Interplay between non equilibrium and equilibrium spin torque using synthetic ferrimagnets

TL;DR

This paper investigates how the interplay between equilibrium and non-equilibrium spin torques in synthetic ferrimagnets influences magnetization dynamics, revealing conditions that induce high-frequency spin torque oscillations without external magnetic fields.

Contribution

It demonstrates how RKKY coupling strength controls the sign of spin torque and induces spin torque oscillator behavior in synthetic ferrimagnets.

Findings

Sign of spin torque is controlled by RKKY coupling strength.

Strong coupling reverses the usual spin torque alignment.

High-frequency oscillations up to 50 GHz observed without magnetic field.

Abstract

We discuss the current induced magnetization dynamics of spin valves F0|N|SyF where the free layer is a synthetic ferrimagnet SyF made of two ferromagnetic layers F1 and F2 coupled by RKKY exchange coupling. In the interesting situation where the magnetic moment of the outer layer F2 dominates the magnetization of the ferrimagnet, we find that the sign of the effective spin torque exerted on the free middle layer F1 is controlled by the strength of the RKKY coupling: for weak coupling one recovers the usual situation where spin torque tends to, say, anti-align the magnetization of F1 with respect to the pinned layer F0. However for large coupling the situation is reversed and the spin torque tends to align F1 with respect to F0. Careful numerical simulations in the intermediate coupling regime reveal that the competition between these two incompatible limits leads generically to spin torque oscillator (STO) behavior. The STO is found in the absence of magnetic field, with very significant amplitude of oscillations and frequencies up to 50 GHz or higher.