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MR-Sensors
Where do you use MR-sensors?
MR-sensors, evaluation electronics and magnetic scales from Sensitec are in use everywhere, where movement is being controlled and steered, where paths, angles, positions, electric or magnetic fields are measured and detected.
What is the MR effect?
The MagnetoResistive Effect, "MR-Effect", has been known for 150 years. However, its use in sensor applications was first made practically possible through the development of thin-film technology some 30 years ago.
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MR SENSOR TECHNOLOGY
The MagnetoResistive Effect, »MR-Effect«, has been known for 150 years. However, its use in sensor applications was first made practically possible through the development of thin-fi lm technology some 30 years ago. Since this time, MR sensors have consistently opened up new application fi elds in magnetic field measurement, be it in an electronic compass, in path- or angle-measuring systems, or in small potential-free current sensors.
The term MR sensor is a collective term for sensors based on a range of different, but related physical principles. All MR effects have in common that the electrical resistance of the sensor changes due to the infl uence of a magnetic field. By adept arrangement of the structure of the sensor quite different tasks can be solved, to sense for example a magnetic field angle, 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 impedance changes with the direction of the applied magnetic field. The resistance change is in the order of a few percent and this effect can be used even for very weak magnetic fields.
The Tunnel MagnetoResistive (TMR) effect, discovered by Julliere in 1975, occurs in layer systems consisting of at least two ferromagnetic layers and a thin isolation layer. The tunnel resistance between both layers depends on the angle of both magnetization
directions.
Change in resistance (R) of a AMR strip as a function of the angle (α) between the current (I) and magnetization (M).
Typical GMR layer arrangement. Both magnetizations M1 and M2 are turned by the external magnetic field H.
The Giant MagnetoResistive (GMR) effect was first discovered in 1988 by Fert and Grünberg, who were awarded with the Nobel Prize for Physics in 2007 for this achievement. This effect occurs in layer systems with at least two ferromagnetic layers and a single non-magnetic, metallic intermediate layer. If the magnetization in these layers is non-parallel, the resistance is larger than if the magnetization is parallel. The difference may reach up to 50 per cent, thus the name “giant”. 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 the characteristic curve to be targeted on the specific requirements of a particular measurement application.
In 1993 von Helmholt et al discovered the Colossal MagnetoResistive (CMR) effect. This effect occurs in perowskitic, manganesebased oxides, which change their resistance in the presence of a magnetic fi eld. 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 enables weak magnetic fields to be detected and delivers a signal with an excellent signal-to-noise relationship.
At Sensitec sensors based on the AMR- and GMR-effects are in series production. The first TMR-sensors are now under development. CMR-technology is still at the pre-industrial research stage.