Diffusive versus local spin currents in dynamic spin pumping systems

TL;DR

This paper uses microscopic theory to distinguish between diffusive and local spin currents in spin pumping systems, showing that diffusive spin currents dominate and their conversion to charge currents depends on the type of spin-orbit interaction.

Contribution

It reveals that diffusive spin currents are the main component in spin pumping and their conversion to charge currents varies with the spin-orbit interaction type, clarifying the nature of spin transport.

Findings

Diffusive spin current dominates in smooth magnetization textures.

Spin-charge conversion depends on the type of spin-orbit interaction.

Local spin current does not significantly contribute to charge current conversion.

Abstract

Using microscopic theory, we investigate the properties of a spin current driven by magnetization dynamics. In the limit of smooth magnetization texture, the dominant spin current induced by the spin pumping effect is shown to be the diffusive spin current, i.e., the one arising from only a diffusion associated with spin accumulation. That is to say, there is no effective field that locally drives the spin current. We also investigate the conversion mechanism of the pumped spin current into a charge current by spin-orbit interactions, specifically the inverse spin Hall effect. We show that the spin-charge conversion does not always occur and that it depends strongly on the type of spin-orbit interaction. In a Rashba spin-orbit system, the local part of the charge current is proportional to the spin relaxation torque, and the local spin current, which does not arise from the spin accumulation, does not play any role in the conversion. In contrast, the diffusive spin current contributes to the diffusive charge current. Alternatively, for spin-orbit interactions arising from random impurities, the local charge current is proportional to the local spin current that constitutes only a small fraction of the total spin current. Clearly, the dominant spin current (diffusive spin current) is not converted into a charge current. Therefore, the nature of the spin current is fundamentally different depending on its origin and thus the spin transport and the spin-charge conversion behavior need to be discussed together along with spin current generation.