Magnetic domain structure across the austenite–martensite interface in Ni50Mn25Ga20Fe5 single crystalline thin foil

The magnetic domain structure (MDS) at the martensite transformation interface is relevant for the complete understanding of functionalities in magnetic shape memory alloys. We study the MDS in Ni50Mn25Ga20Fe5 single crystalline foil using Lorentz transmission electron microscopy. The transformation interface is stabilized by decreasing foil thickness at a thickness of about 40 nm. The martensite in the thicker region contains no twin domains and exhibits dense labyrinth MDS with magnetization out-of-plane. The austenite in the thinner region exhibits broad magnetic domains with magnetization in-plane. The transformation interface is about 1 μm broad, formed by an unusual nanosized laminated structure of austenite and martensite. Within the interface region, the two distinct MDSs interpenetrate each other, with a labyrinth MDS of martensite changing suddenly to broad magnetic domains of austenite. The changes from dense to broad domains do not follow the underlying phase changes exactly but occur on antiphase boundaries, revealing their role in the overall MDS formation.