Magnetoresistance of insulating amorphous NixSi1-x films exhibiting Mott variable-range hopping laws - art. no. 094426

Magnetoresistance (MR) ratios R(B,T)/R(0,T) have been measured in an insula ting three-dimensional amorphous nickel-silicon film that exhibits the Mott variable-range hopping (VRH) law in its zero-field resistance behavior. Su rprisingly, the resistance displayed a decrease in small fields; only in mo derately strong magnetic fields did the resistance exhibit a large increase over its zero-field value. These results are described by a phenomenologic al empirical model of two hopping processes acting simultaneously-the orbit al magnetoconductance (forward-interference) model yielding negative magnet oresistances and the wave-function shrinkage model contributing positive ma gnetoresistances. The fits use numerical values for estimating the R(B,T)/R (0,T) ratios, based upon the wave-function shrinkage model. The model inclu des three fitting parameters, whose magnitudes are extracted from the MR ra tio data at T = 10.5 K. Agreement between the predicted and measured data i s acceptable at high temperatures. A crossover of the conductivity to an Ef ros-Shklovskii (ES) variable-range hopping law is observed around T = 6 K. At lower temperatures for this ES case, predicted values for the R(B,T)/R(0 ,T) ratios are fitted to the data. For a second weakly insulating film, whi ch also exhibits a Mott VRH law in its resistance, the negative magnetoresi stance contribution is greatly depressed.