Magnetoresistive Sensors

Magnetoresistance is the change in the electrical resistance of a material when it is exposed to a magnetic field. Anisotropic magnetoresistive (AMR) sensors make use of this effect in thin films of ferromagnetic alloy, most commonly permalloy, a nickel–iron alloy. The resistance of the film depends on the angle between the direction of the current flowing through it and the direction of its internal magnetisation. When an external field rotates that magnetisation, the resistance changes, and this change can be measured precisely.

To turn this effect into a usable signal, AMR sensors typically arrange four resistive elements in a Wheatstone bridge. The bridge produces a differential output that is largely free of temperature drift and offset, improving accuracy. Compared with simple Hall effect elements, magnetoresistive sensors are considerably more sensitive to weak magnetic fields, which makes them well suited to electronic compasses, angle and rotary position sensing, and detection of the small fields produced by passing ferrous objects.

Magnetoresistive sensors are valued for their contactless operation, fast response and good linearity over a defined range. Because the magnetisation of the permalloy can become disturbed by strong fields, many devices include a set or reset coil that restores a well-defined magnetic state. Related but distinct technologies include giant magnetoresistance (GMR), which uses layered structures to achieve even larger resistance changes and is widely used in data storage read heads.

Frequently asked questions

What is permalloy and why is it used?

Permalloy is a nickel–iron alloy with high magnetic permeability and a strong anisotropic magnetoresistive effect, making it ideal for sensitive field-sensing films.

How is a magnetoresistive sensor more sensitive than a Hall sensor?

It responds to weak fields through resistance changes in a ferromagnetic film, giving it higher sensitivity than a basic Hall element for low-field measurements such as compassing.

Why are the elements arranged in a Wheatstone bridge?

The bridge configuration produces a differential output that cancels much of the temperature drift and offset, yielding a more accurate measurement.