Study of attempt frequency in Sharrock's formula for perpendicular recording hard disk media

Change of the attempt frequency fo in Sharrock's formula as functions of magnetostatic and inter-grain exchange couplings has been investigated using micromagnetic simulation. It was found that fo increases monotonically with increasing the magnetostatic coupling, whose mechanism is not understood yet. It was also found that fo decreases initially with increasing the inter-grain exchange coupling, below a critical value and increases above it. The initial decrease is likely due to that the exchange coupling depresses the effect of the magnetostatic coupling, because the effective fields from the neighboring grains are competing with each other. The increase of fo above the critical value is deeply related to the increase of the thermal activation volume V.