Three-dimensional modelling of metal evaporated tape

Abstract

As developments in magnetic recording increase, the demands for high density recording media continue to rise. One particular medium developed towards meeting these needs is the commercially successftil metal evaporated (ME) tape. To provide a deeper understanding of the magnetic processes occurring in recording media, theoretical studies are carried out to complement experimental observations. Therefore, this thesis is concerned with developing a micromagnetic model to simulate the magnetic behaviour of ME tape.
ME tape consists of bundles of granular columns which are tilted towards the film normal. The morphology of the film influences the magnetic behaviour and a 3D model is developed in this thesis to represent the columnar structure and the granular nature of ME tape. A Monte Carlo algorithm, consisting of a 'move and grow' procedure, is developed to generate an irregular system of spherical grains whose radii were generated from a log-normal distribution. This structure provides the foundation for modelling columns of ME tape.
Initially an isolated column of ME tape was investigated. Grains were considered to be single domain and the dynamic process of the magnetic moments was explored by the Landau-Lifschitz equation. The model demonstrated that as the strength of the exchange coupling parameter increased, then moments which were strongly coupled resisted large negative fields and square hysteresis loops with a high coercivity were produced.
Reversal of the moments was observed to be initiated at the ends of a column and then propagated throughout the column. This was attributed to large demagnetising fields and was particularly evident in the model with aligned easy axes. If the easy axes were randomiy orientated then more disorder was present in the model and moments rotated incoherently. This produced hysteresis loops that were less square than those of the aligned model. The results also showed how the microstructure influenced the reversal processes.
Simulations on assemblies of interacting columns of ME tape highlighted that, when exchange coupling was weak, moments reversed incoherently throughout the structure. However, increasing the strength of the exchange coupling caused a high degree of cooperative reversal to occur in individual columns. As moments reverse their direction they initially relax towards the long axis of a column due to column demagnetising effects. The model demonstrates that as moments reverse there is a changing balance between magnetostatic (demagnetising) and exchange interactions within and between columns.
The model was considered to be a good representation of ME tape, and the results obtained not only complement previous studies but provide a deeper understanding of the reversal processes occurring both within and between columns of ME tape.