In 1988 two solid-state physicists discovered that alternate ultra-thin layers of iron and chromium, each layer a few atoms thick, showed a very large drop in resistivity under an applied magnetic field. The effect had been known for a long time but not at such an amplitude, which is why it was called Giant MagnetoResistance or GMR. The phenomenon has its origin deep in quantum theory.
It has made it possible to measure tiny variations in magnetic field intensities. Practical applications were almost immediate with the technology for magnetic recording and reading of information on hard disk drives. By magnetising a small region on the disk alternating magnetisation in a perpendicular "up" or "down" direction makes it possible to store a series of binary data. The smaller these regions are, the greater the information density and therefore the higher the storage capacity of the hard disk. On the other hand the magnetisation is increasingly weak and becomes more and more difficult to read. By using the GMR effect storage capacity was multiplied by 100!
The two researchers behind these discoveries, the Frenchman Albert Fert and the German Peter Grünberg received the Nobel prize for physics in 2007.
The variation in the electrical resistance of iron/chromium multi-layers depending on the applied magnetic field (first observation of GMR). One kiloOersted is equal to one tenth of a Tesla (Credit: CNRS).