Specific heat of Sm0.55Sr0.45MnO3 manganite in magnetic fields up to 15 T: An anomalous critical behavior of the ferromagnet in magnetic field and the observation of a tricritical point

The specific heat of Sm 0.55 Sr 0.45 MnO 3 manganite is measured in the vicinity of the ferromagnetic phase transition in strong magnetic fields up to 15 T. An anomalous critical behavior of the Sm 0.55 Sr 0.45 MnO 3 ferromagnet in magnetic field is predicted and experimentally observed. The anomalous behavior manifests itself in that, in magnetic fields up to 4 T, the field favors fluctuations and the specific-heat anomaly increases with the field and sharpens, becoming λ-shaped. In fields above 4 T, the behavior becomes classical: the field suppresses the fluctuations and the specific-heat peak is smeared out. The hysteresis of the transition temperature measured in the heating and cooling runs is about 15 K in zero magnetic field. As the field increases, it narrows gradually and vanishes completely when the field reaches 4 T. The results of the experiments are explained in terms of the competition between the hysteresis and the fluctuations of the magnetic order parameter. The H-T phase diagram of Sm 0.55 Sr 0.45 MnO 3 certainly indicates that, at 4 T, Sm 0.55 Sr 0.45 MnO 3 has a tricritical point, at which the ferromagnetic and paramagnetic phases of Sm 0.55 Sr 0.45 MnO 3 are leveled.