Spin current noise of the spin Seebeck effect and spin pumping

TL;DR

This paper provides a theoretical analysis of spin current fluctuations in a bilayer system, revealing how spin-current noise can inform on magnon spin, interface processes, and material properties.

Contribution

It introduces a theoretical framework for calculating spin current and noise at FI--NM interfaces, including spin-non-conserving effects, and links noise to physical parameters.

Findings

Spin-current noise can determine the effective magnon spin.

Noise analysis reveals interface spin-non-conserving processes.

Results help estimate spin Hall angle and interface heating effects.

Abstract

We theoretically investigate the fluctuation of a pure spin current induced by the spin Seebeck effect and spin pumping in a normal metal (NM)/ferromagnet (FM) bilayer system. Starting with a simple FI--NM interface model with both spin-conserving and spin-non-conserving processes, we derive general expressions of the spin current and the spin-current noise at the interface within second-order perturbation of the FI--NM coupling strength, and estimate them for an yttrium iron garnet (YIG) --platinum interface. We show that the spin-current noise can be used to determine the effective spin carried by a magnon, modified by the spin-non-conserving process at the interface. In addition, we show that it provides information on the effective spin of a magnon, heating at the interface under spin pumping, and spin Hall angle of the NM.