Effects of the Internal Magnetic Field on the Magneto-Mechanical Properties of Magnetic Shape Memory Alloys

Shape memory alloys are a class of functional material which recover from large strains without permanent deformation. The strain is accommodated by the displacement of twin boundaries in the martensite phase. The shape memory alloy Ni-Mn-Ga is also ferromagnetic. Ni-Mn-Ga preferentially magnetizes along a certain crystallographic axis. This direction of easy magnetization changes across twin boundaries, such that the directions in neighboring twin domains are nearly perpendicular. The interaction of magnetic moments and interfaces including the crystal surface and twin boundary interfaces has a large role in the magnetization process of the material. The goal of this study is to characterize the relative influence of twin boundaries on the magnetization of the material, and the dependence of the magnetization on the twin domain microstructure. The torque on a single crystal specimen in a homogeneous external magnetic field was characterized with experimental methods. The torque is the negative first derivative of the magnetic energy as a function of angle between the specimen and magnetic field. The torque and magnetic energy strongly depends on the twin domain microstructure. For specimen with two twin boundaries at 3% strain in an external magnetic field of 50 mT, one twin microstructure required 1.7 times more torque to rotate than another twin microstructure. At fields above 100 mT, the torque was asymmetric depending on the direction the direction the sample was rotated. Numerical micromagnetic simulations were performed to gain a qualitative understanding of the difference in magnetization and magnetic energy in different twin microstructures. At low fields, the continuity of magnetization across the twin boundary results in one twin microstructure having completely saturated twin domains, while the other microstructures contained 180° magnetic domains. At larger fields, the asymmetry in torque was due to the angle of the twin boundary with the crystal surface. Both the dependence on magnetization and torque asymmetry are due to the internal magnetic field at the twin boundary. The interaction of magnetic moments across the twin boundary drives the internal magnetic field and magnetization. The twin domain microstructure can be manipulated to drive the magnetization process in order to optimize the performance of the material in a device. The role of the internal magnetic field and specimen magnetization is discussed regarding a low power strain sensing measurement technique