Effect of grain boundaries on the magnetoresistance of magnetite

The effects of grain boundary morphology and stoichiometry had been systematically examined to clarify the role of natural grain boundaries in magnetoresistance of magnetite Fe_3(1-δ)O₄. We found that the excess resistance, caused by presence of the grain boundaries, is negligibly low in stoichiometric polycrystals. Accordingly, there was no grain boundary magnetoresistance detected in dense polycrystals. Moreover, the incorporation of grain boundaries was found to decrease the resistance of polycrystalline samples below the Verwey transition temperature. That was connected to the enhanced conductivity of grain boundaries appearing due to the local suppression of charge ordering. On the other hand, the essential negative magnetoresistance was detected in granular samples, exploring the point contact geometry for intergrain contacts. That magnetoresistance is characterized by large high-field component and appearance over a wide range of oxidation. It has been explained within the model of magnetically inhomogeneous grain boundary with the characteristic magnetic thickness of the order of exchange length. The magnetoresistance effect was connected to the spin-dependent scattering at the transition layers of magnetization formed around hard magnetic defects. The contraction of these transition layers by external magnetic field is supposed to provide the origin of the observed magnetoresistance. The analysis of appropriate microscopic scattering mechanisms reveals the important role of point defects in the spin-dependent scattering. The second magnetoresistance component was separated at highly oxidized grain boundaries and associated with tunneling transport across the isolating grain boundaries. Although the oxidation was shown to improve the isolating properties of natural grain boundaries, the performance of oxidized grain boundary as a tunneling barrier is still poor.