LLB MICROMAGNETIC MODELS OF NANO-GRANULAR MAGNETIC THIN FILMS FOR ULTRA-HIGH DENSITY RECORDING MEDIA

The continuing need for increased information storage capacity has driven a remarkable increase in areal density over the lifetime of hard disk technologies, built on the physical principles of nano magnetic structures. Understanding how the component nano-granular magnetic recording media operates at ultra-high densities and how novel switching mechanisms, such as heat assisted magnetic recording (HAMR), impact the nature of magnetic data storage is essential. In this thesis current state-of-the-art macro scale modeling methods, built on the principles of the Landau-Lifshitz-Bloch equation, are developed and applied to better understand the physical principles that govern the ultra-high density recording media. The modeling method is shown to be in close agreement with experiment in a number of situations. The dependence of magnetic damping, the combination of intrinsic and extrinsic damping, due to inter-granular interaction is shown to be significant. The form of the damping arises due to a change in the degeneracy of the ferromagnetic frequency of spinwaves, as a function of both increasing magnetostatic and inter-granular exchange interactions. The observed damping results in a nontrivial dependence of the magnetic switching time on intergranular interactions, within the range for the intergranular exchange and saturation magnetisation that is likely in ultra-high density recording media. The nontrivial nature of the switching time should be taken in to consideration when selecting materials for the magnetic grains and inter-granular regions. A detailed investigation of the HAMR process is made, concentrating on the thermodynamic limits of the technology. The nature of HAMR is shown to be far more complex than simply magnetisation reversal over a thermally reduced energy barrier. It is shown that, to achieve the required level of magnetisation reversal a number of factors must be considered. The temperature rise must be to the Curie point or above, invoking the linear reversal mechanism, with a cooling rate that is sufficiently low to allow the temperature of the media to remain higher than the blocking temperature for a period of time significantly larger than the relaxation time of the material. Also, the write field must be sufficiently large not only to reverse the magnetisation, but also to ensure no thermally activated back switching of the magnetisation, as in the concept of thermal writability. Also a new method for approximating the magnetostatic field to a high level of accuracy with a computational runtime that is comparable with the tensor form of the dipole approximation has been developed and tested.