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J. K. Glasbrenner, Ab-initio and model studies of spin fluctuation effects in transport and thermodynamics of magnetic metals, Ph.D. dissertation, University of Nebraska (2013).
Magnetic materials are vital to many devices and the manipulation of spins is central to the operation of novel devices such as spin transistors. It is important to understand the effect of spin fluctuations on such systems. In this dissertation, first-principles calculations and models further the understanding of spin fluctuation effects in the transport and thermodynamics of magnetic metals.
A simple classical spin-fluctuation Hamiltonian with a single itinerancy parameter is studied using the mean-field approximation, Monte Carlo simulations, and a generalized Onsager cavity field method. The results of these different methods are in agreement. It is found that the thermodynamics are sensitive to the choice of phase space measure and that short-range order is weak for all degrees of itinerancy.
Spin injection from a half-metallic electrode in the presence of thermal spin disorder is analyzed using a combination of random matrix theory, spin-diffusion theory, and explicit simulations for the tight-binding s-d model. It is shown that spin-flip scattering from the interface destroys spin coherence. Spin injection is possible and is constrained by the mean-free path and spin diffusion length in the semiconductor.
The spin-disorder resistivity (SDR) is calculated for the Gd-Tm series in the paramagnetic state using two complimentary first-principles approaches. The SDR in the series follows an almost universal dependence on the exchange splitting and is underestimated when compared with experiment. Frozen atomic displacements (phonons) are then introduced along with spin disorder and the total resistivity is calculated as a function of the mean-square displacement for Fe and Gd. The resistivity increases non-linearly for small displacements and transitions to a linear dependence at larger displacements that, when fitted, enhances the SDR. The enhancement observed in Gd is substantial. The enhancements are electronic in origin, and the rapid increase observed in Gd is traced to a strong, disorder-induced interaction between the electron and hole Fermi surfaces, while the linear trend at large displacements is a saturation effect brought on by strong disorder.
Adviser: Kirill D. Belashchenko