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Spin valve

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Title: Spin valve  
Author: World Heritage Encyclopedia
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Subject: Spin polarized scanning tunneling microscopy, Exchange bias, Magnetoresistive random-access memory, Spin engineering, Stuart Parkin
Collection: Quantum Electronics, Spintronics
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Spin valve

A schematic diagram of a pseudo spin valve. The free layer is magnetically soft and the fixed layer is magnetically hard. When the magnetic layers are antiparallel the electrical resistance is higher than when they are aligned.

A spin valve is a device, consisting of two or more conducting magnetic materials, whose electrical resistance can change between two values depending on the relative alignment of the magnetisation in the layers. The resistance change is a result of the Giant Magnetoresistive effect. The magnetic layers of the device align "up" or "down" depending on an external magnetic field. In the simplest case, a spin valve consists of a non-magnetic material sandwiched between two ferromagnets, one of which is fixed (pinned) by an antiferromagnet which acts to raise its magnetic coercivity and behaves as a "hard" layer, while the other is free (unpinned) and behaves as a "soft" layer. Due to the difference in coercivity, the soft layer changes polarity at lower applied magnetic field strength than the hard one. Upon application of a magnetic field of appropriate strength, the soft layer switches polarity, producing two distinct states: a parallel, low-resistance state, and an antiparallel, high-resistance state.

How it works

Spin valves work because of a quantum property of electrons (and other particles) called spin. Due to a split in the density of states of electrons at the Fermi energy in ferromagnets, there is a net spin polarisation. An electric current passing through a ferromagnet therefore carries both charge and a spin component. In comparison, a normal metal has an equal number of electrons with up and down spins so, in equilibrium situations, such materials can sustain a charge current with a zero net spin component. However, by passing a current from a ferromagnet into a normal metal it is possible for spin to be transferred. A normal metal can thus transfer spin between separate ferromagnets, subject to a long enough spin diffusion length

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