Critical behavior and magnetocaloric effect in La_0.67Ba_0.33–xSr_xMnO₃ (x = 0.1, 0.2 and 0.25)

We have systematically investigated the critical behavior around paramagnetic-ferromagnetic (PM-FM) phase transition temperature, T_C, and attempted to determine the Curie temperatures and critical exponents β, γ and δ of perovskite manganites La_0.67Ba_0.33–xSr_xMnO₃ (x = 0.1, 0.2 and 0.25).The T_C determined from M(T) is observed to increase from 336.7 to 368.4 K, with Sr doping at Ba. The critical exponents β, γ and δ are determined from the magnetization, M(T,H), using Modified Arrott Plots for theoretically predicted models and critical M(H) isotherms at T = T_C. From the critical exponents we have inferred that La_0.67Ba_0.33–xSr_xMnO₃ behave as a three-dimensional (3D) isotropic nearest heighbor Heisenberg ferromagnet in the critical region. We have further verified that the behavior of 3D isotropic nearest neighbor Heisenberg is in agreement with the values of exponent ‘n’ determined based on the relation between field dependent change in magnetic entropy at the Curie temperature, T = T_C. Sr doping at Ba site in La_0.67Ba_0.33–xSr_xMnO₃ does not lead to any crossover in the type of magnetic interactions present in the materials, indicating a stable behavior of 3D isotropic nearest neighbor Heisenberg ferromagnet in the critical region. The differences in the values of parameters may be attributed to the complex nature of interactions present in these materials.