Date of Award
Doctor of Philosophy
This thesis describes the microscopic, quantum mechanical theory of exchange-dominated spin waves in Heisenberg ferromagnetic and antiferromagnetic thin films including the effects of nonuniaxial single-ion anisotropy. The results constitute a generalization of previous theoretical studies on spin waves in thin films with uniaxial anisotropy and in semi-infinite ferromagnets with nonuniaxial anisotropy.;A spin Hamiltonian containing nearest-neighbour exchange terms, Zeeman terms, and single-ion anisotropy terms is used. The films may be asymmetric with respect to surface exchange and anisotropy parameters which are also assured to be perturbed from the bulk values. The results apply both to cases in which the nonuniaxial anisotropy is an intrinsic aspect of the material and where it arises only at the surfaces as a consequence of lowered symmetry for those sites. Low temperatures are assumed where the linear spin-wave approximation is valid. The formalism is developed for arbitrary film thickness, arbitrary quantum spin number S, and perpendicular magnetization. Simple cubic (001) ferromagnetic systems and body-centered tetragonal (001) antiferromagnetic systems are specifically examined with extensions to other situations outlined.;A theoretical approach based on the equation-of-motion method is used to find Green functions which provide expressions for the dispersion relations for surface and quantized bulk spin waves, the associated spectral intensities, transverse spin correlation functions, and the dynamic response of the system in, for example, light scattering and spin wave resonance experiments. Representative numerical examples are provided for the dispersion relation results, some thermodynamic properties related to the mean-squared amplitude and ellipticity of spin precession, and the static magnetization. The use of the Green function results in calculating the light scattering cross-sections and absorption strength in spin wave resonance is outlined. Procedures developed to manage the increased mathematical complication associated with the nonuniaxial anisotropy are described.
Heron, Paula R., "Theory Of Spin Waves In Heisenberg Ferromagnetic And Antiferromagnetic Thin Films With Nonuniaxial Single-ion Anisotropy" (1995). Digitized Theses. 2557.