MULTILEVEL AND THREEDIMENSIONAL NANOMAGNETIC RECORDING - Bio-Inspired and Nanoscale Integrated Computing - page 176

perpendicular magnetic recording (already commercialized) [5, 6], patterned
medium [7-9], and heat-assisted magnetic recording (HAMR) [10-13]. It is
expected that these technologies can defer the superparamagnetic limit somewhat
above 1 TbitT/in 2 . However, there are other alternatives which the industry might
not seriously consider due to the lack of similarity to the conventional technology.
This somewhat conservative approach (to avoid ''high risk'' projects) is often
dictated by economical reasons. Despite the fact that the high risk, unconven-
tional technologies often have strong potential to revolutionize the industry, they
might not be considered cost effective according to the current standards of the
industry. The university environment, on the other hand, may favor high risk
technologies because of their scientific values and far-future potential. In this
chapter, multilevel (ML) three-dimensional (3D) magnetic recording will be
discussed as one of such high risk data storage systems. To comply with the
multilevel signal configuration, ML magnetic recording exploits a 3D head/media
system powered with next-generation data coding methods. It is believed, when
that combined with novel information processing techniques, cost effective ML
systems could be scaled down to a single-grain spin level, thus enabling memory
with effective areal densities above 100 Tbit/in 2 .
In summary, the purpose of this chapter is to diversify from the mainstream
magnetic technologies and explore a novel nanoscale system suitable for unpre-
cedented data storage densities and rates.
6.2. MULTILEVEL MAGNETIC RECORDING
6.2.1. Introduction
In this section, an unconventional approach to increase the effective areal densities
is discussed. The feasibility of using more than two signal levels to code recorded
information is explored. This is in contrast to conventional recording schemes in
which binary coding is used methods, meaning that the signal recorded into or
read back from magnetic media has only two states: presence or absence of the
magnetization reversal in a bit transition. For example, Figure 6.1a and b
illustrate so called frequency modulation (FM) encoding form, probably the
most trivial encoding scheme, as it could be used in longitudinal and perpendicular
recording, respectively. The only difference between the two recording modes is in
the orientation of the magnetization, along or perpendicular to the plane of the
disk, respectively. In both cases, encoding has a simple one-to-one correspondence
between the bit to be encoded and the magnetization reversal pattern.
ML magnetic recording refers to the use of multiple signal values to encode
data onto a magnetic disk. By using more than two levels, more information could
be put in the minimum feature size. Figure 6.1c illustrates how a multilevel code
could be used in a system with a 3D media with a perpendicular orientation of
the magnetization. This is a simplified case; in general, the magnetization could be
oriented along or at some arbitrary angle to the plane of the disk. As described