Electronic structure of epitaxial CrO$_{2}$(100) and CrO$_{2}$(110) films

Half-metallic ferromagnets are conducting solids whose conduction electrons undergo magnetic ordering with a spin polarization of 100{\%} at 0 K. Although CrO$_{2}$ is predicted to be a half-metallic ferromagnet, previous attempts to make devices using CrO$_{2}$ have resulted in a degradation of performance instead of an enhancement. Using ultra-violet photoelectron spectroscopy at the CAMD synchrotron, we have measured the electronic properties of epitaxial CrO$_{2}$(110)/TiO$_{2}$(110) and CrO$_{2}$(100)/TiO$_{2}$(100) surfaces grown using a CrO$_{3}$ precursor. Clean, stoichiometric CrO$_{2}$ surfaces have been prepared either by exposure to white light, which desorbs OH groups from the surface, or by sputtering and annealing in O$_{2}$. The measurements of the CrO$_{2}$ surfaces show no emission at E$_{F}$ after sputtering and annealing the surfaces in oxygen. However, the white light prepared surfaces show a small density of states at E$_{F}$. Photon energy dependent photoemission experiments show no increase in the density of states at E$_{F}$ as the photon energy is lowered from 50 eV to 15 eV, which increases the bulk sensitivity of these measurements. These results indicate that CrO$_{2}$ behaves more like a semi-metal than a half-metal and that surface disorder can induce a semi-metal to semiconductor transition at its surfaces.