U. Glaeser

To record data on a surface of a disk, the modulated signal current, typically bipolar, is passed through
the electromagnet coils thus generating a fringing magnetic field. The fringing magnetic field creates a
remanent magnetization on the ferromagnetic surface, i.e., the ferromagnetic surface becomes permanently
magnetic. The magnetic domains in the surface act like tiny magnets themselves and create their
own fringing magnetic field above the ferromagnetic surface. The data are recorded in concentric tracks
as a sequence of small magnetic domains with two senses of magnetization depending on a sign of writing
current. In this, so-called saturation recording,
the amplitude of two writing current signal levels are
chosen sufficiently large so as to magnetize to saturation the magnetic medium in one of two directions.
In this way, the nonlinear hysteresis effect does not affect domains recorded over previously recorded ones.
In a simple reading scenario the reading head flies over the disk-spinning surface (at head-to-medium
velocity, v)
and passes through the fringing magnetic fields above the magnetic domains. Depending on
a head type, the output voltage induced in the electromagnet is proportional to the spatial derivative of
the magnetic field created by the permanent magnetization in the material in the case of inductive heads,
or is proportional to the fringing magnetic field in the case of magneto-resistive heads. Today’s hard
drives use magneto-resistive heads for reading, because of their higher sensitivity. Pulses sensed by a head
in response to transition on the medium are amplified and then detected to retrieve back the recorded data.
For both types of heads, it is arranged that the head readback signal responds primarily to transitions
of the magnetization pattern. The simplest, single parameter model for an isolated magnetic transition
response is the so-called Lorenzian pulse
where t50
is a parameter representing the pulse width at 50% of the maximum amplitude. Simplicity and
relatively good approximation of the channel response are the main reasons for attractiveness of this
model. The family of g(
t)
curves for different values is depicted in Fig. 34.3. The width at half amplitude
FIGURE 34.1
FIGURE 34.2
© 2002 by CRC Press LLC
g() t
t
50
(a) Longitudinal recording. (b) Perpendicular recording.
(a)
(b)
Recording Surface
Write Current
Head
Magnetic domains representing bits.
1
2t
1 ⎛------------ ⎞
⎝PW50⎠ 2
= -------------------------
+
(a) (b)
Head
Soft Underlayer
Bit Cell
Magnetic Transition
Orientation of
Magnetic Domains