Coupling phenomena observed in multilayer films composed of two ferromagnetic layers separated by a non-magnetic intermediate
Bruyère, C., Clerc, G., Devenyi, J., Massenet, O., Montmory, R., Néel, L. & Yelon, A. (1964) Coupling phenomena observed in multilayer films composed of two ferromagnetic layers separated by a non-magnetic intermediate. Proceedings of ICM’64, Nottingham, UK, Pub. IOP and Phys. Soc., pg. 789.
Essay about this article
The question that remained after Fert’s studies on how to realize GMR was how can one switch the moments from parallel to antiparallel other than by varying the amount of ternary impurity in an alloy? The answer that eventually evolved was to use multilayered structures that allowed one to rotate the magnetization of one magnetic layer relative to another. Multilayers are grown by sequentially depositing, on a planar substrate, layers of one atomic species followed by a second species and repeating this sequence a number of times. Some of the challenges researchers had to overcome were: 1) existing vacuum chambers were not sufficiently “air tight” to remove foreign elements, 2) deposition techniques were not available to grow and monitor [characterize] continuous and flat layers of atoms so as to avoid channels to form for the following atomic species to fill and form microbridges, and to have reasonably flat interfaces between adjacent layers so as to minimize electron scattering there, and 3) metal atoms are notorious for intermixing at the interfaces between two layers of different elements.
One of the early attempts at growing multilayers of metallic elements, known as “metallic multilayers”, was pursued in Louis Neél’s laboratory in Grenoble France. At the International Conference on Magnetism in Nottingham England in 1964, his group reported on their attempts to grow multilayers of iron and copper. As L.F. Bates, the chairman of the conference, wrote: “Incidentally, it had been hoped that Professor L. NeéI, President of the International Union of Pure and Applied Physics, would be present to give an account of the Grenoble work on multilayer films consisting of two ferromagnetic layers separated by a nonmagnetic layer. In his unavoidable absence a talk on the work was given by A. Yelon who described the strange coupling which occurs when the intermediate layer is not an insulator, and which is attributed to polarized conduction electrons.”* Creating these structures from metallic elements was no mean feat; even though they had deposited non-magnetic spacer layers of copper about 300-400 Å thick between iron layers [which is quite thick] they nonetheless always found a ferromagnetic interlayer coupling which invariably aligned the magnetic layers in parallel. For applications that envisaged switching the layers between parallel and antiparallel this was a non-starter, i.e., by using an external magnetic field it is only possible to switch an antiparallel configuration to parallel. It took a couple of years to understand why the iron layers were always ferromagnetically coupled. It was not what was originally thought, i.e., polarization of the conduction electrons of the intervening copper layer; rather it was the formation of microbridges [pinholes] in the copper layer which were filled by iron atoms when one deposited iron on top of the cooper to form the multilayer.
So in the 1960’s there were a myriad of obstacles that had to be overcome before one could grow metallic multilayers that had sufficiently thin layers, of the order of nanometers, to be useful for electrical transport phenomena, such as GMR. The principal challenge was to grow continuous and smooth layers with few imperfections so that the scattering of electrons is minimized; also one had to eliminate the pinholes, which formed conduits for microbridges.
Bates (1965) Brit J Appl Phys v 16 1-3
Why were the layers produced in 1964 so thick?
The full text of Neél, et al. from the ICM 1964 was provided with kind permission of the Institute of Physics and IOP Publishing Limited.
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Levy, P.M. The Nobel Prize in Physics 2007: Giant Magnetoresistance. An idiosyncratic survey of spintronics from 1963 to the present: Peter Weinberger's contributions. Philosophical Magazine, Volume 88, Issue 18 - 20 June 2008 , pages 2603 - 2613 (2008)
Whereas our convention for the Classic Articles in Context Issues is to list a selection of approximately 10 of the articles that have cited the classic paper presented with the interpretive essay (some papers are cited thousand of times), occasionally, a document of particular significance to the problem at hand falls somewhat outside of the formal system of literature published in peer-reviewed journals, such as within a conference proceedings or a part of a government report. Such is the case for this account of Professor Louis Neél’s group examination of the properties metallic multilayed structures, presented at a symposium at Nottingham in 1964. Such documents/the work they describe are often known to the community of researchers interested in the topic, but, because they have not been published in a journal cataloged by the important Abstractors/Indexers in the discipline, they don't receive wider acknowledgment.