Anisotropic Electron Transport and Weak Localization in Copper/aluminum and Copper/silicon Multilayers.

The aim of this project has been two-fold: to investigate the phenomenon of Weak Localization (WL) in metallic multilayers and to explore the possibility of using it to measure the anisotropy of electron diffusivity in these systems. Sputtered Cu/Al and Cu/Si multilayers were selected as the systems to be investigated. The series of Cu/Al and Cu/Si multilayers were prepared by planar magnetron sputter-deposition technique in the laboratory. Samples of different resistivities were obtained by varying the individual thicknesses of the Al, Si and Cu layers. The individual layer thicknesses were in the range of 10-100 A. Some of the Cu/Al multilayers were doped at the interfaces with a small amount of W (less than a monolayer) in order to enhance the spin-orbit scattering. Structural characterization was done by x-ray diffraction at both small and large angles. WL was investigated by measuring the transverse magnetoresistance of each multilayer sample for temperatures between 2.5K and 20K, in magnetic fields up to 8T. In each case, the magnetoresistance was measured for two different orientations for magnetic fields--parallel and perpendicular to the plane of the sample film. Measurements were carried out in a liquid-He cryostat equipped with a superconducting magnet, using a high-precision, four -terminal, AC bridge. The data were analyzed using Kawabata's theory of WL for 3D systems, modified to account for anisotropic electron diffusion in multilayers. In the case of Cu/Al multilayers, an additional correction due to classical ([omega_{c}tau _{e}]^2) contribution, which was evident in the data at high fields, had to be included in the analysis. Application of this theory to the observed magnetoresistance for the two orientations of the field yielded a measure of the transport anisotropy ratio delta = (D_ {xy}/D_{z}). For the samples studied, values of this ratio ranged from 1 to more than 2. The anisotropy was more clearly revealed in the Cu/Si system compared to the Cu/Al system, and in both the systems anisotropy was largest for the highest conductivity sample.