Scanning tunneling microscopy and spectroscopy study of the oxygen-induced effects on the Fe(001) surface and 3d metals/Fe(001) interface

The uninterrupted efforts devoted over the last decades to continuously reduce the dimensions of devices have led to Nanotechnology, a discipline that aims to create and use structures, devices and systems in the size range of about 0.1 - 100 nm. The two basic approaches to creating surface patterns and devices on sub- strates in a controlled and repeatable manner are the top-down and bottom- up techniques. The top-down methods retain the lithographic design motifs by making use of tools such as nanolithographic techniques and scanning probe micro- scopies, imposing a structure or pattern on the substrate being processed. In contrast, bottom-up methods aim to guide the self-assembly of atomic and molecular constituents into organized surface structures through processes inherent in the manipulated system, with the aim to "engineer" electronic and magnetic materials on the atomic scale. Common to all bottom-up strategies for the fabrication of nanostructures at surfaces is that they are essentially based on growth phenomena. In this respect understanding the processes of nucleation and growth on a substrate surface is of importance for growing high-quality nanostructures. By tuning parameters like the substrate temperature and deposition rate one can change the kinetic conditions of growth and obtain structures that does not represent necessarily the minimum energy configuration of the sys- tem. The stabilization of this metastable structures is of particular interest in the case of magnetic materials, because they can have different magnetic properties depending on the morphological configuration. Many of these features are related to the different mechanisms of strain relaxation in heteroepitaxial systems as well as to specific characteristics of atomic diffusion, such as the presence of Ehrlich - Schwoebel barriers hindering step crossings. Some special growth techniques (use of surfactants and codeposition) can also be useful to overcome the elements natural limitations and produce accurately controlled, custom-designed epitaxial samples. In this frame the preadsorption on the substrate of foreign atoms has been demonstrated to be effective in changing both the kinetics of growth and the minimum energy configuration of the system. In particular the presence of an ordered overlayer of oxygen on the Fe(001) surface affects the magnetic and electronic surface properties. Furthermore, it is well known that oxygen acts also as surfactant, promoting layer by layer growth whereas the growth without oxygen is characterized by a roughness increasing with the coverage. In this thesis the atomistic processes involved in the oxygen mediated homoepitaxial and eteroepitaxial growth on Fe(001) surface will be discussed. In particular the oxygen induced effects on the Fe(001) surface and on the interface between the Fe(001) and two 3d metals, namely Cr and Ni are investigated. In chapter 1 the properties of the oxygen passivated surface, the so called Fe(001)-p(1x1)O will be exposed. Chapter 2 is about the oxygen mediated growth of nickel and chromium ultrathin films on Fe(001). In chapter 3 two different cobalt oxide nanostructure, the cobalt oxide thin films on Fe(001)-p(1x1)O and cobalt oxide nanowire grown on Pd(1 1 19) vicinal surface are presented. The data about cobalt oxide nanowire grown on Pd(1 1 19) have been taken in collaboration with the group of the professor F.Netzer, during the period of six months I have spent at the Karl Franzens University of Graz.

Gli sforzi compiuti negli ultimi decenni per ridurre le dimensioni dei dispositivi hanno portato alla nascita della nanotecnologia. I due approcci per creare strutture di dimensioni nanometriche sono le tecniche top-down e bottom-up. Nel primo caso si usano tecniche litografiche, mentre nel secondo si sfruttano i meccanismi atomici per ottenere strutture autoassemblate. Variando parametri come la temperatura di crescita e la velocità di deposizione del materiale si possono ottenere strutture metastabili con le caratteristiche desiderate. In questa tesi sono discussi i processi atomici presenti nella crescita omoepitasiale e eteroepitassiale sulla superficie Fe(001) passivata con l'ossigeno. In particolare le interfaccie studiate sono quelle tra cromo e ferro e tra nichel e ferro. Nel primo capitolo sono esposti gli effetti indotti dall'ossigeno sulle proprietà elettroniche e strutturale della superficie Fe(001). Nel secondo capitolo sono studiati gli effetti indotti dall'ossigeno sulla crescita di film sottili di cromo e nichel sulla superficie Fe(001). Il terzo capitolo riguarda due differenti nanostrutture di ossidi, film sottili di ossido di cobalto cresciuti sulla superfice del ferro e fili di ossido di cobalto autoassemblati sulla superficie vicinale del palladio. I dati sui nanofili di ossido di cobalto sono stati acquisiti in collaborazione con il gruppo del professor Netzer durante il periodo di sei mesi che ho trascorso alla Karl Franzens University of Graz.