Fluctuation-induced tunneling conductance and enhanced magnetoresistance in polycrystallineCrO2and its composites

Abstract

Intergranular conduction in half metallic ${\mathrm{CrO}}_{2}$ 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 ${\mathrm{CrO}}_{2}$ 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) below $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In this temperature range, FIT is seen to be retained when pure ${\mathrm{CrO}}_{2}$ is diluted by two different insulators ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ and ${\mathrm{Cr}}_{2}{\mathrm{O}}_{5}$. Above $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, a step in conductance is observed in pure ${\mathrm{CrO}}_{2}$ which is seen to be modulated by the magnetic state of the grain boundary as well as applied magnetic field. In the vicinity of this step, a significant enhancement of magnetoresistance is observed. Overall, magnetotransport studies in these compounds reveal that a systematic variation in crystallographic microstructure in granular ${\mathrm{CrO}}_{2}$ leads to a different functional form of conductance which is intimately coupled to the enhanced magnetoresistive properties as observed here.