GMR

In spin valve GMR, two magnetic layers are separated by a thin (~3 nm) non-magnetic (insulating) layer. It is possible to measure and adjust the strength of magnetism between these layers.

The Giant Magnetoresistance (GMR) is the large change in the electrical resistance which is induced by the application of a magnetic field to thin films composed of alternating ferromagnetic and nonmagnetic layers. This change in resistance, in general a reduction, is related to the field-induced alignment of the magnetizations of the magnetic layers. In the first experiments, the film was composed of layers of Fe (ferromagnetic) and Cr (nonmagnetic) with typical thicknesses of a few nm and the current was in the plane of the film. GMR effects can also be obtained with the current perpendicular to the layers. The origin of the GMR is the dependence of the electrical conduction in ferromagnetic materials on the spin state of the carriers (electrons).

A spin valve structure, in its simplest form shown in Figure 5 a, consists of a magnetically soft layer separated by a nonmagnetic layer from a second magnetic layer which has its magnetization pinned by an exchange biasing interaction with an antiferromagnetic (FeMn) or ferrimagnetic layer. The operation of the spin valve can be understood from the magnetization and magnetoresistance curves shown in Figure 5 b. One of the permalloy layers has its magnetization pinned by the FeMn in the negative direction. When the magnetic field is increased from negative to positive values, the magnetization of the free layer reverses in a small field range close to H=0, whereas the magnetization of the pinned layer remains fixed in the negative direction. Consequently, the resistance increases steeply in this small field range. Magnetic multilayers of the spin valve type are used in most applications of GMR, in particular the read heads of hard discs,

Uses:

Spin-valve sensors

Hard disk drives

Magnetic RAM

biosensors

Magnetoresistive insulators for contactless signal transmission between two electrically isolated parts of electrical circuits were first demonstrated in 1997 as an alternative to opto-isolators. A Wheatstone bridge of four identical GMR devices is insensitive to a uniform magnetic field and reacts only when the field directions are antiparallel in the neighboring arms of the bridge.