Using a criterion given by Banerjee [Phys. Lett. 12, 16 (1964)] to distinguish first-order magnetic transitions from second-order ones, we demonstrate that not all the title perovskites exhibit a first-order transition at their Curie temperatures $T_C.$ We observe that, although ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_3$ exhibits a first-order magnetic transition at $T_C,$ ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3$ exhibits a second-order one.

• Received 27 January 1999

DOI:https://doi.org/10.1103/PhysRevB.60.2998