Influence of Superlattice Potentials on Transport in Magnetic Multilayers
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INFLUENCE OF SUPERLATTICE POTENTIALS ON TRANSPORT IN MAGNETIC MULTILAYERS Shufeng Zhang, and Peter M. Levy, Department of Physics, New York University, 4 Washington Place, New York, NY 10003 ABSTRACT We discuss the effect of the superlattice potential on the magnetotransport properties of magnetic multilayers for current parallel and perpendicular to the plane of the layers. While quantum well states affect the magnetotransport, they are not the primary origin of the giant magnetoresistance observed in these materials for currents in the plane of the layers. In general, it is necessary to include both spin-dependent scattering and the effects of superlattice potentials in order to explain the magnetoresistance of multilayered structures.
Giant magnetoresistance in magnetic multilayers is usually understood in terms of spin-dependent scattering [1], i.e., the scattering of a conduction electron by a local magnetic impurity depends on its spin direction relative to that of the local moment. With this picture, one uses unpolarized plane waves to represent conduction electrons in the structures and calculates the conductivity for different magnetic configurations, e.g., ferro and antiferro aligned magnetic layers [2]. Recently, there is an emerging realization that quantum well states may be formed in these multilayers [3], at least for some directions in momentum space. The existence of these states implies that the conduction electrons are not plane waves for some momenta, and that layering changes the electronic states of the conduction electrons. To model these states, Hood and Falicov [4] introduced spin-dependent potentials in different magnetic layers and considered the effects of these potentials on the conduction electrons by using reflection and transmission probabilities at the interfaces ( potential steps). However, as the quantum coherence length (inelastic mean free path) at low temperatures is much larger than the thicknesses of the layers, it is necessary to include effects from the interference of conduction electrons reflected at different interfaces [5]. Therefore, one should first determine the proper wavefunctions and energy spectrum of conduction electrons quantum mechanically, and then evaluate their scattering rates due to impurities in the layers and at the interfaces. Kalmeyer [6] considered the influence of spin-dependent wave functions on the magnetoresistance; by focusing on the scattering at interfaces, he was able to conclude that the spin-dependent potential alone can give rise to large magnetoresistance. To examine this conclusion, we select superlattices as our prototypical structures in order to eliminate the confinement effect of outer boundaries. To obtain complete information on the effect of spin-dependent potentials, one also needs to simultaneously work out the magnetoresistance for the CIP (current in the plane of the layers) and CPP (current perpendicular to the plane of the layers). Although most of the experimental data studied to date are for the CIP geometry, the CPP-MR m
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