Studies on Nickel-Manganese-Gallium Ferromagnetic Shape Memory Alloys and Its Composites

Ferromagnetic shape memory alloys are a new class of active materials which combine the properties of ferromagnetism with those of a diffusionless, reversible martensitic transformation. These materials have been the subject of recent study due to the giant magnetic field-induced strain (MFIS) effect. To fulfill the applications for both sensors and actuators, in this thesis, an extensive study was conducted on Ni-Mn-Ga single crystal and its composites.;In the Ni-Mn-Ga single crystal, its magneto-mechanical properties of Ni-Mn-Ga single crystal were characterized by measuring the magnetic field-induced strain effect in a wide variety of mechanical and magnetic loading conditions, and its mechanical properties were also measured in various magnetic bias field conditions. The damping behaviours in the single crystal were measured in three different crystallographic planes (a-b, a-c, and c-a planes). Besides, the anisotropic resistivity in the single crystal was observed experimentally in the martensitic phase, and its mechanism was explored theoretically by the combination of first-principles calculation and Boltzmann transport theory.;In Ni-Mn-Ga-based composites, First, to order to obtain a large MFIS without any mechanical loading, Ni-Mn-Ga composites of multilayered and sandwiched types were fabricated by laminating Ni-Mn-Ga single crystal plates with polyurethane (PU) polymer plates. The dc- and ac-MFISs in the composites were measured as functions of both magnetic field and mechanical load, and the results were compared with those of the single crystal. Once characterized, the experimental data was used to predict the behavior of the material in a theoretical model. Secondly, once MFIS in Ni-Mn-Ga single crystal was characterized, the MFIS and magnetoelectric (ME) effects in bilayered and sandwiched composites fabricated by laminating Ni-Mn-Ga single crystal plate(s) and piezoelectric PVDF polymer plate(s) were studied. The observed ME effect was attributed to a reversible reorientation of martensitic twin variants. Based on the combination of pseudo-piezomagnetism in Ni-Mn-Ga single crystal and piezoelectricity in PVDF polymer, the ME effect was reasonably predicted. Finally, based on the strong coupling between elastic and magnetic domains in the Ni-Mn-Ga single crystal, the effect of phase transformation on the converse magnetoelectric (CME) properties was studied in a heterostructure of Ni-Mn-Ga and PMN-PT single crystals.