in building thin-film heads with up to 50 turns arranged in
multiple layers, the level of electronic noise was becoming a
limiting factor. In 1991 the first magneto-resistive (MR) head was
introduced. Further innovations have included giant MR (1997)
and then tunnel MR (2005). Sensitivity expressed as DR/R has
gone from a few percent in the first MR heads to around 100% in
today’s tunnel MR heads. Acknowledging the huge significance of
the initial giant MR innovation, the Nobel prize in physics for 2007
¨was awarded to Albert Fert and Peter Grunberg. Similarly large
strides have been made with respect to reducing media noise.
Media noise in thin-film media arises primarily from variations in
placement of a transition away from its ideal location. Media noise
is determined by both the strength and the sharpness of the head
field that creates the transition, as well as the quality of the
recording medium. Head-field magnitudes and gradient have
been pushed up using high-moment pole-tips and very close
spacing. The media characteristics have shown continued im-
provement by scaling to thinner layers with small high-coercivity
grains with more uniform properties.
Since their inception, HDDs employed the longitudinal mode
of recording. However, 2005–2007 saw the long-awaited intro-
duction of a new mode of recording: perpendicular recording. In
perpendicular recording the magnetization axis is orthogonal to
the disk surface rather than lying in plane. Perpendicular
recording has the virtue of writing very sharp transitions into a
relatively thick, high-coercivity medium [2]. The introduction of
perpendicular recording, in particular, has reinvigorated HDD
technology. The areal-density growth curve which had been
flagging has taken an upward tick to around 50% per annum in
very recent products (Fig. 1).
However, there is one fundamental limit that even perpendi-
cular recording cannot avoid and that is the thermal or ‘super-
paramagnetic’ limit [3,4]. In current systems, the noise of the
transitions is not far from the lower limit that occurs when the
transition simply wends its way along the grain boundaries