Effect of spin-glass ordering on conduction in a-Si1-cMnc near the metal-insulator transition

The electrical conductivity and magnetoresistance of amorphous Si films implanted by Mn ions up to a concentration of 22 at% are investigated near tl-ie metal-insulator transition (MIT). We Ve find that there is a characteristic temperature T-SG = 6 to 20 K (dependent or; the Mn concentration) which separates two different conductivity behaviours. As temperature is reduced below T-SG on the insulating side of the MIT: the conductivity mechanism is found to change from variable-range hopping to conductivity with constant activation energy Delta E which decreases linearly in a magnetic field. The activation energy of hopping determined from the data above T-SG, on approaching T-SG, turns out to be smaller than Delta E. In the metallic regime a;; T < T-SG, we observe a shift of the MIT towards higher impurity concentration in respect with that determined from the data at T > T-SG The low-temperature anomalies are ascribed to the spin-glass transition caused by d-d interaction between Mn atoms.