Electrodeposited multilayer films with giant magnetoresistance ( GMR): Progress and problems

Abstract

The giant magnetoresistance ( GMR) effect was discovered in 1988 in nanoscale metallic ferromagnetic/non-magnetic (FM/NM) multilayers. By now, devices based on this phenomenon have been widely commercialized which use multilayered structures manufactured via physical deposition (PD) methods, mainly sputtering. It was shown in the early 1990s that electrodeposition (ED) is also capable of producing multilayered magnetic nanostructures exhibiting a significant GMR effect. These layered structures include multilayer films similar to those prepared by PD methods on macroscopic substrates and multilayered nanowires deposited into nanosized template pores, the latter ones being unique to the ED technique. Whereas ED multilayered nanowires can exhibit a GMR effect comparable to the values obtained on PD multilayer films, the GMR values achieved on ED multilayer films still remain inferior to them and, quite often, require high magnetic fields for saturation. Therefore, in spite of the relative simplicity and cost-effectiveness of the ED method, the GMR characteristics of ED multilayer films are still not competitive with the corresponding parameters of their PD counterparts. The main purpose of the present review is to give a summary of the progress achieved over the last one and a half decades on ED multilayer films with GMR effect and to critically evaluate the GMR results reported for various element combinations accessible to the ED technique for the preparation of FM/NM multilayer films (ED multilayered nanowires will be treated very briefly only). In order to promote an understanding of the inferior behavior of ED multilayer films, a detailed discussion of the magnetoresistance effects occurring in bulk homogeneous ferromagnets as well as in magnetic nanostructures (FM/NM multilayers and granular alloys) will be provided. Particular attention will be paid to the case of non-ideal magnetic nanostructures which contain both FM and superparamagnetic (SPM) regions. This is an essential ingredient in explaining the high saturation field of GMR commonly observed in ED multilayer films. In addition to the GMR magnitude, this is another characteristic decisively influencing the magnetic field sensitivity, a key feature concerning applications in sensor devices. The controversial results reported for the spacer layer thickness dependence of GMR in ED multilayer films will also be discussed. It is pointed out that the still inferior GMR characteristics of ED multilayer films can be to a large extent ascribed to microstructural features leading to the appearance of SPM regions, pinholes in the spacer layers and probably not sufficiently perfect interfaces between the FM and NM layers. The origin of the latter deficiency is not yet well understood although it is clearly one of the main causes of a weak interlayer coupling (if there is any coupling at all) and, thus, a small degree of antiparallel alignment leading to a reduced GMR effect. Works will also be described in which attempts were made to produce ED multilayer films with view on possible applications in GMR sensor devices. Finally, problems will be identified which should still be solved in order to make the properties of ED multilayer films attractive for GMR applications.