Imaging of Domains and Vortices in Multifunctional Materials

Abstract

In this chapter, we describe a series of observations of magnetic domain wall configurations in a Ni2-Mn-Ga--type ferromagnetic shape memory alloy. First we introduce the technique of phase reconstructed Lorentz transmission electron microscopy, both the classical approach and the quantum mechanical determination of the electron wave phase shift using the Transport-of-Intensity Equation formalism. Then we apply this technique to domain wall observations in several phases, including the austenitic state and the (modulated) martensitic state. We conclude the chapter with a preliminary analysis of a magnetization state that is best described as a nearly regular array of magnetic vortices, pinned by martensite variant boundaries (twin boundaries). The magnetization configuration is only observed when the twins are finely spaced, and is likely due to the interaction of the magnetic and strain order parameters in this system. © Springer-Verlag Berlin Heidelberg 2014.