Crossover between the second- and first-order magnetic transition in ferromagnetic manganites

The temperature dependence of the magnetic moment in La0.7Sr0.3MnO3 and La0 .7Ca0.3MnO3 ferromagnetic manganites was studied by means of de magnetic me asurements and by NMR on Mn-55 nuclei. In addition, for temperatures close to T-C, the ac susceptibility and the NMR spin-lattice relaxation were dete rmined, From the results it follows that the phase transition changes from the conventional continuous second order for La0.7Sr0.3MnO3 (T-C = 364 K) t o more abrupt but close to continuous for La0.7Ca0.3MnO3 (T-C = 265 K). The change of the character of the phase transition is explained on the basis of the mean field theory of double exchange taking into account the spontan eous magnetostriction. [S0163-1829(99)04833-X].