Specific heat and thermal expansion of La0.65Ca0.35MnO3:: Magnetic-field dependence, isotope effect, and evidence for a first-order phase transition -: art. no. 024441

The specific heat and thermal expansion of the same polycrystalline samples of La0.65Ca0.35MnO3, with either O-16 or O-18, have been measured for 85 < T< 310 K in zero field and in magnetic fields to 60 kOe. Anomalies at T-C(O-16) = 265 K and T-C (O-18) = 256 K mark the zero-field ferromagnetic transitions. Their shapes, the derived changes in entropy and volume, and a comparison with the pressure dependence of T-C show that the ferromagnetic ordering is a thermodynamic first-order transition. broadened by a distribution in T-C. Magnetic fields change the character of the transition, broaden the anomalies, and shift them to higher temperatures. The shift is greater for O-18 than for O-16, corresponding to a decrease in the isotope effect with increasing field.