Evidence for two-band magnetotransport in half-metallic chromium dioxide

Magnetotransport measurements were made on patterned, (110) oriented ${\mathrm{CrO}}_{2}$ thin films grown by the high-pressure, thermal decomposition of ${\mathrm{CrO}}_{3}$ onto rutile substrates. The low-temperature Hall effect exhibits a sign reversal from positive to negative as the magnetic field is increased above 1 T, which may be interpreted within a simple two-band model as indicating the presence of highly mobile $({\ensuremath{\mu}}_{h}=0.25{\mathrm{m}}^{2}/\mathrm{V}\mathrm{}\mathrm{s})$ holes as well as a much larger number of less mobile electrons $(n=0.4$ electrons/Cr). Between 50 and 100 K, the field at which the sign reversal occurs rapidly increases and a contribution from the anomalous Hall effect becomes significant, while the large, positive transverse magnetoresistance (MR) observed at low temperatures changes over to a predominantly negative MR. These changes correlate with a thermally activated dependence in the resistivity of the form ${T}^{2}{e}^{\ensuremath{-}\ensuremath{\Delta}/T}$ with $\ensuremath{\Delta}\ensuremath{\approx}80\mathrm{K},$ reflecting the lack of temperature dependence in the resistivity at low temperatures and a ${T}^{2}$ behavior above 100 K. The high mobilities at low temperature which result in the observed positive MR reflect the suppression of spin-flip scattering expected for a half-metallic system. However, the changes in magnetotransport above the temperature $\ensuremath{\Delta}$ must be due to the onset of spin-flip scattering, even though ${k}_{B}\ensuremath{\Delta}$ is much less than the expected energy gap in the minority spin density of states. The significance of $\ensuremath{\Delta}$ is discussed in terms of recent models for another half-metallic system, the perovskite manganites, and the possible formation of ``shadow bands.''