Ab initio study of magnetic effects at material interfaces .

Abstract

Fairly little is known from a fundamental first principles level about the role of magnetism at material interfaces. This thesis will address (a) changes in spin density at grain boundaries and surfaces; (b) the impact of these changes on interfacial energies and structures; and (c) the behavior of point defects (including vacancies, impurities and adatoms) at interfaces. We first develop a simple, general energy functional for ferromagnetic materials based upon a local spin density extension to the Stoner theory of itinerant ferromagnetism and use it to explore the physics of grain boundaries in iron, such as interfacial energies, structures, and magnetic effects. Our results show that magnetism, in addition to driving structural relaxation, also greatly enhances intergranular cohesion in iron. To explore the effects of point defects at material interfaces, we present an extensive study of non-magnetic Molybdenum grain boundaries. This trend study which is carried out with an appropriate atomistic potential reveals an important set of structural phase transitions involving the exchange of vacancies with the surrounding bulk material. We also show that the same mechanism of vacancy driven structural phase transitions appear when these systems are studied with first principles techniques. Finally, we explore the role of magnetism in the diffusion of adatoms and vacancy migration at surfaces and steps in a transition path study of Cobalt.