Spin valve

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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. The magnetic layers of the device align "up" or "down" depending on an external magnetic field. Layers are made of two materials with different hysteresis curves so one layer ("soft" layer) changes polarity while the other ("hard" layer) keeps its polarity. In the figures below, the top layer is soft and the bottom layer is hard.

[edit] How it works

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.