Quantum theory of electrical conduction in magnetic multilayers.

摘要

Magnetic multilayered structures known as magnetic superlattices, which are ultrathin films made up of alternating layers of magnetic and nonmagnetic metals, exhibit remarkable physical properties: an unusually large magnetoresistance and an oscillatory interlayer coupling. The interlayer coupling allows researchers to achieve antiferromagnetic configurations which turn into ferromagnetic ones under the action of an externally applied magnetic field; the corresponding electrical resistance undergoes a giant decrease. This phenomenon shows great promise for the hard disk magnetic recording technology.;In this dissertation I discuss a real space quantum linear transport theory that accounts for the giant magnetoresistance of magnetic multilayers. It is based on the Kubo formula for the nonlocal electrical linear conductivity coefficients and on a model Hamiltonian that incorporates spin-dependent bulk and interface scattering in a unified way. I show how a general length scale analysis, combined with the study of limiting regimes and with the use of circuit analogies, already explains the main features of giant magnetoresistance. I also find a general solution via an impurity-averaging procedure, which in the dilute limit yields a local self-energy and reduces the model to a form akin to the Sturm-Liouville problem. This technique combined with the length scale analysis yields general criteria for the characterization of the so-called quasiclassical regime and leads to completely general quasiclassical solutions via a WKB integration. Furthermore, the connection between the quantum Green's functions and the quasiclassical distribution functions is established.;Even though the nonlocal linear transport coefficients are directly given by the Kubo formula, the global or measurable transport properties require algorithms for their evaluation from the linear transport coefficients. For the case of current in the plane of the layers (CIP), general formulas are derived for the electrical resistance and giant magnetoresistance of arbitrary metallic multilayers. Similarly, for the case of current perpendicular to the plane of the layers (CPP), the so-called series resistor model is shown to be exact for all length scales, provided that spin-flip processes be neglected.;Finally, several extensions of this work are discussed briefly: its application to granular solids, the origin of spin-dependent electric fields, transport in noncollinear magnetization configurations, and the inclusion of superlattice potentials.