Theory of mechanical spin current generation via spin–rotation coupling

• We present theoretical details of spin current generation from mechanical motion.
• The principle used in this method relies on “spin–rotation coupling” (SRC).
• We examine spin dynamics based on spin diffusion equation with the SRC.
• Larger spin currents can be obtained in materials with longer spin lifetimes like Cu.
• Possible detection methods for the proposed spin signals are also discussed.

The spin–rotation coupling (SRC) is responsible for angular momentum conversion between the electron spin and rotational deformations of elastic media, whereby spin current generation from mechanical motions is theoretically predicted. Here a surface acoustic wave (SAW) is considered as an example of such elastic deformations. We formulate the spin diffusion equation that is extended to include the influence of the SRC, and from the solution we find that larger spin currents can be obtained in materials with longer spin lifetimes. This distinct feature arises from the fact that the present method is of “purely” mechanical origin, i.e., it relies on neither equilibrium magnetization nor spin–orbit coupling, offering a new route to building the so-called “rare-metal-free” spintronics devices.