Effects of the Zhang-Li Torque on Spin Torque nano Oscillators

TL;DR

This paper investigates how Zhang-Li torque influences magnetic excitations in spin-torque nano-oscillators, revealing modifications in threshold currents and mode sizes, controllable via geometric ratios.

Contribution

It is the first study to analyze the impact of Zhang-Li torque on localized magnetic modes in STNO with nanocontact geometry using micromagnetic simulations.

Findings

Zhang-Li torque alters threshold currents of magnetic modes.

The effective size of magnetic modes is affected by Zhang-Li torque.

Control over effects is possible by adjusting nanocontact size to layer thickness ratio.

Abstract

Spin-torque nano-oscillators (STNO) are microwave auto-oscillators based on magnetic resonances having a nonlinear response with the oscillating amplitude, which provides them with a large frequency tunability including the possibility of mutual synchronization. The magnetization dynamics in STNO are induced by spin transfer torque (STT) from spin currents and can be detected by changes in electrical resistance due to giant magnetoresistance or tunneling magnetoresistance. The STT effect is usually treated as a damping-like term that reduces magnetic dissipation and promotes excitation of magnetic modes. However, an additional term, known as Zhang-Li term has an effect on magnetization gradients such as domain walls, and could have an effect on localized magnetic modes in STNO. Here we study the effect of Zhang-Li torques in magnetic excitations produced in STNO with a nanocontact geometry. Using micromagnetic simulations we find that Zhang-Li torque modify threshold currents of magnetic modes and their effective sizes. Additionally we show that effects can be controlled by changing the ratio between nanocontact size and layer thickness.