Kerr microscopy study of magnetic domains and their dynamics in bulk Ni–Mn–Ga austenite

The observation of magnetic domains on the surface of bulk austenite Ni–Mn–Ga has been a significant challenge for many years. Using advanced Kerr microscopy, with automatic compensation of the sample motion under monochromatic LED light, we were able to observe magnetic domains and follow their evolution with magnetic field on the {100} faces of an austenite single crystal. After mechanical polishing, the variable fine stress-induced domain patterns were observed at different locations. The Kerr loops obtained by visual integration of surface domains indicate a surface coercivity that was two orders of magnitude higher than the bulk coercivity from magnetometry measurements. After additional electropolishing, wide 180° domains could be observed with a width of about 50 μm, and the Kerr loop coercivity decreased to the level consistent with magnetometry measurements. Surprisingly, the magnetic domains were observed only along one of the two ⟨100⟩ cubic axes lying in the surface plane.