Effect of grain boundaries on the magnetoresistance of magnetite

Abstract

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 3͑1−␦͒ O 4 . 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.

Low-field positive and high-field negative magneto-resistances in multiphase Fe-oxide thin films at room temperature

“…The majority of numerous experimental realizations of tunneling MR displayed a very small low-field MR response [30] . The distorted or hard magnetic structure and decreased spin polarization were often suggested as possible reasons for the low value of low-field MR [30][31][32][33] . Almost any kind of magnetic reconstruction will mix the opposite spin states for magnetite as half metal, decreasing the local spin polarization and hence the MR.…”

Tailoring the Interface Properties of Magnetite for Spintronics

“…However, in most cases the MR ratio is much smaller than expected, especially at room temperature. In fact, it is well known that in polycrystalline specimens and powder compacts of Fe 3 O 4 , the surfaces or interfaces at the grain boundaries have rather different magnetic properties and reduced spin polarization than the bulk [85][86][87][88]. This is a consequence of offstoichiometry, surface reconstruction, oxidation, defects, and bonding effects located at or close to the surfaces and interfaces.…”

Magnetoresistance in magnetite: Switching of the magnetic easy axis

“…Although several magnetoresistance studies in magnetite were described (see, e.g. [4][5][6][7]), none of them directly addressed the problem of axis switching. The primary result of our studies is that the electronic transport at low temperatures is affected by the structural disorder rather than magnetic, what is reflected in highly irreproducible * Corresponding author.…”