Advanced Science Research

Generating Magnetic Flows with Sound=Discovery of a Link between Sound and Modern Spintronics=(d)(f)
VResonance frequency = 7.2 MHz(rotation of electron)

ChargeMagnet(b) Charge current
Up-spinelectronPiezoelectric
actuator(e)(c) Spin current

V
Voltage

Up-spinelectron

Platinum
MagnetSpin
current

Soundwave

5.4 MHz
3.6 MHz

7.2 MHz
5.4 MHz
3.6 MHz

0Down-spin
electronDown-spinelectronFig.7-4 Electrons, charge currents, and spin currents(a) Charge and spin, the two properties of electrons. (b)Up-spin and down-spin electrons move in the same
direction for charge current generation. (c) Up-spin anddown-spin electrons move in the opposite direction forspin current generation.

Electronics form the basis of the modern informationsociety. Spintronics, a new electronic technology, has
drawn global attention as a new way to achieve further
reduction in both size and power consumption of electronicdevices. We have found a new phenomenon linking suchmodern spintronics and ubiquitous sounds.
Electrons in a material possess two aspects: charge as asource of electricity and spin as a source of magnetism(Fig.7-4). While conventional electronics have been developedby making use of the charge currents in devices, spintronics
aim to develop an innovative functionality of devices bymaking use of the spin currents , or the flow of spins. The
spin current is indispensable for spintronics, but it is difficult
to drive the spin current in a device. Therefore, only a limited
number of ways for generating spin currents have beenidentified to date.
In this context, we have discovered a new method forgenerating spin currents by merely injecting sound waves
into a material. Our method uses a spin current device,
which is composed of a magnet with a platinum terminal on

Piezoelectric
actuator

0242468Frequency (MHz)6810

Frequency of sound waves (MHz)Fig.7-5 Spin current generation using sound waves(a) Schematic view of the spin current device used in thepresent study. (b) Schematic illustration of the spin current
generation process. (c) Dependence of the spin current signal on
sound wave frequencies. Inset: Piezoelectric actuator oscillates
strongly at a particular frequency.

the top, attached to a piezoelectric actuator that acts as asound wave generator (Fig.7-5). After injecting sound waves
into the spin current device , we measured the spin current
signal induced in the platinum terminal and found that whena sound wave with a certain frequency is injected, a spincurrent is generated in the spin current device .
The following phenomenon is known as the spin pumping
effect : When the spin current device is excited by radio
frequency (rf) waves, a spin current is generated in thedevice. In the present study, we have clarified boththeoretically and experimentally that a similar spin pumping
effect can be observed even if a sound wave is used to excite
the spin current device instead of rf waves.
Sound waves can propagate through materials that canneither conduct electricity nor possess magnetism, and thus
are used as device substrates. Therefore, the method
developed in the present study enables the extraction ofelectromagnetic energy from a substrate material and createsa new route for the development of innovative spintronicdevices.

Reference
Uchida, K., Adachi, H. et al., Long-Range Spin Seebeck Effect and Acoustic Spin Pumping, Nature Materials, vol.10, issue 10, 2011, p.737-741.

84 JAEA R&D Review 201210

Spin current signal due to sound waves

Platinum

Spin

Oscillation amplitudeof the actuator(a) Electron

Electric voltage induced in Pt

7-2