Ellipsometric Study of the Electronic Band Structure of CrO$_2$ across the Ferromagnetic Transition

We present a detailed study of the optical properties of half metallic CrO$_2$ at temperatures below, at, and above the Curie temperature. The films were grown epitaxially on (100) and (110) oriented TiO$_2$ substrates by chemical vapor deposition. Using variable angle spectroscopic ellipsometry we have extracted the optical conductivity of the films in the range from 0.06 to 6 eV. Our data reveal an important anisotropy in the films which is in good agreement with what is know about their crystal structure. The main features of the conductivity spectra in the ferromagnetic state are consistent with existing band structure calculations. However, we observe no temperature dependence of these features across the ferromagnetic transition, posing questions about the electronic structure of the material.