Magnetization switching by current and microwaves

TL;DR

This paper presents a theoretical model showing how combining electric current and microwaves can significantly reduce the magnetization switching current in ferromagnetic multilayers, with practical implications for spin torque devices.

Contribution

The authors develop a novel theoretical framework and analytical formulas for optimizing microwave frequency and minimizing switching current in magnetization switching.

Findings

Switching current is reduced when microwaves are used in a specific frequency range.

An analytical formula for the optimal microwave frequency is derived.

Both optimal frequency and switching current decrease with higher microwave amplitude.

Abstract

We propose a theoretical model of magnetization switching in a ferromagnetic multilayer by both electric current and microwaves. The electric current gives a spin transfer torque on the magnetization, while the microwaves induce a precession of the magnetization around the initial state. Based on numerical simulation of the Landau-Lifshitz-Gilbert (LLG) equation, it is found that the switching current is significantly reduced compared with the switching caused solely by the spin transfer torque when the microwave frequency is in a certain range. We develop a theory of switching from the LLG equation averaged over a constant energy curve. It was found that the switching current should be classified into four regions, depending on the values of the microwave frequency. Based on the analysis, we derive an analytical formula of the optimized frequency minimizing the switching current, which is smaller than the ferromagnetic resonance frequency. We also derive an analytical formula of the minimized switching current. Both the optimized frequency and the minimized switching current decrease with increasing the amplitude of the microwave field. The results will be useful to achieve high thermal stability and low switching current in spin torque systems simultaneously.