Transport in Superconducting/Ferromagnetic Heterostructures

Abstract

In this thesis I present my research on spin and charge transport in ferromagnet, superconductor (F=S) heterostructures using a self-consistent, clean limit theory. The goal is to characterize realistic samples. The primary focus is on the F1=N=F2=S superconducting spin valve. I also consider the S1=F1=N=F2=S2 ferromagnetic Josephson structures. We solve the Bogoliubov deGennes equations (BdG) using a self-consistent, numerical approach and determine the thermodynamic quantities such as the pair potential. For the charge transport, we use the Blonder-Tinhkam-Kapwijk (BTK) method to determine the conductance G. We study the conductance features and their dependence on the physical parameters such as the layer thicknesses and interfacial quality of the sample. The main results are the dependence of G on the misalignment angle of the magnetizations in F2 relative to F1, which constitutes a 'valve eect'. The valve eect in F=S structures is due to the proximity eect, which is angularly dependent. The critical bias (CB), equal to the gap energy, is non-monotonic with due to this proximity eect. The conductance features are split for incoming spin-up and spin-down electrons, which leads to a subgap (below CB) peak in the total conductance. This subgap peak is dependent on the intermediate F2 layer thickness and ferromagnetic exchange eld h in which the peak position oscillates between zero bias and the CB with a periodicity of =h. These subgap peaks are resistant to high interfacial barriers and lead to a monotonic angular dependence on in the peak maxima. In the S1=F1=N=F2=S2 quasiparticle conductance, there are multiple subgap peaks with similar oscillations in the peak positions. In addition, the conductance peak position oscillates with by a quarter phase between the parallel and antiparallel conguration. We also study the spin transport in the F1=N=F2=S system for realistic parameters. The spin transport quantities are not conserved due to the spin transfer torque (STT) within the ferromagnetic layers, and are spatially dependent. There exists a critical bias feature in which no spin current penetrates the S layer for biases below the CB, and the STT becomes quasilinear for biases above the critical bias.