Giant magneto resistance (GMR) - devices sensor spin valve

GIANT MAGNETO RESISTANCE (GMR) - DEVICES SENSOR SPIN VALVE

A device that woks on the principle of the GMR is a spin valve. This device is used in magnetic hard discs for high density data storage (Fig. 2.35 (a)). There are 4 layers altogether in a spin valve (Fig. 2.35 (b)).

Two ferro magnetic layers are separated by a thin spacer layer (Cu or Ru). One ferromagnetic layer is pinned (its direction of magnetization is fixed and is not distributed by changes in field. This layer is pinned by adding a fourth layer: a strong anti ferromagnet.

The other layer, called the free layer, is sensitive to the field produced by the data bit. Permalloy (an alloy of Ni and Fe) is usually chosen for both feromangnetic layers. This structure is called the spin valve.

When a weak magnetic field, such as that from a bit on a hard disk, passes beneath such a structure, the magnetic orientation of the unpinned magnetic layer rotates relative to that of the pinned layer. This generates a significant change in electrical resistance due to the GMR effect.

• As the bit travels under the head, the resistance goes down, the electrons do not scatter very much and the current flow increases.

• As the bit moves on, the resistance increases, the electrons are scattered ore and the current decreases.

• As the bit travels further from the head, the resistance peaks and the current decreases to its lowest point.

• As the resistance change is quiet large, even small data bits can generate quite large resistance changes, thus increasing the capacity to store data bits in the hard disc.