Magnetoresistance through grain boundaries in a resonant-tunneling mechanism

The effects of local spins inside grain boundaries on tunneling magnetoresistance are studied. A spin-dependent multichannel picture is used to describe the magnetotransport processes across antiferromagnetic grain boundaries. Using a resonant-tunnelling model within a mean-field scheme, we calculate the magnetoresistance caused by the change of the magnetic configuration of grains and inside grain boundaries. We show that, unlike paramagnetic barriers, the existence of local spins in an antiferromagnetic grain-boundary always decreases the low-field magnetoresistance. In the limit of complete spin polarization, a high-field slope proportional to the grain boundary magnetization is found, which is consistent with the characteristics of the high-field magnetoresistance phenomenon. By taking the effects of the direct-, resonant-, and inelastic-tunneling processes into account simultaneously, we can explain the experimental results of extrinsic magnetoresistance in highly spin-polarized magnetic oxides qualitatively.