Metal-insulator transition in manganites: mixture of oxygen isotopes versus magnetic field

We have investigated the effect of oxygen isotope substitution on the metal-insulator transition temperature and the resistivity of the narrow band manganite (La0.25Pr0.75)0.7Ca0.3MnO3 in a constant magnetic field. A set of 16 samples having different mixtures of 16O, 17O and 18O isotopes with average mass varying from 16.0 to 17.8 a.m.u. was studied. We have found that the magnetoresistance and the isotope effect can be linked together with a single parameter - effective magnetic field, which decreases linearly with an increase of average oxygen mass with a slope of -2 T/a.m.u. The applicability of the small polaron model is discussed.