Nanomagnet coupled to quantum spin Hall edge: An adiabatic quantum motor

TL;DR

This paper proposes a nanoscale device where a magnetic island coupled to a quantum spin Hall edge acts as an adiabatic quantum motor, converting electrical bias into magnetization precession with high efficiency.

Contribution

It introduces a microscopic scattering matrix approach to model the magnetization dynamics and demonstrates the device's potential as a highly efficient quantum motor.

Findings

Device can achieve unit efficiency when chemical potential is in the magnetization gap.

Scattering theory derives magnetization dynamics including spin-transfer torque and damping.

Characterization of device performance using a thermoelectric-like figure of merit.

Abstract

The precessing magnetization of a magnetic islands coupled to a quantum spin Hall edge pumps charge along the edge. Conversely, a bias voltage applied to the edge makes the magnetization precess. We point out that this device realizes an adiabatic quantum motor and discuss the efficiency of its operation based on a scattering matrix approach akin to Landauer-B"uttiker theory. Scattering theory provides a microscopic derivation of the Landau-Lifshitz-Gilbert equation for the magnetization dynamics of the device, including spin-transfer torque, Gilbert damping, and Langevin torque. We find that the device can be viewed as a Thouless motor, attaining unit efficiency when the chemical potential of the edge states falls into the magnetization-induced gap. For more general parameters, we characterize the device by means of a figure of merit analogous to the ZT value in thermoelectrics.