Temperature Dependent Ferromagnetic Resonance in FePt

Ferromagnetic resonance (FMR) is a technique for measuring the magnetic properties of materials such as, damping, gyromagnetic ratio and anisotropy. The underlying theory was outlined as long ago as the 1950’s by Charles Kittel and has since been extensively studied both experimentally and theoretically. The temperature dependence of ferromagnetic resonance curves and the properties derived from them can often be tricky to predict. By using the Landau-Lifshitz-Bloch (LLB) equation that describes the time-dependence of an ensemble of magnetic moments in a spatially averaged way, we have derived in a recently published article a new equation for the power absorbed during ferromagnetic resonance.

This paper predicts a number of temperature dependent magnetic properties using input functions into the LLB that have been parameterised from ab-initio calculations through atomistic spin dynamics simulations. This provides a link directly between electronic structure calculations to macroscopic observables.

As well as studying the properties analytically we have also extended the model to incorporate the effects of exchange between the macrospins, demagnetising fields and stochastic thermal fluctuations. By utilising GPU acceleration large magnetic structures can be simulated for the long times required to get good enough averages to simulate ferromagnetic resonance. Our results of simulating FMR in thin films have shown that there is a strong variation in the damping when the film thickness is varied. The thinner films show the largest damping at high temperatures due to the dominance of the demagnetising fields. This has a knock on effect in terms of the dynamic properties such as the reversal times, an important property in magnetic storages devices utilising heat assisted magnetic recording.

The GPU model that we have developed is capable of calculating a wide range of scenarios for large magnetic systems for long time-scales. This paves the way for new theoretical studies that can be compared to experimental measurements.