Direct observation of magnetic anisotropy-induced magnetic domain structure evolutions using Lorentz transmission electron microscopy

Abstract

Recent advances in memory device technology have achieved nanometer-scaled magnetoresistive random access memory (MRAM) development for breaking down the conventional charge based memory technology. Spin-based MRAM device ensures many advantages such as fast operation, low power consumption, and non-volatile storage that many scientists have tried to map new type of MRAM device out for enhancing degree of integration and commercialization. In this procedure, control of magnetic anisotropy in ferromagnetic layer is key technique.

However, understanding of magnetic anisotropy induced magnetic domain structure evolution in nanometer-scaled ferromagnetic layer is difficult due to extremely small sizes. Therefore, microscopy based technique is inevitable to look into the internal phenomena in nano-scaled magnetism. Among many types of microscopy for imaging magnetic structure, Lorentz transmission electron microscopy (Loretnz TEM) is most powerful microscopy because of both provision in structural information and magnetic domain information. Lorentz TEM also provides systematic study of dynamical magnetic phenomena in nano-scale under mechanical strain, thermal environment, etc.

In this thesis, magnetic anisotropy effects on magnetic domain structure evolution was sequentially studied by Lorentz TEM. First, Fe-based bulk amorphous ferromagnetic material was analyzed due to easiness of interpretation. Lack of magnetocrystalline anisotropy in bulk amorphous ferromagnet enables similar magnetic anisotropy effect with nano-scaled material. Secondly, self-assembled $BiFeO_3$-$CoFe_2O_4$ thin film system showing distinctive geometrical shape in ferrimagnetic $CoFe_2O_4$ islands was studied. Both unusual shape of pillars and expected complicated strain state in $CoFe_2O_4$ islands enabled systematic study in shape or strain anisotropy induced magnetic domain structure evolution in nano-scale. This study will contribute the fundamental study of magnetic phenomena in nano-scale and also give helpful information in real device construction.