A spin valve is a device consisting of two or more conducting magnetic materials, that alternates its electrical resistance (from low to high or high to low) depending on the alignment of the magnetic layers, in order to exploit the Giant Magnetoresistive effect. The magnetic layers of the device align "up" or "down" depending on an external magnetic field. Layers are made of two materials with different magnetic coercivity, which can be seen in the layers' hysteresis curves. Due to the different coercivities one layer ("soft" layer) changes polarity at small magnetic fields while the other ("hard" layer) changes polarity at a higher magnetic field. As the magnetic field across the sample is swept two distinct states can exist, one with the magnetisations of the layers parallel, and one with the magnetisations of the layers antiparallel. In the figures below, the top layer is soft and the bottom layer is hard.
Spin valves work because of a quantum property of electrons (and other particles) called spin. When a magnetic layer is polarized, the unpaired carrier electrons align their spins to the external magnetic field. When a potential exists across a spin valve, the spin-polarized electrons keep their spin alignment as they move through the device. If these electrons encounter a material with a magnetic field pointing in the opposite direction, they have to flip spins to find an empty energy state in the new material. This flip requires extra energy which causes the device to have a higher resistance than when the magnetic materials are polarized in the same direction.
Spin valves are used in magnetic sensors and hard disk read heads. They are also used in magnetic random access memories (MRAM).