The potential usage of the exchange spring media for magnetic recording
requires the evaluation of both coercive field reduction and thermal
energy barrier. The parameters of the system have to be chosen with the
goal of maximizing both quantities. However, the number of such
parameters, including the intrinsic material ones, does not allow the
systematic search to obtain the best choice. Other parameters, as for
example the grain size, are limited by the current growing techniques.
In this section we will consider a generic soft/hard grain and study
the effect of several combinations of saturation magnetization in the
overall exchange spring performance. We considered the grain sizes with the
soft and hard
material. The anisotropy constants are and . The exchange
interactions were taken from the models I and II for the FePt described
in Section 3.3. For the
saturation magnetization in the hard material we used the value of
FePt, , and alternatively, a low value
, representing an hypothetical magnetic
recording media. Accordingly, in the soft material we varied the
saturation magnetization, starting from that of FeRh, , and
decreasing it up to a low value .
Soft/Hard material: one grain simulation
Coercivity of a soft/hard grain in model I as a function of the
for different values of
corresponding to: (a) and (b)
Hysteresis loops in a soft/hard grain simulated with the model I for and .
First, we will use the model I for the exchange interactions. Fig. 3.30 presents the coercivity field
a function of interfacial exchange parameter . First of all, we note that changing the
saturation magnetization value in the soft magnetic
material, we change its domain wall width since it is determined
by the magnetostatic interaction. Consequently, the coercivity
mechanism undergoes a transition between two types of behavior. In
the first case (high ) the exchange spring is formed. In the
second case (low ), the magnetization reversal is homogeneous
in each grain and can be represented by two-macrospins. The same type
of transition was observed in Section 3.4
changing the thickness of the soft layer (see Fig. 3.13).
In the low case we can distinguish also two
types of behaviors. For low interfacial exchange the soft magnetic
rotates and exerts an additional torque to the hard magnetic
moment. For larger values of
the soft material
will eventually be so coupled to the hard material that will be
unable to rotate independently and the behavior will be collective. The
corresponding demagnetization curves are shown in Fig. 3.31.
This crossover of behaviors in the two-macrospins mechanism results in
a minimum and, posteriorly, larger coercivity with
increasing interfacial exchange as in Ref. [Richter 06]. This minimum has
experimentally in an exchange spring medium based on FeSiO(Soft) and
CoPd(Hard) in Refs. [Wang 05a,Wang 05b]. In these experiments
variation of interfacial exchange is achieved through the
interposition of a non magnetic material of different thicknesses,
resulting in reduction of interfacial exchange with increasing
From Fig. 3.30 it is clearly seen that
the exchange spring formation is much more efficient in the coercive
reduction than the two macro-spins mechanism.
In the exchange spring the coercivity reduction is saturated for the
value of the interfacial exchange higher than . This situation
is favorable for an experiment since the field reduction is achieved
for low exchange value and no tuning of the coupling parameter is
necessary. In the opposite situation, the coercive field reduction
is less, presents a narrow minimum for interfacial exchange values
below and the
coercivity reduction experiences slight increase
for higher values.
To have an optimum coercivity reduction,
the value of the exchange should be tuned to this value which is
hardly affordable. The qualitative behavior does not change, if in the
hard material we consider large or low magnetization, Fig. 3.30(a) and Fig. 3.30(b) respectively, although the
minimum in the coercivity
for small becomes less pronounced. Finally, Fig. 3.32 shows the calculation in model II.
The two mechanisms are also present, but the interfacial exchange
needed for saturation is
of the bulk exchange, which is more difficult to reach. Additionally,
the minimum, although present, is very shallow.
Coercivity of a soft/hard grain, normalized to the anisotropy field (
) of the hard
magnetic phase, in model II as a function of the interfacial exchange for different values
of and .