Experimental Study of a Vortex Spin-Torque Oscillator in an MTJ with a Vortex Polarizer

TL;DR

This study experimentally demonstrates a vortex spin-torque oscillator in a magnetic tunnel junction, where vortex interactions induce auto-oscillations without external magnetic fields, advancing nanoscale microwave source technology.

Contribution

It presents the first experimental realization of a vortex-based spin-torque oscillator driven by vortex-like polarized spin currents in MTJs.

Findings

Oscillations observed at 110-60 MHz in devices 800-1000 nm in diameter.

Auto-oscillations occur without external magnetic fields due to vortex interactions.

Mechanism involves vortex-like spin current inducing local effective fields.

Abstract

Spin-torque nano-oscillators (STNOs) are promising nanoscale microwave sources for spintronic applications, serving as signal generators or elements in neuromorphic computing systems. In this paper, we investigate the experimental realization of an oscillator based on a magnetic tunnel junction (MTJ) comprising two magnetic layers: a reference layer (RL) and a free layer (FL). We demonstrate that when magnetic vortices with opposite chirality and polarity are formed in the layers, the application of a current induces auto-oscillations even in the absence of external magnetic fields. This effect is observed in devices with diameters ranging from 800 to 1000 nm, exhibiting oscillation frequencies between 110 and 60 MHz. The underlying mechanism is attributed to the action of a spin current with vortex-like polarization injected from the RL, interacting with the magnetic vortex in the FL. This interaction generates a local out-of-plane effective field due to spin-transfer torque, which acts on the vortex core and initiates its motion. The observed mechanism differs qualitatively from the case of uniformly polarized spin currents perpendicular to the plane, where the resulting in-plane field acts on the planar components of the vortex magnetization.