The giant magnetoresistance
The giant magnetoresistance, or GMR (giant magnetoresistance) was discovered in 1988 by Peter Grünberg and Albert Fert, who were awarded the Nobel Prize in Physics in 2007.
The GMR effect is based on the spin of electrons, a basic particle property like charge or mass. The spin ensures that electrons are surrounded by a magnetic field, i.e. that they have a north and a south pole. If the electron is located in a magnetic field, the spin can only be set in two directions parallel or opposite to the magnetic field.
An ordinary conductor such as a power cable consists of atoms arranged in a lattice-like pattern (figure above). When a current flows, electrons from the shells of the atoms move away through the lattice. From time to time they collide with the lattice atoms, which prevents their movement and thus the flow of electricity. This effect is called electrical resistance. The resistance depends, for example, on the material of the conductor. If there is a magnetic conductor, the resistance also depends on the spin of the migrating electrons: Electrons whose spin points in the same direction as the magnetic field in the conductor experience a lower resistance than electrons with a spin opposite to the magnetization of the conductor.
GMR sensors consist of very thin layers of metal, hundreds of thousands of times thinner than a human hair, which are alternately magnetic and non-magnetic. They serve as conductors (figure below). If the directions of the magnetic fields in the two layers are equal, only electrons whose spin is in the opposite direction to the magnetic field are strongly scattered; for electrons with spin in the direction of the magnetic field, the resistance is small. However, if the orientations of the magnetic fields are antiparallel, both spin orientations are strongly scattered in each layer, so that the overall resistance is large.
A GMR sensor is installed in such a way that when it is exposed to a magnetic field, only one of the two magnetic layers changes its orientation. Instead of an anti-parallel orientation, the magnetic field of both layers then points in the same direction and the resistance drops abruptly, or in the reverse process it suddenly increases. These changes in resistance can be read out by electronics and used, for example, to determine the position of the sensor.
The most important application of the GMR is its use in the read heads of magnetic computer hard drives: It is only because of the high sensitivity achieved by the GMR that today's standard storage densities could be achieved at all. Today's ubiquitous search engines like Google or video platforms like YouTube would be almost unimaginable without sensors like the GMR.