Magnetocaloric materials with second order phase transition near the Curie temperature can be described by critical phenomena theory. In this theory, scaling, universality, and renormalization are key concepts from which several phase transition order criteria are derived. In this work, the rescaled universal curve, Banerjee and mean field theory criteria were used to make a comparison for several magnetocaloric materials including pure Gd, SmCo1.8Fe0.2, MnFeP0.46As0.54, and La0.7Ca0.15Sr0.15MnO3. Pure Gd, SmCo1.8Fe0.2, and La0.7Ca0.15Sr0.15MnO3 present a collapse of the rescaled magnetic entropy change curves into a universal curve, which indicates a second order phase transition; applying Banerjee criterion to H/σ vs σ2 Arrot plots and the mean field theory relation |ΔSM| ∝ (μ0H/Tc)2/3 for the same materials also determines a second order phase transition. However, in the MnFeP0.46As0.54 sample, the Banerjee criterion applied to the H/σ vs σ2 Arrot plot indicates a first order magnetic phase transition, while the mean field theory prediction for a second order phase transition, |ΔSM| ∝ (μ0H/Tc)2/3, describes a second order behavior. Also, a mixture of first and second order behavior was indicated by the rescaled universal curve criterion. The diverse results obtained for each criterion in MnFeP0.46As0.54 are apparently related to the magnetoelastic effect and to the simultaneous presence of weak and strong magnetism in Fe (3f) and Mn (3g) alternate atomic layers, respectively. The simultaneous application of the universal curve, the Banerjee and the mean field theory criteria has allowed a better understanding about the nature of the order of the phase transitions in different magnetocaloric materials.

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Tunable spin reorientation transition and magnetocaloric effect in Sm0.7−xLaxSr0.3MnO3 series J. Appl. Phys. 113, 013911 (2013); 10.1063/1.4773337 Effect of Fe substitution on magnetic and magnetocaloric effect in Gd(Co1− x Fe x )2B2 compounds

This work presents a study of how the magnetic and temperature variables, as well as the measurement rate, affect magnetic and structural phase transitions for La0.7Ca0.3−xSrxMnO3 (x=0.15, 0.08, 0.07 and 0.06) manganites. A single phase rhombohedral crystal structure for Sr0.15 compound and a single phase orthorhombic crystal structure for Sr0.08, Sr0.07, and Sr0.06 compounds were identified by X ray diffraction. The Curie temperature adjustment to room temperature was reached for Sr0.07 sample, in which a Jahn-Teller transition was observed. This Jahn-Teller transition showed a susceptible response to 0.5, 1, 2, 3, 5 and 7 K/min measurement rates, and was revealed without measurement sequence at 234, 257, 196, 233, 245 and 238 K respectively in thermomagnetic curves (10mT). Also, Curie temperature increases sequentially around 2 K for the same measurement rates. To avoid magnetic field effect, a differential scanning calorimetry for Sr0.07 sample was obtained in the same temperature region, for 2, 3, 5, 7 K/min showing the Jahn-Teller phase transition at 225 K and a vibrational like behavior in the heat flux below 225 K, for all the temperature rates. The latter, is related to the static to dynamic Jahn-Teller transition. Furthermore, Curie temperature appears around 284 K.