Raman spectra of the half-metallic ferromagnet CrO2

Abstract

Chromium dioxide, CrO2, attracts significant interest as it has a unique electronic band structure, resulting in half-metallic ferromagnetism (Tc ≈ 390 K) with completely spin-polarized electrons at EF. In this work we report the polarized Raman spectra of Cro2 between 5 and 420 K, as obtained from (110) and (001) oriented thin films on TiO2 substrates of the same orientation. The Raman lines corresponding to the four Raman allowed phonon modes of A1g, B1g, B2g and Eg symmetry were identified. The two low frequency Raman lines are very narrow, in particular at low temperatures, indicating weak scattering of the corresponding B1g and E8 phonons. The Raman intensities are not strongly affected by the magnetic ordering, whereas the line position versus T dependencies change their slope near TC for the A1g and Eg modes. At low temperatures the variations with T of the phonon linewidths follow the dependence expected for a phonon-phonon scattering. The additional broadening, which appears with approaching TC is tentatively assigned to the scattering of phonons from collective spin fluctuations.