Spin-dependent transport in ${\mathrm{Co}}_x({\mathrm{SiO}}_2{)}_{1-x}$ granular films was investigated just below percolation (volume fraction $x=0.38,$ 0.41, 0.46, and 0.50). ${\mathrm{C}\mathrm{o}-\mathrm{S}\mathrm{i}\mathrm{O}}_2$ is an ideal system for investigating magnetic nanoparticle properties since the ${\mathrm{C}\mathrm{o}-\mathrm{S}\mathrm{i}\mathrm{O}}_2$ 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 ${\mathrm{C}\mathrm{o}-\mathrm{S}\mathrm{i}\mathrm{O}}_2$ interface, independent of metallic volume fraction and temperature. Ferromagnetic correlations among the Co nanoparticles are evident in the zero-field-cooled (ZFC) state of ${\mathrm{C}\mathrm{o}-\mathrm{S}\mathrm{i}\mathrm{O}}_2$ granular films. For $x=0.41,$ the correlation is among isolated particles of 40 Å diameter. For $x=0.46\mathrm{}$ 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 $x=0.46\mathrm{}$ and 0.50, there are ferromagnetic correlations involving particles that are superparamagnetic at room temperature, similar to the correlation observed for $x=0.41\mathrm{}$ at 77 K.

• Received 27 April 2000

DOI:https://doi.org/10.1103/PhysRevB.62.14273