Growth and characterization of high quality functional (Ni-Co-Fe) oxides on Ru(0001)

Resumen

The competition of charge, spin and orbital degrees of freedom in complex oxides leads to intriguing physical phenomena, including ferromagnetism, ferroelectricity or multiferroelectricity. The advances in the growth of oxides gives rise to the controlled synthesis of high-quality oxide heterostructures with monolayer-precision. Designing electronic properties in ultrathin oxide films and interfaces opens up routes to explore novel nanoelectronic functionalities for applications. Particularly, transition metal oxides (TMO) are an interesting group of materials presenting a broad structural variety due to their ability to form phases of varying metal to oxygen ratios reflecting multiple stable oxidation states of the metal ions what influence their chemical, optical, electrical and magnetic properties. A broad range of different properties of these oxides are due to the unique nature of outermost $d-$ electrons. However for the use in devices, often layers only few nm thick are required, in which interface and surface effects as well as defects, can play important roles. In fact, ultrathin spinel oxide films often exhibit disappointing properties compared to their bulk counterparts. Thus, optimized fabrication methods for ultrathin films and a detailed understanding of their magnetic and transport properties at such scale are needed. In this thesis we demonstrate a route for preparing high quality ultrathin ternary transition metal oxide films on a metallic substrate. Mixed nickel, iron and cobalt oxides have been grown on Ru(0001) by high temperature oxygen-assisted molecular beam epitaxy. The nucleation and growth process are observed in real time by means of Low-Energy Electron Microscopy (LEEM), which enables to optimize the growth parameters. A comprehensive characterization is performed combining LEEM and LEED for structural characterization and PEEM (PhotoEmission Electron Microscopy) with synchrotron radiation for chemical and magnetic analysis via X-ray Absorption Spectroscopy, X-ray Magnetic Circular Dichroism and X-ray Magnetic Linear Dichroism (XAS-PEEM, XMCD-PEEM and XMLD-PEEM, respectively). We have found that depending on the chosen conditions, either spinel ferrites or divalent monoxides with rocksalt structure can be prepared.