Oxygen-isotope effect of the paramagnetic-insulating to ferromagnetic-metallic transition inLa1−xCaxMnO3

The oxygen-isotope effect of the ferromagnetic transition in ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ was investigated from $x=20\%$ to $x=43\%.\mathrm{}$ This is the range of the conducting ferromagnetic phase. We find that ${\alpha }_{\mathrm{O}}=-\Delta \ln T_c/\Delta \ln m$ decreases from 0.36 to 0.14 with increasing Ca concentration. A large value of ${\alpha }_{\mathrm{O}}=0.83\mathrm{}$ was found for $x=20\%,$ it is possibly connected with excess oxygen content. The isotope effect decreases with increasing tolerance factor, pointing to the importance of double exchange. The isotope effect at 35% Ca is independent of magnetic field.