Unusual temperature dependence of the oxygen-isotope effect on the exchange-energy of La$_{1-x}$Ca$_{x}$MnO$_{3}$

We report magnetic susceptibility $\chi(T)$ measurements on oxygen-isotope exchanged La$_{1-x}$Ca$_{x}$MnO$_{3+y}$ up to 700 K. The $1/\chi(T)$ data show that the ferromagnetic exchange-energy $J$ depends strongly on the oxygen-isotope mass. The isotope effect on $J$ decreases with temperature up to 400 K and then increases again with temperature above 400 K. This unusual temperature dependence cannot be explained by existing theories of the colossal magnetoresistance effect for doped manganites. We suggest that a correct model for description of the physics of manganites should be based on double-exchange and the formation of localized bipolarons in the paramagnetic state.