Strong oxygen-mass dependence of the thermal-expansion coefficient in the manganites (La1-xCax)(1-y)Mn1-yO3

Thermal-expansion and magnetic susceptibility measurements were performed on oxygen-isotope substituted manganites (La1-xCax)(1-y)Mn1-yO3 with a Mn4+ concentration of similar to 33%. The linear thermal-expansion coefficient beta(T) exhibits an asymmetric peak at the Curie temperature T-c, indicative of a second-order ferromagnetic transition. Upon replacing O-16 with O-18, Te is lowered by about 10 K, while the second-order jump in the thermal-expansion coefficient [Delta beta(T-c)] is raised by about 20%. Such a ''colossal'' oxygen-isotope effect on Delta beta(T-c) is very surprising, but can be explained quantitatively on the basis of double exchange and strong polaronic effects.