ResearchHighlights of Collaborated Research
Collaborative works with outside researches are carriedout in consideration of the joint research program. Accepted
applications are performed in following each division andfacility of IMR.
1) Research Laboratories
Joint research conducted by out side researchers and
IMR staff members at each research laboratory. Four
categories “Research in Priority Areas”, “General Research”,“Exploratory Research for Young Researchers” and “Workshop” are prepared.

2) International Research Center for Nuclear Materials
ScienceThis facility is open to university scientists all over Japanto support experiments using Japan Materials Testing
Reactor, JMTR, the fast experimental reactor, JOYO, andthe test reactor, JRR-3, operated by JAEA (Japan Atomic
Energy Agency). The overseas reactor, BR2, located at theBelgian Nuclear Research Center, is also used for irradiation
experiments. This facility acts as a hub for international
collaborations; specimens irradiated in overseas reactors are
accepted here for post-irradiation examinations byparticipating university researchers. Research subjects
covered here include fundamental studies and R&D on fusion
structural materials, high heat-flux materials, and a variety offunctional materials, as well as engineering-oriented studies
for the safety of light water reactors and basic researches
supporting them. Materials studies utilizing radio-isotopes
are also being conducted.

KINKEN Research Highlights 2010 IMR

3) Advanced Research Center of Metallic Glasses (ARCMG)
ARCMG was established in April 2006, succeeding theLaboratory for Advanced Materials (LAM), investigating
metallic glasses and other materials. This facility is open to
domestic and international collaborations for research on all
aspects of materials science, consisting of two major
divisions: (i) scientific research on fundamental properties ofmaterials, and (ii) development of engineering materials forpractical usage. Micromotors, pressure sensors, fuel cell
separators, soft magnetic material and many other devises
using metallic glasses are proposed for practicalapplications.

4) High Field Laboratory for Superconducting Materials
This laboratory restarted in 2001, succeeding “High Field
Laboratory for Superconducting Materials” which wasestablished in 1981. The main equipment is a hybrid magnet
which generates steady high magnetic fields up to 31T. In
addition, many cryogen-free superconducting magnets whichhave been developed by our laboratory are installed.
Especially, we succeeded in developing the world’s firstcryogen-free hybrid magnet, which generates 27.5 T. The
laboratory also provides instruments for measuring various
physical properties. These facilities are open to scientists andengineers on superconductors and other materials research.

Highlights of Collaborated Research415) Osaka Center for Industrial Materials research
The Osaka Center was established at Osaka as a special
unit in IMR in April 2006 based on the agreement between
Tohoku University and Osaka Prefecture Government. It is alaboratory dedicated to research and development of nanostructured metallic materials from the application viewpoints
based on the basic research in the material science, chemistry
and physics associated with research organizations in the
Kansai region. Research in the Osaka Center is focused inparticular on the understanding of the fundamental propertiesof nano-structured metallic materials and the rapid realizationof their application to industry, in particular, the small andmedium enterprises in Osaka area.

7) International Collaboration Center (ICC-IMR)
ICC-IMR was founded after the International FrontierCenter for Advanced Materials in April 2008. It has evolved
into the international research collaboration center of Institutefor Materials Research. As such, ICC-IMR’s programs havebeen expanded to include collaborative research projects byinternational teams. It works as a gateway of diverse
collaboration between international researchers and IMR
members. Currently, ICC-IMR coordinates five different
programs: 1) International Integrated Project Research, 2)
Visiting Professorships, 3) Short Single Research Visits, 4)International Workshops and 5) Coordination of International
Collaboration. We welcome applicants from around the globe
to participate in these international programs.

6) Center for Computational Materials Science
This center was developed from the Laboratory ofMaterials Information Science which was established in
February 1989. Its main tasks are 1) administration andmaintenance of the supercomputing system in this Institute,2) maintenance of a supercomputing system network, 3)
general support for the usage of the supercomputing system,4) support of materials design by supercomputing simulation
with vectorization and parallelization, 5) construction of
factual database for materials in nonequilibrium phase , and6) support of activity of Nanotechnology group on SINET3
and Asian Consortium for Materials Science, ACCMS-VO.

42

Highlights of Collaborated Research

IMR KINKEN Research Highlights 2010

ResearchFormation of Nanotubes of Carbon by Joule Heating of
Carbon-coated Si Nanochains
Carbon nanotubes (CNTs) are usually synthesized bychemical vapor deposition. We have found a new route tosynthesizing CNTs. Insulating Si nanochains which arecoated with hydrocarbon contamination are transformed into
conducting CNTs by Joule heating. The transformation has
been observed in situ on a TEM. The transformation can beutilized for selective formation of nanowire conducting paths.
Various types of nanostructure have been fabricated via
template methods, in which template nanostructures aretransformed into other types of nanostructure. In particular,
the transformation of insulating nanostructures into conducting
nanostructures has a great impact on nanodevice fabrications.
We have found the transformation of insulating silicon
nanochains into conducting nanotubes of carbon by Joule
heating in a transmission electron microscope (TEM) [1, 2].
In a silicon nanochain, silicon nanoparticles are covered
with and connected alternately by oxide to form a nanowire[3]. The Si nanochains were grown via an advanced vaporliquid-solid (VLS) process using Au-Pb as a catalyst.
Individual Si nanochains were heated by applying an electricalcurrent using micro-manipulator system in a TEM to observe
their structural change in situ. The Joule heating of ananochain resulted in the transformation into a nanotube ofcarbon as shown in Fig. 1. Some Si nanoparticles were
found in the nanotube. The current which was required forthe transformation was typically as large as 101 nA with abias voltage of 101 V.
It is very likely that the nanotube was formed from surface
carbon contamination, followed by the evaporation of thecore oxide. In fact, carbon was detected from the Si
nanochains by means of Energy Dispersive X-ray
spectrometry (EDX). When the Joule heating was insufficient,
we found that the oxide core was only partly evaporated tomake cavities. This evidences that the transformation occurs
in two steps: first the surface graphene shell was formed, andthen the core oxide evaporates.
We estimated thetemperature during the Joule heating. By considering thebalance of Joule heating, thermal radiation, and thermal
conduction to the electrodes, the temperature was estimatedto be of the order of 103 K. This temperature is high enoughfor the evaporation of the oxide core.

Fig. 2 Schematic illustration of the selective formation of
conducting paths of nanotubes by Joule heating of Si
nanochains.
Our finding enables us to change the charge transport
properties of nanowires drastically, from insulating(nanochain) to conducting (nanotube): in a Si nanochain, theSi core is fragmented by insulating oxide, while carbon is agood conductor. We confirmed that the conductance of aformed nanotube was higher by an order of magnitude than
a nanochain. Therefore, the transformation could be used
for fabricating conduction paths (Fig. 2). When dispersing Si
nanochains between predefined electrodes randomly with asufficient density, all the paths are off. By applying a large
bias voltage between a selected pair of electrodes, the path
between the electrodes can be switched to the on state.
References[1] T. Nogami, Y. Ohno, S. Ichikawa, and H. Kohno,
Nanotechnology 20, 335602 (2009).
[2] H. Kohno, T. Nogami, S. Takeda, Y. Ohno, I. Yonenaga,
and S. Ichikawa, accepted for publication in J. Nanosci.
Nanotechnol.
[3] H. Kohno and S. Takeda, Appl. Phys. Lett. 73, 3144 (1998).
Key Words
Si Nanochain, Carbon Nanotube, Joule Heating
Contact toHideo Kohno (Graduate School of Science, Osaka University)
E-mail: kohno@phys.sci.osaka-u.ac.jp
URL: http://tem.phys.sci.osaka-u.ac.jp/
Yutaka Ohno (Physics of Crystal Defects Division)

Fig. 1 Transformation of a carbon contaminated Si nanochain
into a nanotube of carbon by Joule heating in a TEM.

KINKEN Research Highlights 2010 IMR

E-mail: yutakaohno@imr.tohoku.ac.jp

Highlights of Collaborated Research

43Research

We have successfully grown InxAl1-xN films on nearlylattice matched ZnO (0001) substrates at room temperature(RT) by the use of pulsed laser deposition (PLD). We havefound that reduction in growth temperature down to RT
leads to suppression of the interfacial reactions between
InAlN and ZnO and the layer-by-layer growth of InAlN.
InxAl1-xN has attracted much attention because of theband gap ranging from 0.65 eV to 6.0 eV. However, it has
been difficult to grow high quality InAlN films due to two major
problems. The first problem is the phase-separation of InAlN
that stems from the large miscibility gap between InN and
AlN. The other problem is the lack of lattice-matchedsubstrates. ZnO substrates have the same crystal structureand give the small lattice mismatch to InAlN. In spite of theseadvantages, ZnO has not been successfully used, since iteasily reacts with nitrides at temperatures above 500 °C. The
use of PLD low temperature growth technique enables us tosolve the above-mentioned problems. In this study, we reporton growth of high-quality InAlN films on ZnO substrates by
the use of the RT growth technique [1].
Figures 1(a)-(c) show AFM images of InAlN films grown
at 600 °C, 480 °C and RT, respectively. The film grown at
600 °C has quite a rough surface with a root mean square(RMS) value of 7.9 nm. In contrast, the surfaces of the films
grown at 480 °C and RT were found to be smooth. Especially,
the layer grown at RT clearly has a stepped and terraced
surface, which is analogous to the morphology of theannealed ZnO surfaces. The RMS values for the surface
roughness of the annealed ZnO and RT-InAlN terraces were0.16 nm and 0.17 nm, respectively.
We performed XRD 2θ/ω measurements for InAlN layersgrown at 600 °C, 480 °C and RT. The layer grown at 600 °C
exhibits an extremely weak and broad peak, which indicatesthat the film quality is quite poor. The poor structural propertiesof films deposited at higher temperatures can possibly be
attri buted to the rough interface between InAlN and ZnO,
and the phase separation of InAlN. In contrast, a sharp InAlN
0002 peak appears when the growth temperature is reduced

RHEED Intensity (arb. units)

Room Temperature Layer-by-Layer Growth of InAlN Filmson Lattice-matched ZnO Substrates
Layer-by-Layer growth

1 ML

0

50

100

150

200

Growth Time (s)Fig. 2 Intensity profile of the specular spot in the RHEED
pattern during room temperature growth of In0.6Al0.4N on
atomically flat ZnO surfaces. Clear RHEED oscillation
indicates that the growth proceeded in the layer-by-layer
mode from the initial stages.
to RT. Interpreting the peak position of this XRD data using
Vegard’s law showed that the In composition of the InxAl1-xN
layer grown at RT was x = 0.7
In order to evaluate the interfacial abruptness between
InxAl1-xN with a nominal In composition of x = 0.7 and ZnO,
grazing X-ray reflectivity measurements were performed. It
was found that the thickness of the interfacial layer between
the InAlN layer and the ZnO substrate prepared at 600 and
480 °C was 2.7 nm and less than 1.0 nm, respectively. Although
the interfacial layer of the sample grown at 480 °C is quite thin,
the interface was so rough that damping of the X-ray reflectivity
oscillation was observed, indicating that intermixing of InAlN
and ZnO certainly took place. In contrast, the X-ray reflectivity
oscillation of the sample prepared at RT is not seriously
damped and the curve fits well with the two-layer model for an
abrupt interface. This indicates that the interfacial reaction
between the InAlN layer and the ZnO substrate has been
drastically suppressed and an atomically abrupt InAlN/ZnO
interface was obtained by reducing the growth temperature.
We also investigated the growth mode of InAlN on ZnO
substrates at RT. Figure 2 shows the intensity profile of the
RHEED specular spot during the growth of In0.6Al0.4N. We
observed a clear RHEED oscillation, indicating that thegrowth proceeded in the layer-by-layer mode from the initialstages. Using the measured film thickness, one cycle of theoscillation was calculated to be 0.28 nm, which corresponds
to ~1 monolayer of the c-plane.
References[1] T. Kajima, A. Kobayashi, K. Shimomoto, K. Ueno, T. Fujii,
J. Ohta, H. Fujioka, and M. Oshima, Appl. Phys. Express 3,021001 (2010).
Key Words
Group III Nitrides, Crystal Growth, Thin Films

Fig. 1 AFM images of In0.7Al0.3N films grown on ZnO substrates
at (a) 600 °C, (b) 480 °C, and (c) RT by pulsed laser deposition.
Atomically flat InAlN surface was obtained at RT.

44

Highlights of Collaborated ResearchContact toHiroshi Fujioka (Institute of Industrial Science, The University of Tokyo)
E-mail: hfujioka@iis.u-tokyo.ac.jp
URL: http://fujioka.iis.u-tokyo.ac.jp

IMR KINKEN Research Highlights 2010

Research

Laser-enhanced Chemical Vapor Depositionof Alumina for Cutting Tools
A laser-enhanced chemical vapor deposition (Laser
CVD) is a new process to obtain ceramic coatings at
extremely high deposition rates. Effects of deposition
conditions on deposition rates, microstructures andcrystalline phases of alumina coatings have been
investigated in this study in order to apply this high-speed
coating process to alumina coatings on cutting tools [1].
WC-Co composites are widely used as cutting tool
materials, because they have high hardness and ductility athigh temperature. In 1990s, alumina-coated WC-Co cutting
tools were developed to elongate life-time of the cutting tools,
and are generally used today.
In metalworking manufacturing industry, dry and highspeed lathe cutting process are required to improve
productivity. However, dry and high-speed cutting process
gives a severe high temperature and stressed environment
to the cutting tools. Conventional alumina coatings usuallycontain alumina of γ- and κ-phase, and these phases are
known to transform into the α-phase accompanying a volume
change at a high temperature above 1000 K during the
cutting process. Thus, alumina coatings in a single α-phaseshould be prepared on cutting tools for a dry and high-speed
cutting process.
Although a conventional thermal CVD has been commonly
applied for alumina coating on cutting tools, the deposition
temperature of α-alumina coating is usually about 1300 K
and such a high temperature degrades WC-Co substrate.
In this study, we prepared α-alumina films by LCVD andinvestigated the effect of deposition conditions on thedeposition rates, microstructures and crystalline phases ofalumina film. Aluminum tri-acetylacetonate (Al(acac)3) wasused as the precursor and was vaporized at vaporization
temperature (Tvap) . Precursor vapor carried with argon gas
and oxygen gas were separately introduced into the CVD
chamber through a double-tube nozzle. The YSZ substrate

Fig. 2 Cross-sectional SEM image of α-alumina prepared at
Tvap = 433 K.

was heated on a hot stage at pre-heating temperatures (Tpre)
from room temperature to 973 K. An Nd:YAG laser was
irradiated on the whole surface of the substrate.
Figure 1 depicts the effect of deposition temperature(Tdep) and precursor temperature (Tvap) on the phase of
deposited alumina coating. At Tdep < 1060 K, α-alumina in asingle phase were formed independently of Tvap. Alumina
films with a α+θ mixture phase were formed at Tvap = 403 K
and Tdep > 1070 K, whereas alumina films with a α+θ mixture
phase were formed at Tvap = 413 K and Tdep > 1116 K. At Tvap
> 423 K, the phase changed from a γ to a γ+α mixture to an
α-phase with increasing Tdep.
The lowest temperature to obtain α-alumina was 1116 K
in this study, which was about 200 K lower than that bythermal CVD.
A typical cross-sectional SEM image of α-alumina coating
is shown in Fig. 2. The coating had a columnar structure andfaceted surface.
References[1] A. Ito, H. Kadokura, T. Kimura and T. Goto, J. Alloys andCompounds, 489, 469 (2010).
Key Words
Laser CVD, Alumina Coating, Cutting Tool
Contact toTeiichi Kimura (Japan Fine Ceramics Center)
E-mail: t_kimura@jfcc.or.jp
Takashi Goto (Multi-Functional Materials Science Division, IMR,
Tohoku University)
E-mail: goto@imr.tohoku.ac.jp

Fig. 1 Effect of deposition temperature(Tdep) and precursor
temperature (Tvap) on the phase of alumina films.

KINKEN Research Highlights 2010 IMR

Highlights of Collaborated Research

45Research

Ferromagnetism and Superconductivity in UCoGe
Interplay of ferromagnetism and superconductivity has
long been of great interest in solid state physics. Here we
report microscopic coexistence of the two competing longrange ordered states as well as the presence of two nuclearspin-lattice-relaxation rates in UCoGe [1].

4

TCurieUCoGe
59
Co-NQR

1/T1 (1/s)

It has been considered that internal magnetic inductions
in a ferromagnet would suppress superconductivity andhence superconductivity could not coexist in a bulk
ferromagnet. In some uranium-based ferromagnets that wererecently discovered, however, it is expected that parallel-spin
Cooper pairs (spin-triplet state) will form and survive even ina huge internal field. UCoGe is considered a promissing
candidate for such an itinerant ferromagnet, characterized bya low Curie temperature TC~2.5 K and a small orderedmoment M0~0.1μB/U [2]. Superconductivity with TSC~0.5 K
was confirmed in bulk measurements, but it was unclear
whether the superconductivity microscopically coexists withthe ferromagnetism.
We installed a tetra-arc furnace at Laboratory of α-Ray
Emitters, IMR to prepare a single crystalline material of goodquality including uranium element. Figure 1 shows a snap
shot of the single crystal growth. The obtained single crystals
exhibited a jump in the specific heat at TSC, indicating a bulk
phase transition. Then, we carried out 59Co NQR (nuclear
quadrupole resonance) experiments [1]. NQR signals below
1 K indicated ferromagnetism throughout the sample volume.
We also studied the nuclear spin-lattice relaxation rate 1/T1.
As shown in Fig. 2, the rate decreases rapidly below TSC,
roughly T3, suggestive of line nodes in a superconducting
gap. This provides unambiguous evidence for the microscopic
coexistence of ferromanetism and superconductivity.
One may notice that in addition to the slow component as
mentioned above, a fast component at the ferromagnetic
signal emerges in the superconducting state. The latter isroughly proportinal to temperature, consistent with Korringa
behavior characteristic of metals. This indicates that it

10

10

3

10

2

TSC

~T
10

10

10

PM (8.3MHz)
poly crystal
single crystal

1

single crystal
FM (8.1MHz)
above TSC

0

-1

T*

~T

0.1

below TSC

3

fast
slow

1

10

100

Temperature (K)Fig. 2 Temperature dependences of 59Co relaxation rate in
the single crystalline sample, along with the rate in the poly
crystal between 10 and 150 K [1].
originates from non-superconducting regions. One possibilityof this intriguing feature is a nonunitary superconducting
state, as in superfluid 3He. Another possibility is a selfinduced vortex state, which has never been identified
experimentally. These results shed a new light on the“classical” issue of the interplay between ferromagnetismand superconductivity.
References[1] T. Ohta, T. Hattori, K. Ishida, Y. Nakai, E. Osaki, K.
Deguchi, N. K. Sato and I. Satoh. J. Phys. Soc. Jpn. 79,
023707 (2010).
[2] N. T. Huy, A. Gasparini, D. E. de Nijs, Y. Huang, J. C. P.
Klaasse, T. Gortenmulder, A. de Visser, A. Hamann, T. Gorlach,and H. v. Lohneysen, Phys. Rev. Lett. 99, 067006 (2007).
Key Words
UCoGe, Ferromagnetic Superconductor, Heavy Fermion
Contact toNoriaki Sato (Department of Physics, Nagoya University)
E-mail: kensho@cc.nagoya-u.ac.jp
Kenji Ishida (Department of Physics, Kyoto University)

Fig. 1 Picture of single crystal growth using a tetra-arc
furnace installed at IMR.

46

Highlights of Collaborated ResearchE-mail: kishida@scphys.kyoto-u.ac.jp

IMR KINKEN Research Highlights 2010

Research

Microstructures and Reaction Mechanism of Aluminum Hydride
A promising hydrogen storage material, AlH3 wasinvestigated by transmission electron microscopy (TEM)
and associated electron energy-loss spectroscopy (EELS).
Microstructural and electronic structural changes associatedwith hydrogen desorption were in situ observed for the firsttime. In addition, EELS of AlH3 and its derivative phases
revealed the electronic structure changes during thehydrogen desorption process. The hydrogen desorption
mechanism is discussed based on the observation andtheoretical calculations.
Aluminum trihydride (AlH3, alane) is considered as one ofthe promising advanced hydrogen storage materials becauseof its high gravimetric and volumetric hydrogen densities,
relatively low hydrogen desorption temperature and stability
at an ambient environment [1]. It is important to investigate
the microstructural and electronic structural changes during
the hydrogen desorption process, which could be a clue tounderstand the reaction mechanism of this material. It was,however, extremely difficult to observe alane particles in
TEM because high energy electron illumination instantly
starts to decompose the material into metallic aluminum. We
successfully observed the hydrogen desorption process insitu with TEM for the first time by reducing the accelerating
voltage down to 100 kV and minimizing the electron dose,using a high sensitivity CCD camera. At the same time, we
also recorded electron energy-loss spectra (EELS) of alane
and its derivative phases, which convey the chemical bonding
information of those phases [2].
Figures 1(a)-(c) show in situ TEM observation of hydrogen
desorption process by high energy electron irradiation at RT,
and the corresponding low loss EELS spectra are shown in(d) [2,3]. The TEM images clearly show that the hydrogen
desorption process (AlH3 → Al + 3/2H2) proceeds by anucleation-and-growth mechanism of metallic Al particles.
AlH3 and metallic Al exhibit their intrinsic plasmon peaks at
16.7 and 15.1 eV, respectively, due to their different band
structures and valence electron densities [2], as shown in
Fig. 1(d). At the intermediate state of decomposition (Fig.
1(b)), the two plasmon peaks coexist (spectrum #2 in (d)),
indicating that this process is a first order phase transition.
Furthermore, it was found that a single crystalline alane
particle is covered by a thin amorphous layer, which shows
no external shape change before and after the desorption
reaction. This suggests that the layer stabilizes the AlH3 andFig. 2 Al L2,3 core-loss EELS (2p → 3d* transition) from theindicated phases and reference spectra from the known
materials to identify each observed phase.
prevents the particle from the hydrogen desorption reaction
at ambient temperature and pressure. Once a part of alane
particle is decomposed, the hydrogen molecules perforate
the alumina (Al2O3) layer by the increasing internal pressure,which catastrophically triggers the entire decomposition.
Al L2,3 core-loss EELS (2p → 3d* transition) from theindicated phases and reference spectra from the known
materials are shown in Fig. 2. The top spectrum from alane
was restored from the low signal-to-noise ratio experimental
spectra, using the PIXON method [4] because of spectrum
acquisition at the very low electron dose. The spectral profile
was reproduced by a theoretical prediction based on the
α-AlH3 structure [2], which confirmed that alane is a covalentbonding material and weak covalent bonding between thesecond neighboring Al-Al pairs.
The low loss EELS from the surface layer is also shown
as #4 in Fig. 1(d). The surface protecting layer is thus
identified from the low-loss and core-loss EELS to beamorphous alumina (Al2O3), comparing the spectrum withthe reference ones.
References[1] S. Orimo, Y. Nakamori, T. Kato, C Brown and C. M.
Jemsen, Appl. Phys A 83, 5 (2006).
[2] S. Muto, K. Tatsumi, K. Ikeda and S. Orimo, J. Appl. Phys.
105, 123514 (2009).
[3] Kinken Research Highlights 2009, p.22.
[4] S. Muto, R. C. Puetter and K. Tatsumi, J. Electron Microsc.
55, 215 (2006).
Key Words
Aluminum Hydride, Transmission Electron Microscope,Electron Energy-loss Spectroscopy

Fig. 1 (a)-(c) In situ TEM observation of hydrogen desorption
reaction AlH3 → Al + 3/2H2 and corresponding electron
diffraction. (d) Low loss spectra 1-3 correspond to TEM
images (a)-(c), respectively. Spectrum 4 is acquired near thesurface layer of the decomposed particle A in (c).

KINKEN Research Highlights 2010 IMR

Contact toShunsuke Muto (Graduate School of Engineering, Nagoya University)
E-mail: s-mutoh@nucl.nagoya-u.ac.jp

Highlights of Collaborated Research47

Research

Highly-arranged Nanoscale Phase Separation in EpitaxiallyGrown III-V Semiconductor Alloys
We prepared TlInGaAsN/TlInP quantum well structuresusing gas source molecular beam epitaxy and characterized
them by means of transmission electron microscopy andscanning transmission electron microscopy. It was foundthat naturally-formed vertical quantum wells with a periodicity
of ~1 nm are formed in TlInGaAsN layers.
The introduction of structurally and/or compositionally
modulated structures into semiconductor materials is oftechnological importance for adding desirable functionalities
to electronic and optoelectronic devices. Spontaneous
formation of nanostructures during crystal growth, the socalled self-assembly or self-organization, is one of the usefulways to introduce the modulated structure to materials in
atomic scale. Here we demonstrate that nanoscale phaseseparation perpendicular to the growth direction can berealized in III-V semiconductor alloys [1,2].
TlGaInAsN/TlInP multiple quantum well structures weregrown at 420 oC on InP(100) substrates by gas source
molecular-beam epitaxy. Figure 1(a) shows a cross-sectional
bright-field TEM image of the obtained specimen, viewed
along the [110] direction. The TlGaInAsN (bright contrast)
and TlInP (dark contrast) layers are alternatively stacked.
The TlInP layers reveal uniform diffraction contrast, whereasa modulated structure exists in the TlGaInAsN layers. In theelectron diffraction pattern (inset), satellite reflections areapparent on both sides of the Bragg reflections, indicatingthat the modulated structure is highly arranged. The
modulation period was estimated to be ~1 nm. On the other
hand, no modulation was observed in the image anddiffraction pattern viewed along the [110] direction. That is,vertical quantum wells are formed in the TlGaInAsN layers.
From the HAADF-STEM (Fig. 1(b)) and EDX analyses, it was
found that the modulated structure consists of Ga- and Inrich regions, indicating that nanoscale phase separation
occurs along the lateral direction.
We discuss the formation process of nanoscale phaseseparation based on a simple kinetic Ising model in which therearrangement of atoms occurs at the topmost layer only andthe atomic configuration is frozen-in when the surface iscovered with a next monolayer [3,4]. In this model, pairwise
atomic interactions up to the second nearest neighbors are

Fig. 2 (a,b) Pairwise atomic interactions at the topmost layerof an (A1-xBx)0.5C0.5 alloy. (a) Plan-view and (b) crosssectional view. A and B (blue) are group-III elements and C(red) is a group-V element, or vise versa. (c-e) Formation
process of like-atom chains along the growth direction. Only
group-III elements are drawn, and black and gray circles
correspond to A and B atoms, respectively.
considered (Figs. 2(a) and 2(b)). From Monte Carlosimulations, it was found that the pairwise atomic interactions
are defined as follows: V1h<0, V1h’>0, V2h<0, V1v>0, and V2v<0,
where the positive and negative values prefer homonuclear(Ga-Ga and In-In) and heteronuclear (Ga-In) atomic pairs,
respectively. When the growth temperature is low and themigration of atoms is suppressed, the ordering on the surface
proceeds incompletely and homonuclear bonds are inevitably
formed (Fig. 2(c)). Once such homonuclear bonds are formed,
the same atoms preferentially assemble on them in the next
deposited monolayer due to the positive interaction V1v (Fig.
2(d)). The epilayer thus obtained includes like-atom clusters
which elongate along the growth direction (Fig. 2(e)).
References[1] M. Ishimaru, Y. Tanaka, S. Hasegawa, H. Asahi, K. Satoand T. J. Konno, Appl. Phys. Lett. 94, 153103 (2009).
[2] M. Ishimaru, Y. Tanaka, S. Hasegawa, H. Asahi, K. Satoand T. J. Konno, Proc. 21st Int’l Conf. Indium Phosphide andRelated Materials, 253 (2009).
[3] M. Ishimaru, S. Matsumura, N. Kuwano and K. Oki, Phys.
Rev. B 51, 9707 (1995): 52, 5154 (1995): 54, 10814 (1996).
[4] M. Ishimaru, S. Matsumura, N. Kuwano and K. Oki, J.
Appl. Phys. 77, 2370 (1995).
Key Words
Self-assembly, Phase Separation, Ordering
Contact toManabu Ishimaru (Institute of Scientific and Industrial Research,
Osaka University)
E-mail: ishimaru@sanken.osaka-u.ac.jp
Toyohiko J. Konno (Advanced Analysis of Materials Division, IMR,

Fig. 1 (a) Bright-field image and electron diffraction patternof TlGaInAsN/TlInP multiple quantum well structure. (b)HAADF-STEM image of TlGaInAsN layer.

48

Highlights of Collaborated Research

Tohoku University)
E-mail: tjkonno@imr.tohoku.ac.jp

IMR KINKEN Research Highlights 2010

ResearchSi-based Light-emitting Devices Based on Ge Quantum Dots in
Optical Microcavities
Ge self-assembled quantum dots were embedded into
Si microcavities as active centers. Resonant luminescence
was observed at telecommunication wavelengths at roomtemperature. The results show a promising method for Sibased light source for optoelectronic integration on Si
platform.
Optoelectronic hybrid integration on Si platform is one of
the solutions for the next-generation integrated circuits.
Therefore, Si-based light-emitting devices are inevitable forthe hybrid integration. Various proposals have been reported
for Si-based light-emitting devices in recent years. Amongthem, Ge self-assembled quantum dots are one possible
candidate of Si-based light-emitting devices. The advantagesof Ge self-assembled dots are the full compatibility to CMOS
technology and the light emission at the telecommunication
wavelengths. Unfortunately, light emission from Ge dots isstill lack of efficiency and spectrum purity, especially at room
temperature. In order to enhance the light emission, Ge dots
were embedded into various optical microcavities, such as
photonic crystal microcavities and microdisk resonators.
Photoluminescence (PL) measurement results show strong
resonant light emission from Ge quantum dots in thesemicrocavities at room-temperature (RT) [1, 2].
Figure 1 shows the schematic structure of the device.
The devices were fabricated using standard fabrication
techniques. First, three layers of the quantum dots weregrown on Si-on-insulator (SOI) wafers in Stranski-Krastanov
mode using gas-source molecular-beam epitaxy at 600oC.
Electron-beam lithography, reactive-ion etching, and wet
etching were used to pattern optical microcavity structures.
Figure 2 shows the room-temperature photoluminescencefrom Ge quantum dots in a photonic crystal microcavity.
Strong PL signal is observed in the range from 1.3 to 1.6µm.
No signal is observed in a pattern-free region under the same
conditions, which indicates that optical resonance in the cavity
causes significantly enhancements. Multiple sharp peaks
dominate the luminescence spectrum, while no obvious offresonant luminescence is observed. The quality factors (Q) of
these resonant peaks are in the range of 300–600. The
wavelength of the resonant peaks can be adjusted by tuning
the lattice constant of the photonic crystal structure.

µFig. 2 RT PL signal from a T6 photonic crystal microcavity.
The inset shows the SEM picture of the device.

As a step towards practicable devices, recently, a currentinjected light emitting device was fabricated throughembedding Ge dots into a Si microdisk with a p-i-n junction
for current-injection. Resonant electroluminescence spectrum
was recorded at room-temperature [3].
Silicon-based light-emitting devices are highly desired forthe hybrid optoelectronic integration on Si to make it more
powerful. Embedding self-assembled Ge quantum dots into
optical microcavities is a promising direction, which is fullycompatible with CMOS technology.
References[1] J. Xia, R. Tominaga, S. Fukamizu, N. Usami, and Y.
Shiraki, Japanese Journal of Applied Physics, 48, 022102(2009).
[2] J. S. Xia, R. Tominaga, N. Usami, S. Iwamoto, Y. Ikegami,
K. Nemoto, Y. Arakawa, Y. Shiraki, Journal of Crystal Growth,311, 883 (2009).
[3] J. Xia, Y. Takeda, N. Usami, T. Maruizumi, and Y. Shiraki,
Opt. Express, 18, 13945(2010).
Key Words
Si-based Light-emitting Device, Ge Self-assembled Quantum
Dots, Microcavity
Contact toJinsong Xia (Advanced Research Laboratories, Tokyo City University)
E-mail: jinsongxia@gamil.com
Noritaka Usami (Crystal Physics Division, IMR, Tohoku University)

Fig. 1 Schematic structure of a photonic crystal microcavity
with Ge self-assembled quantum dots on slicon-on-insulator(SOI) wafer.

KINKEN Research Highlights 2010 IMR

E-mail: usa@imr.tohoku.ac.jp

Highlights of Collaborated Research49Research

Static Heterogeneity with a Certain Correlation Lengthin Metallic Glasses(b)
P h a se -ve lo city ra tio , v r (= v IX S /v U S )M e a n fre e p a th , L (n m )

(a)

1 .0

PNCP
P dC P
P tC P

0 .8

Phonon excitation energy, h Ω (meV)PNCP
P dC PP tN P

0 .6
0 .4

ZANC
ZANP
ZCA

20
15

1

0.1
1000

~ λ4

~ λ6

~ λ6

0

2

4

6

8

10

12
-1

Wave number, Q (nm )

14

16

0.1

US
Positively deviatedfrom US

1.34
1.32

US

1.30

GM = 20 GPa
GI = 40 GPa
Vf = 0.7
Vf = 0.8
Vf = 0.9

1.28

US

1.26
0

2

4

5

10

GM = 20 GPa
GI = 40 GPa
Vf = 0.7
Vf = 0.8
Vf = 0.9

4

10

3

10

2

10

~ λ6

1

10

~ λ2

0

10
6

8

10

d /λ

0.1

2

3

4 5 6 712

3

4 5 6 7

λ/dFig. 2 (a) Schematic figure illustrating the two-dimensional
heterostructure model of glass, consisting of hard inclusions
embedded in a softer matrix. (b) The effective phase velocity
relative to that in the matrix as a function of the inverse of
wavelength and the propagation length as a function of
wavelength for a composite material calculated with theeffective medium self-consistent theory. The volume fractionof inclusion is set at Vf = 0.7, 0.8, and 0.9. The effective phasevelocity is normalized the phase velocity of the matrix VM. GI= 40 GPa and GM = 20 GPa.

elastic wave scattering theory with the present structural
heterogeneous model, it is strongly supported that the elastic
inhomogeneity with a certain correlation length exists even in
dense metallic glasses, as seen in Fig. 2. The present results
indicate that Zr-based metallic glasses are more homogeneousthan the Pt- and Pd-based metallic glasses [2].

Tetsu Ichitsubo (Department of Materials Science and Engineering,
Kyoto University)~ λ2

E-mail: tichi@mtl.kyoto-u.ac.jp

1

5

10
1.36

Contact to

~ λ4

ZANC
ZANP
ZCA

100

Short −−−− λ −−−− Long
6

Key Words
Metallic Glass, Structural Heterogeneity, Phonon Dispersion

~ λ2

10

10

10

0

100

Long −−−− λ −−−− Short
1.38

References[1] T. Ichitsubo et al., Phys. Rev. Lett. 95, 245501 (2005).
[2] T. Ichitsubo et al., Phys. Rev. B 81, 172201 (2010).

1000

1 .2

(b)

Normailized propagation length, L / dDisordered materials, liquids and glasses, exhibit complex
relaxation processes. The correlation between these dynamic
processes and glass structure is one of main issues in glassscience. Recently, we reported deterioration of the structureof a glass near the glass transition temperature due to the
combined action of external megahertz frequency vibrationsand β relaxation. The partially crystallized microstructure of a
Pd-based metallic glass under sub-megahertz vibrationstrongly indicates that the atomic motions associated with
the Johari-Goldstein β relaxation are spatially localized.
Based on our experimental results on a transmission electron
micrograph, we proposed a microstructural model of metallic
glasses that consists of strongly-bonded regions surrounded
by weakly-bonded regions [1]. To confirm the validity of themodel, here we have investigated systematically the acoustic
properties of some Pd-, Pt-, and Zr-based metallic glasses by
the combination of inelastic x-ray scattering (IXS) andultrasonic (US) measurements with the aid of the elastic
wave scattering theory. In the cases of PdNiCuP, PdCuP,
and PtNiP, the sound velocity of the phonons at low Q
exceeds the US velocity, whereas the former goes close to
the latter in the cases of ZrAlNiP, ZrAlNiCu and ZrCuAl, as
seen in Fig. 1. Moreover, the wavelength (λ)-dependence of
L ~ ΩQ/QΓQ representing the mean free path of phonon was
also evaluated for the six metallic glasses. The L values of all
the glasses tend to increase with increase in λ and steeplychange at a certain value of λ (λc). It is found that the λcvalues for PdNiCuP, PdCuP, and PtNiP glasses are larger
than those for Zr-based metallic glasses. In the light of the(a)Normalized phase velocity, Veff / VM

We have investigated acoustic-phonon behavior of Pd-,
Pt-, and Zr-based metallic glasses with the combination of
inelastic x-ray scattering (IXS) and ultrasonic (US)measurements. The measured acoustic phonon behavior
can be qualitatively explained by the elastic-wave scattering
theory with a structural heterogeneous model.

	URL: http://www.mdsgn.mtl.kyoto-u.ac.jp/htms/members/
0.4

0.5 0.6 0.7 0.8 0.9

2

1
W a ve le n g th , λ (n m )

3

4

5

ichitsubo/index.htm

Fig. 1 (a) The phonon dispersion relation (lower panel) andrelative phase velocity to ultrasound (upper panel) of six
types of metallic glasses, which were obtained from the IXS
spectra. (b) The wavelength (λ)-dependence of the mean
free path L of acoustic phonon obtained for six glasses.

50

Highlights of Collaborated Research

IMR KINKEN Research Highlights 2010

Research

Neutron Irradiation Effect on Superconducting Magnet Materials

Superconducting strands including NbTi, Nb3Sn and
Nb3Al were prepared for neutron irradiation [1, 2]. The strands
were irradiated by 14 NeV neutron at Fusion Neutronics
Source in JAEA and the post irradiated tests were carried out
at High Field Laboratory for Superconducting Materials in
Tohoku University.
The results of the critical current measurements after
irradiation are shown in Fig. 1. The Nb3Sn strand showedthat the critical current in the lower field increased remarkably
as an increment of the neutron fluence. The border magnetic
field (arrows in Fig. 1) where the critical current starts to getaway from the non-irradiated critical current curve shifts tothe higher magnetic field when the fluence increases. So, theincrement of the critical current becomes larger in the lower
field. The results of the critical field measurements under 100
mA showed no clear changes. However, the criticaltemperature measurements presented that the criticaltemperature after irradiation to 3.10 x 1021 n/m2 decreased
about 0.2 K.
The fast neutron knocks on the atoms in the matrix andmakes interstitials and vacancies. When the damage
increases with the higher neutron fluence, the field flux
pinning force would be strengthened and the critical current
would increase in the higher field. At the same time, the A-15
crystal structure would be disordered and the criticaltemperature decreased.
Regarding the electric insulation materials, epoxy resin
blended with cyanate ester was used for trial GFRP
fabrication. It has 10 polyimide film layers and 11 glass cloth
layers. The vacuum impregnation process was adopted andCritical Current (A)

200
150

3.52x1020 (4.5K)
1.78x1021 (RT)
3.10x1021(RT)

Non-irrad.

100

Nb3Sn

NbTi
Non-irrad.

50
0

1.78x1021 (RT)

Non-irrad.
1.78x1021 (RT)Nb3Al

14 MeV neutron
Unit: n/m2

8

12

16
20
24
Magnetic field (T) Border point

70
ILSS (MPa)

New large scale fusion devices require a data base on
neutron irradiation effect of fusion magnet, because thedevices will generate high energy neutrons. To investigate
mechanisms of irradiation effect and pile up the data base,neutron irradiation tests were conducted and post irradiatedtests were performed.

Irradiated at JRR-3

60
50
40
30

Toshiba CTD403. RT
Toshiba CTD403. 77 K
NIFS CTD403. RT
NIFS CTD403. 77 K
Toshiba AroCyL-10/PY306. RT
Toshiba AroCyL-10/PY306. 77 K

Non-irrad.
1018 1019 1020 1021 1022
Neutron fluence (n/m 2)

1023

Fig. 2 Change in ILSS against neutron fluence. CTD403
shows 100 % cyanate ester resin. AroCyL-10/PY306 isblended resin with cyanate ester/epoxy of 40/60 in weight.

it was cured at 150 degree C. After machining the specimen
for short beam test (10 mmW x 15 mmL x 2.5 mmt), thespecimens were irradiated by fission neutron at JRR-3 in
JAEA and the short beam tests were carried out at RT and 77
K at Oarai center.
The results are shown in Fig. 2 [3]. Depending on the
fabricated plate, the interlaminar shear strength (ILSS)
becomes different but the scatter of each data set is rather
small and over 40 MPa of ILSS is obtained even after
irradiation to 1.0 x 1022 n/m2. The RT samples showed thebending fracture at the tension side but the 77 K samples
showed the interlaminar fracture at the mid section of theplate thickness.
References[1] A. Nishimura, T. Takeuchi, S. Nishijima, G. Nishijima, T.
Shikama, K. Ochiai, N. Koizumi, Fusion Engineering and
Design, 84, 1425–1428 (2009).
[2] A. Nishimura, T. Takeuchi, S. Nishijima, K. Ochiai, G.
Nishijima, K. Watanabe, and T. Shikama, Advances in
Cryogenic Engineering, 56, 255-262 (2010).
[3] A. Nishimura, Y. Izumi, S. Nishijima, T. Hemmi, N.
Koizumi, T. Takeuchi, and T. Shikama, Advances in
Cryogenic Engineering, 56, 127-134 (2010).
Key Words
Fusion Magnet, Neutron Irradiation, Nb3Sn
Contact toArata Nishimura (Department of Helical Plasma Research, National
Institute for Fusion Science)
E-mail: nishi-a@nifs.ac.jp

Fig. 1 Change in critical current against magnetic field after
14 MeV neutron irradiation.

KINKEN Research Highlights 2010 IMR

Highlights of Collaborated Research51

Research

Plutonium Migration in Compacted Bentonite Under Reducing
Conditions by Electromigration Method
The electromigration method was applied to study the migration
behavior of plutonium in compacted bentonite under reducing
condition [1]. Plutonium migrated as a chemical form with positive
charge through bentonite. Strong reducing condition changes thechemical form of plutonium, and accelerates the migration of
plutonium in compacted bentonite.

The movement of ions under the influence of the electric potential
gradient, hence electromigration, can be described with thedispersion-convection equation,

∂C
∂ 2C
∂C
(2)
= Da 2 − Va∂t∂x∂xwhere Da is an apparent dispersion coefficient, Va is an apparent
migration velocity including mostly electromigration of iron andCounter electrode(Pt)
Contact solution(NaCl 0.01M)
Porous filter(Stainless steel)Acrylic resin
columnReference electrode(Ag/AgCl)

Bentonite specimen(Ø10 x 10 mm2)

Tracer solution
1kBq of 238Pu
Work electrode(Carbon steel:Ø18x3 mm2)

Pu (Bq/g)

For high-level radioactive waste disposal, corrosion products ofcarbon steel overpack diffuse into buffer materials and then maintainthe reducing environment in the vicinity of the repository. This
reducing condition is expected to retard the migration of redoxsensitive radionuclides. However, a rare study on plutonium diffusion
in compacted bentonite is available, especially under reducing
conditions. Therefore our research group have developed andcarried out electro-migration experiments with source of iron ions
supplied by anode corrosion of iron coupon in compacted bentonite.
A carbon steel coupon was assembled with bentonite saturated
by contacting with water including 0.01M of NaCl into an apparatus
for electromigration as shown in Fig. 1. A few micro liter of tracer
solution containing 1kBq (7x10-12mol) of 238Pu was spiked on theinterface between carbon steel coupon and bentonite before
assembling. There was a reference electrode of Ag/AgCl and acounter electrode of platinum foil in the upper part of the apparatus
with 0.01 M of NaCl solution. The carbon steel coupon wasconnected to a potentiostat as a work electrode and was supplied
electrical potential in the range of -500 to 300 mV vs. Ag/AgClelectrode at 25°C for up to 168 h. After supplying electrical potential,
the bentonite specimen was pushed out from the column and wassliced in steps of 0.3 to 2mm. Each slice was submerged in 1N HCl
solution to extract plutonium and iron, and the liquid phase wasseparated by the centrifugal method. Then the supernatant was
taken to be measured with alpha liquid scintillation counter forplutonium and with atomic absorption spectrometry for iron.
The concentration profiles of plutonium infiltrated in bentonite
specimens at different potentials are shown in Fig. 2. Plutonium
migrated from anode toward cathode as far as 1mm from theinterface, and the penetration length of plutonium grew deeper as
the higher potential supplied. This result indicated plutonium
migrated as a chemical form with positive charge through bentonite.
It has been reported that no movement could be measured forplutonium diffusion in a concrete-bentonite system under anoxidizing condition in an experimental period as long as 5 years[2]. Our results show much faster migration of plutonium thanthose reported. We assume the formation of diffusive trivalent
plutonium converted from non-diffusive tetravalent by ferrous ionsintroduced into bentonite because the total amount of plutonium
penetrated was increased depending on the amount of ferrous
ions introduced into bentonite.
(1)
Pu 4+ + Fe2+ ↔ Pu 3+ + Fe3+

10

4

10

3

10

2

10

1

+300mV0mV-300mV-400mV-500mV

0.0

0.5

1.0
x (mm)

1.5

2.0

Plutonium
profilesin compacted
bentonites
byFig. Fig.2
2 Plutonium
profilesin compacted
bentonites
by electromigration
up to 168 h.
electromigration up to 168 h.
negligible electro-osmotic flow of water. To solve Eq.(2) with theboundary conditions in this experimental configuration, we
considered successive spikes of ion in each time step, M(t), which
is a function of the current. We obtained approximate solution as
follows,C(x,t) =

⎡

⎧⎪

2 ⎫

⎧⎪

2 ⎫⎤

∫ 2 (πD τ) ⎢⎢⎣exp⎨⎪⎩− ( 4D τ ) ⎬⎪⎭ + exp⎨⎪⎩− ( 4D τ ) ⎬⎪⎭⎥⎥⎦dτ  (3)
t M

t −τ

0

a

x − Vaτ⎪ax + Vaτ⎪aFitted curves are also plotted in Fig.2 for plutonium. We carriedout the same analysis for iron.
Apparent diffusion coefficient of plutonium in bentonite can becalculated by the Einstein’s relation.

uRT
D=
(4)
ZF

where T is the temperature, Z is the valence of diffusing cation,
and F is the Faraday constant, C. The mobility, u, could be obtained
from following relation,
(5)
V a = uEwhere E is an electrical gradient in bentonite specimen. However,
the electrical gradient in bentonite cannot be measured directly. So
we tried to estimate the apparent diffusion coefficient of plutonium
by comparing the migration velocity with iron ions. Applying Eq.(4)and Eq.(5) for both ions, following relation is obtained.

DPu Va,Pu Z Fe
=
⋅
(6)
DFe

Va,Fe Z Pu

The apparent diffusion coefficient of plutonium was estimated as
not larger than 10-13 m2/s by the comparison of migration velocityof plutonium and iron.
References[1] K. Idemitsu, H. Ikeuchi, S. A. Nessa, Y. Inagaki, T. Arima, S.
Yamazaki, T. Mitsugashira, M. Hara, Y. Suzuki , Proc. Materials
Research Society Symposium on Scientific Basis for Nuclear
Waste Management XXXII, Vol.1124, 283 (2009).
[2] Y. Albinsson, K. Andersson, S. Börjesson, B. Allard, J.
Contaminant Hydrology 12, 189 (1996).
Key Words
Plutonium, Electromigration, Bentonite
Contact toKazuya Idemitsu (Department of Applied Quantum Physics and Nuclear
Engineering, Kyushu University)
E-mail: idemitsu@nucl.kyushu-u.ac.jp

Lead wire

Fig. 1 Experimental apparatus for electromigration.

52

Highlights of Collaborated Research

IMR KINKEN Research Highlights 2010

Research

Development ofNovel High Temperature Thermoelectric Materials
Doping of zinc to borosilicide was discovered tosignificantly improve the crystal quality even although zincdoes not remain in the final product, and the thermoelectric
power factor was increased by more than 30%. Zinc doping
may lead to a way to improve the crystal quality of not justsuch borides and silicides but other compounds also
Boron is an interesting element, tending to form atomic
network structures such as clusters and 2D atomic nets incompounds. This tendency is similar to carbon, which has
been much more extensively studied for such materials as
fullerenes, nanotubes, graphite-related materials, aromatics,
etc. The rich materials science potential of boron remains to
be tapped. Boron has one less electron than carbon and thus
forms electron deficient atomic networks. However, thiscauses it to have a special affinity with the rare earth atoms,
which are localized and fit in the voids of the atomic networks
while supplying electrons. Numerous new rare earth boride
compounds have been discovered in the past decade [1].
This is a good combination, because the rare earth atoms
supply electrons to the robust boron atomic framework tostabilize and form novel structures, while at the same time
the shell of f electrons will supply interesting properties likemagnetism and can also enable tuning of the electricalproperties (Fig. 1) [1].
One promising application is the use of boron clustercompounds as high temperature thermoelectric materials
since they are quite stable materials and possess intrinsic
low thermal conductivity [2]. Compounds such as p-type
RB44Si2 and the series of RB15.5CN, RB22C2N and RB28.5C4
which exhibit intrinsic n-type behavior and may be the longawaited counterpart to boron carbide, are attracting particularinterest [2]. The RB44Si2 compounds exhibit Seebeckcoefficients in excess of 200 μV/K at high temperatures
above 1000 K and also possess a low thermal conductivity of~0.02 W/cm/K. We carried out a series of transition metal
doping in an attempt to improve the properties. Zinc doping
was found to have a striking effect, with the thermoelectric
power factor increasing by more than 30% [3]. Surprisingly,
analysis revealed that zinc is actually not retained in theFig. 2 SEM pictures of (a) undoped and (b) Zn doped YB44Si2.
sample. However, the addition of zinc to the synthesis isindicated to have an effect to significantly improve thecrystallinity of the sample (Fig. 2(b)). Crystal quality had beena longstanding problem for the borosilicides, withimperfections in the form of Si aggregate regions existing(Fig. 2(a)).
This discovery of the zinc effect is exciting, since it may
offer a simple way to improve crystal quality of not just borides
or silicides but other high temperature compounds also.
References[1] T. Mori, in Handbook on the Physics and Chemistry ofRare Earths, Vol. 38, ed. K. A. Gschneidner Jr., J. -C. Bunzli,
and V. Pecharsky (North-Holland, Amsterdam, 2008) pp.
105-173.
[2] T. Mori and T. Nishimura, J. Solid State Chem., 179,
2908-2915 (2006), T. Mori, Material Matters 4, 37-39(2009).
[3] T. Mori, D. Berthebaud, T. Nishimura, A. Nomura, T.
Shishido, and K. Nakajima, Dalton Transactions 39, 10271030 (2010). (Hot Article).
Key Words
Boron, Thermoelectric, Crystal Quality
Contact toTakao Mori (Materials Nanoarchitectonics Center (MANA), National
Institute for Materials Science (NIMS))
E-mail: MORI.Takao@nims.go.jp
	URL: http://www.nistep.go.jp/achiev/ftx/jpn/stfc/stt103j/
0910_03_featurearticles/0910fa02/200910_fa02.html
Toetsu Shishido (Advanced Research Center of Metallic Glasses,
IMR, Tohoku University)
E-mail: shishido@imr.tohoku.ac.jp
URL: http://www.arcmg.imr.tohoku.ac.jp/topics/shishido.html

Fig. 1 Development strategy of boron network materials.

KINKEN Research Highlights 2010 IMR

Highlights of Collaborated Research

53

Research

Metallic Glass Nanowires: Formation and Characterization
Metallic glasses (MG) are a promising class of materials
that could find potential use as structural components inapplications ranging from mobile phone casings tobiomedical implants. At the same time, the realisation of MG
structures on the nanoscale for novel applications hasremained a challenge. We have developed a new methodthat is capable of producing MG nanowires showing that
outstanding mechanical properties can be inherited into
nanowire [1].
MGs are amorphous materials like window silica glass,but they are made from alloy metals. Although MGs visually
look similar to crystalline metals like a steel, their amorphous
structure leads to many advantages.
For example,
imperfections such as vacancy, dislocation, twin, and grainboundary in the crystalline metals are the main cause of
mechanical failure. Owing to the amorphous structure, MGspossess a combination of unique properties including
ultrahigh strength, high hardness, and high resistance towear corrosion. For the practical engineering applications ofsuch advantageous mechanical properties, there have beenremarkable progresses in exploring alloy compositions thatare best suited for the fabrication of bulk MGs [2]. However,
far less attention has been paid to MG nanostructures, suchas a wire.
We showed the self-organized amorphous nanowires
that are formed on the fracture surfaces after conventional
compression tests of bulk metallic glasses [3]. However, it
was difficult to control their morphologies because they werecreated by instantaneous fracture processes. Recently, we
have developed a mechanism to draw MG ribbons in a much

more controlled fashion [1]. Our process follows a simple
principle that glass blowers have been using for centuries.
The MG is processed when it is still in a viscous liquid state
under a tension that caused by a weight attached to theribbon end drawn out into nanowires (Fig. 1).
We used a scanning laser Dopper vibrometer to measure
the resonant frequences that deduces the Young’s modulus
beased on the Euler-Bernoulli beam theory. The modulus of
the nanowires with the diameter range 250-1100 nm aremuch lower than that of the bulk counterpart owing to thehyper-excess free volume in nanowire and is tunable byheating.
The versatility of the drawing process ensures that MGsof a broad range of compositions can be formed in nanowire.
In terms of future applications that depend on the composition
elements used, Pd-based MG nanowires could be used as ahighly sensitive hydrogen sensor. Fe-based MG nanowires
exhibit soft magnetic properties that could be used for an
ultrahigh magnetic-field sensor. These tough nanowires
could be used in a large number of applications in
nanotechnology.
References[1] K. S. Nakayama, Y. Yokoyama, T. Ono, M. W. Chen, K.
Akiyama, T. Sakurai, and A. Inoue, Adv. Mater. 22, 372(2010).
[2] A. Inoue, Acta. Mater. 48, 279 (2000).
[3] K. S. Nakayama, Y. Yokoyama, G. Xie, Q. S. Zhang, M.
W. Chen, T. Sakurai, and A. Inoue, Nano. Lett. 8, 516(2008).
Key Words
Nanowire, Metallic Glass, Young’s Modulus
Contact toKoji Nakayama (WPI-AIMR, Tohoku University)
E-mail: kojisn@wpi-aimr.tohoku.ac.jp
URL: http://www.wpi-aimr.tohoku.ac.jp/jp/index.php
Yoshihiko Yokoyama (Advanced Research Center of Metallic
Glasses, IMR, Tohoku University)
E-mail: yy@imr.tohoku.ac.jp
URL: http://www.arcmg.imr.tohoku.ac.jp

Fig. 1 A fast drawing of metallic glass ribbons above the grass
transition temperature produces amorphous nanowires [1].
(a) TEM image of Zr based MG nanowire. (b) High resolution
TEM and diffraction pattern reveal the amorphous states. (c)SEM image of Pd based MG nanowire having 40 nm indiameter. (d) The longest one has a total length of 1.3 cm.

54

Highlights of Collaborated Research

IMR KINKEN Research Highlights 2010

Research

Electrical Spin Injection in L10-FePt / GaAs based Ferromagnet
Semiconductor Hybrid Structures
In the field of semiconductor spintronics, electrical spin
injection, manipulation and detection are fundamental
technologies for utilizing spin degree of freedom insemiconductors. Recently, perpendicularly magnetized thinfilm is paid much attention not only for the magnetoresistive
random access memory (MRAM) but also as the spin
injector source to semiconductors. We realize the highremanent magnetization of perpendicularly magnetized L10FePt on a MgO / GaAs structure and apply to the electricalspin injection from L10-FePt to GaAs / AlGaAs light emitting
diode (LED) through a MgO tunnel barrier.
Ferromagnetic metal (FM) / semiconductor (SC) hybrid
structure is attracting much attention for realizing electricalspin injection and spin detection. In addition, since aneffective magnetic field due to spin orbit interaction (SOI) is
electrically controlled in semiconductor two dimensional
electron gas (2DEG) [1], FM / SC hybrid structure is one ofthe promising candidates for realizing novel electronic
devices based on spins. Since the SOI induced effective
magnetic field is along an in-plane direction to the 2DEG,
perpendicular spin injection becomes prerequisite for anelectrical control of spin rotation. However, magnetic easy
axis of conventional ferromagnetic film is the in-plane
direction resulting in the application of an external magnetic
field. L10-ordered FePt shows a large perpendicular magnetic
anisotropy and a moderately high saturation magnetization.
In addition, since the L10-FePt can be grown on a MgO layer
which is used as a tunnel barrier for an efficient spin injection,
L10-FePt is of considerable interest for spintronics materials[2].
We investigated the magnetic properties of L10-FePt on
MgO / (001) n-GaAs structure for the high remanent
magnetization and applied to the electrical spin injection from
L10-FePt to GaAs / AlGaAs LED structure [3]. Perpendicularly
magnetized FePt films were prepared at a substratetemperature Ts = 300 °C by magnetron sputtering and wemeasured the magnetic field dependence of polar magnetooptical Kerr signal in 5nm L10-FePt / 3nm MgO / GaAs. By
post annealing at Ts = 350 °C for 60 min, the ratio of remanetmagnetization to saturation magnetization drasticallyincreases from 0.22 to 0.93, and the magnetization curveshows good squareness in the annealed film resulting in theperpendicularly magnetized FePt at a zero magnetic field.
Then, we tried the electrical spin injection from L10-FePt to
GaAs / AlGaAs LED. Figure 1 shows the magnetic fielddependence of circular polarization in an electroluminescence(EL) and a photoluminescence (PL) at T = 10K. EL polarizationshows the hysteresis loop with good squareness reflecting
L10-FePt magnetic property. The hysteresis of PL polarizationis originated from the magnetic circular dichroism, where thelinear polarized light transmitting through the magnetic thinfilm changes to circularly polarized light due to the spin
dependent density of state in magnetic materials. As a result,circular polarization due to the electrical spin injection
becomes ±1% at B = 0T.

KINKEN Research Highlights 2010 IMR

Fig. 1 Magnetic field dependence of circular polarization in
L10-FePt / MgO / GaAs LED structure. Square and triangle
plots show the results of EL and PL measurements,
respectively.

Recently, MRAM based on current induced magnetization
reversal is promising for the next memory generation.
Perpendicularly magnetized magnetic tunnel junction basedon L10-ordered ferromagnetic thin film is one of the candidates
for realizing a small critical reversal current density and ahigh thermal stability. These MRAM technology combinedwith transistor structures, i.e. spin-MOSFET, will produce
reconfigurable logic and logic-in-memory architecture for low
energy consumption. Although the spin injection efficiency is
still low due to the roughness at MgO / GaAs interface, theperpendicular spin injection into semiconductor will bringadvances for future spintronics integrated systems.
References[1] J. Nitta, T. Akazaki, H. Takayanagi, and T. Enoki, Phys.
Rev. Lett. 78, 1335 (1997).
[2] M. Kohda, A. Ohtsu, T. Seki, A. Fujita, J. Nitta, S. Mitani,
and K. Takanashi, Jpn. J. Appl. Phys. 47, 3269 (2008).
[3] M. Kohda, et al., in preparation.
Key Words
Spintronics, L10-FePt, Electrical Spin Injection
Contact to
Makoto Kohda (Department of Materials Science, Tohoku University)
E-mail: makoto@material.tohoku.ac.jp
URL: http://www.material.tohoku.ac.jp/~kotaib/top_e.html

Highlights of Collaborated Research55

Research

Improvement of Critical Current Properties in MgB2/Fe Tape
Conductors
The critical current density of MgB2 can be greatly
improved through carbohydrate doping. This organic
material can decompose at relatively low temperature, andgenerate reactive carbon atoms before the MgB2 phaseformation. The C can be incorporated into the MgB2 crystal
lattice by replacing boron, and thus Bc2 is significantly
enhanced due to selective tuning of impurity scattering ofthe π and σ bands in the two-band MgB2. Besides the efforts
of increasing Bc2 by carbon doping, the fine grain size andnano-size inclusions caused by doping would create many
flux pinning centres improving the Jc-B property of MgB2.
One promising application of MgB2 superconductor is themagnetic resonance imaging (MRI) magnet that can work at~20 K, a temperature range that can be readily reached by acryogen-free cryocooler. For this reason, a huge amount ofwork has been carried out to improve the current carrying
capability of MgB2 wires/tapes, so that they can be used toconstruct MRI magnets.
However, the pristine MgB2 always shows lower Jc values
because of low upper critical field (Bc2) and poor flux pinning.
In order to improve Jc-B properties, a number of experimental
techniques, including chemical doping, irradiation, magnetic
field annealing, and ball-milling methods, have been attempted.
Compared to other methods employed, chemical doping withcarbon-containing materials was thought as the convenientand effective way to enhance the Jc-B properties of MgB2.
Figure 1 shows transport Jc versus magnetic field curves
for the C9H11NO doped samples sintered at different
temperatures. Compared to those for the undoped samples,the in-field Jc properties of C9H11NO doped tapes were much
improved, indicating an enhanced flux pinning ability. 4%
doped samples show higher Jc than the 2% doped ones at
each corresponding temperature, indicating a high doping
tolerance of C9H11NO in MgB2. The highest Jc values wereobtained for the 4% doped samples annealed under 800°C.
For example, at 4.2 K, 10 T, Jc reached 3.7×104 A/cm2 [1].
This is the highest Jc value observed for carbohydrate doped
MgB2 tapes
The melting point of C9H11NO is only about 70°C,
meaning that the C9H11NO would be in liquid state before
chemical reaction happens, making a homogeneously

Fig. 2 Jc-B properties of pure and organic rare-earth salts
doped tapes.
distribution in raw materials. On the other hand, C9H11NO
decomposes at a temperature below the formation
temperature of MgB2 phase, so it can provide highly reactive
C during the formation of MgB2 phase. The C element
released from decomposition can effectively substitute into B
sites. And the substitution of C for B induces disorder on thelattice sites, leading to the enhancement of the Bc2. This isthought as the main reason for the superior field dependenceof Jc in the C9H11NO doped tapes.
Recently, we also realized the simultaneous introductionof “scattering + pinning” using a single dopant of organic rareearth salt. Figure 2 presents the transport Jc at 4.2 K inmagnetic fields for two organic rare earth salts doped
samples. It is evident that both doped tapes exhibited superior
Jc values, at 4.2 K and in a field of 12 T, the Jc value for LaBenzoate doped tape reached 1.84×104 A/cm2, 23 times
higher than that of undoped samples.
Therefore, the present addition acts as a codopant orreactive nano-carbon, combining a C substitution induced
Bc2 enhancement and stronger flux pinning induced bynanosize precipitates, thus leading to the excellent Jc-B
performance of the doped tapes. The results suggest thatorganic material has to be considered as a promising dopant
for enhancing the current capacity of MgB2 conductors.
References[1] X. P. Zhang, D. L. Wang, Z. S. Gao, L. Wang, Y. P. Qi, Z.
Y. Zhang, Y. W. Ma, S. Awaji, G. Nishijima, K. Watanabe, E.
Mossang, X. Chaud, Supercond. Sci. Technol. 23, 025024(2010).
Key words
MgB2, Doping, Critical Current Density

Fig. 1 Transport Jc-B properties of Fe-sheathed undoped
and doped tapes.

56

Highlights of Collaborated ResearchContact toYanwei Ma (Institute of Electrical Engineering, Chinese Academyof Sciences)E-mail: ywma@mail.iee.ac.cn
Kazuo Watanabe (High Field Laboratory for Superconducting
Materials)
E-mail: kwata@imr.tohoku.ac.jp

IMR KINKEN Research Highlights 2010

Research

Magnon Localization in Quantum Spin System NH4CuCl3
How the ground state of field-induced magnons inquantum spin systems is determined? Two isomorphic
quantum spin systems TlCuCl3 and NH4CuCl3 show thequite different ground state. The former enjoys the realizationof Bose-Einstein Condensation at low temperatures as wellknown, while the latter shows the magnetization plateaus
indicating the crystallization of magnons. We havemicroscopically investigated the ground state of NH4CuCl3
under high magnetic fields and found that magnons localize
at low temperatures to form a crystal.
In last two decades, quantum spin systems have been
attracting much interest both experimentally and theoreticallybecause they exhibit numerous peculiar magnetic features
such as a spin gap and a magnetization plateau, whichcannot be interpreted by conventional classical spin models.
When a magnetic field is applied in a spin gap system, theenergy of the one of the triplet states is lowered due to the
Zeeman interaction so that the spin gap vanishes at thecritical field. Then the ground state becomes magnetic sothat the magnetic ordering occurs if there are threedimensional interactions. Such field-induced magnetic
ordering has been observed in the spin gap systems KCuCl3
and TlCuCl3. The field-induced magnetic ordering was
captured as the Bose-Einstein condensation of magnons by
mapping the spin gap system to the system consisting ofboson with magnetic moment. On the other hand, the title

III
II

III
II
III

I

M (µB/Cu 2+)

1.0

III

I

II
III

I

HS

0.5

HC3

Echo amplitude (arb. unit)

0

5

c

HC2

HC1

0.0

HC4

10 15

20

(a) H (T)

25 30

35

b

(b)
2.3K

13.35T

16.8T

2.3K

16.725T

2.8K
3.4K

α

4.2K
α
β

60

120

13.05T

Takayuki Goto (Department of Physics, Faculty of Science andβRotation angle (deg)Key Words
Quantum Spin System, High-field NMR, Magnetization
Plateau
Contact to

12.6T

120

References[1] H. Inoue, S. Tani, S. Hosoya, K. Inokuchi, T. Fujiwara, T.
Saito, T. Suzuki, A. Oosawa,T. Goto, M. Fujisawa, H. Tanaka,
T. Sasaki, S. Awaji, K. Watanabe, N. Kobayashi, Phys. Rev.
B79, 174418 (2009).

16.65T
12.9T

60

compound NH4CuCl3, which are isomorphous of the former
two show the two-stepped magnetization plateaus at onequarter and three-quarters of the saturation magnetization(Fig. 1), indicating the localization of the magnons, though
their periodic structure is still controversial.
We have carried out 63/65Cu-, 35/37Cl-NMR experiments
under high magnetic field in order to investigate the triplet
localization in NH4CuCl3. In the 63/65Cu-NMR experiments at
the 1/4-plateau phase, the NMR spectra at the singlet Cu sitewere only observed because those at the triplet Cu site are
wiped out due to the strong spin fluctuation of onsite 3d-spins.
It was found that the temperature dependence of the shift of
63/65Cu-NMR spectra at the singlet Cu site deviates from thatof macroscopic magnetization below T=6 K. This deviationreflects the triplet localization in this system, because thelocalized triplets contribute only to the macroscopic
magnetization but to the Knight shift of the NMR-visible singlet
site. The result of 35/37Cl-NMR also supports this idea.
We also performed the 63/65Cu-NMR experiments at highmagnetic fields between the one-quarter and the threequarters magnetization plateaus. There are two kinds ofdimers referred as α, β with different orientation in crystal(Fig. 1). These two can be separately detected by NMR
spectra. Theoretical models proposed so far tried to explain
the plateaus in terms of the three inequivalent dimers (I, II, III)with different critical fields (0, Hc2, Hc4). In those models, it isexpected that the signal amplitude of α and β differs in thefield region Hc1<H<Hc3, where the dimer I is saturated andhence should be wiped out. However, our NMR results [1]articulately show that α and β dimers in singlet state have thesame number density in the field region. This result imposesa strong limit onto theoretical models for the spatial allocationof the localized triplets.

Technology, Sophia University)
E-mail: gotoo-t@sophia.ac.jp
URL: http://www.ph.sophia.ac.jp/~goto-ken
Takahiko Sasaki (Low Temperature Physics Division, IMR, Tohoku

180

University)
E-mail: takahiko@imr.tohoku.ac.jp

Fig. 1 Magnetization curve with the two-stepped plateaus,
spatial allocation of dimers in NH4CuCl3, and Cu-NMR
spectra under magnetic fields Hc2<H<Hc3.

KINKEN Research Highlights 2010 IMR

Highlights of Collaborated Research57

Research

Neutron Diffraction in 30 T Pulsed High Magnetic Fields
A non-trivial magnetic structure is often induced wheninteractions are competing for geometrical frustration. The
neutron diffraction is the key experiment in such case. We
have developed a neutron diffraction instrument under verystrong magnetic field up to 30 Tesla based on the international
collaboration project of ICC-IMR. It has been used at theworld leading neutron facilities such as J-PARC, SNS and
ILL.
Magnetic field is a powerful tuning parameter ofmagnetic properties of matters. It induces new states with
non-trivial magnetic structures in geometrical frustrated
antiferromagnets. When interactions are competing eachother for the special arrangement/network of magnetic
moments, it is impossible to satisfy all interactions. In thiscase, the ground state energy is highly degenerated. When amagnetic field is applied, the degeneracy is partly lifted andsome state falls into the lowest energy. A situation is more
complicated when there is a finite spin-lattice coupling. In
such case, there is another route to lift the degeneracy of theground state. Namely, the geometrical frustration can be
partly resolved by the lattice deformation. A series of Crspinel compounds XCr2O4 (X=Cd, Hg, Zn) belong to thisclass of material. It is known to show the spin Jahn-Teller
effect.
In XCr2O4, a non-trivial half-magnetization plateau has
been found in the ordered state [1]. Different from theconventional plateau stabilized by magnetic anisotropy, it
appears irrespective of magnetic field direction. To understandthe origin of this anomalous behaviour, it is necessary to
determine the magnetic structure at the plateau state. For
X=Hg, the task was completed by means of neutron diffraction
in a steady magnetic field. For X=Cd, the plateau starts at 28
T, which is far beyond the steady magnetic field available forneutron diffraction.

Fig. 2 Field dependence of bragg peak intensity at (a)incommensurate and (b) commensurate positions. The
intensity alternates at 28 T. The commensurateincommensurate transition is directly confirmed by thepresent neutron diffraction.

Recently, we have made a new breakthrough bydeveloping a 30 Tesla portable miniature pulsed magnet forneutron diffraction. Because of the compact design of themagnet, we can use it without any modification of thespectrometer. Thanks to the very small magnet volume, acompact mobile capacitor bank can be used as a power
supply as shown in Fig. 1.
Figure 2 shows the result of neutron diffraction up to 30 T[2]. The bragg peak moves from the incommensurate position
to commensurate one above 28 T. It is the first direct
determination of the magnetic structure at the half-plateau
state. By measuring several different peaks, magnetic
structure can be refined. The obtained structure is found to
be common between Hg and Cd derivatives. It indicates that
there is a universal mechanism of spin-lattice coupling in thisseries of materials.
The present result has established the pulsed high magnetic
field neutron diffraction as a powerful new tool for investigating
magnetic compounds. A technical transfer has been made toforeign institutes by the ICC-IMR collaboration scheme.
References[1] H. Ueda et al., Phys. Rev. Lett. 94, 047202 (2005).
[2] M. Matsuda et al., Phys. Rev. Lett. 104, 047201 (2010).
Key Words
Neutron Diffraction, High Magnetic Field, Spin Jahn-Teller
EffectContact toHiroyuki Nojiri (Magnetism Division)

Fig. 1 A portable capacitor bank of neutron diffraction, whichis installed at JRR3 reactor. It can be set into a limited spaceof a spectrometer.

58

Highlights of Collaborated ResearchE-mail: nojiri@imr.tohoku.ac.jp
URL: http://hfpm.imr.tohoku.ac.jp

IMR KINKEN Research Highlights 2010