Anomalous transport in half-metallic ferromagnetic CrO2

We have investigated the transport properties of CrO2 thin films deposited on TiO2 and sapphire substrates and find subtle differences with respect to earlier reports. The films are good metals down to low temperatures, with residual resistivities of the order of 6 Omega cm for films deposited on TiO2 and two times higher for films on sapphire substrates. Magnetoresistance (MR) measurements in high fields show an as yet unobserved nonmonotonic behavior, which is particularly pronounced around the sign change that takes place from negative to positive at a temperature around 100 K. Moreover, both the ordinary and anomalous Hall coefficients show considerable changes around 100-150 K, suggesting a change in carrier density together with the onset of the influence of spin defects in this temperature window. At lower temperatures, the MR is a linear function of the applied field, which can be explained as intergrain tunneling MR. This interpretation is also suggested by the angular MR. Planar Hall effect measurements reveal that the CrO2 thin films are not in a single magnetic domain state even for films deposited on an isostructural TiO2 substrate.