Current-induced magnetization changes in a spin valve due to incoherent emission of non-equilibrium magnons

TL;DR

This paper models how spin-polarized currents induce magnetization changes in a spin valve through incoherent magnon emission, revealing critical behaviors like magnon avalanches and potential magnetization reversal.

Contribution

It introduces a rate equation approach for non-equilibrium magnon dynamics in spin valves, highlighting critical phenomena and their impact on magnetization switching.

Findings

Magnon emission reduces free layer magnetization.

Critical bias induces magnon avalanche and divergence in magnon temperature.

Magnon-magnon scattering can saturate magnon concentration, affecting magnetization reversal.

Abstract

We describe spin transfer in a ferromagnet/normal metal/ferromagnet spin-valve point contact. Spin is transferred from the spin-polarized device current to the magnetization of the free layer by the mechanism of incoherent magnon emission by electrons. Our approach is based on the rate equation for the magnon occupation, using Fermi's golden rule for magnon emission and absorption and the non-equilibrium electron distribution for a biased spin valve. The magnon emission reduces the magnetization of the free layer. For anti-parallel alignment of the magnetizations of the layers and at a critical bias a magnon avalanche occurs, characterized by a diverging effective magnon temperature. This critical behavior can result in magnetization reversal and consequently to suppression of magnon emission. However, magnon-magnon scattering can lead to saturation of the magnon concentration at a large but finite value. The further behavior depends on the parameters of the system. In particular, gradual evolution of the magnon concentration followed by a magnetization reversal is possible. Another scenario corresponds to a step-like increase of the magnon concentration followed by a slow decrease. In the latter case a spike in the differential resistance is expected due to a contribution of electron-magnon scattering. A comparison of the obtained results to existing experimental data and theoretical approches is given.