TEM Studies of Epitaxial Iron Oxide Thin Films

Abstract

The discovery of the exotic magnetic properties of epsilon ferrite (ε-Fe2O3) has motivated several studies regarding the synthesis and device integration of this nanomaterial. However, due to the metastable nature of this phase, its application has been limited to nanoparticles or nanorods where the size confinement helped its stabilization. Recently the deposition as thin films was achieved on complex oxide substrates. However, in order to favor its integration into electronic devices, high-quality thin films are necessary. Distinct challenges are present in the growth of epitaxial films, the search for single domain and defect-free thin films with a unique magnetic easy axis is desired to obtain a good microstructure and achieve the exotic reported magnetic properties for the other nanocompounds. In this work, a series of Scanning Transmission Electron Microscopy (STEM) investigations are presented with the purpose to gain insight about the microstructure and chemical state of epsilon ferrite thin films grown on vicinal strontium titanite (STO) and yttrium-stabilized zirconia (YSZ). Detailed analysis of high-resolution STEM images and electron diffraction patterns highlighted the, growth of misoriented in-plane domains with its easy magnetic axis pointing in opposite directions and edge dislocations in the grain boundaries. In specific cases, different mechanisms of partial strain relaxation are observed. For the samples grown on STO substrates, a set of misfit dislocations at the interface are visible while in the samples grown on YSZ the growth of a secondary phase at the film/substrate interface was identified. Electron Energy-Loss Spectroscopy (EELS) was used for chemical characterization of the sample helping the identification of the localization of a secondary phase growing at the interface, identified as magnetite. Further EELS analysis showed changes in the bonding conditions of the iron cations allowing to stablish a correlation between the strain effects (such as plastic deformations, the presence of dislocations, lattice distortion and secondary phases) and the magnetic moment of the sample. The presence of these types of defects and their relationship with the reduced magnetic response of the sample and is discussed in the following thesis.