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 dc magnetic measurements and by NMR on ⁵⁵Mn nuclei. In addition, for temperatures close to TC, the ac susceptibility and the NMR spin-lattice relaxation were determined. From the results it follows that the phase transition changes from the conventional continuous second order for La0.7Sr0.3MnO₃(TC=364 K) to more abrupt but close to continuous for La0.7Ca0.3MnO₃(TC=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 spontaneous magnetostriction.