Charge dynamics in the half-metallic ferromagnetCrO2

Infrared spectroscopy is used to investigate the electronic structure and charge carrier relaxation in crystalline films of ${\mathrm{CrO}}_2$ which is the simplest of all half-metallic ferromagnets. Chromium dioxide is a bad metal at room temperature but it has a remarkably low residual resistivity $(<\mathrm{}5\mathrm{}\hspace{0.5em}\mu \Omega \mathrm{cm})$ despite the small spectral weight associated with free carrier absorption. The infrared measurements show that low residual resistivity is due to the collapse of the scattering rate at $\omega <2000\mathrm{}\hspace{0.5em}{\mathrm{cm}}^{-1}.$ The blocking of the relaxation channels at low $\omega$ and T can be attributed to the unique electronic structure of a half-metallic ferromagnet. In contrast to other ferromagnetic oxides, the intraband spectral weight is constant below the Curie temperature.