different atmospheric conditions
was characterized with X-ray absorption spectroscopy and secondary ion mass spectrometry. Through theanalysis, we report selective oxidation of Ga in the Ar-10%H2 atmosphere.
FeGa alloys are new magnetostrictive materials.
They are promising as smart materials for applicationin actuators and sensors. Since these applications
are often expected to be used outdoors, it isnecessary to control the surface conditions toensure anticorrosion. In such cases, Ga2O3 isformed on the surface, which is chemically morestable than Fe2O3. In this study [1], the surface
condition of the Fe-20at%Ga (Fe80Ga20) alloys
annealed under different conditions were
investigated by X-ray absorption spectroscopy(XAS) measurements in the conversion electron
yield (CEY) to elucidate the effect of the atmosphereon the surface properties of these alloys.
Buttons of Fe80Ga20 alloy were prepared by arc
melting. The buttons were cut into discs, diameter
10 mm and thickness 1 mm. After the disk samples
were polished mechanically [condition (a)], they
were annealed in air at 873 K for 3600 s [condition(b)] or in Ar-10 % H2 gas at 873 K for 3600 s[condition (c)]. Figure 1 shows the CEY-XAS
spectrum for Fe80Ga20 samples annealed underconditions (a)-(c) and oxide standards of Fe2O3 andGa2O3. The CEY-XAS spectrum of the Fe and Ga K
absorption edge is different for the metallic state
sample corresponding to condition (a) comparedwith that of the oxide standard. The Fe and Ga K
absorption edges of the (b) are both observed
between the (a) and the oxide standard. On theother hand, the Fe K absorption edge of the sample
treated using (c) shows the spectrum consistent
with that of (a), however, the Ga K absorption edge
is located between the metal and the oxide standard.
This result suggested the selective formation of
Ga2O3 on the surface of Fe80Ga20 annealed undercondition (c). The depth profiles of iron, gallium, andoxygen ions of F80Ga20 annealed in condition (c)measured with the TOF-SIMS technique are shownin Fig. 2. The signal intensity derived from Fe
increased at the interface of the oxide layer. These
results of CEY-XAS and TOF-SIMS suggested that
the Fe concentration in the oxide layer is lower thanFig. 1 CEY-XAS spectrum near of Fe and Ga K edge for
Fe80Ga20 annealed under condition (a)-(c).

Fig. 2 Depth profiles of Fe80Ga20 annealed under condition(c) obtained by TOF-SIMS.

that of Ga and that heat treatments of Fe-Ga alloys
under a low partial pressure of oxygen can effectively
form protective or corrosion-resistant layers, which
are composed of less-noble gallium oxides.
References[1] T. Kawamata, K. Shinoda, K.Sugiyama, and S. Suzuki, Surf.
Interface Anal. 52, 1098 (2020).

Keywords: surface reaction, synchrotron, XAFSToru Kawamata (Chemical Physics of Non-Crystalline Materials Research Laboratory)
E-mail: kawamata@tohoku.ac.jp
URL: http://www.imr.tohoku.ac.jp/en/about/divisions-and-centers/research-division/15.html

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IMR KINKEN Research Highlights 2021

IMR KINKEN Research Highlights 2021

Microstructure Refinement for Superior Ductility of Al–Si
Alloy by Electron Beam Melting

Refining the microstructure to improve the ductility of cast Al–Si alloys is challenging. Recently, we reported

a novel microstructure refinement approach for AlSi10Mg alloy via the synergetic effects of superheating, fastcooling, and preheating using electron beam melting (EBM) technology. A fine granular Si phase (0.5–2 μm)
and bimodal Al grains (40 μm and 0.5–2 μm sub-grains) were obtained in the as-EBM-built AlSi10Mg alloy,
which resulted in an improved tensile elongation exceeding 30%.

Al–Si alloys are widely applied to thin walls,
complex geometries, and parts subjected to large
loads because of their high corrosion resistance,
weldability, and hardenability due to natural or
artificial aging. Refining the Si phase is essential for
improving the ductility of cast Al–Si alloys because
cracks can initiate in the coarse, acicular Si phase.
The ductility improvement of as-cast Al–Si alloy
by adding modification elements is limited. Previous
studies have shown that reheating the Al–Si alloy toa much higher temperature than its melting point
could refine both the Si phase and Al grains under afast cooling rate. This provides a novel approach forrefining the microstructure of Al–Si alloys using
additive manufacturing (AM) technology.
Here, electron beam melting (EBM), an AM
process, was selected to refine the AlSi10Mg alloy
because of its high building temperature, fastcooling rate, and preheating process. In the as-built
alloy, mixed fine island-like and scattered granular
Si phase particles (approximately 2 μm) withrounded corners and edges were embedded into
the Al matrix, as shown in Fig. 1 [1].

An Al matrix with a mean grain size of
approximately 40 µm and fine sub-grain size of 0.5–
2 μm was obtained in the as-EBM-built AlSi10Mg
alloy. During the building process, the hightemperature inner melt pool (>1500 °C) gave rise toa fully homogeneous Al–Si liquid in a true equilibrium
solution state without undissolved Al-enriched andSi-enriched nanoparticles. Afterward, the Si and Al
phases nucleated directly and grew to fine grains
owing to the fast cooling rate of the inner melt pool.
A high tensile elongation of approximately 33% was
obtained owing to the refined microstructure of the
as-EBM-built AlSi10Mg alloy.
This study provides a novel alloy manufacturingmethod to improve the microstructure andmechanical properties of alloys consisting ofelements with significantly higher melting point
differences, such as Al–Si alloy systems, using EBM
technology.
References[1] H. Bian, K. Aoyagi, Y. Zhao, C. Maeda, T. Mouri and A. Chiba,
Addit. Manuf., 32, 100982 (2020).

Fig. 1 The as-EBM-built AlSi10Mg alloy with fine Si and Al grains lead to high tensile ductility. After T6 aging, the tensile
strength increased.

Keywords: additive manufacturing, aluminum alloys
Akihiko Chiba (Deformation Processing Research Laboratory)
E-mail: a.chiba@imr.tohoku.ac.jp
URL: http://www.chibalab.imr.tohoku.ac.jp/english/

IMR KINKEN Research Highlights 2021

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Infrastructural Materials

Hot Deformation and Recrystallization Behaviors of MediumEntropy Alloys

M

ultiprincipal element alloys have attracted significant academic interest, but their hot deformation characteristics
are relatively unknown. We investigated the hot deformation, dynamic recrystallization, and microstructural evolutionof typical medium entropy alloys at single-phase temperatures, which provide guidelines for microstructure refinement
via thermomechanical processing.
High- or medium-entropy alloys (HEAs, MEAs),
which consist of multiprincipal elements, have attracted
much attention because of their unusual structuresand properties. However, the insufficient strength ofthese alloys hinders their application. To further
improve their mechanical performance, a precipitationstrengthened MEA was designed by adding a strongγ´-phase former (Ti, Al) to the CoCrNi MEA [1]. However,the deformation and recrystallization behavior of the
MEA at high-temperature remains unclear.
Here, we reveal the deformation behavior of
typical CoCrNi and (CoCrNi)96Ti3Al3 MEAs by hotcompression at single-phase temperatures [2]. A
constitutive model was constructed and the relationshipbetween the recrystallization, strain rate, andtemperature was clarified. Figure 1 shows thecompressive true stress-strain curves of the two MEAs
at 1173 K. All the curves exhibited work hardening(stress increase with the increase in strain) at a small
strain, and flow softening (stress decrease with theincrease in strain due to dynamic recrystallization) at alarge strain. Further, a sharper flow softening wasobserved in the (Ti, Al)-added MEA compared with the(Ti, Al)-free MEA. In both samples, the flow stressincreased with the increase in the strain rate.
Constitutive equations were also constructed todescribe the flow behaviors. The apparent activationenergies were 401.08 kJ/mol (CoCrNi) and 566.13 kJ/
mol ((CoCrNi)96Ti3Al3), much higher than that of dilutealloys. This indicates the sluggish diffusion of elementsin the MEAs.
The microstructure evolutions were carefully
characterized. As displayed in Fig. 2, the fraction ofrecrystallization has a positive correlation with thetemperature, and a negative correlation with the strain
rate. A higher fraction of recrystallized grains was
obtained at a higher temperature and low strain rate.
The average size of recrystallized grains also has apositive correlation with temperature and a negative
correlation with the strain rate, which demonstrates

Fig. 1 Stress-strain curves of (a) CoCrNi and (b)(CoCrNi)96Ti3Al3 MEA compressed at 1173 K with strain
rate of 0.1, 0.01, and 0.001 s−1.

Fig. 2 The volume fraction (a) and average grain size (b) of
the recrystallized grains in the CoCrNi and (CoCrNi)96Ti3Al3
MEA hot-compressed to a true strain of 1.0 at 1173 K and
1273 K.

faster grain growth at higher temperatures. Fine grains
were produced by continuous and discontinuousrecrystallization mechanisms. The addition of Ti and Al
impeded the grain growth at elevated temperatures.
The present study sheds light on refining the
microstructures of HEAs and MEAs.
References[1] Y. Zhao, T. Yang, Y. Tong, J. Wang, J.H. Luan, Z.B. Jiao, D. Chen,
A. Hu, C.T. Liu, and J.J. Kai, Acta Mater. 138, 72 (2017).
[2] H. Yi, D. Wei, Y. Wang, L. Wang, M. Fang, K. Yang, and H. Kato,
Metals 10, 1341 (2020).

Keywords: deformation, microstructure, mechanical properties
Daixiu Wei (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

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IMR KINKEN Research Highlights 2021

IMR KINKEN Research Highlights 2021

Variation in the Number of PhD Holders in Computational Materials Science:
Importance of Higher Education of Computational Materials ScienceProfessional development Consortium for Computational Materials Scientists (PCoMS) supports young researchers in computational
materials science (CMS) to develop a broader view and gain extensive knowledge of interdisciplinary areas in materials science,
condensed matter physics, molecular science, and materials design. Here, we introduce a recent study by key members of PCoMS.
Prof. Terada and coauthors analyzed the trend of human resource development in doctorate holders in CMS in Japan.

2

0.1

1
0

197019801990 20002010

year

0

3

[×10 ]

0.2

3 (b)
2

ratio of new PhD holders in CMS

3 (a)ratio of new PhD holders in CMS

0.2

number of new PhD holders

3[×10 ]

number of new PhD holders

The recent progress in computational materials science(CMS), materials informatics, and related subjects has
accelerated the discovery of advanced and novel materials.
However, there have been only a few quantitative analyses
regarding the change in the number of doctoral degree
holders (doctors) in CMS in Japan.
We analyzed approximately 150 thousand doctoral
dissertations from the engineering and science (ES) divisionsof all national universities, major public universities, andprivate universities using text data mining techniques last 50
years. We sorted doctoral theses with titles containing
keywords related to CMS and estimated the number ofdoctors in CMS.
In Japan, universities are classified into three
categories: research universities (research Us), local
education-oriented universities (education Us), and special
education-oriented universities (special Us). The majority of
doctors have been traditionally fostered in research Us.
However, new doctoral courses in the education Us havebeen established since the 1990s due to the reconstructionof the graduate school. Thereafter, about 1/3 doctors are
fostered in education Us. The number of doctors fostered in
special Us in Japan is small even today.
From the analysis, we found that research Us areleading the national trend of the number of doctors in ES andCMS and the ratio of the number of doctors in CMS to thoseof doctors in ES in Japan. It increased rapidly in the beginningand then dropped rapidly. However, the rate of decrease in
CMS thereafter is small. Therefore, the ratio of doctors in
CMS to the doctors in ES continued to increase, closing toabout 0.10 in 2018, as shown in Fig.1(a). On the other hand,change in the number of doctors in ES (especially inengineering) in education Us followed those of research Us
and the national trend. However, the ratio of the number ofdoctors in CMS to those of doctors in ES at education Us
was almost constant and small, approximately 0.05, as canbe seen from Fig.1(b). As a reference, the ratio of the doctors
in CMS to the doctors in ES in the California Institute ofTechnology (Caltech), which is one of a research Us in theUSA, is almost constant and approximately 0.10–0.15.
We also investigated the appearance frequency of

0.1

11970 1980 1990 2000 2010

year

0

Fig.1 Number of new PhD holders in Japan. (a) research universitiesand (b) local education-oriented universities. --- : total number of
new PhD holders in ES. - : number of new PhD holders in CMS.
- : ratio of new PhD holders in CMS to PhD holders in ES.

keywords in the titles of theses. We found that keywords
related to application appeared most frequently in theses
from education Us. On the other hand, keywords related totheory appeared most frequently in theses from Caltech
compared to those in research Us in Japan. This indicatesthat compared with education Us, research Us foster
researchers who are more oriented toward fundamental
research. It also indicates that Japanese universities foster
researchers who are more inclined toward applied researchand practical research compared to the universities in theUSA.
Additionally, we found that the ratio of number ofdoctors in CMS in new majors, such as mathematicalscience, information science, and so on, is greater than thatin traditional fields such as ES, though the traditional majors
continue to be dominant in Japan.
These results indicate that the importance of researchin CMS is increasing and that the number of doctors in CMS,
who are oriented toward fundamental research in bothresearch Us and education Us in Japan, should be increased[1].
References[1] Y. Terada and T. Mohri, “Recent changes in doctoral holders ofcomputational materials science in Japan - recent diversity of specialties
in diploma”, JIM Annual Fall Meeting 2020, Sep 16, 2020.

Keywords: computational materials science, higher education
Yayoi Terada (Professional development Consortium for Computational Materials Scientists)
E-mail: terada@imr.tohoku.ac.jp
Momoji Kubo (Project Leader of Professional development Consortium for Computational Materials Scientists)
URL: http://pcoms.imr.tohoku.ac.jp/index_e.html

IMR KINKEN Research Highlights 2021

13Energy-Related Materials

IMR KINKEN Research Highlights 2021

Research

Energy-Related Materials

Effect of {111} Polarity on the Growth Rate During the
Solidification of Compound Semiconductor GaSb

The twin formation and propagation during the directional solidification of GaSb was directly observed, andthe effect of twinning on the growth rate of the GaSb{111} plane was clarified. The growth rate parallel to the<111> direction decreased after twinning, indicating that the polarity reversal by twinning significantly influenced
on the growth kinetics of the {111} plane.
Large-sized single crystals of compound
semiconductors are difficult to grow because
defects, such as twin boundaries and dislocations,
are significant during solidification. As the defectformation mechanism remains poorly understood, auniversal technique for the single-crystal growth ofcompound semiconductors has yet to beestablished. The sphalerite structure in compound
semiconductors, such as GaSb and GaAs, exhibitspolarity in the <111> direction; therefore, the {111}Σ3
twin boundary exhibits two possible atomic
structures, and the difference between twin
structures influences grain growth. However, theeffect of the twin atomic structure on the growth
dynamics of compound semiconductors is notcompletely understood. In this study, we directly
observed
twinning
processes
duringthesolidification of polycrystalline GaSb and clarified
the effect of twinning on the grain growth rate [1].
The direct observation system consisted of a
furnace and digital microscope. The furnace had
two comb-shaped carbon heaters, between whicha sample in a quartz crucible was placed andheated. The solidification of the sample was initiatedby decreasing the currents applied to the heaters.
The top surface of the sample was observed andrecorded using a digital microscope. The moving
distance of the crystal–melt interface was measured
by tracing the position of the interface. The

orientations of grains in the areas observed were
determined by electron backscatter diffraction
method.
Figure 1 shows the effect of twinning on thecompetitive grain growth of GaSb. Three grains
were present in the crystal (Fig. 1(a)). The boundary
between Grains 1 and 2 was identified as a Σ3 twinboundary, while that between Grains 2 and 3 was arandom grain boundary. The shape of the crystal–melt interface remained the same before twinning.
The twin nucleation was identified at the junction of
the crystal–melt interface. The twin boundary
between Grains 1 and 2, and the newly formed twinboundary propagated along the crystal–meltinterface (Fig. 1(b)). The growth rate of Grain 2 along
the direction parallel to <111> was reduced by halfowing to the formation of the twin boundary, which
resulted in the growth of Grain 2 being surpassedby adjacent grains (Fig. 1(c)). This indicates thatpolarity reversal occurred, and the growth direction
turned to <111>A owing to the formation of areflection twin. Thus, the {111} polarity significantly
affected the grain growth rate during the solidificationof GaSb.
References[1] K. Shiga, K. Maeda, H. Morito and K. Fujiwara, Acta Mater.
185, 453 (2020).

Fig.1 Photographic images of a moving crystal–melt interface during directional solidification of GaSb: (a) before twinning,(b) at twinning, and (c) after twinning.

Keywords: crystal growth, defects, kinetics
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 2021

Computational Simulations for Super-Low-Friction and HighWear-Resistant Materials DesignT

he design of low-friction and high-wear-resistant materials requires a deep understanding of multiphysics phenomena,such as reaction, friction, impact, and stress. Therefore, we developed computational simulation methodologies for thetheoretical design of super-low-friction and high-wear-resistant materials based on first-principles calculations andreactive molecular dynamics simulations.
Super-low
frictionandhigh-wear-resistanttechnologies are important not only for reducing energy
loss but also for improving the durability of industrial
machines. For example, carbon-based materials andsilicon-based ceramics exhibit low friction properties.
However, their friction and wear mechanisms are difficult
to elucidate because these processes include
multiphysics phenomena, such as chemical reaction,
friction, impact, and stress. Therefore, we developed
computational simulation methodologies for thetheoretical design of super-low friction and high-wearresistant materials based on first-principles calculations
and reactive molecular dynamics (MD) simulations.
These methods were applied to various friction systems
and successfully 1) established theoretical wear law at
the nanoscale [1]; 2) clarified the low-friction mechanismof Si-doped diamond-like carbon (DLC) [2]; and 3)
proposed a super-low friction mechanism of siliconbased ceramic materials in water lubrication [3].
To prevent energy loss and device failures in
micromachines, the theoretical design of anti-wear
materials at the nanoscale is necessary. The conventional
Archard’s law can depict wear in macroscale contact,but it cannot describe nanoscale wear induced bychemical reactions. Thus, we proposed a non-empiricaland robustly applicable wear law for nanoscale contact[1]. To confirm its robustness and applicability, we
compared the amount of wear according to our proposed
wear law with that by reactive MD simulations. Figure 1ashows the ball-on-disk friction model of DLC; its wear
amounts are shown in Fig. 1b. Figure 1b indicates thatour proposed wear law successfully reproduced thewear amounts obtained by reactive MD simulations,proving the its robustness and applicability.
Experimentally, it is known that Si-doping of DLC
film leads to super-low friction; however, its mechanism
has not been clarified. Therefore, we investigated thestructural changes in DLC by Si-doping via firstprinciples calculations [2]. Replacing Si atoms with C
atoms on a diamond (111) surface caused large stressaround the doped Si atom, and induced the transition ofa six-membered C-ring into a five-membered C-ring.

Fig. 1 (a) Snapshots of MD simulation in ball-on-disk contactsof DLC. The worn atoms are indicated by the red circles. (b)Wear amounts obtained by our proposed wear law and MD
simulation.

Fig. 2 (a) Initial and (b) optimized Si-doped diamond (111)structures, where two C atoms in the first layer are substituted
with two Si atoms.

This led to the generation of the “graphene arch-bridge”(Fig. 2), which differs from a simple graphene structure
because both of its ends are pinned to the diamondsurface. This “graphene arch-bridge” structure affords
the lower wear properties of Si-doped DLC comparedwith simple graphene because its ends are pinned to the
DLC surface.
References[1] Y. Wang, J. Xu, Y. Ootani, N. Ozawa, K. Adachi, and M. Kubo, Adv.
Sci. 8, 2002827 (2021).
[2] S. Bai, J. Xu, Y. Wang, Q. Zhang, T. Tsuruda, Y. Higuchi, N. Ozawa,J. M. Martin, K. Adachi, and M. Kubo, J. Phys. Chem. C 124, 26379(2020).
[3] Y. Ootani, J. Xu, N. Takahashi, K. Akagami, S. Sakaki, Y. Wang, N.
Ozawa, T. Hatano, K. Adachi, and M. Kubo, J. Phys. Chem. C 124,
8295 (2020).

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 2021

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Energy-Related Materials

In Situ Observation of Dislocation Loop Behavior During
Annealing by WB-STEM in Neutron-Irradiated Reactor
Pressure Vessel SteelThe dynamic behavior of loops and interactions between loops as well as loops and dislocations play key

roles in microstructure variation, which is strongly connected to radiation-induced embrittlement in reactor
pressure vessel (RPV) steels. The dedicated weak-beam scanning transmission electron microscope at the
Oarai center was employed to observe and analyze the coalescence of loops and the interactions between
loops and dislocations during annealing in neutron-irradiated RPV steel.
Neutron-irradiation-induced defects, especiallydislocation loops, significantly contribute toembrittlement in reactor pressure vessel (RPV)
steels during the long-term operation of nuclear
power plants [1]. Therefore, understanding thethermal stability of defects is crucial.
The Burgers vector of dislocation loops is an
important factor in microstructural evolution. In ironbased ferritic steels, the formation of <100> loops
becomes more dominant than the formation of ½
<111> loops with the increase in irradiation
temperature. However, the mechanism behind thetransformation of ½ <111> loops to <100> loops
remains unclear.
In the Oarai center, weak-beam scanningtransmission electron microscopy (WB-STEM) was
developed to achieve a more accurate quantificationof irradiation-induced defects [2]. In situ WB-STEM
observation was performed during annealing for asurveillance test specimen of RPV steel, which was
neutron-irradiated to a fluence of 8.2 × 1023
neutrons·m-2.
As shown in Fig. 1(a) and (b), this is the first time
to directly observe that two ½ <111> dislocation
loops collided with each other and coalesced toform a <100> dislocation loop. Small ½ <111>dislocation loops could be absorbed by large<100> dislocation loops, whereas the Burgers
vector of the <100> loops remained unchanged, as
illustrated in Fig. 1(c) and (d). Dislocation decoration
by loops occurred during annealing because of theinteraction between the dislocations and loops. The
dislocations decorated by loops forming helicalconfigurations were fairly stable during thecontinuous annealing process [3].
This study provides direct experimental
evidence for the formation mechanism of <100>dislocation loops and provides a better
understanding of the temperature-dependence ofdislocation loop evolution.

Fig. 1 In situ observation of dislocation loop evolution in
neutron-irradiated RPV steel during annealing at 723 K
with diffraction vector g = [110] near [001] zone axis in
STEM mode: (a) (b) loop coalescence; (c) (d) loop
absorption.

The interaction between loops and dislocations
suggests a new mechanism of loop-impeding
dislocation motion.
References[1] M. Shimodaira, T. Toyama, K. Yoshida, K. Inoue, N. Ebisawa,
K. Tomura, T. Yoshiie, M. J. Konstantinovic, R. Gerard, and Y.
Nagai, Acta Mater. 155, 402 (2018).
[2] K. Yoshida, M. Shimodaira, T. Toyama, Y. Shimizu, K. Inoue, T,
Yoshiie, M. J. Konstantinovic, R. Gerard, and Y. Nagai, Microsc.
66, 120 (2017).
[3] Y. Du, K. Yoshida, Y. Shimada, T. Toyama, K. Inoue, K.
Arakawa, T. Suzudo, M. J. Konstantinovic, R. Gerard, Somei
Ohnuki, and Y. Nagai, Materialia. 12, 100778 (2020).

Keywords: microstructure, nuclear materials, defects
Yasuyoshi Nagai (Irradiation Effects in Nuclear and Their Related Materials Research Laboratory)
E-mail: nagai@imr.tohoku.ac.jp
URL: http://www.imr.tohoku.ac.jp/en/about/divisions-and-centers/research-division/11.html

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IMR KINKEN Research Highlights 2021

IMR KINKEN Research Highlights 2021

Role of Crystal Lattice Defects in Polycrystalline SiC Corrosion

Irradiation-accelerated corrosion is a problem when SiC is exposed to extreme environments, such as particle beams.

The relationship between lattice defects and corrosion was investigated using electrochemistry and electron spinresonance. The corrosion rates increased with dangling bond (DB) density, indicating that DBs were the primary reaction
sites. Further, the lattice distortion associated with dense DBs was found to be the primary source of corrosion acceleration.
spin resonance (ESR) spectroscopy. An immersion
test in high-pressure hot water was also used toevaluate surface recession.
Surface recession, corrosion current, and DB
density increased after irradiation. The changes weremore pronounced at lower irradiation temperature.
The corrosion current increased monotonically withDB density, indicating that DBs were the primary
reaction site. Further, the surface recession rate in
hot water increased exponentially with increasing DB
density. The largest number of DBs were induced
during irradiation at the lowest temperature. These
DBs were surrounded by a distorted lattice becauseof the absence of lattice recovery during irradiation.
These findings [2] indicate that corrosion was
accelerated by the accumulation of excess DBs andthe surrounding lattice distortion.

Materials resistant to extreme conditions, suchas oxide dispersion-strengthened alloys [1], ceramics[2], and semiconductors [3], are crucial to realize
advanced nuclear systems. Silicon carbide (SiC)
exhibits outstanding irradiation stability and strength
at high temperatures, which has led its numerous
applications along with its composites in structuraland core components of fusion reactors, and nuclear
fuels. These environments are highly corrosive andinvolve exposure to hot water and high-temperature
steam. Our previous study first reported that thecorrosion rate increased when various lattice defects
were created by ion irradiation. Then, researchers
subsequently reported the irradiation-accelerated
corrosion of SiC in various coolants.
Longer irradiation times lead to the accumulationof lattice damage, thereby accelerating corrosion.
However, the reason for this acceleration remains
unknown, and there are no clear guidelines forcorrosion protection. We hypothesized that unpaired
electrons (or dangling bonds, DBs) that formed at thevacancies contributed to the reaction.
The relationship between lattice defects and thecorrosion of SiC irradiated with heavy ions wasinvestigated using electrochemistry and electron

References[1] H. Wang, H. Yu, S. Kondo, N. Okubo, and R. Kasada, Corros
Sci. 175, 108864 (2020).
[2] S. Kondo, K. Seki, Y. Maeda, H. Yu, K. Fukami, and R. Kasada,Scr. Mater. 188, 6-9 (2020).
[3] Y. Okuno, N. Ishikawa, M. Akiyoshi, H. Ando, M. Harumoto, and
M. Imaizumi, J. App. Phys. 59, 074001 (2020).

－
－

－－

－
－

C

－
－

－
－

C

－

－－

Si

V

Si

C

Si

－－

－
－
－

－－

－－

Si

－－

－－

－－

Si

－
－

－

C

－
－

C

－

Si

Si

－
－

－－
－
－

C

C
－－

－
－

Si

Red

Significant increase in
the number of unpaired
electrons was found in
the irradiated specimen.
Corrosion current was
found to be increased
with the unpairedelectron density.

Ox

As received
IrradiatedAnnealed at
900℃

ESR signal

Preferential corrosion at theunpaired electron formed in
vacancies

Pre-introduction of latticedefects by heavy ion-irradiation

Corrosion acceleration at
the irradiated regions

ESR results
Δh=2.8µm

326

331

336
341
Magnetic Field [mT]346

Fig.1 A schematic view of SiC with irradiation-induced defects and corrosion starting from the defects (left). ESR results
show an increase in DB with irradiation and a decrease with defect recovery (right).

Keywords: ceramic, corrosion, defects
Ryuta Kasada, Sosuke Kondo, Hao Yu, and Yasuki Okuno (Nuclear Materials Engineering Research Laboratory)
E-mail: s-kondo@imr.tohoku.ac.jp
URL: http://www.imr.tohoku.ac.jp/en/about/divisions-and-centers/research-division/13.html

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Energy-Related Materials

Novel Hierarchical Nanoporous Graphene Nanoplatelets with Excellent
Rate Capabilities Produced via Self-Templating Liquid Metal Dealloying

Hierarchical nanoporous graphene nanoplatelets (HNGNPs) with high crystallinity and a moderate specific surface area aresynthesized by liquid metal dealloying. The large surface area, high crystallinity, and structural robustness of the mutuallyinterconnected hierarchies of the HNGNPs result in their excellent rate capabilities and cycling stability in lithium-ion batteries.

Significant efforts have been made to develop carbon
nanomaterials that meet the demands of high capacity, long
cycle performance without degradation, and high-rate charge/
discharge performance to realize advanced lithium-ion
batteries (LIBs). In particular, the good electric conductivity
and short diffusion path of lithium ions are necessary toenhance the rate capability. A hierarchical open porous
structure has been proposed to facilitate efficient electrolyte
access and accelerate electron percolation to obtain superior
performance, especially the high-rate capability required for
LIBs. Such hierarchical structures are prepared using specifictemplates, but the shapes of templating materials are generally
restricted.
In this work, we designed a liquid metal dealloying (LMD)
process to develop hierarchical nanoporous graphene
nanoplatelets (HNGNPs) as anode active materials of LIBs toachieve high rate capability [1]. Based on the miscibility among
the elements, a Mn78C22(at%) precusor and a Bi melt wereselected for the LMD reaction, in which the selective dissolutionof Mn in Bi melt and formation of porous carbon occurred.
Figure 1(a) shows the microstructure of the productdealloyed in the Bi melt at 1200 °C for 3 min. The mesoporous
network of GNPs formed the lower hierarchy, which was nested
within the much larger network (macropores) of the upperhierarchy. This LMD process enabled the synthesis of welldefined hierarchical nanoarchitectures on two different lengthscales without any additives. Such hierarchical morphology
results from the self-template effect due to the unique phasetransformation during the LMD reaction. During the Mn
depletion, the precursor transformed into the intermetallic
Mn7C3 compound with the development of the upper hierarchyof the micron-sized large ligament. With the further depletion ofMn, this intermetallic compound transformed into graphite withthe development of the lower hierarchy of the nanometer-sizedpores within the ligaments.
The optimized microstructural characteristics of the
HNGNPs include high crystallinity (interlayer distance of the(002) plane of ~0.341 nm and intensity ratio of the Raman Gand D-bands of ~0.552); a moderate specific surface area(~152.7 m2 g−1); and a bimodal pore size distribution with thehighest peak at 126 nm and an additional peak at 457 nm.
The rate capability of the HNGNPs was evaluated atvarious rates from 0.2C to 20C (1C = 0.372 A/g) and compared
Keywords: dealloying, porosity, energy storage
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

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IMR KINKEN Research Highlights 2021

Fig.1 (a) Microstructure and (b) rate capability of HNGNP anodeprepared by LMD process [1]. The inset in (a) shows crosssectional structure of the ligaments.

with a commercial artificial graphite sample. As shown in
Fig.1(b), the HNGNP electrodes exhibited superior capacity
retention to the commercial artificial graphite, especially at ahigh C-rate, demonstrating the beneficial role of theirhierarchical nanoporous structure developed via LMD. Thesefavorable features of the HNGNP electrode can be attributed
to its mutually interconnected hierarchical structure with large
surface area, moderate crystallinity, and high stability.
References[1] W.-Y. Park, T. Wada, S.-H. Joo, J. Han, and H. Kato, Mater. Today Commun.
24, 101120 (2020).

IMR KINKEN Research Highlights 2021

Pseudorotating Hydride Complexes with High Hydrogen Coordination:
A New Class of Rotatable Polyanions in Solid Matter

Solid-state materials containing rotatable polyanions constitute a peculiar class of ionic conductors owing totheir unique transport behavior, i.e., rotating polyanions promote phase transitions to disordered phases withcation conductivities enhanced by several orders of magnitude. A major drawback is the high temperature
required to activate rotation and the corresponding low conductivities at room temperature. Here, we propose
a mechanism to drastically reduce the temperature based on the use of pseudorotation in high-H coordination
hydride complexes. We demonstrate this mechanism for an existing hydride Li5MoH11 containing MoH93– anddemonstrate that this material can exhibit an unprecedentedly high lithium-ion conductivity at room temperature.
There is currently a strong demand to replaceorganic liquid electrolytes used in conventional
rechargeable batteries with solid-state electrolytes
owing to their advantages, such as improved safety
and higher energy density. Much effort has been
devoted to identifying materials with superiortransport properties.
Among the diverse solid-state electrolytes, thosecontaining polyanions, such as B12H122– (Fig 1 (a)),
constitute a peculiar class of materials because of
their unique transport behavior, i.e., polyanions rotate
at an elevated temperature, thereby promoting phasetransitions with cation conductivities enhanced byseveral orders of magnitude. However, a major
drawback is the high temperature required to activate
rotation and the corresponding low conductivities atroom temperature (RT).
In this study, we adopt a novel approach to
reduce the transition temperature based on the useof homoleptic transition metal hydride complexes,
wherein hydrogen is the sole ligand species,
covalently binding to single transition metals (Fig. 1(b)). The rationale behind this choice is that, unlike
other polyanions, the rotation only requires small
displacements of highly mobile hydrogen, which
could therefore be expected to occur with low
activation energy.
We found that MoH93− in Li5MoH11 exhibits
pseudorotation with a remarkably low activationenergy of ~20 meV. This motion yields a large
entropic contribution of 19.1 J/mol・K, which stabilizes
the lithium-ion conducting phase even below RT. The
RT lithium-ion conductivity reached 7.9 × 10−2 S cm−1[1], which is more than three times greater than thehighest value of 2.5 × 10−2 S cm−1 previously reported
for a sulfide Li9.54Si1.74P1.44S11.7Cl0.3 [2] (Fig. 2).
The mechanism is quite general in principle andwould be useful in lowering the temperature toactivate the rotation of polyanions. This is expected

Fig. 1 Typical polyanions found in solids: (a) B12H122–, and (b)MoH93–.

Fig. 2 Arrhenius plot of lithium-ion conductivity of Li5MoH11.
Data for typical sulfides are shown for comparison.

to help identify compositions that are amenable to RT
superionic conductors.
References[1] S. Takagi, T. Ikeshoji, T. Sato, and S. Orimo, Appl. Phys. Lett.
116, 173901 (2020).
[2] 2. Y. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M.
Yonemura, H. Iba, and R. Kanno, Nature Energy 1, 16030 (2016).

Keywords: solid-state electrolytes, hydride, first principles calculations
Shigeyuki Takagi and Shin-ichi Orimo (Hydrogen Functional Materials Research Laboratory)
E-mail: shigeyuki.takagi@imr.tohoku.ac.jp
URL: http://www.hydrogen.imr.tohoku.ac.jp/en/index.html

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Energy-Related Materials

Quantitative Analysis of Hydrogen in High-Hydrogen-Content
Materials via Laser-Induced Breakdown Spectroscopy

An analytical approach that can rapidly determine the hydrogen concentrations in solid-state materials is needed

to contribute to the hydrogen economy. We established a method that can estimate hydrogen concentrations
ranging from 0.2 to 7.6 mass% within a few seconds via laser-induced breakdown spectroscopy.

The demand for the quantitative analysis ofhydrogen in solid-state substances has recently
grown owing to the increasing studies on materials
related to hydrogen production, transportation,
storage, and utilization. Several analytical techniques
are currently being used for this purpose, but theirapplication areas are limited. Moreover, they are not
rapid analytical methods. This study developed
laser-induced breakdown spectroscopy (LIBS),
which can rapidly quantify hydrogen concentrations
in solid-state substances within a few seconds.
Magnesium hydride (MgH2) was selected as thetarget sample.
A straight-line calibration curve was obtainedby measuring the hydrogen emission intensity (at656.28 nm) for standard Mg/MgH2 mixturescontaining 0, 0.1, 0.2, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, and7.6 mass% of hydrogen in a 3000 Pa helium
atmosphere (Fig. 1) [1]. In the LIBS measurement,
the residual water molecules in the samples and the
LIBS gas line were removed using a desiccator
containing silica gel and using a moisture trap,
respectively. The lower limit of quantification was
estimated to be 0.20 mass% from the calibration
curve, indicating a determinable hydrogenconcentration range of 0.2–7.6 mass%. We also
found that reducing the residual water molecules in
the gas line (including the gas cylinder), by heating
the gas line and/or attaching more moisture traps, isimportant to quantify ppm concentrations with the
LIBS system.
By using the calibration curve (Fig. 1), we
estimated the hydrogen concentrations of MgH2
samples heated at 350 °C for 2, 4, and 6 h in an
argon atmosphere. The results were consistent withthose obtained through inert gas fusion/gas
chromatography, which is the conventional method
for measuring the hydrogen content of solid-state
substances (Table 1) [1]. Our results demonstrate
the feasibility of the rapid quantitative analysis ofhydrogen in MgH2 via LIBS. The LIBS technique isalso applicable to two- and three-dimensional
analyses.

Fig. 1 Calibration curve based on the intensity of thehydrogen emission line (H I 656.28 nm) measured in
reduced helium (3000 Pa) atmosphere.

Table 1 Intensity of hydrogen emission Line (H I 656.28 nm) via
LIBS spectra from MgH2 samples heated at 350 °C for
different times under argon flow, and the derived hydrogenconcentration, along with the hydrogen content estimatedvia inert gas fusion/gas chromatography.

References[1] S. Imashuku, T. Kamimura, S. Kashiwakura, and K.
Wagatsuma, Anal. Chem. 92, 11171 (2020).

Keywords: atomic emission spectrometry, hydrogen
Susumu Imashuku and Kazuaki Wagatsuma (Analytical Science Research Laboratory)
E-mail: susumu.imashuku@imr.tohoku.ac.jp
URL: http://www.imr.tohoku.ac.jp/ja/about/divisions-and-centers/research-division/27.html

22

IMR KINKEN Research Highlights 2021

IMR KINKEN Research Highlights 2021

Development of High-Performance Spinel-Oxide CathodeMaterials for Magnesium Rechargeable BatteriesMagnesium rechargeable batteries (MRBs) are a promising candidate for next-generation batteries. However,their development has been hindered by the lack of cathode materials. We proposed a new design concept anddemonstrated that defect spinel ZnMnO3 has significantly high cyclability among the high-potential (2–3 V class)spinel-oxide materials for MRBs.
To realize a future sustainable society, it isessential to develop next-generation batteries
combining high safety and energy density. Magnesium
rechargeable batteries (MRBs) have attracted muchattention because Mg can be deposited without
dangerous dendritic growth, enabling its use as ahigh-capacity anode material. However, establishingoxide cathode materials that allow reversible Mg
insertion/extraction is a challenge because thesereadily transform into irreversible MgO-like rock salt
phases by Mg insertion. Although we have beendeveloping spinel-oxide cathode materials for MRBs,
they exhibit rapid capacity fading, possibly due to thespinel-to-rock-salt transition.
In this study [1], we propose the concept ofutilizing "defect" spinel oxides to prevent the irreversible
transformation by Mg insertion. Defect spinel oxides
contain cation deficiencies at octahedral sites in thespinel structure. The cation-deficient sites are
expected to provide migration pathways andaccommodation sites for Mg2+ ions, as illustrated inFig. 1a. Defect spinel oxides, denoted as ABO3,
generally consist of divalent A(II) and tetravalent B(IV)cations. We chose Zn and Mn as the A and B cations,
respectively, because Zn stabilizes the spinel phase
due to the strong preference for the tetrahedral sites,
while Mn is stable at a relatively high valence state.
Nanocrystalline ZnMnO3 was synthesized for use

in preparing the electrodes. Detailed electrochemicalmeasurements and structural analysis of the electrodes
after discharge/charge revealed that Mg2+ ions werepreferentially inserted into the cation-deficient sitesthat maintain the spinel-based structures. This is instrong contrast with the stoichiometric spinel oxidematerials for MRBs, e.g., MgCo2O4, which readily
undergo the spinel-to-rock-salt transition.
To investigate the cyclability of defect spinel
ZnMnO3, a battery cycling test was carried out at 150°C to facilitate Mg2+ diffusion in the cathode material.
As shown in Fig. 1b, the discharge capacity of ZnMnO3
was ~100 mAh/g even after 100 cycles with a highcoulombic efficiency. Such long-term cyclability has
never been achieved using stoichiometric spineloxides. By assuming the ideal redox potential of the
Mg-metal anode, the energy density of Mg/ZnMnO3cell was estimated to be 200–300 Wh/kg, which iscomparable to that of conventional Li-ion batteries.
Because of the advantages in terms of safety andelement resources, this battery system is promising
for large-scale appreciations, such as electric vehicles
and stationary energy storage systems.
References[1] K. Shimokawa, T. Atsumi, N. L. Okamoto, T. Kawaguchi, S.
Imashuku, K. Wagatsuma, M. Nakayama, K. Kanamura, and T.
Ichitsubo, Adv. Mater. 33, 2007539 (2021).

Fig. 1 (a) Schematic illustration showing the expected migration pathway of Mg2+ ions in defect spinel ZnMnO3. In contrast to
stoichiometric spinel oxides, Mg2+ ions can be stored in the cation-deficient sites that suppress the spinel-to-rock-salt
transition. (b) Long-term cyclability of the defect spinel ZnMnO3 in the electrolyte of (Mg10/Cs90)-TFSA at 150 °C.

Keywords: energy storage, defects, oxide
Kohei Shimokawa (Exploratory Research Laboratory, IMR, and Creative Interdisciplinary Research Division, Frontier
Research Institute for Interdisciplinary Sciences)E-mail: shimokawa.kohei@imr.tohoku.ac.jp
URL: http://www.imr.tohoku.ac.jp/en/about/divisions-and-centers/research-division/33.html

IMR KINKEN Research Highlights 2021

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Electronic Materials

IMR KINKEN Research Highlights 2021

ResearchElectronic Materials

Hopfield Neural Network with Intrinsic Learning in Magnetic Films

M

acroscopic spin ensembles possess brain-like features, which offer opportunities for neuromorphic
computing by spintronics devices. We propose a four-node Hopfield network based on magnetic textures. Its
synapses do not require external input to update weights as this occurs intrinsically via physical feedback
mechanisms built into the texture.
Over the last decade, tremendous progress haspropelled neuromorphic computing to the forefrontof information technology. However, brain-inspired
algorithms are mainly emulated by conventional von
Neumann architectures with physically separated
computing and storage units, thereby limiting thepower of artificial intelligence algorithms.
Spintronics is a leading candidate for hardwareimplemented neuromorphic computing, in whichanalogue devices emulate neurons and synapse
functions. However, practical implementations
require the improved efficiency of the training andinference processes. To the best of our knowledge,
the main problem of existing approaches is the needfor external algorithms running on conventional
computers to update the weights of the synapses
during training.
We propose a neuromorphic computing
platform with superior performance based on theplasticity of magnetic textures [1]. The physicalrequirements are a magnetic film with chiral domain
patterns generated by the Dzyaloshinskii–Moriya
interaction, anisotropic magnetoresistance, andcurrent-induced spin-transfer torque [1]. We
simulated the device performance by finite element
simulations, which have been successfully applied
to problems in spin cavitronics [2].
We show that an electrically conducting
magnetic film can operate as a collection of artificial
synapses with weights encoded by the conductance
between external electrodes in a matrix configuration.
A voltage applied to the magnetic thin film (Fig. 1(a))
drives the magnetic textures by the spin-transfer
torque. With changes in the magnetic configuration,
the conductance (or resistance) is modified by theanisotropic magnetoresistance. Figure 1(b) showsthe enhanced horizontal conductance over time
under a current, as the vertical conductancedecreases. This “plasticity” encodes “weights” into
magnetic textures.
We implemented this concept as a four-node
Hopfield network in Fig.1(c). The value of fourneurons were encoded as voltage applied to fourelectrodes. The weight matrix was encoded into theFig. 1 (a) Configuration of magnetic texture at an initialtime. Black (white) color represents up (down)
magnetization perpendicular to the film and the arrows(inset)represent
in-planemagnetization.
(b)
Conductance change in either the horizontal or verticaldirection in the presence of an electric current. (c)Schematic four-node Hopfield network.

effective conductance matrix inside the magnetic
film. The four-node Hopfield network can operate as
an associative memory to memorize and recall afour-pixel binary picture. In the training stage, only
the pattern to be memorized was applied in terms ofbinary voltages, and the network naturally updated
the weights without requiring external computations.
In the inferring stage, the network is fixed, and thestored patterns can be recalled by standard
methods.
The learning rule is based on physical laws
rather than human interference. The concept can begeneralized to other materials with current-induced
plasticity. Our work paves the way for hardwarebased neuromorphic computing devices with onchip learning.
References[1] W. Yu, J. Xiao, and G.E.W. Bauer, arXiv: 2101.03016 (2021).
[2] W. Yu, T. Yu, and G.E.W. Bauer, Phys. Rev. B 102, 064416(2020).

Keywords: spin dynamics, magnetoresistance, spintronics
Weichao Yu (Theory of Solid State Physics Research Laboratory)
E-mail: wcyu@imr.tohoku.ac.jp
URL: http://www.bauer-lab.imr.tohoku.ac.jp/

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IMR KINKEN Research Highlights 2021

High Magnetic Field Element Specific Magnetization to Resolve theMagnetization Process of a Hard-Intermetallic Ferrimagnet

Nonmonotonic magnetic-field-dependence of magnetic moments in TmFe Al were determined using high magnetic field
5

7

X-ray circular magnetic dichroism. The complex behavior was caused by the competition between two different magnetic
anisotropies and exchange coupling.

Rare-earth transition metal intermetallic compounds
have been studied intensively for their strong anisotropies
and large magnetic couplings. These are an importantclass of materials for hard magnets used in a wide range
of applications, such as energy conversion and transport.
High magnetic fields have been used to investigate theanisotropies and exchange couplings of these magnets,
and important material parameters have been evaluated.
TmFe5Al7 has a ThMn12-type structure with tetragonal
symmetry. The alignment of the magnetic moments of Fe
and Tm is determined by the balance between the easyplane type anisotropy of Fe, the easy-axis type anisotropyof Tm, and the exchange coupling between Fe and Tm. It
should be noted that the easy-axis type ferrimagnet
ground state at low temperature is rare in the RFe5Al7
family, when R (rare earth element) is heavy rare-earth.
A magnetization curve in high magnetic fields wasmeasured to examine the magnetic parameters of thesystem at the Dresden High Magnetic Field Laboratory.
The bulk magnetization showed an upturn curvature up to
12 T, where a small jump in magnetization was observed.
The curvature then became negative above 12 T. This
indicates that there is a flop of magnetic moments at
approximately 12 T. Despite the characteristic features
found in the magnetization curve, the magnetic parametersor the alignment of magnetic moments could not be
inferred.
The key to understanding the magnetization process
is element-specific magnetization measurement with X-ray
circular magnetic dichroism (XMCD) in a pulsed highmagnetic field, which was developed by the collaboration
between the magnetism division of the Institute forMaterials Research, Tohoku University, and BL25 atSPring-8 [1]. In XMCD, the magnetic moment of a specificmagnetic ion is deduced by tuning the X-ray energy to therelated absorption edge. When soft X-rays are employed,
the final states of the edge absorptions are the 3d orbitalsof the transition element and the 4f orbital of the rare earth
element, respectively. This is the advantage of using soft
X-rays in XMCD despite the difficulty of using ultra-highvacuum. In BL25 at SPring8, the XMCD can be measured
up to 40 T using a compact bipolar pulsed field generator.
Figure 1 presents the results of the element-specificFig. 1 Element-specific magnetization of Tm and Fe and thealignment of magnetic moment determined by XMCD [2].

magnetization curve and the change in the magnetic
moment alignment in the magnetization process. It showsthat Fe moments always align along the magnetic field withfield-dependent tilting, while the Tm moment changes
from collinear to spin-flopped-like arrangements aroundthe transition found by the small bulk magnetization jump.
This result can be understood as the balance between themagnetic anisotropies and exchange coupling. Suchcomplex nonmonotonic magnetic-field-dependence of the
Fe moment can be evaluated only by the XMCD techniquein high magnetic fields. This method contributes to thestudy of various intermetallic magnetic compounds.
References[1] H. Yasumura, Y. Narumi, T. Nakamura, Y. Kotani, A. Yasui, E. Kishaba,
A. Mitsuda, H. Wada, K. Kindo, and H. Nojiri, J. Phys. Soc. Jpn. 86,
054706 (2017).
[2] S. Yamamoto, D. I. Gorbunov, H. Akai, H., Yasumura, Y. Kotani, T.
Nakamura, T. Kato, N. V. Mushnikov, A. V. Adreev, and H. Nojiri, Phys.
Rev. B 101, 174430 (2020).

Keywords: high magnetic field, hard magnet, XMCD
Hiroyuki Nojiri (Magnetism Research Laboratory)
E-mail: nojiri@imr.tohoku.ac.jp
URL: http://www.hfpm.imr.tohoku.ac.jp

IMR KINKEN Research Highlights 2021

27

Electronic Materials

Conductive Oxide with Surface Polarity for Device Applications

A layered delafossite, PdCoO , is one of the most conductive oxides with a bulk conductivity comparable with2

those of Au metals. Charged layers of Pd+ and [CoO2]− alternate in the PdCoO2 crystal structure, generating
surface polarities that strongly influence the surface/interface properties. Using pulsed laser deposition, we
successfully grew high-quality thin films of PdCoO2 and fabricated functional devices: a nanodevice showing
quantum transport and a diode that can operate at elevated temperatures. The superior properties of thesedevices are attributed to the anisotropic electrical conductivity of PdCoO2 that originates from its unique layered
crystal structure.
8

Conductivity σ (107 S/m)

The layered oxide PdCoO2 has attracted
significant attention as a highly conductive metalcomparable to the noble metals (Fig. 1). Using
PdCoO2 thin films [1], we are currently exploringnovel electronic functionalities that originate fromtwo key features of PdCoO2: the high electron
mobility (~51000 cm2/Vs for bulk single crystals)
and surface polarity induced by the ionic layered
structure (inset of Fig. 1). By fabricating submicronscale Hall-bar devices using electron beamlithography, we carried out the first observation ofquantum transport in PdCoO2 thin films [2]. We
found that the surface polarity of PdCoO2
significantly affects the surface electronic states,
giving the inherently nonmagnetic PdCoO2 uniquemagnetic states in the vicinity of the Pd-terminatedsurfaces [3]. The interplay between surface
magnetic states and quantum effects would be aninteresting subject for future study.
The surface polarity of PdCoO2 can improve
the performance of practical and useful devices.
We fabricated a crystalline heterointerface of
PdCoO2 and a wide-bandgap semiconductor
β-Ga2O3. We found that the PdCoO2/β-Ga2O3
interface can function as a Schottky diode that
performs current rectification even at a hightemperature of 350 °C [4]. The PdCoO2/β-Ga2O3
diode can operate in the MHz frequency, whichshows promise for high-frequency power electronic
devices [5]. The PdCoO2 exhibits superior Schottky
properties by forming a bilayer with an elemental
metal (tandem electrode). The junction between thetandem electrode and β-Ga2O3 has a wide range of
Schottky barrier heights that can be fine-tuned bychanging the thickness of PdCoO2 [6]. The interface
dipole effect was also found in the junction between
β-Ga2O3 and PdCrO2, a sister compound of PdCoO2[7], suggesting the universal role of surface polarity
in these polar oxides.
These results demonstrate that PdCoO2 andrelated metallic delafossites are a fascinating class

IMR KINKEN Research Highlights 2021

Cu

Pd+
[CoO2]─

PdCoO2
PtCoO2

4

Au
Al

c
a

2

b

SrMoO3ReO3

Pt Pd Rb0

Cs IrO2

Fig. 1 Electrical conductivity of PdCoO2 and various
metals at room temperature. Inset: the crystal structureof PdCoO2. The ionic layers of Pd+ and [CoO2]− areindicated.

of materials for basic physics and various
applications. We will continue to explore exotic
phenomena and functions in the heterostructures ofmetallic delafossites, which have high-mobility
electrons and surface polarity in their simple (andbeautiful) alternating layered crystal structures.
References[1] T. Harada, K. Fujiwara, and A. Tsukazaki, APL Mater. 6,046107 (2018).
[2] T. Harada, P. Bredol, H. Inoue, S. Ito, J. Mannhart, and A.
Tsukazaki, Phys. Rev. B 103, 045123 (2021).
[3] T. Harada, K. Sugawara, K. Fujiwara, M. Kitamura, S. Ito, T.
Nojima, K. Horiba, H. Kumigashira, T. Takahashi, T. Sato, and
A. Tsukazaki, Phys. Rev. Res. 2, 013282 (2020).
[4] T. Harada, S. Ito, and A. Tsukazaki, Sci. Adv. 5, eaax5733(2019).
[5] T. Harada and A. Tsukazaki, Appl. Phys. Lett. 116, 232104(2020).
[6] T. Harada and A. Tsukazaki, APL Mater. 8, 041109 (2020).
[7] T. Miyakawa, T. Harada, S. Ito, and A. Tsukazaki, J. Appl.
Phys. 128, 025302 (2020).

Keywords: oxide, thin films, devices
Takayuki Harada (Low Temperature Physics Research Laboratory)
E-mail: t.harada@imr.tohoku.ac.jp
URL: http://mu.imr.tohoku.ac.jp/index_en.html

28

6

Ag

IMR KINKEN Research Highlights 2021

Nanoscale Polarized π -Electron Clusters Probed by Noise Spectroscopy

We investigated the low-frequency charge-carrier dynamics of a molecular dimer-Mott insulator β‘-(BEDT-TTF) ICl , in
2

2

-1
2

The molecular organic dimer-Mott insulator β’(BEDT-TTF)2ICl2 is a model system for studying theunderlying charge fluctuations on the dimers, where
electric-field-induced charge disproportionation has
been demonstrated in dielectric and pyroelectric
properties [1] and Raman scattering [2]. β’-(BEDTTTF)2ICl2 exhibits relaxor-type ferroelectric behavior witha strong frequency-dependent peak of the dielectric
constant developing below T~125 K. At approximately
below 60 K, pyrocurrent measurements reveal the onsetof electric-field-induced macroscopic polarization. The
P−E curves that show small values of the fully polarized
moment and relaxation effects in Raman spectroscopy
suggest a dipole-glass state at low temperatures.
We employed conduction current noise spectroscopy
to study the dynamical charge fluctuations on stronglycorrelated charge carriers in β’-(BEDT-TTF)2ICl2 at low
frequencies, i.e., in the mHz to kHz regime [3]. The method
is complementary to dielectric spectroscopy and allows
the extraction of spectroscopic information from electronictransport measurements. Our aim is to establish aconnection between the fluctuating conductivity in the
Mott insulating phases and relaxor ferroelectric properties.
Figure 1 demonstrates the normalized current noise
power spectral density SI at 130 K. In the temperature
region of 110 K < T < 140 K, a single Lorentzian spectrum,
which is characteristic for a fluctuating two-level system,
was found to be superimposed on the “background” 1/f
noise spectrum. At temperatures higher and lower than
this temperature region, we observed only generic 1/fnoise, which is independent of the applied electric field,
i.e., the expected scaling SI∝I2 was obeyed. On the other
hand, the Lorentzian excess noise indicates the formationof polar nanoscale regions (PNRs) that possess distinct
two-level fluctuations that are dependent on the applied
electric fields. The corner frequency fc is related to thecharacteristic energy of the two-level fluctuator andthermally-activated states of a double-well potential withan activation energy Ea. This energy is very close to theintradimer transfer integral and optical charge gap of~2900 K [4], which indicates that the observed switchingprocesses are of electronic origin. We suggest that thecompeting inter- and intradimer Coulomb interactions

SI /I (Hz )which the freezing of charge fluctuations on the dimers gave rise to electronic ferroelectricity. Conductance noise
spectroscopy proved the formation of a polar nanoscale region leading to glassy and relaxor-type ferroelectric behavior.
Thus, conductance noise spectroscopy is a suitable tool for investigating electric-polarization dynamics in molecular andother charge-driven ferroelectrics.

10

-10

10

-11

10

-12

10

-13

10

-14

10

-15

exp

1/f noise +Lorentzian excess noise

1

10
100
1000
Frequency (Hz)Fig. 1 shows normalized current noise PSD SI at 130 K, which
consists of 1/f noise and Lorentzian excess noise fromferroelectric fluctuations of PNRs (conceptual picture).

cause coherent fluctuations of electrons within a cluster ofdimers, switching between the unpolarized dimer-Mottstate and a charge-disproportionated state, as indicatedby the bottom figure in Fig. 1.
These findings demonstrate that conductance noise
spectroscopy is a sensitive tool for probing PNR formation
or fluctuating mesoscopic domains and their coupling toelectric fields to study the physics of materials that giverise to electronic ferroelectricity.
References[1] S. Iguchi, S. Sasaki, N. Yoneyama, H. Taniguchi, T. Nishizaki, and T.
Sasaki, Phys. Rev. B 87, 075107 (2013).
[2] Y. Hattori, S. Iguchi, T. Sasaki, S. Iwai, H. Taniguchi, and H. Kishida,Phys. Rev. B 95, 085149 (2017).
[3] J. Müller, S. Iguchi, H. Taniguchi, and T. Sasaki, Phys. Rev. B 102,
100103(R) (2020).
[4] K. Hashimoto, R. Kobayashi, H. Okamura, H. Taniguchi, Y. Ikemoto, T.
Moriwaki, S. Iguchi, M. Naka, S. Ishihara, and T. Sasaki, Phys. Rev. B
92, 085149 (2015).

Keywords: organic, electronic properties, metal-insulator transition
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

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Electronic Materials

Probing the Spin Current Carrier through Polarized Neutron
ScatteringIn insulating magnets, the spin current (i.e., flow of the spin degree-of-freedom in matter) can be propagatedby the precessional motion of ordered magnetic moments. This motional direction, i.e., magnon polarization, isa unique and important signature of magnets, yet hitherto eluded experimental observations. Here, we present
the first observation of magnon polarization on yttrium iron garnet through polarized neutron scattering.

The spin current in insulators can be carried by
the precessional motion of ordered magnetic
moments. According to the Landau–Lifshitz
equation, a magnetic moment precesses counterclockwise around the effective magnetic field
direction. This motion can be described as“positively” polarized. The collective excitations insingle-domain ferromagnets also precess counterclockwise; hence, all ferromagnetic magnons havea positive polarization (Fig. (a)). Simple collinear
antiferromagnets have two magnon modes with
opposite polarization (Fig. (b)), but these degenerate
unless large magnetic fields or Dzyaloshinskii–
Moriya interactions are turned on.
Simple ferrimagnets have two anti-aligned
sublattices and support two magnon polarizations;however, the intersublattice exchange field naturally
separates the branches of opposite polarization into
acoustic and optical modes (Fig. (c)). The energy
gap between these modes can be large.
Spectroscopic studies have the potential to observe
this characteristic; however, there is no direct
experimental proof of the magnon polarization.
We observed the magnon polarization on
yttrium iron garnet Y3Fe5O12 (YIG) [1]. YIG is aferrimagnetic insulator with a complex crystal
structure, and is a quintessential magnet for
spintronics and magnonics. To detect the magnon
polarization, neutron spins were aligned parallel or
antiparallel to the scattering wave vector (Q).
Magnetic neutron scattering can only detect the
spin components perpendicular to Q. These
projections are supposed to be tiny because appliedmagnetic fields also align the moments along thisdirection. The component detected by thisconfiguration corresponds to the area covered by
the precessional motion. The use of the chiral termin polarized neutron scattering can distinguishbetween clockwise and counter-clockwise motion.
YIG is a good material to attempt such measurements
because it has a sufficiently large moment. Figure

Fig. Sketch of polarization for (a) a ferromagnet, (b) an
antiferromagnet, and (c) a ferrimagnet. (d) Calculated
scattering cross-sections overlaid with experimentallyestimated peak positions.

(d) shows the observed neutron scattering spectra.
Together with theoretical calculations, magnon
polarization for positive and negative signs is clearlyobserved for the acoustic and optical mode,
respectively.
We observed the magnon polarization in thecollinear ferrimagnet. The magnon polarization is
directly related to the propagation direction of thespin current. Our experimental findings provide
momentum/energy-resolved information on the
propagation. The method we developed can beapplied to more complex magnets to obtain
additional microscopic views of magnetism.
References[1] Y. Nambu, J. Barker, Y. Okino, T. Kikkawa, Y. Shiomi, M.
Enderle, T. Weber, B. Winn, M. Graves-Brook, J. M. Tranquada,
T. Ziman, M. Fujita, G. E. W. Bauer, E. Saitoh, and K. Kakurai,
Phys. Rev. Lett. 125, 027201 (2020).

Keywords: neutron scattering, spintronics, magnetism
Yusuke Nambu (Quantum Beam Materials Physics Research Laboratory)
E-mail: nambu@tohoku.ac.jp
URL: http://qblab.imr.tohoku.ac.jp/index-e.html

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IMR KINKEN Research Highlights 2021

Electric Current Control of Spin Helicity in an Itinerant Helimagnet

In a helimagnet, the ordered direction of the spin moment rotates along the wave vector. The sense of rotation,
which is denoted as helicity, can be viewed as an internal degree-of-freedom when the crystal structure has
inversion symmetry. We successfully controlled the helicity in an itinerant helimagnet MnP.
FAN

CONICAL− helicity+ helicity

Fig. 1 Illustrations of magnetic transition from fan to conicalmagnetic structure. Helicity degree-of-freedom exists inthe conical magnetic state but is absent in the fanmagnetic structure [2].
58(a) jac = 5.1 × 10 Am

–2

(b)

51 K

Hp > 0 , jp < 0(nΩ cm)4

2f

In a helimagnet, the helical structure hosts thechirality because any mirror operation reverses thesense of rotation, referred to as helicity. The helicity
degree-of-freedom is robust against disturbances
and is therefore useful for retaining information.
Recently, spintronics based on antiferromagnets has
attracted much attention because of its high-speed
dynamics and robustness against stray fields. The
helical magnetic structure is one form of
antiferromagnetic structure that seems to share thecharacteristics mentioned above. Realization ofcontrol and helicity detection could pave way for new
possibilities of magnetic memory applications basedon helimagnets.
It is of great interest to understand how the spinrotation direction couples with electromagnetic fields.
In insulators, the spiral magnetic structure often
induces ferroelectric polarization, as studied in thefield of multiferroics. Nevertheless, the coupling
between the spin-rotation direction and electric
current in itinerant spiral magnets remains unknown.
Theoretical work has suggested that electric current
along the propagation vector induces finite
magnetization in helimagnets owing to the damping
torque, and that the magnetization direction depends
on the helicity [1]. Based on these results, we
conjecture that the degeneracy relevant to helicity
should be lifted by the simultaneous application of amagnetic field and electric current.
In this work, we experimentally confirmed theconjecture [2]. To increase the electric current
density, we fabricated a micrometer-scale singlecrystalline sample of the helimagnet MnP using a
focused ion-beam technique. We applied a large
electric current jp parallel or antiparallel to themagnetic field Hp in the high magnetic state of the
fan-type magnetic structure (see Fig. 1). Then, we
decreased the magnetic field below the transition
field to the conical magnetic state (tilted helicalmagnetic state). Finally, we turned off the electric
current. To probe the helicity, we measured themagnetic field dependence of the second harmonic
resistivity. The field-odd component corresponds tothe electrical nonreciprocity, whose sign reflects thehelicity. Figure 2 shows the second harmonic

Hp > 0 , jp > 0

3
5

(c)

Hp < 0 , jp > 0(d)4
Hp < 0 , jp < 03–3 –2 –1

0

1

2

3 –3 –2 –1
0H (T)

0

1

2

3

Fig. 2 (a)–(d) Magnetic field dependence of the second
harmonic resistivity ρ2f after the poling procedure withthe positive and negative magnetic fields Hp and dc
electric currents jp [2].

resistivity after the poling procedures with positive
and negative jp and Hp. The sign of the field-odd
component depends on whether jp and Hp areparallel or antiparallel. This result demonstrates theconjectured helicity control with a magnetic field andelectric current.
References[1] O. Wessely, B. Skubic, and L. Nordström, Phys. Rev. B 79,
104433 (2009).
[2] N. Jiang, Y. Nii, H. Arisawa, E. Saitoh, and Y. Onose, Nat.
Commun. 11, 1601 (2020).

Keywords: spin transfer, multi-ferroics, spintronics
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

IMR KINKEN Research Highlights 2021

31

Electronic Materials

Red-Emission Scintillator Exploration

Scintillators with emission bands in the red or near-infrared region are novel materials with several important

applications. We explored such materials and discovered their outstanding properties. Several cameras havebeen developed with these materials for applications such as a dose monitor used in extremely high radiationdose conditions.

Scintillators are widely used in radiation detectors
coupled with photosensors, such as a photodiodeand photomultiplier tube. Under extremely highradiation-dose conditions, conventional photosensors
can be damaged immediately owing to the ionizationof semiconductors or circuits. The decommissioningof the Fukushima Daiichi nuclear plant is a major
challenge, and optical fibers and scintillators areused to conduct internal surveys of the dose ratio
and to identify the distribution of nuclear debris.
In such applications, the length of the opticalfiber should be approximately 100 m, and effective
light output (intensity) through the fiber is an importantfactor. The transition loss of red or near-infrared
photons is lower compared with shorter wavelength
photons in the optical fiber. Moreover, a large numberof scintillation and Cherenkov photons can begenerated by such high-dose radiation in the fiber asa noise, which are expected to dominate in a shorter
wavelength region (below 600 nm).
Conventional scintillators have emission
wavelength regions of 200–600 nm, and typical light
output of up to 60,000 photons/MeV. Impurity
emissions, such as Ce-doped materials, have goodquantum efficiency, while the typical emission energy
is green or blue.
Light output is basically inversely proportional tobandgap energy. The gaps of most conventional
scintillators with impurity emissions cannot be smaller
than the energy gaps of the emission center.
Impurity-doped materials are hard to have redemission band(s) with good quantum efficiencies
because of overlapping between conduction bandand the excitation level of the emission center forallowed transition. Thus, other emission mechanisms
are required to realize the red scintillators.
We focus on the charge transition transfer andself-trapped exciton luminescence, which can begenerated only by host materials. In general, iodide
materials tend to have a small band gap energy, andhost emission for iodides tend to have a longer

Fig 1. Photograph of setup for demonstration test of themonitor with red-emission scintillator

wavelength.
The scintillation properties of several iodide
materials, such as A2HfI6, were investigated by the
Bridgeman method. For example, Cs2HfI6 has anemission wavelength of ~700 nm and light output ofover 60,000 photons/MeV. However, the transmittance
was poor (<50%) because of the low quality of raw
materials (99.9%) [1]. We also successfully grew
several novel crystals and crystal structures, andinvestigated their scintillation properties [2-3].
Because these samples have a hygroscopic
nature, a piece was embedded in optical cement, asshown in Fig. 1. In addition, a real-time remote
gamma-ray-dose monitor for extremely highradiation-dose conditions was developed. We
successfully tested the monitor using a long opticalfiber and gamma-ray source with a radioactivity of
approximately 100 TBq (Fig. 1) [1].
References[1] S. Kodama, S. Kurosawa, and A. Yoshikawa et al., Appl. Phys.
Express 13, 047002 (2020).
[2] S. Kodama, S. Kurosawa, and A. Yoshikawa et al., IEEE T NUCLSCI 67(6), 1055 (2020).
[3] S. Kodama, S. Kurosawa, and A. Yoshikawa et al., Opt. Mater.
106, 109942 (2020).

Keywords: scintillator, radiation imaging, crystal growth
Shunsuke Kurosawa (New Industry Creation Hatchery Center, Tohoku University)
E-mail: kurosawa@imr.tohoku.ac.jp
Akira Yoshikawa (Advanced Crystal Engineering Research Laboratory)
E-mail: yoshikawa@imr.tohoku.ac.jp
URL: http://yoshikawa-lab.imr.tohoku.ac.jp/index-e.html

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IMR KINKEN Research Highlights 2021

Thermalization Dynamics of Photo-Excited Resonantly
Bonded PbTe

T

he ultrafast optical response of resonantly bonded systems should be investigated to gain a further understandingof the ultrafast amorphization of phase-change materials induced by femtosecond laser irradiation. Our recent
work proposed a methodology for detecting the ultrafast thermalization dynamics of non-equilibrium electron andlattice systems relaxed into a thermal state, following the photoexcitation of resonantly bonded PbTe.
Chalcogenide phase-change materials (PCMs)
have been applied to memory devices such as DVDs,
Blu-ray discs, and PCRAMs owing to their reversible
crystalline and amorphous phase-change properties
and the sharp contrast between their optical andelectrical properties. It has been reported that one of
the representative PCMs, Ge2Sb2Te5 (GST), can be
amorphized within 10 ps after femtosecond (fs) laser
excitation. However, it remains unclear whether thisprocess is thermal or nonthermal. In PCM systems,“resonant bonding” is considered as one form ofbonding that involves the linear long-range alignmentof p-orbitals, which plays crucial role in the ultrafast
phase-change process. To investigate such ultrafast
dynamics following laser irradiation, we studied theultrafast optical response and temperaturedependence of the optical properties of PbTe, which
has a cubic lattice and resonant bonding like GST [1].
Figure 1 shows the temperature-dependenceof the optical-density (OD) change of PbTe thin film
deposited onto a quartz substrate in Ar atmosphereby the spattering method. The ΔODT [= OD(T) –OD(78 K)] value characterizes the degree of photoabsorption. Figure 1 shows that PbTe became more“transparent” with the increase in temperature. This
result can be explained from the viewpoint ofresonant bonding. The enhanced atomic vibration
at elevated temperature can disturb the long-rangealignment of the p-orbital, which deteriorates or
weakens the resonant bonding. Interestingly, the
ΔODT ratio of ΔOD at 2.2 eV to ΔOD at 1.2 eV
became a constant value of approximately 2.6 in thepresent temperature region.
On the other hand, after fs-laser irradiation at
78 K, the OD decreased over time. The decrease in
ΔODp [= OD(t) – OD(t = 0)] at 1.2 eV was more
prominent than that at 2.2 eV after irradiation. Figure
2 shows the temporal variation of the ΔODp ratio [=ΔODp (2.2 eV) / ΔODp (1.2 eV)]. This clearly explores
relaxation, i.e., the thermalization process between
non-equilibrium electron and lattice systems toward
a thermal state. In the case of PbTe, the ΔODp ratio

Fig. 1 The temperature-dependence of OD relative to 78
K. The ratio of ΔOD (2.2 eV) to ΔOD (1.2 eV) becomes
constant of 2.6 in all temperature regions studied.

Fig. 2 The time evolution of ΔOD ratio after photo-excitation
at 78 K. In the region where ΔOD is less than 2.6,
electron and lattice systems are not in equilibrium.

reached 2.6 at 12 ps after photoexcitation. At thispoint, the system may be regarded as reaching athermal state and that its temperature can be welldefined.
Summing up, we proposed a methodology todetermine the time domain for thermalization after
photoexcitation. Our results provide a further
understanding of the photo-induced dynamics ofPCMs.
References[1] H. Tanimura, S. Watanabe, and T. Ichitsubo, Adv. Funct. Mater.
30, 3003932 (2020).

Keywords: optical properties, amorphous, phase transformation
Hiroshi Tanimura (Structure-Controlled Functional Materials Research Laboratory)
E-mail: tanimura@imr.tohoku.ac.jp
URL: http://ilab.imr.tohoku.ac.jp/

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Electronic Materials

Alchemy for Magnets: CO2 Switches Magnetism

S

witchable magnets, with magnetic properties that can be reversibly changed in response to external stimuli,
have promising potential for applications such as switches, memories, and sensors. Here, we report a gasresponsive magnet, i.e., a paramagnet on CO2 adsorption (with its magnetism "switched off") and transforms
into a ferrimagnet on CO2 desorption (with its magnetism "switched on").
Metal–organic frameworks (MOFs), with theirdiverse designability and high porosity, could be
ideal for gas-responsive materials. From theperspective of magnetic materials, MOFs are
expected to change their magnetic properties
depending on the presence or absence of guest
molecules in the pores. However, obtaining selective
and drastic magnetic responses to the sorption ofcommon gases, such as O2 and CO2, remains
challenging because the interactions between
common gases and MOFs are often very weak. An
effective method to induce a drastic magnetic phasechange is to use our proposed magnetic changing
mechanism, that is, manipulating the electronic
state modulation in strongly correlated electronicsystems [1,2].
A layered magnetic MOF, consisting oftrifluorobenzoate-bridged
paddlewheel-typeruthenium (II) dimers as electron donors (D)coordinated in-plane to 2,5-diethoxy-7,7,8,8tetracyanoquinodimethane (TCNQ(OEt)2) units aselectron acceptors (A) in a D2 A formula, shows
ferrimagnetic ordering at Tc = 110 K through a oneelectron-transferred form of [D–A− –D+] ∞ (Figs. 1aand 2a) [3]. The magnetic ordering is attributed tothe strong superexchange interactions between theparamagnetic D (S = 1 for a [RuII, RuII] cluster) andD+ (S = 3/2 for a [RuII, RuIII]+ cluster) subunits via A−
with S = 1/2 (TCNQ(OEt)2•−) (Fig. 1a). When CO2 isadsorbed in a stepwise manner within and between
layers, the [D–A− –D+] framework undergoes
stepwise structural transitions, with the second
transition involving an intralattice electron transfer(D+ ← A−) to produce a paramagnetic neutral formof [D–A–D] ∞ with diamagnetic A (Figs. 1b and 2a).
In addition to magnetism, this sudden electrontransfer involving CO2 uptake/release induces
modulations of electronic conductivity (Fig. 2b) andpermittivity, revealing a favorable example of thefeasibility and versatility of MOFs for guest
molecules. Theoretical calculations using density
functional theory suggest that accommodated CO2
molecules have significant electronic interactions
with the framework. These findings provide insights

Fig. 1 Crystal structures of layered magnetic MOF withoutgas (a) and within CO2 (b).

Fig. 2 (a) The CO2 pressure-dependence of field-cooledmagnetization curves at Hdc = 100 Oe. (b) Temperaturedependence of σdc measured on a pelletized sample
under P = 100 kPa of He (black) and CO2 (blue).

into the mechanism by which switchable properties
can be controlled by gas sorption, which may be
helpful in future applications, such as gas sensors
or memory storage.
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).
[3] J. Zhang, W. Kosaka, Y. Kitagawa, and H. Miyasaka, Nat.
Chem. 13, 191 (2021).

Keywords: crystal structure, 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

34

IMR KINKEN Research Highlights 2021

IMR KINKEN Research Highlights 2021

Highly Ordered L10-FeNi Films with Large Uniaxial Magnetic
Anisotropy Fabricated by a Denitriding Method

L1 -ordered FeNi alloy is a candidate material for applications in rare-earth-free and noble-metal-free permanent0magnets and magnetic storage technology. This paper reveals that denitriding FeNiN films is an effective method
to form L10 -orderd FeNi films with a high degree of order and a large uniaxial magnetic anisotropy energy.

Ferromagnetic materials that possess large
uniaxial magnetic anisotropy energy (Ku) and consistof earth-abundant elements are required to realize thenext-generation permanent magnets and magnetic
storage. An L10 -ordered FeNi alloy has been
considered as a rare-earth-free high Ku material [1].
Recently, the synthesis of polycrystalline L10 -FeNi
powder by denitriding FeNiN powder was reported(Fig. 1), and a high degree of order (S) of 0.71 wasachieved [2]. However, the exact Ku value of L10 -FeNi
formed by the denitriding method remains unclear. The
preparation of a single-crystal L10 -FeNi is indispensablefor the evaluation of Ku. In this study, epitaxial FeNiN
films were grown by molecular beam epitaxy. Then,
L10 -FeNi films were fabricated using nitrogen topotactic
extraction from the FeNiN, which allowed us toquantitatively characterize their S and Ku values.
FeNiN films with a thickness of 20 nm were grown
on SrTiO3(001) substrates at 200, 250, and 350 °C by
supplying Fe, Ni, and RF N2 plasma, simultaneously[3]. Denitriding was performed by ex-situ furnaceannealing at 300 °C for 4 h under a H2 gas flow rate of
1 l/min at ambient pressure. The structure of thesamples was characterized by x-ray diffraction
measurements using Cu-Kα radiation and synchrotron
radiation (hν = 7.11 keV), and cross-sectional scanningtransmission
electronmicroscope(STEM)
measurements. The saturation magnetization (Ms) andthe Ku were estimated by magnetic torquemeasurements.
The epitaxial L10 -FeNi films oriented with thea-axis perpendicular to the film plane were fabricated,which had two variants with the in-plane c-axes
orthogonal to each other. Figure 2 shows the growthtemperature dependence of Ms and Ku. For the sample
grown at 350 °C, the S and Ku values were evaluatedas 0.87 and 5.9 × 105 J/m3, respectively. The large S
exceeding those reported previously [1,2] may allow
us to expect a larger Ku value. The exchange length of
L10 -FeNi (3.2 nm) is comparable to the variant sizes ofsamples estimated by the STEM measurements. Thissuggests that the obtained macroscopic Ku value
averages out owing to the exchange coupling ofFig. 1 Crystal structures of FeNiN and L10 -FeNi. L10 -FeNi
is fabricated by topotactic extraction of N atoms fromFeNiN.

Fig. 2 Growth temperature dependence of Ms and Ku ofthe samples.

variants with different easy axes and is smaller than theactual Ku of each variant [4].
References[1] K. Takanashi, M. Mizuguchi, T. Kojima, and T. Tashiro, J. Phys. D:
Appl. Phys. 50, 483002 (2017).
[2] S. Goto, H. Kura, E. Watanabe, Y. Hayashi, H. Yanagihara, Y.
Shimada, M. Mizuguchi, K. Takanashi, and E. Kita, Sci. Rep. 7,13216 (2017).
[3] K. Ito, M. Hayashida, M. Mizuguchi, T. Suemasu, H. Yanagihara,
and K. Takanashi, J. Magn. Soc. Jpn. 43, 79 (2019).
[4] K. Ito, M. Hayashida, H. Masuda, T. Nishio, S. Goto, H. Kura, T.
Koganezawa, M. Mizuguchi, Y. Shimada, T. J. Konno, H.
Yanagihara, and K. Takanashi, Appl. Phys. Lett. 116, 242404(2020).

Keywords: magnetic properties, thin films, molecular beam epitaxy (MBE)
Keita Ito and Koki Takanashi (Magnetic Materials Research Laboratory)
E-mail: itok@imr.tohoku.ac.jp, koki@imr.tohoku.ac.jp
URL: http://magmatelab.imr.tohoku.ac.jp

IMR KINKEN Research Highlights 2021

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Electronic Materials

Growth of a Super Uniform Congruent-Melting LiNbO3 Crystal

A super uniform congruent-melting LiNbO (cs-MgO:LN) was grown, which was simultaneously stoichiometric3

So-called “high-quality” crystals are always
grown from congruent melts in which the crystal
maintains exactly the same composition as that of
the melt. The congruent composition is represented
in a phase diagram as the intersection of the liquidus
line and solidus line. Then, the equilibrium partition
coefficient k0 is unity for the components in thephase diagram. The congruent point, which isinvariant with zero degrees-of-freedom, is located at
the summit of the liquidus lines showing the highest
temperature. Although it appears unstable, it is very
stable because the congruent point is located at thelowest position of the molar free energy curves ofthe solid and liquid as a common tangent point.
However, more care and attention is needed when
dealing with the oxide melt. The oxide melt contains
a variety of ionic species [1] whose k0s are notnecessarily unity, although the k0s of the constituent
components of the phase diagram (systemcomponents) are unity. The segregation of ionicspecies occurs at the interface, even for congruent
growth. Interestingly, the crystal composition isalways congruent during steady-state growth [1].
However, a perturbation of the heat and mass flow
beneath the interface always occurs during growth,which changes the growth velocity and crystal
composition. Here, we should know when the k0s of
all ionic species are unity; their populations at theinterface never change but remain constant as in
the bulk melt, even if a growing crystal experiences
an abrupt change in growth velocity. Thus, thecrystal composition was surprisingly uniform
throughout the growth. Such bulk crystals werecreated by doping MgO into LiNbO3, which isreferred to as cs-MgO:LN (Li2O:Nb2O5:MgO =45.30:50.00:4.70) [2]. It has a congruent point
coincident with the stoichiometric composition,
which makes k0 unity for all ionic species. To
construct cs-MgO:LN, we introduced the extended
concept of “stoichiometry,” that is, “a material inwhich the activities of all the constituent elementsthat can be unity is stoichiometric.” This concept

Compositional
Growth velocity
uniformity

and congruent. The equilibrium partitioning coefficient k0 is unity for all ionic species; therefore, the segregationof ionic species is not expected to occur during crystal growth even when cs-MgO:LN exhibits non-steadystate growth. This confirms the improved compositional stability compared with conventional LiNbO3 under all
growth conditions.

IMR KINKEN Research Highlights 2021

(a) Conventional congruent LN (c-LN)(b) Super congruent LN (cs-MgO:LN)
Time

Fig. 1 Compositional stability in (a) conventional and (b)super congruent LN when confronting a growth
perturbation.

expands the stoichiometric composition from apoint to a line, so that coincidence with the congruent
point can occur much more easily. In Fig. 1, the csMgO:LN crystal shows no composition fluctuations
even if it confronts non-steady-state growth because
it has a unity of k0 for every chemical species, which
ensures the super uniform distribution of therefractive index with ∆n < 10 –5 /cm; this is beneficial
for developing future optical devices with extremely
high precision. However, conventional c-LN shows
a compositional bump. In conclusion, cs-MgO:LN
has a great advantage over c-LN in terms ofcompositional uniformity.
References[1] S. Uda and C. Koyama, J. Cryst. Growth 548, 125837 (2020).
[2] Q. Shi, J. Nozawa, and S. Uda, J. Cryst. Growth 549, 125864(2020).

Keywords: crystal growth, crystal structure, doping
Satoshi Uda and Qilin Shi (Crystal Chemistry Research Laboratory)
E-mail: uda@imr.tohoku.ac.jp
URL: http://www.uda-lab.imr.tohoku.ac.jp/index-e.html

36

Steady state Perturbation Steady stateIMR KINKEN Research Highlights 2021

Multiple Superconducting Phases in Novel Spin-Triplet
Superconductor UTe2

We discovered multiple superconducting phases in the novel spin-triplet superconductor UTe under a magnetic field2

Heavy fermion superconductivity in UTe2 has attracted
much attention because of recently discovered
superconducting phenomena. UTe2 is a paramagnet with aheavy electronic state in the proximity of the ferromagnetic
order. The microscopic coexistence of ferromagnetism andsuperconductivity has already been established for
uranium-based compounds, namely UGe2, URhGe, andUCoGe. The newly discovered UTe2 has many similarities
with these ferromagnetic superconductors. One of thehighlights of UTe2 and ferromagnetic superconductors is
the huge superconducting upper critical field Hc2. In UTe2,
when the field is applied along the hard magnetization axis
(b-axis), the superconducting phase persists at an extremely
high field up to 35 T, which is one order of magnitude larger
than usually expected at the superconducting transition
temperature 1.6 K. Furthermore, the Hc2 curve displays field
re-entrant behavior; Tc at 35 T is two times higher than thatat 15 T. Therefore, spin-triplet superconductivity is expected
because Hc2 can be enhanced by tuning the ferromagnetic
fluctuation without the Pauli paramagnetic effect. Anotherimportant aspect of spin-triplet superconductivity is the spin
degree-of-freedom. In this case, multiple superconducting
phases are theoretically expected, as is well known insuperfluid 3He. However, obtaining experimental evidence
for multiple superconducting phases is a difficult task.
We performed AC calorimetry measurements underpressure and for the field along the easy-magnetization axis
(a-axis) in UTe2 to clarify whether the superconducting
phases are a thermodynamic response. Figure 1 shows the
H-T phase diagram at 0.54 GPa for the H || a-axis. The
superconducting transition temperature was enhanced to Tc= 2.5 K compared with that at ambient pressure (Tc = 1.6 K).
When the field was increased, Tc decreased with a strong
convex curvature. At temperatures below 0.4 K, Hc2 abruptly
increased and reached 8 T at 0 K. This Hc2 curve was also
confirmed by magnetoresistance [2]. The Pauli limitation
exhibited by Hc2 could be attributed to the appearance of
the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state, inwhich the pair-breaking due to the Pauli paramagnetic
effect is reduced. However, this is unlikely because the
FFLO state never produces the sharp kink observed in

Magnetic Field (T)and pressure. Owing to the thermodynamic response shown by AC calorimetry measurements, H-T phase diagrams underpressure were determined for the field along the a-axis in the orthorhombic structure. The unusual enhancement of theupper critical field Hc2 was observed as a consequence of multiple superconducting phases. This behavior of Hc2 requires
the state of the superconducting order parameter to be more complex than the spin-triplet state with equal-spin pairing.

UTe 2
0.54 GPa
H || a-axis

10
SC4

PM

5

0

SC3
0

SC2
2
1
Temperature (K)3Fig. 1 H-T phase diagram for H || a-axis at 0.54 GPa [1]. PM, SC
denote paramagnetism and superconductivity, respectively

UTe2. Moreover, UTe2 is a three-dimensional superconductor,
different
fromthetwo-dimensional
iron-based
superconductor. Most importantly, the abrupt increase in
Hc2 emerged as a prolongation of the lower superconducting
transition existing at zero field, where no paramagnetic
limitation exists. Our results demonstrate multiple
superconducting phases, which may require different orderparameters for each phase. Several theoretical models to
explain this phase diagram have been proposed based on
the existence of point node gaps at ambient pressure
indicated by field-angle resolved specific heatmeasurements [3].
This work was done in collaboration with F. Honda, G.
Knebel, D. Braithwaite, A. Nakamura, D.X. Li, Y. Homma, Y.
Shimizu, Y. J. Sato, J.P. Brison, and J. Flouquet.
References[1] D. Aoki, F. Honda, G. Knebel, D. Braithwaite, A. Nakamura, D.X. Li, Y.
Homma, Y. Shimizu, Y. J. Sato, J. P. Brison, and J. Flouquet, J. Phys. Soc.
Jpn. 89, 053705 (2020).
[2] G. Knebel, K. Motoi, M. Valiska, F. Honda, D.X. Li, D. Braithwaite, G.
Lapertot, W. Knafo, A. Pourret, Y. J. Sato, Y. Shimizu, T. Kihara, J. P.
Brison, J. Flouquet, and D. Aoki, J. Phys. Soc. Jpn. 89, 053707 (2020).
[3] S. Kittaka, Y. Shimizu, T. Sakakibara, A. Nakamura, D. X. Li, Y. Homma,
F. Honda, D. Aoki, and K. Machida, Phys. Rev. Res. 2, 032014(R) (2020).

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

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37

Electronic Materials

Self-Organized Solid-State Epitaxy of PZT Thin Films by
Chemical Solution DepositionT

he film–substrate interface is critical for crystal growth via solid-state epitaxy. We have elucidated the local
structure and crystallization mechanism of amorphous phase PbZrxTi1-x O3 (x = 0.3) thin films deposited onto
SrTiO3 (001) single-crystal substrates by chemical solution deposition. The amorphous phase involves a shortrange order of (Ti/Zr)O6 octahedra and Pb-O. The first unit cells of the film have already crystallized with acube-on-cube epitaxial relationship with the substrate.
Chemical solution deposition (CSD) is a thinfilm growth technique with several advantages overgas-phaseprocesses,
suchas
excellentcomposition transferability from mixed raw materials
to thin films, simple facilities, and low cost. We havestudied metal-organic deposition as the simplest
CSD. This process involves the following steps:
metal-organic precursors with long-chain organic
ligands were dissolved in an organic solution, andcoated by spraying, dipping, or spin coating. These
were then dried to remove solvents, decomposed
into an amorphous oxide thin film as a precursor,
and heated to crystallize. In this CSD process, it isimportant to establish the structure and bonding
states of amorphous oxide thin films, and theinterface structure between the film and substrate,
immediately after decomposition. However, theinitial stage of the crystal growth mechanism usingchemical solvents has not been elucidated because
crystal growth cannot be monitored in real time.
Figure 1 shows the (a) high-angle annular darkfield scanning transmission electron microscopy(HAADF-STEM) and (b) annular bright field STEM
(ABF-STEM) images of the amorphous PbZrxTi1xO3 (PZT) thin film after pyrolysis at 350 °C, whichis considerably lower than the crystallization
temperature of 650 °C [1]. The first Pb-O layer of
PZT crystallized coherently, while the second Pb-O
layer showed a weaker contrast.
These results correspond to a situation in which
the first Pb-O layer widely covers the SrTiO3 surface,
while the second layer covers it locally, and theinterplane distance increased. Figure 1(b) shows
the incorporation of atoms in the second Pb-O layer.
Figure 2(a) shows the Ti-L3 electron energy loss
near-edge structures (ELNES) of the amorphous
phase, interface layer, tetragonal phase, andrhombohedral phase of PZT thin films [1]. The
pyrolyzed films show ligand field splitting under Oh
local symmetry, that is, the TiO6 octahedroncoordination structure already exists in the pyrolyzed

Fig. 1 (a) HAADF- and (b) ABF-STEM images of theamorphous PZT thin film. Arrows point out justincorporating atoms on the second Pb-O layer.

Fig. 2 (a) Ti-L3 ELNES of amorphous phase, interface
layer, tetragonal phase, and rhombohedral phase of
PZT thin films. (b) Schematic model of the crystallization
process of the amorphous PZT thin film.

PZT thin film, which is apparently an amorphous
phase, as shown in Fig. 1. These results indicatethat the amorphous PZT thin film has a short-rangeorder and was incorporated into the crystalline films
via interface layers as self-organized nucleationsites (Fig.2(b)).
The present study demonstrates that theinterface layer and short-range order in amorphousfilms are among the essential factors governing thesolid-state epitaxy of oxides by CSD.
References[1] T. Kiguchi, T. Shimizu, T. Shiraishi, and T. J. Konno, J. Ceram.
Soc. Jpn. 128, 501 (2020).

Keywords: thin films, epitaxial growth, electron microscopy
Takanori Kiguchi and Toyohiko J. Konno (Materials Science of Non-Stoichiometric Compounds Research Laboratory)
E-mail: tkiguchi@imr.tohoku.ac.jp, tjkonno@imr.tohoku.ac.jp
URL: http://www.imr.tohoku.ac.jp/en/about/divisions-and-centers/research-division/26.html

38

IMR KINKEN Research Highlights 2021

Research Centers

IMR KINKEN Research Highlights 2021

ResearchResearch Centers

Dynamics of Absorption/Emission of Hydrogen Isotope in NeutronIrradiated Tungsten with the Compact Divertor Plasma Simulator
International Research Center for Nuclear Materials Science

A compact divertor plasma simulator combined with thermal absorption spectroscopy was developed in the radiation-

controlled area of the IRCNMS. The dynamics of hydrogen isotope absorption/emission for neutron-irradiated tungsten arebeing investigated, which will be essential for ongoing studies on plasma-facing materials in fusion reactors.
Studies on fusion reactors are ongoing worldwide.
These include the development of plasma-facing materials
that requires an understanding of hydrogen isotope
behavior in the materials. Systematic experiments on theabsorption/emission of hydrogen isotopes in neutronirradiated materials are of importance for this matter, but
few are available worldwide.
Recently a compact divertor plasma simulator(CDPS), as shown in Fig. 1, was developed in the radiationcontrolled area of the International Research Center for
Nuclear Materials Science (IRCNMS), Tohoku University,
as a result of collaborations with the National Institute for
Fusion Science and other universities [1]. The CDPS
produces steady-state deuterium (D) and/or helium
plasmas with densities above 1018 m-3 at a well-controlled
sample temperature of approximately 5 K. The CDPS has
a sample-carrier system, which makes it possible totransfer a plasma-exposed sample from the sample holder
to an infrared heater for analysis using thermal desorption
spectroscopy, without exposing it to the air.
We performed neutron-irradiation for tungsten, whichis the primary candidate for plasma-facing material. Then,
we investigated the dynamics of D absorption andemission [2, 3]. The total D retention was found to be
proportional to the square root of D fluence, as shown in
Fig. 2. The results suggest that the implanted D atoms firstoccupied the defects near the surface, and then thedefects located in deeper regions. The effects of postplasma annealing on D emission was also studied. It was
found that 30 h of annealing at 573 K caused D emissions,suggesting that the heat treatment of the plasma-facing
component of a fusion reactor at moderately elevatedtemperatures can contribute to the removal of accumulatedhydrogen isotopes. These results were obtained throughinternational collaborations at the IRCNMS and will beessential for ongoing and future studies of plasma-facing
materials in fusion reactors.

Fig. 1 Schematic view of the CDPS.

Fig. 2 Correlation between the plasma exposure time and D
retention in neutron-irradiated pure tungsten.

References[1] N. Ohno, T. Kuwabara, M. Takagi, R. Nishimura, M. Yajima, A. Sagara,
T. Toyama, K. Suzuki, H. Kurishita, T. Shikama, Y. Hatano, and N.
Yoshida, Plasma Fusion Res. 12, 1405040 (2017).
[2] V. Kh. Alimov, Y. Hatano, T. Kuwabara, T. Toyama, S. Someya, and A.
V. Spitsyn, Nucl. Fusion 60, 096025 (2020).
[3] T. Toyama, M. Yajima, N. Ohno, T. Kuwabara, V. Kh. Alimov, and Y.
Hatano, Plasma Fusion Res. 15, 1505081 (2020).

Keywords: nuclear materials, radiation effects, hydrogen
Takeshi Toyama (Corresponding Author, International Research Center for Nuclear Materials Science)
E-mail: ttoyama@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/40

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IMR KINKEN Research Highlights 2021

Photoactivity of an Anodized Biocompatible TiNbSn Alloy
Cooperative Research and Development Center for Advanced MaterialsAbioactive TiNbSn alloy was anodized in an electrolyte of sodium tartrate containing hydrogen peroxide(H2O2). Highly crystallized porous rutile-structured TiO2 that formed on TiNbSn exhibited superior photoactivity
and generated hydroxyl radicals under ultraviolet illumination. The photoactivity was attributed to the highly
crystallized TiO2 formed by the application of a high voltage during anodization, thus suppressing therecombination of the photogenerated carriers.
density of defects owing to self-heating during
anodization, which is attributed to the high electric
resistivity of the TiNbSn alloy. Galvanostatic
anodization increased the voltage due to the highresistivity of the alloy, and micro-arc oxidation andspark discharge associated with oxide dissolution
occurred. As a result, porous anodic oxide with arough surface was formed on TiNbSn alloy. Depth
analysis by glow discharge optical emission
spectroscopy revealed the presence of alkaline
metals Na and K originating from the electrolyte
near the interface between the oxide and TiNbSn
alloy. This suggests that these alkaline metals wereincorporated into the oxide after adsorption to thesurface of the TiNbSn alloy.
It was concluded that the application of a highvoltage promoted anodization, resulting in theformation of well-crystallized porous TiO2 and theimprovement of photocatalytic activity.

Metallic implant materials can release metallicions into biological fluids and tissues, causing
allergies and possibly granuloma formation.
Abraded micron-sized wear particles can also
cause fretting, impinging inflammatory reactions,and eventual bone resorption through theiraccumulation in surrounding tissues following longterm exposure to the biological environment ortransport to distant organs by synovial fluid, blood,or urine. Thus, the demand for biocompatible
surface coatings on metal implants has increased.
Figure 1(a) shows the plots of the photoactivity(degradation rate of methylene blue) against the
average amount of •OH generated upon ultraviolet
illumination (measured by electron spin resonance)
for as-anodized and annealed anodic oxides on Ti
and TiNbSn. Two separate regions could be
contoured for the anodic oxides on Ti and TiNbSn,
respectively. This result indicates that thephotogenerated electrons to generate •OH were nottransferred to H2O2 for TiO2 on Ti, and transferred to
H2O2 for rutile TiO2 on TiNbSn, and is related to thedefects in TiO2 inducing charge recombination [1].
The highly crystallized rutile TiO2 contains a low(a)80

MB degaradation rate / %

70
60
50

References[1] N. Masahashi, Y. Mori, H. Kurishima, H. Inoue, T. Mokudai, S.
Semboshi, M. Hatakeyama, E. Itoi, and S. Hanada, Appl. Surf.
Sci. 543, 148829 (2021).

(b)

(c)

Ti TiNbSn△ □ As_C 4 H4 Na2 O6▲ ■ An_C 4 H 4 N a2 O 6

TiO2 on TiNbSn〇 ▽ A s_C 4 H 4 N a2 O 6 + H2 O2● ▼ A n_C 4 H4 Na 2 O6 + H2 O2

40
30

TiO2 on Ti

20
10
0

0.0

0 .2

0.4

0.6

0.8

1.0

1.2

1 .4

1.6

1.8

Amount of generated hydroxyl radicals / 6M

Fig. 1 (a) Plots of the degradation rate of MB against the amounts of generated •OH upon UV illumination for as-anodized
and annealed anodic oxides on Ti and TiNbSn. (b) Schematic structural and (c) compositional illustration of the evolutionof anodic oxide on TiNbSn [1].

Keywords: biomaterial, oxide, photocatalytic
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

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Research Centers

Magnetization of YBCO and MgB2 Bulks Under High
Magnetic Fields
High Field Laboratory for Superconducting Materials

High magnetic field trapping has been studied for Y-Ba-Cu-O (YBCO) and MgB2 superconducting bulks. We
achieved the high magnetic field trapping of 15.1 T in YBCO bulks and 5.58 T in Ti-doped MgB2 bulks by thefield-cooled magnetization method. The trapped field of 5.58 T is a world record among MgB2 bulk magnets.

FCM (Bex=18 T@HM, Ts=28K)

B (T)

20

B

0

ramp endT

15.1 T (HP#1)

Bex

10

YBCO bulks
0

100

200

8
5.58 T (HP#2)

6
42

Ti-doped MgB bulks
2

0
0

10

20

IMR KINKEN Research Highlights 2021

30

40

T (K)
Fig. 2 Temperature-dependence of the trapped field forthe triple-stacking Ti-doped MgB2 bulks.

References[1] T. Naito, H. Fujishiro, and S. Awaji, J. Appl. Phys. 126, 243901(2019).
[2] T. Naito, Y. Takahashi, and S. Awaji, Supercond. Sci. Technol.
33, 125004 (2020).

Keywords: high temperature superconductor, high magnetic fieldTomoyuki Naito (Corresponding Author, Faculty of Science and Engineering, Iwate University)
E-mail: tnaito@iwate-u.ac.jp
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

42

300

Time (min.)Fig. 1 The filed trapping process during FCM from 18 T at
28 K for the YBCO bulk pair.

BT (T)

High-temperature superconducting (HTS) bulk
magnets can be used as a quasi-permanent
magnet. In particular, they can trap very highmagnetic fields that cannot be obtained byconventional permanent magnets. High-field HTS
bulk magnets have novel superconducting
applications, such as a compact bench-top NMR/
MRI. The trapped magnetic field is limited by thecritical current density Jc, the mechanical strength,
and the superconducting stability of bulk materials.
The stacking of the Y-Ba-Cu-O (YBCO)
superconducting bulk-pair reinforced by a stainlesssteel (SUS) container was magnetized by fieldcooled magnetization (FCM) under magnetic fields
of up to 22 T using the 28 T hybrid magnet (28THM). Numerical simulations were used to optimize
the structure of the SUS container against theelectromagnetic hoop stress of approximately
several hundred MPa. We obtained the trappedmagnetic field of 15.1 T by FCM from 18 T at 28 K at
the center of the YBCO pair, as shown in Fig. 1.
However, cracking partially occurred at the
periphery of both YBCO bulks during FCM from 22
T at 23 K. The bulk fracture originated from thethermal shock by the flux jumps [1].
Ti-doped MgB2 bulks were prepared by a hot
isostatic pressing method. We succeeded in thehigh-field trapping of 5.58 T by FCM from 6 T at 11.3
K for the triple-stacking Ti-doped MgB2 bulks [2].
We found that the Ti-doped MgB2 bulks had theability to trap a 7 T class magnetic field attemperatures below 4.2 K, as shown in Fig. 2;
however, the FCM experiments at both 2.2 and 4.2
K were disturbed by flux jumps.
The experimental results suggest that thermal
instability triggered the large-scale vortex jumps
and large heat generation. Hence, it is important toimprove the thermal stability of the bulks to realize astronger trapped field magnet.

IMR KINKEN Research Highlights 2021

Scientific Breakthrough Toward 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
institute-wide effort that pairs research groups at IMR across the innovation spectrum to solve current industrial
problems and prepare for future global energy problems. The Center was established in 2016 by an agreement
between IMR, the Osaka Prefecture government, and enterprises in the Kansai area sponsored by government;
it took over Osaka and Kansai center projects.
In April 2016, the Trans-Reginal Corporation
Center was established in Osaka as a special unit of
the Institute for Materials Research (IMR), Tohoku
University, as a result of an agreement with the
Osaka Prefecture government. The Center issponsored by the Ministry of Education, Culture,
Sports, Science and Technology, which took over
the Kansai Center and carried out its mission from2012 to 2016. The Center has three missions. First,
to solve technical problems that industries havestruggled with. Second, to introduce academic
output to industries for application to society. Third,
to help educate next-generation materials scientists
and researchers in universities and enterprises. The
Center organized the forum “Monodukuri Kisokoza,”
which focuses on special topics in materials andprocessing. Projects are conducted throughcollaborations with the government, universities,
research institutions, and other organizations. The
Center has four venues to cover the Kansai area: theOsaka office at Osaka Prefecture University, theHyogo office at University of Hyogo, the Sendai
office in IMR, and Monozukuri Bussiness Information
Center Osaka in the Creation Core Higashi-Osaka.
The Center has developed materials andprocesses in collaboration with industries. These
includeathermal-sprayedamorphousmagnetostrictive film developed by IMR and USUICo., LTD. New magnetic amorphous alloys with ahigh glass-forming ability have been developed,
such as the (Fe, Co)-Si-B-Nb alloy. Amorphousalloys are known to exhibit low coercivity, although
their magnetostriction is relatively large. Some
properties of amorphous magnetostrictive films withhigh glass-forming ability and the constitution of thetorque sensor have been investigated. The
amorphous (Fe, Co)-Si-B-Nb spraying films withhigh adhesion strength were formed on the surface

Fig. 1 (a) The shaft for rotating-torque sensor with thermalspraying film of (Fe, Co)72Si4B20Nb4 amorphous alloy, and(b) the developed product of the rotating-torque sensor.

of the shaft using a spray-quenching system and bycontrolling the substrate temperature (Fig. 1(a)). The
shaft developed with the amorphous thermal
sprayed alloy generated an output voltage five times
higher compared with the Fe shaft machined into
the surface morphology of the chevron pattern [1].
These products of the torque sensor (Fig. 1(b)),
which assembled the shaft with the spraying films,
have excellent sensor properties, such as highsensitivity and low hysteresis measurement error[2].
Similar collaborative research for practicalapplications utilizing academic knowledge andindustrial technology are in progress. The Center
will make an effort to innovate in the metallic material
industry through alliances and by promoting
research on materials science.
References[1] M. Komaki, H. Ashino, S. Kamata R. Tsunemi, Y. Saotome, and
K. Amiya, Japan Patent, 5940531-B2, 2016-5-27.
[2] Tohoku Techno Arch Co., Ltd., Success Stories, https://www.ttechnoarch.co.jp/content/licence_T12-094.html(15. Nov. 2020)

Keywords: metallic glass, magnetic properties, devices
Kenji Amiya (Corresponding Author, Trans-Regional Corporation Center for Industrial Materials Research)
E-mail: amiya.k@imr.tohoku.ac.jp
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

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Research Centers

Comparison of Spin and Polarization Caloritronics
Collaborative Research Center on Energy Materials

T

he spontaneous order of electric and magnetic dipoles in ferroelectrics and ferromagnets, even at high temperatures, is bothfascinating and useful. Spintronics studies the transport of magnetic order in the form of spin currents, but the polarization
current of the ferroelectric order has escaped attention. We present a time-dependent diffusion theory for heat and polarizationtransport in a planar ferroelectric capacitor with parameters derived from a one-dimensional phonon model. We predicted thesteady-state Seebeck and transient Peltier effects.
Ferromagnetism and ferroelectricity describe the orderof magnetic and electric dipoles that spontaneously form,
often far above room temperature and have much incommon [1]. The robustness of the order and the associatedstray magnetic and electric fields give rise to numerous
technological applications that affect our daily lives.
However, the physics appears to be very different. The
Amperian electric (Gilbertian magnetic) dipoles break the(conserve) inversion symmetry but conserve the (break)time-reversal symmetry. Nevertheless, the phenomenologyof these material classes displays similar analogies. The
dipolar order is staggered in antiferromagnets andantiferroelectrics. The electrocaloric (magnetocaloric) effectis based on the dependence of the entropy of electric(magnetic) dipolar ensembles as a function of the applied
electric (magnetic) field and temperature [2]. Both themagneto- and electrocaloric heat pumps appear to beclose to the market. However, an equivalent for spin
caloritronics [3] in ferroelectric materials does not exist.
Motivated by the need to find new uses for ferroelectricmaterials in energy applications and supported by the coappointment scheme between NIMS and Tohoku University,
we studied the caloritronics of ferroelectric capacitors [4],i.e., ferroelectric insulators sandwiched between two metal
contacts.
The linear response relation in a ferromagnetic
insulator (Ohm’s law) is as follows [3]:where Js is the spin current, Jq is the heat current, G is
the spin conductance, T is the temperature, S is the Seebeckcoefficient, Π is the Peltier coefficient, Κ the heat
conductance ∆H is the magnetic field difference, and ∆T is
the temperature difference between the devices. For a
ferroelectric insulator, the equation is almost the same [4].

Fig. 1 A temperature gradient over a ferroelectric capacitor (left)drives a polarization current. For opaque interfaces, this leads topolarization accumulation µ on the scale of the diffusion length
λ. The latter may generate an observable thermovoltage V.

However, the spin current is replaced by a polarization
current, the drive is an electric instead of a magnetic field
difference, and the material constants have other units.
Excited magnetic or electric polarizations are notconserved but characterized by material-dependent
lifetimes. In diffusion approximation, this leads to spin (ormagnon) accumulation in magnets and polarization
accumulation in ferroelectrics, parameterized by nonequilibrium chemical potentials. The solution of the diffusion
equation shows that the accumulations exist near interfaces
on the scale of the diffusion length λ, as shown in Fig. 1. The
spin diffusion length can be tens of micrometers in yttrium
iron garnets, but there is no data on ferroelectrics. In theabsence of experiments, we estimated the model
parameters using a simple one-dimensional phonon modelof elastically coupled electric dipoles [4].
References[1] N. A. Spaldin, Topics Appl. Phys. 105, 175 (2007).
[2] S. Crossley et al., AIP Advances 5, 067153 (2015).
[3] G.E.W. Bauer, E. Saitoh, and B.J. van Wees, Nature Mat. 11, 391 (2011).
[4] G.E.W. Bauer, R. Iguchi, and K. Uchida, Phys. Rev. Lett., (2021) inpress.

Keywords: dielectric, electronic material, electronic properties
Gerrit Bauer (Corresponding Author, Theory of Solid State Physics Research Laboratory)
E-mail: g.e.w.bauer@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

44

IMR KINKEN Research Highlights 2021

IMR KINKEN Research Highlights 2021

Unique Power-Law Scaling in Nanoindentation Pop-ins
Center for Computational Materials SciencePower laws are obeyed in many scientific fields, including the plasticity of materials. Large-scale molecular
dynamics nano-indentation simulation and nano-indentation testing showed that the power law statistics in thedisplacement burst (pop-in) magnitude of the nano-indentation is unique. Thus, the power law exponents aremuch higher than those typically observed in plasticity.
Power laws are ubiquitous and actively studiedin many scientific fields, including the plasticity ofmaterials. We first performed large-scale molecular
dynamics (MD) simulation and nano-indentation
testing, and then reported the power law statistics inthe second and subsequent pop-in (displacement
burst) during the load-controlled nano-indentation
test [1]. The first pop-in was characterized by
Gaussian-like statistics with a well-defined averagevalue. The transition from Gaussian-like to a powerlaw is because of the change in the deformation
mechanism from dislocation nucleation to dislocation
network evolution in the sharp-indenter-induced
abruptly decaying stress and dislocation density
fields.
The nano-indentation tests on the (100) and(111) surfaces of body-centered cubic (BCC) ironshowed that the scaling exponents of the powerlaws were 5.6 and 3.9, respectively. These powerlaw exponents are much higher than those typicallyobserved in micropillar plasticity (1.0 - 1.8),
suggesting that the nano-indentation plasticity
belongs to a different universality class than
micropillar plasticity.
Figure 1 shows the large-scale molecular
dynamics nano-indentation simulation results for the
first pop-in, and the subsequent pop-in observed in
the nano-indentation simulation on the Fe (100)surface. Dislocation nucleation, dislocation network
activities, and the corresponding stress-fieldchange, which are the origins of the high power-law
exponent, were clearly simulated. Figure 2 shows
the power-law statistics of the subsequent pop-in
magnitude obtained by MD and actual nanoindentation testing. The MD results reasonably
agreed with the experiment. Both MD andexperiment showed very high exponents of 5.0 and
5.6, respectively.

Fig. 1 Molecular dynamics nano-indentation simulation
results: (a) first pop-in and (b) subsequent pop-in. The
upper panels show dislocation activities. The lower
panels show stress field change beneath the indentor
tip [1].

Fig. 2 Power law statistics in nano-indentaion pop-ins for
BCC Fe(100): (a) MD (load drop) and (b) experiment(displacement burst). β is the power-law exponent [1].

References[1] Y. Sato, S. Shinzato, T. Ohmura, T. Hatano, and S. Ogata,
Nature Communications 11, 4177 (2020).

Keywords: mechanical properties, molecular dynamics
Shigenobu Ogata (Corresponding Author, Graduate School of Engineering Science, Osaka University)
E-mail: ogata@me.es.osaka-u.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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Research Centers

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 a background in novel materials
science at the Institute for Materials Research, Tohoku University. This center manages three neutron instrumentsin the research reactor facility JRR-3, under a general user program; and a brand new spectrometer POLANO
at the Japan Proton Accelerator Research Complex/Materials and Life Science Experimental Facility. In thisreport, we describe the current status of the research program and the preparations for the restart of JRR-3.
The Center of Neutron Science for Advanced
Materials (CNSAM) was established in 2010 topromote novel materials science research using
neutrons. CNSAM operates one diffractometer(HERMES) and two neutron spectrometers (TOPAN
and AKANE) at the JRR-3 research reactor of theJapan Atomic Energy Agency. HERMES can
precisely determine the arrangement of atoms andspins, while their motions are examined by AKANE
and TOPAN. These are used for target energy andmomentum transfer regimes. POLANO, located at
the Materials and Life Science Experimental Facility(MLF) at the Japan Proton Accelerator ResearchComplex (J-PARC), is a brand new spectrometer
designed to perform polarization analysis in higher
energy regions beyond those measured byconventional triple-axis spectrometers. POLANO
and TOPAN are equipped with polarized options
and can disentangle nuclear and magnetic
contributions. We aim to contribute to condensed
matter physics and materials science with themultipurpose neutron instruments of PATH. Figure 1
shows the PATH, which is a collective term forneutron instruments managed by the CNSAM.
To achieve this purpose, we established aninternational collaborative research program in IMRfrom FY 2019. We provide opportunities not only forexperiments with neutron beams but also for thedevelopment of neutron devices and analysis
methods, and even for theoretical work with ourneutron data. Proposals can be submitted under
several categories, such as “Neutron experiment at
MLF,” “Instrumental development,” “Development inanalysis method,” “Structural analysis and data
handling,” and “Others”. Under the category“Others,” proposals on the complementary use ofneutron and other quantum beams, such as X-ray
and muon beams, are accepted. In FY 2020, we
received 17 and 6 proposals from domestic andoverseas researchers, respectively. Despitedifficulties in implementing the program this year,

Fig. 1 A photograph of neutron scattering instruments
managed by the Center of Neutron Science for Advanced
Materials, Tohoku University.

we continue to develop a compact spin-exchange
optical pumping system under a collaboration with
the High Energy Accelerator Research Organization.
This system will be installed in POLANO as a spin
filter for high-energy neutrons [1]. Furthermore, as
the restart of the JRR-3 is scheduled at the end of
FY 2020, we are preparing for the general user
program at JRR-3 and checking the operation of theabove instruments. We made fact sheets for each
instrument, which were broadly distributed toresearchers and potential users, and promoted theuse of these instruments during online meetings.
References[1] T. Okudaira et al., Nucl. Inst. Meth. A 977, 164301 (2020).

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, http://qblab.imr.tohoku.ac.jp

46

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IMR KINKEN Research Highlights 2021

Superconductivity, Quantum Phase Transitions, andFermi Surface of Strongly Correlated f-Electron Systems
International Collaboration CenterWe investigated Ir-doped CeRhIn5, in which quantum criticality occurs at a critical concentration xc = 0.6 by means of the deHaas–van Alphen (dHvA) effect. Our results suggest that that the Fermi surfaces (FSs) and effective masses in CeRh1-xIrxIn5
remain unchanged up to at least the critical concentration of Ir. This study will be continued, if necessary, in high magnetic fields
up to 36 T at Grenoble. To this end, new samples of CeRh1-xIrxIn5 are being grown.
CeRhIn5 is among the best-studied heavy fermion(HF) materials. This tetragonal antiferromagnetic (AFM)
compound with TN = 3.8 K can be tuned to a quantum
critical point (QCP) by pressure, chemical substitution, andmagnetic field. The f electrons of CeRhIn5 are localized atambient pressure. As the critical pressure Pc = 2.3 GPa isreached, all dHvA frequencies observed at P < Pc change
discontinuously, indicating an abrupt FS reconstruction as aconsequence of f-electron delocalization.
Another way to tune CeRhIn5 to quantum criticality is tosubstitute Rh by Ir [1], as shown in Fig. 1(a). Upon doping
CeRhIn5 with Ir, the antiferromagnetic order is suppressedat the critical concentration xc = 0.6, giving rise to a QCP.
Above the Ir concentration of approximately x=0.3, acommensurate magnetic order develops and co-exists with
the incommensurate structure up to xc. Finally,
superconductivity was observed over a wide concentration
range 0.3 < x < 1. Note that the commensurate AF wave
vector was found in 10% Cd-doped CeIrIn5 in our previous
studies [2].
The FSs and effective masses of the end compounds,
CeRhIn5 and CeIrIn5, are well known. CeRhIn5 ischaracterized by FSs with localized f electrons and slightly
enhanced effective masses. In contrast, CeIrIn5 has FSs
with itinerant f electrons and strongly enhanced effective
masses. How and where the FSs and effective masseschange as a function of Ir concentration is one of thequestions that we addressed. To this end, high-quality
single CeRh1-xIrxIn5 crystals with different Ir concentrations
were grown. Figure 1(b) shows the Fourier spectra of thedHvA oscillations in CeRh0.8Ir0.2In5 and CeRh0.4Ir0.6. For
both compositions, the observed dHvA frequencies andeffective masses were the same as in pure CeRhIn5. Thissuggests that the FSs and effective masses in CeRh1-xIrxIn5
do not change up to at least the critical concentration of Ir.
This finding should, however, be treated with caution, as amore recent microprobe analysis revealed a significant
difference between the real and nominal Ir concentrations insome samples. Therefore, new samples of CeRh1-xIrxIn5 are

Fig. 1 (a) Temperature-composition phase diagram of CeRh1-xIrxIn5
with three distinct long-range orders: superconducting (SC),incommensurate antiferromagnetic (I), and commensurate
antiferromagnetic (C) (adopted from [2]). (b) Fourier spectra of
the dHvA oscillations in CeRh0.8Ir0.2In5 and CeRh0.4Ir0.6In5.

being grown. All the new crystals will be tested by
microprobe and specific heat measurements prior to dHvA
studies, and, if necessary, in high magnetic fields up to 36
T at Grenoble.
References[1] A. Llobet, A. D. Christianson, W. Bao, J. S. Gardner, I. P. Swainson, J. W.
Lynn, J.-M. Mignot, K. Prokes, P. G. Pagliuso, N. O. Moreno, J. L. Sarrao,
J. D. Thompson, and A. H. Lacerda, Phys. Rev. Lett. 95, 217002 (2005).
[2] K. Beauvois, N. Qureshi, R. Tsunoda, Y. Hirose, R. Settai, D. Aoki, P.
Rodière, A. McCollam, and I. Sheikin, Phys. Rev. B 101, 195146 (2020).

Keywords: heavy fermion, fermi surface, electronic structure
Ilya Sheikin (Corresponding Author, LNCMI-Grenoble, CNRS)
E-mail: ilya.sheikin@lncmi.cnrs.fr
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/

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Research Centers

Dynamical Vortex Phase Diagram of Two-Dimensional
Superconductor in Ion-Gated MoS2
Laboratory of Low Temperature Materials ScienceT

wo-dimensional superconductors with high crystallinity, realized by exfoliation and ionic gating, have revealed
various intrinsic properties of quantum vortices. Recently, we found a current-induced phase transition at zeromagnetic field in ion-gated MoS2. This dynamical phase transition can be explained by the appearance of thephase-slip line with Josephson-like ultrafast vortex-antivortex flow.
Two-dimensional (2D) materials with highcrystallinity provide ideal platforms to explore thenovel phenomena of vortex matter when they
become 2D superconductors by exfoliation andionic gating. While various vortex phases [1] andtheir dynamics [2] under magnetic fields have been
intensively studied in such 2D superconductors, thedynamical properties at zero magnetic field are
relatively unexplored. In conventional dirty 2Dsuperconductor films, it is known that the truesuperconducting state with zero ohmic resistance isachieved at the low-current limit by binding thermally
excited vortex and antivortex (V-AV) pairs throughthe Berezinskii–Kosterlitz–Thouless (BKT) transition.
However, the V-AV dynamics beyond the lowcurrent region remains not completely understood,
probably because of the effect of disorder on
vortices. We dealt with this issue using a gateinduced 2D superconductor, MoS2, with minimal
disorder by measuring the current-voltage (I-V)
characteristics [3].
In the low-current region, we obtained thepower law behavior V ∝ Iα below the mean-fieldtransition temperature Tc0 = 7.6 K, and firm evidenceof the BKT transition with the universal jump of α
from 1 to 3 at TBKT = 6.0 K, which indicate that oursystem is a purely 2D superconductor. With
increasing I, we found successive step-likeincreases in V(I) at the characteristic currents Ic1, Ic2,
and Ic3; and in the T-dependence of the nonlinear
resistance R NL at T1 and T2, respectively. Eventually,
we obtained the I-T dynamical phase diagram
shown in Fig. 1. Based on the calculation of thevortex velocity vf, the anomaly at Ic1 and T1 can beconsidered as the dynamic transition of the V-AV
pairs from the BKT state with vf ~ 103 m/s (Abrikosov
vortex flow) to the ultrafast dynamical state with vf ~105 to 10 6 m/s (Josephson-like vortex flow)

C: phase slip line region
Phase slip line

I
Josephson-like V-AV
ﬂow

A: BKT region

I

Abrikosov V-AV ﬂow

IMR KINKEN Research Highlights 2021

anti-vortex(AV)I

vortex(V)
Abrikosov V-AV ﬂow

Fig. 1 Current-temperature (I-T) phase diagram of iongated MoS2 at zero magnetic field [3]. Color mapping
displays the magnitudes of dR NL /dT. Ic1, Ic2, and Ic3 arecharacteristic currents at which the step-like increase in
I-V curves are observed. T1 and T2 are the peaktemperatures of dR NL /dT. T3 is the onset temperature of
R NL. Schematic images of the flow of vortices andantivortices in region A (BKT region: lower left), region B(BKT critical region: lower right), and region C (phaseslip line region: upper left) are shown.

accompanying the phase-slip line across the
conducting plane (upper left in Fig. 1). I3 and T2
correspond to the transition to the partially normal or
fluctuation region. The unexpected richness of the
dynamical vortex diagram in Fig. 1 originates fromthe combination of the weak pinning force, lowresistance, and enhanced fluctuations at the 2D
limit in our high-crystallinity gated superconductors.
References[1] Y. Saito, T. Nojima and Y. Iwasa, Nat. Commun. 9, 778 (2018).
[2] Y. M. Itahashi, Y. Saito, T. Ideue, T. Nojima, Y. Iwasa, Phys.
Rev. Research 2, 023127 (2020).
[3] Y. Saito, Y. M. Itahashi, T. Nojima, Y. Iwasa, Phys. Rev. Mater.
4, 074003 (2020).

Keywords: superconducting, 2D materials, field-effect transistor
Tsutomu Nojima (Corresponding Author, Laboratory of Low Temperature Materials Science)
E-mail: nojima@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/

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B: BKT cri�cal region

IMR KINKEN Research Highlights 2021

Research Facility for Physics and Chemistry of Radioactive
and Nuclear Materials
Laboratory of Alpha-Ray EmittersOver 170 species of radioisotopes and nuclear materials are available for study at the Laboratory of Alpha-Ray
Emitters. This laboratory is one of the most important centers in the world for studying the physical and chemical
properties of radioactive materials, such as actinide compounds. Researchers from many of the leading universities
and institutes all over Japan visit this facility every year to prepare a variety of materials and carry out chemical andphysical measurements.
The Laboratory of Alpha-Ray Emitters offers a
research environment for the study of 170 radionuclides
and elements, especially alpha-ray emitters, such as
actinide. This lab is a source for pure crystals of
actinide compounds, supplying them to otheruniversities and synchrotron orbital radiations. Overthe past decade, Actinium-225 (Ac-225) has been
provided to other universities and research centersthrough inter-university collaborations. The radiationcontrolled area of this laboratory includes threechemical rooms, three physical rooms, etc., which areequipped with local exhaust ventilation systems thatmake the handling of various kinds of radioactive
materials possible. Many spectrometers, including
those for gamma rays and alpha rays, are available.
Ac-225 (half-life: 9.92 d) and Radium-223 (Ra223, half-life: 11.4 d) are known for their effectivenessin targeted alpha therapy (Fig. 1). Despite theavailability of testimonies on the effectiveness ofradiopharmaceuticals, very few investigations havebeen conducted in Japan. One reason is the fact that
Ac-225 is subject to rigid regulation as an alpha-ray
emitter. On the other hand, Ra-223 chloride salt has
been approved in Japan as a drug. To the best of ourknowledge, information on the dispersion of both Ac225 and Ra-223 from an aqueous solution is still rare.
In our laboratory, we developed a special
ventilation system for measuring the dispersion of
those nuclides and their progeny nuclides (Fig. 2) [12]. This evaluation is especially important to establish
guidelines for the control standard, especially for
useful short-life alpha-ray emitters in practical use.
Recently, we evaluated the dispersion of Ac-225 using
an alpha-ray spectrometer. To obtain the starting
material of Ac-225, we used the Thorium-229 (Th-229)
milking method. Th-229 was separated from
Uranium-233 stored in our laboratory. The dispersion
rate of Ac-225 and its progeny nuclides was wellidentified. The dispersion rate was obtained in thefollowing order: DOTA complex in neutral > salt in

Fig. 1 The decay scheme of Ac-225 and Ra-223.

Fig. 2 Special ventilation system for dispersion measurement.

neutral > acidic, in the liquid condition. Even in thehighest, the dispersion rate was less than 10 -4/h,
which was 1/100 of the current regulation value.
References[1] T. Yamamura, K. Shirasaki, H. Kikunaga, K. Nagata, Zi. J. Zhang,
K. Washiyama, A. Toyoshima, T. Yoshimura, and A. Shinohara,
Radiation Safety Management. 19, 35 (2020).
[2] K. Nagata, K. Shirasaki, A. Toyoshima, K. Ooe, T. Yamamura, A.
Shinohara, and T. Yoshimura, Radiation Safety Management 19, 1(2020).

Keywords: actinide, alpha-ray spectroscopy
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

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Research Centers

Direct Observation of Atomic Columns and Identification of
Elemental Species in New Layered Perovskite [RbCeTa2O7]
by Cs-Corrected STEM
Analytical Research Core for Advanced MaterialsTo examine the crystal structure of a novel Ce-based layered perovskite, RbCeTa2O7, the detailed atomic arrangement
was observed under a spherical aberration collected by a scanning transmission electron microscope (Cs-corrected
STEM). The metallic atom and oxygen columns were identified by energy-dispersive spectroscopy mapping and STEMannular-bright-field methods, respectively.
Layered perovskite compounds with two-dimensional
structures consisting of lantha-nide and transition metals
exhibit various properties. In particular, Dion–Jacobson and
Ruddlesden–Popper-type ion exchangeables havereceived much attention for photo-functional applications.
In this study, we developed a new Ce-included layered
perovskite compound RbCeTa2O7 [1]. XRD analyses
showed that it has a Dion–Jacobson-type layered perovskite
structure with the space group of P4/mmm, as shown in Fig.
1(a).
For further crystallographic characterization, direct
observation of atom columns and identification of theelements occupying each column were carried out using
scanning transmission electron microscopy (STEM). The
thin film specimen for STEM was prepared by an Ar ion
slicer method using resin-filled powder samples. STEM
observations were performed at an acceleration voltage of
200 kV using the JEOL JEM-ARM200F equipped with aspherical aberration corrector in both the illuminating andimaging systems. The low magnification observation image
in Fig. 1(b) shows that the samples with flake-like morphology
were clustered together; hence, a single crystal region
without overlap was selected for high-resolution
observations. The high-resolution high-angle annular dark
field (HAADF) images in Fig. 1(c) show stacked periodic
structures with bright contrast. These bright spots
correspond to relatively heavy elements: Ta, Ce, and Rb. In
contrast, in the annular bright field (ABF) image, lighter
elements have bright contrast, as shown in Fig. 1(d); even
oxygen columns exhibit gray contrast. Energy-dispersive
spectroscopy (EDS) mapping was performed to identify theelemental species in each column consisting of metals andlanthanides, as suggested by HAADF observations. The
high-resolution EDS mapping result is shown in Fig. 2.
The STEM results show good agreement with thestructural model of the layered perovskite compound
RbCeTa2O7 based on the x-ray diffraction experiment.

Fig. 1 (a) Schematic drawing of crystal structure of RbCeTaO7,(b) low-magnification, (c) HAADF, and (d) ABF-STEM imagesof RbCeTaO7.

Fig. 2 EDS column mapping of RbCeTaO7. Crystal structure
model of RbCeTaO7 (inset).

References[1] T. Hasegawa, A. Shigee, Y. Nishiwaki, M. Nagasako, A. T. Hanindriyo, K.
Hongo, R. Maezono, T. Ueda, and S. Yin, Chem. Commun. 56, 8591
(2020).

Keywords: electron microscopy, crystal structure
Makoto Nagasako (Corresponding Author, Analytical Research Core for Advanced Materials)
E-mail: nagasako@tohoku.ac.jp
Toyohiko J. Konno (Head of Analytical Research Core for Advanced Materials)
E-mail: tjkonno@imr.tohoku.ac.jp
URL: http://www.imr.tohoku.ac.jp/en/about/divisions-and-centers/service-divisions/03.html

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International Collaborative Research

IMR KINKEN Research Highlights 2021

Research

International Collaborative Research

Toroidal State in {Fe8Dy8} Ring Identified by High-Frequency
Electron Paramagnetic ResonanceW

e synthesized a new ring shaped magnet {Fe8Dy8} in which the toroidal magnetic state is realized by the cooperationof a strong anisotropy of Dy ion and a substantial ferromagnetic exchange coupling between Dy and Fe ions. The highfrequency electron paramagnetic resonance measurement revealed the ferromagnetic exchange, otherwise thedetermination is difficult.
In magnetic recording materials, small ferromagnetic
particles or ferromagnetic domains with unidirectionallyaligned magnetic moments have been used for decades.
When the size of the recording approaches the nanoscale,such uniform domain materials show a fundamental
vulnerability for weak magnetic fields in the environment.
In future societies with advanced electrification, themagnetic fields around us will increase substantially. Thishighlights the need to develop magnetic recording
materials based on a novel concept.
Magnetic molecules are promising candidates forultra-dense information storage applications. One of thefeatures is the diversity of structures, such as linear,
zigzag, star, and ring. Among them, the ring structure
shows promise for the possible realization of toroidal
magnetic structures. There are two distinguished states ina toroidal structure: the left and right windings. When
information is stored as these windings, such states arestable and insensitive to magnetic perturbations. This
would be an ideal candidate for nanoscale molecularmagnetic storage.
We synthesized a new toroidal magnet called{Fe8Dy8}, in which 16 magnetic metal ions can bealternately arranged into a macrocycle [1]. The possibilityof the toroidal arrangement is given by the magnetization
curve, which shows no spontaneous ferromagnetic
moment at zero field, but a slow increase toward higher
fields. The magnetization at 7 T was only 40% of the fullsaturated moment, which indicates two possible
arrangements of magnetic moments: ferri-magnetic andtoroidal. In the ferrimagnetic case, Dy3+ and Fe3+ ionswere coupled antiferromagnetically, and the strong
anisotropy was responsible for the slow increase inmagnetization. Note that the easy axis of the anisotropy of
Dy3+ must rotate in the ring from one Dy3+ site to the next,
because there is no large spontaneous moment. When astrong magnetic field is applied, the change from theferrimagnet to ferromagnet will appear in the magnetization
curve because the exchange coupling between 4f and 3dions is often weak.
In the toroidal arrangement, the magnetic momentsof both Dy and Fe were aligned along the tangential

Fig. 1 Structure of {Fe8Dy8} molecule. Fe and Dy are represented
by large red and sky blue balls. The Dy easy axis is located in
the molecular plane.

direction of the ring and were confined in the ring planeowing to the strong anisotropy of Dy ions. The collaboration
between ferromagnetic coupling and the strong anisotropy
of Dy ions is the key to realizing a toroidal arrangement.
The key experiment distinguishing these two cases
is the determination of Dy-Fe exchange coupling by highfrequency electron paramagnetic resonance (EPR) [2].
We observed the systematic frequency-dependence of
the EPR spectra at frequencies between 135 GHz and
405 GHz using a THz-ESR system developed by themagnetism division of the Institute for Materials Research,
Tohoku University. It shows the linear frequency-field
relation with a zero field gap of approximately 110 GHz(5.3 K). The g-values determined by the slope of themode shows that the signal is the transition in the Fe3+
multiplet, and the level is exchange-biased by theferromagnetic coupling between Fe and Dy ions. The totalmagnetic coupling energy is 26.4 K for one Fe-Dy pairand over 400 K for the whole molecule. The present
results provide a useful recipe to design a toroidal
magnetic molecule.
References[1] H.-L. Zhang, Y.-Q. Zhai, L. Qin, L. Ungur, H. Nojiri, and Y.-Z. Zheng,
Matter 2, 1481 (2020).
[2] H. Nojiri, Y. Ajiro, T. Asano, and J.-P. Boucher, New J. Phys. 8, 218(2006).

Keywords: high magnetic field, toroidal magnet, electron paramagnetic resonance (EPR)
Hiroyuki Nojiri (Magnetism Research Laboratory)
E-mail: nojiri@imr.tohoku.ac.jp
URL: http://www.hfpm.imr.tohoku.ac.jp

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IMR KINKEN Research Highlights 2021

Origin of Luminescence in Y2SiO5:Ce And Lu2SiO5:Ce Single Crystals

We studied the photoluminescence characteristics of Ce -doped rare-earth oxyorthosilicates RE SiO (RE:Y, Lu) and
3+

2

5

evaluated the concentrations of different Ce3+-related centers from Electron Paramagnetic Resonance (EPR) spectra. We
elucidated the role of Ce3+-related centers in the luminescence of these crystals and proposed a new interpretation of theirluminescence spectra.
Ce3+-doped rare-earth (RE) oxyorthosilicates RE2SiO5(RE:Y, Lu) have been extensively studied as promising
scintillators. Owing to their high density, short luminescencedecay time, high light yield, high stopping power, and goodmechanical and chemical properties, single crystals(Lu,Y)2SiO5:Ce (LSO:Ce and LYSO:Ce) have been
employed in detectors in medical imaging, nuclear physics,
and several other applications [1]. Y2SiO5:Ce (YSO:Ce)possesses comparatively poor scintillation characteristics.
However, it has a lower melting point, which is preferablefrom a technological point of view, and it has been
employed in applications such as detectors and imaging
screens in high-tech devices.
In their luminescence spectra, the intense violet (V)doublet emission band is located near 3 eV (Ce1 emission)and is ascribed to the allowed 5d → 4f transitions of the
Ce3+ ions located at the RE1 site (the Lu1 site in the LSO
lattice). A weaker broad blue (B) emission band wasobserved at approximately 2.75 eV. The origin of the B
emission is not yet understood and is usually ascribed to
the Ce3+ ions at the RE2 site (Ce2 centers) [2].
The investigated high-quality single crystals weregrown at Tohoku University [3].
The Ce3+-related emission spectra (Fig. 1) indicate astrong difference in the shapes of the Ce3+-related V and B
emission bands. The decomposition of the V emission
band results in two doublet bands with significantly different
intensities arising from the Ce3+-related centers of two
types, Ce1, and Ce2 centers. The decomposition of the Bband into three Gaussian components indicates a complex
structure of the B band.
Under excitation approximately 3.85 eV, the B and V
emission intensities are proportional to the absorption
coefficient, and consequently, to the number of the centers
responsible for these emission bands. The intensity of thedominant V emission depends linearly on the Ce3+ content,
which is characteristic for the single Ce3+ ions. According
to the EPR data, the Ce2 fractional content decreases withincreasing Ce3+ content. The data obtained indicate that B
emission cannot arise from single Ce3+ centers.
The detailed analysis of the experimental data on theluminescence and EPR characteristics allows a novel

Fig. 1. (a) V emission and (b) B emission spectra and theirapproximate decomposition into Gaussian components(dashed and dotted lines).

conclusion that is summarized in Ref [4]:(i) B emission cannot arise from Ce2 centers, as therelative number of these centers and B emission intensity
have opposite dependence on Ce3+ concentration;(ii) A considerable difference in the luminescencecharacteristics of the V and B emissions cannot be
explained by different positions (Ce1 and Ce2) of single
Ce3+ ions;(iii) The Ce1 and Ce2 centers in LSO:Ce and YSO:Ceshould have similar luminescence characteristics.
References[1] C.L. Melcher and J.S. Schweitzer, IEEE Trans. Nucl. Sci. 39, 502 (1992).
[2] C.L. Melcher, M. Schmand, M. Eriksson, L. Eriksson, M. Casey, R. Nutt,
J.L. Lefaucheur, and B. Chai, IEEE Trans. Nucl. Sci. 47, 965 (2000).
[3] A. Yoshikawa, M. Nikl, G. Boulon, and T. Fukuda, Opt. Mater. 30, 6(2007).
[4] V. Babin, V.V. Laguta, M. Nikl, J. Pejchal, A. Yoshikawa, and S.
Zazubovich, Opt. Mater. 103, 109832 (2020).

Keywords: scintillator, EPR, luminescence
Vladimir Babin (Corresponding Author, Institute of Physics, The Czech Academy of Sciences)E-mail: babinv@fzu.cz
Akira Yoshikawa (Advanced Crystal Engineering Research Laboratory)
E-mail: yoshikawa@imr.tohoku.ac.jp
URL: http://yoshikawa-lab.imr.tohoku.ac.jp/index-e.html

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International Collaborative Research

Undetectably Low Concentration of Quenched-In Vacancies in
High-Entropy Alloys/Mechanical Property Evolution in BetaTitanium Alloys via Diffusionless Isothermal Omega Transformation

In this report, we introduce two of our research topics: (i) high-entropy alloys (HEAs) and (ii) titanium alloys.

Understanding the atomic transport mechanisms in HEAs, including the formation and migration of equilibrium
point defects, is essential to clarify their extraordinary properties, such as high irradiation resistance. Positronannihilation lifetime spectroscopy measurements revealed that the concentration of quenched-in thermal vacancies
in HEAs with a face-centered cubic structure is as low as below the detection limit (in the order of 10 -6). We recently
defined the diffusionless isothermal omega (DI-ω) transformation, which gradually proceeds in beta (β)-titanium
alloys even at above the ω-transformation temperature. The evolution of mechanical properties, including Vickers
hardness, submegahertz internal friction, and elasticity, with the progression of the DI-ω transformation can be
explained by the concept of quenched-in thermal concentration ﬂuctuations.
Single-phase solid-solution alloys consisting ofmultiple principal elements in (nearly) equimolar
fractions, often referred to as high-entropy alloys(HEAs), have attracted extensive attention owing totheir extraordinary properties, such as high strength,
corrosion, and irradiation resistance. Although theformation and migration behavior of vacancies in
HEAs is important to understand bulk diffusion andradiation tolerance, there are contradictions between
previous reports on the concentration of thermal
vacancies in HEAs. Thus, positron annihilation lifetime
spectroscopy measurements were conducted toinvestigate the concentration of thermally induced andquenched-in vacancies in a series of single-phasesolid-solution alloys (Cr0.3FeNi0.2, CrFeNi, CoCrFeNi,
CoCrFeMnNi, and Al0.3CoCrFeNi) quenched fromtemperatures close to their melting point as a functionof the configurational entropy. The quenched-in
vacancy concentration estimated by the mean positron
lifetime decreased with the increase in configurational
entropy and could fall below the detection limit (in theorder of 10 -6) of quaternary and quinary alloys [1].
Understanding the ω-transformation mechanismin β-titanium alloys is essential because the ω-phase
dominates the microstructure and mechanicalproperties of titanium alloys. We recently proposed
the diffusionless isothermal ω (DI-ω) transformation
mode [2], which can gradually occur near room
temperature. We investigated variations in the
microstructure and mechanical properties of β-Ti–V
alloys with the progression of the DI-ω transformation
using transmission electron microscopy, differential
scanning calorimetry, and measurements of theVickers hardness, internal friction, and inelastic X-ray

Fig.1. (a) Variation in Vickers hardness as a function of
annealing time at 373 K for Ti-15, 21, and 27 at.%V alloys.
(b) Schematics of the concentration distribution of the
β-stabilizing element (V) for the interpretation of thehardness increase.

scattering [3]. As shown in Fig. 1(a), the Vickers
hardness increased with aging at 373 K, where no
atomic diffusion occurred. The relative increase inhardness is larger as the vanadium composition islower. This hardening behavior can be attributed to
the DI-ω transformation and interpreted by the conceptof the quenched-in concentration distribution of the
β-stabilizing element (Fig. 1(b)) [3].
References[1] L. Resch, M. Luckabauer, N. Helthuis, N. L. Okamoto, T.
Ichitsubo, R. Enzinger, W. Sprengel, and R. Würschum, Phys.
Rev. Materials 4, 060601(R) (2020).
[2] 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).
[3] N. L. Okamoto, S. Kasatani, M. Luckabauer, R. Enzinger, S.
Tsutsui, M. Tane, and T. Ichitsubo, Phys. Rev. Materials 4, 123603(2020).

Keywords: high entropy alloy, vacancy, phase transformation
Tetsu Ichitsubo, Norihiko L. Okamoto (Structure-Controlled Functional Materials Research Laboratory), and Martin
Luckabauer (University of Twente)
E-mail: nlokamoto@tohoku.ac.jp, tichi@imr.tohoku.ac.jp
URL: http://ilab.imr.tohoku.ac.jp/

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IMR KINKEN Research Highlights 2021

Strong Antiferromagnetic Coupling of Cobalt(II)–Semiquinone
Radical in a Dinuclear Complex with 2,2′-Bipyrimidine Ligands

In this study, a new dinuclear cobalt(II)–radical complex [Co (3,5-DBSQ) (bpym)]・(3,5-DBCatH ) (1) (3,5-DBSQ
2

4

2 4

= 3,5-di-tert-butylsemiquinone, 3,5-DBCatH2 = 3,5-di-tert-butylcatecholato, bpym = 2,2′-bipyrimidine) was
synthesized and characterized using single-crystal X-ray crystallography. Simulated results indicated relatively
strong antiferromagnetic exchange coupling between cobalt(II) centers and semiquinone radical ligands.

The combination of organic radicals with metal
ions has received considerable attention in the fieldof molecular magnetism owing to the construction ofvarious molecular magnetic materials. Among
molecular magnetic materials, complexes containing
semiquinone radicals have become the focus
because of their ease of chemical modification,
diverse coordination structures, and various
properties. Among all the transition metal complexes
with the semiquinone ligand, cobalt complexes
have been extensively investigated because of theirinteresting behavior in the presence of a magnetic
field.
Here, dinuclear cobalt(II) complexes with a2,2′-bipyrimidine
bridging
ligand[Co2(3,5DBSQ)4(bpym)](3,5-DBCatH2)4 were synthesized,
and their single-crystal structures were characterizedby single-crystal X-ray diffraction [1]. The
multidentate ligand, 2,2′-bipyrimidine (bpym), is abis(α-diimine)-type ligand that can exhibit single
chelating and bis-chelating coordination modes in
its metal complexes. The organic radical ligand,
3,5-DBSQ, can act as a terminal radical ligand tocouple with metal ions and form an almost symmetricbinuclear cobalt(II)–radical complex.
Plots showing the magnetic properties of thedinuclear complex with respect to the magnetic
susceptibility (χ MT vs. T) were obtained between 1.8
and 400 K under an applied field of 1000 Oe. Upon
cooling, the value of χ MT continuously and rapidly
decreased. The experimental susceptibility (10–400K) was fitted using the PHI program to determine
the nature and magnitude of the spin Hamiltonianparameters. Thus, Eq. 1 and Eq. 2 are the
Hamiltonians used to fit the experimental magnetic
data.

Fig. 1 Experimental χ MT vs. T plot for 1 and the best-fit result(red line) using the PHI.
^

H = −2J1 (SCo1SCo2) − 2J2 (SCo1SR1 + SCo1SR2+ SCo2SR3 + SCo2SR4) − 2J3 (SR1SR2 + SR3SR4)+ D(S2Co1z + S2Co2z) + gβH·(SCo1 + SCo2).(1)

χM =

χ fit
1 − (2zJ' / Ng2β 2)χfit

(2)The fitting procedure extracted the fitted valuesof J1 = −2.67 (±0.004) cm−1, J2 = −90.25 (±0.023)
cm−1 and J3 = −2.14 (±0.36) cm−1 with D CoII = 87.41(±0.23) cm−1, gCoII = 2.49 (±0.0002), and zJ ′ = 0.02(±0.005) cm−1 (by fixing gR = 2.0, R = 1.85 × 10−4)(Fig. 1). The large negative value of J2 indicatedrelatively strong antiferromagnetic properties
between the Co(II) ions and semiquinone radical
ligands.
References[1] J. H. Wang, J. W. Dai, Z. Y. Li, and M. Yamashita, New J.
Chem. 44, 8471 (2020).

Keywords: magnetic properties, antiferromagnets, crystal structure
Zhao-Yang Li (Corresponding Author, Nankai University)
E-mail: zhaoyang@nankai.edu.cn
Hitoshi Miyasaka (Solid-State Metal-Complex Chemistry Research Laboratory)
E-mail: miyasaka@imr.tohoku.ac.jp
URL: http://www.miyasaka-lab.imr.tohoku.ac.jp/cn5/HOME-Eng.html

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International Collaborative Research

Effect of the Synthesis Conditions of Ce0.9Gd0.1O1.95 Powder on its
Morphology and Characteristics of the Oxygen Ion-Conducting
Ceramics Obtained by Spark Plasma Sintering

In this work, two 10 mol.% Gd-doped ceria powders were obtained by thermal decomposition of the same plate-like

oxalate precursor under different reaction conditions. The powders with different morphologies were sintered using spark
plasma sintering. Electron microscopy was applied to investigate the ceramic samples obtained; their ionic conductivities
were measured and compared.
The material 10 mol.% Gd-doped ceria (10GDC) is widelyused as a solid electrolyte in solid oxide fuel cells (SOFC). The
high oxygen conductivity of 10GDC at intermediate
temperatures 500–700 °C is ideal for use in SOFCs. However,ceria powders usually have poor sinterability that complicatesthe manufacturing of materials. To improve the sintering of thepowder, one can change its morphology or/and use
nonconventional methods for sintering, such as spark plasma
sintering (SPS). In this work, 10GDC powders were obtained bythermal decomposition of the plate-like oxalate precursor
Ce1.8Gd0.2(C2O4)3·10H2O. This work aims to investigate theinfluence of the thermal decomposition conditions on the
10GDC morphology and the properties of the ceramics sinteredby SPS.
The main factor that influenced the morphology was foundto be water vapor pressure during the dehydration stage at 125°C. Upon dehydration with a fast water removal rate, the crystal
retained its shape and size (pseudomorph, termed as "pseudo"),forming an X-ray amorphous product (Fig.1a). Slow dehydration
implemented under an elevated water vapor pressure (QuasiEquilibrium conditions, termed as "QE") led to the intensive
shattering of initial crystals and the formation of a crystalline
intermediate product (Fig.1b). The observed morphologicalchanges reflected two different topotaxial transformations.
These transformations were characterized by different
deformation patterns of the crystal structure, causing significant
differences in the morphology evolution during dehydration.
Following the oxidation of the two dehydrated products in
air at 300 °C, the 10GDC-pseudo and 10GDC-QE powders
were obtained with no further significant morphologicalchanges. Crystallite sizes calculated from the X-ray diffraction
peak broadening were approximately 5–6 nm in each case,which correlated with the transmission electron microscopy
data obtained using the JEM 2010 microscope. The sizes andshapes of the nanoparticle agglomerates were tailored at thedehydration stage. The specific surface area and pore volumeof the powders were measured by ASAP 2020.
The 10GDC powders were compacted and sintered using

Fig. 1 Morphology of the air dehydrated product (a) and the productobtained by dehydration under an elevated water vapor pressure (b)SPS under identical conditions. Then, the samples wereannealed in air at 1200 °C to oxidize them and eliminate the restof carbon. The obtained ceramics were investigated by an
ULTRA 55 microscope. Unlike the 10GDC-pseudo sample, the
10GDC-QE sample obtained from the precursor dehydrated
under high vapor pressure had no visible fractures and showeda higher relative density. This sample also had higher ionic
conductivity (2.07 S/m vs. 0.27 S/m at 700 °C) and lower
activation energy (66 kJ/mol vs. 85 kJ/mol).
Thus, the dehydration conditions of the chosen precursor
were found to be an essential factor in controlling themorphology of 10GDC powder. Changing the powder
morphology yielded ceramics with various characteristics [1].
References[1] D.V. Maslennikov, A.A. Matvienko, A.A. Sidelnikov, D.V. Dudina, M.A. Esikov,
R.V. Belosludov, and H. Kato, Ceramics International 47, 2557 (2021).

Keywords: sintering, oxide, ionic conductor
Daniel Maslennikov and Alexander Matvienko (Corresponding Authors, Group of Reactivity of Solids, Institute of Solid
State Chemistry and Mechanochemistry, Novosibirsk)
E-mail: daniel@solid.nsc.ru, matvienko@solid.nsc.ru
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

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Anisotropy of the Upper Critical Field Under Pressure in the Heavy
Fermion Superconductor UTe2
International Research Center for Nuclear Materials ScienceWe studied the anisotropy of the superconducting upper critical field Hc2 under pressure in UTe2 in the spin-triplet state. The
superconductivity collapsed at 1.5 GPa, and a magnetically ordered state appeared. The Hc2 curve for the field along the a-axis
showed an abrupt enhancement at the high-field and low-temperature regions, implying different order parameters. The Hc2 curve forthe b-axis showed the cutoff of superconducting phases above the metamagnetic transition field Hm~35 T at ambient pressure withthe field-reentrant behavior, which was immediately suppressed with pressure. Our results demonstrate the novel interplay between
spin-triplet superconductivity and magnetism.
The recent discovery of superconductivity in the heavy
fermion paramagnet UTe2 has attracted much attention becauseof its novel phenomena. The superconducting state below Tc = 1.6
K is proposed to be a spin-triplet state, as it is established inferromagnetic superconductors, such as URhGe and UCoGe. In
UTe2, a field-robust superconductivity, which significantly exceeds
the Pauli limit, was observed at the verge of the ferromagnetic
order. Thus, the spin-triplet state is most likely to be realized.
We measured the magnetoresistance under pressure forthe field along the a-, b-, and c-axes in the orthorhombic
structure of UTe2 [1]. The measurements were performed in
Oarai, Sendai, and Grenoble within the framework of aninternational collaboration project between IMR and UGA/CEAGrenoble. Figure 1 shows the H-T phase diagram of Hc2 atdifferent pressures for H || a and b-axes. For the H || a-axis
corresponding to the easy-magnetization axis, an unusualenhancement was detected at low temperature and in the highfield region at 0.5 and 0.86 GPa. At a higher pressure (1.4 GPa),
the initial slope of Hc2 was almost vertical or even positive,
indicating that superconductivity was stabilized by the magnetic
field. For the H || b-axis, the field-reentrant superconductivity
was suppressed owing to the suppression of the metamagnetictransition field Hm. At 1.32 GPa, the Hc2 curve showed a strong
convex curvature, and Hc2(0) was reduced to 9 T, which isapproximately equal to Hm.
These typical Hc2 curves under pressure for the a andb-axes were attributed to the multiple superconducting phases
originating from the spin degree-of-freedom for the spin-triplet
state. The multiple superconducting phases were experimentally
detected as a thermodynamic response [2], indicating different
order parameters. Another important factor is the interplay withmagnetism, which developed above 1.5 GPa in a long-rangeorder. The Fermi surface reconstruction induced by a magnetic
field and pressure, such as the Lifshitz transition [3], will play animportant role in spectacular superconducting phases.
This work was done in collaboration with G. Knebel, K.
Motoi, M. Valiska, F. Honda, D.X. Li, D. Braithwaite, G. Lapertot,

Fig.1: Upper critical field Hc2 for the field along a and b-axes atdifferent pressure in UTe2 [1]. The inset of panel (a) show the T-P
phase diagram at zero field.

W. Knafo, A. Pourret, Y. J. Sato, Y. Shimizu, T. Kihara, J. P. Brison,and J. Flouquet.
References[1] G. Knebel, K. Motoi, M. Valiska, F. Honda, D.X. Li, D. Braithwaite, G. Lapertot,
W. Knafo, A. Pourret, Y. J. Sato, Y. Shimizu, T. Kihara, J. P. Brison, J. Flouquet,and D. Aoki, J. Phys. Soc. Jpn. 89, 053707 (2020).
[2] D. Aoki, F. Honda, G. Knebel, D. Braithwaite, A. Nakamura, D.X. Li, Y.
Homma, Y. Shimizu, Y. J. Sato, J. P. Brison, and J. Flouquet, J. Phys. Soc. Jpn.
89, 053705 (2020).
[3] Q. Niu, G. Knebel, D. Braithwaite, D. Aoki, G. Lapertot, M. Valiska, G.
Seyfarth, W. Knafo, T. Helm, J. P. Brison, J. Flouquet, and A. Pourret, Phys.
Rev. Res. 2, 033179 (2020).

Keywords: superconducting, heavy fermion, actinide
Dai Aoki (Corresponding Author, Actinide Materials Science Research Laboratory)
E-mail: aoki@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 2021

57

International Collaborative Research

Evaluation of Electromechanical Properties in Practical
REBCO Coated Conductor Tapes Under Magnetic Field forSuperconducting CC Coils
High Field Laboratory for Superconducting Materials

Mechanical and electromechanical properties of practical REBCO CC tapes were evaluated, and were foundto vary as the test temperature decreased from 77 K to 35 K. Each CC tape showed a significant increase inelastic modulus, yield strength, and irreversible stress limit as a result of a hardening effect at low temperature.
All practical CC tape samples evaluated are suitable for the expected operating conditions in a large-scale
superconducting wind power generator, given that all exceeded the critical values required for the irreversible
strain limit (~0.4%).
The electromechanical properties of various
CC tapes were evaluated at 77 K and self-field; and35 K and 2 T, as shown in Fig. 1(a) and (b),respectively. The Ic decreased gradually, showing astrain-sensitive behavior at 77 K and a self-field
condition, as seen in Fig. 1(a). The strain sensitivity
was slightly different depending on the fabrication
process adopted, and the alignment direction of thegrains on the REBCO layer to the loading axis
influenced its sensitivity. At 35 K and 2 T, Ic wasalmost uniform across all samples before gradually
decreasing. This is the result of decreased strain
sensitivity in Ic at low test temperatures and anincreasing externally applied magnetic field. At 35
K, although Ic remained nearly flat against either theapplied strain or stress, it suddenly decreased when
its critical value was exceeded. Fig. 1(b) shows that
although Ic decreased by 10% to 20% under theapplied strain, it recovered when the applied load

was removed.
The electromechanical properties of all CC
tape samples are shown in [1]. All samples showed
increased irreversible stress limits when the test
temperature decreased from 77 K to 35 K. The ISD/RCE sample showed larger irreversible limits at 35 K
and 2 T (εirr = 0.44%; σirr = 655 MPa) than at 77 K
and self-field (εirr = 0.30%; σirr =370 MPa). The
opposite was true for the other two samples. Bothshowed lower limits for εirr at 35 K and 2 T (0.60% for
RCE-DR and 0.30% for PLD) than at 77 K and selffield.
Among the three practical CC tape samples
tested, RCE-DR showed a superior irreversible
strain limit of εirr = 0.60% at 35 K and 2 T.
References[1] H. S. Shin, M. A. Diaz, M. Velasco, and S. Awaji, IEEE Trans.
Appl. Supercond. 30, 8400405 (2020).

Fig. 1 Normalized critical current as a function of tensile strain at (a) 77 K and self-field, and (b) 35 K and 2T.

Keywords: high temperature superconductor, mechanical properties, high magnetic field
Hyung-Seop Shin (Corresponding Author, Andon National University)
E-mail: hsshin@anu.ac.kr
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

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IMR KINKEN Research Highlights 2021

Ultimate Impedance of Coherent Heat Conduction in Van
Der Waals Graphene-MoS2 Heterostructures
Center for Computational Materials Science

Herein, we investigate the controllability of coherent phonon transport in van der Waals graphene-MoS

2

heterostructures with different stacking orders using molecular dynamics simulations. Using Bayesian
optimization–based materials informatics, the optimal stacking order of graphene and MoS2 was identified fromtens of thousands of candidates with varying degrees of phonon localization.

There are growing efforts to control thermaltransport via coherent phonons in the onedimensional superlattice. However, the difference
between the intrinsic lattice structures of constituent
materials generate interface disorders during the
fabrication
process,
hinderingthe
directexperimental observation of coherent phonontransport. The flexible integration and atomistic
interlayer smoothness of the van der Waals
heterostructure provides an ideal platform forcoherent phonon transport manipulation. Thus, inthe current work, non-equilibrium molecular
dynamics simulations were used to investigate thecoherent phonon transport in a van der Waals
graphene-MoS2 heterostructure with different
stacking orders [1]. The histogram of phonontransmissions in different disordered structures
exhibits a log-normal distribution, which reveals the
localization of coherent phonons. The optimal
stacking order of the graphene and MoS2 wasefficiently identified from tens of thousands ofcandidates by machine learning. The suppressionof phonon transmission in the low-frequency (<5THz) phonons of the optimized structure led to asignificant reduction (>95%) of the thermal
conductance compared with pristine graphite.
Finally, the effects of the defect, size, andtemperature on the thermal conductivity of graphite
and optimized structures were discussed. Our work
provides deep insights into coherent phononstransport behavior in the atomistically smooth vander Waals structure, which is beneficial for thefurther development of phononics.

Fig. 1. Schematics of the machine learning method to
combine molecular dynamics and Bayesian optimization.

Fig. 2. Comparison of the thermal conductivity between
pristine MoS2, graphene, MoS2-graphene superlattice,and optimum heterostructure.

References[1] S. Hu, S. Ju, C. Shao, J. Guo, B. Xu, M. Ohnishi, and J. Shiomi,
Mater. Today Phys. 16, 100324 (2021).

Keywords: 2D materials, molecular dynamics, nanostructure
Junichiro Shiomi (Corresponding Author, School of Mechanical Engineering, The University of Tokyo)
E-mail: shiomi@photon.t.u-tokyo.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/

IMR KINKEN Research Highlights 2021

59

International Collaborative Research

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

he neutron spectrometer POLANO at J-PARC is a powerful instrument that will enable polarization analysis inhigher energy regions up to 100 meV to elucidate the detailed spin and lattice dynamics. Utilizing POLANO, we
measured the phonon density of states in the T*-type cuprate superconductor Nd1.34Ce0.26Sr0.4CuO4 and clarified
the negligible oxidation annealing effect on high-energy bond-stretching phonon.
The interplay between spin, charge, and latticedynamics is an important subject in the research on
high-transition-temperature superconductivity incuprate oxides [1]. Recently, we synthesized T*-type
cuprate, which contains a CuO5 pyramid in the unit
cell, and determined a phase diagram for assintered and oxidation annealed samples.
Superconductivity in T*-type cuprate appeared
after the suppression of the short-range magnetic
order due to annealing. Thus, phonon modes could
be varied by carrier doping [2] and introducing
oxygen brought by annealing. To elucidate thephononvariations
and
their
relationtosuperconductivity, we performed inelastic neutron
scattering measurements on the chopper
spectrometer POLANO, which was recently
supplied for a user program at MLF.
Figures 1(a) and (b) show the neutron scattering
intensity (I) in the energy (E) and momentum (Q)spaces of as-sintered and annealed polycrystal
samples of Nd1.34Ce0.26Sr0.4CuO4, respectively.
The intensity was obtained after subtracting thebackground and multiplying it by E2 to enhance the
I at the higher E region. In both samples, the opticalphonon corresponding to the bond-stretching modein the CuO2 plane was approximately 100 meV.
Figure 1(c) shows the constant-Q spectra at Q = 7.5
Å-1. The peak position of the optical phonon isalmost the same for the two samples, indicating thenegligible annealing effect on this phonon mode.
This result contrasts with the softening of opticalphonons due to carrier doping in T-type La2xSrxCuO4, which has a CuO6 octahedron in the unit
cell. These results show that phonon softening
develops differently in T*- and T-type cuprates.

Fig. 1. Neutron scattering intensity for (a) as-sintered and (b)annealed T*-type Nd1.34Ce0.26Sr0.4CuO4. (c) Constant-Q
spectra at Q = 7.5 Å-1.

References[1] S. Asano, K. Ishii, K. Yamagami, J. Miyawaki, Y. Harada, andM. Fujita, J. Phys. Soc. Jpn. 89, 075002 (2020).
[2] K. Sato, K. Ikeuchi, R. Kajimoto, S. Wakimoto, M. Arai, and M.
Fujita, J. Phys. Soc. Jpn. 89, 114703 (2020).

Keywords: high temperature superconductor, neutron scattering
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

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IMR KINKEN Research Highlights 2021

2021

Index

Author Index
NamepageNamepageA
Amiya, KenjiAoki, Dai4337, 57

B
Babin, Vladimir
Bauer, Gerrit E. W.

Nagasako, MakotoNaito, Tomoyuki
Nambu, Yusuke
Nojima, Tsutomu
Nojiri, Hiroyuki

53
44, 47C
Chiba, Akihiko

11

F
Fujita, Masaki

46, 60H
Harada, Takayuki

28

I
Ichitsubo, Tetsu
Imashuku, Susumu
Ito, Keita

44, 5422
35

K
Kasada, RyutaKato, HidemiKawamata, Toru
Kiguchi, Takanori
Kondo, Sosuke
Konno, Toyohiko J.
Koyama, Motomichi
Kubo, MomojiKurosawa, Shunsuke
LLi, Zhao-Yang
Luckabauer, MartinM
Masahashi, Naoya
Maslennikov, Daniel
Matvienko, AlexanderMiyamoto, Goro
Miyasaka, Hitoshi
N
Nagai, Yasuyoshi

62

IMR KINKEN Research Highlights 2021

19
12, 20, 56
1038
19
38, 50913, 17, 45, 59
32

55
54

41, 4356
56
8
34, 5518, 40, 57

O
Ogata, Shigenobu
Okamoto, Norihiko L.
Okuno, Yasuki
Onose, Yoshinori
Orimo, Shin-ichi

50
42
30
48
27, 42, 52, 58

45
54
19
31
21

S
Sasaki, Takahiko
Sheikin, Ilya
Shi, Qilin
Shiga, Keiji
Shimokawa, KoheiShin, Hyung-Seop
Shiomi, Junichiro
Shirasaki, Kenji

29, 48
47
36
16
23
58
59
49

T
Takagi, Shigeyuki
Takanashi, Koki
Tanimura, Hiroshi
Terada, Yayoi
Toyama, Takeshi

21
35
33
13
40

U
Uda, Satoshi

36

W
Wagatsuma, Kazuaki
Wei, Daixiu

22
12

Y
Yoshikawa, AkiraYu, Hao
Yu, Weichao
Z
Zhang, Jun32, 5319
26

34

Keyword Index
Keywordpage

Keywordpage

A
actinide37, 49, 57
additive manufacturing
11
alpha-ray spectroscopy
49
aluminum alloys
11
amorphous33
antiferromagnets55
atomic emission spectrometry22high magnetic field
27, 42, 52, 58
high temperature superconductor 42, 58, 60
hydride21hydrogen
9, 22, 40

Bbiomaterial41
Cceramic19
computational materials science
13
corrosion19
crystal growth
16, 32, 36
crystal structure34, 36, 50, 55
Ddealloying20
defects
9, 16, 18, 19, 23
deformation12
devices
28, 43dielectric44
doping36
Eelectrical properties
29, 44
electron microscopy
38, 50electron paramagnetic resonance (EPR)52, 53
electronic material
44
electronic structure47
energy storage
20, 23
epitaxial growth38
Ffermi surface
field-effect transistor
first principles calculations
H
hard magnet
heavy fermion
higher education
high entropy alloy

47
48
21

27
37, 47, 57
13
54

I
instrumentation46
interface17
ionic conductor
56
K
kinetics16
L
luminescence53
Mmagnetic properties
34, 35, 43, 55
magnetism30
magnetoresistance26
mechanical properties
12, 45, 58
metal-insulator transition
29
metallic glass
43
microstructure
8, 12, 18
molecular beam epitaxy (MBE)
35
molecular dynamics
17, 45, 59
multi-ferroics31
N
nanostructure59
neutron diffraction
46
neutron scattering30, 46, 60
nuclear materials
18, 40O
optical properties
33
organic29
oxide
23, 28, 41, 56
Pphase transformation
8, 33, 54
photocatalytic41
porosity
20, 34

IMR KINKEN Research Highlights 2021

63

Keywordpage
R
radiation effects
radiation imaging

40
32

S
scintillator
32, 53simulation17
sintering56
solid-state electrolytes
21
spin dynamics
26
spin transfer
31
spintronics
26, 30, 31
steel
8, 9
superconducting37, 48, 57
surface reaction
10synchrotron10
T
thin films
toroidal magnet
2D materials

28, 35, 38
52
48, 59V
vacancy54
X
XAFS10
XMCD2764

IMR KINKEN Research Highlights 2021

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
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