Universal curves in assessing the order of magnetic transition of La_0.7−xPr_xCa_0.3MnO₃ compounds exhibiting giant magnetocaloric effect

The magnetic properties of La_0.7-xPr_xCa_0.3MnO₃ (x = 0.0, 0.2, 0.3, 0.4, and 0.5) compounds fabricated by a solid-state reaction are studied by using Arrott plots and universal curves. The Arrott plots near the Curie temperature show negative slopes over the entire magnetic field range up to 50 kOe, indicating a first-order magnetic phase transition (according to Banerjee's criterion) for the x = 0.0, 0.2, 0.3, and 0.4 samples. For the x = 0.5 sample, however, the plots show positive slopes at low magnetic fields below 10 kOe, indicating a second-order transition. These results are further checked by universal curves of the normalized entropy change versus reduced temperature. The universal curves show a divergence among the curves with different magnetic fields for a first-order magnetic transition, but for a second-order transition, they collapse onto the same curve. The Curie temperature, around which the maximum magnetic entropy change occurs, decreases continuously from 260 to 110 K as x increases from 0.0 to 0.5. In the same x range, the maximum entropy change measured at a magnetic field span of 50 kOe decreases from 10.70 to 7.14 J/kg K, but the relative cooling power increases from 278 to 380 J/kg under the same magnetic field span.