Ab initio study on the coupling of lattice and magnetic degrees of freedom and the role of interfaces in magneto-caloric materials

The aim of this project is the development of a complete first-principles based methodology to study the magneto-caloric effect (MCE) close to the technologically and scientifically relevant regime of first order magneto-structural transitions.

Within a concerted action with the partners of our package, we mainly focus on an in-depth understanding of the coupling of lattice and magnetic degrees of freedom, which plays a critical role at the transition and for the subsequent MCE.

Ab initio calculated phase diagram of Ni2+xMn1-xGa
Schematic representation of the cycle for the magneto-caloric effect

Investigations are performed for those Heusler alloys that are employed by our experimental partners. Starting with stoichiometric compositions, a systematic change in the composition will lead towards off-stoichiometric alloys which are of prime interest for applications. Our studies will be performed in the frame work of density functional theory in combination with thermodynamic concepts, which have been developed in our group (see Fig. 1) and will be used in this project to derive temperature dependent entropies in MCE systems. Necessary method developments include the incorporation of magnetic fields and magnetic excitations as well as the determination of phonons in disordered systems.

Ni2MnGa-Ni2MnSn layered structure

A second central goal of the project is to understand the coupling of different layers of magnetocaloric materials in experimentally investigated multilayer systems. We will study the influence of the interfaces on the entropy and the resulting thermodynamic properties in such a system. For this purpose, our ab initio based investigations of vibrational and magnetic excitation mechanisms will be extended from bulk to superlattice structures containing an interface between two Heusler alloys.

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