| Understanding domain wall pinning centers and the resultant mobility of ferroic walls is of central importance to actuator applications of magnetic shape memory alloys (MSMAs). The main objective of this thesis is to characterize the interactions of ferroic walls under an applied magnetic field. The magnetic domain structure inside two ferromagnetic martensites with different magnetic properties has been investigated by in-situ Lorentz TEM. Field-induced changes in the magnetic domain wall structure were recorded over a field range of [−500,+300] Oe.;So far, there have been no reports about the nature of dislocations and the influence of dislocation density on the twin boundary motion in the MSMAs. The dislocations in Ni-Mn-Ga based alloys were characterized in the conventional mode TEM and their Burgers vectors were identified. Even a marginal increase in the dislocation density appears to adversely affect the mobility of twin boundaries thereby significantly reducing the magnetic field induced strain (MHS). The phenomenon of aging, i.e., twin stabilization due to repeated twin boundary movement, was studied by TEM analysis of aged samples. These studies revealed that the dislocations in aged samples form complex networks that give rise to small angle grain boundaries (SAGBs). In-situ heating and cooling studies in aged samples suggested that the twin interface mobility can be lowered because of the dislocation networks and a higher incidence of undesirable compound twins.;At lower fields, the magnetization process occurred through the movement of un-pinned 180° domain walls. Further increments in the field led to the collapse of magnetoelastic walls, i.e., magnetic domain walls pinned on twin boundaries. In the Fe68Pd30Co2 alloy, both magnetoelastic walls and magnetic domain walls show remarkable recovery on reducing the field. While the restoring force for the recovery of 180° walls was found to be of magnotostatic origin, the recovery of magnetoelastic walls occurred through the contribution from magnetoerystalline anisotropy energy. Finer details of magnetization processes such as: collapse of magnetoelastic walls, nucleation of new domains walls, evolution of lens shaped domains, magnetostatic coupling through anti-phase boundaries, have been presented in a hitherto unknown detail. |
