Optical response of ferromagnetic YTiO3 studied by spectral ellipsometry

We have studied the temperature dependence of spectroscopic ellipsometry spectra of an electrically insulating, nearly stoichiometric YTiO3 single crystal with ferromagnetic Curie temperature T-C=30 K. 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 3.7 eV. Although the optical conductivity spectra are only weakly temperature dependent below 300 K, we are able to distinguish high- and low-temperature regimes with a distinct crossover point around 100 K. 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 d(i)(1)d(j)(1)-> d(i)(2)d(j)(0) optical transitions. In particular, Kramers-Kronig consistent changes in optical spectra around 2.9 eV can be associated with the high-spin-state (T-3(1)) optical transition. This indicates that other mechanisms, such as weakly dipole-allowed p-d transitions and/or exciton-polaron excitations, can contribute significantly to the optical band at 2 eV. The recorded optical spectral weight gain of the 2.9 eV optical band is significantly suppressed and anisotropic, which we associate with complex spin-orbit-lattice phenomena near the ferromagnetic ordering temperature in YTiO3.