This paper reports direct microscopic evidence of magnetoelastic coupling between ferroelastic twin domains and ferromagnetic Weiss domains in magnetic shape memory alloys, using the Ni-Mn-Ga Heusler alloys. In a martensitically transformed ${\mathrm{Ni}}_{2+x}{\mathrm{Mn}}_{1-x}\mathrm{Ga}$ single crystal, the magnetic domains were found to be superimposed upon the martensite twin domains. Simultaneous observation of magnetic domains and twin domains as a function of applied magnetic field shows that concomitant with the reconfiguration of magnetic domains in an applied magnetic field, ferroelastic twin domains also readjust their relative volume fraction in order to accommodate to the externally applied magnetic field. This readjustment is shown to occur by the displacement 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 the magnetic-field-induced macroscopic strain and the microscopic self-adjustment of twin domains.

• Received 11 February 2000

DOI:https://doi.org/10.1103/PhysRevB.61.R14913