Magnetothermal Effects and Critical Behavior of Rare Earth Manganese Oxides La 0.65 Ca 0.35 Mn 1-x Ni x O 3 (x=0, 0.2)

This study used the conventional solid-phase reaction method to prepare polycrystalline samples: La 0.65 Ca 0.35 Mn 1−x Ni x O 3 (x = 0, x = 0.2). The effects of Ni doping on the preformed cluster phases, magnetothermal effects, and critical behavior were systematically investigated. The results showed that the prepared polycrystalline samples all had cubic chalcogenide structures. The substitution of Ni 2+ ions for a certain number of Mn 3+ ions reduced the changes in the lattice parameters, unit cell volume, Curie temperature (Tc), magnetic entropy, and magneto-entropy. The changes in these properties originated from the partial substitution of Ni 2+ for Mn 3+ ions, which changed the Mn 3+ /Mn 4+ 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 primary to secondary 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 J·kg − 1 ·K − 1 and 2.08 J·kg − 1 ·K − 1 , respectively.