Modelling of Exchange Spring Media magnetic properties (Vol. 44 No. 2)

The authors analyze the magnetic properties of strongly interacting hard and soft magnetic thin films: so called Exchange Spring Media (ESM). Such materials are characterized by a good thermal stability, while keeping the coercive field of the total structure below the value of the hard magnetic layer. FePt hard/soft bilayers are used, of which crystallographic nanostructure is very complex, as shown by Transmission Electron Microscopy (TEM). For instance, a rough interface between hard and soft layers is created with obstacles of one phase embedded in the second phase (Fig. left).

To understand how such a complex nanostructure influences magnetic properties of the ESM, the experimental phase distributions were transferred into numerical models (Fig. right) with which the magnetization reversal processes were studied using finite element micro-magnetic simulations. This analysis was supported by the analytical consideration of the magnetic pinning field (damping of the magnetization reversal). The combination of analytical and numerical approaches showed that observed nanostructures could significantly alter the coercive field of the media. For each distribution the coercive field minimum was found when nucleation field was equal to the maximal pinning field within the ESM structure. Moreover, the influence of the obstacles on the magnetic properties of the media was strongly restricted by their size and position within the material.

The present model of magnetic properties analysis is general enough to be applied to other ESM materials, such as CoPt or FePt/FeAu hard/soft structures.