Peter M. Levy

Conductivity and magnetoresistance in magnetic granular films (invited)

The transport properties in magnetic granular films are modeled by considering the spin‐dependent impurity scattering within the granules and the interface roughness scattering at the boundaries of the granules. The magnetoresistance for these films is derived by using the formalism developed for layered structures with currents perpendicular to the plane of the layers and which is applicable to random systems. With this model, various features of the magnetoresistance observed in recent experiments can be explained and the optimal choice of parameters to maximize the magnetoresistance can be determined.

Mechanisms of spin-polarized current-driven magnetization switching

The mechanisms of the magnetization switching of magnetic multilayers driven by a current are studied by including exchange interaction between local moments and spin accumulation of conduction electrons. It is found that this exchange interaction leads to two additional terms in the Landau-Lifshitz-Gilbert equation: an effective field and a spin torque. Both terms are proportional to the transverse spin accumulation and have comparable magnitudes.

Conductivity perpendicular to the plane of multilayered structures

Due to the inhomogeneous nature of multilayered structures, the measured conductivities for currents parallel and perpendicular to the layers are different, even if the local conductivity is isotropic. The perpendicular conductivity is derived and compared to the parallel conductivity which has been calculated previously. When our formalism is applied to Fe/Cr superlattices, interesting features of the magnetoresistance are found that can be verified by experiments.

Theory of magnetic superlattices: Interlayer exchange coupling and magnetoresistance of transition metal structures (invited)

Theoretical calculations and models to explain two unusual features of Fe/Cr magnetically layered structures are presented: (1) Strong antiferromagnetic (AF) couplings between Fe layers separated by Cr layers have been found in Fe/Cr/Fe sandwiches and Fe/Cr superlattices. These AF couplings are too strong to be accounted for by dipolar interactions and have to be ascribed to exchange interactions through the Cr layers. The interlayer exchange coupling from numerical calculations of the electronic structure of Fe/Cr superlattices based on the local density approximation is derived. (2) Recently, giant magnetoresistance effects have been found in Fe/Cr magnetically layered structures for currents in the plane of the layers. The spin‐dependent scattering at the Fe/Cr interfaces that comes from interface roughness, as well as that in the bulk of the layers are considered. The resistivity of these magnetic superlattices are calculated by adapting the quantum treatment of the electrical conductivity of ultrathi...

Self-consistent treatment of nonequilibrium spin torques in magnetic multilayers

It is known that the transfer of spin angular momenta between current carriers and local moments occurs near the interface of magnetic layers when their moments are noncollinear. However, to determine the magnitude of thetransfer, one should calculate the spin transport properties far beyond the interface regions. Based on the spin-diffusion equation, we present a self-consistent approach to evaluate the spin torque for a number of layered structures. One of the salient features is that the longitudinal and transverse components of spin accumulations are intertwined from one layer to the next, due to the presence of the much longer longitudinal spin-diffusion length and thus, the spin torque could be significantly amplified with respect to treatments which concentrate solely on the transport at the interface. We conclude that bare spin currents do not properly estimate the spin angular momentum transferred between the magnetic background; the spin transfer that occurs at interfaces should be self-consistently determined by embedding it in our globally diffuse transport calculations.

Identification of Transverse Spin Currents in Noncollinear Magnetic Structures

In this Letter we construct a spinor transport theory and derive the equations of motion for the distribution functions for currents in noncollinear magnetic multilayers. We find the length scale which characterizes the transverse spin current is of the order of 3 nm for a ferromagnetic 3d transition metal such as Co; this alters ones prediction of the spin torque generated for free magnetic layers less than 3 nm. In the limit of large exchange splitting we reproduce the results previously found for spin currents across noncollinear multilayers inasmuch as there are no transverse spin currents in the layers themselves in this limit.

Origin of anisotropy in transition metal spin glass alloys (invited)

The strong enhancement of anisotropy produced in CuMn by the addition of impurities with strong spin‐orbit coupling is explained by Dzyaloshinsky‐Moriya interactions. Estimates of the anisotropy have been obtained by approximate analytic methods as well as Monte Carlo studies. Although D‐M interactions seem to be dominant, we discuss some other sources of anisotropy, e.g., pseudo‐dipole and crystal‐field‐like terms.

Spin Hall effect induced by resonant scattering on impurities in metals.

The spin Hall effect is a promising way for transforming charge currents into spin currents in spintronic devices. Large values of the spin Hall angle, the characteristic parameter of the yield of this transformation, have been recently found in noble metals doped with nonmagnetic impurities. We show that this can be explained by resonant scattering off impurity states split by the spin-orbit interaction. By using as an example copper doped with 5d impurities we describe the general conditions and provide a guide for experimentalists for obtaining the largest effects.

Ab-initio calculation of indirect multipolar pair interactions in intermetallic rare-earth compounds

Abstract We present the results of an extensive ab-initio calculation of the indirect bilinear and quadrupolar pair couplings in a cubic rare earth intermetallic compound, DyZn. Spin and orbital effects in the complete rare earth-conduction electron interaction have been taken into account and we used conduction electron wave functions and energy bands obtained by a self-consistent augmented plane wave calculation. Our results underline the dominant role played by the d conduction electrons in the bilinear and quadrupolar matrix elements. The ferromagnetism as well as the ferroquadrupolar tendency in the rare earth-zinc series are well described by our calculations. The dependences of the couplings on the Fermi energy and on the rare earth-zinc series investigated.

Current understanding and open questions on giant magnetoresistance

Abstract After reviewing our current understanding of the phenomena of giant magnetoresistance in magnetic multilayered structures, we discuss some of the outstanding unresolved issues. Principal amongst these are the importance of the spin-dependent potentials (density of states) relative to the spin dependence of the scattering potential of impurities in the layers and at interfaces.