Read Back Signals in Magnetic Recording - Research Group Fidler

Output [V]
0.108
0.107
0.106
0.105
0.104
0.103
0.102
0.101
-0.10 -0.05 0.00 0.05 0.10
H z [T]
FEM Simulations
ΔR/R [%]
Current in pos. y-direction
Current in neg. y-direction
Current in pos. y-direction, Hex = 0.1T
1
0
-1
-2
-3
-4
-0.10 -0.05 0.00 0.05 0.10
Hz [T]
Figure 6.6: The output voltage (left) and the relative change in resistance (right, in respect
to the equilibrium state) over the external applied field in z-direction for both current
direction and a larger exchange bias field.
The corresponding outputs for both current directions and a stronger exchange bias are given
in Figure 6.8. A positive external field leads to a parallel alignment of the magnetizations of
both layers, resulting in a low resistance. If the external field is applied in negative z-
direction, the resistance increases, because the magnetic configuration approaches the
antiparallel state. However, if the magnetic field overcomes the exchange bias field, the
pinned layer magnetization is not fixed anymore and rotates towards negative z-direction. So
for large negative external fields we have again a parallel state and therefore low resistance.
As expected the favorable current direction shows less asymmetry in change of resistance
around the equilibrium state than the opposite current direction. Moreover the GMR sensor is
more sensitive, because the slope of the transfer curve at zero field is larger. The output
voltage for this case is also higher, because here the magnetization vectors are more likely
aligned antiparallel (see equilibrium state in Figure 6.4), leading to larger resistance.
In case of a stronger exchange bias field, H ex = 0.1T , we have a much better transfer curve
for higher external fields. (Unfortunately such high exchange fields are rather unrealistic. To
increase the bias field for the pinned layer, synthetic antiferromagnets, as described in Section
71