Collective thermal fluctuations in nominally "independent" granular recording media: application to thermal decay

We present initial results of a study of a system of fine magnetic grains with resonance interactions. For the case of magnetostatic and exchange interactions a general criterion for collective magnetization motion of a pair of grains is derived. Simulations demonstrate a dynamic cluster structure in both longitudinal and perpendicular recording media. These clusters may significantly reduce thermal degradation of recorded patterns. An indirect interaction between grains through an antiferromagnetic substrate is also discussed.