Reduced effective magnetization and damping by slowly-relaxing impurities in strained $\gamma$-$\mathrm{Fe_2O_3}$ thin films

TL;DR

This study investigates how slowly relaxing impurities affect the magnetic properties of strained $ ext{Fe}_2 ext{O}_3$ thin films, revealing reduced magnetization and a temperature-dependent anisotropy transition.

Contribution

It introduces a model based on slowly relaxing impurities to explain non-linear ferromagnetic resonance linewidth behavior in strained maghemite thin films.

Findings

Observation of negative effective magnetization due to strain-induced anisotropy.

Identification of a sign change in effective magnetization with temperature.

Evidence of a transition from perpendicular to in-plane magnetic anisotropy.

Abstract

We study the static and dynamic magnetic properties of epitaxially strained $\gamma$-$\mathrm{Fe_2O_3}$ (maghemite) thin films grown via pulsed-laser deposition on MgO substrates by SQUID magnetometry and cryogenic broadband ferromagnetic resonance experiments. SQUID magnetometry measurements reveal hysteretic magnetization curves for magnetic fields applied both in- and out of the sample plane. From the magnetization dynamics of our thin films, we find a small negative effective magnetization in agreement with a strain induced perpendicular magnetic anisotropy. Moreover, we observe a non-linear evolution of the ferromagnetic resonance-linewidth as function of the microwave frequency and explain this finding with a model based on slowly relaxing impurities, the so-called slow relaxor model. By investigating the magnetization dynamics in our maghemite thin films as a function of frequency and temperature, we can isolate the temperature dependent contribution of the slowly relaxing impurities to the resonance linewidth and, in particular, observe a sign change in the effective magnetization. This finding provides evidence for a transition of the magnetic anisotropy from a perpendicular easy axis to an easy in-plane anisotropy for reduced temperatures.