Optical response of ferromagneticYTiO3studied by spectral ellipsometry

Abstract

We have studied the temperature dependence of spectroscopic ellipsometry spectra of an electrically insulating, nearly stoichiometric $\mathrm{Y}\mathrm{Ti}{\mathrm{O}}_{3}$ single crystal with ferromagnetic Curie temperature ${T}_{C}=30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. 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 $3.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Although the optical conductivity spectra are only weakly temperature dependent below $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, we are able to distinguish high- and low-temperature regimes with a distinct crossover point around $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. 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 ${d}_{i}^{1}{d}_{j}^{1}\ensuremath{\rightarrow}{d}_{i}^{2}{d}_{j}^{0}$ optical transitions. In particular, Kramers-Kronig consistent changes in optical spectra around $2.9\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ can be associated with the high-spin-state $(^{3}T_{1})$ optical transition. This indicates that other mechanisms, such as weakly dipole-allowed $p\text{\ensuremath{-}}d$ transitions and/or exciton-polaron excitations, can contribute significantly to the optical band at $2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The recorded optical spectral weight gain of the $2.9\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ optical band is significantly suppressed and anisotropic, which we associate with complex spin-orbit-lattice phenomena near the ferromagnetic ordering temperature in $\mathrm{Y}\mathrm{Ti}{\mathrm{O}}_{3}$.