Magnetic domain configurations in spark-eroded ferromagnetic shape memory Ni-Mn-Ga particles

Spherical particles of the ferromagnetic shape memory material Ni51 Mn29 Ga20 obtained by spark erosion transform during cooling from the high-temperature Heusler L 21 cubic phase into tetragonal martensite. Using the Fresnel (i.e., Lorentz) imaging mode, magnetic domains with an average width of 100 nm are observed in the modulated martensitic phase in the absence of a magnetic field. The magnetization distribution within individual particles is determined using electron holography. The magnetic flux lines change direction when crossing boundaries between crystallographic twin variants. These boundaries, where the easy c -axis of the crystallographic variants rotate by 86.5°, coincide with quasi-90° magnetic domain walls, with thickness of approximately 17 nm. The magnetization saturation determined by electron holography is about 0.57 T.