We present low field thermoremanent magnetization (TRM) measurements in granular CrO2 and composites of ferromagnetic (FM) CrO2 and antiferromagnetic (AFM) Cr2O3. TRM in these samples is seen to display two distinct time scales. A quasi static part of remanence, appearing only in low field regime exhibits a peculiar field dependence. TRM is seen to first rise and then fall with increasing cooling fields, eventually vanishing above a critical field. Similar features in TRM have previously been observed in some antiferromagnets that exhibit the phenomenon of piezomagnetism. Scaling analysis of the TRM data suggest that presumably piezomoments generated in the AFM component drive the FM magnetization dynamics in these granular systems in low field regime.
Intergranular conduction in half metallic CrO2 is known to occur through a combination of spin dependent tunneling (driven by Coulomb Blockade (CB) effects) together with certain spin independent (SI) hopping processes. We present evidence that in polycrystalline CrO2 with enhanced grain size, both these process (CB effect and SI Hopping) are suppressed and the functional form of conductance is best described by Fluctuation Induced Tunneling (FIT) in a wide temperature range. Similar features are observed when grain boundary density is increased by Cr2O3 or Cr2O5.The spin dependent tunneling driven by FIT results in the observation of significant enhancement and monotonic temperature dependence of magnetoresistance. Overall, the magnetotransport measurements in a thus far unexplored crystallographic regime of CrO2 reveal that the functional form of conductance strongly influences its magnetoresistive properties.
We report magnetotransport measurements on high purity sintered samples of spintronic CrO2 in an unexplored crystallographic regime between 5-300 K. The negative magnetoresistance (MR) as derived from RH isotherms is observed to be unhysteretic up to temperatures as high as 200 K. Between 240-290 K, RH isotherms exhibit some unusual features including a positive MR and strong pinning effects. These feature disappear above 290 K and is apparently related with the antiferromagnetic ordering of the insulating grain boundary. Qualitatively similar features with significantly enhanced MR are also observed when the GB density is increased. These results bring out the role played by the magnetic and crystallographic microstructure on the magnitude, sign and hysteresis of the magnetoresistance in this technologically important material.
