Characterizing Twin Structure and Magnetic Domain Structure of Ni-Mn-Ga Through Atomic Force Microscopy

Ni-Mn-Ga is a ferromagnetic shape memory alloy that deforms by twin boundary motion. The magneto-mechanical properties depend strongly on the twin microstructure. A thermomechanical treatment was applied to a Ni-Mn-Ga single crystal with coexisting 10M and 14M martensite structures to create twin boundaries and align the short crystallographic c direction preferentially perpendicular to the surface. The resulting twin structure was characterized using atomic force microscopy (AFM) to obtain the surface relief caused by twinning. Magnetic force microscopy (MFM) was used to find the direction of easy magnetization (which coincides with the crystallographic c direction) of each twin. Among 18 possible twinning histories, ac twinning in 10M martensite was identified as the unique solution. The combination of AFM and MFM after thermomechanical treatment provides a non-destructive characterization of the twin microstructure including the identification of the crystallographic orientation of each twin and of each twin boundary plane