= Solution
A <spin valve> contains a magnetically soft free <ferromagnet>, a thin nonmagnetic metallic spacer and a reference <ferromagnet> pinned by <exchange bias> from an <antiferromagnet>. The recording track's stray field rotates or reverses the free-layer moment while the reference remains approximately fixed. Spin-dependent scattering makes the resistance lower for parallel moments and higher for antiparallel moments. A sense current converts this into a <voltage> signal, or a fixed <voltage> into a current signal.
A useful angular approximation is $R(\theta)=R_P+(R_{AP}-R_P)(1-\cos\theta)/2$. Biasing the equilibrium moments near $90^\circ$ gives an approximately <linear response> to small track fields. Pinning stability, <magnetic shielding> and the sensor's spatial extent matter for reading adjacent narrow bits.<Scientific Background on Giant Magnetoresistance by the Royal Swedish Academy of Sciences>[Spin-dependent transport and spin-valve read-head principle]{ref}
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