Different magneto-optical response of magnetic sublattices as a function of temperature in ferrimagnetic bismuth iron garnet films

Abstract

In this paper we investigate the magneto-optical (MO) and magnetic properties of bismuth iron garnet $\mathrm{B}{\mathrm{i}}_{3}\mathrm{F}{\mathrm{e}}_{5}{\mathrm{O}}_{12}$ thin films over a wide range of photon energies (1.6--3.5 eV) and temperatures (5--740 K). Depending on the photon energy range, the Faraday rotation (${\mathrm{\ensuremath{\Theta}}}_{F}$) and ellipticity (${\ensuremath{\varepsilon}}_{F}$) vary nonmonotonously with temperature. This behavior cannot be explained by a magnetization variation that can only decrease with increasing temperature. ${\mathrm{\ensuremath{\Theta}}}_{F}$ and ${\ensuremath{\varepsilon}}_{F}$ spectra have therefore been analyzed using a model based on two optical transitions of a diamagnetic nature, representing the tetrahedral and octahedral iron sites. Thus, the contribution of each magnetic sublattice has been extracted from the global macroscopic MO response and investigated as a function of temperature. The magnetic properties of octahedral and tetrahedral sublattices depend differently on temperature, suggesting a different anisotropy due to oxygen coordination. We have demonstrated that this relatively simple macroscopic measurement with a subsequent analysis can grant access to the information on the properties at a microscopic level. These results can advance the fundamental understanding of MO properties in multisublattice magnetic materials.