The GMR-Effect

To detect a giant magnetoresistive effect two ferromagnetic layers and a non -magnetic conductive intermediate metal layer are needed. The resistance depends on the difference between the layer magnetization angles. With a parallel magnetization arrangement in the ferromagnetic layers a higher resistance is reached in contrast to an anti- parallel arrangement of the magnetization directions.

There are several types of GMR multilayers leading to different sensor designs, the basic types will be described in the following:

Simple GMR Multilayers

The oldest type is the antiferromagnetically (AF) coupled multilayer. It consists of alternatingly ferromagnetic (f.e. NiFe or Co) and non magnetic layers(f.e. Cu or Ag), of which the thicknesses are chosen such that the exchange coupling between the magnetic layers is antiparallel.

An external magnetic field will rotate all magnetization directions towards a parallel configuration, and thus reduce the resistance. This effect is independent of the direction of the magnetic field (neglecting anisotropy fields, which are usually relatively small).
The rotation of the magnetization in a bilayer configuration will be shown in the following video.

Magnetic field
Magnetization1
Magnetization2

 

GMR-Spin Valve Systems
In this kind of multilayer, the magnetization direction of one ferromagnetic layer is pinned via the exchange biasing effect, usually by an adjacent antiferromagnetic layer. The magnetization of the free layer separated by a thin intermediate layer (f.e. Cu) from the pinned layer is almost free to rotations induced by external magnetic fields.

A couple of new simulations of spinvalve - layers and spin valve based sensors are available in the download area.
The magnetization rotations of a spin valve system in an extrenal field that at the beginning grows up and then changes its direction are shown in the following video

exchange biased magnetization
Magnetization 1
Magnetization 2
extrenal magnetic field

Due to the free magnetization rotation in the second layer in contrast to the almost unmovable pinned magnetization in the biased layer, the resistance of the multilayer predominantely depends on the angle of the external field. Angle detection sensors are therefore preferable to construct.

Further informations are available under articles.