Spin-Transfer Excitations of Permalloy Nanopillars for Large Applied Currents

TL;DR

This study investigates magnetic excitations in permalloy nanopillars under large currents, revealing new modes and complex dynamical states, with experimental results aligning qualitatively with theoretical models.

Contribution

It extends measurements of spin-transfer excitations to higher currents and identifies additional modes and non-uniform states in permalloy multilayers.

Findings

Identification of new excitation modes at higher currents

Observation of spatially non-uniform dynamical states

Qualitative agreement between experiment and theory

Abstract

Using measurements of the spectra of microwave-frequency resistance oscillations, we determine the phase diagram of magnetic excitations caused by torques from DC spin-polarized currents in (thin permalloy)/copper/(thick permalloy) multilayer samples. We extend the measurements to larger values of current than have been reported previously. We find several additional modes that we are able to identify with motion of the thick magnetic layer as well as the thin one. Peaks in the microwave spectra at multiple frequencies suggest that spatially non-uniform dynamical states can be important in some circumstances. We compare the experimental phase diagram with simple theoretical models and achieve a good qualitative agreement.