A device with a reference magnetic layer whose magnetization is comparatively fixed and another layer whose magnetization responds more readily to an applied field. The resistance reads their relative alignment. Fert's account describes how such giant magnetoresistance sensors entered hard-disk read heads in 1997.[ref]
Nobel Lecture: Origin, development, and future of spintronics by Albert Fert. p.1519, spin-valve structure and historical read-head adoption
A spin valve is a type of device that exploits the phenomenon of spin-dependent electrical resistance, which is related to the spin of electrons. It consists generally of two ferromagnetic layers separated by a non-magnetic metal or semiconductor layer. The key principle behind a spin valve is that the electrical resistance of the device changes depending on the relative magnetization orientations of the two ferromagnetic layers.
Basic component in spintronics, used in both giant magnetoresistance
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