Research Group for Mechanical Control of Materials and Spin Systems
Group Leader : Eiji SaitohMembers : Satoru Okayasu, Masao Ono, Rie Haruki, Hiroyuki Chudo, Kazuya Harii,Hiroshi Yasuoka

The research objective of our Group “Mechanical Control ofMaterials and Spin Systems” is to develop new methods forcontrolling spin currents by combining electron spins andmechanical rotation, and/or by coupling spins and nuclear
magnetic resonance (NMR) techniques. To achieve this, weexpand spintronics frontier using a high-speed rotation
technique and NMR method. Our goals are1) Magnetization manipulation using mechanical rotation.
2) Detection of a spin current that is generated from rotating
objects using the spin Hall and spin torque effects.
3) Detection of spin transfer between nonmagnetic andferromagnetic layers by NMR.

Voltage generation from mechanical motion based on a fluid
spin-pumping
We have succeeded in generating voltage from a mechanicalmotion of a liquid metal; a fluid spin-pumping through vorticity
gradient in the liquid metal. The injected spins are converted toa voltage via the inverse spin Hall effect (ISHE).
Figure 1 illustrates a schematic diagram of a voltage generation
using a jet emission method. The liquid metal is sprayed on aspin detection layer (a Pt thin film) by a commercial airbrush.
Figure 2 shows results of the voltage measurement in the fluid
spin-pumping based on the jet emission method. A voltage isgenerated during the liquid metal being sprayed and its signchanges as the flow direction is reversed [Fig.2(a) and (b)].

(b)
1Voltage (μV)

Voltage (μV)

531-1-10 -5 0 5Time (s)10-1-3-5-10 -5 0 5Time (s)Fig.2 A result of the jet emission experiment.

1)

2)

3)

yvx0vx0xω4)

yvxx05)
VISHE

Fig. 3 Schematic diagram of the fluid spin-pumping.

This scenario explains the sign change of the observed
voltages; the inversion of the velocity reverses the polarity ofthe vorticity, and hence the spin polarization of the injected spincurrent, resulting in the sign change of the ISHE signal.
In the jet emission method, however, it is difficult to control thespeed of the droplets vxo, which hinders quantitative estimationof experimental results. To analyze the measured voltage VISHE
more precisely, we have developed a new method using a flow
in a narrow channel. We have also constructed a theoreticalframework based on hydrodynamics, spin diffusion, and thephysics of the ISHE. The measured voltages in the channels
with different diameters can be well-described by the theory.
These findings bridge spintronics and fluid motion, and open up
a new research field called fluid spintronics.

Fig. 1 Setup for a fluid spin-pumping.

(a)

The mechanism of the fluid spin-pumping can be depicted asfollows (corresponding illustrations are shown in Fig. 3):
1) Very high speed motion (~10m/s) of liquid droplets can beachieved by a jet emission method.
2) After the droplet touchdowns on a film, it starts sliding.
3) From the boundary condition of the droplets’ speed at thedroplet/film interface, vx =0, the gradient of vx develops
perpendicular to the film within a length scale determinedby the viscosity, leading to a finite vorticity ω of fluid.
4) Here, the vorticity can be regarded as an effective magnetic
field (Barnett field) Heff.
5) The gradient of Heff injects a spin current into a Pt film andthe spin current can be detected as an ISHE voltage along
the flow direction.

10

Detecting the interaction between a mechanical rotation andnuclear spins by using a spectroscopic method
In a rotating object, spins are subject to an effective magnetic
field known as a Barnett field. This is a quantum mechanicalanalogue of the Coriolis force. By using NMR techniques wehave succeeded in measuring the effective magnetic field thatwas predicted previously but has been elusive thus far. The coilspinning method, rotating the NMR measurement system and asample simultaneously, has allowed us to access a shift of theNMR frequency due to rotation in various nuclear isotopes,
demonstrating its high potential for further investigations of thecoupling between nuclear spins and mechanical rotations