The colossal magnetocaloric effect in Mn1-xFexAs: What are we really measuring?

Recently, a so-called colossal magnetocaloric effect (MCE) was reported in Mn1-xFexAs [A. de Campos , Nat. Mater. 5, 802 (2006)]. However, the value of Delta S that was determined appears markedly overestimated since it results from the inadequate use of the Maxwell relation. Here, we report on recent measurements of Delta S in Mn1-xFexAs from which a correct MCE value is deduced by using the Clausius-Clapeyron equation. This result is asserted by careful use of the Maxwell relation.