Determination of the entropy changes in the compounds with a first-order magnetic transition

Entropy changes in the compounds of La1−xPrxFe11.5Si1.5 (x=0.3 and 0.4) have been experimentally studied. A tower-shaped entropy change of the height of ∼27J∕kgK is obtained based on the analyses of heat capacity, while the Maxwell relation predicts an extra entropy peak of the height of ∼99J∕kgK, slightly varying with Pr content. A careful study indicates that the Maxwell relation cannot be used in the vicinity of the Curie temperature because of the coexistence of paramagnetic and ferromagnetic phases, and the huge entropy peak is a spurious result. Similar conclusions are applicable to MnAs and Mn1−xFexAs, for which huge entropy changes have been reported. Appropriate methods for the determination of entropy change of the compound with phase separation are discussed based on the magnetic data.