Browsing by Subject "Proximity effects"

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    Proximity Effects in Ferromagnet/Superconductor Layered Heterostructures with Inhomogeneous Magnetization
    (2014-07) Wu, Chien-Te
    In this thesis, we present a theoretical investigation of the proximity effects in ferromagnet/superconductor heterostructures with inhomogeneous magnetization, including ferromagnet/ferromagnet/superconductor (F1F2S) trilayers and conical-ferromagnet/superconductor bilayers. We numerically obtain the self-consistent solutions of the Bogoliubov-de Gennes (BdG) equations and use these solutions to compute the relevant physical quantities. In F1F2S trilayers, we find that the critical temperature, Tc, can be a non-monotonic function of the angle &alpha between magnetizations in F layers. The minimum Tc(&alpha) often occurs when magnetizations are mutually perpendicular (&alpha=&pi/2). In addition, we demonstrate that the Tc minimum corresponds to the maximum of the penetration of the long-range triplet amplitudes. We compare our theoretical results with experiment and find that they are in excellent agreement. We also study other aspects of proximity effects such as the local density of states, local magnetizations, and thermodynamic functions. In conical-ferromagnet/superconductor bilayers, we obtain the relation between Tc and the thickness dF of the magnetic layer, and find that the Tc(dF) curves include multiple oscillations. Moreover, for a range of dF, the superconductivity is reentrant with temperature T: as one lowers T the system turns superconducting, and when T is further lowered it returns to normal. We demonstrate that the behavior of both m=0 and m=±1 triplet amplitudes are related to the intrinsic periodicity of conical ferromagnet. Our theoretical fits of Tc(dF) are in good agreement with experimental data. The transport properties, including the tunneling conductance and the spin polarized transport, in F1F2S trilayers are investigated. To fully take into account proximity effects, we adopt a transfer matrix method incorporated with the Blonder-Tinkham-Klapwijk formalism and self-consistent solutions to the BdG equations. We show that our method ensures that conservation laws are properly satisfied. Our results indicate that the behavior of tunneling conductance depends on the misorientation angle between magnetizations, and also exhibits resonance effects. We also investigate the bias dependence of non-equilibrium spin transfer torque and its connection to both spin currents and local magnetizations.
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    Transport in Superconducting/Ferromagnetic Heterostructures
    (2019-07) Moen, Evan
    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.