UCLA

This thesis advances the theory of spintronics by exploring ways in which collective spin dynamics can be manipulated in magnetic materials, with an emphasis on sytems which can exhibit low dissipation spin currents and nontrivial response to external driving.

In Chapter 1, we provide an introductory review of mesoscopic ferromagnetic dynamics. The Landau-Lifshitz equation is motivated and derived, with a brief discussion on its application in the presence of dissipative forces and external driving. Then we discuss the notion of spin superfluidity in easy plane ferromagnets and compare this to traditional Ginzberg-Landau superconductivity.

In Chapter 2, we further extend the analogy between ferromagnetism and superconductivity, and apply this anology to the study of the ferromagnet with strong coherent easy-plane anisotropy and weak in-plane coherent anisotropy. The low bias non-equilibrium phase diagram is mapped, and the potential for producing applications with superconductor-based circuit functionality at elevated temperatures is discussed.

In Chapter 3, the spin superfluid is studied in strong driving regimes in which the anaology to superconductivity begins to break down. Multiple non-equilibrium phases are discovered which are associated with the onset of non-linear effects, such as choatic oscillations and stationary soliton formation, near the spin torque injection region. Using numerical simulations and analytical modeling, we observe a robustness in spin superfluid transport and its high bias phase diagram despite the presence of symmetry breaking dipole-dipole interactions, finite size effects, and dissipation.

In Chapter 4, we turn to microscopic magnetic lattices, in particular weakly coupled arrays of quantum spin chains with fermionizable, via a Jordan-Wigner transformation, Hamiltonians. Using mean field theory, we find models which may exhibit dissipationless spin currents closely analogous to that of superconductivity and the quantum Hall effect.

In Chapter 5, we study the manipulation of magnetic domain walls on a wire with mechanical waves. We show how ferromagnetic and anitferromagnetic domain walls can be driven by circularly and linearly polarized waves, respectively. We note the potential for applications using mechanical waves as a means for manipulating magnetic solitons in insulators.

For Chapter 6, in collaboration with an experimental study on a cleaved edge overgrowth sample, we analyze the chiral edge states of the quantum Hall effect and their tunneling properties. Our modeling provides a theoretical fooundation from which the experimental method of momentum resolved spectroscopy can provide insight into how the quantum Hall effect evolves with changing magnetic field. Spin splitting of the chiral edge states is observed in the presence of a strong in-plane magnetic field.

We close with Chapter 7 giving an outlook for future work which could build off of the research presented in this thesis.