Research

Spin Currents in Ferromagnetic Insulators
We proposed that the spin current can transmit inferromagnetic insulators by means of a propagating spinwave [1]. By attaching Pt probes on the ferromagnetic
insulators Y2Fe5O12(YIG), we observed that the electric
signal can transmit through the insulators. The Pt probes areused for converting the electric signal to the magnetic
signals by using the spin Hall effect.
Manipulating of the spin degrees of freedom is the key
issue of the spintronics field. So far, the spin of the conduction
electrons in a metallic sample is the carrier of the spin current.
However, it is well known that the coherent length of the spinof the conduction electrons is too short to use in electric
devices. In this work, we propose the new carrier of the spincurrent which is realized even in the ferromagnetic insulators(FI). We consider the magnetic excitation, called as amagnon, in the non-equilibrium situation. The coherent lengthof the magnon is order of several millimeters that is enoughlong to use in devices. We show that the magnon can carry
the angular momentum through sample and can be detectedby using the normal metal (NM) probes. By using this spincurrent, we show that two electrically separated NM probes
can communicate each other.
In order to investigate the spin current in ferromagnetic
sample theoretically, we focus on the dynamics of themagnetizations. The magnetization dynamics obeys theLandau-Lifshitzs-Gilbert (LLG) equation,dMdM,= −γ (M × H ) + αM ×dt
dt
where M is the magnetization of the sample, H is theeffective magnetic field acting on the magnetization, γ isgyromagnetic ratio, α is the Gilbert damping coefficient. H
includes the applied magnetic field, the shaped anisotropic
field that comes from the dipole-dipole interaction between
unit cells, and the random magnetic field due to the finite
temperatures. We consider the thin film sample. Two normal
metal (NM) probes are attached to the FI for injecting anddetecting the spin current (Fig. 1). The NM probe converts
the spin current to the charge current due to the spin-orbit
interactions. To formulate the spin injection from NM to FI,
we consider the exchange coupling at the interface between
NM and FI. After the straightforward calculations, we foundFig. 2 Transverse components of the magnetization obtainedby the numerical simulation. The spin current transmits in thex direction.

that the spin torque term τ=M×∇M acts on the interface of FI.
The magnitude of the spin torque is proportional to theapplied voltage. By including the spin torque term, we
calculate the dynamics of the magnetization. Figure 2 shows
the time average of the transverse components of themagnetization. Although the spin torque acts on only theinterface of FI and NM, the transverse components appears
in whole sample. We found that this component is theevidence of the spin current which is excited by the spin
injection of NM. This new concept of the spin current can
open a possibility of the new spintronics devices.
References[1] T. Kajiwara et al, Nature 464, 262 (2010).
Key Words
Spintronics, Spin Current, Spin Wave
Contact to
Sadamichi Maekawa* (Theory of Solid State Physics Division)  * Present Address: Advanced Science Research Center, Japan
Atomic Energy Agency
E-mail: maekawa.sadamichi@jaea.go.jp
URL: http://asrc.jaea.go.jp/

Fig. 1 Schematic view of the sample configuration. Twonormal metal (NM) probes are attached to the Ferromagnetic
Insulator (FI). The spin current mediated by the magnon
transmits through the FI.

22

Electronic Materials

IMR KINKEN Research Highlights 2010

Research

Intrinsic Magneto-Resistance of Metallic Carbon Nanotube
The intrinsic magneto-resistance of the single-walledcarbon nanotube (SWNT) is obtained by using a noncontact
method. The origin of the magneto-resistance is identified tobe the Aharonov-Bohm effect of metallic nanotubes. It is thefirst clear observation of pure magneto-transport behavior in
metallic SWNT.
A single-walled carbon nanotube (SWNT) is a uniqueconductor with nanoscale diameter. The electronic propertyof SWNT is governed by the periodic boundary conditions of
the wave functions along the tube. Depending on the diameteror helical arrangement, metallic or semiconducting behaviors
are expected. Another fascinating feature of the SWNT is themagnetic field effect. When a field is applied parallel to thetube, the quantum states of the electrons are influenced by
the so-called Aharonov-Bohm (AB) phase. Namely, theperiodic boundary condition depends on the magnetic flux
inside the tube and the band gap varies as a function of themagnetic field.
There have been considerable efforts to verify those
properties. Indeed, the optical spectroscopy has revealed thesplitting of energy gap of semiconducting SWNT caused by
the AB effect. In turn, the experimental investigation of AB
effect on metallic nanotube is still controversial. It is because
the magneto-resistance of SWNT is always contaminatedwith various extrinsic effects. The crucial difficulty is in themaking of good electric contact on a nanotube.
Our concept is to use a contactless method to evaluate
the intrinsic magneto-resistance of metallic SWNT. By
employing the cavity perturbation technique, high-frequency
conductivity of the SWNT can be obtained without making
any contact. As is well-known, the cavity resonator has aparticular resonant spectrum, which is described by theresonant frequency: f and the quality factor: Q. These two

Fig. 2 Magneto-resistance of carbon nanotube for magnetic
fields parallel and perpendicular to the tube axis. A large
positive magneto-resistance is observed only in the parallel
configuration.

factors vary according to the permittivity or conductivity ofsample inside the cavity. Thus, high-frequency conductivityof the SWNT can be obtained by comparing f and Q of thecavity with or without the sample.
Figure 2 shows the magneto-resistance evaluated fromthe field dependence of the inverse Q factor for the highlyoriented SWNT thin film at 4.2 K [1]. The tubes are grown by
the high-pressure CO conversion method and the averagetube diameter is about 0.95 nm. The liner positive magnetoresistance is clearly observed as expected for AB-effect ofmetallic SWNT. Moreover, the magneto-resistance iscompletely suppressed in perpendicular condition where
magnetic flux does not penetrate inside the tube. The result
indicates that he intrinsic magneto-resistance of metallic
SWNT is evaluated successfully for the first time. It highlights
the powerfulness of microwave technique in investigating
nano materials.
References[1] Y. Oshima et al., Phys. Rev. Lett. 104, 106803 (2010).
Key Words
Carbon Nanotube,
Resistance

Aharonov-Bohm

Effect,Magneto-Contact toHiroyuki Nojiri (Magnetism Division)

Fig. 1 A cylindrical cavity with the resonant frequency around
6.3 GHz used for the cavity perturbation technique to evaluate
the intrinsic magneto-resistance of carbon nanotube. The
sample is located at the position where the microwave electricfield is along the SWNT.

KINKEN Research Highlights 2010 IMR

E-mail: nojiri@imr.tohoku.ac.jp
URL: http://hfpm.imr.tohoku.ac.jp

Electronic Materials

23Research

Spin Current and Electrical Signal Transmissionin Magnetic Insulator

A flow of spin angular momentum is called a spin current.
In a metal the spin current is typically carried by the diffusive
motion of conduction electrons (Fig. 1(a)). However, it canalso be carried as spin waves—the collective motion ofmagnetic moments in a material (Fig. 1(b)). Here we call aspin current carried by spin waves a “spin-wave spin current”.
In a metal, the spin-wave spin current genarally quickly
decays—typically within a few microns. In contrast, spin–wave spin currents in magnetic insulators such as Y3Fe5O12(YIG) can travel as far as several centimeters.
To make use of the spin-wave spin currents in insulators,
it is necessary to find methods of getting a d.c. spin current
into and out of the insulators. We used the spin pumping andspin-transfer torque (STT). Spin pumping refers to thetransfer of spin angular momentum from magnetization
precession motion to conduction-electron spin, a phenomenon
which allows generation of a spin current from magnetization
motion. STT is, in contrast, the reverse process of this spin
pumping, that is, the transfer of angular momentum fromconduction-electron spin to magnetization: the magnetization
receives torque by absorbing a spin current. These two
phenomena enable the mutual conversion of angularmomentum between conduction-electron spin andmagnetization. However, up to now, experiments on these
phenomena have been limited to electric conductors. In thepresent work, we demonstrate that by using YIG/Pt films,
both phenomena occur even at insulator/metal interfaces,
and the phenomena allow transmission of a d.c. electric
signal through the insulator for a long distance.
First, we established the spin pumping and STT processes
at the interface between a magnetic insulator YIG and aparamagnetic metal Pt separately. A microwave was applied
to YIG/Pt film in order to inject a spin current into the Pt layer
via the spin pumping. The Pt layer is used as a spin-current
detector, in which the spin-Hall effect (SHE) converts a spincurrent into electromotive force. We successfully observed a
SHE signal induced by spin pumping from YIG [1]. We then

(b)

(a)
spin current

spin-wave
spin current

(a)

(b)

V

Pt (o)

ESHE
H

spin-wave
spin current

Pt/Y3Fe5O12
H = 2.3 kOe

Y3Fe5O12
Pt (i)J

V (nV)

We present the electric-signal transmission in a magnetic
insulator, in which a “spin-wave spin current” is used. An
insulator has, in general, a large charge gap, which prevents
electron excitation and an electric current. However, in amagnetic insulator, a spin current can flow via spin waves: aspin-wave spin current. Here, we demonstrate that this spinwave spin current enables an electric-signal transmission
even in an insulator, establishing reversible spin transfer
processes across a metal/insulator interface.

10−1 −0.50

0.5

j (GA/m2)

1

Fig. 2 (a) A schematic illustration of the electric-signaltransmission through an insulator. ESHE, H and J denoteelectromotive force induced by ISHE, the external in-planemagnetic field and the electric current, respectively. (b) V as afunction of the electric-current density in the Pt film. V denotes
the voltage difference between the ends of the Pt layer.
applied an electric current to the Pt layer to demonstrate STT
across the YIG/Pt interface. In the Pt layer, an electric current
induces a spin current via SHE. This spin current exerts STT
on the magnetization in YIG, which can be perturbed whenmagnitude of STT is greater than that of the damping torque.
This means that when a sufficient amount of current is applied,
the magnetization spontaneously oscillates and emits
electromagnetic waves. We attached an antenna on the YIG/
Pt film and succeeded in detecting a microwave emission
induced by an electric current applied to the Pt film [1].
Finally, we demonstrate the electric-signal transmission in
the insulator YIG film by making use of spin transfer across a
YIG/Pt interface and spin-wave spin current in YIG [1]. Figure2(a) shows a schematic illustration of the experimental setup.
Two Pt films (i and o) are sputtered on YIG film and an electric
current is applied to the Pt film i. The distance between thefilms i and o is 1 mm. In this setup, the electric current applied
to the Pt film i induces magnetization oscillation in the YIG
layer due to the STT across the YIG/Pt interface. Thismagnetization oscillation then propagates in the YIG layer via
a spin-wave spin current. When the oscillation reaches thesecond interface, it generates electric voltage in the Pt film ovia spin pumping and ISHE. Figure 2(b) shows the voltage
difference V between the ends of the Pt film o as a function of
the electric current density j in the Pt film i. When themagnetization of the YIG layer is perpendicular to the electric
current, the voltage V signal appears. This hybrid electricaltransmission method potentially offers a means of innovative
signal delivery in electrical circuits and devices.
Reference[1] Y. Kajiwara, K. Harii, S. Takahashi, J. Ohe, K. Uchida, M.
Mizuguchi, H. Umezawa, H. Kawai, K. Takanashi, S.
Maekawa and E. Saitoh, Nature (London) 464, 262 (2010).
Key Words
Spintronics, Spin Current, Spin Transfer

Fig. 1 (a) A schematic illustration of a conduction-electron
spin current: spin angular momentum carried by electron
diffusion. (b) A schematic illustration of a spin-wave spincurrent: spin angular momentum carried by collective motionof magnetic moment.

24

Electronic Materials

Contact toEiji Saitoh (Surface and Interface Research Division)
E-mail: saitoheiji@imr.tohoku.ac.jp

IMR KINKEN Research Highlights 2010

Research

Scanning Tunneling Microscopy/Spectroscopy
in Iron-Pnictide Superconductor Ba(Fe0.93Co0.07)2As2
The recent discovery of high transition temperature (Tc)iron-pnictide superconductor LaFeAsO1-xFx [1] has provided
the new route to researches on high-Tc superconductivity.
To understand the electronic structure and the mechanismof the superconductivity in high-Tc iron-based pnictide
superconductors,
scanning
tunneling
microscopy/
spectroscopy (STM/STS) experiments have been performedon Ba(Fe0.93Co0.07)2As2 single crystals [2].

superconductors (2Δ/kBTc = 3.53) and is close to that ofcuprate superconductors (2Δ/kBTc ~ 6-10). The results
indicate that Ba(Fe0.93Co0.07)2As2 is in a strong coupling
regime and the pairing symmetry is unconventional.
Figure 1(c) indicates tunneling spectra measured at the
depression region which is characterized by the one
dimensional atomic defect with a typical width of 2a-10a. The
tunneling spectra show the strong peak structure at ~ 2.5
meV without any features of superconductivity. The peak
spectra can be associated with the scattering resonancepeak due to the defect structure and/or the characteristicfeature of the next atomic layers (i.e., nonsuperconducting
Ba layer).

STM/STS measurements [2] were performed on the coldcleaved surface of Ba(Fe0.93Co0.07)2As2 single crystals [3] at
4.9 K in the ultrahigh vacuum (UHV, P ~ 10-10 Torr) condition.
The STM images indicate two kinds of atomic arrangements:(i) the square lattice structures with the distance of ~ 5.4 Å
–(i.e., √–2 × √2 structure) and (ii) the one-dimensional stripe-likestructures. Since the former surface does not show thesuperconducting tunneling spectra, we focus on the
superconductivity on the latter surface below.
Figure 1(a) shows a STM topographic image of the onedimensional stripe-like structures. The stripe-like structures
have a periodicity of ~ 7.97 Å which corresponds to twice the
As-As (or Ba-Ba) distance (a = 3.96 Å). Figure 1(b) shows
the tunneling conductance (dI/dV) spectra measured at
different positions along the stripe-like structure. The
tunneling spectra exhibit a superconducting energy gap 2Δ, aclear coherence peak, and an asymmetric background at|V| > Δ. The energy gap 2Δ ~ 15.2 meV is extracted from the
distance between the coherence peaks. The single gap
structure is consistent with the previous STM/STS results but
is different from the multiple gaps (Δ ~ 7 meV and ~ 4.5 meV)
reported by angle resolved photoemission spectroscopy(ARPES).
In this study, the gap ratio is estimated to be 2Δ/kBTc ~7.3 in Ba(Fe0.93Co0.07)2As2. The value is about two times
larger than that of the weak coupling s-wave BCS

References[1] Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, J.
Am. Chem. Soc. 130, 3296 (2008).
[2] T. Nishizaki, Y. Nakajima, T. Tamegai, and N. Kobayashi,
Physica C, to be published (2010).
[3] Y. Nakajima, T, Taen, and T. Tamegai, J. Phys. Soc. Jpn.
78, 023702 (2009).
Key Words
Scanning Tunneling Microscopy/Spectroscopy, High-Tc Ironbased Pnictide Superconductors, Energy Gap
Contact to
Terukazu Nishizaki (Low Temperature Physics Division)
E-mail: terukazu@imr.tohoku.ac.jp
URL: http://ltp.imr.tohoku.ac.jp/index.htm

1.5

1.5

Ba (Fe0.9Co0.1)2As 2

Ba (Fe0.9Co0.1)2As 2

T = 4.9 K

1

0.5

A

(a)

B

C

dI/dV (nS)dI/dV (nS)

T = 4.9 K

B

0.5

A

C

D

E

7.9Å

0
-40

1

D

(b)
-20

0

V (mV)

20

E

40

0
-40

(c)
-20

0

V (mV)

20

40

Fig. 1 (a) Constant-current STM image of Ba(Fe0.93Co0.07)2As2 with one dimensional structure. V = 100 mV, I = 20 pA, 390 Å ×
390 Å. Tunneling spectra taken at (b) the positions A, B, C and (c) positions D, E marked in (a). Each subsequent spectrum isoffset for clarity.

KINKEN Research Highlights 2010 IMR

Electronic Materials

25Research

Electric Field Induced Superconductivity at 15 K
The electric double layer capacitor at the interface
between an electronic conductor and an ionic conductor
has an ability of high density charge accumulation reaching
8 x 1014 cm-2, which is almost two orders of magnitude larger
than conventional solid capacitors. By using an electric
double layer as a gate dielectric of a transistor, wedemonstrated an electric field induced superconductivity
occurring at a critical temperature of 15 K, on atomically flat
surface of a layered material, ZrNCl. This result indicates an
emergence of novel materials science under extremely highelectric field.
The electric double layer formed at an electrochemicalinterface between liquid and solid is recently attracting
considerable interest from the view point of charge capacitor
applications. However, its ability of high density charge
accumulation can be used as a transistor with high capacity
and high maximum carrier density. Importantly, themechanism of the charge accumulation at the electric double
layer is electrostatic, providing a novel opportunity to search
for new states of matter, which are not accessible byconventional chemistry. We named this transistor, electric
double layer transistor (EDLT).
In 2008, we succeeded in demonstrating the first electricfield induced superconductivity in an oxide semiconductor,
SrTiO3, in a tight in-house collaboration with Prof. Kawasaki’s
group (WPI), Prof. Nojima’s group (IMR), and Prof. Aoki’s
group (Physics) [1]. Although this work is conceptuallyimportant, it is of very limited applicability to materials science
as it dealt with the very special case of a material whichbecomes superconducting at very low carrier densities andfor which atomically flat surfaces can be easily fabricated. To
generalize this technique to a broader range of materials, wehave employed ionic liquid (Fig. 1 right), which was found to
exhibit an electrostatic charge accumulation of 8 x 1014 cm-2[2]. This value is eight times larger than the case of a polymer
electrolyte used for the demonstration of SrTiO3-EDLT.
Another key technique required for EDLTs is the preparationof atomically flat surfaces of solid semiconductors, since itworks as a carrier transport channel. For easy preparation of
atomically flat surfaces, we chose a layered material, ZrNCl,
and introduced a mechanical cleavage method, used in theFig. 2 Temperature dependence of resistivity of a ZrNClEDLT for various gate voltages.
fabrication of graphene devices (Fig. 1 left).
Figure 2 displays temperature dependence of resistivityof a ZrNCl single crystal flake of the 12 monolayers inthickness. With increasing gate voltage VG, the resistivity
displays a dramatical reduction. At VG = 3.5 V, we found a
resistivity drop, followed by zero resistivity above VG = 4 V,
providing convincing evidence for the occurrence of electricfield induced superconductivity [3]. Tc is much higher than thecase of SrTiO3. The result implied that the EDLT could be a
versatile tool applicable to variety of materials and thus could
be useful for searching for new superconductors.
Reference[1] K. Ueno et al., Nat. Mater. 7, 855 (2008).
[2] H. T. Yuan et al., Adv. Funct. Mater. 19, 1046 (2009).
[3] J. T. Ye et al., Nat. Mater. 9, 125 (2010).
Key Words
Electric Double Layer Transistor, Superconductivity, Ionic
LiquidContact toYoshihiro Iwasa* (Low Temperature Condensed State Physics
Division)  * Present Address: Quantum-Phase Electronics Center, Universityof TokyoFig. 1 Left: a micro cleavage technique with Scotch Tape formaking atomically flat flakes of single crystals of layeredmaterials. Right: an EDLT device using ionic liquid.

26

Electronic Materials

E-mail: iwasa@ap.t.u-tokyo.ac.jp

IMR KINKEN Research Highlights 2010

Research

A New CZ Method for Ge Crystals
with Extremely Low Dislocation Density
A new CZ method for growing Ge crystals with anextremely low dislocation density was explored. A Ge melt
was partially covered with liquid B2O3 only in the outersurface region of the melt. GeO2 particles generating
dislocations in the grown crystal were removed by the B2O3covered melt. From a particle-free “melt-window” in thecentral portion of the melt, Ge single crystals with extremely
density were successfully grown.
Recently, there is a renaissance of interest in germanium(Ge) for possible use in the next generation of high-speed
ULSI due to its superior carrier mobility in semiconductor
industries, as well as low temperature processing [1]. Itsapplication as substrates for GaAs solar cells is also
investigated as it enables higher energy-conversion efficiency
than Si [1]. For use, Ge is expected free from dislocations to
avoid their demerit as severe carrier killers in GaAs epitaxial
layers. Since Ge and GaAs well lattice-match (~ 0.12%),dislocation-free Ge crystals are desirable for the substrates
with that purpose. Currently, large-sized dislocation-free Ge
crystals are grown by the Czochralski (CZ) method in Umicore,
world-wide Belgium company. However, it is commerciallywell recognized that growth of dislocation-free Ge crystal isvery difficult. Germanium oxide (GeO2) particles floating on
the Ge melt surface attach to the Ge crystal during growth,
resulting in generation of dislocations. The melting point of
GeO2 is 1115˚C, higher than that of Ge (938˚C). They may beformed in the melt originating from the surface oxide layer ofcharged Ge raw material, air leakage of the chamber orresidual moisture in the growth furnace. We explored a new
CZ method for growing Ge crystals with an extremely low
dislocation density using boron oxide (B2O3) with controlling or
preventing such particle formation on the melt surface [2].
Figures 1(a)–(c) illustrate the arrangement of Ge, B2O3
and the silica crucible in charging, melting and growing
stages, respectively. First, B2O3 melted at 480˚C, and thenGe melted at 938˚C. Due to fine wetting ability between B2O3
and silica glass, the Ge melt and B2O3 liquid maintained thearrangement shown in Fig. 1(b). Only the outside region of
the Ge melt was partially covered with the liquid B2O3.
Particles were caught by the liquid B2O3 and move outside of
the Ge melt surface with the B2O3 liquid. Finally, a clean Ge
melt surface free from any particles is realized in the central
region of the crucible, making it possible to grow from such a

Fig. 2 A photograph of a Ge single crystal grown from B2O3partially-covered Ge melt.
“clean” Ge melt as shown in Fig. 1(c).
Figure 2 shows a photograph of a Ge boule grown fromGe melt partially covered with B2O3. The diameter is about 1
inch and the length about 50 mm. Three habit lines wereclearly observed. The dislocation density at the top of thecrystal shown in Fig. 2 was evaluated by the preferential
etching method to be almost zero.
The crystal was n-type and the resistivity, carrier
concentration and electron mobility were 18 Ωcm, 6.8×1014
cm-3 and 3.2×103 cm2/Vs, respectively. B and O
concentrations in the grown crystals were lower than thedetection limit (2×1015 cm-3) in the SIMS analysis, implyingthat the contamination level of B from B2O3 liquid is very
low. Any negative influences of B2O3 liquid on chemical andelectrical properties were not detected.
One of the reasons why B2O3 liquid does not dissociate
to B and O, and is stable on Ge melt may be that the Gibbs
standard free energy of B2O3 formation is the lowest among
those of isolated B, O atoms and GeO2 at a temperature
close to the melting point of Ge [3]. In the present method,
B2O3 is being used as a remover of GeO2 floating on the Ge
melt. Its use is very effective for reduction of grown-in
dislocation density. This effect can be also originating inwetting ability and low reactivity among Ge melt, silica
crucible and B2O3 liquid. Realization of complete dislocationfree Ge crystal growth is now in progress.
References[1] B. Depuydt, A. Theuwis and I. Romandic, Mater. Sci.
Semicond. Process. 9, 437 (2006).
[2] T. Taishi, Y. Ohno and I. Yonenaga, J. Crystal Growth311, 4615 (2009).
[3] H. Okamoto, In: T.B. Massalski (Ed.), Binary Alloy Phase
Diagrams, 2nd. Vol. 2, 1976 (1990).
Key Words
Germanium, Dislocation-Free, Czochralski Growth
Contact toIchiro Yonenaga (Physics of Crystal Defects Division)

Fig. 1 Illustrations of arrangement of Ge, B2O3 and silica
crucible: (a) after charging, (b) after melting and (c) during the
growth.

KINKEN Research Highlights 2010 IMR

E-mail: yonenaga@imr.tohoku.ac.jp
URL: http://lab-defects.imr.tohoku.ac.jp/

Electronic Materials

27Research

Quantum Transport Properties in Nanoscale Materials
by First-Principles Calculations
Transistor density of silicon-based semiconductor chips
is approaching the ultimate size limitation. Innovative
materials such as organic molecules, single-walled carbon
nanotubes, and graphenes have been suggested as
substitutions of conventional semiconductors to implement
promising electronic devices in nanoscale where quantum
mechanics dominates the electron kinetic behavior. The
present work showed an interesting behavior of quantum
conductance, which is based on nanoscale-structured
material. The method used in these papers is based on thenonequilibrium Green’s function and the code has beendeveloped originally in our research group. All the numericalcalculations have been conducted by using the HITACHI
SR11000 supercomputing system at the Center for
Computational Materials Science, IMR.
A systematic analysis of electron transport characteristics
for 1D heterojunctions with two nitrogen-doped (N-doped)
capped carbon nanotubes (CNTs) facing one another at
different conformations is presented considering the chiralityof CNTs (armchair(5,5) and zigzag(9,0)) and spatial
arrangement of N-dopants. Figure 1 shows schematic
illustration of the two facing N-doped armchair CNTs indifferent sites. The remarkable feature emerging from the I-V
characteristics of the N-doped CNT junctions is that thetunneling current between two N-doped capped CNTs is
dramatically increased by N-doping and negative differential
resistance (NDR) behavior is observed as in the Esaki-like
diode, that is, tunneling diode. The ab initio calculations showthat the modification of the molecular orbitals by the N-dopants
generates a conducting channel in the designed CNT
junctions. The facing conformation and the N-doping site
significantly affect electron transport characteristics; when
the N-doped CNT junction has the pentagon-facing
conformation and a nitrogen atom is substituted at the cap
region, especially the b doping site (See Fig.1), it provides
strong NDR behavior with large peak-to-valley ratio and Vpeak
values. Furthermore, a clear interpretation is presented for
the NDR behavior by a rigid shift model of the HOMO- and
LUMO-filtered energy levels in the left and right electrodes
under the applied biases. It is concluded that the doped
nitrogen atom plays an important role in the electron transportcharacteristics of the designed CNT junctions by modifying
their molecular orbitals so as to have NDR behavior. These
results give an insight into the design and implementation ofvarious electronic logic functions based on CNTs forapplications in the field of nanoelectronics [1].
Structure modulation of graphene nanoribbons (GNRs) isalready possible, by using direct focused electron writing orplasma etching. Defects in GNR structures, such as vacancy
or doped atoms, are also reported to modify GNR electronic
properties significantly. In the present study, we perform an
ab initio calculation to investigate transport properties of
GNR junctions self-consistently with nonequilibrium Green’s
function approach combined with density functional theory[2]. Here, we fabricate GNR edge defects by removing threecarbon atoms from the edges of zigzag GNR, with thedangling bonds saturated by hydrogen atoms (See Fig. 2).
We obtain I-V curves of edge-defect junction in modulation of
the intensity of transverse electric field. From the results oftransport property of an edge-defect ZGNR junction, It is
concluded that the tunable tunneling current can be sensitively
controlled by transverse electric fields. In the presence of
external fields, the junction almost turns off the conducting

28

Electronic Materials

Fig. 1 Schematic of the two facing N-doped capped CNTs indifferent conformations according to the chirality of thearmchair(5,5) CNTs. The NDR behavior significantly depends
on the N-doping site and the facing conformations of the
N-doped capped CNT junctions.

Fig. 2 LUMO of edge-defect graphene nanoribbons junction,with external field indicated by blue arrows.
channel when external field exceeds 1.0 V/Å exhibiting the
maximum of the on/off ratio up to 104. Consequently,
electronic devices based on edge-defect ZGNR junctions
have the advantage of easy control through the transverse
electrostatic fields. The study of small and narrow GNR
devices benefits graphene integrated circuit engineering thatmight be realized by ultrafine GNR fabrication technologies
in the future.
References[1] S. U. Lee, R. V. Belosludov, H. Mizuseki, and Y. Kawazoe,
Small 5, 1769 (2009).
[2] G. Yin, Y. Y. Liang, F. Jiang, H. Chen, P. Wang, R. Note,
H. Mizuseki, and Y. Kawazoe, J. Chem. Phys. 131, 234706(2009).
Key Words
Molecular Electronics, Molecular Devices, First Principles
Calculations
Contact toYoshiyuki Kawazoe (Materials Design by Computer Simulation
Division)
E-mail: kawazoe@imr.edu
URL: http://www.kawazoe.imr.edu/

IMR KINKEN Research Highlights 2010

Research

3D Analysis of Dopant Distributions in MOSFET Structures
by Laser-assisted Atom Probe Tomography
The dopant distributions in metal-oxide- semiconductor
field effect transistor (MOSFET) structures (gate, gate oxide,
channel, source/drain extension, and halo) were analyzed
by laser-assisted atom probe tomography (Laser APT). The
segregations of P atoms on the grain boundaries in poly-Si
gate and on the interface between gate and gate oxide wereobserved. B atoms were enriched near the edge of thesource/drain extension. These results are very important tounderstand the origin of variability in MOSFET
characteristics.
As the minimum feature size of MOSFET decreases to
65 nm or less, variability in MOSFET characteristics has
become a serious problem. The major factor is supposed to
be inhomogeneous distribution of discrete dopant in thechannel and/or gate region. Therefore, the information on theexact location of each dopant in the channel region is strongly
desired. In this work, we report dopant distributions overentire regions of an n-type MOSFET structure with atomic
scale resolution using Laser APT (IMAGO: LEAP3000XHR)[1].
The samples were long gate length n-type MOSFETs(line-and-space pattern), consisting of a poly-Si gate, a gate
oxide film, and a Si substrate with source/drain extension
and halo regions. After B atoms were implanted into thechannel region, the gate-oxide film and P-doped poly-Si gate
were formed. The sample was then patterned and etched byconventional lithography and dry etching with the gate lengthof 60 nm. As and B atoms were implanted for a source/drain
extension and punch-through-stopper, respectively, followedby spike annealing at 1,050˚C to activate the implanted
dopants.
A 3D elemental map of the line-and-space pattern sample
is shown in Fig. 1(a). A cross-sectional transmission electron
microscopy image is also shown in Fig. 1(b). The observed
region in Fig. 1(a) corresponds to the region inside the broken
line in Fig. 1(b). The MOSFET structure, which consists of apoly-Si gate electrode, gate oxide, Si channel, and source/
drain extension, was clearly observed. Elemental maps of P
in a 10-nm-thick slice parallel to the X-Z plane are shown in
Fig. 1(c). P atoms were not uniformly distributed in the polySi gate electrode due to the segregation on grain boundaries
in the poly-Si gate, at the interface between the poly-Si gate
and the gate oxide [1,2].
In order to clarify the detailed dopant distribution aroundthe source/drain extension, the enlarged view of 3D elemental
map, in which the edge of the source/drain extension was
nearly in the center of the needle specimen, is shown in Fig.
2. B atoms were enriched near the edge of the source/drain
extension because the B atoms were implanted (halo) after
extension implantation for the punch-through-stopper. These
results are very important to understand the origin of variability
in MOSFET characteristics, especially in threshold voltage
fluctuation.

(c)Fig. 1 The 3D elemental map of an n-type MOSFET from aline-and-space pattern sample. (b) Cross-sectionaltransmission electron microscopic image. (c) Elemental mapof P in a slice 10nm thick.

10nm

O
SiO
SiO2
As
B (Sphere)
Fig. 2 The enlarged view around the edge of the source/
drain extension. Red circle indicates the B enriched region.
References[1] K. Inoue, F.Yano, A. Nishida, H. Takamizawa, T.
Tsunomura, Y. Nagai, M. Hasegawa: Ultramicroscopy 109,
1479 (2009).
[2] K. Inoue, F. Yano, A. Nishida, H. Takamizawa, T.
Tsunomura, Y. Nagai, M. Hasegawa, Appl. Phys. Lett. 95,
043502 (2009).
Key Words
Laser Atom Probe, MOSFET, Dopant Distribution
Contact toYasuyoshi Nagai (Irradiation Effects in Nuclear and Their RelatedMaterials Division)
E-mail: nagai@imr.tohoku.ac.jp
URL: http://wani.imr.tohoku.ac.jp/

KINKEN Research Highlights 2010 IMR

Electronic Materials

29Research

Nitride semiconductors such as InN, GaN, AlN and their
alloys are very promising for fabricating devices with low
energy consumption, high-power, and high-frequency.
InGaN-based blue light-emitting-diodes (LEDs) and laserdiodes (LDs) have been already commercially available.
The new trends of nitride semiconductors in the region ofdeep ultraviolet, green, and infrared wavelength, will
promote the device applications saving energy and creating
new energies. Our researches are related with these newtechnology trends, challenging the nitride semiconductorbased LDs for the optical fiber communications system,
phosphor-free white LEDs, brilliant green LEDs, and highefficiency solar cells. One of the key tasks for implementing
the above devices is to improve the crystalline quality of the
InN film, using the pressurized-reactor metalorganic vapor
phase epitaxy (PR-MOVPE) technology suitable for mass
productions.
The development of sciences and technologies hasbrought large conveniences to our life, but also problems to
our planet, such as the globe warming effect due to the overexhausting of the carbon dioxide. To solve this problem, we
have to reduce the energy consumption and create newenergies, for example, from solar energy. A strong candidate
for these purposes is nitride semiconductor material, which
covers the bandgap energy from about 0.63 to 6.03 eVcorresponding to the wavelength of 0.21 to 1.97 μm. These
series of materials can be used to fabricate new devices
which save much more energy, create new energies and areenvironmental-friendly (free of arsine, phosphorus, selenium,
and mercury). For energy saving, we propose solid-state
lighting with phosphor-free white LEDs based on nitride
semiconductor layers, as shown in Fig.1(a). This structure
provides higher emission efficiency than that of thephosphorus type. If all the conventional lighting equipments
are substituted by the solid-state-lighting, the reduction of theenergy consumption will be about 1/10. On the other hand,for creating new energies, we propose nitride semiconductorp-ELECTRODE
n-ELECTRODE

BLUE LED
ELECTRODE LAYERGREEN EMITTERRED EMITTERGaN TEMPLATE
SUBSTRATEWHITE LIGHT(a)n-InGaN
p-InGaN
n-Si
p-Si
n-InN
p-InN
(b)

Fig. 1 A phosphor-free nitride semiconductor-based white
LED (a) and a solar cell (b) combined with silicon substrate.
For the white LED structure, the current is injected to only
blue active layer and its blue light excited green and red
active layers.

30

Electronic Materials

GROWTH TEMPERATURE (℃)

Nitride Semiconductors: Saving Our Planet

700 50AMMONIA TO TRIMETHYLINDIUM RATIO（×104）
5

650

3

2

METAL-INDIUM PRECIPITATION600
SINGLE

550 CRYSTAL
650 Torr

500
0

SINGLE CRYSTAL
2400 Torr
METAL-INDIUM PRECIPITATION

5
10
15
20
25
30
TRIMETHYLINDIUM FLOW RATE (µmol/min)

35

Fig. 2 Phase diagram of InN growth as functions of growthtemperature and trimethylindium flow rate at 650 Torr and2400 Torr.
silicon combined solar-cell structure, that a silicon substrateis sandwiched with high indium composition InGaN and InN
layers as shown in Fig.1(b). This cell can cover the wholesolar spectrum and lead to a theoretical conversion efficiency
of about 36%. In addition, we are exploring the possibility ofInN-based laser diodes for optical-fiber communications
systems. The temperature stability of InN band-gap is higher
than that of InGaAsP, promising for Peltier-free and dense
wavelength- division-multiplexing applications.
The above applications require high-quality InN and highindium composition InGaN layers, which are difficult to growusing the conventional low-pressure (LP) and atmosphericpressure (AP) MOVPE. The barriers lie in the extremely highequilibrium vapor pressure of nitrogen between solid andvapor phases for InN, and the phase separation for InGaN.
We have proposed and designed the pressurized-reactor(PR) MOVPE and obtained InN films with extended growthcondition-window as shown in Fig.2 compared with LP and
AP-MOVPE [1, 2]. Here, ammonia and trimethylindium(TMIn) as nitrogen and indium sources are used, respectively.
PR-MOVPE can be expected to make it possible to further
improve the InN and InGaN crystalline quality for device
applications.
References[1] Y. H. Liu, T. Kimura, T. Shimada, M. Hirata, M. Wakaba,
M. Nakao, S. Y. Ji, and T. Matsuoka, phys. stat. sol. (c), 6,
S381 (2009).
[2] Y. H. Liu, T. Kimura, Y. T. Zhang, M. Hirata, Masaki Hirata,
K. Prasertusk, R. Katayama, and T. Matsuoka, The 37th
International Symposium on Compound Semiconductors,
FrD1-2 (2010), Takamatsu, Japan, invited talk.
Key Words
Nitride Semiconductors, White LEDs, Solar Cells
Contact toTakashi Matsuoka (Physics of Electronic Materials Division)
E-mail: matsuoka@imr.tohoku.ac.jp
URL: http://www.matsuoka-lab.imr.tohoku.ac.jp/

IMR KINKEN Research Highlights 2010

Research

Observation of Charged Exciton Statesin ZnO Modulation-doped Quantum Wells
We report on the effect of electrical injection from Ga
delta-doped layer of an MgZnO barrier on the magnetooptical properties of a ZnO/MgZnO single quantum well
(SQW). Approaching the delta-doped layer to the SQW layer
resulted in the photoluminescence (PL) blueshift due to thescreening of piezoelectric and macroscopic polarization
field by electrons from the MgZnO barriers. Circularly
polarized PL was observed in magnetic fields of up to 50 Twith a Faraday configuration when the delta-doped layer islocated 6 nm away from the SQW layer. The observed
Zeeman splitting and the degree of circular polarization inmagnetic fields revealed that the PL from the negatively
charged excitons (X-) for the first time in a ZnO system.
Magneto-optical spectroscopy is a spectacular tool instudying the electronic structure in low-dimensional systems[1,2], especially for visualization of the excitation spectrum of
Landau quantized states in interface two-dimensional
electron states. Recently ZnO has attracted attention in this
aspect to look for potential electronic application since theobservation of quantum Hall effects. It was also known
through spectroscopic research that Coulomb effects
between the interface electrons and a hole play an importantrole. As a prototypical model system where the hole position
is well defined, we studied the optical properties of 5-nmthick modulation-doped ZnO/MgZnO single quantum wells(SQWs) to understand the properties of such an interface(charged) exciton. Circular polarization in PL from thecharged exciton was enhanced by a magnetic field up to 50
T with a Faraday configuration at 4.2 K, as shown in Fig. 1(a)and 1(b). The PL peak energies for both polarization
components (σ±) are almost coincident and independent of
the magnetic field. Almost zero-splitting of the PL andmagnetic-circular dichroism suggest existence of the X-

states and its importance of Coulomb effects (Fig. 1(c)-(e)).
This is a first-time observation of the negatively charged
exciton in ZnO-based quantum structures [2]. Based on thedetailed analysis, a binding energy of the charge exciton was
evaluated to be approximately 5 meV. We also observed
magneto-PL from a collective state in a ZnO single
heterojunction containing high-mobility electrons. The current
postulation has a possibility to be used in a versatile mannerto get in-depth insight for the optical properties of other widegap oxides [3-5].
References[1] T. Makino, Y. Segawa, A. Tsukazaki, A. Ohtomo, Appl.
Phys. Lett. 93, 121907 (2008).
[2] T. Makino, Y. Furuta, Y. Segawa, A. Tsukazaki, A.
Ohtomo, Y. Hirayama, R. Shen, S. Takeyama, Y. Takagi, M.
Kawasaki, Phys. Rev. B 80, 155333 (2009).
[3] T. Yamasaki, T. Fukumura, Y. Yamada, M. Nakano, K.
Ueno, T. Makino, and M. Kawasaki, Appl. Phys. Lett. 94,
102515 (2009).
[4] H. Hiraga, T. Fukumura, A. Ohtomo, T. Makino, A.
Ohkubo, H. Kimura, and M. Kawasaki, Appl. Phys. Lett. 95,
032109 (2009).
[5] H. Hiraga, T. Makino, T. Fukumura, A. Ohtomo, and M.
Kawasaki, Appl. Phys. Lett. 95, 211908 (2009).
Key Words
Exciton, Modulation-doped Structure, Photoluminescence
Contact to
Takayuki Makino (Superstructured Thin Film Chemistry Division)
E-mail: tmakino@imr.tohoku.ac.jp
URL: http://www.kawasaki.imr.tohoku.ac.jp/English/top.html

Fig. 1 Spectral evolution of PL with the applied magnetic field perpendicular to the QW plane (B=0-50 T) in different circularpolarizations for modulation-doped ZnO SQW taken at T=4.2 K with Ls= 6 nm. (c) Degree of circular polarization (ρ) and (d) the
peak energies of the PL spectra in both polarizations. (e) An optical transition model, a charged exciton as an initial state of theemissive transition.

KINKEN Research Highlights 2010 IMR

Electronic Materials

31Research

Giant Magnetoresistance and Interface Spin-asymmetry in
Co2MnSi/Ag/Co2MnSi Magneto-Resistive Devices
Epitaxial-Co2MnSi/Ag/Co2MnSi current-perpendicularto-plane magnetoresistive (CPP-GMR) devices were
fabricated by UHV magnetron sputtering systems. The
maximum MR ratio of about 36% at room temperature wasobserved, indicating high spin-polarization of Co2MnSiclearly. A quantitative analysis of bulk and interface spinasymmetry coefficients (β and γ) based on Valet & Fert’s
model showed a large contribution of spin-asymmetricelectron scattering at Co2MnSi/Ag interfaces to the MR
effect, which could be originated from good band matchingof up-spin electrons between Co2MnSi and Ag(001)-planes.

32

Electronic Materials

Fig. 1 High-resolution TEM image for a CMS/Ag/CMS fully
epitaxial film.
20

15∆RA (mΩ·μm2)

Half metals, which have perfectly spin-polarized
conduction electrons, are very important materials in
Spintronics research field, because they are able to enhancevarious kinds of spin-dependent phenomena such as tunnel
and giant magnetoresistance effects, etc. CPP-GMR
structures, which consist of two ferromagnetic layers
separated by a nonmagnetic layer, are promising to observe
half-metallicity at room temperature (RT) because electron
scattering both in the bulk region and at the interface
contributes to the MR effect. In this study, we fabricated thefully epitaxial CPP-GMR devices with a candidate of halfmetal Co2MnSi (CMS) and an Ag spacer and investigatedthe magneto-transport properties systematically.
Figure 1 shows high-resolution TEM images of CMS/Ag/
CMS annealed at 350°C. Fully epitaxial growth with very flatand sharp interfaces was clearly confirmed. In CMS/Ag/CMS,
it was possible to increase the annealing temperature up to
550°C due to small solubility of Ag with CMS. Finally, weobserved maximum MR ratios of 36% and 68% at RT and
100 K, respectively [1], indicating the high spin-polarizationof CMS not only at low temperature but also at RT. CMS
thickness dependence of resistance change area product(ΔRA) for CMS/Ag/CMS is shown in Fig.2. The contributions
from bulk and interface scattering to MR represented by βand γ, respectively, can be quantitatively estimated by thefitting based on Valet-Fert’s 2-current model [2]. The fitting
result shows γ at the CMS/Ag is very large both at RT and
100K, which contributes dominantly to the large MR ratio.
Miura et al. [3] calculated a ballistic conductance in a (001)CMS/Ag/CMS structure and found a large conductance inthe majority spin channel in the parallel state. This good band
matching can be responsible for the observed large γ and the
MR ratio.
Our recent studies on magnetic tunnel junctions basedon CMS have also demonstrated the successful tuning of EF
in the half-metallic gap by Al-doping to CMS [4] and theimportance of CMS composition to the half-metallicity [5].
Improvement of CMS electrodes will further increase the spinpolarization of CMS, and consequently the MR ratio in CPPGMR at RT.

)
00K(@1 = 0.87
°C
500 .72, γ
0
β=

10

)
100K 6C (@350° 5, γ = 0.8
.6
β=0

RT)
500°C (@
, γ = 0.84
β = 0.50

350°C (@RT)
2
β = 0.46, γ = 0.8

5

0

0

2

4

6
8
tCMS (nm)1012Fig. 2 CMS thickness dependence of ΔRA for CMS/Ag/CMS
annealed at 350°C (■) and 500°C (▼).

References[1] T. Iwase, Y. Sakuraba, S. Bosu, K. Saito, S. Mitani, and K.
Takanashi. Appl. Phys. Express 2, 063003 (2009).
[2] T. Valet and A. Fert, Phys. Rev. B, 69, 144413 (2004).
[3] K. Miura et al, unpublished.
[4] Y. Sakuraba et al., Phys. Rev. B, 81, 094402 (2010).
[5] Y. Sakuraba et al., Appl. Phys. Lett., 96, 092511 (2010).
Key Words
Spintronics, Half-Metal, CPP-GMR
Contact toKoki Takanashi (Magnetic Materials Division)
E-mail: koki@imr.tohoku.ac.jp
URL: http://www.magmatex.imr.tohoku.ac.jp/~mml/index.html

IMR KINKEN Research Highlights 2010

Research

Remarkable Benefits Obtained When the Congruent Point of a
Compound Meets the Stoichiometric Point

Many oxides, particularly those that have excellent
properties for devices, often form solid solutions to someextent and their congruent points differ from their stoichiometricpoints. LiNbO3 is a typical oxide that has superior optical andpiezoelectric properties, but its congruent composition has a
Li2O content of 48.5 mol%, which is slightly lower than that of
the stoichiometric composition (50.0 mol%). The congruent
composition is ideal for growing crystals because the crystal
has the same composition as the source melt. However,
because of its nonstoichiometry, congruent LiNbO3 does nothave optimal properties for device applications. In contrast,
stoichiometric LiNbO3, which is very hard to grow, has an
ideal crystal structure in which each element (Li, Nb and O)
completely occupies its own site. Consequently, it has
excellent physical properties so that it is used in various
optoelectronic devices. It has been a long-held dream to makethe congruent point of LiNbO3 coincident with its stoichiometricpoint. However, for over half a century the stoichiometric pointof LiNbO3 has been considered to be fixed.
We have discovered a simple solution that involves
adding MgO to LiNbO3 powder in the ratio Li2O:Nb2O5:MgO= 45.30:50.00:4.70 [1] and use it as the source melt. Using
this method, it is possible to grow an ideal LiNbO3 crystal for
device applications, cs-MgO:LN. LiNbO3 has three sites for
its three constituent elements (Li, Nb and O). If each site iscompletely occupied by its element, the crystal will have thestoichiometric structure (s-LN in Fig. 1). This means thateach site has an activity of unity in the stoichiometric crystal,and thus the activity of each constituent element will also be
unity. The converse is also true, namely if each constituent
element has an activity of unity, then the crystal will have thestoichiometric structure. This is a renovation of Dalton’s
principle of stoichiometry.
We apply this new concept to the development of the ideal
LiNbO3 for device applications (i.e., cs-MgO:LN). cs-MgO:LN
is located on the isoconcentration line of Nb2O5 (Fig. 2) andhas unique sites for O and Nb with activities of unity. In
contrast, Li, Mg and lattice vacancies occupy the Li site (csLi site

Nb siteO sites-LN

c-LN

6MgO:LN

Iso-concentration lineof Nb2O5MgO

7

6MgO:LN

6
mol% MgO

We have developed an ideal LiNbO3 crystal for device
applications half a century after the birth of LiNbO3 in Bell
Laboratories. This involved a renovation of Dalton’s principles
on the stoichiometry of materials, which has been uncritically
accepted for two centuries. The simple addition of MgO
resulted in the formation of a congruent-melting LiNbO3
crystal with a stoichiometric structure.

cs-MgO:LN

5
4(Li2O : Nb2O5 : MgO= 45.3 : 50 : 4.7)

3

c-LN

2

s-LN

1

Nb2O5

46

47

48 49
mol% Li2O

50

Li2O

∆φEMF = 0 @cs-MgO:LN
Fig. 2 cs-MgO:LN on the isoconcentration line of Nb2O5 has
a c-EMF of zero, which demonstrates the realization of bothcongruency and stoichiometry.

Fig. 3 The occurrence of a crystallization EMF, ∆φEMF, due tothe segregation of ionic species in the melt near the interface.
MgO:LN in Fig. 1). The question is whether it is possible to
make the activities of these elements (including vacancies)
unity. The answer is ‘yes’ by selecting appropriate standardstate chemical potentials for them [2]. This has been
demonstrated both analytically and experimentally. We
measured the interfacial potential, crystallization electromotive
force (c-EMF), caused by segregation of ionic species near
the interface (Fig. 3) during crystallization of ionic LiNbO3
melts. cs-MgO:LN has highest melting temperature and ac-EMF of zero, which indicates that there is no segregation at
the congruent point. This result demonstrates the simultaneous
occurrence of congruency and stoichiometry in cs-MgO:LN.
References[1] H. Kimura and S. Uda, J. Cryst. Growth, 311, 4094 (2009).
[2] S. Uda, J. Cryst. Growth, 310, 3335 (2008).

cs-MgO:LN

Key Words
Stoichiometry, Congruent, LiNbO3
：Li+

：Nb5+

：O2‐

：Mg2+

：Vacancy
Fig. 1 Site occupancies in various lithium niobates including
cs-MgO:LN.

KINKEN Research Highlights 2010 IMR

Contact toSatoshi Uda (Crystal Chemistry Division)
Fig. 1E-mail: uda@imr.tohoku.ac.jp

Electronic Materials

33Research

3D Imaging of Magnetic Nanoparticles by Electron Tomography
3D shapes and distribution of FePd nanoparticles (NPs)have been studied by single-axis tilt tomography using
atomic number (Z) contrast. We demonstrate that weighted
back-projection (WBP) yields a better estimation of theparticle size in the z-direction than simultaneous iterative
reconstruction technique (SIRT) does, most likely because of
the presence of a “missing wedge” in the original data set.
Recent developments in ultrahigh density magnetic
storage technology rely on novel recording media with a highmagnetocrystalline anisotropy energy (MAE), in order toincrease storage density and to reduce recording noises.
FePd alloy NPs with the L10-type ordered structure is one of
the candidate materials suitable for the ultrahigh density
magnetic storage media. Here the atomic ordering is a key
issue for improving the hard magnetic properties [1, 2].
Therefore previous studies have focused on the atomic
structure inside the NPs. On the other hand, the high-areal
density packing of NPs and the control of magnetostaticinteraction among the NPs are considered as the next step torealize ultrahigh density magnetic storage media. For thispurpose, it is desired to visualize and understand 3D
structures of NPs and spatial configuration of neighboring
NPs precisely by means of electron tomography.
The FePd alloy NPs studied were formed as epitaxial
islands on a NaCl(001) substrate using sequential depositionof Pd and Fe followed by postdeposition annealing. The tilt
series of Z-contrast images were obtained using an FEI Titan
80-300 scanning transmission electron microscope (STEM)
operating at 300 kV. Alignment of the tilt axis for the obtained
data set by an iterative cross-correlation technique andsubsequent 3D reconstruction were performed by using theInspect3D software package. As for the algorithm for 3D
reconstruction, we employed WBP, as well as SIRT.
Figure 1 compares reconstructed images processed by
WBP (a) and SIRT (b), viewed from an oblique direction [3].
The tilt axis is the x axis, about which the specimen film issequentially tilted toward the y direction. It is noted that somefloating dotlike artifacts, as indicated by the arrows, are seenin the reconstructed image obtained with WBP, while theFig. 2 The relation between particle diameter and thickness(height) for the FePd NPs estimated by using several different
techniques. The elongation factor (eyz) is 1.42 for the present
experimental conditions.
result by the SIRT shows a smooth surface with little apparent
artifacts. Besides the floating artifacts, a significant difference
between these two images can be noticed. NPs in the upper
image show oblate 3D-shapes, while those in the lower
image are prolate.
To examine a possible elongation of a reconstructedparticle height in the z direction, we therefore measured
projected particle length in the y direction as a function of tilt
angle and deduced the true particle height by extrapolating the
projected length to the value expected at the tilt angle α = 90°.
Using the aforementioned procedure, here termed “tiltseries extrapolation (TSE) method”, we obtained a relation,
which summarizes the relation between particle diameterand thickness estimated by using several different techniques(Fig.2). Note that the deduced thicknesses obtained by the
TSE agree well with those obtained by WBP (tz / eyz) as wellas those by electron holography. On the other hand, thethicknesses suggested by SIRT are much larger than thevalues deduced by the TSE method or electron holography.
Therefore, within a framework of single-axis tilt geometry, it isdemonstrated in a semiquantitative manner that the WBPgives a better result in terms of the accuracy of the particle
length in the z direction than that predicted by SIRT [3]. It isdemonstrated that TSE method is found to be reliable anduseful in order to examine the accuracy and resolution of 3D
reconstructed information provided by different algorithms.
References[1] K. Sato, Nature Mater. 8, 924 (2009).
[2] A. Kovács, K. Sato et al., Phys. Rev. Lett. 103, 115703(2009).
[3] K. Sato, K. Aoyagi, and T. J. Konno, J. Appl. Phys. 107,024304 (2010).

Fig. 1 Oblique-view of the reconstructed volume processed
by WBP (a) and SIRT (b). Large discrepancy in particle
thickness (height) is apparent between these two images.
For 3D reconstruction, 81 images taken at tilt angles between-66º and +64º were employed.

34

Electronic Materials

Key Words
Electron Tomography, 3D Structure, Nanoparticle
Contact toKazuhisa Sato (Advanced Analysis of Materials Division)
E-mail: ksato@imr.tohoku.ac.jp

IMR KINKEN Research Highlights 2010

Research

Observation of Magnetic Excitation in
Quantum Spin System at J-PARC
Entire excitation spectrum of the quantum spin system
CuGeO3 was successfully observed by using the new
chopper spectrometer 4SEASONS in J-PARC. This
demonstration opens up the possibility of study on complex
dynamics in the strongly correlated electron systems, whichplays an important role for the emergence of new functions.
In strongly correlated electron systems, new phenomena
can appear due to the complex combination among degrees
of freedom of electron (charge, spin and orbital). To
understand the mechanism, it is important to obtain theinformation of not only basic structure but also the dynamics
in intriguing materials, which is strongly coupled withelectrons. For the study of dynamics, neutron-scatteringexperiments, which can measure the collective behavior ofspins or nuclei in a system, provide important information.
However, up to now, because of the insufficient intensityof available neutron beam, the observation of excitation
spectrum was limited in a part of energy-momentum space.
In order to make a progress in the study of dynamics by theneutron-scattering measurement, we developed a new
method, which can efficiently collect the date with several
different experimental conditions. We have first attempted
the method to the observation of spin excitation spectrum ina quantum spin system CuGeO3, on the chopper spectrometer
4SEASONS installed in J-PARC [1]. The sample used in theexperiment was assembled single crystals with the total
weight of ~30 grams. (See Fig.1)
As clearly seen in Fig. 2, in a single measurement, theentire spectrum of two-spinon continuum was detected above
the low-lying mode. This observation is consistent with thepreviously obtained result at the ISIS facility in UK, whichused to provide the most intense pulsed neutron beam when
the first high-energy neutron-scattering measurement on
CuGeO3 was done [2]. Moreover, the intensity map with thelower incident neutron energy and better experimental

Fig. 2 Spin excitation spectrum in the quantum spin system
CuGeO3. Color indicates the measured neutron intensity.
Whole excitation continuum of two-spinon was successfully
observed in a wide energy-momentum space.

resolution, which was collected at the same time, indicatesan opening of spin gap (Eg~2meV) due to a formation of spin
singlet below 14K. (Not shows.)
The simultaneous observation of entire and detailed
structure of excitation spectrum is expected to be effective
for the exploration of complex dynamics originated from acoupling among plural degrees of freedom of the electron ina condensed matter. With focusing on a study of spin
excitations and phonons in high-transition-temperature
superconductivity, we are developing the technique ofneutron-scattering measurement at J-PARC.
References[1] M. Nakamura, R. Kajimoto, Y. Inamura, F. Mizuno, M.
Fujita, T. Yokoo, and M. Arai, J. Phys. Soc. Jpn. 78, 093002(2009).
[2] M. Arai, M. Fujita, M. Motokawa, J. Akimitsu, and S. M.
Bennington, Phys. Rev. Lett. 77, 3649 (1996).
Key Words
Neutron-scattering Measurement, Quantum Spin System,
Spin Excitation
Contact to
Masaki

Fujita(Materials

Processing

andCharacterization

Division)
E-mail: fujita@imr.tohoku.ac.jp
	URL: http://www.imr.tohoku.ac.jp/jpn/research/bumon/m_
proces/06.html

Fig. 1 Assembled single crystals of the spin-Peierls compound
CuGeO3.

KINKEN Research Highlights 2010 IMR

Electronic Materials

35Research

AC Impedance Analysis of a Ni-Nb-Zr-H Glassy Alloy with
Femtofarad Capacitance Tunnels
A Nyquist diagram of a (Ni0.36Nb0.24Zr0.40)90H10 glassy
alloy shows a semi-true circle, indicating that it is aconducting material with a total capacitance of 17.8 μF. The
Bode plots showing the dependencies of its real andimaginary impedances, and phase on frequency suggest asimpler equivalent circuit having a resistor in parallel with acapacitor. Dividing the total capacitance (17.8μF) by thecapacitance of a single tunnel (0.9fF), we deduced that thismaterial has a high number of dielectric tunnels, which canbe regarded as regular prisms separated from the electricconducting distorted icosahedral Zr5Ni5Nb3 clusters by anaverage of 0.225 nm [1].
Following the discovery of the Coulomb blockade effectby quantum-dot tunneling at low temperature, a numberof studies have reported the achievement of roomtemperature oscillation. Recently, Fukuhara et al. haveobserved the electric current-induced voltage oscillation in((Ni0.6Nb0.4)0.7Zr0.3) 1-yHy glassy alloys (with 0.052 ≤ y ≤0.152) at temperatures below 240 K [2] and at room
temperature [3]. Thus, we regarded it as a DC/AC converting
device with a large number of nanometer size capacitors.
To investigate the distribution state of vacancies in
eccentric glassy alloys, we determined an AC impedance ofa glassy alloy consisting of nanometer-sized clusters [4].
Significantly, no research work has been carried out
previously on the AC impedance analysis of Ni-Nb-Zr-H
glassy alloys with femtofarad capacitances. Our results
indicate that the glassy alloys have potential applications as
nonwired, room-temperature quantum devices such asbatteries, amplifiers, and memory switches.
To nondestructively analyze electronic contribution of theglassy alloy without grain boundaries, we measured the AC
impedance in a simple circuit in which the specimen was
integrated at room temperature, as a function of the appliedfrequency. The Nyquist (complex impedance plane) plot forthe (Ni0.36Nb0.24Zr0.40)90H10 alloy is shown in Fig. 1(a). The
alloy’s variation of impedance with frequency followed asemi-true circle, showing an ideal Debye relaxation peak. A
reactance, R, of 6.6 Ω and a relaxation time, RCtotal, of 1.2 x
10-4 s at the summit of the semicircle were derived from theformula RCtotal = 1/(2πfmax), where fmax (= 1,350 Hz) is thefrequency of the peak. Thus, the total capacitance, Ctotal, of0.225 nm
0.23
nm

1nm
nm
electrong ～0.55
elect rotunnelinn tunneling
electronelect ro nis lan dZrislandNi

NbislandGlassy alloy
nanotunnels

is landislandFig. 2 Configuration pattern of insulating zigzag tunnels
among the distorted icosahedral Zr5Ni5Nb3 clusters (quantum
dot) arranged at intervals of 0.225 nm (a). A glassy alloy has
numerous insulating nanotunnels among conducting
clusters (b).
the specimen was calculated to be 1.8 x 10-5 F.
The Bode plots (b) illustrate the dependencies of the
impedance (Z) and phase (θ) on the frequency (f). The
impedance and phase remained constant as the frequency
rose to 600 and 25,000 Hz, respectively. Then, each of themremarkably decreased as the frequency increased up to 2
MHz: the impedance decreased as a third-order polynomial,and the phase decreased as a parabolic. From these results,
we hypothesize the existence of numerous dielectric tunnels
in the glassy alloy. Dividing the total capacitance (17.8μF) by
the capacitance of a single tunnel (0.9 fF), the number oftunnels was calculated to be 2.0 x 1010. Because the volumeof the specimen was 0.60 mm3 (0.04 mm x 1.0 mm x 15 mm),the free volume of 2.57% was 0.015 mm3. Assuming that thetunnel is a regular prism of side d and a length of 15 mm, we
obtain the side d = √0.015/2.0E10/15= 0.225 nm.
References[1] M.Fukuhara, M.Seto and A.Inoue, Appl.Phys.Lett., 96,043103 (2010).
[2] M.Fukuhara, A.Kawashima,S.Yamaura and A.Inoue,
Appl.Phys.Lett., 90, 20311 (2007).
[3] M.Fukuhara and A.Inoue, J.Appl.Pjys., 105, 063715(2009).
[4] M.Fukuhara, N.Fujima, H.Oji, A.Inoue and S.Emura, J.
Alloy Comp., 497, 182 (2010).
Key Words
Nano Quantum Dot Tunnelling, Nanocluster, Nanocapacitor
Contact toMikio Fukuhara*  *	Present Address: Advanced Materials Development andIntegration of Novel Structured Metallic and Inorganic Materials
E-mail: fukuhara@imr.tohoku.ac.jp

Fig. 1 Nyquist plots (a), and Bode plots showing theimpedance (log Z) and the phase (θ) vs the frequency (log f)(b) for the (Ni0.36Nb0.24Zr0.40)90H10 glassy alloy.

36

Electronic Materials

IMR KINKEN Research Highlights 2010

Research

Transport Properties in Ferromagnetic TM-Al-O (TM = Fe, Co)
Granular Films with High TM Concentrations
The magnetic and transport properties of TM-Al-O (TM =Fe, Co) granular films strongly depend on the TM
concentration. We found that the transport properties offerromagnetic TM-Al-O films with high TM concentration canbe described by the scattering of conduction electrons dueto thermally excited ferromagnetic spin waves arising in thestrongly coupled granular structure, which is a contrast to
the spin-dependent tunneling process observed insuperparamagnetic films with low TM concentration.
TM-Al-O (TM = Fe, Co) granular films, consisting of nanosize TM granules and an insulating Al-O matrix, haveattracted much attention as materials for various magnetic
devices because of their high resistivity and soft magnetization.
The magnetic and transport properties strongly depend on
the TM concentration x. When x is as low as 40 - 60 at. %, thefilms are superparamagnetic and exhibit giant magnetic
resistance. In this regime, the electrical resistivity ρ increases
drastically with decreasing temperature T, following therelation ln ρ ∝ (1/T)1/2. With increasing x, the films become
ferromagnetic with a soft magnetic property and show ametallic transport property around x = 60 - 70 at. %. Until
now, the transport mechanism in the superparamagnetic TMAl-O films is well established to be the spin-dependent
electron tunneling between TM granules. However, details oftransport properties of ferromagnetic TM-Al-O films have notbeen fully examined yet. We systematically investigated ρ ofthem and clarified their characteristic properties [1].
Fe-Al-O and Co-Al-O granular films of 1-2 μm thick,containing Fe (Co) granules with diameter of 2-3 nm, wereprepared on glass substrates using radio frequency
magnetron sputtering. Figure 1 displays the low temperatureρ of Fe-Al-O films for various x as a function of T2. We found
the relation described as ρ(T) = ρ(0) + AT2 for all the samples
in the temperature range of 50 - 150 K with the coefficient A
decreasing drastically with increasing x. At temperatures
lower than 50 K, ρ deviates from the T2 behavior, andincreases logarithmically according to the formula ρ(T) = ρ(0)Fig. 2 Temperature dependence of the electrical resistivity offerromagnetic Fe-Al-O granular films on an expanded logT
scale for Fe concentrations of (a) 62, (b) 64, (c) 76 at. %. The
solid lines are fits to the formula ρ(T) = ρ(0) - BlogT.
– BlogT as shown in Fig. 2. The same types of results wereobserved also in Co-Al-O films [1]. Therefore, the resistivity
described with the combination of T2 and –logT terms can bea characteristic of the ferromagnetic TM-Al-O films.
The T2 behavior is theoretically predicted to occur in pure
ferromagnetic metals as a consequence of the scattering ofconduction electrons by long spin waves. Recently, Yoshihara
et al. observed thermally excited spin waves in ferromagnetic
TM-Al-O films using Brillouin light scattering (BLS), [2]. The
obtained wavelength is about 300 nm, which is much longer
than the diameters of granules, leading to the possible
existence of the spin waves even in granular system. Since
the decrease in the coefficient A with increasing x is
consistently related to the increase in the exchange field Hex
characterizing the spin wave, which is derived from theanalyses of BLS spectra [1,2], the T2 behavior in the
ferromagnetic TM-Al-O films can be ascribed to the scatteringof conduction electrons by thermally excited spin waves. Thisis a good contrast to the spin-dependent tunneling process insuperparamagnetic TM-Al-O films. Although the origin of thelogarithmic behavior remains open question, we note that
this is robust against the magnetic field up to 8 T.
References[1] S. Nakamura, T. Nojima, A. Yshihara, S. Ohnuma, and H.
Fujimori, J. Phys. Soc. Jpn. 78, 074708 (2009).
[2] A. Yoshihara, S. Ohnuma, H. Fujimori, S. Nakamura and
T. Nojima, J. Phys. Soc. Jpn. 77, 094704 (2008).
Key Words
Ferromagnetic Granular Films, Electrical Resistivity, Spin
Wave

Fig. 1 Temperature dependence of the electrical resistivity offerromagnetic Fe-Al-O granular films as a function of T2 for
Fe concentrations of (a) 62, (b) 64, (c) 76 at. %. The solidlines are fits to the formula ρ(T) = ρ(0) + AT2.

KINKEN Research Highlights 2010 IMR

Contact to
Tsutomu Nojima (Laboratory of Low Temperature Materials
Science )E-mail: nojima@imr.tohoku.ac.jp

Electronic Materials

37Research

Ferromagnetism in Semihydrogenated Graphene Sheet
Graphene has been one of the most interesting andexciting discoveries in recent years, which is also be viewed
as the next generation of nano-electronic devices. Firstprinciples calculation is a powerful investigation tool topredict and design novel graphene-based nanostructures.
The present work showed an interesting behavior of semihydrogenated graphene sheet, which possesses uniform
2D ferromagnetism. The method used in this paper is basedon density functional theory. All the numerical calculations
have been conducted by using the HITACHI SR11000
supercomputer at the Center for Computational Materials
Science, IMR.
Fully hydrogenated graphene sheet has been
experimentally synthesized. However, studies of magnetic
behavior of graphene haven’t been achieved to a satisfactory
level. With the help of first-principles calculation, geometry,stability, magnetism and electronic structure ofsemihydrogenated graphene have been predicted. And
uniform 2D ferromagnetism can be achieved [1].
Figure 1 shows the optimized geometric structure ofsemihydrogenated graphene. The sheet has a slightly buckle
and the C1 atoms are sp3 hybridized, while C2 atoms areapproximated sp2 hybridized.
Figure 2 shows the spin density of the structure in 2D
slice and iso-surface forms. It can be seen that C2 atoms areferromagnetically coupled with each other, with magnetism
contributed mainly by pz orbital. Each C2 atom gives around
1 μB magnetic moment. The mechanism of magnetism is dueto the broken of delocalized π orbital of graphene by H atoms,
thus pz electrons from unsaturated C2 atoms are unpaired.
States of antiferromagnetism and non-magnetism areenergetically unfavorable by 38 meV and 123 meV per unit
cell, respectively. And correspondingly, the Curie temperature
can be approximated through mean field theory between 278
and 417 K, which shows that the magnetism can be retained
under room temperature.
The calculated band structure are plotted in Figure 3,
which indicate that the semihydrogenated graphene is anindirect semiconductor, with band gap of ~0.46 eV.

Fig. 2 2D slice and iso-surface of spin density of ferromagnetic
semihydrogenated graphene.

Fig. 3 Electronic band structure of semihydrogenatedgraphene sheet. G (0, 0, 0), K (-1/3, 2/3, 0), and F (0, 1/2, 0)
are high symmetric points in reciprocal space.

References[1] J. Zhou, Q. Wang, Q. Sun, X. S. Chen, Y. Kawazoe, P.
Jena, Nano Lett. 9, 3867 (2009).
Key Words
Graphene, ab Initio Calculation, 2D Ferromagnetic Material
Fig. 1 Geometry of semihydrogenated graphene.
Contact toYoshiyuki Kawazoe (Materials Design by Computer Simulation
Division)
E-mail: kawazoe@imr.edu

38

Electronic Materials

IMR KINKEN Research Highlights 2010

Research

Possible d0 Ferromagnetism in MgO Doped with Nitrogen
Using recently developed numerical method we study
the possibility of d0 ferromagnetism in MgO doped withnitrogen. By realistic parameters, we observe a finite local
moment for an N impurity, and long-range ferromagnetic
correlations between two N impurities. The ferromagnetic
correlations are strongly influenced by the impurity boundstate [1].
The discovery of ferromagnetism (FM) in dilute magnetic
semiconductor (DMS) has created much activity in the fieldof spintronics. There is the so called d0 DMS, where themagnetism is coming from moments induced in the p orbitalsof the oxygen band instead of from partially filled d orbitals.
The experimentally observed d0 DMS are quite few now. As
a promising candidate, the FM in MgO by nitrogen doping
has been observed in recent experiment.
Considering the importance of spin and charge
fluctuations in d0 DMS, it is important, therefore, to develop
methods which treats correlations correctly and that do not
rely on arbitrary approximations or on the separation between
charge, spin and orbital fluctuations. Such a method has
been introduced and applied to models of the DMS [2],namely combined approach of tight-binding approximation(TBA) or density functional theory (DFT) and Hirsch-Fye
quantum Monte Carlo (QMC) method.
When two N impurities are introduced in the MgO host,the magnetic correlation function <Mz1Mz2> between ξ = x
orbitals of the impurities versus the impurity separation attemperature T = 200 K is shown in Fig. 1. The impurity spins
exhibit ferromagnetic (FM) correlations at chemical potential
μ= 0.0 eV. By increasing μ to 0.06 eV, the FM correlations

become larger, and the range of the FM correlations becomes
longer. Further increasing μ, the FM correlations become
weaker. This is because the impurity bound states of Mg(O,N)
becomes occupied when μ is increased to above 0.06 eV.
Results in Fig. 1(b) are obtained with fixed impurity energy
εp = -0.5 eV. The impurity spins exhibit quite weak
antiferromagnetic (AFM) correlations due to thesuperexchange interaction at μ = 0.0 eV. The impurity boundstates of Mg(O,N) lies around 0.35 eV, deep in the band gap(not shown here). Increasing μ to 0.3 eV, the FM correlations
appear with stronger magnitude and longer range.
References[1] B. Gu, N. Bulut, T. Ziman, and S. Maekawa, Phys. Rev. B
79, 024407 (2009).
[2] B. Gu, N. Bulut, and S. Maekawa, J. Appl. Phys. 104,
103906 (2008); J. Ohe, Y. Tomoda, N. Bulut, R. Arita, K.
Nakamura, and S. Maekewa, J. Phys. Soc. Jpn. 78, 083703(2009).
Key Words
Diluted Magnetic Semiconductor, MgO, Quantum Monte
Carlo Method
Contact to
Sadamichi Maekawa* (Theory of Solid State Physics Division)  * Present Address: Advanced Science Research Center, Japan
Atomic Energy Agency
E-mail: maekawa.sadamichi@jaea.go.jp
URL: http://asrc.jaea.go.jp/zz<M1M2>

0.2(a) R//(0.5,0,0.5), ξ = x, T = 200 K
ε p = -1.5 eVµ = 0.00 eV
0.1
0.02 eV0.06 eVzz<M1M2>

00.2(b)

ε p = -0.5 eVµ = 0.00 eV
0.25 eV
0.30 eV

0.1
0
0

1

R/a

2

3

Fig. 1 Magnetic correlation function <Mz1Mz2> between ξ = x
orbitals of the N impurities versus distance R/a for (a) the
impurity 2p energy εp= -1.5 eV and (b) εp= -0.5 eV at T = 200
K. Direction R//(0.5,0,0.5) is along one of the 12 nearest N-Nneighbors in rocksalt structure.

KINKEN Research Highlights 2010 IMR

Electronic Materials

39