Read Back Signals in Magnetic Recording - Research Group Fidler

3.2.2 Signal Field
Analytical Calculations
Now we are able to calculate the effective signal field from (3.4)
eff 1
n
Hsig = () ⋅ H () dV
V ∫ Mr H r . (3.18)
SL V
DL
In case of a moving head with velocity v the effective signal field can be written as function
of time
eff 1
n
Hsig () t = ( ) ⋅ H ( − t) dV
V ∫ Mr H r v . (3.19)
SL V
DL
The advantage of using the reciprocity principle is that the head field has to be calculated only
once. Then the signal field can be determined by folding the head field with the magnetization
of the recording layer.
3.2.3 Signal of a Perfect Transition
The maximum signal field is achieved for perfect transitions, which are defined in case of
longitudinal recording by
J ( x, z) =± J ⋅ sgn( x)
. (3.20)
x s
Here J s is the spontaneous polarization, which is in the range of 0.3 T to 0.5 T for common
recording layers. For the head field shown in Figure 3.3 we get a pulse for the effective signal
field, which is shown in Figure 3.4 for a 10 nm thick data layer positioned 10 nm below the
ABS. The maximum signal field is reached whenever the center of the head is exactly over
the transition. The full width at half maximum is about 50 nm and is therefore in the order of
the distance between the two shields (60 nm).
34