3.12 Conclusions

In summary, we have studied the field and thermal magnetization switching in soft/hard bilayers for magnetic recording application. Both can consist in a complex reversal mechanism involving the penetration of an exchange spring from the soft into the hard magnetic material. Special attention has been given to the case when the hard material is FePt and the soft material is FeRh. From our results we can conclude the following:

• The atomistic model indicates that penetration to the length scale of few atomic units may be sufficient to induce the domain wall propagation in a hard material. However, the latter is not allowed in the micromagnetic model due to the lack of discretization and this limitation is responsible for the failure of the micromagnetic model to predict the coercivity at low interfacial exchange. This emphasizes the necessity of atomistic and, in general, multiscale models to describe properly the role of interfaces in the magnetization reversal process. The result may have a general importance for all cases where the influence of small exchange on the switching properties is considered, such as two-phase permanent magnets [Fischer 98,Schrefl 94].

• From the point of view of an FePt/FeRh bilayer we have demonstrated that a small amount of interfacial exchange could suffice to decrease the coercivity of FePt by up to $2-5$ times. The prediction of coercivity saturation at relatively low interfacial exchange energy is of practical significance. Clearly, the production of bilayer systems with this level of exchange energy is necessary in order to maximize the reduction in coercivity. A further consideration arises from the fact that the reduction in $H_c$ is very rapid for small $J_s/J$. This means that in this region any local fluctuations in the exchange energy strength will give a contribution to the switching field distribution (SFD) over and above those arising from the dispersion of the intrinsic properties, principally, the anisotropy and the grain volume. Given that a narrow SFD is required for good recording properties, it would appear to be desirable to develop bilayers with exchange energy in the saturation region. The degree of exchange energy in real experimental bilayer systems is not well known and difficult to quantify. However, the interfacial exchange is generally rather small, and it may be that even such relatively small values as $J_s /J=0.04$ may be beyond the abilities techniques such as sputtering, and thus it may be possible that successful composite media may require molecular-beam epitaxy or some other advanced technique.

• The granular structure of FePt necessary for magnetic recording application does not modify this conclusion, at least provided that the grain size of the soft material is much larger than that of FePt. However, continuous soft layers are shown to give rise to an effective exchange coupling within the grains of the hard FePt layer.

• The domain-wall assisted mechanism (high $M_s^{soft}$), in agreement with the previous predictions [Dobin 06], is demonstrated to be more efficient in the reduction of the coercivity than the two-macrospin rotation mechanism (low $M_s^{soft}$), considered in Ref. [Victora 05a].

• The differences between an isotropic model for exchange (Model I) and the one with more realistic parameters for the hard phase ($L1_0$ FePt) (Model II) in the energy barrier calculations demonstrate the importance of using correct material properties for simulations of composite media. This again emphasizes the role of multiscale modeling, when the ab-initio results are incorporated into micromagnetics.

• We also showed that in the generic bilayer some moderate exchange amount ($20-40\%$) is necessary in order to achieve low coercivity and the maximum figure of merit via the exchange spring effect. This differs from the FePt/FeRh case showing that the exchange saturation value depends on the real material parameters.

• The direct calculation of the magnetostatic interactions in a multigrain film, more realistic than the mean-field approximation, confirmed that in conventional perpendicular thin film recording the use of low-magnetization hard phase is necessary and that the state of the neighboring grains is altered by the state of the central one.