Giant magnetoresistance and magnetic aspects in granular structures

The microstructural influence of binary melt-spun AuCo and pseudo-binary AlNiCo5 permanent magnets in the optimum magnetic state on the giant magnetoresistance (GMR) have been compared to identify the origin of the magnetotransport in these metal/metal granular systems. In general, their high field GMR dependence is originated from small ferromagnetic clusters distributed within the nonmagnetic well-conducting matrix phase. These "heterogeneities" are determined by the corresponding phase diagrams and hence are inherently present. AlNiCo5 as an example further shows that these "heterogeneities" also determine the GMR behavior in systems were the magnetic ordering takes place on a much larger length scale compared to the electron mean free path. The microstructure and magnetic properties of well-ordered metal (Co)/organic granular superlattices are discussed in comparison so as to show their high potential for future studies of granular transport and tunneling properties in these structurally better-defined systems. It is also demonstrated that alloying of (FeCo)/organic granular superlattices is possible on a length scale of a few nanometers only. (C) 2003 Elsevier Science B.V. All rights reserved.