  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.
