AMR head
The first read head that sensed a magnetic field by changing its electrical resistance rather than inducing a voltage — the ancestor of today's GMR and TMR heads.
Anisotropic magnetoresistance is the fact that a conductor's resistance changes with the angle between the current and its magnetisation. William Thomson (Lord Kelvin) discovered the effect back in 1857, but it took until 1991 for IBM to put it in a read head. The point was that a resistive read element gives a signal that doesn't depend on how fast the disk spins, unlike the older inductive head — crucial once platters grew denser and bits grew weaker.
AMR delivered only a few percent of resistance change, which sufficed for a while. When the GMR head arrived in 1997 with tens of percent, AMR was overtaken within a few years, and later the TMR head took over entirely. Write and read have been separate ever since: an inductive writer and a magnetoresistive read element on the same slider.