Modeling specific heat and entropy change in La(Fe-Mn-Si)13-H compounds

In this paper we model the magnetocaloric effect of LaFexMnySiz-H1.65 compound (x+y+z=13), a system showing a transition temperature finely tunable around room temperature by Mn substitution. The thermodynamic model takes into account the coupling between magnetism and specific volume as introduced by Bean and Rodbell. We find a good qualitative agreement between experimental and modeled entropy change −Δs(H,T). The main result is that the magnetoelastic coupling drives the phase transition of the system, changing it from second to first order by varying a model parameter η. It is also responsible for a decrease of −Δs at the transition, due to a small lattice contribution to the entropy counteracting the effect of the magnetic one. The role of Mn is reflected exclusively in a decrease of the strength of the exchange interaction, while the value of the coefficient β, responsible for the coupling between volume and exchange energy, is independent on the Mn content and it appears to be an intrinsic property of the La(Fe-Si)13 structure.