Effects induced by 16O-18O oxygen isotope exchange in manganite ceramics and films

Unique sensitivity of electrical resistivity to the O-16-->O-18 isotope exchange was observed for (La1-yPry)(0.7)Ca0.3MnO3 ceramics and epitaxial thin films. At y = 0.7, substitution of O-16 by O-18 in ceramic samples results in the reversible transition from a metal-like to insulating state. In thin films, this metal-insulator transition occurs at y = 0.5 due to the strains induced by substrate. The applied magnetic field (H greater than or equal to 2 T) transforms the sample with O-18 back to the metallic state. In such magnetic fields the temperature dependence of resistivity exhibits a peak corresponding to the onset of metallic state. The shift of the peak toward low temperatures after the O-16 --> O-18 exchange exceeds 50 K at 3 T for both ceramic and thin film samples. The results are interpreted in terms of the isotope dependence of the effective electron bandwidth which shifts the balance between the charge ordered insulating and the ferromagnetic metal-like states.