Evidence for a gap in the excitation spectrum of CrO2

Magnetization and resistivity of CrO2 powders and oriented CrO2 films on different TiO2 single-crystal substrates are reported, and the low-temperature specific heat is measured on powders. The M(T) data follow a Bloch T3/2 law, with no sign of a spin-wave gap. The residual resistivity depends strongly on film texture, but a T2 temperature dependence of the resistivity is found in all samples above a cut-off temperature of the order of 70 K. The behavior is attributed to electron-magnon spin-flip scattering which is suppressed at low temperature in a half-metallic ferromagnet.