Ferromagnetic-insulator-ferromagnetic tunneling: Spin-dependent tunneling and large magnetoresistance in trilayer junctions

Tunneling between ferromagnet-insulator-ferromagnet (FM-I-FM) trilayer thin-film planar junctions has been successfully studied. Tunnel current was observed to be dependent on the relative orientation of the magnetization (M). Co, CoCr, CoFe, Fe0.7Pt0.3, and NiFe were tried as the FM electrodes with Al2O3 or MgO as the barrier layers for the above studies. Large magnetoresistance (MR) was observed as the M alignment of the two ferromagnets changed from being parallel to antiparallel orientation. At room temperature, the highest change in junction MR was 18%, field sensitivity factor reaching 5%/Oe in the best cases. The MR value increased to 25.6% at 4.2 K, and decreased as the de bias was increased to a fraction of the barrier height. The angular dependence of MR varied nearly as the cosine of The relative angle of M, as predicted by Slonczewski's theory. The magnitude of MR agrees well with that given by Julliere's model, which predicts that the MR varies as the product of the conduction electron spin polarization of the FMs. These trilayer junctions can find application as high-density, nonvolatile storage media or as field sensors. (C) 1996 American Institute of Physics.