Novel caloric materials by mastering hysteresis: microscopic understanding by element-specific studies

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The origin of the magnetocaloric effect, i.e. a reversible temperature change in a suitable material caused by a magnetic field, shall be revealed on a microscopic lengthscale bridging the results determined by commonly employed macroscopic thermodynamic measurements and theory investigations.We will focus on ternary and quaternary Heusler alloys like Ni2MnSn, Ni2MnInxCo1-x, but also on La(Fe1-xSix)13-based compounds, Mn-Fe-P-Si compounds with hexagonal Fe2P-type structure and Mn3GaC systems exhibiting giant magnetocaloric effects. In these cases, the magnetocaloric effect is accompanied by a structural phase transition and all the systems may exhibit different temperature dependencies of magnetic and structural properties for the different elements along the ferroic transition. The X-ray absorption spectroscopy offers the unique possibility to measure spin and orbital magnetic moments, changes in the electronic structure and crystal structure as static properties as well as dynamic properties like lattice vibrations element-specifically. In addition, the Mössbauer spectroscopy will be used for Fe-containing systems to characterize the local structural and magnetic properties. With this approach, we plan to study the relation between (spin resolved) electronic structure and the magnetocaloric effect, the contributions of the lattice vibrations to the total entropy, the temperature-dependence of spin and orbital magnetic moments, and verify the relevant microscopic degrees of freedom relevant for the order character of the transition by comparing our temperature-dependent spectroscopies to theoretical results.