For sensor application.

Magnetic sensors indirectly measure properties such as direction, position, rotation, angle and current by detecting the magnetic field and its changes. The first application of permanent magnet was a 3rd century BC Chinese compass, which is a direction sensor. Compared to other direct methods such as optical or mechanical sensor, most magnetic sensors require some signal processing to get the property of interest. However they provide reliable data without physical contact even in adverse conditions such as dirt, vibration, moisture, hazardous gas and oil, etc.

The most widely used magnetic sensors are variable reluctance, Hall effect and reed switch sensors. Automotive crash saftey systems use sensors based on a holding mechanism that can be closed or open using electrical current.

With the rapid development of GMR in the past decade, a new family of magnetic sensors has emerged - magnetoresistive sensors. Similar to Hall effect sensors, the major difference is that MR sensors detect current change instead of voltage. Permanent magnets can also be used in MEM sensor and CMOS magnetic sensor arrays to supply the magnetic field.