Research Group for Spin-Energy Transformation ScienceGroup Leader: Eiji Saitoh
Members: Michiyasu Mori, Hiroaki Onishi, Jun’ichi Ieda, Mamoru Matsuo, Bo Gu, Yuichi
Ohnuma, Koki Nakata, Kei Yamamoto, Satoru Okayasu, Masao Ono, Hiroyuki Chudo, Yudai
Ogata, Kazuya Harii, Masaki Imai

Spin and orbital angular momentum are the origin of magnetism.
We quest to extend these degrees of freedoms for energy
sustainability. One strategy is to use for transformation between
thermal and electric energies via spin, i.e., spin Seebeck effectand its inverse one called spin Peltier effect [1]. This subject isexplained in Research Highlights. Another effort is devoted toconversion between magnetism and mechanical motion such asrotation and vibration. Two interesting results are reported
below.
Spin Current Generation by Surface Acoustic Wave [2]In a rotating frame, a mechanical rotation is equivalent to amagnetic field. In fact, a conversion from a magnetic moment ofspin to a macroscopic rotation was experimentally demonstrated
in a ferromagnet by Einstein and de Haas, and its inverse effectis done by Barnett. We successfully generate an alternating spincurrent (SC) using a surface acoustic wave (SAW) in a Cu film
via spin-rotation coupling (SRC). A SAW generates a gradientof vorticity, i.e., rotational deformation of lattice, in a depth
direction. The SRC directly converts the vorticity to electron
spin in the metallic film. Hence, we can generate a spin current
in a nonmagnetic material by the SAW. Figure 1 (a) shows abasic principle of our experiment to create an alternating SC
generated by a SAW. The Cu and NiFe layers shown in Fig. 1(a)act as source and detector of the SC, respectively. An injected
SC into NiFe layer is observed by its ferromagnetic resonance(FMR) and the interdigital transducers (IDTs) are used fortransmitter and detector (see Fig.1 (b)). When a frequency of
SAW matches to that of FMR frequency, the FMR is excited ina NiFe layer. It is confirmed that the resonance is suppressed byinserting a SiO2 film at the interface of Cu and NiFe layers. Thisis the clear evidence that the SC is injected by the SAW. The
intensity of the resonance depended on the angle between thewave vector of the SAW and the magnetization of the Ni-Fefilm. This angular dependence is associated with the presence ofspin transfer torque from a SC generated via SRC. We further
found that the amplitude of SC increases with the thickness ofCu layer. Our experimental results open pathways to thegeneration of alternating SC in SAW devices without
ferromagnets and/or large spin-orbit interaction.

Magnetic Control of Cantilever [3]A cantilever is a long rigid plate, whose one end is supported
tightly and the other end can mount a load. Thanks to its highsensitivity and feasible fabrication in large area, the cantilever isa good tool to study an interplay between magnetism andmechanical motion. We fabricate a cantilever of ferrimagnet,
Y3Fe5O12 (YIG) with 0.8×0.9×80 m. Vibration spectra aremeasured by a laser Doppler vibrometer at room temperature asshown in Fig. 1 (a). The obtained minimum detectable force isas small as 10-16 N. This is much less than that usedcommercially in the atomic force microscopy. In a magnetic
field, resonance frequencies of both (horizontal and vertical)
vibrations are changed as shown in Fig. 1 (b), where a case ofnon-magnetic cantilever using gadolinium gallium garnet(GGG) is also shown as reference. The resonance frequency ofthe horizontal mode steeply increases with magnetic field. Also
for the vertical mode, the similar strong magnetic fielddependence is observed especially at low magnetic fields.
However, on the contrary to the horizontal mode, the resonancefrequency of the vertical mode decreases with increasing
magnetic fields. Hence, the clear difference in the response tomagnetic field is observed between the two modes. Thisdifference can be understand by an effective spring constantassociated with a magnetic force gradient, which is induced by astray field of surrounding YIG film (See also Fig. 4 in Ref. [2]).
Since YIG has been typically used for various spin current
phenomena, the YIG cantilever will be useful for the mechanicaldetection of spin currents.

Fig.2 (a) A schematic illustration of cantilever and setup of ourmeasurement. (b) The frequency shifts with magnetic fields areplotted as a function of external magnetic fields for the YIG andGGG cantilevers. The solid and dashed curves are fitting results ofthe experimental data.

Fig.1 Schematic illustrations of (a) spin-current (Js) generation via
spin-rotation coupling in surface acoustic wave (SAW) and (b)experimental setup for measuring microwave absorption caused by
the ferromagnetic excitation.

References[1] Y. Ohnuma et al., Phys. Rev. B 96, 134412 (2017).
[2] D. Kobayashi et al., Phys. Rev. Lett. 119, 077202 (2017).
[3] Y. Seo et al., Appl. Phys. Lett. 110, 132409 (2017).