GIANT MAGNETORESISTANCE IN HYBRID MAGNETIC NANOSTRUCTURES INCLUDING BOTH LAYERS AND CLUSTERS

We report on giant magnetoresistance (GMR) effects in a type of magnetic nanostructure including both layers and clusters. Our structures are prepared by sputtering and include both continuous permalloy layers and cobalt layers broken into small clusters (islands). We obtain magnetoresistance ratios as large as 35% in less than 10 Oe, with slopes of about 6.5% per Oe in the steepest part of the curve. The advantage of such hybrid structures including layers and clusters, and the origin of the observed GMR effects are the following: (a) lateral decoupling of the thin Co layers into islands prevents ferromagnetic bridging by pinholes or other defects from extending to a significant part of the sample; (b) the field dependence of the GMR is controlled by the magnetization reversal in the thick and soft permalloy layers; (c) the antiparallel magnetic ordering is better as the contrast between the hard magnetic properties of the clusters and the soft ones of the layers is higher. The drawback of these hybrid systems is the crossover to superparamagnetic behavior as one goes to room temperature [clearly shown by superconducting quantum interference device (SQUID) measurements] and the resulting deterioration of the GMR slopes. © 1994 The American Physical Society.