Micromagnetics of exchange spring media: Optimization and limits

The magnetic properties of exchange spring media are compared with perpendicular recording media using simple analytical estimates and using micromagnetic simulations. For a hard layer exchange coupled to a perfectly soft layer, the theory predicts a coercive field of the entire system of 14 of the anisotropy field of the hard layer in the limit of infinite layer thicknesses. A value of the coercive field close to the maximum reduction can be achieved for soft layers that are thicker than lh where lh2=2π2As/Kh,As is the exchange constant in the soft layer and Kh the anisotropy constant of the hard layer. For multilayer structures with gradually decreasing anistropy in each layer the coercive field can be decreased even further. A genetic optimization algorithm is used to design a bilayer structure as close as possible to the theoretical limit. The optimization shows that a gain in energy barrier of about 70% can be expected practically in exchange spring media with two layers and zero anisotropy in the soft layer. The switching time of exchange spring media for external fields applied 10° off the easy axis is about 0.4 ns which is comparable to single-phase media.