Critical Behavior Near the Paramagnetic-Ferromagnetic Phase Transition in La_1−xSr_xCoO₃ (x = 0.3 and x = 0.5) Cobaltites

In this work, we present the study of structural, magnetic and critical behavior of La_1−xSr_xCoO₃ (x = 0.3 and 0.5) cobaltites. X-ray diffraction analysis using the Rietveld refinement shows that the sample La_0.7Sr_0.3CoO₃ crystallizes in a rhombohedral distorted perovskite structure with $R\overline{3}c$ space group, while La_0.5Sr_0.5CoO₃ exhibits both phases rhombohedric and cubic, with $R\overline{3}c$ and $Pm\overline{3}m$ space groups. Our samples exhibit thermo-magnetic irreversibility behavior in the magnetization data under a magnetic applied field of 0.05 T. The temperature dependence of magnetic susceptibility at higher temperatures for La_0.7Sr_0.3CoO₃ sample reveals the presence of the Griffiths' phase above the Curie temperature, while it follows the Curie–Weiss law perfectly for La_0.5Sr_0.5CoO₃. Different techniques including the modified Arrott plot, the Kouvel-Fisher method and the critical isotherm analysis have been used to analyze the paramagnetic-ferromagnetic phase transition. The values of the critical exponents associated to this transition were close to those expected in the mean-field model. The validity of the calculated critical exponents has been confirmed by the scaling equation of state. The determined critical exponents using various methods are found to follow the scaling equation with the magnetization data scaled into two different curves below and above the critical temperature.