We report on the doping dependence of the order of the ferromagnetic metal to paramagnetic insulator phase transition in ${\mathrm{L}\mathrm{a}}_{1-x}{\mathrm{C}\mathrm{a}}_x{\mathrm{M}\mathrm{n}\mathrm{O}}_3$. At $x=0.33$, magnetization and specific heat data show a first order transition, with an entropy change ($2.3\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{K}$) accounted for by both volume expansion and the discontinuity of $M\sim 1.7{\mu }_{\mathrm{B}}$ via the Clausius-Clapeyron equation. At $x=0.4$, the data show a continuous transition with tricritical point exponents $\alpha =0.48\pm 0.06$, $\beta =0.25\pm 0.03$, $\gamma =1.03\pm 0.05$, and $\delta =5.0\pm 0.8$. This tricritical point separates first- ($x<0.4$) from second-order ($x>0.4$) transitions.

• Received 20 June 2002

DOI:https://doi.org/10.1103/PhysRevLett.89.227202