Magnetoresistivity is the phenomena of change of resistivity of some metals and semiconductor when it is exposed to external magnetic field. This phenomenon is observed at low temperature. The material which possesses magnetoresistivety are called Magnetoresistors.
Magnetoresistivity is observed due to lengthening of path of moving electron when it experiences a force due to external magnetic field. This property depends on the mobility of electrons. It is highest in semiconductor such as Indium Antimonide which has a mobility of 7.8 m2/Vs. as compared to 50×10-4m2/Vs for a metal.
Magnetoresistors are sensitive to the total magnetic field and not to its rate of change. The value of resistance depends upon the direction of magnetic field. The maximum change in resistance occurs when the crystal of Indium Antimoide is parallel to each other and the magnetic & electric field are mutually orthogonal. Figure below shows the characteristics of a magnetoresistive element.
When there is no magnetic field, the magnetization is along the elemental length O. As the strength of magnetic field is increased, the magnetization increases till point b is reached. Beyond point b, the material gets saturated.
Magnetoresistors are built by depositing a film of about 25 micro meter of indium antimonide/ nickel antimonide into a 0.1 mm thick substrate. The film is in the form of meander and the value of resistance at zero magnetic field can be varied by changing the dimension of this meander and the number of loops. The film is insulated from the substrate, which may be made from a magnetic base coated with a thin insulating film of a non-magnetic material, such as ceramic or plastic.
The magnetoresistive effect, in some metals, is even noticeable at room temperature, for example bismuth. The resistivity of bismuth doubles when it is exposed to a field of 1.5×106 A/m. Therefore, this metal could be used for the measurement of flux density.