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 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
is very rapid for small . 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
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
), 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
), 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 ( 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 () 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.