Spin Caloritronic Phenomena Driven by Spin-orbit Coupling

In this thesis, we report several effects in spintronics and spin caloritronics related to relativistic spin-orbit coupling. In Chapter 2, we discuss the relativistic spin caloritronicHall effects in terms of a semiclassical theory for anomalous thermoelectric effects in ferromagnetic metals due to spin-orbit scattering at impurities, including the anomalous Nernst and Ettingshausen effect, the planar thermalHall effects, and thermolectric anisotropic magnetoresistance. The linear response relations between the currents and driving forces are derived for out-of-plane and in-plane magnetizations, respectively. In the out-of-plane configuration, there are anomalous thermoelectric Hall effects linear to the spin-orbit constant, while the thermoelectric anisotropic magnetoresistance and the planar Hall effect in the in-plane configuration are of second order in the spin-orbit coupling. The extrinsic theory systemizes the competing effects/mechanisms from a microscopic point of view and identifies the parameters needed to describe experiments. We developed a diffusion theory in Chapter 3 for the spin Hall magnetoresistance (SMR) in multilayers made from an insulating magnet F such as yttriumiron garnet (YIG), and a normal metal N with spin-orbit interactions, such as platinum (Pt). In an N|F bilayer system, the SMR requires spin-flip in N and spin-transfer at the N|F interface. Our results explain the SMR both qualitatively and quantitatively with transport parameters that are consistent with other experiments. The degrees of spin accumulation in N that can be controlled by the magnetization direction is found to be very significant. In the presence of an imaginary part of the spin-mixing conductance Gi we predicted an AHE-like signal (SHAHE), which has been observed experimentally and can be explained with values of Gi that agree with first principles calculations. We furthermore analyzed F|N|F spin valves for parallel and perpendicular magnetization configurations. The SMR torques under applied currents in N are expected to lead to magnetization dynamics of N|F and F|N|F structures. In Chapter 4,we generalized the SMR theory in Chapter 3 to a thin-film made of a metallic ferromagnet and take into account the out-of-plane spin currents generated by the spinHall effect, which were disregarded in Chapter 2. We predict a new contribution to the anisotropic magnetoresistance by the simultaneous action of the anomalous Hall effect and its inverse. By diffusion theory, we compare this contribution with the conventional anisotropic magnetoresistance, demonstrating that they can be distinguished experimentally by studying its dependence on the film thickness. The extra contribution to the magnetoresistance has a magnetization dependence different from that of the conventional AMR. While the conventional AMR is usually positive, the new contribution is always negative. In order to analyze the effect of interface and boundary roughness that was disregarded in Chapter 3, we reports in Chapter 5 a Boltzmann study to quantify how the surface/interface scattering affects the spin Hall physics. In a bilayer system made of N and FI, we observe an AHE-like transverse voltage induced by the spin dependent scattering at the FI|N interface, which is competing with the imaginary SMR predicted in Chapter 3. We further show that the spin diffusion equation on which the SMR in Chapter 3 is based, has to be corrected by the surface/interface roughness in the limit of thin-films. Our model provides an approach to analyze the role of roughness in recent measurements on layered systems. Even though the theories developed in Chapters 3–5 are not directly related to spin caloritronics, they can be easily generalized for their thermoelectric analogues by the formulation spelt out in Chapter 2, and can be useful for prospective research in spintronics and spin caloritronics.Quantum NanoscienceApplied Science