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6.2.5.2. Patterned Soft Underlayers and Interlayers.
The main motiva-
tion underlying the system modification described below is to substantially
increase the maximum recording field, H
max
, so that the range of the field,
H
max
to +H
max
, controlled via the variation of the drive current could also be
substantially increased.
Traditionally, SUL has a flat surface boundary with the recording layer. The
image model could be used to describe the effect of SUL on the recording field within
the recording layer (Fig. 6.8a) [47, 48]. Ideally a factor-of-two increase in the field is
expected, although from the same image model one could observe that the image is
located further away from the center of the recording layer as compared to the real
head. The difference between the separations of the real and image heads from the
center of the recording layer (the spacing loss) is
equal to the thickness of the
recording layer. The closer to the recording media the head is placed, the stronger
and more localized the recording field would be in the media. Because the image
head is located further away compared to the real head, the net effect will result in a
deteriorated signal compared to the signal, due to two equally separated heads. The
reciprocity principle states that the detrimental effect exists for both write and read
processes [49]. The following concept is proposed to resolve the spacing loss issue.
One could recall that magnetic imaging is very much like regular mirror
imaging [50]. Therefore, exactly as in the case of a convex mirror, one could use
a SUL with a convex boundary to move the image closer to the ''mirror's'' (SUL's)
boundary with the recording layer [51]. To illustrate this effect, a diagram
comparing the positions of the images in the two cases is shown in Figure 6.8b.
Using a convex SUL instead of a flat SUL could make the image head ''be closer'' to
the recording layer and thus increase the areal density. Based on the above described
concept, it is proposed to use a patterned SUL with each patterned island having a
convex shape. Patterning is necessary to implement the novel concept on the
physical scale of one bit. The cross-sectional dimensions of each patterned island
correspond to the cross-sectional dimensions of the targeted bit cell. For example,
for an areal density of 1 Tbit/in
2
, assuming a square symmetry in the plane, each of
the (x and y) period values T should be approximately 26 nm. Via numerical
simulations, it has been discovered that this favorable effect exists not only for
patterns with convex islands but also for patterns with rectangular islands (Fig. 2.8c)
[52]. This is attributed to the combined effect of patterning and periodicity.
Both continuous (conventional) and patterned recording layers could be
integrated with a patterned SUL, as shown in Figure 6.9a and b, respectively.
Patterning in the latter case combines the described below advantages of patterned
SULs with the well known advantages of patterned recording layers [53].
The key practical advantages due to the use of a convex patterned SUL are the
following and described below in detail:
B
SNR drastically increases, as demonstrated below.
Patterning of SUL increases both the recording and sensitivity field
gradients. The latter is critical for maximizing the areal density during
writing and reading, respectively.
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