Charge conservation in spin torque oscillators leads to a self-induced torque

TL;DR

This paper introduces a self-induced torque in spin torque oscillators resulting from charge conservation, which modifies the conventional LLGS equation and better aligns theory with experimental observations.

Contribution

It reveals a previously overlooked self-induced torque caused by charge conservation, improving the theoretical model of spin torque oscillators.

Findings

Self-induced torque originates from pumping current in magnetic tunnel junctions.

In-plane magnetized nanopillars show qualitative corrections with the new torque.

The modified LLGS equation aligns better with experimental results.

Abstract

Spin torque oscillators are conventionally described by the Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation. However, at the onset of oscillations, the predictions of the conventional LLGS equation differ qualitatively from experimental results and thus appear to be incomplete. In this work we show that taking charge conservation into account leads to a previously-overlooked self-induced torque, which modifies the LLGS equation. We show that the self-induced torque originates from the pumping current that a precessing magnetization drives through a magnetic tunnel junction. To illustrate the importance of the self-induced torque, we consider an in-plane magnetized nanopillar, where it gives clear qualitative corrections to the conventional LLGS description.