KINKEN
Research Highlights
2015

Institute for Materials Research,Tohoku University

KINKEN Research Highlights 2015
Contents
Preface .......................................................................................................................................................... 4

Research Highlights

1. Infrastructural Materials
Microstructure Design of Structural Metallic Materials Division .................................................................... 8
Chemical Physics of Non-Crystalline Materials Division .............................................................................. 9
Biomaterials Science Division .................................................................................................................... 10
Multi-Functional Materials Science Division ................................................................................................11
Deformation Processing Division ............................................................................................................... 12
Materials Science of Non-stoichiometric Compounds Division.................................................................. 14
Analytical Science Division ........................................................................................................................ 15
Advanced Materials Development and Integration ofNovel Structured Metallic and Inorganic Materials............................................................ 16
Computational Materials Research Initiative (CMRI) .................................................................................. 17

2. Energy-related Materials
Theory of Solid State Physics Division ....................................................................................................... 20
Physics of Crystal Defects Division ............................................................................................................ 21
Materials Design by Computer Simulation Division .................................................................................... 22
Nuclear Materials Science Division ........................................................................................................... 23
Nuclear Materials Engineering Division...................................................................................................... 24
Physics of Electronic Materials Division .................................................................................................... 25
Non-equilibrium Materials Division ............................................................................................................. 26
Crystal Chemistry Division ......................................................................................................................... 27
Hydrogen Functional Materials Division ..................................................................................................... 28

3. Electronic Materials
Magnetism Division .................................................................................................................................... 30
Surface and Interface Research Division ................................................................................................... 31
Low Temperature Physics Division ............................................................................................................. 32
Low Temperature Condensed State Physics Division ................................................................................ 33
Metal Physics with Quantum Beam Spectroscopy Division ....................................................................... 34
Irradiation Effects in Nuclear and Their Related Materials Division ............................................................ 35
Solid-State Metal-Complex Chemistry Division .......................................................................................... 36
Magnetic Materials Division........................................................................................................................ 37
Advanced Crystal Engineering Laboratory ................................................................................................ 38
Actinide Materials Science Division ........................................................................................................... 39

Research Centers
International Research Center for Nuclear Materials Science .................................................................... 42
Cooperative Research and Development Center for Advanced Materials ................................................ 43
High Field Laboratory for Superconducting Materials................................................................................ 44
Kansai Center for Industrial Materials Research ........................................................................................ 45
Center for Computational Materials Science .............................................................................................. 46
Collaborative Research Center on Energy Materials .................................................................................. 47
International Collaboration Center (ICC-IMR)............................................................................................. 48
Center of Neutron Science for Advanced Materials ................................................................................... 49
Research and Development Center for Ultra High Efficiency Nano-crystalline Soft Magnetic Material .... 50
Laboratory of Low Temperature Materials Science .................................................................................... 51
Laboratory of Alpha-Ray Emitters .............................................................................................................. 52
Analytical Research Core for Advanced Materials ..................................................................................... 53

Author Index ............................................................................................................................................... 55

Keyword Index ........................................................................................................................................... 56

Preface

Dear Colleagues,

We are pleased to present KINKEN Research Highlights 2015, 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.

Professor Kotaro Honda established the IMR at the Tohoku Imperial University in 1916 as the 2nd
Division of the Provisional Institute of Physical and Chemical Research. At that time the primary research
focus was steel. Thereafter, the research domains gradually broadened to include various types ofalloys and metals. The name was changed into the Research Institute for Iron, Steel and Other Metals(RIISOM) in 1922. Subsequently, the institute developed into a global center for fundamental and appliedresearch covering all types of materials, including nonmetals. In 1987 the institute was reorganized intoa national collaborative research institute affiliated with Tohoku University, and consequently renamed tothe present Institute for Materials Research (IMR).

IMR has greatly contributed to the advancement of materials science and engineering. The inventionof KS steel in 1916, the strongest permanent magnet at that time, was the first great achievement. In
subsequent years, many different types of practically useable materials have successfully beendeveloped, including new KS steel, Sendust alloy, SiC fibers, various intermetallic compounds, andmore recentry, amorphous alloys. In addition, considerable effort was dedicated to basic research formaterials development, which paved the way for pioneering research in magnetism, superconductivity,

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IMR KINKEN Research Highlights 2015

Director

optical properties, and microstructure analyses of materials. Recently, IMR has created a wide array of
new materials, including high-performance, high-quality, and multifunctional materials such as bulk
metallic glasses, nanostructured materials, nanocomposites, ceramics, crystals, oxides, nitrides,
hydrides, complexes, organic materials, etc., which are useful for electronic, optical, magnetic, spintronic,
biological, energetic, and infrastructural applications.

In the 21st Century, we face worldwide environmental problems such as global warming and thedepletion of resources and energy. There is an increasing need to preserve the environment and work
towards achieving sustainable societies. IMR upholds these themes with the objective of “contributing
to the well being of the human race and the development of civilization through the creation of newmaterials that are truly useful to society”.

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 2015

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Infrastructural Materials

IMR KINKEN Research Highlights 2015

Infrastructural Materials

Controlling Phase Transformation of Steels using
Microalloying Elements

Advanced high-strength steels with high toughness are demanded for reliable construction and for lowering

the weight of automobiles. Microalloying technology, that is, addition of very small amounts of specific elements
such as B and Mo, has been developed to control properties of steels. In our group, the effects of microalloying
on phase transformation kinetics have been investigated systematically. Here, we show strong effects ofaddition of a small amount of B and Mo, and their local enrichment at interphase boundaries.

More than 100 million tons of steel is produced
every year in Japan for use in industries such as
automobile, construction, and vessels. In order toachieve the various properties required for theseapplications, novel alloy designs combined withsophisticated thermomechanical treatments havebeen developed. It is known that the addition of avery small amount of specific alloying elements,
such as Nb, B, and Mo can change the kinetics ofphase transformation significantly, and thus,
influence the mechanical properties. Such a“microalloying” technique has been attracting muchattention because of a strong demand in developing
advanced high-strength steels with reduced amountof elemental consumption.
Recently, our group has studied the effects of
microalloying Mo, Nb, and B to low-carbon welding
steels [1-3], since the microalloying of theseelements effectively yields a bainite structure in lowcarbon, low-alloy steels under slow cooling, which
is important for strengthening steels. Figure 1 showsa significant influence of a very small amount of Mo(0.5%) and Boron (10 ppm) on bainite transformation
kinetics. It was found that bainite transformation issuppressed strongly by the addition of Mo while theeffect of B is small. On the other hand, a synergetic
effect appears by the combined addition of B and
Mo. In order to clarify the mechanism of the retarding
effect of Mo on bainite transformation, element
segregation at the bainite/matrix interface
boundaries are investigated using aberrationcorrected scanning TEM/EDS (Fig. 2). In the earlystage of transformation (Fig. 2(a)), there is nosegregation of either Mn or Mo at the boundaries. At
the onset of stasis of transformation (Fig. 2(b)), onlya small segregation of Mn is seen, whereas Mo
does not exhibit a detectable enrichment.
Remarkable segregation of Mo and Mn takes place
at longer holding time (Fig. 2(c)).
Since there is clearly a time lag between the
beginning of transformation stasis and the beginning

Fig. 1 Effect of a small amount of Mo (0.5%) and B (10ppm) on transformation kinetics [2].

Fig. 2 Element segregation at the interphase boundary inthe specimen with Mo transformed for (a) 10 s, (b) 60 s,
and (c) 1.8 ks [2].

of Mo segregation, this suggests that the effect of
Mo segregation at the interface is weak, despite themarked effect of Mo in the appearance oftransformation stasis.
References[1] T. Furuhara, T. Yamaguchi, G. Miyamoto, and T. Maki, Mater.
Sci. Tech. 26, 392 (2010).
[2] T. Furuhara, K. Tsuzumi, G. Miyamoto, T. Amino, G. Shigesato,
and Metall. Mater. Trans. A 45, 5990 (2014).
[3] Y. Xia, G. Miyamoto, Z.G. Yang, C. Zhang, and T. Furuhara,
Acta Mater. 91, 10 (2015).

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

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IMR KINKEN Research Highlights 2015

Structural Modeling of Fe80B20 Amorphous Alloy by Using
AXS TechniqueT

he three-dimensional structural model of an Fe80B20 amorphous alloy was simulated using a reverse Monte
Carlo (RMC) simulation using a combination of anomalous X-ray scattering (AXS) and neutron diffraction (ND).
The fundamental structural features agree well with those obtained using the common dense random packing(DRP) model. However, the atomic arrangement around B is characterized by an anti-prism-like cluster [1].

Fe-based metal–metalloid amorphous alloys
are used as soft magnetic materials. Their glassforming abilities are associated with the atomic
arrangement of constituents [2]. We prepared an
ingot with a nominal quantity of Fe80B20 by theconventional arc melting of a mixture of pure iron(99.99%) and boron (99.4 % 11B). Ribbon samplesof thickness ~20 µm and width 1 mm were preparedusing a melt-spinning technique. Ordinary X-ray
diffraction and AXS for the Fe K-absorption edge
were carried out at the BL-7C beam-line of the
Photon Factory at IMSS. The ∆QiFe(Q) and Qi(Q)X-ray
interference functions were obtained from theobserved scattering intensities. The Qi(Q)neutronfunction was also obtained by using the time-offlight technique at NOVA of MLF, J-PARC. Threeinterference functions were used to model theatomic arrangement by applying the AXS–ND–
RMC method. An initial model, consisting of 10,000
atoms (Fe: 8000 and B: 2000) placed within a cubic
hypercell (L = 4.737 nm, 7.32 Mg/m3), was produced
using the DRP model of hard spheres.
Figure 1 shows the experimental and calculated
interference functions ∆QiFe(Q), Qi(Q)X-ray, andQi(Q)neutron. There is no pre-peak signal at the lower
Q-region, indicating the tendency for compound
formation. The fundamental atomic arrangement
can then be well demonstrated using a random
packing structure. To determine the local structuralunits, further advanced AXS–ND–RMC modelingwas carried out under experimental constraints forthe obtained interference functions. Although theprevious study reported the first B–B distance ofabout 0.355 nm [3], the present AXS–ND–RMC
modeling indicates a shorter nearest-neighbor B–B
correlation at 0.21 nm. The Voronoi polyhedral
analysis around Fe clearly shows that the deformedfcc and icosahedron-like local structural units arecommon, and this feature is readily reproduced by
the common DRP model. On the other hand, theatomic arrangement around B prefers an anti-prism-

Fig. 1 Three interference functions for Fe80B20 amorphousalloy. Solid lines: experimental data. Dotted lines: AXS–
ND–RMC results.

like coordination. These characteristic local
structures in Fe80B20 represent the chemical shortrange order around B.
References[1] H. Arima, T. Kawamata, Y. Yokoyama, K. Sugiyama, and T.
Ohtomo, JPS Conf. Proc., (2015) in press.
[2] E. Nold, P.Lamparter, H. Olbrich, G. Rainer-Harbach, andS.Steeb, Z. Naturforsch. 36A, 1032 (1981).
[3] A. S. Clarke and J. D. Wiley, Phys. Rev. B 35, 7350 (1987).

Keywords: amorphous alloy, structure, synchrotron
Hiroshi Arima and Kazumasa Sugiyama (Chemical Physics of Non-Crystalline Materials Division)
E-mail: arimah@imr.tohoku.ac.jp
URL: http://www.xraylab.imr.tohoku.ac.jp/

IMR KINKEN Research Highlights 20159

Infrastructural Materials

Wear Behavior of Biomedical Titanium Alloys in Simulated
Body Fluid and AirW

ear behavior of combinations comprised of an (α+β)-type titanium alloy, Ti-6Al-4V ELI (Ti64) and a β-type
titanium alloy, Ti-29Nb-13Ta-4.6Zr (TNTZ) in simulated body fluid and air were investigated for biomedical
applications. Although the volume loss of metallic materials often increases in air compared to simulated body
fluid, the volume loss in simulated body fluid is larger than that in air for TNTZ disc/TNTZ ball combination. In
this case, delamination wear is predominant for this combination. On the other hand, the predominantly abrasive
wear occurs in a Ti64 disc/Ti64 ball combination so that its volume loss in simulated body fluid is less than thatin air.
Titanium alloys are currently used for biomedical
applications because of their excellent mechanicalproperties and biocompatibility. Such applications
sometimes include components with metal-to-metal
contact, although wear resistance of titanium alloys
is generally poor. It is well known that wear occurringin human body induces implant loosening, and thepresence of wear debris causes inflammatory
reactions, resulting in pain in the patient. Thus, animprovement in wear resistance is required forbiomedical titanium alloys so that wear
characteristics of titanium alloys can be understoodto improve their wear properties.
Two types of titanium alloys were examined inthis study. The first is a conventional (α+β)-type Ti6Al-4V ELI alloy (Ti64), which accounts forapproximately 45–60% of the total titanium alloys
produced globally, and the second is a β-type Ti29Nb-13Ta-4.6Zr alloy (TNTZ) developed forbiomedical applications, which is comprised of only
non-toxic and allergy-free alloying elements, andlower Young’s modulus compared to that of Ti64.
Figure 1 shows the volume loss of discs andballs made of Ti64 and TNTZ subjected to thefrictional wear tests in simulated body fluid, Ringer’s
solution at 310 K [1], and air at room temperature [2].
When the TNTZ disc slides against the TNTZ ball,the volume loss of both the disc and ball is the
maximum in both Ringer’s solution and air. In
addition, although the volume loss often decreases
in Ringer’s solution compared to air because
Ringer’s solution can act as a liquid lubricant duringsliding, the volume loss in Ringer’s solution increases
compared to that in air for the TNTZ disc/TNTZ ball
combination.
In order to clarify the difference in the wear
behaviors between the TNTZ disc/TNTZ ball andthe Ti64 disc/Ti64 ball combinations, morphologicalanalysis of the wear debris collected from the discs
Keywords: alloy, biological, mechanical properties
Mitsuo Niinomi (Biomaterials Science Division)
E-mail: niinomi@imr.tohoku.ac.jp
URL: http://biomat.imr.tohoku.ac.jp/

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IMR KINKEN Research Highlights 2015

Fig. 1 Volume losses of Ti64 and TNTZ discs and balls
obtained from frictional wear tests in Ringer’s solution
and air.

and balls at the end of frictional wear tests in Ringer’s
solution and air was performed. Cutting chip-likewear debris were observed in the Ti64 disc/Ti64
ball combination, indicating that a microcutting
action occurs during sliding. This result suggeststhat abrasive wear occurs in both Ringer’s solution
and air for this combination. In contrast, large
amounts of flake-like wear debris were observed in
the TNTZ disc/TNTZ ball combination, indicatingthat delamination wear occurs in both Ringer’s
solution and air. Therefore, it is considered that thevolume loss during the frictional wear tests isenhanced in Ringer’s solution compared to air whendelamination wear is predominant for eachcombination.
References[1] Y.-S. Lee, M. Niinomi, M. Nakai, K. Narita, and K. Cho, Mater.
Trans. 56, 317 (2015).
[2] Y.-S. Lee, M. Niinomi, M. Nakai, K. Narita, J. Hieda, and K.
Cho, J. Mech. Behav. Biomed. Mater. 41, 208 (2015).

IMR KINKEN Research Highlights 2015

Laser Oscillations from Transparent Polycrystalline
Ceramics Produced by Spark Plasma Sintering

Laser oscillations were demonstrated for the first time using Nd -doped Lu O transparent polycrystalline
3+

2

3

ceramics produced by spark plasma sintering. The laser performance confirms that spark plasma sintering is
a promising technique for producing laser-quality transparent ceramics.

Transparent polycrystalline ceramics arepromising optical media for solid-state lasers andscintillators. Spark plasma sintering is anadvantageous technique for producing transparent
polycrystalline ceramics because of a fasterdensification process compared to conventionalsintering methods.
Although extensive research have been
undertaken on the preparation of transparent
polycrystalline ceramics using spark plasma
sintering, no one has successfully demonstrated
laser oscillation thus far. For the spark plasma
sintering process, a short holding time and a low
sintering temperature prevent grain coarsening andfavor the fabrication of fine-grained ceramics withhigh transparency. However, a deep understanding
is required for optimizing powder processing andsintering parameters to produce a laser-qualitytransparent ceramic by inhibiting the defectformation and eliminating pores.
We proposed the two-step pressure sintering
combined with a low heating rate to produce
transparent Lu-based oxides, such as Lu2O3 [1],Lu2Ti2O7 [2], Lu3NbO7, and Lu3Al5O12. The preloadpressure affected the intermediate relative density,and the transmittance in the visible range was moresensitive to preload pressure than that in the infrared
range. The combination of the low heating rate andthe two-step pressure profile resulted in a hightransparency (Fig. 1(a)).

Fig. 2 (a) Straight-type and (b) V-type laser cavities for
end-pumping configurations with fiber coupled diodelaser (LD) sources. OC: output coupler, RM: rear mirror,
and FM: folding mirror.

We report another discovery that LiF can promote
reaction sintering and eliminate carbon contamination
through vaporization of LiF at elevated temperatures[3]. Nd3+-doped Lu2O3 ceramic with 0.2 wt% LiF
showed a transmittance of 99.5% of the theoretical at
λ = 1080 nm (Fig. 1(b)), which corresponds to thelaser lines with 4F3/2 to 4I11/2 transition.
Laser oscillations were demonstrated by twotypes of laser cavities using end-pumping
configurations (Fig. 2) [4, 5]. For example, theobtained maximum output power was 0.2 W, andthe slope efficiency was 14%, which is comparable
to that of Nd3+-doped Lu2O3 ceramics and single
crystals [4]. These laser performances confirm thatspark plasma sintering is a promising technique for
producing laser-quality transparent ceramics.
References[1] L.Q. An, A. Ito, and T. Goto, J. Am. Ceram. Soc. 93(3), 695(2011).
[2] L.Q. An, A. Ito, and T. Goto, J. Eur. Ceram. Soc. 31(1-2), 237(2011).
[3] L.Q. An, A. Ito, and T. Goto, J. Asian Ceram. Soc. 2(2), 154(2014).
[4] L.Q. An, A. Ito, J. Zhang, D. Tang, and T. Goto, Opt. Mater.
Express 4(7), 1420 (2014).

Fig. 1 (a) Lu2O3 and (b) Nd3+-doped Lu2O3 transparent
polycrystalline ceramics produced by spark plasma
sintering. The diameter of the ceramics is 10 mm.

[5] G. Toci, M. Vannini, M. Ciofini, A. Lapucci, A. Pirri, A. Ito, T.
Goto, A. Yoshikawa, A. Ikesue, G. Alombert-Goget, Y. Guyot,
and G. Boulon, Opt. Mater. 41, 12 (2015).

Keywords: ceramic, sintering, powder processing
Akihiko Ito, Hirokazu Katsui and Takashi Goto (Multi-Functional Materials Science Division)
E-mail: goto@imr.tohoku.ac.jp
URL: http://www.goto.imr.tohoku.ac.jp/

IMR KINKEN Research Highlights 201511

Infrastructural Materials

FCC (γ)→HCP (ε) Martensitic Transformations Dominate Strain andDamage of Co–Cr–Mo Alloy with Nagative Stacking Fault Energy

Co–Cr–Mo alloys have been used for various biomedical implants such as artificial hip and knee joints.

Negative stacking fault energy (SFE) of the alloy gives rise to unique mechanical behaviors, which differentiate
the alloy from other conventional FCC alloys. Here, we introduce the FCC (γ)→HCP (ε) martensitic transformations,originating from the negative SFE, in strengthening and fatigue behaviors.
Local strains developed by athermal γ→εmartensitic transformation were investigated for Co–
29Cr–6Mo (wt%) alloys with or without added nitrogen[1]. Electron backscatter diffraction (EBSD) based
strain analysis revealed that the N-free alloy consistingof the γ matrix and athermal ε-martensite plates
showed large elastic and plastic strains in the γ matrix.
Transmission electron microscopy (TEM) (Fig.1)
showed the ε-martensite plates were aggregates ofthin ε-layers formed by three different Shockley partial
dislocations that canceled each other’s shear strains.
We have revealed that the elastic strains in the γ matrix
originate from interactions among the ε-martensite
phase, extended dislocations, and/or thin ε-layers.
The dislocations highly dissociated into stacking
faults, making stress relaxation at intersections difficultand further introducing local strain.
The mechanism of extrusion and intrusion
formation, which causes fatigue fracture, wasinvestigated with special focus on the role of thestrain-induced γ→ε martensitic transformation (SIMT)(Fig. 2(a)) [2]. We found that the SIMT leads to
extrusions and intrusions whose morphologies werequite different from those seen in other γ alloys (Fig.
2(b) and (c)), and it was attributed to the localization ofplastic strain within thin layers of SIMTed ε-phase.
Also, it was found that the localization of plastic strain
could be avoided by decreasing the grain size, andthe techniques of grain refinement is important not
only for increasing the tensile strength but also
improving fatigue strength. These findings areimportant for producing reliable devises by electron
beam additive manufacturing (i.e. 3D printing) inwhich coarse grains are formed [3].
Based on these fundamental studies, we aredeveloping novel technologies for improving theproperties, such as the heat treatment decreasing
grain size after fabrication with keeping the shape [4].
Also, we are extending the application area of Co-CrMo alloy to the fields beyond the biomedical area,
including metal die-casting, injection-molding, andfuel-cell application [5]. Moreover, novel Co-Cr based
Keywords: phase transformation, microstructure, defects
Akihiko Chiba (Deformation Processing Division)
E-mail: a.chiba@imr.tohoku.ac.jp
URL: http://www.chibalab.imr.tohoku.ac.jp/english/index.html

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IMR KINKEN Research Highlights 2015

Fig. 1 (a) TEM image showing interfaces between γ matrixand ε martensite in N-free Co–29Cr–6Mo alloy quenchedfrom 1473 K. (b) Magnified image of γ/ε interface growing
through nucleation and densification of thin ε-layers.

Fig. 2 (a) Schematic of SIMT and cross-sectional TEM, (b)bright field image (BFI) and (c) dark field of surface
reliefs on fatigued Co-Cr-Mo alloy [2].

alloys consisting other elements, such as Co-Cr-W
alloy, are being developed [6].
References[1] K. Yamanaka, M. Mori, Y. Koizumi, and A. Chiba, J. Mech.
Behav. Biomed. Mater. 32, 52 (2014).
[2] T. Mitsunobu, Y. Koizumi, B.-S. Lee, K. Yamanaka, H. Matsumoto,Y. Li, and A. Chiba, Acta Mater. 81, 377 (2014).
[3] S. H. Sun, Y. Koizumi, S. Kurosu, Y. Li, H. Matsumoto, and A.
Chiba, Acta Mater. 64, 154 (2014).
[4] S. Kurosu, H. Matsumoto, and A. Chiba, Mat. Letter. 64, 49 (2010).
[5] Y. Li, P. Tunthawiroon, N. Tang, H. Bian, F. Wang, S. H. Sun, Y. Chen,
K. Omura, Y. Koizumi, and A. Chiba, Corros. Sci. 84, 147 (2014).
[6] K. Yamanaka, M. Mori, and A. Chiba, J. Mech. Behav. Biomed.
Mater. 37, 274 (2014).

IMR KINKEN Research Highlights 2015

Synthesis of Ultrafine-grained Copper Alloys with Nanoscale
TiC Dispersion by High Energy Ball-Milling and Hot Pressing

Copper is being widely used in industries because of its high electrical and thermal conductivity, and excellent

resistance to corrosion. Considerable efforts have been directed to the development and design of novel
copper-based alloys exhibiting high electrical conductivity and mechanical strength for practical applications.
The nanoscale TiC-strengthened copper alloys were manufactured by an internally developed two-step ballmilling process, and the properties at room and elevated temperature were investigated.

Recently, many studies have been conducted todesign new copper-based alloys with high electricalconductivity and high strength using precipitationand dispersion-strengthening methods. However, it
has been proven that dispersion strengthening ismore effective in providing outstanding properties atelevated temperatures, as precipitates could coarsen
or dissolve into the copper matrix at elevatedtemperatures.
In this study, TiC was selected as the reinforcing
particle because of its high melting point, highthermal, and chemical stability. The mechanism forhigh conductivity and high strength was analyzed(Fig. 1) [1]. The Ti-C reaction leads to a highly
purified matrix, which guarantees high conductivity.
It was found that the carbon particle size plays asignificant role in the formation of TiC during theprocess. The ultrafine-grained (UFG) microstructurewith an average grain size of around 300 nm

Fig. 1 Reaction mechanism for high conductivity andstrength of Cu-Ti-C system: (a) Fine-grained mixture of
Cu-Ti-C after ball-milling; (b) Interdiffusion of Cu and Ti,
dissociation of the supersaturated Cu(C) during spark
plasma sintering (SPS); (c) Chemical reaction of Ti-C
during SPS and heat treatment; (d) Final microstructure.

Fig. 2 (a) Representative microstructure showing UFG
microstructure and nanoscale TiC particles, and (b)microhardness of the prepared samples compared toother Cu materials after 1 h annealing at different
temperatures.

developed in this process [2], and the nanoscale
TiC particles in the grain interior result in highstrength through the Hall-Petch and Orowan
strengthening mechanisms, as shown in Fig. 2(a).
The manufactured Cu-TiC alloys showed a
combination of conductivity-tensile strength ofaround 71%IACS-712 MPa. Furthermore, the TiC
particles located at the grain boundaries could
contribute to the thermal stability by the Zener
pinning effect. Stagnation of the matrix grain growth
was observed through annealing experiments at
different temperatures, and the room temperature
microhardness compared to other Cu-basedmaterials after 1 h annealing was analyzed (Fig.
2(b)). Compared to other Cu materials, the preparedalloys showed excellent thermal stability at theannealing temperature of 1223 K, while there is onlya slight drop in HV with annealing temperature of1323 K for 1 h.
References[1] F. Wang, Y. Li, K. Wakoh, Y. Koizumi, and A. Chiba, Mater.
Des. 61, 70 (2014).
[2] F. Wang, Y. Li, K. Yamanaka, K. Wakoh, K. Harata, and A.
Chiba, Mater. Des. 64, 441 (2014).

Keywords: powder processing, nanostructure, deformation
Akihiko Chiba (Deformation Processing Division)
E-mail: chibalab@imr.tohoku.ac.jp
URL: http://www.chibalab.imr.tohoku.ac.jp/english/index.html

IMR KINKEN Research Highlights 201513

Infrastructural Materials

Microstructure and crystallographic features of nodular
bainite in the Cu-Sn alloy system

We have investigated the microstructure and orientation relationship (OR) of the α and δ phases in an as-cast

ingot and nodular bainite of Cu-14.9 at% Sn alloy using transmission electron microscopy (TEM). It was foundthat the α and δ phases in nodular bainite exhibit both (111) α //(110)δ, [111]α //[110]δ orientation relationship as wellas (111) α //(110)δ, and the emergence and variations of these ORs between the α and δ phases have beeninterpreted from the viewpoints of minimum strain.

The phase transformation behavior of the CuSn alloy system with 5–30% Sn is complex becauseof the similarity of the structures and narrowcomposition ranges of the phases involved [1, 2].
The β, γ, and δ phases are all based on body
centered cubic (bcc) arrangements of Cu and Sn
atoms, which are known as electron compounds.
When β alloys are relatively slowly quenched, they
decompose into a mixture of α and δ phases,
displaying a heterogeneous microstructure, which issometimes called bainite. The present study was
initiated to elucidate the crystallography of theseheterogeneous reactions in the Cu-Sn alloy at theeutectoid β composition using a focused-ion beam
(FIB) technique, which allows local sampling of anarea of interest [3]. During the course of the research,however, we realized that the morphology of thenodular bainite in fact resembles that of the as-cast
ingot, and the subsequent crystallographic study
revealed a striking similarity with a subtle difference.
Figure 1 (a) and (b) are bright field (BF) anddark field (DF) images of a FIB specimen of anodular bainite region, showing that behind theapparently heterogeneous microstructure, there
exists an orientation relationship between the twophases. Figure 1 (c) is a corresponding diffraction
pattern obtained along the 110 axis of the δ phase,
as well as the 111 axis of the α phase. As seen, two
OR, i.e., Nishiyama-Wasserman (NW) andKurdjumov-Sachs (KS) coexist. The spread of the
220 α spot and the like indicates that there is aspread in the OR.
These variations in ORs between the α and δ
phases can be understood by considering: (i) the
matching of (110)β (or (660)δ and (111) α) spacings,
which favors the planes being parallel to each other,(ii) the minimum strain criteria put forward by theinvariant line theory for K-S OR, and (iii) the matchingof atomic positions in the respective layers in the αand δ phases.

Fig. 1 Microstructure and crystallography of nodular
bainite in Cu-Sn alloy (a) BF, (b) DF images, (c)diffraction pattern viewed along the 110 axis of the δphase, showing the OR between the α and δ phases.

References[1] Z.-J. Wang and T. J. Konno, Phil. Mag. 93, 949 (2013).
[2] Z.-J. Wang and T. J. Konno, Phil. Mag. 93, 1618 (2013).
[3] Z.-J. Wang and T. J. Konno, Phil. Mag. 94, 420 (2014).

Keywords: microstructure, metal, transmission electron microscopy (TEM)
Toyohiko J. Konno (Materials Science of Non-stoichiometric Compounds Division)
E-mail: tjkonno@imr.tohoku.ac.jp
URL: http://konno-lab.imr.tohoku.ac.jp/

14

IMR KINKEN Research Highlights 2015

IMR KINKEN Research Highlights 2015

Measurement of Excited Non-Metallic Atoms in Glow Discharge Plasma
by Diode Laser Coherent Forward Scattering Spectrometry (DL-CFS)

Excited non-metallic atoms could be sensitively probed by diode laser coherent forward scattering

spectrometry (DL-CFS) over the 640–930 nm wavelength range. The signal intensity depended on the 4th
power of the number density of argon in the helium plasma. CFS spectrometry was found to be very sensitive
to the excitation mechanisms of atoms in glow discharge plasma.

Coherent forward scattering (CFS) spectrometry
technique is based on two crossed polarizers, thepolarizer and the analyzer, which block the probe
light path when there is no atomic interaction.
Because of the anisotropy of resonant analyteatoms, which is induced by the applied external
magnetic field, the state of light polarization ischanged upon passing through the atomic vapor
between the polarizer and the analyzer, and light istransmitted. Without light interaction with theresonant atoms, the transmitted intensity is zero.
Transmitted CFS intensity is approximately
proportional to the square of the number density ofatoms. An illustration of this is shown in Fig. 1.

Fig. 1 Illustration of coherent forward scattering spectrometry.

CFS spectra of excited argon atoms, whichwere produced in a glow discharge plasma, at
842.46 nm were investigated [1]. The CFS signal
intensity attained a maximum at 20 mA discharge
current at 160 mT external magnetic field in 3 Torr
(399 Pa) of argon. Although the lifetime of the lower
level of the 842.46 nm line is short (8.4 ns), a
relatively strong absorption transition could beobserved probably because the population densityof the lower level of the 842.46 nm line may besustained by the electron quenching of argon
metastable atoms. A calibration curve of argon was
prepared to mix a small amount of argon into 6 Torr
(798 Pa) of helium as shown in Fig. 2. The CFS
intensity at 842.46 nm depended on the 4.4th powerof the number density of argon. This dependence is
too large to be explained by the theoretically
predicted quadratic dependence.
The dependence may be because of theFig. 2 Calibration curve of admixed argon with helium.

influence of the production processes of argon
metastable atoms connecting to the lower level of Ar(I) 842.46 nm. When a small amount of molecular
nitrogen was admixed into the argon plasma, astrong suppression of the CFS intensity at 842.46
nm was observed because of the reduction in argon
metastable atoms by collisions with molecular
nitrogen. Molecular gases such as air were found to
be unsuitable to excite target atoms. Noble gases
should be used for the plasma gas to produce
excited atoms.
CFS spectrometry was found to be very
sensitive to the excitation mechanisms of atoms inglow discharge plasma.
References[1] H. Matsuta, ISIJ Int. 55, 213 (2015).

Keywords: spectroscopy, glow discharge plasma, magnetic properties
Hideyuki Matsuta (Analytical Science Division)
E-mail: matsuta@imr.tohoku.ac.jp
URL: http://wagatsuma.imr.tohoku.ac.jp/
IMR KINKEN Research Highlights 201515

Infrastructural Materials

Development of Mg-based Bulk Metallic Glasses forApplication as Biodegradable MaterialsN

ew Mg-based bulk metallic glasses (BMGs) with high strength and high corrosion resistance have beendeveloped. The corrosion rate and strength can be controlled by a small addition of the element (Ag). Using a
spark plasma sintering process, large Mg-based BMGs with high strength and high corrosion resistance havebeen fabricated, making it possible to use the Mg-based BMGs in biomedical fields.
Bulk metallic glasses (BMGs) have attracted
great interest because of their properties such as
high mechanical strength, high hardness, highfracture toughness, excellent corrosion resistance,and high wear resistance, compared to conventional
crystalline metallic alloys. Many BMG systems havebeen developed so far. Among them, Mg-based
BMGs are of great interest for use as biodegradable
implant materials, such as cardiovascular or
orthopedic devices. Recently, Mg-based (Mg-ZnCa) BMGs have demonstrated a great reduction inhydrogen evolution during in vitro and in vivo
degradation [1], showing a potential for application
as biomedical materials.
We developed new Mg-based BMGs with highstrength and high corrosion resistance. Effects ofminor Ag addition on the mechanical properties andcorrosion behavior of the Mg-Zn-Ca glassy alloys
were investigated. The results reveal that the as-cast
Mg-Zn-Ca BMG exhibits a high fracture strength ofover 720 MPa. It is further increased with 1 at.% Agaddition, as shown in Fig. 1(a). The corrosion rate insimulated body fluid (SBF) Hanks’ solution andstrength of the Mg-based BMGs can be controlled
by adjusting the Ag content (Fig. 1). The addition of 1
at.% Ag and a homogeneous structure areresponsible for improved strength and corrosion
resistance. With more Ag addition, the strength of thealloy samples decreases, which originates from adecrease in the glass forming ability of the alloys [2].
It is known that the BMGs are commonly
produced using solidification techniques such ascopper mold casting. A rather high cooling rate isrequired to suppress the formation of more
thermodynamically stable crystalline phases.
Sample size and shape are limited, thereby limiting
the range of their applications. On the other hand, it
is possible to produce larger metallic glassy alloyparts in a variety of shapes using the consolidation
technique [3]. Using a spark plasma sintering (SPS)
technique, we developed Mg-based (Mg-Zn-Ca)
BMGs and composites in a cylindrical shape with a

(a)

(b)

Fig. 1 (a) Compressive stress-strain curves of as-cast MgZn-Ca-Ag alloy rods with a diameter of 2 mm under astrain rate of 5×10 -4 s-1, (b) corrosion rates of pure Mg
and Mg-Zn-Ca-Ag metallic glasses in Hanks’ solution.

diameter of over 15 mm [4]. The consolidated Mgbased BMGs exhibited a high strength of over 450
MPa. The corrosion resistance of sintered Mgbased BMGs and the composites was evaluated tobe higher than that of pure Mg and commercial
AZ31 alloy in Hanks’ solution.
References[1] B. Zberg, P. J. Uggowitzer, and J. F. Löffler, Nature Mater. 8,887 (2009).
[2] F. X. Qin, G. Q. Xie, Z. H. Dan, S. L. Zhu, and I. Seiki,
Intermetallics 42, 9 (2013).
[3] G. Q. Xie, J. Powder Metall. Min. 2, e109 (2013).
[4] F. X. Qin, Z. H. Dan, and G. Q. Xie, Mater. Sci. Forum 783,
1931 (2014).

Keywords: metallic glass, powder metallurgy, mechanical properties
Guoqiang Xie (Advanced Materials Development and Integration of Novel Structured Metallic and Inorganic Materials)
E-mail: xiegq@imr.tohoku.ac.jp
URL: http://amdi-pro.imr.tohoku.ac.jp/en/index.html

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IMR KINKEN Research Highlights 2015

Liquid-solid Phase Transformation Studied through a Large
Scale Computer SimulationLiquid-solid phase transformation is of fundamental importance in controlling the microstructure of engineering
materials. Molecular dynamics simulation has been recognized as a powerful tool for analyzing transformation
behavior. Introduced below are two recent simulations attempted by key members of CMRI. One is on thesolidification process of monolayer colloids by Prof. Terada and the other is the analysis of anisotropy ofinterfacial velocity for Cu-Al alloy by Dr. Deb Nath. A large-scale computer simulation reveals processes thatcannot be readily accessed by ordinary experiments.
Effect of Impurities on the Solidification Processof Monolayer Colloids*
It is important to understand the behavior of
impurities on the solidification process for controlling
segregation. We constructed a simulation model ofthe formation of monolayer colloidal crystals from
uniform dilute suspensions based on the experimentsof an advection accumulation method. By performinga large-scale Brownian dynamics simulation of thesolidification process of monolayer colloids with theaddition of differently sized impurities, we discussed
the effect of size difference between normal colloids
and impurities. We found that big impurities induce
defects in the crystal region, and they move from
more dense regions to less dense regions near theinterfaces [1]. Thus, the fraction of big impurities in
the crystal region is smaller than that in the initialsuspensions.
Melting, Solid-Liquid Interfacial Velocity of CuAl Alloys**
Cu-Al alloys are widely used in many industrial
applications; but details of the solid-liquid interfacial
properties of these alloys are still unknown. Here,we evaluate the melting point and solid-liquidinterface velocity of Cu-Al alloys using molecular
dynamics simulations. The obtained melting point of
Cu-Al alloys will be compared with the experimental
results to verify the present simulation results. Deb
Nath and his co-authors [2] presented an outline of
the molecular dynamics simulation to estimate thesolid-liquid interfacial properties of Cu-Al alloys.
Figure 2 illustrates the snapshot of solid-liquidinterface movement at step 200,000 when Cu50Al50
is heated above the melting point. The anisotropy ofthe solid-liquid interfacial velocity above and below
the melting point is observed, as shown in Fig. 3.

Fig. 1 Picture of 98% normal colloids with radius a and 2%
big impurities with radius 1.5a at the late stage ofsolidification process. Each color of the normal colloid
indicates the number of nearest neighbor colloids. Dark
red particles indicate a large impurity.
Solid-liquid interface

Fig. 2 Picture of Cu50Al50
at step 200,000.

Fig. 3 Solid-liquid interfacial
velocity of Cu50Al50.

References[1] Y. Terada, “Brownian dynamics simulation of monolayer
colloids with repulsive interactions in crystal growth”, 2014
MRS Fall Meeting, USA/Boston, Dec. 3, 2014.
[2] Presented by S. K. Deb Nath, “Investigation of solid-liquidinterface of Cu-Al alloys during melting”, 11th CMSI Young
Researcher Technical Workshop, Shizuoka, Feb. 5, 2015.

Keywords: simulation, phase transformation, interface
Tetsuo Mohri, *Yayoi Terada and ** Sankar Deb Nath (Computational Materials Research Initiative)
E-mail: tmohri@imr.tohoku.ac.jp
URL: http://www.cmri-tohoku.jp/

IMR KINKEN Research Highlights 201517

Energy-related Materials

IMR KINKEN Research Highlights 2015

Energy-related Materials

Magnon Interactions Modifying Spin CurrentsT

he spin Seebeck effect (SSE) causes spin currents to flow in an insulator when a thermal gradient is applied.
Simple theories can describe this effect, but it is not well known how the spin is generated and transported incomplicated ferrimagnetic garnets. The spin wave modes present in these materials can contribute to the SSE.
Using detailed numerical calculations, we are beginning to understand the important role played by spin waves
in the ferromagnetic order of these materials.

f (THz)8[1] K. Uchida, J. Xiao, H. Adachi, J. Ohe, S. Takahashi, J. Ieda, T.
Ota, Y. Kajiwara, H. Umezawa, H. Kawai, G. E. W. Bauer, S.
Maekawa, and E. Saitoh, Nature Mater. 9, 894 (2010).

Keywords: spintronics, insulator, spin wave
Joseph Barker (Theory of Solid State Physics Division)
E-mail: joseph.barker@imr.tohoku.ac.jp
URL: http://www.bauer-lab.imr.tohoku.ac.jp/

20

IMR KINKEN Research Highlights 2015

6

4

2

0

Γ

N

160K
120K
100K
80K
60K
40K
20K
0.0

References

T = 20K

10PSD (arb.)

The spin Seebeck effect [1] is typically observedin ferrimagnetic insulators, such as yttrium iron garnet(YIG); however, other materials, such as gadolinium
iron garnet (GdIG), are also being explored. A
temperature gradient across the insulator pumps a net
spin current into an attached metal such as Pt. Thiseffect has potential uses in spintronics and energy
scavenging, but more material-specific understanding
is required to fully exploit this.
Recent experiments conducted in Munich, Mainz,
and Sendai revealed that the SSE changes sign twicein the GdIG | Pt system; once at the magnetizationcompensation temperature and a second time at alower temperature where there are no significant
changes in the macroscopic magnetic properties.
Using numerical simulations based on detailed
atomistic spin models, we studied how the spin wave
spectra of YIG and GdIG change with temperature;
this was done to better understand the role of magnon
interactions in the thermally induced transport of spincurrent. We found that the polarization of the spin wave
modes determines the sign of the contribution to theSSE. The frequency and amplitude of the spin wave
modes change with temperature.
The many-body nature of the numerical approachprovides information about dispersion and amplitudeof all spin wave modes. Most of them broaden but
remain almost stationary in frequency as thetemperature is increased. However, we observe in Fig.
1 that one optical mode is significantly red-shifted andthe magnon gap at 2 THz increasingly closes withincreasing temperature. The amplitude of this mode
also increases, giving rise to an increasing contribution
to the SSE with opposite sign compared to that of thelowest ferromagnetic magnon band. In GdIG, thisleads to the observed second sign change.

0.5

1.0

1.5 2.0 2.5 3.0
Frequency (THz)

3.5

4.0

Fig. 1 (top) Low-frequency part of the GdIG spin wave
spectrum. The magnon-gap at 2 THz is caused by thepresence of the Gd. (bottom) Line scan at k = 0 showsthat this gap reduces with increasing temperature.

IMR KINKEN Research Highlights 2015

Cu Precipitation Conditions at Small Angle Tilt Boundaries in Si

I

mpurity gettering at grain boundaries plays a decisive role in determining macroscopic properties of mostpolycrystalline materials, as well as in fabricating stable nanostructures. We have recently shown the Cu precipitation
conditions at small-angle tilt boundaries (SATBs) in Si crystals depending on Cu contamination levels, which aresimilar to the Cu precipitation conditions in metallic alloys. Here, we highlight the nanoscopic mechanism of the Cu
precipitation, which may be universal in non-insulating polycrystalline materials with metallic impurities.
Impurity gettering at grain boundaries in Si
crystals influences the photovoltaic properties of solar
cells fabricated with polycrystalline Si. Accordingly, acomprehensive knowledge of impurity gettering
conditions is essential for engineering size and spatial
distributions of impurity-related nanostructures at grainboundaries in a controlled manner in order to produce
cost-effective functional solar cells.
Here, we determined the Cu precipitation
conditions at small-angle tilt boundaries (SATBs) on(220) in a Czochralski-grown p-type Si crystal [1] byhigh-resolution transmission electron microscopy(HRTEM) and ab-initio calculations [2] combined withatom probe tomography (APT) [3]. The Cu precipitation
process was explained in terms of the balance of the
precipitation and interface energies for the precipitates
as well as the strain energy due to lattice mismatchand edge dislocations in the SATBs (EDLs). During theinitial stage of precipitation, Cu atoms agglomerated
along SATBs forming coherent layers of Cu3Si with abody-centered cubic structure in a meta-stable state(a = 0.285 nm) (Fig. 1). As the layers thickened, they
became semi-coherent layers with misfit dislocations
on the (220) interphase boundaries to reduce
coherency strains (Fig. 2). Subsequently, as Cu atoms
agglomerated further, the metastable layers converted
into incoherent polyhedrons of orthorhombic η”-Cu3Si
in the thermal equilibrium state, forming interphaseboundaries on {112} in Si (Fig. 3).
The Cu precipitation process observed in Si
appears to be similar to that in metallic alloys; theformation of Guinier-Preston (GP) zones followed byconversion into the equilibrium θ-phase. The
precipitation process may be universal in noninsulating polycrystalline materials with metallic
impurities. Our finding can provide a general guide tocontrol Cu contaminants in polycrystalline Si, which
will enable lower-quality Si feedstock to be fabricatedinto cost-effective functional solar cells. In addition, it
has potential applications in Cu-Si interface
nanotechnologies such as nanoscale field-emittersand interconnects.

Fig. 1 (a) HRTEM of a coherent bcc-Cu3Si layer at a SATB
in an as-grown Si. (b) Lattice fringes of (220) in (a).

Fig. 2 (a) HRTEM of a semi-coherent bcc- Cu3Si layer at a
SATB in an annealed Si. (b) Lattice fringes of (220) in (a).

Fig. 3 Projected 3D distribution of incoherent polyhedronsof orthorhombic η”-Cu3Si at a SATB in a annealed Si
viewed (a) parallel to and (b) normal to the SATB. (c)Diffraction pattern of the polyhedrons for [220]incidence.

References[1] Y. Ohno, K. Inoue, K. Kutsukake, M. Deura, T. Ohsawa, I.
Yonenaga, H. Yoshida, S. Takeda, R. Taniguchi, H. Otubo, S. R.
Nishitani, N. Ebisawa, Y. Shimizu, H. Takamizawa, K. Inoue, and Y.
Nagai, Phys. Rev. B 91, 235315 (2015)[2] Y. Yamamoto, K. Togase, Y. Ohno, I. Yonenaga, and S. R.
Nishitani, Jpn. J. Appl. Phys. 53, 061302 (2014).
[3] Y. Ohno, K. Inoue, K. Fujiwara, M. Deura, K. Kutsukake, I.
Yonenaga, Y. Shimizu, K. Inoue, N. Ebisawa, and Y. Nagai, Appl.
Phys. Lett. 106, 251603 (2015).

Keywords: solar cells, grain boundaries, defects
Yutaka Ohno (Physics of Crystal Defects Division)
E-mail: yutakaohno@imr.tohoku.ac.jp
URL: http://lab-defects.imr.tohoku.ac.jp/index.html

IMR KINKEN Research Highlights 201521

Energy-related Materials

Multiphysics Simulation Based on Quantum Chemical
Molecular Dynamics Method

Recent material and system technologies constitute of multiphysics phenomena including chemical reaction,

friction, impact, stress, fluid, photon, electron, heat, electric field etc. and then a deep understanding of theabove multiphysics phenomena is strongly required. Therefore, we have pioneered multiphysics simulation
technologies based on our quantum chemical molecular dynamics method.

To solve current energy and environmental
problems, high-functional and high-performancematerials are highly demanded in a wide variety of
research fields, such as tribology, aerospace
instruments, fuel cell, solar cell, micromachine, etc. In
particular, recent material and system technologies
constitute of multiphysics phenomena including
chemical reaction, friction, impact, stress, fluid,
photon, electron, heat, electric field etc. and then adeep understanding of the above multiphysics
phenomena is strongly required. However, traditional
simulation methods, such as first-principles
calculations and fluid mechanics, cannot simulate
the above multiphysics phenomena. Therefore, wehave developed new multiphysics computational
simulation technologies based on our quantum
chemical molecular dynamics method for pioneering
next-generation material design [1-3].
Diamond-like carbon (DLC) is expected to be
used as a solid lubricant. Thus, to design low-friction
DLC films, we investigated the effects of H- andF-termination on the friction behavior of DLC films [1].
Figure 1 shows that the friction coefficients for the
H-terminated DLC (H-DLC) and F-terminated DLC(F-DLC) films are 0.38 and 0.10, respectively, under ahigh contact pressure of 7 GPa. Moreover, we foundthat C–C bond formation during friction causes thefriction coefficient of the H-DLC film to be higher thanthat of the F-DLC film and the strong repulsive
interaction between the F atoms is the reason for less
C-C bond formation of the F-DLC film than that of the
H-DLC film. Thus, we suggest that F-termination
improves the friction properties of DLC films.
Thin-film Si is expected to be a next-generation
solar cell. The surface smoothening during thechemical vapor deposition (CVD) process is key for
its performance. Therefore, we performed Si CVD
simulations and proposed a dangling bond (DB)diffusion model as an alternative to the SiH3 diffusion
model, which does not match the experiments. In ourmodel, the DBs diffuse rapidly along an upper layer
consisting of Si-H3 sites and then migrate from theFig. 1 Friction coefficients for H- and F-DLC films under apressure of 7GPa.

Fig. 2 Snapshots of Si CVD simulation at (a) 35 ps, (b) 36
ps, (c) 39 ps, and (d) 40 ps.

upper layer to a lower layer consisting of Si-H sites.
The subsequently incident SiH3 radical is thenadsorbed onto the DB in the lower layer, causing twodimensional growth. Our findings can explain thereason why the layer-by-layer growth occurs [2].
References[1] S. Bai, H. Murabayashi, Y. Kobayashi, Y. Higuchi, N. Ozawa, K.
Adachi, J. M. Martin, and M. Kubo, RSC Adv. 4, 33739 (2014).
[2] T. Kuwahara, H. Ito, K. Kawaguchi, Y. Higuchi, N. Ozawa, and
M. Kubo, Sci. Rep. 5, 9052 (2015).
[3] H. Ito, T. Kuwahara, K. Kawaguchi, Y. Higuchi, N. Ozawa, S.
Samukawa, and M. Kubo, J. Phys. Chem. C 118, 21580 (2014).

Keywords: simulation, molecular dynamics, surface reaction
Momoji Kubo (Materials Design by Computer Simulation Division)
E-mail: momoji@imr.tohoku.ac.jp
URL: http://www.simulation.imr.tohoku.ac.jp/

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IMR KINKEN Research Highlights 2015

Effects of Ion Implantation on Hydrogen and Helium in SiC
for Fusion Applications

T

he retention/release behavior of H isotopes and He is important in controlled fusion devices. We found asignificant accumulation of hydrogen in a damaged layer upon air exposure after completion of the thermal
release of He implanted in SiC single crystals.

10 keV He+
10 keV D2+

Detector for
Backscattering
Analyzing beam
5.0 MeV O++++

Detector for
Recoil particles
Absorbing foilαϕDepth xSample surface

60

He implanted SiC50

0 nm
1270 K
1170 K

40

Recoil yield

Silicon carbide (SiC) is a promising material forcomponents such as first wall, tritium breedingblankets, and insulating materials, in nuclear fusion
reactors owing to its inherent properties, such as
low activation, good thermal conductivity, andstability at high temperatures. As a first wall material,
SiC is subjected to severe high-energy neutronsand energetic He bombardment from the plasma.
The retention of tritium in breeding materials is a
critical issue from the viewpoint of tritium recycling[1]. We investigated the ion irradiation effects andthe interactions between H and He in SiC crystals
using the elastic recoil detection analysis [2].
The depth profiling during isochronal annealing
experiments showed that the D retention in He+
implanted SiC at 1370 K was smaller than that for D
implanted into the un-irradiated SiC crystal. The
reduction in D retention at 1370 K can be explained
by He implantation-induced damage, which
suppressed the formation of C–D bonds. Besides,
the morphological changes during thermal releaseof He may contribute to accelerate the recombination
process of D atoms. A significant increase in He
release temperature was found in the D2+ implanted
SiC, indicating suppression of He bubble formation
by D remaining in SiC at high temperature. The
damage induced by He and D implantation was notrecovered from the saturating concentration even at1370 K. The release behavior of ion-implante D and
He has no relevance to the annealing behavior of
the implantation-induced disordered layer observed
by Rutherford backscattering combined with the ionchanneling technique. The H atoms wereaccumulated in the damaged layer upon air
exposure after completion of the thermal release of
He in a vacuum, probably due to the dissociation ofwater molecules at the surface. However, H was notobserved in the D implanted SiC even after D wasreleased.

H

He

20

0

400

600

800

1000

1200

Detected energy (keV)
Fig. 1 (Top) Schematic illustration of H and He detection
using the ion beam. (Bottom) Recoil spectra from He
implanted SiC after annealing at 1170 and 1270 K for 10
min in vacuum.

For the implantation at 770 K, the D retention
and D-ion-induced disorder were drasticallydecreased compared with implantation at 300 K.
The He pre-implantation induced damage did not
influence the D retention at 770 K, while the He-ioninduced displacements were considerably larger
than those induced by the D implantation alone.
References[1] S. Nagata, H. Katsui, K. Hoshi, B. Tsuchiya, K. Toh, M. Zhao,
T. Shikama, and E. R. Hodgson, J. Nucl. Mat. 442, 501 (2013).
[2] Y. Ishikawa, S. Nagata, M. Zhao, and T. Shikama, J. Nucl. Mat.
455, 512 (2014).

Keywords: hydrogen implantation, ion beam analysis, nuclear materials
Shinji Nagata (Nuclear Materials Science Division)
E-mail: nagata@imr.tohoku.ac.jp
URL: http://www-lab.imr.tohoku.ac.jp/~wshikama/index.html

IMR KINKEN Research Highlights 201523

Energy-related Materials

Development of an Advanced Expansion due to Compression(A-EDC) Test Method for Safety Evaluation of Nuclear Fuel Cladding

The expansion due to compression (EDC) test was modified to evaluate the performance of tubular materials likenuclear fuel claddings where pellet-cladding mechanical interaction (PCMI) triggers the larger hoop deformationof the pellets, particularly during accidental transients. The purpose of this study is to modify the EDC test by
specimen volume reduction and other methods to overcome its shortages and disadvantages. Using ring-shaped
specimens and cylindrical metal pellets as the inner pellets, expansion of the specimens due to the inner pellet
compression was performed. Together with the analysis using the finite element method, the experimental set-up
was optimized. The ideal uniaxial tension condition in the hoop direction up to the failure, as well as excellent
reproducibility with less error, was confirmed. Stress–strain curves in the hoop direction were successfully derived.

Tubular materials like nuclear fuel cladding are
generally required to maintain their mechanical strength
during exposure to an environment where corrosion,hydrogenation, and/or irradiation damage are present[1]. In the case of nuclear fuel claddings, an increase in
the stress and strain in the hoop (circumferential) direction
is caused by an increase in the inner pressure becauseof accumulation of fission product gas in the gap
between the fuel pellet and the cladding, pellet-cladding
mechanical interaction (PCMI) by swelling of fuel pellets,
and the resulting bonding. Though the EDC and othertests have been applied to evaluate the material
performance, shortages and disadvantages werepointed out, such as limitation of the test temperatureand limitation in post irradiation experiments arising fromthe radioactivity. The purpose of this study is, therefore,
to modify the EDC test to describe PCMI, specimen
volume reduction, and others [2].
Ring-shaped specimens were cut from Zry-4
cladding tubes. Cylindrical metal pellets 8 mm indiameter and 15 mm in maximum height were used as
inner pellets. Expansion of the specimens because of
inner pellet compression was performed at room
temperature. The experimental data were further
analyzed by the finite element method. Through asurvey of the variation of the specimen and inner pellets,
the geometry of the specimens and inner pellets were
optimized. Excellent reproducibility with less error was
confirmed. The uniaxial tension condition in the hoop
direction up to specimen failure was confirmed. Hoop
stress–hoop strain curves were successfully derived.
In this work, the problems of the conventional EDC
test were elucidated, and an advanced EDC (A-EDC)
test was developed. Employing metallic inner pellets
and ring-shaped specimens, a great reduction in thesample volume was achieved. Reduction in radioactivity
is expected in material test reactors or accelerator
irradiation experiments and post-irradiation experiments

Fig. 1 Load-displacement curves of A-EDC test. Inset
shows the experimental setup.

Fig. 2 Hoop stress and hoop strain curves in Cu innerpellets with 1 and 2 mm specimen width.

under varied environmental conditions. Using
experiments and FEM simulations, ideal uniaxial hoop
tension up to the fracture of specimen, with better
reproducibility and less error, was confirmed.
References[1] H. L. Yang , J. J. Shen, Y. Matsukawa, Y. Satoh, S. Kano, Z.S. Zhao,
Y. F. Li, H. Abe, J. Nucl. Sci. Tech., (2015) available online.
[2] H. Abe, T. Abe, S. Kishita, S. Kano, Y. Satoh, Y. F. Li, H. L. Yang, K.
Tawara, and Y. Matsukawa, J. Nucl. Sci. Tech., (2015) in press.

Keywords: nuclear materials, mechanical properties, zirconium alloy
Hiroaki Abe (Nuclear Materials Engineering Division)
E-mail: abe.hiroaki@imr.tohoku.ac.jp
URL: http://www-lab.imr.tohoku.ac.jp/~abeken/

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IMR KINKEN Research Highlights 2015

High-efficiency N-polar InGaN/GaN Solar Cells with
Promotion of Photocarrier Extraction by Polarization Fields

N

Ga
PSP PPZ

N

Ga
Ga

Ga

N
N

N
(a) Ga-polarity<0001̄>The nitride semiconductors InGaAlN are promisingmaterials not only for light-emitting devices, but also forhigh-efficiency solar cells because their band-gap canencompass the entire solar spectrum. In nitride
semiconductors with a wurtzite crystal structure, astrong polarization is generated along the <0001> axis
as shown in Fig. 1. By adopting the nitrogen polar (0001)
plane during growth for fabricating solar cells, thepolarization field can be incorporated in the built-in field,
which drives photo-carriers to the metal electrodes in aconventional solar cell with a p-i-n structure. Thesepolarization fields are expected to improve theconversion-efficiency. For this purpose, the techniquefor N-polar growth is extremely useful. However, all thedevices developed until now and available commerciallyuse Ga-polarity. We have already reported N-polar
growth with a smooth surface [1, 2] and p-typeconduction [3] by metalorganic vapor phase epitaxy(MOVPE). In addition, we have succeeded in the
fabrication of LEDs with InGaN/GaN multiple quantum
wells (MQWs) from red to blue colors [4]. In this paper,an N-polar solar cell fabricated on a sapphire substrateis described. Their characteristics are also shown andcompared to the Ga-polar cells.
The N-polar solar cell consists of undoped GaN,
Si-doped n-type GaN, InGaN/GaN MQWs, and p-type
GaN, which is the same as a conventional GaN-based
light-emitting diode.
The current- and the power-density versus voltage
characteristics were evaluated under 1 sun of the AM1.5
equivalent illumination. The N-polar InGaN/GaN solar
cell operation was demonstrated for the first time. The
open-circuit voltage and the short-circuit current density
were 0.85 V and 62.4 µA/cm2, respectively. The biasdependent spectral responses are shown in Fig. 2. For
the Ga-polar solar cell, the photocurrent strongly
depended on the bias voltage. This indicates that boththe built-in field and the polarization field are not effective
for the extraction of photo-carriers. For the N-polar solarcell, the photocurrent does not depend on the bias
voltage and both fields can be considered to effectively
play a role in the extraction of photo-carriers.

<0001>-polar nitride-based devices have a strong electric field due to both spontaneous polarization andpiezoelectric polarizations. In a p-i-n structure of solar cells, this field is applied along the same direction as thebuilt-in field, and is expected to enhance the extraction of photo-carriers. We have demonstrated N-polar
InGaN/GaN solar cells for the first time and confirmed this effect.

N
N
N
PSP PPZ Ga

N
Ga

Ga
Ga

(b) N-polarity

Fig. 1 Atomic arrangements and direction of spontaneous(PSP) and piezoelectric (PPZ) polarization for GaN undercompressive strain.

(a) Ga-polarity(b) N-polarity

Fig. 2 Bias-dependent spectral response for InGaN/GaN
solar cells with different crystallographic polarity.

In conclusion, N-polar solar cells have the potential
to achieve high-efficiency solar cells.
References[1] T. Matsuoka, T. Mitate, H. Takahata, S. Mizuno, Y. Uchiyama,
A. Sasaki, M. Yoshimoto, T. Ohnishi, and M. Sumiya, phys. stat.
sol. (b) 243, 1446 (2006).
[2] T. Aisaka, T. Tanikawa, T. Kimura, K. Shojiki, T. Hanada, R.
Katayama, and T. Matsuoka, Jpn. J. Appl. Phys. 53, 085501 (2014).
[3] T. Tanikawa, K. Shojiki, T. Kimura, S. Kuboya, T. Hanada, R.
Katayama, and T. Matsuoka, Jpn. J. Appl. Phys. 53, 05FL05 (2014).
[4] K. Shojiki, J. H. Choi, H. Shindo, T. Kimura, T. Tanikawa, T.
Hanada, R. Katayama, and T. Matsuoka, Jpn. J. Appl. Phys. 53,
05FL07 (2014).

Keywords: nitride, solar cells, vapor phase epitaxy (VPE)
Takashi Matsuoka (Physics of Electronic Materials Division)
E-mail: matsuoka@imr.tohoku.ac.jp
URL: http://www.matsuoka-lab.imr.tohoku.ac.jp/?TOPPAGEIMR KINKEN Research Highlights 201525

Energy-related Materials

Bulk Nanoporous Silicon Negative Electrode with Extremely High
Cyclability for Lithium-Ion Batteries Prepared Using a Top-Down ProcessA

top-down process utilizing dealloying in a metallic melt was used to obtain freestanding three-dimensional
nanoporous bulk Si. We fabricated a high capacity negative electrode (capacity approaching the theoreticallimit) for lithium ion battery with nanoporous Si. Compared to conventional electrodes made from Si nanoparticles,
our electrode material greatly extended the lifetime, and significantly improved the battery performance.
Nanoporous structures that are capable of accommodating large changes in volume during charging are
responsible for the prolonged lifetime cycle and superior performance.
Si is a promising active material as the negativeelectrode of Li-ion batteries (LIBs) because it has atheoretical Li capacity of ~4200 mAh/g, approximately 10
times that of carbon-based electrodes. However, Si
electrodes suffer from volume changes of more than 400%
with Li insertion and removal. This large volume change
pulverizes the conductive network between the active
material and the current collector, rapidly degrading thecyclic performance. Many researchers have studied ways
to accommodate this volume change by using
nanostructured Si including thin films, nanotubes,
nanowires, and nanoporous structures. The nanoporous
Si-based negative electrodes exhibited improved cyclic
performance because the many open channels in such
structures act as ideal volume expansion buffers. Despite
such promising performance of the nanoporous Si-based
electrodes, the preparation has generally required several
complicated steps, such as fabricating a template fordeposition of material, making the preparation expensive,
slow, and difficult to scale up. To overcome these issues,
we originally developed dealloying in a metallic melt [1, 2],a simple top-down process for fabricating nanoporous Si.
We synthesized freestanding 3D nanoporous Si byimmersing a precursor of Mg2Si intermetallic compoundin a pure Bi melt at 1123 K. Because the Mg is miscible in
Bi but Si is not, the Mg atoms in the precursor selectively
dissolved into the Bi melt, and the remaining unstabilized
Si atoms spontaneously organized into a 3D nanoporous
structure with average pore size and porosity of 400 nm
and 60.4%, respectively, as shown in Fig.1.
Using this nanoporous Si, we fabricated negativeelectrodes for LIBs. Figure 2(a) shows the cyclic
performance of the electrode operated in a constantcurrent (CC) mode. The electrode exhibited almost
theoretical capacity (3579 mAh/g), and excellent capacity
retention of >2000 mAh/g over 200 cycles, significantly
improved compared to electrodes fabricated from acommercial Si nanoparticle. We further improved theperformance of the electrodes by controlling the depth oflithiation based on the relation between the volume
change and the porosity of nanoporous Si. Figure 2b
shows cyclic performance of the electrode operated at aconstant charge capacity of 1000 mAh/g, in which thevolume change and the pore volume become nearly
equal. The capacity of the electrode didn’t degrade over
Keywords: foam, porosity, energy storage
Hidemi Kato (Non-equilibrium Materials Division)
E-mail: hikato@imr.tohoku.ac.jp
URL: http://www.nem.imr.tohoku.ac.jp/

26

IMR KINKEN Research Highlights 2015

Fig. 1 SEM image of the 3D nanoporous Si prepared byimmersing a precursor of Mg2Si intermetallic compoundin a pure Bi melt at 1123 K.

Fig. 2 Cyclic performance of the nanoporous Si electrodes.
(a) constant current mode at 0.5 C and (b) constantcharge capacity mode at 1000 mAh/gSi. The 1 C rate
corresponds to 3579 mA/gSi.

1500 cycles, which is greatly prolonged compared to that
operated by CC mode, indicating that the 3D nanoporous
structure effectively accommodated volume expansion
due to lithiation, which was helpful for improving capacity
and life cycle of LIB electrodes.
References[1] T. Wada, T. Ichitsubo, K. Yubuta, H. Segawa, H. Yoshida, and H.
Kato, Nano Lett. 14, 4505 (2014).
[2] J. W. Kim, M. Tsuda, T. Wada, K. Yubuta, and H. Kato, Acta Mater.
84, 497 (2015).

IMR KINKEN Research Highlights 2015

Improvement in Crystal Quality of Protein Crystals under anExternal Electric Field

C

rystal quality and crystal homogeneity of protein crystals were improved by application of an external
electric field. This method utilizing an external electric field is a novel crystallization technique for proteins,which can be employed not only to enhance the nucleation rate but also to improve the crystal quality.
In order to allow both structure-guided drug
design and controlled drug delivery, it is important
to determine the 3D structures of protein molecules.
At present, these structures are primarily determinedusing X-ray diffraction (XRD) analysis, and therefore,high-quality single crystals of proteins are required,even though these are quite difficult to obtain. Thus,
the establishment of a crystallization technique thatcan provide high-quality single crystals of proteinsis required.
Previously, we have concentrated on controllingthe nucleation rate of proteins by applying an external
electric field. Consequently, the control of both theincrease and the decrease in nucleation rate forproteins was successfully achieved under applicationof an electric field by focusing on the chemical
potentials of the liquid and solid [1,2]. Therefore, if
crystal quality could also be improved by applicationof an external electric field, then, this crystallization
technique could be an excellent tool.
Figure 1 shows the two-dimensional maps ofthe full width at half maximum (FWHM) measured
for the 440 and the 12 12 0 reflections from tetragonal
hen egg white (HEW) lysozyme crystals preparedwith and without an external electric field. As shownin Fig. 1, it was found that the values of FWHM withan external electric field at 1 MHz were smaller thanthose without an external field [3], and the overall
crystal quality was homogeneous for the crystals
prepared with an external electric field [4]. Thissuggests that the crystal quality and crystal
homogeneity of protein crystals can be improved by
the application of the external electric field at 1 MHz.
Moreover, from the FWHM analysis of the rocking
curves, it was determined that improvements in thequality of tetragonal HEW lysozyme crystals are
attributed to a decrease in the misorientation
between subgrains in the crystal during crystal
growth [5]. This crystallization technique is expected
to enhance the resolution of protein molecular
structure analysis by X-ray diffraction.
References[1] H. Koizumi, K. Fujiwara, and S. Uda, Cryst. Growth Des. 9,2420 (2009).
[2] H. Koizumi, Y. Tomita, S. Uda, K. Fujiwara, and J. Nozawa, J.
Crystal Growth 352, 155 (2012).
[3] H. Koizumi, S. Uda, K. Fujiwara, M. Tachibana, K. Kojima, and
J. Nozawa, J. Appl. Cryst. 46, 25 (2013).
[4] H. Koizumi, S. Uda, K. Fujiwara, M. Tachibana, K. Kojima, and
J. Nozawa, AIP Conference Proceedings 1618, 261 (2014).
[5] H. Koizumi, S. Uda, K. Fujiwara, M. Tachibana, K. Kojima, and
J. Nozawa, Cryst. Growth Des. 14, 5662 (2014).

Fig. 1 Two-dimensional maps of FWHMs measured for the 440 and the 12 12 0 reflections from tetragonal HEW lysozyme
crystals prepared with and without an external electric field. (a) and (c) No electric field, and (b) and (d) applied fields at
1 MHz. The upper-right scale bar for each two-dimensional map represents 0.3 mm.

Keywords: crystal growth, X-ray diffraction (XRD), biologicalHaruhiko Koizumi and Satoshi Uda (Crystal Chemistry Division)
E-mail: h_koizumi@imr.tohoku.ac.jp
URL: http://www.uda-lab.imr.tohoku.ac.jp/

IMR KINKEN Research Highlights 201527

Energy-related Materials

Hydrogen Storage Materials with Optimal Thermodynamic Stability

Among the methods spanning many approaches, solid-state hydrogen storage using complex transition-

metal hydrides as a storage medium has a clear advantage in weight and volume efficiencies. The major
disadvantage is the high thermodynamic stability that makes reversible room temperature applications difficult.
Here, we present a strategy for designing complex hydrides with thermodynamic stability suitable for hydrogenstorage.

Dimagnesium iron(II) hydride (Mg2FeH6, left
panel in Fig. 1) is a prototypical member of a familyof complex transition-metal hydrides, where covalent
Fe–H interactions in a tetravalent [FeH6]4– anion arestabilized by charge transfer from two divalent Mg2+
cations. While this material reversibly stores a large
amount of hydrogen, the relatively highthermodynamic stability makes room temperaturehydrogen storage applications challenging. In thisregard, we recently reported a linear relationshipbetween the standard heat of formation and cationelectronegativity for the (M,M ’)2FeH6 system; i.e.,
the thermodynamic stability can be tuned bysubstituting Mg with elements of different
electronegativity [1]. While this relationship provides
a useful insight into the rational design of newcomplex hydrides, the tunability of thermodynamic
stability is practically limited to some extent becauseof a small number of possible combinations ofcations to compensate the tetravalent charge stateof the [FeH6]4– anion. One strategy for increasingthese combinations is to raise the total anioniccharged state. To this end, here we introduce anovel method for increasing the charge state byincorporating H – anions in complex hydrides, as
shown in the right panel in Fig. 1 [2].

Fig. 2 Standard heat of formation as a function of cationelectronegativity
forthecomplexhydrides(M,M ’)2[FeH6]4–(blackopen
circle)and
4––(M,M ’)4[FeH6] H 2 (blue solid circle). The values weretaken from Ref. 1 and 2.

Figure 2 shows the standard heat of formationof (M,M ’)4FeH8, composed of an [FeH6]4– anion,
two additional H – ions, and a total of four cations
that compensate the hexavalent anionic charge
state, as a function of cation electronegativity. As isevident from the blue solid circles, a linear relation
can be seen between the heat of formation andcation electronegativity as well as the (M,M ’)2FeH6
system without the incorporation of H – ions. Thus,
the chemical flexibility provided by incorporating H –ions enhances the tunability of thermodynamic
stability, which will be beneficial in obtaining
materials with optimal stability for reversible room
temperature hydrogen storage.
References[1] K. Miwa, S. Takagi, M. Matsuo, and S. Orimo, J. Phys. Chem.
C 117, 8014 (2013).
[2] S. Takagi, T. D. Humphries, K. Miwa, and S. Orimo, Appl.

Fig. 1 Crystal structures of Mg2FeH6 (left), andNa2Mg2FeH8 containing two H – ions (right).

Phys. Lett. 104, 203901 (2014).

Keywords: hydride, electronic structure, first-principles calculations
Shigeyuki Takagi and Shin-ichi Orimo (Hydrogen Functional Materials Division)
E-mail: shigeyuki.takagi@imr.tohoku.ac.jp
URL: http://www.imr.hydrogen.ac.jp/

28

IMR KINKEN Research Highlights 2015

Electronic Materials

IMR KINKEN Research Highlights 2015

Electronic Materials

Giant Artificial Spin on a Molecule-Ferromagnetic Cage
Cluster Fe42

The ferromagnetic ground state of a molecular cluster showing S = 45 high spin state is found by low-temperaturemagnetization, X-ray spectroscopy, and high-frequency EPR. This is the largest artificial spin found ever. The
ferromagnetic coupling is mediated by the highly symmetric network of the mixed-valence Fe ions.

Synthesis of new giant spins on single molecularclusters is one of the most important fields ofmagnetism. Such ferromagnetic molecular clusters
are essential for realizing ultra-high density
molecular storage and/or quantum spin devices.
Coordination bonds among magnetic ions mediatedby anions are, in many cases, antiferromagnetic.
This makes the creation of a high-spin ground statechallenging. A possible approach is to use rareearth elements that have large magnetic moments.
However, use of ubiquitous elements such as Fe3+
S =5/2 is desired, for environmental safety.
The present material contains 42 Fe ions, 18 ofwhich are in Fe3+ S = 5/2 states. To align these highspins into a ferromagnetic cluster, the charge andthe spin states of the Fe ions are tuned [1]. As aresult, ferromagnetic coupling arises for theinteractions between mixed-valence Fe ions. The
high symmetry of the cluster is another key point of
the ferromagnetic ground state as a whole cluster.
The synthesis was made in the Sato group at theInstitute for Materials Chemistry and Engineering,
Kyushu University. The material was then transferredto the Magnetism division of the Institute for Materials
Research for further physical analysis.
Figure 1 depicts the magnetization curvemeasured at 0.5 K, which shows a steep increase in
low magnetic fields and saturation at 90 µB (S = 45,
g = 2). This is the fingerprint of the ferromagnetic
ground state. In many ferromagnetic molecularclusters, bulk ferromagnetic ordering takes place.
The possibility of such ferromagnetic ordering must
be excluded to establish that the ferromagnetic
ground state is realized on an isolated single
molecule. The first evidence is the temperature andmagnetic field dependence of the magnetization
showing no critical temperature. The second
evidence is the absence of an anomaly related tomagnetic phase transitions by means of calorimetry.
Another characteristic of this Fe42 cluster is verysmall hysteresis. High frequency EPR reveals that theanisotropy is very small in this compound thoughhighly anisotropic Fe2+ ions are involved. This

Fig. 1 The magnetization curve measured at 0.5 K, whichshows the steep increase in low magnetic fields and thesaturation at 90 µB (S = 45, g = 2). The inset shows theshape of the molecule with high symmetry.

isotropic nature is caused by the cancelation of thelocal anisotropies for the highly symmetric cluster
shape. The symmetry of the cluster is shown in theinset of Fig. 1. The network has a star-like side view,and the crystal has a highly symmetric cubic lattice.
Finally, the differentiation of the magnetic
moments and the charge states of the complex
mixed-valence network is made using X-ray
spectroscopy at SPring8. The obtained ratio amongfour different states containing Fe2+/Fe3+ and high/low spin states is consistent with the ferromagnetic
ground state.
These intensive studies using different probes
finally establish the giant spin ground state of the
Fe42 cluster. Possible photo-control of electrontransfer to modify the spin ground state is inprogress.
References[1] S. Kang, H. Zheng, T. Liu, K. Hamachi, S. Kanegawa, K.
Sugimoto, Y. Shiota, S. Hayami, M. Mito, T. Nakamura, M.
Nakano, M. L. Baker, H. Nojiri, K. Yoshizawa, C. Duan, and O.
Sato, Nat. Commun. 6, 5955 (2015).

Keywords: high magnetic field, novel magnetic materials, ferromagnetic
Hiroyuki Nojiri (Magnetism Division)
E-mail: nojiri@imr.tohoku.ac.jp
URL: http://www.hfpm.imr.tohoku.ac.jp30

IMR KINKEN Research Highlights 2015

IMR KINKEN Research Highlights 2015

Topological Insulators as Inborn Spintronics Devices

D

etection and manipulation of electron spins are key prerequisites for spintronics. Since spin currents decay
on a length scale less than a few micrometers, thin-film nanostructures have been indispensable in spintronics
devices. Here, we experimentally demonstrate that, contrary to this limitation, a bulk topological insulator acts
as a spin detector without forming into films.

spin injection

Voltage
topologicalinsulator

Bi1.5Sb0.5Te1.7Se1.3|Ni81Fe19
Voltage

A topological insulator is a new state of quantum
matter that possesses a surface metallic state andan insulating interior. This characteristic of
topological insulators is the structure commonlyused in spintronics, i.e., a conductor film artificiallymade on an insulator substrate. Furthermore,
conduction electrons on the surface state are helical
Dirac fermions that have a novel property called
spin–momentum locking. On the surface states of
topological insulators, the direction of the electron
motion determines its spin direction and vice versa.
Hence, if a spin imbalance is induced by the spin
injection, a charge current is expected to be
produced along the Hall direction, as illustrated inFig. 1.
By exploiting these features combined withspin pumping, we experimentally demonstratespin–electricity conversion in a bulk topologicalinsulator Bi1.5Sb0.5Te1.7Se1.3 (BSTS), where injected
spins are converted into electric voltage along theHall direction despite the bulk nature of the samples[1]. Figure 1 shows the results of the electromotiveforce measurement with a magnetic field at various
temperatures for BSTS|Ni81Fe19. At ferromagnetic
resonance of Ni81Fe19, voltage peaks whose signs
are odd with respect to external magnetic fields areobserved. This sign reversal of the voltage signals is
consistent with the spin–electricity conversion
induced by spin–momentum locking on the surface
states of topological insulators.
The physical origin of the observed spin–electricity conversion effect is the spin–momentum
locking. This mechanism is fundamentally new,because there is no flow of injected spins in
topological insulators, unlike in conventional
materials. Moreover, because of the one-to-one
correspondence between spin and momentum
directions, 100% spin–electricity conversion is, inprinciple, possible for topological insulators. The
present results reveal a great advantage of
topological insulators as inborn spintronics
detectors.

voltage
peaks

0
5 µV

15K

-50
0
50
Magnetic field (mT)Fig. 1 (top) Concept of spin–electricity conversion on
surface states of topological insulators. (bottom)
Electromotive force for Bi1.5Sb0.5Te1.7Se1.3 (BSTS)
attached with a Ni81Fe19 film at 15 K. Clear antisymmetricvoltage peaks are observed on the surface state at
ferromagnetic resonance of Ni81Fe19.

References[1] Y. Shiomi, K. Nomura, Y. Kajiwara, K. Eto, M. Novak, K.
Segawa, Y. Ando, and E. Saitoh, Phys. Rev. Lett. 113, 196601(2014).

Keywords: spintronics, electronic material, spin current
Yuki Shiomi (Surface and Interface Research Division)
E-mail: shiomi@imr.tohoku.ac.jp
URL: http://saitoh.imr.tohoku.ac.jp/

IMR KINKEN Research Highlights 201531

Electronic Materials

Tuning the threshold voltage of a polar-semiconductor
field-effect transistorPolar-semiconductor heterostructures host two-dimensional electron accumulation, which can be applied asa conducting channel in a high–electron mobility transistor. Such spontaneously induced charges at theinterface yield normally-on characteristic behavior in a field-effect transistor. Towards an electrical device
application, we have recently found that the internal electric field in superlattices causes threshold voltage shift
in transistor operations. Here, we introduce a new approach to tune the transistor characteristics of a polarsemiconductor field-effect transistor.

Keywords: oxide, field-effect transistor, devices
Atsushi Tsukazaki (Low Temperature Physics Division)
E-mail: tsukazaki@imr.tohoku.ac.jp
URL: http://mu.imr.tohoku.ac.jp/

32

IMR KINKEN Research Highlights 2015(a) Bilayer
Gate(b) MgZnO/ZnO (c) MgZnO/ZnO
D

MgZnO

[0001]

S

ZnO

2DEG

(d)

3.0

-3

1/2

(10 A )

1/2

35 / 15 (nm)15 / 35 (nm)

Gate

Gate

Superlattices

Superlattices

ZnO

ZnO

MgZnO/ZnO(nm)
35 / 15

T = 10 K

2.5

(ID)

ZnO-related compounds are one type of polaroxide semiconductors, whose spontaneouspolarization are the largest among all Wurtzite
crystal structures. At the heterointerfaces of MgZnO/
ZnO, high-mobility two-dimensional (2D) electrons
are accumulated [1] that exhibit integer and fractional
quantum Hall effects [2]. By applying the 2D
electrons as a conducting channel in a field-effecttransistor (FET) [3], we can obtain normally-ontransistor operation for FETs based on ZnO and
GaN. Tuning the threshold voltage (Vth) is of great
importance for reliable operation of the FET.
Although the bilayer structure displayed in Fig.
1(a) is a representative example of polarsemiconductor FETs, the Vth of the FET is negative,
indicating normally-on operation (orange line in Fig.
1(d)). The tunable parameters being limited to only
the Mg-composition and thickness of the structure[2,3], tuning the Vth of the bilayer FET is not easy. To
address this problem, we have recently examined
the performance of superlattice FETs for tuning theVth.
By applying superlattices to the FET, we can
expand the number of tunable parameters to include
the total thickness of MgZnO and ZnO, thickness
ratio, and number of periods in addition to Mg
composition. In fact, two types of FETs composedof different superlattice structures, as shown in Fig.
1(b) and (c), exhibit a clear Vth shift, shown as red
and blue lines, respectively, in Fig. 1(d). The best
performance is displayed as a green line in Fig.
1(d), providing a large saturation current withnormally-off operation.
The systematic tuning of the Vth in FET operation
is demonstrated by the insertion of polarsemiconductor superlattices. Although we needfurther investigation to clarify the dominantparameter in these superlattice FETs, this tuning
concept can be widely applied to electrical andoptical devices based on polar semiconductors.

Bilayer

2.0

25 / 251.5
1.0

15 / 35

0.5
0.0
-0.5

0.0

0.5
1.0
VG (V)

1.5

2.0

Fig. 1 (a-c) Schematic cross-sectional FET structures
composed of a bilayer (a) or superlattices (b and c). S
and D indicate source and drain electrodes. White
broken lines represent two-dimensional electron gas(2DEG) at the interface. (d) Drain current is presented
as a function of gate voltage. Threshold voltage iscontrolled via internal electric field in the superlattices.

References[1] A. Tsukazaki, A. Ohtomo, T. Kita, Y. Ohno, H. Ohno, and M.
Kawasaki, Science 315, 1388 (2007).
[2] Y. Kozuka, A. Tsukazaki, and M. Kawasaki, Appl. Phys. Rev. 1,
011303 (2014).
[3] M. Nakano, A. Tsukazaki, K. Ueno, S. Akasaka, H. Yuji, K.
Nakahara, T. Fukumura, and M. Kawasaki, Adv. Mater. 22, 876(2010).

IMR KINKEN Research Highlights 2015

Charge Fluctuations in Multiorbital Organic Molecules in(TTM-TTP)I3 with One-Dimensional Structure

Charge fluctuation is a fundamental issue for strongly correlated organic molecular systems. Recent

discoveries of charge dynamics in molecular dimers in Mott insulators reveal new aspects of charge degrees
of freedom in organic molecular systems. We have detected charge fluctuations and freezing in multiorbital
molecules in an one-dimensional organic crystal of (TTM-TTP)I3 by observing its dielectric properties.

(a)
short
H3CS

S

S

S

S

SCH3

H3CS

S

S

S

S

SCH3

stacking

long
(b)

short

E || molecular long axis

E || molecular short axis

(TTM-TTP)I 380

6ε/ ε(4.2 K)

60

ε/ ε0

Organic molecular systems have numerous
charge degrees of freedom compared with alloys
and transition-metal oxides because of thecomplexities of their molecular shapes andarrangements. Recent observations of charge
degrees of freedom in molecular-dimer structures[1, 2] provide possibilities of new functionalities anddeep insights into the fundamental understandingsof Mott transition. In a Mott insulator, when a charge
in a molecular dimer shifts to either molecule, anelectrical dipole emerges and its corrective
dynamics change the dielectric properties. Even ina molecule with a multiorbital structure, an analogy
can be drawn as shifts of charges within different
orbitals.
(TTM-TTP)I3 is a strongly correlated systemcomposed of TTM-TTP molecules. TTM-TTP has
two segments on the basis of which the molecularelectronic states can be considered. The fragments
play a critical role in the charge ordering transition inthis system [3]. Therefore, this system is suitable fora study of the charge degrees of freedom within themolecule. Here, we have measured the dielectric
properties of (TTM-TTP)I3 and observed charge
fluctuations and freezing in the fragments of themolecules [4].
Figure 1(a) shows a schematic of the moleculararrangement of the (TTM-TTP)I3 crystal. The arrows
indicate the molecular long and short axes. The two
fragments are represented by broken lines. The
temperature dependence of the dielectric constantis shown in Fig. 1(b). In the direction of the molecularlong axis, we can see a large anomaly at around
15–30 K, but it is small along the molecular shortaxis. These results correspond to the directions of
the fragments in the crystal. As the frequency
becomes higher, the peak-top temperature
increases, as shown by the open circles in Fig. 1(b).
This is due to the emergence of a relaxor-likeinhomogeneous electronic state. Below 10 K, theanomaly is suppressed and the dielectric constant

40

0

7k
10k
20k
50k
70k
100k
200k

2

20
0

4

200 Hz
500
700
1k
2k
5k

10 20 30 40
Temperature (K)00

10 20 30 40
Temperature (K)Fig.1 (a) Schematics of (left) TTM-TTP molecular and(right) crystal structure. (b) Temperature dependence ofdielectric constant for each direction.

becomes independent of temperature. This
indicates the freezing of the glassy dynamics of thecharges in the TTM-TTP molecules. Our results
confirm that charge dynamics exist in the multiorbital
molecular system.
References[1] M. Abdel-Jawad, I. Terasaki, T. Sasaki, N. Yoneyama, N.
Kobayashi, Y. Uesu, and C. Hotta, Phys. Rev. B 82, 125119(2010).
[2] S. Iguchi, S. Sasaki, N. Yoneyama, H. Taniguchi, T. Nishizaki,
and T. Sasaki, Phys. Rev. B 87, 075107 (2013).
[3] M. Tsuchiizu, Y. Omori, Y. Suzumura, M.-L. Bonnet, V. Robert,
S. Ishibashi, and H. Seo, J. Phys. Soc. Jpn. 80, 013703 (2011).
[4] S. Sasaki, S. Iguchi, T. Kawamoto, T. Mori, and T. Sasaki, J.
Phys. Soc. Jpn. 83, 094709 (2014).

Keywords: dielectric, organic, metal-insulator transition
Satoshi Iguchi (Low Temperature Condensed State Physics Division)
E-mail: satoshi.iguchi@imr.tohoku.ac.jp
URL: http://cond-phys.imr.tohoku.ac.jp/index.html

IMR KINKEN Research Highlights 201533

Electronic Materials

Combined Study of Neutron and X-ray Inelastic Scattering
Measurements on Electron-Doped Copper-Oxide SuperconductorsT

he evolution of electronic (spin and charge) excitations in carrier doping is an extremely important issue insuperconducting copper oxides. Here, we combined neutron and X-ray inelastic scattering measurements onelectron-doped superconductors and have revealed a highly itinerant character in the spin and charge
excitations, that contrasts with the hole-doped superconducting compounds.
High-Tc
copper-oxide
superconductivity
appears with carrier doping in Mott insulators,
regardless of the sign of the carrier. Doping evolutionof spin and charge correlations is crucially importantfor understanding the emergence of superconductivity.
Neutron and X-ray inelastic scattering measurements
have played a major role in this. For hole-doped
cuprates, the spin-wave-like magnetic dispersion
originating from a local spin character of the undoped
parent compound changes with hole doping to theso-called "hourglass" shape at low-energy regions
below Ecross; however, the magnetic dispersion above
Ecross remains almost unchanged with the spin wave.
In the representative electron-doped superconductor
R2-x Cex CuO4 (R = Pr, Nd, Eu, ...), the magnetic
dispersion changes in a different way from hole
doping:themagnetic
excitation
remains
commensurate at low energies. For more information
about higher energy magnetic excitation, we
exploited complementarily a combination of resonant
X-ray scattering (RIXS) and neutron inelastic
scattering (INS) to cover high-energy and widemomentum transfers [1].
From the energy spectra of RIXS at the Cu L3 edge, we have observed a dominant single-magnon
excitation peak for electron-doped Nd2-x Cex CuO4.
Figure 1(a) shows the magnetic dispersion of theobserved single-magnon excitation. The magnetic
dispersion of Pr1.4-x La0.6Cex CuO4 (PLCCO) with x =0 observed from INS is also plotted. We find that themagnetic excitation shifts to a higher energy in awide momentum space upon electron doping.
Another additional excitation mode has been
observed in the L3 -edge RIXS spectra at a higher
energy than the single-magnon excitation. The peak
position of this excitation is plotted in Fig. 1(b) on the
background of the color map of the Cu K-edge RIXS
for x = 0.15. Considering that spin-flip transitions are
forbidden in K-edge RIXS, this additional excitation
mode in the L3 -edge RIXS spectra can be assigned
to a charge excitation. As deep mixing of themagnetic and charge excitation is observed, we

Fig. 1 (a) Peak positions of magnetic excitation for Nd2x Cex CuO4 (NCCO) observed from RIXS at L3 -edge andfor Pr1.4La0.6CuO4 observed from INS. (b) Peakpositions of charge excitation for NCCO and color plot
for NCCO with x = 0.15 observed from RIXS [1].

conclude that the excitation is dominated by itinerant
electron nature. These results are in stark contrast to
the hole-doped compounds, where localized spin
excitation persists up to high doping levels.
References[1] K. Ishii, M. Fujita, T. Sasaki, M. Minola, G. Dellea, C. Mazzoli, K.
Kummer, G. Ghiringhelli, L. Braicovich, T. Tohyama, K. Tsutsumi,
K. Sato, R. Kajimoto, K. Ikeuchi, K. Yamada, M. Yoshida, M.
Kurooka, and J. Mizuki, Nat. Commun. 5, 3714 (2014).

Keywords: superconducting, neutron scattering, X-ray scattering
Masaki Fujita (Metal Physics with Quantum Beam Spectroscopy Division)
E-mail: fujita@imr.tohoku.ac.jp
URL: http://qblab.imr.tohoku.ac.jp/

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IMR KINKEN Research Highlights 2015

IMR KINKEN Research Highlights 2015

Atom Probe Analysis of Dopant and Contaminant Diffusion
on Silicon Introduced by the Molecular Doping Method

Fabrication of next-generation nanoscale semiconductor devices requires precise placement of dopant atoms

into their crystal lattice. Molecular doping combined with a conventional spike annealing method provides abottoms-up approach that is potentially viable for large-scale production. Here, we highlight atom probe studiesof phosphorus doping on silicon.
HF in H2O(a)

Si sub.

Amorphous
28Si layer

SiO2

H terminated Si

SiO2
DPP in C6H3Me3

DPP Monolayer

SiO2

Sii
S
SiO2

P

Annealing
800 – 1025 oC (5 s)PC

Needle specimen
for APTO
20 nm

107 nm

(b)

O C

C

SiO2

P diffusion(c)

196 nmUltrashallow doping of Si is one of the key
issues of future metal oxide semiconductor
microelectronics. Recently, molecular doping has
attracted much attention and promises fast dopingof entire wafers, and has proven to be effective forthe introduction of P (n-type dopant) based onmolecular diethyl 1-propylphosphonate, (DPP) at
the near surface of semiconductor materials bytaking advantage of the self-limiting surface reaction
properties of Si. An understanding of the behavior of
doped P is essential to confirm the scalability of themethod.
Atom probe tomography (APT) enabled us toobtain three-dimensional distributions of individual
atoms in semiconductor materials on the order of
atomic-scale resolution [1, 2]. Here, we report on the
diffusion of P and contaminants such as C and O
atoms forming the precursor molecules in the firstatomic layers by means of APT [3].
Figure 1(a) shows a series of P doping using
DPP and the location of needle specimens for APT
analysis. Figure 1(b) and 1(c) show APT maps ofsamples annealed at 800 and 1025°C, respectively.
Interestingly, P diffuses more into Si substrates,
although the diffusion coefficients of C and O arelarger than that of P in intrinsic Si. While C is directly
provided by the decomposition of DPP molecules at
the Si interface, O diffuses mostly from the SiO2 cap
layer. However, because of the high reactivity to Si,
C and O diffusion appears to be limited to theinterfacial region, which extends to 2–3 nm.
Directed self-assembly may be used in largescale industrial processes provided one includes anadditional step to remove the first sacrificial layers inorder to eliminate the contaminants.

Fig. 1 (a) Sample preparation by molecular doping andposition of the needle-shaped specimen for APT. P, C,
and O distributions in the samples spike-annealed at(b) 800°C and (c) 1025°C.

Nishida, T. Mogami, K. Kitamoto, T. Miyagi, J. Kato, S. Akahori,
N. Okada, M. Kato, H. Uchida, and Y. Nagai, Appl. Phys.
Express 6, 046502 (2013).
[3] Y. Shimizu, H. Takamizawa, K. Inoue, F. Yano, Y. Nagai, L.
Lamagna, G. Mazzeo, M. Perego, and E. Prati, Nanoscale 6,
706 (2014).

References[1] Y. Shimizu, H. Takamizawa, Y. Kawamura, M. Uematsu, T.
Toyama, K. Inoue, E. E. Haller, K. M. Itoh, and Y. Nagai, J. Appl.
Phys. 113, 026101 (2013).
[2] K. Inoue, H. Takamizawa, Y. Shimizu, T. Toyama, F. Yano, A.

Keywords: atom probe tomography, 3D structure, electronic material
Yasuo Shimizu, Kazuaki Nagumo, Koji Inoue and Yasuyoshi Nagai (Irradiation Effects in Nuclear and Their RelatedMaterials Division)
E-mail: yshimizu@imr.tohoku.ac.jp
URL: http://wani.imr.tohoku.ac.jp

IMR KINKEN Research Highlights 201535

Electronic Materials

Electrical Monitoring of Gas Adsorption in Structurally
Flexible Porous Coordination Polymers

T

he gate-opening adsorption behavior of a one-dimensional chain compound was electronically monitored insitu by measuring its permittivity and impedance under applied AC electric fields. The gate-opening transition
was accurately detected by a sudden change in the real part of permittivity. Furthermore, the imaginary part ofpermittivity effectively captured the electronic perturbations of the sample induced by the introduced guest
molecules. Only the introduction of NO resulted in the increase in the sample’s electrical conductivity.

In recent years, porous molecular materials,
referred to as porous coordination polymers (PCPs)
or metal organic frameworks (MOFs), have attracted
much attention as selective gas adsorption
materials. Selective gas adsorption is largely
achieved by creating host-guest interactions and/or
guest-induced
gate-opening
structuraltransformations that only occur in the presence of a
specific gas molecule [1, 2]. Here, we propose theuse of in situ AC electric field measurements not
only for accurately monitoring such gate-opening
gas adsorption of PCPs/MOFs, but also forunderstanding the electronic effect due to the hostguest interaction [3].
The gate-opening adsorption behavior of the
one-dimensional chain compound [Ru2(4-Cl-2OMePhCO2)4(phz)] (4-Cl-2-OMePhCO2– = 4-chloroo-anisate; phz = phenazine) [1] for various gases(He, O2, NO, and CO2) was electronically monitoredin situ by applying AC electric fields to pelletizedsamples (~5 mg). The gate-opening and -closing
transitions induced by gas adsorption/desorption,
respectively, were monitored by a sudden change inthe real part of permittivity (ε’, Fig. 1(a)). Furthermore,
the imaginary part of permittivity (ε’’) effectively
captured the electronic perturbations in the samples
induced by the introduced guest molecules. The
introduction of NO resulted in an increase in thesample’s electronic conductivity (σac, Fig. 1b),
indicating that NO molecules interact with theframeworks
throughelectronic
host-guest
interactions.
This technique involving the in situ applicationof ac electric fields is useful not only for rapidly
monitoring gas sorption responses accompanied
by gate-opening/-closing structural transitions, butalso potentially for the development of molecular
framework materials such as chemically-driven
electronic devices.

Fig. 1 (a) Temperature dependence of the dielectric
permittivity (ε’) under 100 kPa of various gases. (b)Arrhenius plots of AC electric conductivity (σac), which
was significantly enhanced under NO atmosphere.

References[1] W. Kosaka, K. Yamagishi, A. Hori, H. Sato, R. Matsuda, S.
Kitagawa, M. Takata, and H. Miyasaka, J. Am. Chem. Soc. 135,
18469 (2013).
[2] W. Kosaka, K. Yamagishi, R. Matsuda, S. Kitagawa, M. Takata,and H. Miyasaka, Chem. Lett. 43, 890 (2014).
[3] W. Kosaka, K. Yamagishi, J. Zhang, and H. Miyasaka, J. Am.
Chem. Soc. 136, 12304 (2014).

Keywords: dielectric properties, electrical properties, molecular devices
Wataru Kosaka and Hitoshi Miyasaka (Solid-State Metal-Complex Chemistry Division)
E-mail: miyasaka@imr.tohoku.ac.jp
URL: http://www.miyasaka-lab.imr.tohoku.ac.jp/

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IMR KINKEN Research Highlights 2015

IMR KINKEN Research Highlights 2015

Nano-Oscillator Using Spin Precession in Half-metallic
Heusler AlloysS

pin torque, which originates from the spin angular momentum transfer between local spins and conduction
spins, induces spin precession in a nanomagnet. This provides us with a novel spintronic application of a nanooscillator, called “Spin Torque Oscillator.” We successfully enhanced the output power together with a narrow
oscillation linewidth by exploiting a giant magnetoresistance stack with half-metallic Heusler alloys that havehigh spin-polarization of conduction electrons.
A spin torque oscillator (STO) is an intriguing device
based on spin dynamics in a nanomagnet triggered byspin torque. This is a prospective nanometer-scaled
radio frequency (RF) oscillator, e.g., for on-chipcommunications or radar. When a direct electric current(Idc) is applied to a giant magnetoresistance (GMR)
device or a magnetic tunnel junction consisting of two
ferromagnets separated by a nonmagnetic metal or an
insulator, spin torque induces steady spin precession in
the ferromagnet; this results in time-dependent device
resistance (R(t)) through the magnetoresistance (MR)effect. The product of Idc and R(t), therefore, gives the
RF voltage (Vrf).
Although its simple architecture composed of asingle nanometer-scaled element is one of the greatest
advantages of the STO, several crucial issues need tobe solved before STOs can be put into practical use.
One of them is the enhancement of RF output power.
We also need to narrow the oscillation linewidth. One
promising way to enhance the RF output power is to use
ferromagnetic materials showing a large MR effect, i.e.,highly spin-polarized materials.
We fabricated current-perpendicular-to-plane(CPP) GMR devices using the half-metallic Heusler
alloy of Co2Fe0.4Mn0.6Si (CFMS) in order to develop
high-power STOs. Figure 1(a) shows an illustration of
nanopillar-shaped STO with the CPP-GMR stack of
CFMS / Ag / CFMS. When Idc was injected into the CPPGMR stack, the spin torque excited spin precession in
the upper CFMS layer, leading to the Vrf output. Becauseof a large GMR ratio of 30%, we enhanced the output
power "to the maximum of 23.7 nW (shown in Fig. 1(b))
with the magnetic field (H) applied normal to the device
plane [1]. The micromagnetic simulation suggested thatan out-of-plane precession mode was induced.
However, non-uniform magnetic structures due to the
non-uniform effective magnetic field (Heff) were also
observed during the spin precession, which is notsuitable for a stable operation of the STO. We alsofabricated a point-contact STO having uniform Heff, inwhich the µm-sized CFMS layer had an nm-sized point

Fig. 1 (a) A schematic illustration of a nanopillar-shaped
spin torque oscillator (STO) with the giant
magnetoresistance stack of Co2Fe0.4Mn0.6Si (CFMS) /Ag / CFMS. (b) An RF spectrum exhibiting a large
output power and a narrow oscillation linewidth.

contact region and the Idc flow was confined at thecontact. For the point-contact STO, a vortex-likeoscillation mode was excited. The uniform Heff enabled
us to obtain narrower oscillation linewidth than thenanopillar-shaped STO [2]. We believe that our STOs
with CFMS layers have paved the way to realize a highperformance nano-oscillator.
References[1] T. Seki, Y. Sakuraba, H. Arai, M. Ueda, R. Okura, H. Imamura, andK. Takanashi, Appl. Phys. Lett. 105, 092406 (2014).
[2] T. Yamamoto, T. Seki, T. Kubota, H. Yako, and K. Takanashi, Appl.
Phys. Lett. 106, 092406 (2015).

Keywords: half-metal, spin transfer, spintronics
Koki Takanashi (Magnetic Materials Division)
E-mail: koki@imr.tohoku.ac.jp
URL: http://magmatelab.imr.tohoku.ac.jp/

IMR KINKEN Research Highlights 201537

Electronic Materials

Growth of Al-doped Ca3TaGa3Si2O14 Single Crystals forApplications in Piezoelectric Acoustic Devices

Al-doped Ca TaGa Si O single crystals with various Al concentrations were grown by the micro-pulling3

3

2

14

down method and Czochralski method. The potential of these crystals as a low-power consuming oscillator
alternative to a quartz crystal was demonstrated through experiments and calculations using accurate material
constants and their temperature coefficients measured by the ultrasonic microspectroscopy technology.

Langasite-type single crystals belonging to point
group 32 of the trigonal crystal system with A3BC3D2O14
structure are promising materials for pressure sensors
operating in a high temperature environment as well as
high stability acoustic devices for future communication
applications. In particular, they are potential candidates
as alternative materials for α-quartz from the viewpointof large electromechanical coupling factor, low
impedance, and superior temperature stability. In
2000, compounds with ordered structure, such as
Ca3TaGa3Si2O14 (CTGS) and Ca3NbGa3Si2O14(CNGS) were developed because compounds withdisordered structure, such as La3Ga5SiO14 (LGS),
La3Ta0.5Ga5.5O14 (LTG), and La3Nb0.5Ga5.5O14 (LNG)
cause higher acoustic losses and non-uniform
mechanical properties. Although CTGS and CNGS
have no rare-earth elements unlike previous langasitetype crystals such as LGS, LTG, and LNG, they still
include the expensive element Ga. The substitution of
Al for Ga in CTGS and CNGS crystals can dramaticallydecrease the cost of manufacturing. Besides, there
are some reports that Al-substitution provides better
piezoelectric properties. Hence, our research grouphas been engaged in the development of Al-doped
CTGS and CNGS single crystals.
We successfully grew highly transparent
Ca3Ta(Ga1-xAlx)3Si2O14 crystals with various Al
concentrations of x = 0, 0.2, 0.4, 0.6, 0.8, and 1 by the
micro-pulling-down method (Fig. 1(a)) [1]. Their crystal
structures were analyzed by powder X-ray
diffractometry resulting in a single phase of thelangasite-type structure. Their lattice parameters
related to the a- and c-axes systematically decreased
with increase in Al concentration. It followed from the
EPMA analysis that the actual Al concentration in the
crystals almost corresponded to the nominal
compositions. From this result, it is confirmed that Alsubstituted CTGS single crystal can be grown from astoichiometric melt.
Based on this result, we attempted to grow
Ca3Ta(Ga0.5Al0.5)3Si2O14 (CTGAS) single crystal with adiameter of 1 in. by the Czochralski method [2]. The
Keywords: crystal growth, electronic material
Yuji Ohashi (Advanced Crystal Engineering Laboratory)
E-mail: ohashi@imr.tohoku.ac.jp
URL: http://yoshikawa-lab.imr.tohoku.ac.jp/index-e.html

38

IMR KINKEN Research Highlights 2015

Fig.1 Ca3Ta(Ga1-x Alx)3Si2O14 single crystals grown by theµ-PD method (a) and by Czochralski method (b).

grown crystal was transparent with a yellow color andhad no cracks (Fig. 1(b)). Elastic constants,piezoelectric constants and its temperature
coefficients were accurately determined from bulk
acoustic wave velocities as well as dielectric constants
and the density measured for the CTGAS specimens
using ultrasonic microspectroscopy (UMS). It wasdemonstrated that the constants determined could
provide calculation accuracy within ±0.15% in the
leaky surface acoustic wave (LSAW) velocity.
Numerical calculations using the constants determined
predicts that there is a cut angle where the temperature
coefficient of shear wave velocity becomes zeroaround 148°Y-cut exhibiting an electromechanicalcoupling factor of k2 = 3.2%, which is about four times
larger than that of AT-cut quartz. This new crystal willcontribute to achieving superior piezoelectric devices
for future mobile communications.
References[1] T. Kudo, Y. Yokota, M. Sato, K. Tota, K. Onodera, S. Kurosawa, K.
Kamada, and A. Yoshikawa, J. Cryst. Growth 401, 173 (2014).
[2] T. Kudo, Y. Yokota, Y. Shoji, Y. Ohashi, A. Medvedev, S. Kurosawa,
K. Kamada, and A. Yoshikawa, Proc. IEEE ISAF, DOI: 10.1109/
ISAF.2014.6922990 (2014).

IMR KINKEN Research Highlights 2015

Field Reentrant Superconductivity and Fermi Surface
Instabilities in Uranium Ferromagnetic SuperconductorsT

he field reentrant superconductivity of uranium ferromagnets has recently attracted significant interest
because an unconventional mechanism of superconductivity is realized. The enhancement of the effective
mass and field-induced ferromagnetic fluctuations play key roles for the reentrant superconductivity. In thisstudy, we found that the associated Fermi surface reconstructions, which are often detected as Lifshitz-typetransitions, also play a key role through transport measurements under extreme conditions.
The coexistence of ferromagnetism andsuperconductivity is currently a topic of great interest in
condensed matter physics because an unconventional
mechanism of superconductivity has been realized.
One can naturally consider the triplet spin state withequal-spin pairing in ferromagnetic superconductors
because it is robust against the strong internal field
causedbyferromagnetism.
Moreover,thesuperconducting upper critical field Hc2 is not governedby the Pauli paramagnetic effect. Thus, unusual fieldinduced superconducting phenomena are expected.
Figure 1 shows the temperature–field phase diagram of
URhGe when the field is applied along the hardmagnetization axis. Surprisingly, the superconductivity
reappears at high magnetic fields from 10 to 13 T.
Because the superconducting transition temperature israther low (Tsc = 0.25 K), Hc2 at 0 K is expected to beless than 1 T. However, the reentrant superconductivity
appears at much higher fields. More surprisingly, themaximum temperature (approximately 0.4 K) of thereentrant superconducting dome in Fig. 1 is even higher
than Tsc at zero field. This confirms that the magnetic
field reinforces the superconductivity. The key point is
the field-induced ferromagnetic fluctuations. As shownin Fig. 1, the ferromagnetic Curie temperature TCurie issuppressed with increasing field and finally reaches
zero with a weak first-order transition. The suppressionof TCurie indicates that the ferromagnetic fluctuations arestrongly induced by the magnetic field together with theenhancement of the effective mass.
Recently, we more precisely studied the fieldreentrant superconductivity and ferromagnetic
fluctuations by means of measuring the Hall effect,
thermopower, and quantum oscillations. Figure 2
shows the field dependence of the Hall resistivity at
different temperatures [1]. The Hall resistivity
originates from both the normal and anomalous Hall
effects. After subtracting the anomalous Hall effect, it
is found that the carrier number is significantly
changed at the suppression of TCurie, indicating thedrastic reconstruction of the Fermi surface. Further

PM

10

FM
URhGe
H // b-axis

1

1st order

PPM

SC
0.1

RSC
0

5

10

15

Fig. 1 Temperature-field phase diagram of the ferromagnetic
superconductor URhGe for the field along b-axis. The
reentrant superconductivity (RSC) appears at high fields. SC,
PM, FM PPM denote superconductivity, paramagnetism,
ferromagnetism and polarized paramagnetism, respectively.

Fig. 2 Field dependence of Hall resistivity in URhGe at
different temperatures.

direct evidence was also obtained from quantum
oscillation and thermopower measurements. This
Fermi surface reconstruction is most likely caused by
the Lifshitz-type transition associated with magnetic
instabilities related to significant changes in the
ferromagnetic sublattice moment.
This work was conducted in collaboration with G.
Knebel, A. Pourret, and J. Flouquet in CEA Grenoble.
References[1] D. Aoki, G. Knebel, and J. Flouquet: J. Phys. Soc. Jpn. 83,
094719 (2014).

Keywords: superconducting, Fermi surface, heavy fermion
Dai Aoki (Actinide Materials Science Division)
E-mail: aoki@imr.tohoku.ac.jp
URL: http://actinide.imr.tohoku.ac.jp/

IMR KINKEN Research Highlights 201539

Research Centers

IMR KINKEN Research Highlights 2015

Research Centers

Progresses in Ultrasonic Measurements for Studying Actinide
Physics and Development of Hydride Material for Nuclear Reactors
International Research Center for Nuclear Materials ScienceIn the field of actinide science, an ultrasonic measurement system for studying actinide compounds underpressure has been developed. The elastic constant of URu2Si2 has been measured at a pressure of 1.5 GPa.
In the field of nuclear materials, a hydride neutron absorption material has been developed. An irradiation testof the hydride in the BOR-60 reactor in Russia has been successful.
Pressure experiments are powerful tools tocontrol the electronic state in actinide compounds.
In collaboration with the groups of Prof. Settai andProf. Nemoto at the Niigata University, we havesucceeded in developing a pressure cell up to 3
GPa for conducting electric, magnetic, and elastic
experiments for relatively large actinide compounds.
Ultrasonic experiments under pressure were limited
to about 1 GPa before now, because of the difficultyin sample setting with a transducer and coaxial
cable for a high-frequency signal in a small space.
Figure 1 shows the temperature dependence of theelastic constant of the (C11-C12)/2 mode in URu2Si2
at (a) ambient pressure and (b) 1.5 GPa. The single
crystal of URu2Si2 was grown in Prof. Aoki’s group at
Oarai. URu2Si2 is an intriguing compound with ahidden ordered phase at To = 17.5 K. With increasingpressure, the antiferromagnetic state appears at 0.5
GPa and the hidden order state disappears fromabout 1.3 GPa. The elastic constant of the (C11-C12)/2
mode shows softening below 100 K indicating thatthe crystalline electric field states relate to thehidden order phase. The peak position moves to ahigher temperature at 1.5 GPa. Experiments underpressure and detailed analyses will be performed in(b) 1.5 GPa
Fig. 1 Temperature dependence of the elastic constant of(C11-C12)/2 mode in URu2Si2.

the near future to further study the hidden orderphase of URu2Si2.
Metal hydrides have a high hydrogen-atomdensity equivalent to that of liquid water. Hafnium
hydride has been investigated as a neutron absorberin Fast Breeder Reactors (FBRs). Fast neutrons are
efficiently moderated by hydrogen in Hf hydrides
and are absorbed by Hf. Because three isotopes of
Hf have large cross sections, the life of a control rod
can be increased using Hf hydride. Results of adesign study of a core with Hf hydride control rods
show that a long-lived hafnium hydride control rod is
feasible in a large sodium-cooled FBR. Results ofan irradiation test conducted in the fast experimental
reactor BOR-60 in Russia have demonstrated theintegrity of the capsules during irradiation.
Compared with a He-bonded capsule, a Na-bonded
capsule has an advantage in terms of theconfinement effect of hydrogen [1].
References[1] K. Konashi, K. Itoh, T. Yokoyama, and M. Yamawaki, Advances(a) ambient pressurein Science and Technology. 94, 23 (2014).

Rikio Settai (Corresponding Author, Department of Physics, Niigata University)
E-mail: settai@phys.sc.niigata-u.ac.jp
Yasuyoshi Nagai (Head of International Research Center for Nuclear Materials Science)
E-mail: o-jime@imr.tohoku.ac.jp URL: http://www.imr-oarai.jp/

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IMR KINKEN Research Highlights 2015

Towards Four-Dimensional Electron Tomography (4D-ET)
Cooperative Research and Development Center for Advanced MaterialsWe have developed a newly designed straining holder for in situ transmission electron microscopy compatible
with electron tomography. The holder can deform a specimen under tensile stress with variable strain rates.
The maximum tilt angle reaches ±60°. The new specimen holder, termed as “straining and tomography holder,”will have wide range potential applications in materials science.

Electron tomography (ET) was introduced inmaterials science in the past decade and has openeda new prospect: the technique can reveal threedimensional (3D) structural information [1-2]. However, amajor roadblock exists to combine the in situ experimentswith electron tomography, which is expected to reveal
real-time 3D structural changes. Towards dynamic 3D(i.e., “4D-ET”) visualization of material microstructures
under various straining conditions with a time scale of afew minutes or less, we designed and developed a new
specimen holder compatible with tensile tests and highangle tilting, termed as “straining and tomography(SATO)” holder [3].

actuator deforms a newly designed cartridge-typeblade on which a specimen is glued. The deformation
velocity of the blade is designed to be 1/3 of that of theactuator. Figure 1(b) explains the motion of blade. The
trajectory (dotted line) is an arc, but the radius ofcurvature (R) is so large (3 mm) that the tensile axis isperpendicular to the holder for a nanometer-scale
object whose center is located at O.

Fig. 2 (a) A developed specimen holder and (b) amagnified photo of a cartridge-type blade.

Fig. 1 (a) A schematic illustration of a newly designed
specimen holder and (b) a cartridge-type blade on whicha specimen is glued. The area of gluing is marked by gray.

Figure 2 shows (a) a developed specimen holder
and (b) a magnified photo of the cartridge-type blade.
The holder motion is fully computer-controlled via agraphical user interface developed for this system.
We measured the deformation velocity of theblade and deduced the strain rate. The minimum andthe maximum values obtained were 1.5 × 10 -6 and 5.2
× 10-3 s-1, respectively. We confirmed that the blade
and the holder are robust and that multiple acquisitions
do not raise any technical problem. This result
demonstrates the stability and reliability of the holder
as a novel in situ experimental instrument for 4D-ET.
References

The basic concept of this development is a single
tilt-axis holder that has a tensile mechanism and iscapable of electron tomography. To achieve straining
and high-angle tilting simultaneously, we developed anovel mechanism as shown in Fig. 1(a). A linear-motion[1] K. Sato, S. Semboshi, and T. J. Konno, Materials 8, 1924 (2015).
[2] K. Sato, S. Matsunaga, S. Tashiro, Y. Yamaguchi, T. Kiguchi, andT. J. Konno, Mater. Trans. 56, (2015) in press.
[3] K. Sato, H. Miyazaki, T. Gondo, S. Miyazaki, M. Murayama, and
S. Hata, Microscopy 64, (2015) in press.

Kazuhisa Sato (Corresponding Author, Cooperative Research and Development Center for Advanced Materials)
E-mail: ksato@imr.tohoku.ac.jp
Tadashi Furuhara (Head of Cooperative Research and Development Center for Advanced Materials)
E-mail: crdam@imr.tohoku.ac.jp URL: http://www.crdam.imr.tohoku.ac.jp/index2.html

IMR KINKEN Research Highlights 201543

Research Centers

Optical Spectroscopy in High Magnetic Fields Using theHybrid Magnet
High Field Laboratory for Superconducting Materials

High-magnetic-field optical spectroscopy is a very useful method for exploring microscopic states of various

condensed matter systems. This kind of measurement, by utilizing an optical fiber and a solenoid-type highfield magnet, has been used since the 1980s; this was recently resumed with the newly installed spectrometerand detection systems at the High Field Laboratory for Superconducting Materials (HFLSM), IMR, Tohoku
University. We have developed apparatuses for optical transmission and Raman scattering experiments in very
high fields, up to 30 tesla, by using the hybrid magnet.

Optical spectroscopy, which can determine
quantum energy levels with high accuracy, hasbeen applied to a wide range of materials. A
combination of a high-field magnet and optical
spectroscope is a powerful tool to study exotic
states induced by high magnetic fields. It also offers
unique opportunities to detect intriguing opticalphenomena, such as magnetochromism or
directional dichroism. We have been able to perform
optical spectroscopy measurements in very highfields, up to 30 T, by using the hybrid magnet at theHigh Field Laboratory for Superconducting Materials(HFLSM), IMR, Tohoku University. The measurement
system for optical transmission spectroscopy isshown in Fig. 1(a). Optical fibers are employed tolead light to the sample in the magnet. The system
consists of a halogen-tungsten and deuterium lamp
as light sources, that cover the spectrum from thenear-infrared to the ultraviolet region; a single grating
spectrometer; and a thermoelectrically cooled
multichannel CCD camera acquire of spectroscopy
data with high efficiency. We can also perform
Raman scattering experiments in the infrared region,
down to 100 cm-1, by using an argon laser as thelight source.
Figure 1(b) shows the results of opticaltransmission spectroscopy measurements ofchromium spinel oxide (CdCr2O4), which is a typicalexample of a three-dimensional geometricallyfrustrated magnet. A field-induced transition to thenontrivial magnetization plateau phase is known tooccur at around 29 T in this material in a magnetic
field at low temperature. The inset of Figure 1(b)shows the spectrum at zero magnetic field. We
assigned the cause of a weak absorption peak at
14115 cm-1 to an excition excitation, and two
absorption peaks at 14130 and 14149 cm-1 tosimultaneous excitations of an excition and a magnon[1]. By applying magnetic fields, we observed that(a)Tungstenhalogen lamp
+ Deuterium lamp
Convex lens 1
Convex lens 2
SampleConvex lens 3
Plane mirror(b)Excitonmagnon
excitation
30T
29T
28T
27T

IMR KINKEN Research Highlights 2015

Excitonexcitation
0T

Fig.1 (a) High-field optical transmission spectroscopy
system. (b) Field variations of absorption spectra of
CdCr2O4.

the exciton peak splits into several peaks, as indicatedby the solid arrows. The spectrum is largely changed
at 30 T by the transition to the magnetization plateau
phase, as indicated by the dashed arrows. The
emergence of new peaks at 30 T is attributed to an
abrupt change in the density of state of a magnon
excitation due to the field-induced transition.
References[1] Y. Sawada, S. Kimura, K. Watanabe, and H. Ueda, J. Phys.:Conf. Ser. 568, 042028 (2014).

Yuya Sawada (Corresponding Author, High Field Laboratory for Superconducting Materials)
E-mail: yuya_sawada@imr.tohoku.ac.jp
Hiroyuki Nojiri (Head of High Field Laboratory for Superconducting Materials)
E-mail: hflsm@imr.tohoku.ac.jp URL: http://www.hflsm.imr.tohoku.ac.jp/

44

Spectrometer

IMR KINKEN Research Highlights 2015

Scientific Breakthrough towards Creation of New Industrial Materials
Kansai Center for Industrial Materials ResearchInstitute for Materials Research (IMR) has launched the Kansai Center, a comprehensive institute-wide effort
that pairs IMR research groups across the innovation spectrum to solve today’s industrial challenges andtransform tomorrow’s global energy problems. The center was established in April 2011, assuming the
responsibilities of the Osaka Center project, based on an agreement between the IMR, the Osaka Prefecture
Government, and enterprises in the Kansai area sponsored by the government.
The Kansai Center was established in Osaka as a
special unit within the Institute for Materials Research(IMR), Tohoku University, in April 2011 based on anagreement with the Osaka Prefecture Government.
The Center is sponsored by the government (Ministryof Education, Culture, Sports, Science andTechnology), and it has assumed the responsibilitiesof the Osaka Center, which carried out its mission from2006 to 2011. The mission of the Osaka Center was tosupport small and medium enterprises in the Osaka
area. The area covered by the Kansai Center expandedfrom the Osaka area to the entire Kansai area. The
Center has three missions. First, to solve technicalproblems that industries have struggled to resolve.
Solving such problems is occasionally accomplishedthrough collaboration across universities, research
institutions, fields, and institutional boundaries.
Second, the Center introduces academic research toindustries, with the aim of applying them in society.
Third, the Center helps educate next-generationmaterials scientists and researchers in universities andenterprises. To this end, we have organized a bimonthly
forum named “Monodukuri Kisokoza,” which focuseson special topics regarding materials and processing.
Projects are conducted through widespreadcollaborations between the government, universities,
research institutions, and other organizations. Amongthem, the Development Center for New Metallic
Materials at the Osaka Prefecture University and the
Research Center for Nano-Micro Structure Scienceand Engineering at the University of Hyogo are close
partners in collaborative projects, and the Center staff
participates in educating students at both universities.
The Center has four locations for serving the Kansai
area: the Osaka office at Osaka Prefecture University,
the Hyogo office at the University of Hyogo, the Sendai
office at the IMR, and Monodzukuri BusinessInformation Center Osaka (MOBIO) in the Creation
Core Higashi-Osaka, a governmental body where
sixteen local universities and one college are locatedto facilitate collaboration with various industries.
The Center has developed various materials andFig. 1 Prototype device (left), plasma generator (middle),
and its schematic structure (right).

processes under collaboration with industry. One ofthem is to use a plasma system for water purification.
Figure 1 shows a prototype device (left) and a plasma
generator (middle and right) that generates plasma inwater flow and can continuously treat a large amountof water. When water goes through the orifice of theplasma generator, air is spontaneously taken up fromthe air inlet to provide a cavity of air between theelectrodes, which induces breakdown and canmaintain plasma in water flow. Because flowing water
comes in contact with the plasma core, an enhanced
effect can be expected compared to a conventional
batch plasma system. In the case of pure water
treatment, atmospheric nitrogen molecules entering
the plasma were decomposed to form nitrate ions,which is expected to be used as a hydroponic culture
nutrient medium. OH radicals and UV generated in theplasma are effective in the sterilization of water or
degradation of pollutants in water [1].
Forty similar collaborative studies with industry forpractical application utilizing academic knowledge
and industrial technology have been in progress. The
Kansai Center will make an effort to innovate in themetallic material industry through the alliance with thepartners in Kansai area, and promote scientificresearch in material science.
References[1] Y. Mizukoshi, R. Katagiri, H. Horibe, S. Hatanaka, M. Asano, andY. Nishimura, Chem. Let. 44, 495 (2015).

Naoya Masahashi (Head of Kansai Center for Industrial Materials Research)
E-mail: masahasi@imr.tohoku.ac.jp
URL: http://www.kansaicenter.imr.tohoku.ac.jp

IMR KINKEN Research Highlights 201545

Research Centers

Supercomputing for discovery and design of new materials
Center for Computational Materials ScienceT

he Center for Computational Materials Science at IMR supports various advanced studies on computational materials
science by providing users with an efficiently tuned high-performance super computer. The total number of users isapproximately 150 every fiscal year, producing various fascinating results. Among these, two recent outcomes havebeen selected and are introduced below. One is by R. Yamada, a graduate student at IMR and the other is by Dr. N.
Fujita, a theoretician, at the Institute of Multidisciplinary Research for Advanced Materials, Tohoku University.
First-Principles calculation of Coefficient ofThermal Expansion in Fe-Ni alloys*
First-principles calculation enables us toinvestigate various physical and thermal properties.
The following results were obtained by combining firstprinciples electronic structure total energy calculations
with the Cluster Variation Method.
Fe-Ni alloy is known as Invar, and the temperaturedependence of the Coefficient of Thermal Expansion(CTE) at Fe-35at.%Ni shows a non-monotonic
temperature dependence. The cause of this abnormal
behavior is hypothesized to be the temperature
variation in the alignments and magnitudes of magnetic
moments [1]. Non-monotonic abnormal behavior,
however, was observed even without considering any
spin configurations, as shown in Fig. 1. The cause ofthis temperature dependence in CTE is largely
attributed to a change in atomic configurations. This
fact gives us new insights into the abnormal behavior
of Invar alloys.
Geometrical search algorithm of complex threedimensional tilings**
As a geometrical framework for describing thecluster packing in icosahedral quasicrystals,‘canonical-cell tiling’, or tiling with four types of basic
polyhedra as the proto-tiles, has been proposed [2]. In
X-ray crystallography, approximant crystals with achemical composition similar to icosahedral
quasicrystals are shown to have crystal structures inwhich clusters occupy the vertices of a periodic
canonical-cell tiling. For representing an icosahedral
quasicrystal, a quasiperiodic canonical-cell tiling is
indispensable. However, it has never been proved
theoretically that the canonical cells can tile the spacequasiperiodically. In this study, a computer algorithm
for expanding the dimensions of the unit cell of aperiodic canonical-cell tiling by a factor of the cube of
the golden mean was implemented. An example ofFig. 1 Coefficient of thermal expansion as a function oftemperature.

Fig. 2 A new tiling generated by using an original program.
This is a complex yet beautiful tiling with space group
Pa-3 containing 2464 vertices within the unit cell.

new tilings generated using this program is shown in
Fig. 2. This is a complex yet beautiful tiling with spacegroup Pa-3 containing 2464 vertices within the unit
cell. A similar approach may lend itself to an attempt toprove the existence of quasiperiodic canonical-celltiling, that is, a tiling with an infinitely large unit cell.
References[1] M. van Schilfgaarde, I. A. Abrikosov, and B. Johansson, Nature
400, 46 (1999).
[2] C. L. Henley, Phys. Rev. B 43, 993 (1991).

*Ryo Yamada (Corresponding Author, Center for Computational Materials Science)
E-mail: b3tm5638@imr.tohoku.ac.jp** Nobuhisa Fujita (Corresponding Author, Institute of Multidisciplinary Research for Advanced Materials)
E-mail: nobuhisa@tagen.tohoku.ac.jp
Tetsuo Mohri (Head of Center for Computational Materials Science)
URL: http://www-lab.imr.edu/~ccms/Jpn/

46

IMR KINKEN Research Highlights 2015

IMR KINKEN Research Highlights 2015

Research and Education on Cutting-Edge Energy Materials
by Structural and Carrier Control at the Atomic Level
Collaborative Research Center on Energy Materials

In order to attain clean and economical energy systems, new technology is needed to improve efficiency ofenergy generation and consumption. To that end, it is essential to create technological breakthroughs, such as
the discovery of high-performance energy materials that efficiently convert and transport energy.
At the Center, research and development will
focus on the creation of advanced energy materials
as well as the design of devices that make optimaluse of superior material properties. Through joint
efforts in science and engineering, we will makeinroads into new research frontiers. In addition topromoting top-level research, efforts will be made to
expand the human resource potential of young
talents trained in trans-disciplinary integrated
collaborations.

Spin Current

Heat Current

The following four research divisions areconstructed by mixing science- and engineeringfield researchers.
1) The Spin Energy Materials Division, led by Prof.
G.E.W. Bauer, will carry out theoretical andexperimental studies on materials, such as
ferromagnets, in which the spin degree of
freedom significantly affects the charge andenergy transport. This division will strive tounderstand multi-scale physics and materials
science in order to develop new material
combinations that realize highly effective
conversion between heat and electronic
currents.
2) The Ion Energy Materials Division, led by Prof.
H. Miyasaka, will conduct research in both
fundamental and applied studies of materials
for effective ion transport and chemical energy
conversion. The development of novel solid
electrolytes and electrode materials toward therealization of high-performance all-solid-state
secondary batteries and multi-functional
batteries will contribute to building a sustainable
society.
3) The Light Energy Materials Division, led by Prof.
K. Fujiwara, will contribute to the developmentof a sustainable renewable energy society
through the fusion of research on crystal growth
physics and electronic engineering.

Fig. 1 Realization of environmental spin power generation,
advanced all-solid-state secondary batteries, and highefficiency solar cells, by joint efforts in science andengineering.

4) TheMaterial
Processing
andSocialImplementation Research Division, led by Prof.
H. Kato, will make research efforts in material
processing to produce high-quality and costeffective energy materials and in developing
evaluation
techniquestodemonstrateperformance materials and device abilities.
This division will contribute to the constructionof a new energy system based on cutting-edgeenergy materials with the aim of applying the
research output of this center to a sustainable
society.

Shin-ichi Orimo (Head of Collaborative Research Center on Energy Materials)
E-mail: e-imr@imr.tohoku.ac.jp
URL: http://www.e-imr.imr.tohoku.ac.jp/ index.html

IMR KINKEN Research Highlights 201547

Research Centers

Electrochemical Biocompatibilities of Beta-Type Ti-Nb-Ta-Zr
Alloys through Microstructural Refinement
International Collaboration Center (ICC-IMR)W

e aimed to investigate the electrochemical behavior of β-type Ti-Nb-Ta-Zr (TNTZ) alloys in this collaboration.
In-vitro corrosion susceptibilities of the alloys were compared with traditional implant materials such as Ti-6Al4V and CP Ti in this study. In addition, the effect of high-pressure torsion (HPT) on the corrosion susceptibilitiesof the alloys was investigated using a simulated body fluid.
Recently, the most common corrosion resistantmetallic biomaterials are being made of stainlesssteel (SS), cobalt–chromium (Co–Cr) alloys, andtitanium (Ti) alloys. Ti alloys are frequently used inbiomedical applications, particularly because of
their higher corrosion resistance, biocompatibility,
higher specific strength, and much lower elastic
modulus than the other metallic biomaterials [1].
Duringthevisiting
period,corrosion
susceptibilities of β-type TiNbTaZr (TNTZ) alloys
were compared with commercial implant materials
such as Ti-6Al-4V and pure Ti (CP Ti). The effect of
the high-pressure torsion (HPT) process on the
corrosion susceptibilities was also investigatedunder in-vitro conditions at body temperature. HPT
is a common technique for the fabrication of Ti alloys
with sub-micron grain sizes [2].
Figure 1 shows the potentiodynamic scanning(PDS) curves of the samples in Ringer’s solution(NaCl=8.60 g/L, CaCl2·2H2O= 0.33 g/L and KCl=0.30
g/L). The PDS profiles of the samples are quite similar
to each other and all samples attain their respective
stable passive current densities as the potential
increases. In other words, the protective passive films
formed on the surfaces were stable. It is well known
that protective surface films on the alloys play a keyrole in the corrosion of metallic implants. However, thepassive layers on Ti-6A-4V alloy are different from thaton TNTZ since the oxide layer contains oxides of thealloying elements. The relatively higher corrosion
resistance of TNTZ compared to the Ti-6Al-4V alloy is
attributed to the formation of Nb2O5, which is chemicallymore stable, less soluble, and more biocompatible
compared to the V2O5 formed on the Ti-6Al-4V alloy.
Most investigations showed that the corrosion
resistance of an ultrafine, grained alloy is lower thanthat of its coarse-grained counterpart [3]. TNTZHPT shows lower Epp (passivation potential) value
and a wider passivation range as compared to theothers. This result indicates that the severely strained
sample (TNTZ-HPT) is more resistant to corrosion

Fig. 1 Potentiodynamic polarization curves of the Ti alloys
in Ringer’s solution.

than the other Ti counterparts in Ringer’s solution.
Clearly, there is a strong connection between
corrosion resistance and the microstructural featuresof TNTZ. The supersaturated solid solution
decomposes completely and possibly reaches theequilibrium state after the HPT process [4].
I am thankful to the Institute for Materials
Research (IMR), especially Professor Niinomi for his
valuable contributions to the research. I hope that
our future collaboration continues.
References[1] K. U. Mudali, T. M. Sridhar, and B. Raj, Sadhana 28, 601(2003).
[2] D. Kuroda, M. Niinomi, M. Morinaga, Y. Kato, and T. Yashiro,
Mater. Sci. Eng. 243A, 244 (1998).
[3] S. J. Thorpe, B. Ramaswami, and K. T. Aust, J. Electrochem.
Soc. 135, 2162 (1998).
[4] B. Dikici, H. Yilmazer, M. Niinomi, M. Nakai, I. Ozdemir, and M.
Gavgali, Corrosion-2012, Physicochem. Mech. Mater. 2012
spec. 9, 95 (2012)

Burak Dikici (Corresponding Author, Yuzuncu Yil University, Department of Mechanical Engineering, Van-Turkey)
E-mail: burakdikici@yyu.edu.tr URL: http://www.yyu.edu.tr/ http://www.burakdikici.com
Hiroyuki Nojiri (Head of International Collaboration Center)
E-mail: icc-imr@imr.tohoku.ac.jp URL: http://www.icc-imr.imr.tohoku.ac.jp/

48

IMR KINKEN Research Highlights 2015

IMR KINKEN Research Highlights 2015

Neutrons for Advanced Materials Science
Center of Neutron Science for Advanced Materials

T

he Center of Neutron Science for Advanced Materials is a unique neutron facility, which has a background in
novel materials science at the IMR. This center operates two neutron spectrometers for materials science
investigations in a research reactor facility, and it is constructing a new neutron spectrometer at J-PARC/MLF,
which provides the world’s brightest neutron beam.

The Center of Neutron Science for Advanced
Materials (CN-IMR) was established in 2010 with theaim of promoting novel materials science by using
neutrons. The CN-IMR is now constructing a newneutron spectrometer (Fig. 1) at an intense neutron
source at the Japan Proton Accelerator ResearchComplex (J-PARC) [1]. Coupled with state-of-the-art
techniques in neutron beam and spin-polarization
devices, our spectrometer will provide a unique
opportunity to measure microscopic state in materials
along with separation of pure magnetic correlations
and atomic correlations. The important parts of aspectrometer, such as vacuum chamber andpolarization analyzer, were successfully installed in
2014FY, under collaboration with the High Energy
Accelerator Research Organization (KEK). We expect
to receive the first neutron beam in 2015FY, and theFig. 2 Group photo of participants in the neutron scienceschool organized by CN-IMR.

spectrometer will be available for the collaborative
research programs of IMR after commissioning.
Furthermore, the CN-IMR is operating two neutron
instruments at the JRR-3 research reactor at the Japan
Atomic Energy Agency. Utilizing one of the instruments,
a three-day school of neutron science for new users
was held from June 2, 2014. Over 20 students andyoung researchers at Tohoku University attended theprogram (See Fig. 2) and learned the fundamentals ofneutron scattering techniques. Several staff at JAEA,
Tokyo University, and J-PARC kindly supported theschool by giving lectures and introducing the Materials
and Life Science Experimental Facility at J-PARC. The
students interacted with senior researchers at thefacilities through various discussions. The school
received favorable feedback from all the participants,
indicating the success of the workshop
References[1] T. Yokoo, K. Ohoyama , S. Itoh, J. Suzuki, K. Iwasa, T. J. Sato, H.
Kira, Y. Sakaguchi, T. Ino, T. Oku, K. Tomiyasu, M. Matsuura, H.
Hiraka, M. Fujita, H. Kimura, T. Sato, J. Suzuki, M. Takeda, K.

Fig. 1 Illustration of new spectrometer being constructed
at J-PARC [1].

Kaneko, M. Hino, and S. Muto, J. Phys.: Conf. Ser. 502, 6012051(2014).

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/

IMR KINKEN Research Highlights 201549

Research Centers

Nanocrystalline Fe-Based Alloys for Commercial Applications
Research and Development Center for Ultra High Efficiency Nano-crystalline Soft Magnetic MaterialN

anocrystalline Fe81.2Co4Si0.5B9.5P4Cu0.8 alloy (at.%) with a low coercivity of 7 A/m and high saturation
magnetic flux density of 1.84 T has successfully been reproduced in 50 mm wide ribbons. This suggests thatthe nanocrystallized Co-containing alloy has high applicability in high-efficiency magnetic core materials.
Modern society is full of a variety of electric
products, in which motors, cores, and so forth areused as principal parts. Electricity is an integral partof everyone’s life, and thus, we need to use electric
power efficiently without wastage under a conditionof limited power generation. Use of efficient materials
for electric products can decrease the losses. A
research strategy in our group is to develop novel Febased alloys that contribute to reducing the losses.
Recently, our group has succeeded indeveloping a novel Fe-based nanocrystalline soft
magnetic alloy named “NANOMET®” [1] with highsaturation magnetic flux density (Bs), low core loss(W), and low coercivity (Hc). This report includes a
study of the NANOMET family alloys for use inproducts by modifying the alloy components.
Figure 1 shows the outer shape and surface
appearance of wide ribbons of 50 mm width, 65 m
length, and 25 µm thickness. When using theseribbons in magnetic core applications, W as well as
Hc and Bs are of great importance. Figure 2 shows
W at 50 Hz as a function of the maximum induction(Bm) of nanocrystalline Fe81.2Co4Si0.5B9.5P4Cu0.8(at.%) alloy ribbons where the widths are 10 and 50
mm. Referential data for comparison include theplots of the alloy with commercially-used, nonoriented magnetic steel and Fe-based Fe78Si9 B13(at.%) amorphous alloy ribbon (Metglass). Thisnanocrystalline alloy exhibited W characteristics
independent of the ribbon width. The W at a Bm of
1.5 and 1.7 T was determined to be 0.52 and 0.66
W/kg, respectively, for the 50 mm-wide ribbon, and0.44 and 0.66 W/kg, respectively, for the 10 mmwide ribbon. These W values are almost the sameas those of the ribbon specimens in the laboratory
at a scale of several mm. Comparisons of these W
data with those of commercial alloys revealed thatthe present nanocrystalline alloy ribbons showedmuch lower W at Bm, in particular, above 1.5 T. It is,therefore, concluded that the nanocrystalline
Fe81.2Co4Si0.5B9.5P4Cu0.8 alloy with excellent soft
magnetic properties can be fabricated into thickFig. 1 Outer appearance of a 50 mm-width ribbon
fabricated by melt-spun technology [2].

Fig. 2 Core loss at 50 Hz of the nanocrystalline
Fe81.2Co4Si0.5B9.5P4Cu 0.8 (at.%) alloy wide ribbon as afunction of maximum induction [2]. The data of Fe-3
mass% Si and Fe78Si9 B13 (at.%) amorphous alloy ribbon
are also shown for comparison.

and wide ribbons, which is required for practicaluse. This makes this alloy a promising candidate forhigh-efficiency magnetic core materials.
References[1] A. Makino, T. Kubota, C. Chang, M. Makabe, and A. Inoue, J.
Mag. Mag. Mater. 320, 2499 (2008).
[2] K. Takenaka, A. D. Setyawan, Y. Zhang, P. Sharma, N.
Nishiyama, and A. Makino, Mater. Trans. 56, 372 (2015).

Akira Takeuchi (Corresponding Author, Research and Development Center for Ultra High Efficiency Nano-crystalline
Soft Magnetic Material)
E-mail: takeuchi@imr.tohoku.ac.jp
Akihiro Makino (Head of Research and Development Center for Ultra High Efficiency Nano-crystalline Soft Magnetic Material)
E-mail: nanoc@imr.tohoku.ac.jp URL: http://nanoc.imr.tohoku.ac.jp/eng/index.html

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IMR KINKEN Research Highlights 2015

IMR KINKEN Research Highlights 2015

Visualization of Quantized Vortices in Mesoscopic
Superconductors
Laboratory of Low-Temperature Materials ScienceT

he scanning superconducting quantum interference device (SQUID) microscope is a useful tool thatvisualizes the magnetic field distribution at the sample surface. Using this tool, we succeeded in directly
imaging quantized vortices confined in mesoscopic superconductors. In this paper, we highlight the evolutionof the vortex pattern with the applied magnetic field in square and triangular superconducting dots.
The physical phenomenon caused by theinteraction of a small number of particles in aconfinement potential has attracted significant
interest. A typical model is a system of quantizedvortices in a mesoscopic superconductor, wherethe vortices are subject to lateral confinement due tothe shielding current flowing along the sample edgein addition to the intrinsic repulsive vortex–vortex
interaction. The combination of these interactions
results in unique vortex states that strongly reflect
the sample shape, unlike the well-known Abrikosov
triangular lattice in bulk superconductors. To verify
this, we employed a scanning superconducting
quantum interference device (SQUID) microscope,
which visualizes the magnetic field distribution witha high resolution and enables us to directly observe
the vortices though their magnetic flux. Here, we
demonstrate how the vortex pattern evolves with anapplied magnetic field in square [1] and triangular[2] dots of superconducting amorphous Mo78Ge22
films.
Figure 1 shows typical vortex images for L =1–16, where L is the number of vortices, in a 76 × 76
µm2 square dot [1]. At L = 4, 9, and 16, the vortices
form 2 × 2, 3 × 3, and 4 × 4 square lattices,
respectively, instead of the typical triangular lattices,
according to the shape of the superconductor. We
experimentally found that for L ≥ 5, the vortex pattern
evolves with a multi-shell configuration based on thefollowing rule. With increasing L up to L = 9, theouter shell is first filled while maintaining one innershell vortex. Increasing L further up to L = 12, theinner shell grows while keeping 8 vortices in theouter shell. We also confirm that for 13 ≤ L ≤ 16 (notshown), the outer shell grows again while keeping 4
vortices in the inner shell. Thus, the inner and outershells grow in turn with increasing L, as in Hund’srule, and the corresponding changes occur with theformation of square polygons with multiples of 4
vortices.
In the case of triangular dots, it is found that the

(b)

(a)

(d)

(c)

I

(e)

L

50 mΦ054 mΦ 064 mΦ0(f)(j)

56 mΦ060 mΦ0J(k)

69 mΦ056 mΦ064 mΦ0K(l)78 mΦ054 mΦ061 mΦ079 mΦ0Fig. 1 Typical SQUID images of vortices in the square dot
for L = (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, (f) 6, (g) 7, (h) 9, (i)10, (j) 11, (k) 12, and (l) 16 [1]. The color bars indicate
the magnitude of magnetic flux per pixel of size 5 × 5µm2 normalized by the magnetic flux quantum Φ0.

field evolution of the vortex pattern occurs in a different
way; the vortex configuration can be described as a
combination of a triangular pattern and a symmetriclinear chain parallel to the base, reflecting the triangularshape of the superconductor [2].
The above insight is obtained by performing
repeated observations of vortices in the same
samples with continuously increasing magnetic
fields. This is made possible by the visualization
technique using the scanning SQUID microscope.
References[1] N. Kokubo, S. Okayasu, T. Nojima, H. Tamochi, and B.
Shinozaki, J. Phys. Soc. Jpn 83, 083704 (2014).
[2] N. Kokubo, H. Miyahara, S. Okayasu, and T. Nojima, J. Phys.
Soc. Jpn 84, 043704 (2015).

Tsutomu Nojima (Corresponding Author, Laboratory of Low-Temperature Materials Science)
E-mail: nojima@imr.tohoku.ac.jp URL: http://ltsd.imr.tohoku.ac.jp/
Takahiko Sasaki (Head of Laboratory of Low-Temperature Materials Science)

IMR KINKEN Research Highlights 201551

Research Centers

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

More than 170 species of radioisotopes and nuclear materials are available in the Laboratory of Alpha-Ray

Emitters. This laboratory is one of the most important centers in the world for studying physical and chemical
properties of radioactive materials such as actinide compounds. Researchers, from many leading universities
and institutes all over the country, have visited this facility every year to prepare a variety of materials and carryout their chemical and physical measurements.
(a)dI / dV ( a.u. )The laboratory has provided a research
environment using 170 radionuclides (RI) and nuclear
elements, specifically exemplified by alpha-ray
emitters such as actinides. The laboratory started
operation in 1960 on the basement floor of the old
Main Building 1 of IMR, which was succeeded by thepresent building in 1978. The laboratory’s capacity of
RI, including actinide elements, is ranked as the one of
the largest among Japanese universities. Recently,
this lab has functioned as an important base forpreparation of pure crystals of actinide compounds tobe provided to other universities and to Synchrotron
Orbital Radiation facilities (SORs).
The controlled area of the laboratory consists ofthree chemistry lab rooms, three physical lab rooms, aroom for radioactivity measurements, a contamination
test room, and storage rooms for RIs and nuclear
materials. Each chemistry lab is equipped with local
exhaust ventilation systems. Gamma-ray spectrometers, a liquid scintillation counter, and an alpha-ray
spectrometer are installed in the radioactivity
measurement room.
Point-contact spectroscopy (PCS) is a method of
obtaining differential conductance as a function of abias voltage (dI/dV(V)). The bias voltage V accelerates
a passing electron through a point-contact andprovides an excess energy eV, which results in theenergy spectrum dI/dV(V). When the current I depends
on the electrical density of states (DOS) of the system,
the dI/dV(V) measurement is suitable for studying theevolution of DOS, such as superconducting gap
structure, Kondo resonance peak, etc. Our simple
PCS method dramatically widens the measurement
conditions to include ultra-low temperature, highmagnetic fields, and high pressures. The PCS wasapplied to actinide compounds with strongly correlated
f-electrons, single crystals of which were prepared by
the tetra-arc furnace in this lab.
Figure 1 shows the PCS spectra of URu2Si2 under
two pres-sures [1]. At 4.2 K, both spectra, under P =0.41 and 0.98 GPa, show a similar structure, with kinks
and swells, compared to those at 20 K. Under the

IMR KINKEN Research Highlights 2015

0.98 GPa

HF

20 K

0.4

0.4

16 K
12 K

0.2

8.0 K

HO

-20

0

20

V ( meV )

4.2 K

AFM

-20

0

20

0.2

V ( meV )Fig.1 PCS spectra of URu2Si2 at (a) HO state at P = 0.41
GPa compared with (b) AFM state at P = 0.98 GPa.

pressures of 0.41 and 0.98 GPa, URu2Si2 exhibits theso-called hidden ordered (HO) state, i.e., the orderedstate with an unknown order parameter, andantiferromagnetic (AFM) ordered states at 4.2 K,
respectively. At 20 K, URu2Si2 shows heavy fermion(HF) behavior under both pressures. The energy
dependences of the DOS of HO and AFM are very
similar at all temperatures examined. Our results haveprovided a new relationship between the DOS of the
HO and AFM states.
A high-resolution Compton profile (HRCP)
measurement, which provides the electron occupation number density (EOND) in k-space, wascarried out in the Spring-8 BL08W to visualize theelectronic structure in the first Brillouin zone (BZ). We
observed that the 2D EOND of Ce-Ru2Si2 significantly
changed around the corner of the projected BZ fromroom temperature to 5 K [2]. We continue to apply PCS
and HRCP measurements for investigations on otheractinide compounds, such as URu2Si2 [3].
References[1] Y. Sakai, G. Motoyama, A. Yamaguchi, A. Sumiyama, T.
Yamamura, and I. Sato, J. Phys. Soc. Jpn 81, SB068-1-4 (2012).
[2] A. Koizumi, G. Motoyama, Y. Kubo, T. Tanaka, M. Itou, and Y.
Sakurai, Phys. Rev. Lett. 106 136401 (2011).
[3] A. Sumiyama, K. Hikawa, J. Gouchi, G. Motoyama, A. Yamaguchi,
T. Honma, E. Yamamoto, Y. Haga, and Y. Onuki, J. Phys. Soc. Jpn
82, 025005-1-2 (2013).

Tomoo Yamamura (Head of Laboratory of Alpha-ray Emitters)
E-mail: yamamura@imr.tohoku.ac.jp URL: http://alpha.imr.tohoku.ac.jp

52

(b)

0.41 GPa

HF

IMR KINKEN Research Highlights 2015

Direct Imaging of Atomic Columns by Using Electron
Microscope
Analytical Research Core for Advanced Materials

Quantitative compositional analysis provides useful information in materials sciences. This section supports
research activities of IMR by managing and servicing such analytical facilities and techniques. Furthermore, we
are also conducting microstructural and crystallographic analyses of materials by using electron microscopefor more-reliable research and analysis. Recent observation of NiMnSb and Ni2MnSb alloys by using highangle annular dark field scanning transmission electron microscopy (HAADF–STEM) in an atomic scale isshown here.

(a)

C
B

BD

D

D B

B

A

C

C

B

D

B

D

D

B

C

A

C

B

D

B

A

C

A

110 D
D
1-10

B

D

D

A
B

001 DB C B D A D B
010 D A B CD
100 D

B

B

001

NiMnSb alloy is a typical half-metal having ahalf-Heusler (C1b) structure. Figure 1(a) shows amodel of a bcc-based ordered structure that has
dimensions of 2 x 2 x 2, including 4-fcc-typesublattices: A, B, C, and D. The A, B, and D sites areoccupied by Ni, Mn, and Sb, respectively, with the Csites being vacant in NiMnSb. In the case of fullHeusler (L21)-type Ni2MnSb, which is known as aferromagnetic shape memory alloy, the C sites areoccupied by Ni.
Although L21 and C1b structures belong todifferent space groups—Fm-3m and F-43m,
respectively—both structures have the same
extinction rules. Therefore, it is hard to distinguishbetween these structures by using diffractometry.
Thus, the HAADF–STEM method using a JEMARM200F is employed here to determine theordered structures accurately.
The HAADF–STEM images taken from L21Ni2MnSb and C1b-NiMnSb specimens are shown in
Fig. 1(b) and (c), respectively. In order to identify thefour atomic sites occupied by Ni, Mn, Sb, or vacancy,
as shown in Fig. 1(a), the [1-10]bcc axis was chosen
as the incident beam direction. All the atomic
columns were successfully observed as white dots.
Here, the intensity of the atomic column in the
HAADF–STEM image is almost proportional to thesquare of the average atomic number Z in eachatomic column. The order of brightness for theatomic columns in the present case is Sb > Ni > Mn,
as labeled in Fig. 1(b) and (c). In the C1b-NiMnSb
alloy, the vacant C sites clearly show a dark contrast,
while the C sites in the L21- Ni2MnSb alloy show analmost equivalent atomic contrast as that of the Ni A
sites.
Furthermore, each atomic contrast patternobserved in these experiments is in almost perfectagreement with that predicted by the simulations asD(b) L2 1 -Ni 2 MnSb

001

Sb Mn SbNi Ni Ni
Mn Sb Mn
Ni Ni Ni
Sb Mn Sb

calc.

1nm

110

(c) C1 b -NiMnSb

Sb Mn Sbvac Ni vac
Mn Sb Mn
Ni vac Ni
Sb Mn Sb

001
110

calc.

1nm

Fig. 1 (a) A schematic of the bcc-based crystal structure.
(b) and (c) HAADF–STEM images of Ni2MnSb and
NiMnSb, respectively.

shown in the insets of Fig. 1(b) and (c). Thus, theexistence of the vacant site can be satisfactorily
confirmed using the HAADF–STEM technique forthe first time [1].
References[1] M. Nagasako, Y. Taguchi, T. Miyamoto, T. Kanomata, K. R. A.
Ziebeck, and R. Kainuma, Intermetallics 61, 38 (2015).

Makoto Nagasako (Corresponding Author, Analytical Research Core for Advanced Materials)
E-mail: nagasako@imr.tohoku.ac.jp
Kazuaki Wagatuma (Head of Analytical Research Core for Advanced Materials)
URL: http://bunseki-core.imr.tohoku.ac.jp

IMR KINKEN Research Highlights 201553

IndexIMR KINKEN Research Highlights 2015

Author Index
Name

page

A
Abe, Hiroaki
Aoki, Dai
Arima, Hiroshi
Barker, Joseph

24
39
9
20

C
Chiba, Akihiko12, 13

D
Dikici, Burak
Deb Nath, Sankar

48
17

F
Fujita, Masaki
Fujita, Nobuhisa
Furuhara, Tadashi

34, 4946
43

G
Goto, Takashi

11

I
Iguchi, Satoshi
Inoue, Koji
Itoi, Akihiko

33
35
11

K
Kato, Hidemi
Katsu, Hirokazu
Koizumi, Haruhiko
Konno, J. Toyohiko
Kosaka, Wataru
Kubo, Momoji

26
11
27
14
36
22

M
Makino, Akihiro
Masahashi, Naoya
Matsuoka, Takashi
Matsuta, Hideyuki
Miyamoto, Goro
Miyasaka, Hitoshi
Mohri, Tetsuo

50
45
25
15
8
36
17, 46

Name

page

N
Nagai, Yasuyoshi
Nagasako, Makoto
Nagata, Shinji
Nagumo, Kazuaki
Niinomi, Mitsuo
Nojima, Tsutomu
Nojiri, Hiroyuki
O
Ohashi, Yuji
Ohno, Yutaka
Orimo, Shin-ichi

35, 42
53
23
35
10
51
30, 44, 48

38
21
28, 47

S
Sasaki, TakahikoSato, Kazuhisa
Sawada, Yuya
Settai, Rikio
Shimizu, Yasuo
Shiomi, Yuki
Sugiyama, Kazumasa

51
43
44
42
35
31
9

T
Takagi, Shigeyuki
Takanashi,Koki
Takeuchi, Akira
Terada, Yayoi
Tsukazaki, Atsushi

28
37
50
17
32

U
Uda, Satoshi

27

W
Wagatuma, Kazuaki

53

X
Xie, Guoqiang

16

Y
Yamada, Ryo
Yamamura, Tomoo46
52

IMR KINKEN Research Highlights 201555

Keyword Index
KeywordA
alloy
amorphous alloy
atom probe tomography

10
9
35

B
biological

10, 27C
ceramic
crystal growth1127, 38D
3D structure
defects
deformation
devicesdielectric
dielectric properties

35
12, 21
13
32
33
36

E
electrical properties
electronic material
electronic structureenergy storage

56

page

36
31, 35, 38
28
26

F
Fermi surface
ferromagnetic
field-effect transistor
first-principles calculations
foam

39
30
32
28
26

G
glow discharge plasma
grain boundaries

15
21

H
half-metal
heavy fermion
high magnetic field
hydridehydrogen implantation

37
39
30
28
23

I
insulator
interfaceion beam analysis

20
17
23

K
kinetics

8M
magnetic properties

15

IMR KINKEN Research Highlights 2015

Keywordpage

mechanical properties
metal
metal-insulator transition
metallic glass
microstructure
molecular devices
molecular dynamics

10, 16, 24
14
33
16
12, 1436
22

N
nanostructure
neutron scattering
nitridenovel magnetic materials
nuclear materials

13
34
25
30
23, 24O
organic
oxide

33
32

P
phase transformation
porosity
powder metallurgy
powder processing

8, 12, 1726
16
11, 13Ssimulation
sintering
solar cells
spectroscopy
spin current
spin transfer
spin wave
spintronics
steel
structure
superconducting
surface reaction
synchrotron

17, 22
1121, 251531
37
20
20, 31, 37
8
934, 3922
9Ttransmission electron microscopy (TEM) 14
Vvapor phase epitaxy (VPE)

25

X
X-ray scattering
X-ray diffraction (XRD)

34
27

Z
zirconium alloy

24

Editors:
Atsushi Tsukazaki (IMR), Naoya Masahashi (IMR),
Gerrit E.W. Bauer (IMR), Kozo Fujiwara (IMR),
Satoshi Iguchi (IMR), and Kazuhisa Sato (IMR)

Editorial Staff:
Misa Yokoyama (IMR), Yoshiko Takahashi (IMR),
and Kaori Kikuchi (IMR)

Printing:
HOKUTO Corporation

Organization
Institute for Materials Research
Tohoku University
2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
Tel. +81-(0)22-215-2144 Fax. +81-(0)22-215-2482
URL: http://www.imr.tohoku.ac.jp/
E-mail: pro-adm@imr.tohoku.ac.jp© IMR August 2015