Visualization of Magnetic Domains and Magnetization Vectors in Magnetic Shape Memory Alloys Under Magneto-Mechanical Loading

Ni2MnGa magnetic shape memory alloys have a microstructure consisting of marten- site variants, and magnetic domains exist in each martensite variant. In the absence of a magnetic �eld, the magnetic domains are equally distributed so that the net magnetization of the material is zero. Application of a magnetic �eld or mechanical stress can rearrange the magnetic domains and martensite variants. This thesis is focused on understanding the effects that magnetic �field, compressive stress and magneto-mechanical loading have on the magnetic domains, by measuring and quantifying magnetic domain volume fraction and magnetization vector rotation, using direct imaging. A loading device was designed and built in-house, to allow for the application and measurement of magnetic fi�eld and compressive stress, either individually or simultaneously. A magneto-optical indicator fi�lm, in conjunction with polarization microscopy, was used to visualize the evolution of the magnetic domains and magnetization vector rotation of a Ni2MnGa sample, with the help of the loading device. The magnetic domain patterns exhibited by different faces of the MSMA sample, in the absence of field or stress, were studied �first, followed by different loading cases, namely varying magnetic fi�eld at constant strain, and varying compressive stress at a constant magnetic fi�eld. Our experiments revealed that the applied magnetic fi�eld causes change in domain volume fraction at different rates in each variant, and that domain wall motion is not always fully reversible in the presence of compressive stress. Magnetization vector rotation, however, was found to be reversible for all loading cases tested. Preliminary magneto-mechanical tests also showed the presence of the third variant and formation of perpendicular twin boundary, which appears to diminish with increase in magneto-mechanical loading cycles