Simulation of the response of particles in recording media to rotating magnetic fields

In a series of experiments, a section of commercial recording magnetic tape made with metal particles is magnetized first and then subjected to rotating magnetic fields. Much of the observed response is mimicked by the results of calculations for a small group (up to 729 members) of interacting single-domain uniaxial particles embedded in a Lorentz sphere with the region outside the sphere reflecting the properties of the small group. The calculations are compared with the experiments in some detail. There is a noteworthy effect that is not well represented by the calculations. The memory of the initial magnetization decreases slowly with cycle number n for very large numbers of cycles of rotation. The experiments can be described by a power law in which the residual of the initial magnetization approaches its final value as {1}/{n ^p} with p as small as {1}/{6} for fields near the coercivity, apparently requiring many millions of cycles to approach a limit cycle. In simulations with small numbers of particles the decay of the initial magnetization appears to occur on the scale of tens of cycles. This indicates that the power law requires the interaction of large numbers of particles for its existence.