The generic problem of extracting information on intrinsic particle properties from the whole class of interacting magnetic fine particle systems is a long standing and difficult inverse problem. As an example, the Switching Field Distribution (SFD) is an important quantity in the characterization of magnetic systems, and its determination in many technological applications, such as recording media, is especially challenging. Techniques such as the first order reversal curve (FORC) methods, were developed to extract the SFD from macroscopic measurements. However, all methods rely on separating the contributions to the measurements of the intrinsic SFD and the extrinsic effects of magnetostatic and exchange interactions. We investigate the underlying physics of the FORC method by applying it to the output predictions of a kinetic Monte-Carlo model with known input parameters. We show that the FORC method is valid only in cases of weak spatial correlation of the magnetisation and suggest a more general approach.

In order to enhance the performance of advanced granular recording media and understand the physics behind the mechanism of the reversal process, an atomistic spin dynamics simulation is used to investigate theoretically the magnetic properties and the magnetization reversal behaviour for a composite media design. This model allows to investigate the efect of the magnetostatic interaction and inter/intra layer exchange coupling for realistic system. The composite granular medium investigated consists of hard and soft composite layers in which the grains are well segregated with a continuous layer capping layer deposited to provide uniform exchange coupling. We present a detailed calculation aimed at revealing the reversal mechanism. In particular, the angular dependence of the critical ield is investigated to understand the switching process. The calculations show a complex reversal mechanism driven by the magnetostatic interaction. It is also demonstrated, at high sweep rates consistent with the recording process, that thermal efects lead to a signiicant and irreducible contribution to the switching ield distribution.

Transition jitter is one of the most important parameters for evaluating the potential areal density of Heat Assisted Magnetic Recording. However, obtaining this either experimentally or through micro-magnetic simulation is time consuming. Here, we show that the more easily calculated thermal switching probability distribution (SPD) can serve as a substitute. In particular, we provide an equation relating the standard deviation of write temperature to the jitter. The equation is verified for three critical noise sources: Tc variance, HK variance, and grain size distribution. In addition, the SPD subject to cooling at various cross-track positions is investigated. A method to determine the magnetic grain size is proposed.

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Recording simulations on heat-assisted magnetic recording (HAMR) are implemented to study the dependences of grain size and Gilbert damping on the recording performance. Modified damping and temperature shift for renormalized media cells are needed to address the non-equilibrium nature of HAMR process. It is found that using smaller grains helps reduce transition jitter but increases track width. Larger damping is also found effective in reducing the jitter. Finally, the applicability of this simulation method is corroborated by comparing with experimental results.