The effects of grain boundary morphology and stoichiometry was systematically examined to clarify the role of natural grain boundaries in magnetoresistance of magnetite Fe3(1–δ)O4. It was found that the excess resistance, caused by the presence of the grain boundaries, was negligibly low in stoichiometric polycrystals. Accordingly, no grain boundary magnetoresistance was detected in dense polycrystals. Moreover, the incorporation of grain boundaries was found to decrease the resistance of polycrystalline samples below the Verwey transition temperature. This was related to an enhanced conductivity of grain boundaries which appeared due to the local suppression of charge ordering. On the other hand, an essentially negative magnetoresistance was detected in granular samples; associated with the point contact geometry of intergrain contacts. The magnetoresistance was characterized by a large high-field component and its appearance over a wide range of oxidation. It was explained within a model for a magnetically inhomogeneous grain boundary with a characteristic magnetic thickness of the order of the exchange length. The magnetoresistance effect was related to spin-dependent scattering in the transition layers of magnetization formed around hard magnetic defects. The contraction of these transition layers by external magnetic field was suggested to provided the observed magnetoresistance. An analysis of the microscopic scattering mechanisms revealed the important role played by point defects in spin-dependent scattering. A second magnetoresistance component was separated in highly oxidized grain boundaries and was associated with tunneling transport across the isolating grain boundaries. Although oxidation improved the isolating properties of natural grain boundaries, the performance of an oxidized grain boundary as a tunneling barrier remained poor.

Effect of Grain Boundaries on the Magnetoresistance of Magnetite. S.I.Rybchenko, Y.Fujishiro, H.Takagi, M.Awano: Physical Review B, 2005, 72[5], 054424 (14pp)