Tricritical point and the doping dependence of the order of the ferromagnetic phase transition of La1-xCaxMnO3.

We report on the doping dependence of the order of the ferromagnetic metal to paramagnetic insulator phase transition in La1-xCaxMnO3. At x=0.33, magnetization and specific heat data show a first order transition, with an entropy change (2.3 J/mol K) accounted for by both volume expansion and the discontinuity of M approximately 1.7mu(B) via the Clausius-Clapeyron equation. At x=0.4, the data show a continuous transition with tricritical point exponents alpha=0.48+/-0.06, beta=0.25+/-0.03, gamma=1.03+/-0.05, and delta=5.0+/-0.8. This tricritical point separates first- (x<0.4) from second-order (x>0.4) transitions.