Magnetoresistance

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Magnetoresistance is the property of some materials to change the value of their electrical resistance when an external magnetic field is applied to them. The effect was first discovered by William Thomson in 1857, but he was unable to lower the electrical resistance of anything by more than 5%. This effect was later called ordinary magnetoresistance (OMR). More recent researchers discovered materials showing giant magnetoresistance (GMR), colossal magnetoresistance (CMR) and magnetic tunnel effect (TMR).

[edit] Anisotropic magnetoresistance (AMR)

A material property demonstrating a dependence of electrical resistance on angle between the direction of electrical current flow and orientation of magnetic field. The effect is attributed to a larger probability of s-d scattering of electrons in the direction of magnetic field. The net effect is that the electrical resistance has maximum value when the direction of current is parallel to the applied magnetic field.

In a semiconductor with a single carrier type, the magnetoresistance is proportional to (1+ (μB)²), where μ is the semiconductor mobility (units m^2·V^-1·s^-1 or T^-1) and B is the magnetic field (units teslas). Indium antimonide, an example of a high mobility semiconductor, could have an electron mobility above 4 m²·V^-1·s^-1 at 300 K. So in a 0.25 T field, for example the magnetoresistance increase would be 100%.

To compensate the nonlinear characteristics and inability to detect the polarity of magnetic field, somewhat more complex structure is used for sensors. It consists of stripes of aluminium or gold placed on a thin film of permalloy (feromagnetic material showing AMR effect) inclined by angle of 45°. This structure forces the current not to flow along the “easy axes” of thin film, but at angle of 45°. The dependence of resistance now has a permanent offset and it's linear around the nul-point. Because of its appearance, this structure is called barber-pole.

AMR effect is used for wide array of sensors for measurement of Earth magnetic field (electronic compass), for electrical current measuring (by measuring the magnetic field created around the conductor), for traffic detection and for linear position and angle sensing. The biggest AMR sensor manufacturers are Honeywell and Philips Semiconductors.