Spin-dependent transport of Co-SiO2 granular films approaching percolation

Spin-dependent transport in Co-x(SiO2)(1-x) granular films was investigated just below percolation (volume fraction x = 0.38, 0.41, 0.46, and 0.50). C o-SiO2 is an ideal system for investigating magnetic nanoparticle propertie s since the Co-SiO2 interfaces are of high quality with no evidence of inte rmixing, and the saturation magnetization is consistent with bulk values. T ransport in these films involves tunneling or hopping. The magnetoresistanc e is consistent with a spin polarization of 0.26 for the electrons tunnelin g across the Co-SiO2 interface, independent of metallic volume fraction and temperature. Ferromagnetic correlations among the Co nanoparticles are evi dent in the-zero-field-cooled (ZFC) state of Co-SiO2 granular films. For x = 0.41, the correlation is among isolated particles of 40 Angstrom diameter . For x = 0.46 and 0.50, at room temperature, there is some ferromagnetic c orrelation due to dipolar fields from short chains of connected particles. In the ZFC state at 77 K for x = 0.46 and 0.50, there are ferromagnetic cor relations involving particles that are superparamagnetic at room temperatur e, similar to the correlation observed for x = 0.41 at 77 K.