Results of structure, electrical and magnetoresistive investigations of amorphous metal-dielectric granular composites (cermets) are presented. Amorphous nanoclasters of Fe₄₅Co₄₅Zr₁₀, Co₄₁Fe₃₉B₂₀ or Co₈₄Nb₁₄Ta₂ alloys randomly distributed in dielectric SiO₂ or Al₂O₃ amorphous matrix, have been obtained by ion-beam deposition on NaCl or glassceramic substrates. Simultaneous deposition of the metal and dielectric materials from a compound target with various distances between quartz or aluminium-oxide plates allows to form the granular structure with wide and continuous concentration range. The electronic microscopy has shown that the sizes of the granules are being changed within the range of 2 - 6 nm. The average granule sizes increase with rising of the metal phase concentration as well as substrate temperatures. One should note, that granules in the composites forming at high metal concentration are not isolated absolutely in the dielectric matrix, and form a small conglomerates and chains which in turn form labyrinthiform structure. Concentration dependences of the cermets electrical resistivity, measured at room temperature, have S-like curves, which are typical for percolating systems. Prominent feature of such systems is the increase of the resistivity at thermal annealing of the cermets, which are before the percolating threshold, and reduction of the resistivity in the composites located behind the threshold. Based on the point where the concentration dependences of the cermets resistivity in initial and annealed conditions are crossed the threshold positions for various systems were determined. It has been shown, that concentration position of the percolating threshold is not a constant and depends on the composite structure and the deposition conditions: substrate temperature, pressure and composition of sputtering gases, speed of substrate rotation etc.

Electrical resistivity of the cermets, which are before the threshold, decreases exponentially with the temperature rising. Two conductivity mechanisms are realized in the studied cermets in the temperature range of 77 – 300 K: tunneling of a spin polarized electrons between granules through the dielectric barrier and hope conductivity via localized states in the amorphous dielectric (like the same in amorphous semiconductors – Mott conductivity). For the cermets, which are behind the threshold (with metal conductivity), the growth of the resistivity with the temperature is not observed. It has been shown that all the cermets with granular structure exhibit giant magnetoresistance (GMR). The concentration dependence of the GMR at room temperature is non-monotonous, with the clear maximum while at 77 K GMR has no maximum and grows with increase of the dielectric phase. The maximum GMR values at 300 K reaches seven percents in a field of 10 κέ and is observed in the vicinity of the threshold. Reducing of the temperature leads to increase of the GMR values.

The work was supported by the Russian Foundation for Basic Research (Grant N 02-02-16102).