FERROMAGNETIC RESONANCE IN UNIAXIAL NANOPARTICLES: THE ROLE OF THERMAL FLUCTUATIONS

Abstract

A consistent theory of ferromagnetic resonance in superparamagnetic nanoparticles with uniaxial anisotropy of arbitrary strength is developed. The approach is free from restrictions on the spectrometer characteristics and the material parameters of the particles and is well suitable for the interpretation of experimental data as well as for the validation of approximate models. Ferromagnetic resonance spectra of ensembles of non-interacting randomly oriented nanoparticles are presented. It is shown that an absorption line can consist of several maxima if the temperature is below a certain level defined by the interplay between the intensity of thermal fluctuations and the anisotropy magnitude. The resolution of individual components of the spectrum goes up with an increase in the quality factor of the particles. It is shown that the temperature dependence on the spectral width is non-monotonic: heating of the system first causes narrowing of the absorption line but then its expansion. The temperature at which the width is minimal increases when the anisotropy of dispersed particles goes up.