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

Abstract

Magnetotransport measurements were made on patterned, ͑110͒ oriented CrO 2 thin films grown by the high-pressure, thermal decomposition of 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 ( h ϭ0.25 m 2 /V s) holes as well as a much larger number of less mobile electrons (nϭ0.4 electrons/Cr͒. 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 ͑MR͒ 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 Ϫ⌬/T with ⌬Ϸ80 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 ⌬ must be due to the onset of spin-flip scattering, even though k B ⌬ is much less than the expected energy gap in the minority spin density of states. The significance of ⌬ is discussed in terms of recent models for another half-metallic system, the perovskite manganites, and the possible formation of ''shadow bands.''

Anomalous transport in half-metallic ferromagnetic CrO2

“…It is remarkable that the value of falls in the temperature range of about 100 K where we find a dip in dR/dT . This suggests a certain electronic phase change in CrO 2 around 100 K. That is reinforced by the high-field MR data, which show a field dependent sign change around 100 K. In contrast, we do not find a sign reversal in the Hall data observed in earlier work, 12 which is possibly due to the fact that the films in that study where quite thick (0.5 μm), and grown with a slightly different method (high-pressure 085123-4 synthesis). Our data allow us to extract a carrier density using a one-band model as shown in Fig.…”

“…We find resistivity behavior that is subtly different from earlier reports, with an anomaly around 100 K. We do not see a sign change in the Hall effect reported in Ref. 12, although we do find a sign change in the high-perpendicular-field magnetoresistance (MR) as reported in that work. New in our MR data is the strongly nonmonotonous behavior around the crossover temperature of 100 K. We also study the low-field magnetoresistance behavior and come to a similar conclusion as König et al, that intergrain tunneling magnetoresistance (ITMR) takes over from anomalous magnetoresistance (AMR) when the temperature decreases to below 100 K. 15 Data on the planar Hall effect (PHE) confirm that the magnetization does not switch in single-domain fashion in these films, different from one particular case reported by Gönnenwein.…”

“…Also, different results have been reported with respect to the Hall effect. Watts et al presented data showing a sign reversal at low temperatures, 12 which they interpreted as evidence for two-band transport, but this was not found in later studies. 13,14 In this article we return to the issue of magnetotransport in high-quality thin films of CrO 2 , with proper attention to the different crystallographic axes of the material.…”

“…[8][9][10] However, the electronic properties of CrO 2 are still not fully understood. For instance, the resistivity between 10 and 300 K is usually described in terms of an excitation gap, 11,12 but a clear connection with an electronic or spin gap excitation cannot be made. Also, different results have been reported with respect to the Hall effect.…”

Charge dynamics in the half-metallic ferromagnetCrO2

“…The collapse of the scattering rate at low temperatures is consistent with recent magnetoresistance measurements on films of CrO 2 . 21,22 It can be argued that the evolution of the 1/ ( ) spectra with temperature is a natural consequence of the halfmetallic DOS. Indeed the DOS sketched in the inset of Fig.…”