Spin Tunneling Magnetoresistive Effect In Ferromagnet/AI/sub 2/O/sub 3/Ferromagnet Junctions

Abstract

The giant magnetoresistance (GMR) found in magnetic multilayers and its fundamental problems as well as its promising application have raised considerable interest for the field of spin-polarized electron transport. Spin-polarized tunneling effect (magnetic tunneling valve effect) in ferromagnet/insulator/ferromagnet is a very similar phenomenon to that of the GMR. The pioneering work of the spin-polarized tunneling was conducted first by Julliere' for Fe/ GeKo junction. The relative change of conductance with magnetic field, AG/G, was approximately 14 % at low temperature, where AG is the difference of conductance between the parallel and antiparallel magnetizations of two ferromagnetic electrodes and G is the conductance in the state of antiparallel magnetization. After this report several challenges were carried out in order to prepare a junction with large conductance (or resistance) ratio. However, the highest value of MR ratio at room temperature was only 2.7 %2, far less than the expected value from spinpolarizations. Here the MR ratio is defined as = (Rap R,) E,, where Rp (RU,, is the resistance for like (unlike) direction of magnetization in the two electrodes. Recently, junctions with MR ratio more than 10 9% even at room temperature were prepared by our g r ~ u p ~ , ~ and by Moodera et al.5. From the phenomenological and theoretical treatments1Z6x7 the MR ratio can be expressed as