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

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In this project, our prime focus will be to develop and employ 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 aim at an in-depth understanding of the coupling of lattice and magnetic degrees of freedom at this transition.Identifying und understanding these couplings critically depends on an accurate treatment of finite temperatures, which is a challenge for first principles approaches, such as density functional theory applied in this project. A variety of methods that have been developed and applied over the last couple of years in our group will be used to derive temperature dependent entropies in MCE systems. However, further method development in particular towards the incorporation of magnetic fields and magnetic excitations as well as the determination of phonons in disordered systems will be necessary.Correspondingly, the investigations in this project will start with the established Heusler alloys Ni2MnGa and Ni2MnSn, for which the basics of the MCE are already known. When going to more complex systems, our theoretical research is triggered by the intension of our experimental partner project to structurally combine thin films of Ni54.5Mn20.5Ga25 and Ni47Co3.1Mn36.6Sn13.3, which show conventional and inverse MCE, respectively. We will therefore systematically investigate offstoichiometric Ni-Mn-Ga and Ni-Co-Mn-Sn Heusler alloys. A second central goal of our proposal is to study the influence of the interfaces, occuring in the experimentally investigated multilayer systems. 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.The such obtained insights will contribute to a knowledge-driven improvement of magneto-caloric materials, to be used for innovative cooling devices in future applications.