It has been known for nearly 150 years that the resistivity of bulk ferromagnetic metals and alloys can change by a few percent in an external magnetic field. Due to its dependence on the relative orientation of the magnetization and the magnetic field, this effect is called anisotropic magnetoresistance (AMR). The AMR effect has widely found application in magnetic field sensors.
With the immense development of thin film deposition techniques in the 1970s-80s, it became possible to produce nanoscale layered structures in which ferromagnetic (FM) and non-magnetic (NM) metals alternate with individual layer thicknesses smaller than the electron mean-free path in the corresponding bulk metals. Dut to the latter feature, new physical phenomena may occur in these in such nanoscale structures. A striking example is the discovery of the giant magnetoresistance (GMR) effect in FM/NM multilayers in the late 1980s and in magnetic granular alloys in the early 1990s. Not only the underlying physical mechanisms are different for the AMR and GMR, but also the change of resistance due to the GMR effect can be an order of magnitude higher than for the AMR effect. In a decade after the discovery, specific layered structures (so-called spin-valves) based on the GMR effect have found unique application in read heads of hard disk drives and enabled maintaining an enormous rate of magnetic storage density increase even until today. The discovery of GMR has been awarded by the Nobel Prize in Physics in 2007.
In the present talk, in addition to a historical perspective on the discovery of the GMR effect, we intend to give a description of spin-dependent scattering processes occurring in various magnetic nanostructures, which form the unerlying physical mechanism for explaining the phenomenon of GMR. It will also be discussed how the GMR effect revolutionized magnetic data storage by allowing a drastic decrease of the magnetic bit size.
We will also give a general overview of spin-dependent transport processes in ideal magnetic nanostructures. The two limiting cases are (i) perfect nanoscale metallic multilayers in which FM layers are separated by NM layers and (i) classical granular metals in which nanoscale non-interacting ferromagnetic regions with superparamagnetic (SPM) characteristics are embedded in a NM matrix. Then, we will turn attention to some results achieved by our group in the last two decades in studying the GMR effect in electrodeposited multilayer films [1]. Our efforts were partly oriented towards understanding the commonly observed strongly non-saturating behaviour of the field-dependence of GMR in these systems. It turned out that this feature originates from the presence of SPM regions in the predominantly ferromagnetic layers. We have identified a physical model elaborated originally for granular metals [2,3] and could adapt it for the case of electrodeposited multilayers [4]. On this basis, the latter systems can be considered as non-ideal magnetic nanostructures. Along this line, the GMR effect can be discussed in a unified view for both layered and granular magnetic nanostructures as well as for a mixture of the two types of magnetic nanostructure.

 

1. I. Bakonyi, L. Péter: Electrodeposited multilayer films with giant magnetoresistance (GMR): progress and problems. Progr. Mater. Sci. 55, 107-245 (2010)
2. B.J. Hickey, M.A. Howson, S.O. Musa and N. Wiser, Phys. Rev. B 51, 667 (1995)
3. N. Wiser, J. Magn. Magn. Mater. 159, 119 (1996)
4. I. Bakonyi, L. Péter, Z. Rolik, K. Kiss-Szabó, Z. Kupay, J. Tóth, L. F. Kiss and J. Pádár, Phys. Rev. B 70, 054427 (2004)