Spin valve

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]

A spin valve detects a small field by rotating a free layer relative to a pinned magnetic reference. The chain rule separates transport angular response from the magnetic rotational susceptibility. An angular response proportional to $1-\cos \theta$ has zero slope at perfect parallel and antiparallel alignment but a nonzero slope near perpendicular alignment. The smallest detectable field also depends on electronic and magnetic noise, thermal stability, hysteresis and the chosen bias current; a large magnetoresistance ratio alone does not determine sensitivity.

Fluctuations that limit the detectable field of a magnetoresistive read sensor. Thermal magnetization noise, domain jumps, electronic thermal noise and resistance fluctuations compete with the field-dependent signal; magnetic bias and adequate anisotropy stability are important.

A spin valve with two ferromagnetic layers of different magnetic coercivities, allowing separate reversal without requiring one layer to be rigidly exchange-pinned. A continuous weakly coupling spacer permits a controllable antiparallel state.