Read Back Signals in Magnetic Recording - Research Group Fidler

5 Read Head DesignEquation
5.1 Bias Schemes
Read Head Design
Section (Next)
For AMR and GMR reading heads, magnetic layers with a uniform predefined magnetization
direction are necessary. Besides a single domain state is desirable, to avoid Barkhausen noise
due to moving domain walls. Therefore the magnetizations of these layers have to be forced
into a certain direction by biasing. The most important bias schemes used in MR read heads
are explained below.
5.1.1 Demagnetizing Factor
The field of a magnetized body, the stray field, or also called demagnetizing field H S , acts on
the magnetization itself. This field is proportional to the magnetization M and opposite
directed within the body. So the demagnetizing field can be written as
N =− H M (5.1)
S d
Here N d is the demagnetizing factor, which is generally a tensor and only depends on the
shape of the body. Principal axis transformation leads to three demagnetizing factors a N , N b ,
N c for each axis one respectively. For their sum
Na + Nb + Nc
= 1
(5.2)
always holds. The longest axis corresponds to the smallest demagnetizing factor. It is also the
preferred direction for the magnetization, because then the stray field energy has its minimum
(see (2.37)).
A GMR element basically exists of two ferromagnetic layers, which should be magnetized
perpendicular in respect to each other in ground state. Moreover the free layer should be able
to rotate freely. Therefore the shape of these layers has to be almost quadratic (with an aspect
ratio close to 1) in order not to discriminate a magnetization direction.
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