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

Magnetotransport measurements were made on patterned, (110) oriented CrO2 t hin films grown by the high-pressure, thermal decomposition of CrO3 onto ru tile substrates. The low-temperature Hall effect exhibits a sign reversal f rom positive to negative as the magnetic field is increased above 1 T, whic h may be interpreted within a simple two-band model as indicating the prese nce of highly mobile (mu(h) = 0.25 m(2)/V 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 con tribution from the anomalous Hall effect becomes significant, while the lar ge, 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(-Delt a/T) with Delta approximate to 80 K, reflecting the lack of temperature dep endence 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 pos itive MR reflect the suppression of spin-Rip scattering expected for a half -metallic system. However, the changes in magnetotransport above the temper ature Delta must be due to the onset of spin-Rip scattering, even though k( B)Delta is much less than the expected energy gap in the minority spin dens ity of states. The significance of Delta is discussed in terms of recent mo dels for another half-metallic system, the perovskite manganites, and the p ossible formation of "shadow bands.".