Observation of anomalous spin-torque generated by a ferromagnet

TL;DR

This paper reports the discovery of a novel out-of-plane spin torque in spin valves driven by in-plane current, originating from interface effects rather than conventional spin current mechanisms, with potential applications in spintronic memory devices.

Contribution

It introduces a new class of spin torque caused by an out-of-plane magnetic field, distinct from known mechanisms like Slonczewski or Rashba torques.

Findings

Observed out-of-plane magnetic field driven torque in spin valves.

The torque is not due to spin current injection but from interface effects.

Potential application in switching out-of-plane magnetic bits in spintronics.

Abstract

In this work we report observation of in-plane current induced out-of-plane magnetic field driven torque in spin valve structure. Since ferromagnet has high spin orbit coupling it is expected to be the source of spin-orbit-torque as it possesses anomalous-Hall-effect (AHE: equivalent to spin Hall effect in heavy metal). So we have carried out spin-torque ferromagnetic resonance (ST-FMR) experiment in a spin valve (consists of a fixed magnet and a free magnet which are separated by Cu spacer), passing in-plane radio frequency current and measuring DC voltage. Our experimental results eventually indicate that spin torque exerted on the free magnet is not caused due to the spin current injection by the fixed magnet owing to its AHE, but it is originated from in-plane current driven out-of plane effective magnetic field. This is new class of spin torque which is completely different from Slonczewski-spin transfer torque and Rashba like field like torque. The effective out-of plane magnetic field depends on the direction of current (in-plane) and magnetization (in-plane) of the pinned layer. One possible mechanism behind this unconventional torque could be intefacial spin-scattering which is also origin of current in-plane GMR effect. Most importantly this effective out-of plane torque can be useful to switch out-of plane magnetic bits in spintronic memory application.