Investigation of the crystallographic and magnetic properties of Mn1+xNi2-xGa ferromagnetic shape memory alloys

This thesis work addresses the characterisation of structural and magnetic properties of Ni2MnGa based Heusler alloys. The alloy series Mn1+xNi2-xGa has been investigated experimentally by magnetization measurements, X-ray and neutron scattering. The systematics is established as a function of Mn content of the magnetic and structural properties. Of particular interest is the development of the martensitic phase as a function of Mn content, its hysteresis behaviour and the influence of chemical order as established by a suitable heat treatment. A comprehensive experimental characterisation is given of the alloys series Mn1+xNi2-xGa for 0≤x≤1. The experimental investigation is complemented by modelling of domains in pure Ni2MnGa. Based on a detailed experimental determination of transformation matrices for the austenite martensite transformation a characterisation is given of domain walls and their crystallographic orientation. The descriptions and models which exist in the literature are extended to enable the modelling of individual domain walls within a single crystal of Ni2MnGa and its extension to several domain walls. The framework for this modelling is extended to enable the description of domain wall networks. Particular domain wall network configurations are identified and described. Within the topic of shape memory materials in general, and of the ferromagnetic shape memory Ni2MnGa based alloys in particular, this thesis work offers insight into some important aspects of their physical properties. A range of approaches is used to address some of the characterisation issues for these compounds.