UCLA

This thesis advances the theory of quantum and semiclassical transport in magnetic heterostructures. In the solid state, angular momentum can be carried by individual electrons and collective modes. The flow of angular momentum (a spin current), central to the operation of spintronic devices, is generated by the application of electric and magnetic fields and temperature gradients. In what follows, we explore the physics of such nonequilibrium spin currents in magnetic structures, involving an interplay of charge and magnetic dynamics and thermoelectric effects.

Chapter 1 provides an introduction to the transport of spin in magnets, carried by electrons and collective excitations of the magnetic order. Chapters 2-6 study the role of thermal fluctuations in transport and magnetic dynamics. In Chapter 2, we describe how incoherent thermal fluctuations of the spin density (magnons), which open inelastic scattering channels, contribute to spin and energy transport between a normal metal and a magnet. Such (temperature-dependent) transport may arise from a thermal gradient applied across the metal/magnet interface or a spin accumulation inside the normal metal and may alter or even drive magnetic dynamics.

Chapter 3, is dedicated to the realization of Bose-Einstein condensed magnons (previously observed by microwave pumping) in a normal metal/insulating ferromagnet heterostructure. As is described in Chapter 2, the combination of a temperature gradient and normal metal spin accumulation can drive spin into the insulating ferromagnet, accumulating as magnons; upon reaching a critical density, the magnons, which are bosonic, spontaneously form a quasi-equilibrium condensate.

Chapter 4 focuses on thermally driven spin-torques in electrically insulating structures, wherein direct electric control of magnetic dynamics is prohibited. In contrast to the interfacial transport described in Chapter 2, where a spin accumulation is to necessary to observe magnetic dynamics, here we demonstrate how spin-torques can arise from a pure thermal gradient in a heterostructure. These spin-torques can be measured by ferromagnetic resonance and can, under a sufficiently strong bias, actuate magnetic switching.

Chapter 5 concerns charge transport in a single-electron transistor, consisting of a magnetic quantum dot in contact with magnetic and normal metal leads. Microwave-driven precession by the dot induces a pumped electric current, which can be enhanced and made highly nonlinear by electron interactions (Coulomb blockade). The dependence of the resulting electrical response on the power and spectrum of microwave irradiation may be utilized to develop nanoscale microwave detectors analogous to single-electron transistor-based electrostatic sensors and nanoelectromechanical devices.

In Chapter 6 we study bilayers, composed of a nonmagnetic conducting and a magnetic layer. We develop a general phenomenology for the magnetic and charge dynamics, which are coupled by spin-orbit interactions. In contrast to Chapters 2-4, we focus on the long-wavelength magnetic dynamics, which is subject to current-induced torques and produces fictitious electromotive forces that drive charge dynamics.}