Resistivity minimum in granular composites and thin metallic films

We analyze the temperature dependence of conductivity in thick granular ferromagnetic compounds NiSiO2 and in thin weakly coupled films of Fe, Ni, and Py in the vicinity of the metal-to-insulator transition. Development of a resistivity minimum followed by a logarithmic variation of conductivity at lower temperatures is attributed to the granular structure of compounds and thin films fabricated by conventional deposition techniques. The resistivity minimum is identified as a transition between temperature dependent intragranular metallic conductance and thermally activated intergranular tunneling.