Investigation into magnetocaloric effects and critical behavior of La0.65Ca0.35-xBaxMnO3(x=0,0.1,0.2,0.3) prepared by high-pressure heat treatment

In this study, a series of polycrystalline La 0.65 Ca 0.35- x Ba x MnO 3 ( x = 0,0.1,0.2,0.3) samples were synthesized through high-pressure heat treatment processing. Their magnetic properties, magnetocaloric effects, and critical behaviors associated with phase transitions were investigated. The substitution of Ba2+ for Ca2+ resulted in lattice expansion, which in turn increased the Mn-O-Mn bond angle and enhanced the double-exchange interaction within the system. It was revealed by experimental magnetic studies and Density functional theory calculations that the magnetic interactions, dominated by Mn-3d in the ferromagnetic ground state, were strengthened, leading to an increase in the Curie temperature from 219 K (x = 0) to 249 K (x = 0.1), 272 K (x = 0.2) and 300 K (x = 0.3). Under a magnetic field of 7 T, the maximum magnetic entropy change was found to be 4.47 J & sdot; kg- 1 & sdot; K- 1 (x = 0), 4.25 J & sdot; kg- 1 & sdot; K- 1 (x = 0.1), 3.60 J & sdot;kg- 1 & sdot;K-1 (x = 0.2) and 3.28 J & sdot; kg- 1 & sdot; K- 1 (x = 0.3), respectively. Notably, compared to solid-state reaction method samples, the refrigerating temperature range broadened by 131.25 %, 57.44 %, and 47.45 % for x = 0, 0.1, and 0.2, respectively, using high-pressure heat treatment. Additionally, the x = 0.1 sample exhibited a first-to-second-order phase transition. Analysis of Arrott plots confirmed the first-order phase transition in the parent sample and the second-order phase transition in the doped samples. Furthermore, the Modified Arrott plot, Kouver-Fisher, and local exponent methods revealed that, as the Ba content increased, the critical exponents beta, gamma, and delta of the samples with second-order phase transition characteristics (doped samples) approached 0.5, 1.0, and 0.667, respectively. The introduction of Ba was found to facilitate the establishment of a second-order phase transition pattern that is more consistent with the mean- field theory model.