|Physics of Magnetic Storage|
Magnetic recording consists of leaving patterns of remanent magnetization on a track within the surface of magnetic media that
sits on top of a non-magnetic substrate. There are three main orientations of the magnetization with respect to the track, lateral
(hardly ever used), longitudinal (the current standard), and perpendicular (Figure 2).
Figure 1: Magnetic Write Head Schematics
Most common is longitudinal recording. For this type of recording, the transducer, or recording head, is a ring-shaped
electromagnet with a gap at the surface facing the media. When the head is fed with a current, the fringing field
from the gap magnetizes the magnetic media, as is shown in Figure 3. The media moves at constant velocity under the
recording head, as is shown in Figure 1. The temporal changes in the current leave spatial variations in the remanent
magnetization along the length of the track.
Figure 2:Longitudinal, perpendicular, and lateral recording.
Figure 3: Magnetic Write Head Schematics: Functioning of the Gap
The recorded magnetization creates a pattern of magnetic fields that can be picked up be the media moving under the read head. Under ideal circumstances, the magnetization corresponds to that of bars of permanent magnetic material aligned as shown in Figure 4.
Traditionally, the read head was the same type of a ring-shaped electromagnet with a gap, in practice the write head doubled
as the read head. The changes in the magnetic fields on the track would induce a read current that reflects the changes in
the magnetic flux. Figure 5 shows the idealized relation between the writing of a track and the subsequent reading of the track.
The write current controls directly the orientation of the magnetic field emanating from the gap. Under ideal circumstances, the
track magnetization switches orientation exactly under the head position when the write current switched. Since however the
head is some distance of the track, and since thus the head magnitizes the media some distance away from the head, the switch
in the current overwrites a small portion of the immediate preceeding magnetization. (This is dealt with in write precompensation,
where the write current is switched slightly later to take this effect into account.) In addition, the magnetization of
surrounding material affects the magnetization at any given point. As a result, the border between magnetization zones
is in practice never straight but shows a wild zig-zag pattern. If the same head passes over the track, the
flux changes emanating from the magnetization pattern on the track induce a current. Since the head picks up flux changes and
ultimately magnetization changes from some distance, the induced current is not a single spike, but is somewhat spread out. Other
than that, the read current is the derivative of the write current. By "peak detection" (e.g., electronic differnentiation and
zero crossing detection), a magnetic read discovers the
flux changes and ultimately the historic changes in the write current that encode the stored data.
Figure 4:Ideal Magnetization of Track
Perpendicular recording is done with a probe device (Figure 6). Perpendicular recording has no inherent advantages over
longitudinal recording, but is being pursued in parallel by the magnetic recording industry because of the potential of
better magnetization retention in some materials. Devices like MEMS are probe devices and would use perpendicular recording.
Figure 5:Writing and Reading with a Gap Head. From top to bottom: write current, magnetization pattern, read current.
Nowadays, reading magnetic tracks is done using the magneto-resistive effect (Figure 7). The resistance of certain materials
changes noticeably if a magnetic field is applied to them. The magnetic field alters the direction of a current slightly. The
resistance of the magneto-resistive compound increases noticeably if the current direction is not
aligned with the cristalline structure. This effect can be used to directly measure the magnetic flux, and hence the magnetization
patterns on the track.
Figure 6:Perpendicular Recording with a Probe Head.
Figure 7:Magneto-Resistive Effect: The magnetic field (red) moves the electron flow in the sense current (yellow) up by an angle θ. The magneto-resistive material is in blue, the current conducts in brown.
|© 2003 Thomas Schwarz, S.J., COEN, SCU SCU COEN T. Schwarz COEN 180|