Magnetocaloric Effects and Critical Behavior of Rare Earth Manganese Oxides La0.65Ca0.35Mn1-xNixO3 (x = 0, 0.2)

This study used the traditional solid-phase reaction method to prepare polycrystalline samples: La0.65Ca0.35Mn1-xNixO3 (x = 0, x = 0.2). The effects of Ni doping on the preformed cluster phases, magnetocaloric effects, and critical behavior were systematically investigated. The results showed that the prepared polycrystalline samples all had cubic chalcogenide structures. The substitution of Ni2+ ions for a certain number of Mn3+ ions reduced the changes in the lattice parameters, unit cell volume, Curie temperature (TC), magnetic entropy, and magnetic entropy change. The changes in these properties originated from the partial substitution of Ni2+ for Mn3+ ions, which changed the Mn3+/Mn4+ ratio and decreased the Mn-O-Mn bond angle, thus weakening the double exchange interaction. Both samples had a preformed cluster phase above the low-temperature magnetic transition temperature. The critical behaviors of both samples fit the tricritical model well. A transition from first- to second-order phase transition existed for both the parent and doped samples, and the maximum magnetic entropy changes of the samples at an applied magnetic field of 7 T were 7.70 and 2.08 J·kg−1·K−1, respectively.