Spin-polarized transport and magnetoresistance in magnetic oxides

Magnetic oxide materials possessing a high degree of spin polarization have been found to exhibit enhanced spin-dependent transport properties. For example, interfaces created by naturally occurring or artificial grain boundaries in the perovskite manganites, La(1-x)A(x)MnO(3) result in a large magnetoresistance (MR) at low fields. Extrinsic grain boundary MR has also been reported in other conducting magnetic oxides, such as chromium dioxide (CrO2), pyrochlore Tl2Mn2O7. magnetite (Fe3O4) and the ordered double-perovskite Sr2FeMoO6. Even larger changes in resistances at low fields are obtained in the layered manganite, La2-2xSr1+2xMn2O7 for x = 0.3, which possesses a natural tunnel junction structure in the c-axis direction. But, by far, the largest MR effect to date has been observed in thin film ferromagnetic/insulating/ferromagnetic tunnel junction structures, where resistance changes as high as a factor of 10 at low fields has been reported for the strontium-doped perovskite manganites. In most cases, the enhanced MR effect in these magnetic oxides is limited to rather low temperatures and decreases rapidly with increasing temperature. With a better understanding of the nature of the boundaries in these junctions, it might be possible to enhance the MR at higher temperatures. (C) 1999 Elsevier Science B.V. All rights reserved.