MAGNETIC-ORDERING IN GRANULAR NI-AL2O3

We have produced granular Ni-Al2O3 solids by magnetron sputtering, and the entire composition range has been studied. The percolation threshold (p(c)) has been identified to be near 55 percent Ni by volume, in agreement with other granular metal systems. Below p(c), the samples display typical single domain magnetic behaviors, including superparamagnetism and enhanced coercivity. However, above percolation the samples display ferromagnetic transitions which differ substantially from the transition in pure Ni. We have performed a scaling analysis of the ferromagnetic transition in several such materials, and have obtained values of the Curie temperatures and the critical exponents. These are the first such analyses of the ferromagnetic-paramagnetic transition in granular magnetic solids. We have observed a depression of the Curie temperature as the Al2O3 Content of the samples is increased, as well as a substantial variation of the critical exponents. The variation of T(c) may be understood in terms of finite size scaling.