Observation of fluctuation induced tunneling conductance in polycrystalline CrO2

TL;DR

This study demonstrates that in polycrystalline CrO2 with larger grains, fluctuation induced tunneling dominates conductance, significantly affecting magnetoresistance, especially when grain boundary density is increased by oxide layers.

Contribution

It provides evidence that fluctuation induced tunneling (FIT) governs conductance in polycrystalline CrO2 with larger grains, replacing previous models involving Coulomb blockade and spin independent hopping.

Findings

FIT describes conductance over a wide temperature range.

Enhanced magnetoresistance correlates with FIT-driven tunneling.

Grain boundary modifications influence tunneling mechanisms.

Abstract

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.