The term MR sensor is a collective term for sensors based on a range of different physical principles. All MR effects have in common that the electrical resistance of the sensor changes due to the influence of a magnetic field. By skillfully configuring the sensor structure, it is possible to construct quite different sensors to record, for example, a magnetic field angle, a magnetic field strength or a magnetic field gradient.
The Anisotropic MagnetoResistive (AMR) effect was discovered by Thomson in 1857 and occurs in ferromagnetic materials, whose specific resistance varies depending on the angle between the magnetic field direction and the current direction. The resistance change is in the order of a few percent and this effect can be used even for weak magnetic fields.
The Tunnel MagnetoResistive (TMR) effect was discovered by Julliere in 1975.In this effect, the tunnel resistance between two ferromagnetic layers changes depending on the angle of magnetization of both layers. Bei diesem Effekt ändert sich der Tunnelwiderstand zwischen zwei ferromagnetischen Schichten in Abhängigkeit des Winkels der Magnetisierung der beiden Lagen.
The Giant MagnetoResistive (GMR) effect was first discovered in 1988 by Fert and Grünberg, who were awarded the Nobel Prize for Physics in 2007 for this achievement. The electrical resistance of two thin ferromagnetic layers, separated by a thin non-magnetic layer, changes in relation to the angle of magnetization in both of the ferromagnetic layers to each other. This magnetoresistance may reach up to 50 %. The electrical resistance is at its highest if the magnetization is non-parallel. The change in resistance does not depend on the direction of the current.
The characteristics of GMR sensors can be modified by stacking several layers with different properties and magnetizations. This allows a targeted adjustment of the characteristic curves to the requirements of a particular measurement application.
In 1993, Helmholt et al. discovered the Colossal MagnetoResistive (CMR) effect. This effect occurs in manganese-based oxides, which change their electrical resistance in the presence of a magnetic field.
Of all the known physical effects, by which a solid changes its properties due to magnetism, MR technology has particularly interesting and convincing advantages. The MR effect makes it possible to detect weak magnetic fields and delivers a signal with an excellent signal-to-noise ratio.
At Sensitec, AMR and GMR sensors are in series production. The first TMR sensors will soon be entering series production. CMR technology is still at a pre-industrial research stage.