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

磁热效应的起源，即由磁场引起的适当材料的可逆温度变化，应在微观尺度上揭示，将通常采用的宏观热力学测量和理论研究的结果联系起来。我们将重点研究三元和四元Heusler合金，如Ni2MnSn，Ni2MnInxCo1-x，但也关注La(Fe1-xSix)13基化合物，具有六方Fe2P型结构的Mn-Fe-P-Si化合物和表现出巨大磁热效应的Mn3GaC系统。在这种情况下，磁热效应伴随着结构相变，所有的系统都可能表现出不同的温度依赖关系，不同的元素沿铁性相变的磁性和结构性质。X射线吸收光谱提供了独特的可能性来测量自旋和轨道磁矩、电子结构和晶体结构的变化作为静态属性以及动态属性，如晶格振动-元素。此外，穆斯堡尔谱将用于含铁体系，以表征局部结构和磁性。利用这种方法，我们计划研究(自旋分辨的)电子结构与磁热效应的关系、晶格振动对总熵的贡献、自旋磁矩和轨道磁矩的温度依赖性，并通过比较我们的温度相关光谱和理论结果来验证与相变有序性相关的微观自由度。

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.