Finite-size and Surface Effects in Iron Platinum Nanograins for Applications in Heat-Assisted Magnetic Recording Media

Heat-Assisted Magnetic Recording (HAMR) has emerged as a promising next-generation approach for magnetic recording. The functioning of HAMR requires the recording media to be made of magnetic materials with high anisotropy and suitable Curie temperature. Iron Platinum in the L10-phase (L10-FePt) has been found to satisfy these two requirements, thus having been attracting extensive attention and investigation. This Thesis presents a coherent and comprehensive computational research on the impacts of finite-size and surface effects in L10-FePt for potential HAMR applications. Simulations are performed by VAMPIRE an atomistic simulation code developed in the University of York and several key findings have been obtained. First, the existence of a size threshold at 3.5 nm is discovered below which finite-size effects are found to permeate into the centre of the L10-FePt grains leading to the well-known Curie temperature dispersion of the recording medium. A correlation between the Curie temperature dispersion and surface disorder is formulated which can be extended beyond L10-FePt to be applicable to different crystal structures. Second, a novel fourth-order anisotropy component is found in a phase-coupled core-shell structured L10/A1-FePt grain which exhibits strong size and geometry-dependence unseen in previous literature. Additionally, the scaling to magnetisation of this fourth-order anisotropy is found to disobey the classical Callen-Callen power law. These properties can be successfully explained by an analytic model which demonstrates the origin of this novel fourth-order anisotropy to be from the canting of the core and shell magnetisation. The applicability of this analytic model, also, is shown to extend to be valid in a generic soft-hard coupled nanocomposite magnetic system. Finally, the switching efficiency of this phase-coupled core-shell structured L10/A1-FePt grain is investigated. Available data demonstrate the existence of a non-negligible switching error rate in all tested grain configurations. Reducing the grain size and using shorter write pulses are found to induce higher switching error rate. However, it is shown that these detriments can be mitigated by surface engineering. Overall, the studies presented in this Thesis have addressed and provided many helpful insights into outstanding challenges on the path toward the realisation of L10-FePt in HAMR recording media.