Transport Phenomena in Metal Nanocomposites

Ferromagnetic nanocomposites are the special case of metal composites; they are of practical interest for spintronics. Temperature dependences of resistivity rho(T) and thermoelectric power alpha(T) of ferromagnetic nanocomposites with the composition Co-x(Al2O3)(100-x) (36.6 <= x <= 52.5 at %) are investigated near the percolation threshold (x(p) approximate to 43.3 at %) in a temperature range of 77 300 K. Sizes of Co nanoparticles are no larger than 25 nm. Specific features are observed in the dependences alpha(T) in the form of a kink at T-b approximate to 170 K. The analysis of the structural and electrical schematics as well as energy diagrams of percolation channels of electrons shows that only the diffusion thermoelectric power appears in Co nanoparticles, at which alpha(T) is the linear function. No mechanisms of the thermoelectric power caused by nanosizes of Co particles or by electron tunneling between them are found. The kink of the alpha(T) linear dependence is explained by the existence of the oxide shell of Co nanoparticles. It is assumed that the temperature dependences of energy barriers of oxide shells of metal nanoparticles (including ferromagnetic ones) in oxygen-containing dielectric matrices determine the features of alpha(T) and rho(T) dependences of such nanocomposites.