Tunnel Magnetoresistance (TMR)
When two ferromagnetic layers are separated by an insulator thin layer,
electrical resistance of the multilayer in the perpendicular direction
to the film changes depending on the orientations of the magnetizations
of ferromagnetic thin layers because of spin dependent electron tunneling
between the two ferromagnetic layers. When the directions of the magnetizations
of the two ferromagnetic electrodes are the same, the possibility of electron
tunneling between the two ferromagnetic electrode through the insulator
layer becomes larger, resulting in larger tunneling current. However, if
the directions of the magentizations of the two ferromagnetic electrodes
are opposite, the electron with opposite spin orientation with respect
to the magnetization of the ferromagnetic electrode cannot be tunneled.
Then the tunneling electron current become smaller compared to the case
for the same directions of the magnetizations. This phenomenon is called
tunneling magnetoresistance (TMR), because its value is reaching to 100%
at room temperature.
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 magnetoresistive devices such as magnetic
random memory (MRAM). However, due to its large electrical resistance,
the applications to a read head for hard disk drive (HDD) is considered
to be difficult. So the effort to decrease the electrical resistance by
thinning the tunneling barrier is being attempted for potential applications
to read heads.
