Crossover from first-order to second-order phase transitions and magnetocaloric effect in La0.7Ca0.3Mn0.91Ni0.09O3

Abstract

We have prepared La0.7Ca0.3Mn0.91Ni0.09O3 and then studied its critical behavior and magnetocaloric effect. Analyzing temperature and field dependences of magnetization around the ferromagnetic-paramagnetic transition reveals the sample undergoing the second-order magnetic phase transition with the critical parameters TC ≈ 199.4 K, β = 0.171 ± 0.006, and γ = 0.976 ± 0.012. A considerable difference of these critical exponents compared with those expected for the standard models is due to the sample exhibiting the crossover property (tricriticality); its exponent values are more close to those expected for the tricritical mean-field theory with β = 0.25 and γ = 1. Under the field 40 kOe, the maximum magnetic entropy change (−ΔSmax) around TC is about 7.1 J·kg−1·K−1, corresponding to a refrigerant capacity RC ≈ 170 J/kg. Particularly, its magnetic-field dependence obeys a power law |ΔSmax| ∝ Hn, where n = 0.55 is quite far from the value calculated from the relation n = 1 + (β − 1)/(β + γ).