Magnetic correlations in non-percolated Co-SiO2 granular films

The zero-field-cooled state of Co-x(SiO2)(1-x) granular films was investigated with small-angle neutron scattering, low-field AC magnetic susceptibility chi>(*) over bar * (T), and magnetoresistance measurements. Films with metallic volume fraction x = 0.41, which consist of 40 Angstrom isolated particles that are superparamagnetic at room temperature, show an ordered magnetic state below 150 K consisting of 550 Angstrom correlated regions that are aligned antiferromagnetically. The ordering is attributed to dipolar interactions. The temperature for the onset of correlated behavior, obtained from SANS (faster than 10(-10) s), coincides with the frequency-independent maximum of chi'(T) for frequencies ranging from 0.1 to 1000 Hz. Furthermore, consistent with ferromagnetic correlations, the ZFC resistance is not the maximum resistance. Films with x = 0.32 exhibit evidence of magnetic interactions but no extended magnetic correlation - the magnetic correlation length corresponds to the particle size. (C) 2000 Elsevier Science B.V. All rights reserved.