Research Highlights

Discovery of simple and versatile methods for spin current
generation
H. Adachi1,2), J. Ieda1,2), S. Takahashi3), K. Uchida3), K. Ando3), E. Saitoh2,3,4), and S. Maekawa1,2)
1) R. G. for Condensed Matter Theory, ASRC, JAEA 2) CREST, JST. 3) Tohoku University4) R. G. for Mechanical Control of Materials and Spin Systems, ASRC, JAEA

Electronics is indispensable in our daily life in theinformation society. As the next-generation technology,“spintronics” has attracted global interest. The “spin current” isthe main ingredient of spintronics, and we have devised simple
and versatile methods of generating spin current.
As the name implied, electronics is the art of controllingelectrons in solids. The electron has two aspects: “charge” and“spin”. The charge is the origin of electricity, and its flow leads
to an electric (charge) current. On the other hand, the spin gives
rise to magnetism and its flow is called a spin current. To date,developments in the field of electronics have been based solely
on the charge current. Spintronics aims to improve the currenttechnology by harnessing both charge and spin currents equally.
It has been recognized, however, that it is quite a hard task toutilize spin currents unlike the case of charge currents. In thiscontext, we have discovered new methods for generating spincurrents.
Our method employs the spin transfer without chargetransfer from a ferromagnet into an attached nonmagnetic
conductor (metal or semiconductor), whose process is driven
by the magnetization dynamics in the ferromagnet. When amagnetization in a ferromagnet is excited, it starts precession.
Since a nonmagnetic conductor acts as a spin absorber, attachinga nonmagnetic conductor to the excited ferromagnet results
in an emission of a certain amount of spins into the conductor
as a result of the angular momentum conservation. Therefore,
the spin current is generated in the nonmagnetic conductor
without charge transfer across the interface of the ferromagnet/
nonmagnetic conductor bilayers. This is a simple and versatilemethod because any perturbation exciting the magnetization
dynamics enables this new type of spin current generation. We
have demonstrated that this is indeed the case in the followingways.
First, we have shown that the spin current can be generated
by sound waves. In a hybrid structure of a nonmagnetic metal(Pt) and an insulating ferromagnet (Y3Fe5O12) as schematicallyshown in Fig. 1, a propagating sound wave is generated bya piezoelectric actuator attached to the ferromagnet. The
sound wave then excites magnetization dynamics through themagneto-elastic coupling, thereby injects a spin current into
the nonmagnetic metal. The spin current obtained is converted
into an electric current via the spin-Hall effect. Thus, we haveestablished a new route for generating spin and electric currents
by ubiquitous sounds. This mechanism will be useful for
constructing new spintronic and energy-saving devices.
Second, we have succeeded in injecting spin currents into
semiconductors with a very high efficiency (103 times larger
than before). So far, any attempt to generate spin currents insemiconductors has relied on the spin-polarized charge transferfrom ferromagnets into semiconductors. However, the prevous
attempts suffer from the so-called impedance mismatch problem
arising from the extreme difference of the electric conductivity
between ferromagnets and semiconductors. Using a hybrid
structure of a semiconductor (GaAs) and a ferromagnet(Ni81Fe19) shown in Fig. 2 and by applying a microwave to theferromagnet, we have shown that a high efficiency spin injection
into GaAs is possible. This technique relies purely on the spintransfer caused by microwave-excited magnetization dynamics,
and is free from the impedance mismatch problem that arises in
the charge transfer. Since semiconductors are basic materials forconventional electronic devices, the impact of this finding on thespintronics community is astonishingly large.
A variety of such high-efficiency methods for spin current
generation can make progress in spintronics, which in turncontributes to the development of energy-saving society as
spintronics offers a route to reduce power consumption of solidstate logic devices.

Fig. 1 Schematic illustration of the device structure to realize
the new way of spin current generation by sound waves.

Fig. 2 Schematic illustration of the device structure to realize
the new way of spin current generation by microwaves.
References[1] K. Uchida et al., Nature Materials 10, 737 (2011).
[2] K. Ando et al., Nature Materials 10, 655 (2011).

ASRC 2