Domain structure investigations in multiferroic Heusler ferromagnetic shape memory alloys

Multiferroic ferromagnetic shape memory alloys form an interesting subset of functional materials that exhibit two ferroic properties, ferromagnetism and ferroelasticity, in the same phase. They exhibit a spontaneous magnetization as a result of a ferromagnetic transformation (paramagnetic → ferromagnetic) and a spontaneous deformation as a result of a ferroelastic martensitic transformation (austenite → martensite), both of which can be reoriented by their respective magnetic and stress fields. The resulting high strains are a consequence of mobile magnetic boundaries and structural boundaries associated with the ferroic transformations. While these materials are promising for high strain applications, a microscopic study of their magnetic and structural boundaries is still lacking.; This thesis focuses on the interplay between the microstructure and the magnetic structure in Heusler ferromagnetic shape memory alloys (FSMA), especially Ni2MnGa, the alloy that has gained significant scientific and technological interest. The FSMA Ni2MnGa undergoes an atomic ordering transformation to a Heusler structure prior to the ferroic transformations. A systematic investigation of the atomic ordering defects (antiphase boundaries) was performed, which was then followed by the study of their interaction with the ensuing magnetic domain walls and structural twin boundaries. Transmission electron microscopy was used as the main experimental technique to characterize the antiphase boundaries, magnetic domain walls and twin boundaries.; The experimental work in this thesis clearly demonstrates a strong interaction between the microstructural antiphase boundaries and the magnetic domains of the atomically ordered Heusler phase (austenite). The change in antiphase boundary density upon high temperature annealing was correlated to the change in magnetic properties (coercivity) of the alloy. The antiphase boundaries act as strong pinning sites for the magnetic domain walls in both austenite and martensite.; The evolution of the different structures observed by in-situ dynamic experiments throws light on the effect of antiphase boundaries on the ferroic transformations. The magnetic domain structure of the Heusler phase exhibits complete domain memory across the magnetic and structural martensitic transformation, a phenomenon hitherto unknown. The ability to tailor the different structures by heat treatment provides us a methodology to tune the alloy properties.