Magnetic-field-induced twin boundary motion in magnetic shape-memory alloys

This paper reports direct microscopic evidence of magnetoelastic coupling b etween ferroelastic twin domains and ferromagnetic Weiss domains in magneti c shape memory alloys, using the Ni-Mn-Ga Heusler alloys. In a martensitica lly transformed Ni2 divided by xMn1-xGa single crystal, the magnetic domain s were found to be superimposed upon the martensite twin domains. Simultane ous observation of magnetic domains and twin domains as a function of appli ed magnetic field shows that concomitant with the reconfiguration of magnet ic domains in an applied magnetic field, ferroelastic twin domains also rea djust their relative volume fraction in order to accommodate to the externa lly applied magnetic field. This readjustment is shown to occur by the disp lacement of twin domain walls. As a result, the volume fraction of one set of favorably oriented twin domains (with respect to magnetoelastic energy) increase at the expense of another set of unfavorably oriented twin domains . These studies would enable subsequent quantitative correlation between th e magnetic-field-induced macroscopic strain and the microscopic self-adjust ment of twin domains.