Raman spectra of the half-metallic ferromagnet CrO₂

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 E_F. 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 A_1g, B_1g, B_2g and E_g symmetry were identified. The two low frequency Raman lines are very narrow, in particular at low temperatures, indicating weak scattering of the corresponding B_1g 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 T_C for the A_1g and E_g 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 T_C is tentatively assigned to the scattering of phonons from collective spin fluctuations.