Influence of sputtering atmosphere on hopping conductance in granular nanocomposite (FeCoZr)x(Al2O3)1−x films

The present paper investigates conductivity σ for a wide temperature range of 2 K < T < 300 K in the nanocomposite (FeCoZr)x(Al2O3)1−x films (1-6 μm thicknesses, 0.30 < x < 1.00) which were sputtered on glass-ceramic substrates from the compound target in vacuum chamber filled with Ar (set 1 films) or Ar + O2 gas mixture (set 2 films). TEM and HRTEM images revealed that granular films of set 1 containing nanosized crystalline metallic bcc α-FeCo-based nanoparticles embedded in the amorphous Al2O3 matrix. In case of Ar + O atmosphere of deposited α-FeCo-based crystalline “cores” with stabilized sizes were covered with an amorphous FeCo-based oxide “shells”. The σ(T) dependences were found to be in agreement with this phase structure. They show transition of the set 1 films σ(T) curves with x from tunneling (hopping) behavior below the percolation threshold x < xC ≈ 0.46 to metallic one for x > xC. Moreover, as opposed to such behavior, for the set 2 films hopping mechanism of σ(T) was revealed far beyond xC due to the formation of semiconducting FeCo-based oxide “shells” separating electrically contacting FeCoZr cores.