Abstract
The oxygen-isotope effect of the ferromagnetic transition in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ was investigated from $x=20%$ to $x=43%.$ This is the range of the conducting ferromagnetic phase. We find that ${\ensuremath{\alpha}}_{\mathrm{O}}=\ensuremath{-}\ensuremath{\Delta}\mathrm{ln}{T}_{c}/\ensuremath{\Delta}\mathrm{ln}m$ decreases from 0.36 to 0.14 with increasing Ca concentration. A large value of ${\ensuremath{\alpha}}_{\mathrm{O}}=0.83$ 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.
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