The aim of this theoretical project was to advance understanding of the physical mechanisms by which the interaction of the microscopic magnetic moments in magnetic materials influences their observable properties, such as the magnetic ordering, magnetocrystalline anisotropy, and spin-dependent transport in magnetic nanostructures. Computational tools developed in the course of this project, along with the knowledge gained from their application to specific systems, extends our ability to predict the properties and design new materials for various applications, such as more efficient and inexpensive permanent magnets and emerging spintronic device technologies exploiting electron spin for information processing. This research is based on the first-principles electronic structure theory, contributing to the broader vision of being able to design materials with desired properties through computer simulations based on the fundamental principles of quantum mechanics.

The magnetocrystalline anisotropy and magnetic interaction have been investigated in Fe₂B and Fe₂P-based alloy systems. Microscopic mechanisms responsible for the unusual concentration and temperature dependence of the magnetocrystalline anisotropy in (Fe-Co)₂B alloys have been explained in detail. In particular, it was found that spin fluctuations alone can lead to anomalous temperature dependence of the magnetocrystalline anisotropy. For the Fe₂P-based alloy system, the mechanisms through which the Curie temperature and magnetocrystalline anisotropy can be optimized for specific applications have been elucidated, and a strategy for maximizing the magnetocrystalline anisotropy at and above room temperature has been proposed.

A novel approach to the construction of magnetic phase diagrams of metallic alloys was developed, enabling the prediction of the magnetic state of an alloy at a given concentration and temperature. Application to the (Fe-Mn)Pt “magnetic chameleon” system yielded the complicated magnetic phase diagram in good agreement with experiment, while demonstrating the importance of going beyond the Heisenberg model in describing the magnetic interactions.

The magnetoelectronic circuit theory describing spin transport in nanoelectronic devices has been extended to account for spin-non-conserving scattering at interfaces and applied to elucidate the microscopic meaning of the phenomenological “spin memory loss” parameter measured in metallic multilayers. First-principles calculations have been used to compute the relevant parameters for specific material combinations. This work contributes to better understanding of spin-dependent transport in multilayers and other heterostructures.

The effects of thermal spin disorder and excess manganese on the electronic spectrum of NiMnSb, a half-metallic ferromagnet where only states of one spin projection are present at the Fermi level at zero temperature in the absence of defects. It was found that thermal spin disorder of the weakly coupled spins of excess Mn on the Sb sites destroys the half-metallic gap at low temperatures. This property of the antisite Mn/Sb defect may be the source of the observed low-temperature transport anomalies.

It was shown that magnetic proximity effects can be tuned by electric gating, and the spin polarization may even be reversed in a two-dimensional heterostructure, such as a layer of graphene with a hexagonal boron-nitride underlayer on the surface of cobalt metal. This suggests the possibility of implementing an electrically reversible spin injector as an alternative to using the applied magnetic field or spin transfer torque in spintronic devices, turning a spin valve into a spin transistor as a result.

The stability of skyrmion lattices has been studied in quasi-two-dimensional magnets where the symmetry of the Dzyaloshinskii-Moriya interaction has a lowered symmetry, and the corresponding phase diagrams have been constructed. It was found that the skyrmion phase remains present in the absence of axial symmetry, but its geometry is modified. Under certain conditions, a square vortex-antivortex lattice is stabilized, while interaction of a certain type produces an antiskyrmion lattice.

During the course of this project, efficient computational tools have been developed for the studies of temperature-dependent electronic structure and magnetocrystalline anisotropy in alloys, and of electronic transport in nanostructures in the presence of spin-orbit coupling. These tools have been implemented as part of the Questaal software package, which is available for the broader computational materials research community. This project also provided educational experiences in advanced materials theory for graduate students at the University of Nebraska-Lincoln. The results have been disseminated through publications in professional journals and conference presentations.

Last Modified: 09/26/2017
Modified by: Kirill D Belashchenko