Micromagnetics of exchange spring media: Optimization and limits

Abstract

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 1 4 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 l h where l h 2 = 2 π 2 A s / K h , A s is the exchange constant in the soft layer and K h 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.