Metal–insulator transition induced by 16O–18O oxygen isotope exchange in colossal negative magnetoresistance manganites

The effect of 16O→18O isotope exchange on the electric resistivity was studied for (La1−yPry)0.7Ca0.3MnO3 ceramic samples. It was found that at y=0.75, the substitution of 16O by 18O results in the reversible transition from a ferromagnetic metal (FM) to charge ordered (CO) insulator at zero magnetic field. The applied magnetic field (H⩾2 T) transformed the sample with 18O again to the metallic state and caused the increase in the FM transition temperature TC of the 16O sample. As a result, the isotope shift of TC at H=2 T was as high as 63 K. Such unique sensitivity of the system to oxygen isotope exchange, giving rise even to the metal–insulator transition, is discussed in terms of the isotope dependence of the effective electron bandwidth which shifts the balance between the CO and FM phases.