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

Unique sensitivity of electrical resistivity to the O-16-->O-18 isotope exc hange 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 thi n films, this metal-insulator transition occurs at y = 0.5 due to the strai ns induced by substrate. The applied magnetic field (H greater than or equa l to 2 T) transforms the sample with O-18 back to the metallic state. In su ch magnetic fields the temperature dependence of resistivity exhibits a pea k corresponding to the onset of metallic state. The shift of the peak towar d 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 o f the isotope dependence of the effective electron bandwidth which shifts t he balance between the charge ordered insulating and the ferromagnetic meta l-like states.