Spin-Transfer Torque Induced Vortex Dynamics in Fe/Ag/Fe Nanopillars

TL;DR

This paper investigates how spin-transfer torque induces vortex dynamics in Fe/Ag/Fe nanopillars, revealing new mechanisms for vortex excitation and analyzing their behavior under magnetic fields through experiments, simulations, and analytical models.

Contribution

It introduces a novel mechanism for exciting vortex gyrotropic modes in nanopillars with homogeneously magnetized layers, supported by experiments, simulations, and analytical theory.

Findings

Spin-transfer torque can excite vortex dynamics in nanopillars.

The vortex center shifts with applied magnetic field, affecting excitation frequency.

Asymmetric spin-transfer efficiency enables vortex energy absorption in homogeneously magnetized layers.

Abstract

We report experimental and analytical work on spin-transfer torque induced vortex dynamics in metallic nanopillars with in-plane magnetized layers. We study nanopillars with a diameter of 150 nm, containing two Fe layers with a thickness of 15 nm and 30 nm respectively, separated by a 6 nm Ag spacer. The sample geometry is such that it allows for the formation of magnetic vortices in the Fe disks. As confirmed by micromagnetic simulations, we are able to prepare states where one magnetic layer is homogeneously magnetized while the other contains a vortex. We experimentally show that in this configuration spin-transfer torque can excite vortex dynamics and analyze their dependence on a magnetic field applied in the sample plane. The center of gyration is continuously dislocated from the disk center, and the potential changes its shape with field strength. The latter is reflected in the field dependence of the excitation frequency. In the second part we propose a novel mechanism for the excitation of the gyrotropic mode in nanopillars with a perfectly homogeneously magnetized in-plane polarizing layer. We analytically show that in this configuration the vortex can absorb energy from the spin-polarized electric current if the angular spin-transfer efficiency function is asymmetric. This effect is supported by micromagnetic simulations.