Ab Initio Calculations of the Magnetic Properties of Bimetallic Alloys

Magnetic recording technology is ubiquitous in the modern world and constitutes a corner stone of current research and development. Recent inventions such as heat-assisted magnetic recording and exchange-coupled media has focused magnetic recording research towards alloys exhibiting strong magnetocrystalline anisotropies. In this thesis, we investigate, from first principles using the screened Korringa-Kohn-Rostoker method, the effect of a range of compositional and microstructural defects and features upon the magnetocrystalline anisotropy energy (MAE) of FePt and CoPt alloys. We show that localised Pt alloying affects the MAE of bulk Co primarily through the Pt-induced effects on the Co sites. We demonstrate that stacking faults often reduce the Co MAE and that the effect of composite stacking faults upon the MAE is not necessarily additive, but synergistic. By varying the unit cell geometry and the compositional parameters of FePt, we show that the formation of complete Fe layers is, generally, the dominant factor in maintaining a large MAE. We investigate the magnetic properties (spin moments, magnetocrystalline anisotropy and magnetic exchange) across an Fe/FePt/Fe multi-layer and show that the effective exchange exhibits a strong reduction at the Fe/FePt interfaces and that the MAE of the whole multilayer system is very slightly reduced by the presence of the Fe/FePt interfaces. Across all systems, we observe that localised features such as stacking faults, interfaces and localised alloying incur relatively long-ranged spatial oscillations in the MAE, which may, in turn, cause significant finite-size effects on the nano-scale.