A spin valve is a device, consisting of two or more conducting magnetic materials, whose

electrical resistance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual paralle ...

can change between two values depending on the relative alignment of the magnetization 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 Ferromagnetism is a property of certain materials (such as iron) which results in a large observed magnetic permeability, and in many cases a large magnetic coercivity allowing the material to form a permanent magnet. Ferromagnetic materials ...

, one of which is fixed (pinned) by an

antiferromagnet In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usually related to the spins of electrons, align in a regular pattern with neighboring spins (on different sublattices) pointing in opposite directions. ...

which acts to raise its magnetic

coercivity Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. Coercivity is usually measured in ...

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 In solid state physics and condensed matter physics, the density of states (DOS) of a system describes the number of modes per unit frequency range. The density of states is defined as D(E) = N(E)/V , where N(E)\delta E is the number of states i ...

of electrons at the

Fermi energy The Fermi energy is a concept in quantum mechanics usually referring to the energy difference between the highest and lowest occupied single-particle states in a quantum system of non-interacting fermions at absolute zero temperature. In a Fermi ga ...

in ferromagnets, there is a net spin polarisation. An electric current passing through a ferromagnet therefore carries both

charge Charge or charged may refer to: Arts, entertainment, and media Films * '' Charge, Zero Emissions/Maximum Speed'', a 2011 documentary Music * ''Charge'' (David Ford album) * ''Charge'' (Machel Montano album) * ''Charge!!'', an album by The Aqu ...

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 Spin diffusion describes a situation wherein the individual nuclear spins undergo continuous exchange of energy. This permits polarization differences within the sample to be reduced on a timescale much shorter than relaxation effects. Spin ...

length. Spin transmission depends on the alignment of magnetic moments in the ferromagnets. If a current is passing into a ferromagnet whose majority spin is spin up, for example, then electrons with spin up will pass through relatively unhindered, while electrons with spin down will either 'reflect' or spin flip scatter to spin up upon encountering the ferromagnet to find an empty energy state in the new material. Thus if both the fixed and free layers are polarised in the same direction, the device has relatively low electrical resistance, whereas if the applied magnetic field is reversed and the free layer's polarity also reverses, then the device has a higher resistance due to the extra energy required for spin flip scattering.

Antiferromagnetic and non-magnetic layers

An antiferromagnetic layer is required to pin one of the ferromagnetic layers (i.e., make it fixed or magnetically hard). This results from a large negative exchange coupling energy between ferromagnets and antiferromagnets in contact. The non-magnetic layer is required to decouple the two ferromagnetic layers so that at least one of them remains free (magnetically soft).

Pseudo spin valves

The basic operating principles of a pseudo spin valve are identical to that of an ordinary spin valve, but instead of changing the magnetic coercivity of the different ferromagnetic layers by pinning one with an antiferromagnetic layer, the two layers are made of different ferromagnets with different coercivities e.g., NiFe and Co. Note that coercivities are largely an extrinsic property of materials and thus determined by processing conditions.

Applications

Spin valves are used in magnetic sensors and hard disk read heads. They are also used in magnetic random access memories (

MRAM Magnetoresistive random-access memory (MRAM) is a type of non-volatile random-access memory which stores data in magnetic domains. Developed in the mid-1980s, proponents have argued that magnetoresistive RAM will eventually surpass competing tec ...

References

{{Reflist Quantum electronics Spintronics

How it works

Antiferromagnetic and non-magnetic layers

Pseudo spin valves

Applications

See also

References