U. Glaeser

longitudinal fields such as ring heads. Similarly, some media are composed of crystallites oriented
perpendicularly to the field. Such media have a much higher remanent magnetization in the perpendicular
direction, and favor perpendicular recording. If a head design promotes perpendicular fields, such as
single pole heads, the result is perpendicularly recorded magnetization.
Some recent experiments have shown that media that favor perpendicular recording have better thermal
stability. This is why, lately, perpendicular recording is attracting a considerable attention in magnetic
recording community. Typically, in perpendicular recording a recording surface is made of a hard
ferromagnetic material, i.e., material requiring large applied fields to permanently magnetize it. Once
magnetized, the domains remain very stable, i.e., large fields are required to reverse the magnetization.
The recording layer is made thick so that, since each magnetic domain contains a large number of
magnetic particles, larger energy is required for demagnetization. The low remanence, low coercivity,
materials (the so-called soft materials) are placed beneath hard ferromagnetic surface (soft underlayer)
and used to conduct magnetic field back to another electromagnet pole. A pole-head geometry is used,
so that the medium can effectively travel through the head gap, and be exposed to stronger magnetic
field. A pole-head/soft-underlayer configuration can produce about twice the field that a ring head
produces. In this way sharp transitions can be supported on relatively thick perpendicular media, and
high frequencies (that get attenuated during readback) are written firmly. However, effects of demagnetizing
fields are much more pronounced in perpendicular recording systems, because in longitudinal
media the transitions are not that sharp.
Physical Limits on Recording Density
At extremely high areal densities each bit of information is written on a very small area. The track width
is small and magnetic domains contain relatively small numbers of magnetic particles. Because the particles
have random positions and sizes, large statistical fluctuations or noise on the recovered signal can occur.
The signal-to-noise ratio is proportional to the track width, and is inversely proportional to the mean size
of the particle and the standard deviation of the particle size. Therefore, increasing the track size, increasing
the number of particles by increasing media thickness, and decreasing the particle size will improve the
signal-to-noise ratio. Uniaxial orientation of magnetic particles also gives higher signal-to-noise ratio;
however, the requirement for thermal stability over periods of years dictates a lower limit to the size of
magnetic particles in a magnetic domain because ambient thermal energy causes the magnetic signals to
decay. Achieving both small particle size and thermal stability over time can be done by using magnetic
materials with higher coercivity, but there is a strong practical upper limit to the coercivity that can be
written, and it is determined by the saturation magnetization of the head material.
In addition to the basic physics, a number of practical engineering factors must be considered at extremely
high densities. In particular, these factors include the ability to manufacture accurately the desired head
geometries and control media thickness, the ability to closely follow the written tracks, to control head
flying height, and the ability to maintain a very small, stable magnetic separation.
The Future
The hard drive areal densities have grown at an annual rate approaching 100%. Recently a 20 Gbit/in. 2 has
been demonstrated [5], and some theoretical indications of feasibility of extremely high densities approaching
1 Tbit/in. 2 have been given [8,9]. Although the consideration related to user needs including higher
capacity, speed, error performance, reliability, environment condition tolerances, etc. are important, the
factors affecting cost tend to dominate read channel architecture and design considerations. Thus, achieving
highest recording density with lowest component costs at high manufacturing yields is the ultimate goal.
With areal densities growing at an annual rate approaching 100%, real concern continues to be
expressed that we may be approaching a limit to conventional magnetic recording technology; however,
as long as the read channel is concerned, large opportunities are available to improve on the existing
signal processing, both with detectors better matched to the channel and by applying more advanced
detection, modulation, and coding schemes.
© 2002 by CRC Press LLC