

Fig. 3.8 represents a hysteresis cycle of FePt/FeRh magnetic grain calculated for two different thicknesses of FePt. The nucleation process starts in FeRh grain, which is softer but has an additional shape anisotropy due to the grain elongation. The domain wall (exchange spring) of the Néeltype propagates and gets pinned at the FePt/FeRh interface. An additional field is necessary to push the center of the domain wall into the FePt (see Fig. 3.9). Associated with the movement of the domain wall, there is a change of the domain wall width illustrated in Fig. 3.10. The initial process is a compression of the domain wall against the interface [Dobin 06] that is reflected in the reduction of its width. Once the center of the domain wall penetrates inside the hard medium, this produces the complete magnetization reversal in FePt.
Fig. 3.11 represents the coercivity reduction in the case of the exchange spring medium as a function of the FePt thickness. For a thinner () FePt layer, the domain wall could not be completely formed in the hard magnetic material (with the domain wall thickness ) as can be observed in Fig. 3.12. This is important since the total domain wall formation is an implicit assumption of some analytical models [Asti 06,Loxley 06,Kronmüller 02] that consider infinite thickness, included the one used to obtain the pinning field Eq. (3.4). This reduces the coercivity because the pinning is less effective. A remarkable reduction of the switching field (more than times) than that of the pure FePt grain could be obtained even with small interfacial exchange value for thin FePt layer.


Fig. 3.13 represents the coercive field reduction as a function of the thickness of FeRh. It is clearly seen that the exchange spring formation (thick soft layer) is more efficient in decreasing the coercive field value than the case of thin soft layer. The coercivity saturates when the length of soft material is larger than its domain wall width. Fig. 3.14 represents the coercivity reduction as a function of interfacial exchange in grains with different thicknesses. Substantial reduction of the switching field could be achieved with interfacial exchange of the order of of the bulk value. However, more interfacial exchange is necessary for thicker FePt layer as compared to a thinner one to get the same reduction.


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