Research Group for Spin-Energy Transformation ScienceGroup Leader: SAITOH Eiji
Members: MORI Michiyasu, ONISHI Hiroaki, IEDA Jun’ichi, NAKATA Koki, YAMAMOTOKei, ARAKI Yasufumi, OKAYASU Satoru, ONO Masao, CHUDO Hiroyuki, SATO Nana,
HARII Kazuya, IMAI Masaki

Our research objective is to mutually transform various types ofenergy, e.g., heat, mechanical vibration, and electricity by using
spin. The spin Seebeck effect is a kind of thermoelectric
generation using a bilayer of metal and ferromagnet. We foundthat this effect can induce a mechanical vibration as well [1].
Reciprocity is one of important characteristics of spin, which
can be seen in our Research Highlight. Some ferrimagnets will
be useful for magnetic memory since a magnetic domain-wallmoves very fast around an angular momentum compensation
temperature, which is observed by the Barnet effect and iscontrolled to room temperature by substitution of ions [2].
Spin Seebeck Mechanical Force [1]Magnetic properties of materials originate from a spin ofelectron. As with an orbital angular momentum, roughly
speaking, a spin is also regarded as microscopic rotation. Then,a question arises: Is it possible to induce mechanical rotation byspin? Spin injection can be done by a spin current, which is aflow of magnetism. In the spin Seebeck effect, the spin current
is generated by a temperature gradient. The spin Seebeck effectis a kind of thermoelectric generation using a bilayer of metal
and magnet. By applying a temperature gradient perpendicular
to the bilayer, for example, the spin current is generated in themagnet and is injected into the metal, where the spin current isconverted into a charge current. We fabricated a cantilever ofyttrium iron garnet (YIG) by using ion focus beam (Fig.1). In
order to inject a spin current into this cantilever, a heater madeof platinum (Pt) thin wire was formed near the base of thecantilever as a heat source. A heat flow is generated by analternating current through this heater, and the spin current
generated by the spin Seebeck effect propagates toward thecantilever. When a spin current was not injected, only small
vibrations due to environmental noise were observed. When aspin current was injected, a clear signal appeared in thevibration of the cantilever. By measuring a current anddirectional dependence of the magnetic field, it was confirmedthat the signal is consistent with the force generated by the spincurrent injection.

Angular momentum compensation of ferrimagnet observedby Barnett effect [2]Ferrimagnet is a kind of ferroganet theoretically predicted byNeel. Among several types of ferrimagnets, one called N-type
shows a magnetization compensation, at which a totalmagnetization becomes zero below Curie temperature. Since amagnetization corresponds to a sum of total angular momentum
and spin angular mometum, a angular momentum compensation(AMC) is also possible and becomes a key characteristic ofspintronics. In fact, the high-speed magnetic response at theAMC temperature is reported. We reported the direct
observation of AMC by using the Barnet effect. On a rotating
frame, the rotational frequency couples to an angularmomentum itself instead of magnetization. This is the reason
why we could successfully observed AMC and we must use theBarnet effect. Figure 2 (left) shows our apparatus. The rotor
system was placed in magnetic shields to exclude thegeomagnetic field. The sample was rotated using compressed air
with angular velocity Ω and magnetized to MΩ by the rotation.
We measured the stray field from MΩ using a fluxgate magneticsensor. By applying this method, we observed AMC in theinsulating rare earth iron garnet (RIG) Ho3-xDyxFe5O12 as shownin Fig. 2 (right), where the temperature dependences of MΩ areplotted. The AMC can be controlled by Dy substitution, and it
found that AMC coincides with room temperature at x~1.5 The
advantage of the Barnett effect is that microfabrication of thesample is not necessary regardless of metal or insulator. The
Barnett effect enables us to explore candidate materials for highspeed magnetic devices by exploiting fast-magnetization
reversal at AMC.

Pt
YIG

Fig.1 Schematic illustration of spin Seebeck mechanical force. When a partof a magnetic insulator is heated, a spin wave is excited and flows out of thepart. A spin wave carries angular momentum. Relaxation of spin waves
generates macroscopic mechanical torque and force due to the angularmomentum transfer

Fig.2 Left is schematic illustration of the apparatus for the Barnett effectmeasurement using an air-driven rotor system. The rotor system wasplaced in magnetic shields to exclude the geomagnetic field. The sample
was rotated using compressed air with angular velocity Ω andmagnetized to MΩby the rotation. We measured the stray field from MΩusing a fluxgate magnetic sensor. Right is Temperature dependence ofMΩ of Ho3-xDyxFe5O12 (x=0 and 0.5). The solid lines are guides to theeye. The magnetization and the angular momentum compensation
temperatures are denoted by TM and TA, respectively.

References[1] K. Harii et al., Nat. Commun. 10, 2616 (2019).
[2] M. Imai et al., Appl. Phys. Lett. 114, 162402 (2019).