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We report magnetic susceptibility $\chi(T)$ measurements on oxygen-isotopeexchanged La$_{1-x}$Ca$_{x}$MnO$_{3+y}$ up to 700 K. The $1/\chi(T)$ data showthat the ferromagnetic exchange-energy $J$ depends strongly on theoxygen-isotope mass. The isotope effect on $J$ decreases with temperature up to400 K and then increases again with temperature above 400 K. This unusualtemperature dependence of the isotope effect cannot be explained by existingtheories of the colossal magnetoresistance effect for doped manganites. Thepresent results thus provide essential constraints on the physics ofmanganites.

Unusual temperature dependence of the oxygen-isotope effect on the exchange-energy of $La_{1-x}Ca_xMnO_3$ at high temperatures