Anomalous behavior of spin wave resonances in Ga_{1-x}Mn_{x}As thin films

TL;DR

This study investigates ferromagnetic and spin wave resonance behaviors in high-quality Ga_{1-x}Mn_{x}As thin films, revealing unexpected linear dispersion likely caused by spatially varying magnetic anisotropy.

Contribution

It provides the first detailed analysis linking anomalous linear spin wave dispersion to non-uniform magnetic anisotropy in GaMnAs thin films.

Findings

Multiple resonances indicate robust ferromagnetic order.

Observed linear dispersion contrasts with expected quadratic behavior.

Spatially dependent anisotropy explains the anomalous dispersion.

Abstract

We report ferromagnetic and spin wave resonance absorption measurements on high quality epitaxially grown Ga_{1-x}Mn_{x}As thin films. We find that these films exhibit robust ferromagnetic long-range order, based on the fact that up to seven resonances are detected at low temperatures, and the resonance structure survives to temperatures close to the ferromagnetic transition. On the other hand, we observe a spin wave dispersion which is linear in mode number, in qualitative contrast with the quadratic dispersion expected for homogeneous samples. We perform a detailed numerical analysis of the experimental data and provide analytical calculations to demonstrate that such a linear dispersion is incompatible with uniform magnetic parameters. Our theoretical analysis of the ferromagnetic resonance data, combined with the knowledge that strain-induced anisotropy is definitely present in these films, suggests that a spatially dependent magnetic anisotropy is the most likely reason behind the anomalous behavior observed.