Spin-Dependent Transport in Ferromagnet-Based Devices

In this study three projects are described that investigate the transport properties of the ferromagnetic semiconductor, EuS, as well as devices based on the half metal, CrO2. These materials are expected to be highly spin-polarized sources of conduction electrons, and the resultant transport properties described herein are found to heavily depend upon the magnetic state of these materials in the samples. In the first project, thin films of EuS have been fabricated and characterized, and the magnetotransport properties of the films have been investigated. The films are naturally doped due to sulfur deficiencies that occur at elevated substrate growth temperatures. These deficiencies result in higher carrier concentrations, and the transport properties of these films are found to vary systematically with the substrate growth temperature. At low temperatures, a large nonlinear component in the Hall effect is observed. The Hall data at these temperatures scales with the magnetization which implies that this effect is an anomalous Hall effect rather than a change in carrier concentration with magnetic field. The extracted anomalous Hall coefficient is found to scale linearly with the resistivity which indicates that it is due to skew scattering of the conduction electrons by the defects. The second project consists of an attempt to incorporate both EuS and CrO2 into a magnetic tunnel junction. While the large expected tunneling magnetoresistance is not observed, a surprising relationship between the junction resistance and the magnetic state of the EuS layer is discovered. The third project investigates the effects of spin-polarized injection into a type I superconductor using a ferromagnet/superconductor/ferromagnet-based double tunnel junction. The resistance of the junctions below the superconducting transition temperature at zero bias is found to depend upon the relative orientations of the ferromagnetic electrodes.