Giant Magnetoresistance (GMR)
When two ferromagnetic layers are separated by a paramagnetic thin layer,
electrical resistance of the multilayer changes depending on the orientations
of the magnetizations of ferromagnetic thin layers. When the directions
of the magnetizations are the same, the possibility of electron scattering
at the interface of paramagnetic/ferromagnetic layers become smaller, resulting
in low electrical resistance. However, if the directions of the magentizations
of the two ferromagnetic layers are opposite, the electron with opposite
spin orientation with respect to the magnetization of the electrode layer
is scattered (spin dependent electron scattering). The electrical resistance
of the multilayer becomes higher than the case for the same directions
of the magnetizations. This phenomenon is called giant magnetoresistance
(GMR), because its value is much higher than the MR value obtained from
anisotropic MR that appear from normal ferromagnetic materials.
By sputter depositing ferromagnetic film on top of antiferromagnetic layer,
the orientation of the magnetization of thin films can be "pinned"
by the exchange coupling between the moment of the anitiferromagnetic layer
and the thin ferromagentic layer. The thickness of the ferromagentic layer
must be thinner than the exchange length of the material. The magnetization
of the other ferromagnetic layer can be easily changed by applying external
field if the film is made of soft magnetic thin film. By this configuration,
the MR changes sensitively depending on the external magnetic field, thus
can be used as high sensitive read head for hard disk drives. As the areal
density of magnetic recording media increases, the size of the bit is becoming
more and more smaller. So the development of very sensitive GMR devices
is required.
