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

磁场感应应变(MFIS)是基于孪晶界在磁场作用下相对容易运动和较高的磁各向异性。为了开发新的MFIS材料，分别优化已有材料的MFIS效应，理解决定这些材料中孪晶界运动的物理过程是一个挑战。孪晶界运动和一般的晶界运动一样，受有序、化学计量比偏差、调制、析出物、晶粒度、织构和磁畴结构等参数的影响。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，是因为它们表现出了迄今为止最大的微细结构，而且铸造可以很容易地产生大量材料。计划进行的调查包括对孪晶边界的结构以及上述结晶学和磁性微结构特征的详细描述。将通过常规的透射式电子显微镜、洛伦茨电子显微镜和电子全息术，在中高分辨率下以最高标准进行表征。这些问题的基础研究将全面利用单晶进行。由于微晶材料在技术上的预期应用，例如用于执行器和传感器，多晶材料似乎更合适，因此必须考虑特定的多晶方面：将分析微晶材料的颗粒尺寸和织构对微晶效应的影响。此外，还将通过大温度范围内的力学试验以及同时施加磁场来研究它们对孪晶边界运动的影响。

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.