Abstract
We have studied the temperature dependence of spectroscopic ellipsometry spectra of an electrically insulating, nearly stoichiometric single crystal with ferromagnetic Curie temperature . The optical response exhibits a weak but noticeable anisotropy. Using a classical dispersion analysis, we identify three low-energy optical bands at 2.0, 2.9, and . Although the optical conductivity spectra are only weakly temperature dependent below , we are able to distinguish high- and low-temperature regimes with a distinct crossover point around . The low-temperature regime in the optical response coincides with the temperature range in which significant deviations from a Curie-Weiss mean-field behavior are observed in the magnetization. Using an analysis based on a simple superexchange model, the spectral weight rearrangement can be attributed to intersite optical transitions. In particular, Kramers-Kronig consistent changes in optical spectra around can be associated with the high-spin-state optical transition. This indicates that other mechanisms, such as weakly dipole-allowed transitions and/or exciton-polaron excitations, can contribute significantly to the optical band at . The recorded optical spectral weight gain of the optical band is significantly suppressed and anisotropic, which we associate with complex spin-orbit-lattice phenomena near the ferromagnetic ordering temperature in .
10 More- Received 9 May 2007
DOI:https://doi.org/10.1103/PhysRevB.76.155125
©2007 American Physical Society