Magnetic Domain Structure Across the Austenite-Martensite Interface in Ni50Mn25Ga20Fe5 Single Crystalline Thin Foil by Lorentz Microscopy

The magnetic domain structure (MDS) at martensite transformation interface is relevant for complete understanding of functionalities in magnetic shape memory alloys. We study the MDS in Ni50Mn25Ga20Fe5 single crystalline foil using Lorentz transmission electron microscopy. The austenite-martensite interface is stabilized by decreasing foil thickness at the thickness of about 40 nm. The martensite in thicker region contains no twin domains and exhibits dense labyrinth MDS with magnetization out of plane. The austenite in thinner region exhibits broad magnetic domains with magnetization in plane. Within the interface region, these two distinct MDSs interpenetrate to each other, with labyrinth MDS of martensite gradually changing to broad magnetic domains of austenite. The changes from dense to broad domains do not follow exactly the underlying phase changes but occur on antiphase boundaries, revealing their role in the overall MDS formation and potential to decrease the transformation thermal hysteresis.