GMR Spin-Valve Read Head

The read head is what turns a magnetised bit back into a voltage, and each density step shrinks the bit's signal. Reuters quoted IBM's Bob Scranton in November 1997 saying that shrinking the bit area is gradual but the more sensitive reader needed to follow it comes in major steps. Before GMR, the Computer History Museum records an MR read element shipping in IBM's Sawmill drive in the 9345 DASD system in 1990 at 107 Mbit/in2, and 132 Mbit/in2 in the 1991 Corsair.

The physics came from Europe: the Nobel Academy dates the discovery of giant magnetoresistance to 1988, by Albert Fert and Peter Grunberg independently, and says the first GMR read head launched in 1997. IBM Research says their layers gave 6% and 50% resistance changes at low temperature and high field, and that Parkin and Kevin Roche then saw GMR in sputtered multilayers. Bruce Gurney and colleagues then settled on the simplest arrangement: two weakly coupled magnetic layers with a spacer, plus an antiferromagnet that pins one layer. IBM named it the spin valve.

The 1997 head used a more elaborate stack than the simple spin valve. In the CHM oral history Mustafa Pinarbasi says that on 4 June 1997 he was told to lead the project and that the product heads came from a batch with an antiparallel-pinned (synthetic antiferromagnet) structure, chosen for thermal and magnetic stability. He says the rest of the industry later adopted that structure, after first using simple spin valves with platinum manganese pinning.

Scranton's note says the team deliberately put the head first in a niche high-capacity 3.5-inch desktop drive, code-named Titan, so low yields would be survivable. The note says rest of the industry began commercial GMR in 1998 and that by 2000 every HDD produced worldwide used GMR sensors. The Museum places TMR heads in the 2004 Seagate Momentus II and ties the three MR head generations together with PMR media to the 1 Tbit/in2 areal density reached by 2014.