Observation of heat flow at transition temperature in La1-xCaxMnO3+delta Oxides

Heat-flow changes of La1-xCaxMnO3+delta (x=0.25, 0.33, and 0.375) magnetore sistive oxides at their Curie temperatures have been detected using differe ntial scanning calorimetry in the temperature range from 173 to 293 K. Howe ver, the transition does not occur in the samples of x=0.125 and 0.5. It wa s found that the heat flow reflected the transition behavior and the enthal py change of the transition decreased as the transition temperature increas ed. These results indicate that the La1-xCaxMnO3+delta oxides (x=0.25, 0.33 , and 0.375) undergo an endothermic phase transition from a low-temperature ferromagnetic metal to a high-temperature paramagnetic insulator. It is as sumed that the transition is strongly associated with a local structure cha nge, which is correlated with the metal-insulator transition. (C) 2001 Amer ican Institute of Physics.