Magnetic Characterization of Ni-Mn-Ga in the Martensite and Austenite Phase

Ni-Mn-Ga is a magnetic shape memory alloy (MSMA) that deforms via twin boundary motion. The magneto-mechanical properties depend strongly on the twin-microstructure. It has been shown that the combination of atomic force microscopy (AFM) for analyzing the surface relief caused by twinning and magnetic force microscopy (MFM) for identifying the direction of easy magnetization (which coincides with the crystallographic c direction) provides a non-destructive characterization of the twin-microstructure in the martensite phase. The magnetic characterization of austenitic Ni-Mn-Ga has proven difficult due to the weak anisotropy of the cubic austenite phase, which causes the magnetic moments to align parallel to the surface. This study aims to characterize the magnetic domain structure of Ni-Mn-Ga in the austenite phase using two MFM approaches: standard MFM on a single crystal and MFM on a single crystal that is subjected to a magnetic bias field. While preliminary results with standard MFM are not satisfactory, first results obtained with MFM and a bias magnetic field show a magnetic signature, which presumably is due to the austenite phase