Electronic structure of TiO2 thin films and LaAlO3-SrTiO3 heterostructures: the role of titanium 3d1 states in magnetic and transport properties

TL;DR

This thesis investigates the electronic structure and magnetic properties of TiO2 thin films and LaAlO3-SrTiO3 heterostructures, revealing the role of Ti 3d1 states and defects in magnetism and conductivity.

Contribution

It provides new insights into the origin of ferromagnetism and metallicity in Ti-based oxides through combined experimental and theoretical analysis.

Findings

Weak room-temperature ferromagnetism linked to Ti3+ states

Resonant photoemission reveals Ti3+ contribution to conductivity

Structural disorder influences interface electronic properties

Abstract

In this Thesis, a study of the electronic structure of two Ti-based oxide systems, TiO2 thin films and the ultra-thin LaAlO3-SrTiO3 (LAO-STO) heterojunctions, is given. A weak room-temperature ferromagnetism (FM) has been detected in slightly reduced TiO2 thin film and in other oxides; as these materials are insulating closed-shell systems, this phenomenon has been classified as "d0 magnetism". Since this magnetism could be related to the growth process and to the presence of defects (oxygen vacancies, VO), an analysis of Ti electronic states (especially of Ti3+ energy levels) is mandatory. The first part of this work is devoted to the magnetic characterization of a set of TiO2 and N-doped TiO2 samples, together with the analysis of Ti 3d-related states carried out with X-ray photoemission (XPS) and resonant photoemission (ResPES). The hypothesis of clustered VO as the source of FM is then discussed in the light of the experimental and theoretical results. Another interesting oxide system in which the stoichiometry of Ti ions play a fundamental role is the LAO-STO interface. In fact, while LAO and STO separately are two band insulators, the interface created by growing LAO on the top of STO (001) has found to become metallic, hosting a 2D electron gas. The second part of this Thesis is devoted to the study of conductive and insulating LAO-STO interfaces, carried out by XPS, X-ray absorption (XAS) and with ResPES. The sample stoichiometry is evaluated through a comparison with LAO and STO single crystals. A resonance enhancement of the conductive Ti states, associated to Ti3+ ions, is reported and compared to theoretical calculations. On the basis of these results, the origin of metallic states in ultra-thin LAO-STO interfaces is properly addressed. In addition, a characterization of the structural disorder at the interface is shown, carried out with angle-resolved XPS.