Magnetic domain percolation in granular Co-Ag

The most dramatic changes in the properties of granular metals occur near the percolation threshold x(c), which generally occurs at a volume fraction of approximately 0.50. There has been much speculation concerning the evolution of magnetic domains at x(c), however, no direct observations have been produced. Using scanning electron microscopy with polarization analysis, we investigate the formation of domains in a series of granular Co-Ag samples. We find x(c) to lie in the range [0.50, 0.55], and we report on the size and morphology of the observed domains. Below x(c), a domain pattern appears which may be due either to interparticle correlations or to residual Co in the silver matrix; we have previously discussed the ramifications of this observation on an understanding of GMR. In this paper, we place limits on the latter of these two models. Above x(c), the domains resemble ''stripe domains'' which have been observed in amorphous materials. This observation suggests the presence of anisotropy perpendicular to the film plane. (C) 1996 American Institute of Physics.