In-situ XRD for studying the effects of external fields on the structure of magnetocaloric materials

磁热材料的性质与其结构密切相关。所有尺度上的变化，从原子距离到微观结构再到宏观尺度，都会对磁热效应的大小、磁滞和材料的长期性质产生很大影响。在这个项目提案中，我将探讨如何最有效地将伴随着磁结构转变的结构变化用于磁热冷却应用。现场X射线技术将被用来了解外加磁场时晶格的变化如何有利于磁热效应的大小，同时可以消除或减少降低材料寿命的影响。在该项目中，两种已有的磁热材料(La(Fe，Si)13和NiMnGa型Heusler合金)将成为研究的基础，而从长远来看，将探索新型的富YCo5和Fe的CEFE基合金的室温磁热冷却潜力。使用这种方法，我们将能够从单层的状态及其对温度的依赖关系中推断出接近双层界面的结构状态。在最好的情况下，接近界面的晶格自由度和磁自由度的耦合将导致磁熵变的大幅增加。从长远来看，我们将尝试将这种在界面产生额外贡献的原理转移到两相块体系统中。一阶型磁热材料结构的突然变化带来了磁热性能的增加，这些属性使这些材料在实际器件中的应用具有挑战性，如在外场循环作用下的劣化和磁滞。同样，了解结构如何与磁性一起变化是改进的关键。在此背景下，我们将研究外场对磁热块体材料结构的影响。对于La(Fe，Si)13，我们将覆盖两个紧密相连的长度尺度。一方面，原位X射线衍射将探测原子距离随磁场和温度的变化，探测相变过程中微观体积的变化。另一方面，原位X射线层析成像将探索材料在现场应用时的形貌，成像在磁结构转变过程中经常出现的裂纹和气孔。这些技术结合在一起，将对外场如何作用于磁热材料提供补充的见解，为解决目前阻碍磁热材料在实践中应用的问题提供了一个独特的机会。

The properties of a magnetocaloric material depend very sensitively on its structure. Alterations at all scales, ranging from atomic distances through microstructure to macroscale, can have a large impact on the magnitude of the magnetocaloric effect, its hysteresis and the long-term properties of the material. In this project proposal, I will explore how structural changes which accompany a magnetostructural transition can be used most efficiently for magnetocaloric cooling applications. In-situ X-ray techniques will be applied to learn how changes in the lattice upon application of an external field can benefit the magnitude of the magnetocaloric effect while at the same time effects reducing the materials life span can be eliminated or lessened. In the project, two established magnetocaloric materials (La(Fe,Si)13 and NiMnGa-type Heusler alloys) will form the basis of the investigations, while in the long run, novel YCo5- and Fe-rich CeFe-based alloys will be explored for their room temperature magnetocaloric cooling potential.The structure of the Heusler alloy thin films with intrinsically different states of stress will be studied using stress and temperature-dependent XRD. Using this approach, we will be able to infer the structural state close to a bilayer interface from the state of the single layers and its dependence on temperature. In the best case, the coupling of lattice and magnetic degrees of freedom close to an interface will lead to a substantial increase of the magnetic entropy change. In the long run we will attempt to transfer this principle of generating additional contributions at interfaces to a twophase bulk system.The abrupt changes in the structure of a first-order-type magnetocaloric material brings – alongside an increased magnetocaloric performance - attributes which can make it challenging to apply these materials in actual devices, like deterioration under cyclic application of external fields and hysteresis. Again, knowing how the structure changes together with the magnetic properties is the key for improvement. In this context, we are going to study the effect of external fields upon the structure of magnetocaloric bulk materials. For La(Fe,Si)13, we will cover two length scales which are intimately connected. On one hand, in-situ X-ray diffraction will probe the atomic distances as a function of magnetic field and temperature, probing the microscopic volume changes during the transition. On the other hand, in-situ X-ray tomography will explore the morphology of the material upon field application, imaging cracks and pores which often appear during the magnetostructural transition. These techniques together will give a complementary insight into how an external field acts upon the magnetocaloric material, giving a unique opportunity to tackle the issues that currently hamper the application of magnetocaloric materials in practice.