题 目：A Short Series of Talks on Topics in Spintronics
报告人：Prof. Peter M. Levy （New York University, Department of Physics）
时 间：11月23日（周二），下午2:00； 11月26日（周五），下午2:00；
11月30日（周二），下午2:00； 12月02 日（周四），下午2:00
In a series of four talks the origins and current developments in the emerging field of Spintronics will be presented. This field received its impetus from the simultaneous discovery of giant magnetoresistance, GMR, by Albert Fert and Peter Grünberg in 1988; this was recognized by their being awarded the Nobel Prize in Physics in 2007.
In the first talk the basics of Spintronics will be reviewed. Simple theories of electrical conduction in solids will be covered; then their extension to account for the role of the spin of the electron in conduction will be introduced. Central to spintronics is the concept of a spin current. Multiple manifestations of the role of the spin’s influence on electrical conduction in solids will be covered; amongst them GMR, magnetic tunnel junctions, and current induced switching of magnetic layers. The theoretical methods used to describe electrical and spin currents in ballistic [Landauer-Büttiker] and diffusive [Boltzmann] transport regimes will be described. The central role of spin accumulation attendant to transport across inhomogeneous magnetic media will be stressed.
Transport in magnetic multilayers, in which GMR was first discovered, will be covered in detail in a second talk. The two prevalent geometries, current parallel to the plane of the layers [CIP], and current perpendicular to the plane of the layers [CPP] will. In the latter the role of spin accumulation is present and we present the Valet-Fert theory of how to account for this accumulation in the regime of diffusive transport. Of signal importance in their treatment is the concept of a spin diffusion length; the characteristic length over which spin currents adapt themselves to the background electronic structure of the medium, metal, in which they are propagating. Different approaches to solving for the transport across multilayers, e.g., the whole structure and layer-by-layer approaches are introduced. A derivation of the Boltzmann equation of motion for the electron transport distribution function when a system is driven out of equilibrium is derived from the quantum mechanical equation if motion for density matrices. The approximations needed to derive the diffusion equation of electron transport from the Boltzmann equation are discussed.
Electron transport in noncollinear magnetic multilayers [where the magnetic moments of adjacent layers are not parallel] present new challenges that are not present in collinear structures. In particular linear response theory in which the current response is proportional to the electric field, now involves electron states that do not lie on the Fermi surface, i.e., linear response goes beyond equilibrium states. The Ziman theory of the electrical resistivity based on a thermodynamic formulation of resistance is introduced to stress that spin accumulation is such so as to create a state of maximum current, or minimum resistance [maximum entropy production], when a system is driven out-of-equilibrium by an electric field. This leads to the introduction of spin accumulation transverse to the local magnetization which is completely counterintuitive on the basis of the electron band structure. This accumulation that is transverse to the local magnetization produces a torque on the latter and acts so as to rotate the magnetization in a direction do that the current is increased. Model calculations of the electron and spin currents in noncollinear magnetic multilayers are presented in both the layer-by-layer and whole potential [structure] approaches.
When the spacer layer between magnetic layers is insulating or semiconducting electron transport proceeds via tunneling processes, and a ballistic description of the electron and spin transport is appropriate. In fact, fifteen years before GMR was discovered in metallic multilayers, magnetic tunnel junctions [MTJ’s] were found to have similar magnetoresistive properties. The electron and spin currents across a MTJ are derived based on the Landauer theory of conduction. Inasmuch as finite voltage differences can be sustained across an MTJ inelastic processes occur, and we show how to take account of the ensuing magnon production. Differences in spin torques due to elastic and inelastic processes are highlighted, and discussed in light of recent experimental results. The major differences are discussed between the spin currents in ballistic transport in MTJ’s, which is understood in terms of equilibrium electronic states, and that in metallic junctions in which out-of-equilibrium accumulation is needed.