Entropy determinations and magnetocaloric parameters in systems with first-order transitions: Study of MnAs

We present a study of the giant magnetocaloric effect in MnAs produced by a magnetostructural first-order phase transition. Results deduced from magnetization, M, and heat capacity, Cp,B(T), are compared and discussed. Some spurious effects are explained, and especially a spike in the isothermal entropy change, ΔST, occurring at TC when obtained via the Maxwell relation (∂S/∂B)T=(∂M/∂T)B. Alternative determination methods are given to circumvent this problem. The spike is explained as an artifact due to the incorrect application of the Maxwell relation to path dependent thermodynamic functions that are not state functions. The added wrong contribution to ΔST has been calculated using calorimetric data, giving a good agreement with the result from the magnetization measurements.