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

Ni ₂MnGa magnetic shape memory alloys have a microstructure consisting of martensite variants, and magnetic domains exist in each martensite variant. In the absence of a magnetic field, the magnetic domains are equally distributed so that the net magnetization of the material is zero. Application of a magnetic field or mechanical stress can rearrange the martensite variants and magnetic domains. This study focuses 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. In particular, a magneto-optical indicator film in conjunction with polarization microscopy was used to visualize the evolution of the magnetic domains and magnetization vector rotation of a Ni ₂MnGa sample for different load cases, namely varying magnetic field at constant strain, and varying compressive stress at a constant magnetic field. Our experiments revealed that the applied magnetic field causes change in domain volume fraction at different rates in each variant, and that domain wall motion is not always fully reversible. Magnetization vector rotation, however, was found to be reversible for all loading cases tested.