I. INTRODUCTION
ARECENT study forecasts explosive growth of the digital
universe from (281 Exa) Bytes in 2007 (about
45 GB per person) to (1.8 Zetta) Bytes in 2011 [1]. It
is thus vitally important to ensure the continued rapid increases
in capacity of the ubiquitous hard disk drive (HDD) that provides
the foundation for this digital universe. The biggest lever
for higher HDD capacities is to increase the areal density. A
paper [2] was published predicting that conventional recording
would reach a limit at around 1 Tb/in and that view is still
widely held although current products already boast areal densities
around 380 Gb/in and recent growth is at a remarkable
30–50% per year. The limiting factor is the onset of the superparamagnetic
limit as researchers strive towards smaller grained
recording media. This imposes a tradeoff between signal-tonoise
ratio, , and thermal stability of small grain media and
the writability of a narrow track head, which limits the ability to
continue to scale conventional magnetic recording technology
to higher areal densities. The highest demonstrated areal density
with a continuous perpendicular medium is 612 Gb/in [3].
Fortunately, several new technology options are being explored
that promise to increase the areal density beyond these limits.
These new options include bit patterned magnetic recording