Giant magnetoresistive effect

The Giant Magnetoresistance Effect (GMR) is a quantum mechanical effect observed in thin film structures composed of alternating ferromagnetic and nonmagnetic metal layers.

The effect manifests itself as a significant decrease in resistance from the zero-field state, when the magnetization of adjacent ferromagnetic layers are antiparallel due to a weak anti-ferromagnetic coupling between layers, to a lower level of resistance when the magnetization of the adjacent layers align due to an applied external field. The spin of the electrons of the nonmagnetic metal align parallel or antiparallel with an applied magnetic field in equal numbers, and therefore suffer less magnetic scattering when the magnetizations of the ferromagnetic layers are parallel.

Discovery

Magnetoresistive effects due to what was only later recognized to be Giant Magnetoresistance, were probably first seen by Sato in Co/Ag multilayers 1987 (H. Sato et al. "Galvanomagnetic properties of Ag/Co layered metallic films", Superlattices and Microstructures, Vol. 4, No. 1, 1988). However, the true genesis of the effect involved studies of Fe/Cr/Fe trilayers in 1988 by a research team led by Peter Grünberg of the Jülich Research Centre, who owns the patent, and of Fe/Cr multilayers by the group of Albert Fert of the University of Paris-Sud, who first saw the large effect in multilayers that led to its naming, coined the name, and first correctly explained the underlying physics.

Types of GMR

Multilayer GMR

Two or more ferromagnetic layers are separated by a very thin (about 1 nm) non-ferromagnetic spacer (e.g. Fe/Cr/Fe). At certain thicknesses the RKKY coupling between adjacent ferromagnetic layers becomes antiferromagnetic, making it energetically preferable for the magnetizations of adjacent layers to align in anti-parallel. The electrical resistance of the device is normally higher in the anti-parallel case and the difference can reach several 10% at room temperature. The interlayer spacing in these devices typically corresponds to the second antiferromagnetic peak in the AFM-FM oscillation in the RKKY coupling.

The GMR effect was first observed in the multilayer configuration, with much early research into GMR focusing on multilayer stacks of 10 or more layers.

Spin-valve GMR

Two ferromagnetic layers are separated by a thin (about 3 nm) non-ferromagnetic spacer, but without RKKY coupling. If the coercive fields of the two ferromagnetic electrodes are different it is possible to switch them independently. Therefore, parallel and anti-parallel alignment can be achieved, and normally the resistance is again higher in the anti-parallel case. This device is sometimes also called spin-valve.

Spin-valve GMR is the configuration that is most industrially useful, and is the configuration used in hard-drives.

Granular GMR

Granular GMR is an effect that occurs in solid precipitates of a magnetic material in a non-magnetic matrix. In practice, granular GMR is only observed in matrices of copper containing cobalt granules. The reason for this is that copper and cobalt are immiscible, and so it is possible to create the solid precipitate by rapidly cooling a molten mixture of copper and cobalt. Granule sizes vary depending on the cooling rate and amount of subsequent annealing. Granular GMR materials have not been able to produce the high GMR ratios found in the multilayer counterparts.

Applications

As stated above, GMR has been used extensively in the read heads in modern hard drives. Another application of the GMR effect is in non-volatile, magnetic random access memory (MRAM).

References

Magnetic properties of superlattices formed from ferromagnetic and anitferromagnetic materials, L. L. Hinchey and D. L. Mills, Physical Review B, vol. 33, no. 5, pp 3329, March 1986.

Layered Magnetic Structures: Evidence for Antiferromagnetic Coupling of Fe Layers across Cr Interlayers, P. Grünberg, R. Schreiber, Y. Pang, M. B. Brodsky, and H. Sowers, Physical Review Letters, vol. 57, no. 19, pp 2442, November, 1986.

Antiparallel coupling between Fe layers separated by a Cr interlayer: Dependence of the magnetization on the film thickness, C. Carbone and S. F. Alvarado, Physical Review B, vol. 36, no. 4, pp 2433, August 1987.

Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlattices, M. N. Baibich, J. M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Eitenne, g. Creuzet, A. Friederich, and J. Chazelas, Physical Review Letters, vol. 61, no. 21, pp. 2472, November 1988.

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