Spin-Valve Effect in Soft Ferromagnetic Sandwiches and Giant Magnetoresistance in Soft Ferromagnetic Multilayers

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Dieny, B., Speriosu, V.S., Gurney, B.A., Parkin, S.S.P., Wilhoit, D.R., Roche, K.P., Metin, S., Peterson, D.T. & Nadimi, S., 1991. Spin-Valve Effect in Soft Ferromagnetic Sandwiches J. Magn. Magn. Mater. 93, 101 and Dieny, B., Speriosu, V.S., Parkin, S. S.P., Gurney, B.A., Wilhoit, D.R. & Mauri, D., 1991. Giant Magnetoresistance in Soft Ferromagnetic Multilayers. Phys. Rev. B 43,1297

Essay about these articles

Both Fert and Grünberg understood the impact of their discovery to increase the sensitivity of the magnetoresistive read heads used in the hard disk drives in computers. Originally read heads worked on the principle of magnetic induction or Lenz’s law, which holds that one creates an electromotive force in a circuit that is proportional to the rate of change of the magnetic field. The magnetic flux emanating from a bit of data stored on a disk is fixed by the size of the bit, which is related to the density of the storage [bits]. As the density increases the bit size and flux decreases; therefore to obtain a sizeable signal one needs to rotate the disk faster. By the late eighties, the densities had reached their limit because if engineers increased the speed of the disk--so as to produce a sufficiently high rate of change of the flux--they risked the hard drives mechanically failing and flying apart.

A new generation of heads based on anisotropic magnetoresistance was introduced in the early nineties. The material used for this application was permalloy, a mixture of nickel and iron. However, there was no way of further increasing their sensitivity to meet the demands of higher density storage. For this reason the advent of GMR was immediately recognized by several research groups in academic as well as industrial laboratories as offering a solution.

While Fert’s findings highlighted the spectacular result one could achieve with superlattices, Grünberg’s was the geometry that led to the eventual development of the magnetic sensor in the read heads of the disk drives in our computers. A group at IBM (including Virgil Speriosu, Stuart Parkin, Bruce Gurney, Bernard Dieny and others) engineered a magnetoresistive sensor, called a spin valve, that achieves a magnetoresistance (MR) ratio of 8-17% with a field as small as 1 oersted. They arrived at this by using an antiferromagnet to pin one of the magnetic layers, a soft magnetic layer made of permalloy and a non-magnetic spacer whose thickness is such that there is no coupling between magnetic layers. In this manner the group minimized the magnetic flux needed to switch the spin valve between antiparallel and parallel. Also, they dusted the interfaces between the magnetic layers and non-magnetic spacer with a monolayer of Co to enhance the GMR ratio by increasing the spin dependent scattering at these interfaces. These sensors found themselves into commercially available computers by 1997; about nine years after the discovery of GMR, the effect on which they are based.

Spin valves resolved the question of whether interlayer coupling was a necessary ingredient for GMR. As much of the original work on GMR involved multilayers in which the magnetic layers were coupled, spin valves showed that this coupling was not necessary to produce the effect. The unpolarized Fermi sea of electrons in a nonmagnetic spacer transmits the interlayer coupling, as described in the 1986 Phys. Rev. Lett. paper by Grünberg, Schreiber, Pang, Brodsky, and Sowers, and it acts as a conductor for electrons; however GMR is not dependent on the interlayer coupling.

See Also:

(1991);B. Dieny, V. S. Speriosu, S. Metin, S. S. P. Parkin, B. A. Gurney, P. Baumgart, and D. R. Wilhoit, J. Appl. Phys. 69, 4774 (1991); B. Dieny, J. Magn. Magn. Mater. 136, 335 (1994). Read heads based on this idea (US patent no. 5 159 513) went into production in 1997.

Discussion Question

In which ways does a spin-valve differ from Grünberg’s original Fe/Cr/Fe trilayer?

The above article is reprinted with permission from B. Dieny, V. S. Speriosu, S. S. P. Parkin, B. A. Gurney, D. R. Wilhoit, and D. Mauri, (1991) Phys. Rev. B 43,1297. Copyright (1991) by the American Physical Society. Readers may view, browse, and/or download material for temporary copying purposes only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modified, adapted, performed, displayed, published, or sold in whole or part, without prior written permission from the American Physical Society.

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Thompson, S. M. (2008). "The discovery, development and future of GMR: The Nobel Prize 2007." Journal of Physics D-Applied Physics 41(9).

Levy, P. M. (2008). "The Nobel Prize in Physics 2007: Giant Magnetoresistance. An idiosyncratic survey of spintronics from 1963 to the present: Peter Weinberger's contributions." Philosophical Magazine 88(18-20): 2603-2613.

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Dieny, B. (1994). "GIANT MAGNETORESISTANCE IN SPIN-VALVE MULTILAYERS." Journal of Magnetism and Magnetic Materials 136(3): 335-359.

Gurney, B. A., V. S. Speriosu, et al. (1993). "DIRECT MEASUREMENT OF SPIN-DEPENDENT CONDUCTION-ELECTRON MEAN FREE PATHS IN FERROMAGNETIC METALS." Physical Review Letters 71(24): 4023-4026.

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