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

The observation of magnetic domains on austenite Ni-Mn-Ga bulk samples has been a big challenge for many years. Using advanced Kerr microscopy, with automatic compensation of the sample motion and monochromatic LED light we were able to observe magnetic domains and follow their evolution with magnetic field on the 100 faces of an austenite bulk single crystal. After mechanical polishing variable fine stress-induced domains patterns were observed at different locations. The surface coercivity visualized by the Kerr loop was two orders higher than the bulk coercivity from magnetometry measurement. After additional electropolishing, wide 180 domains were observed with a width of about 50 micrometers and the Kerr loop coercivity decreased to the level determined from the magnetometry. Surprisingly, the magnetic domains were observed only along one of two 100 cubic axes lying in the surface plane.