Magnetoresistance of RuO2-based thick film resistors

In a series of samples whose composition was systematically changed, we have studied the magnetoresistance (Delta R/R = [R(B)-R]/R) of RuO2-based thick film resistors (TFR's) in magnetic fields (B) up to 20 Tesla and in a temperature range (1.2K<T<40K) in which their resistance exhibits insulating behavior at the boundary between rite strong and the weak localization. At the higher temperatures, Delta R/R exhibits a positive bump, that does not depend on the RuO2 concentration but it changes with the concentration of magnetic Mn impurities diluted in the glassy matrix. For T less than or equal to 20 K magnetoresistance is entirely negative and it has a quadratic magnetic field dependence at low field. We use the weak localization theory to relate these features of the high temperature magnetoresistance to the composition of TFR's. At low temperature (T<4.2K) the negative magnetoresistance shows some peculiarities. The quadratic term shrinks, within a vanishing magnetic field range and the magnetoresistance linearly increases in a wide range of B. Ar the strongest fields and the lowest temperature Delta R/R shows a tendency to saturation. Although these results do not enable to discriminate among different mechanisms, we note that the low temperature (T