IMR KINKEN Research Highlights　2020

概要

IMR KINKEN Research Highlights 2020
著者
journal or
publication title
year
URL

Institute for Materials Research Tohoku
University
IMR KINKEN Research Highlights
2020-07
http://hdl.handle.net/10097/00135222

Electronic Materials

IMR KINKEN Research Highlights 2020

Research

Electronic Materials

Electrical Manipulation of Magnetization in Topological
Semimetals

We theoretically study the spin-transfer torque induced by an electric field and analyze the dynamics of the

magnetic domain walls in magnetic Weyl semimetals. Using several models for magnetic Weyl semimetals, we
predict that the spin-transfer torque can be significantly enhanced. Moreover, due to the suppressed longitudinal
conductivity by thin domain-walls, the dissipation by Joule heating associated with the spin-transfer torque
becomes much smaller than that in bulk metallic ferromagnets. Consequently, domain wall motion can be
efficiently controlled with low energy consumption in Weyl semimetals, as required for spintronic devices.

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

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

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

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

24

IMR KINKEN Research Highlights 2020

IMR KINKEN Research Highlights 2020

The Beginning of Paramagnetic Spintronics

I

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

Current
B

GGG

Pt

150

V (nV)

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

Pt/GGG/Pt
T=5K

100
50
T = 300 K
0
-4

-2

0
B (T)

2

4

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

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

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

IMR KINKEN Research Highlights 2020

25

Electronic Materials

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

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

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

Hall voltage (mV)

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

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