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Materials analysis based on partially correlated magnetic switching

Periodic Report Summary - MAPACOMAS (Materials analysis based on partially correlated magnetic switching)

The present reintegration grant project is aimed at developing an accurate characterisation technique for perpendicular magnetic recording (PMR) media based upon a quantitative analysis of their complex magnetisation reversal behaviour. In general, the design of characterisation methods for PMR media relies on devising accurate microscopic models and on solving the 'inverse problem', in which materials parameters can be determined unambiguously from a certain set of measurements.

The specific challenge here is that grain-to-grain interactions in PMR media are rather strong and result in hysteresis properties that are characterised by partially correlated reversal - a rather poorly explored regime in contrast to weakly or strongly correlated cases.

Correspondingly, the following objectives for the proposal were originally identified:
i. Verify the sufficiently realistic nature of a previously devised microscopic interacting hysteron model as the base for numerical and analytic method development.
ii. Derive accurate analysis schemes for partially correlated magnetisation reversal as a foundation for reliable characterisation methods for PMR media.
iii. Extend the methodology developed in (i) and (ii) to include thermal activation processes.
The work implies an understanding of magnetic phenomena on very small length and very short time scales such as the unexplored physics of partially correlated magnetisation reversal.

While the method has been shown to be accurate for modest levels of inter-granular interaction, it fails for completely arbitrary input parameters. However, its failure mode has been demonstrated as being well behaved and occurring in a parameter range that is close to that of real PMR media materials, which suggests possibilities for improving the method to cover even a broader range.

For this purpose, and in accordance with the aims of the objective, (i) we verified the suitability of the microscopic non-mean-field IHM, by comparing it to more realistic calculations based on micromagnetics. Micromagnetic modelling itself is impractical for the key purpose of the proposal because it contains too much detailed information and, as a consequence, does not allow for computational analysis of sufficiently large system sizes and long time-scales, or the use of analytical methods. The comparison between IHM and micromagnetics has been performed by using the least squares fitting and the statistical confidence level assessment. A good agreement was found as long as exchange energies remain smaller than the average particle anisotropy energy.

In accordance with the objectives of the goal (ii), we have successfully developed a framework for identification of PMR material properties, such as the exchange and magneto-static interactions and intrinsic switching field distributions.

Contact information:
Person in charge of scientific aspects:
Dr Andreas Berger
Tel: 943 57 4000
Fax: 943 57 4001