Exchange interaction energy in magnetic recording simulation

Based on a phenomenological theory, micromagnetic simulations and experiments are used to evaluate an improved function for the exchange interaction between magnetic particles in perpendicular recording media. Assuming diluted spin layers in the particle boundary and a gradual rather than abrupt rotation of magnetization between grain cores, the exchange energy is better described by an even power series of θ, rather than a cosine function. The conventional cosine function does not have a restoring torque near θ = π and adjacent grains tend to align strictly antiparallel. In contrast, using a power series of θ, adjacent grains tend to align at a small angle away from θ = π. This gives rise to a small in-plane magnetization component and therefore a distinct peak in in-plane susceptibility is observed around H = 0. From magnetization measurements of a real medium, a peak is observed around H = 0, which matches with an assumption of 2 or 3 spin layers. In some situations, the exchange interaction between di...