Fundamentals of MR Technology
It took about 100 years from the discovery of the "anisotropic magnetoresistive effect" (AMR) (1857 by Thomson) to a first technical application. At the end of the sixties the AMR effect was used in read heads of magnetic bubble memories. Since 1980 the development of commercial AMR sensors started.
In 1988 Grünberg
and Fert discovered the giant magnetoresistance effect (GMR), leading to closer
research and further discoveries in the field of magnetic based resistance
Extensive national, international and industrial funded research is currently under progress to develop new types of magnetoresistive sensors.
Until now several MR effects become known and get classified under XMR.
Magnetoresistance in non magnetic Materials (MR - effect)
The magnetoresistive effect will be found in every conductive material. The electrical resistance changes under the influence of magnetic fields. The carriers of the electrical charges, the electrons, experience field induced forces, their trajectories are lengthend. In high conductive materials as Cu, very high magnetic fields are needed to generate considerable resistance effects. The highest rate of resistance change will be found in Bismuth. Special semiconductors, called "Fieldplate" show values of 100 % and more.
The Extraordinary MagnetoResistance effect was detected in the near past in semiconductor structures.
in ferromagnetic materials (XMR-Effects)
The anisotropic magnetoresistive effect (1857 by Thomson) is effective in ferromagnetic materials. The specific restistance depends on the angle between current flow and magnetization. In magnetization direction, several % higher values will be found in comparison to the perpendicular direction. Inside thin film materials the magnetization becomes easily rotatable, sensors can be realized.
To detect a giant magnetoresistive effect two ferromagnetic layers and a non -magnetic conductive intermediate metal layer are needed. With a parallel arrangement of the magnetizations in the ferromagnetic layers a higher resistance is reached in contrast to anti- parallel arrangement of the magnetization directions. The difference may reach up to 50 percent.These possible high values leads to the acronym "Giant".
The tunnel magnetoresistive effect was found in some multilayer systems, that consist of two ferromagnetic and a very thin non conductive intermediate layer.The resistance of the tunnel junction depends, similar to GMR, on the angle between the magnetization directions in the ferromagnetic layers.
The collosalmagnetoresistive effect is a bulk effect, especially found in perowscovitic materials. Near the transition temperature from the metallic to the semiconductive phase resistance changes of several hundert percent are sometimes observable. Unfortunately the transition temperature ranges always below 100 K.
The giant magnetic inductance effect was observed predominantly in amorphous ferromagnetic wires or in conductive wires coated with a magnetic layer. A ring shaped magnetization direction inside the wire is sufficient. Longitudinal magnetic fields rotate the magnetization into the wire direction. Giant impedance changes will be found at high working frequencies, resulting from skin effects on magnetic surfaces and/or interfaces. Similar effects, with reduced impedance changes in comparison to magnetic wires, were observed in magnetic multilayers.