Basic research of the influence of real structure on the magnetic field induced strain in NiMnGa alloys - Structure and properties of twin boundaries in NiMnGa alloys

磁致应变（MFIS）是基于磁场作用下孪晶界面的非线性运动和高磁各向异性。为了开发新的MFIS材料，分别优化已知材料的MFIS效应，理解这些材料中决定孪晶界运动的物理过程是一个挑战。孪晶界运动，像晶界运动一般，是受参数，如秩序，偏离化学计量，调制，沉淀物，晶粒尺寸，纹理和域结构。因此，本项目的任务是系统地研究这些参数对NiMnGa合金中MFIS效应的影响。之所以选择这些合金，是因为它们显示出迄今为止最大的MFIS，并且铸造可以很容易地以散装形式生产材料。计划中的调查包括对孪晶界结构以及上述晶体学和磁性微观结构特征的详细表征。表征将通过常规透射电子显微镜（TEM）、洛伦兹电子显微镜和电子全息术（中分辨率和高分辨率）以最高标准进行。关于这些课题的基础研究将使用单晶体进行综合。由于MFIS在技术上的预期应用，例如致动器和传感器，多晶材料似乎更合适，因此必须考虑特定的多晶方面：将分析晶粒尺寸和纹理对MFIS效应的修改。此外，它们对孪晶界运动的影响将在大的温度范围内通过力学试验进行研究，同时施加磁场。

The magnetic field induced strain (MFIS) is based on the comparably easy motion of twin boundaries under magnetic fields, and a high magnetic anisotropy. To develop new MFIS materials, respectively optimize the MFIS effect of known materials, it is a challenge to understand the physical processes determining the motion of twin boundaries in these materials. Twin boundary motion, like grain boundary motion in general, is affected by parameters such as order, deviation from stoichiometry, modulation, precipitates, grain size, texture and domain structure. Therefore, it is the task of the project to systematically investigate the effect of these parameters on the MFIS effect in NiMnGa alloys. These alloys are chosen, because they are showing the biggest MFIS so far, and casting can quite easily produce materials in bulk form. The investigations planned comprise a detailed characterization of the structure of the twin boundary as well as of the crystallographic and magnetic microstructural features mentioned above. Characterization will be done at highest standards by means of conventional transmission electron microscopy (TEM), Lorentz electron microscopy, and electron holography, both at medium and high resolution. Basic research on these issues will comprehensively be performed using single crystals. Since for the intended application of MFIS in technology, e.g. for actuators and sensors, polycrystalline materials seem to be more appropriate, the specific polycrystalline aspects have to be considered: The modifications of the MFIS effect by grain size and texture will be analyzed. Furthermore their influence on twin boundary motion will be studied by mechanical tests in a large temperature range, also with simultaneously applying a magnetic field.