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

Spin-dependent transport in CoX_YxXY(SiOX_Y2XY)X_Y1-xXY granular films was investigated just below percolation (volume fraction x=0.38, 0.41, 0.46, and 0.50). Co-SiO₂ is an ideal system for investigating magnetic nanoparticle properties since the Co-SiO₂ interfaces are of high quality with no evidence of intermixing, and the saturation magnetization is consistent with bulk values. Transport in these films involves tunneling or hopping. The magnetoresistance is consistent with a spin polarization of 0.26 for the electrons tunneling across the Co-SiO₂ interface, independent of metallic volume fraction and temperature. Ferromagnetic correlations among the CO nanoparticles are evident in the zero-field-cooled (ZFC) state of Co-SiO₂ granular films. For x=0.41, the correlation is among isolated particles of 40 Å diameter. For x=0.46 and 0.50, at room temperature, there is some ferromagnetic correlation 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 correlations involving particles that are superparamagnetic at room temperature, similar to the correlation observed for x=0.41 at 77 K.