Ferromagnetism and local electronic properties of rutile Ti 1-x Fe x O 2 single crystals L. Sangaletti, 1 M. C. Mozzati, 2 G. Drera, 1 P. Galinetto, 2 C. B. Azzoni, 2 A. Speghini, 3 and M. Bettinelli 3 1 Dipartimento di Matematica e Fisica, Università Cattolica, Via dei Musei 41, 25121 Brescia, Italy 2 CNISM and Dipartimento di Fisica “A. Volta” Università di Pavia, Via Bassi 6, 27100 Pavia, Italy 3 Dipartimento Scientifico e Tecnologico, Università di Verona, Strada Le Grazie 15, 37134 Verona, Italy Received 13 February 2008; revised manuscript received 4 June 2008; published 28 August 2008 An experimental evidence of the effects of iron on the magnetic and local electronic properties of TiO 2 single crystals is provided. When ferromagnetic rutile TiO 2 single crystals are doped with Fe ions, an enhance- ment of the saturation magnetic moment is observed, along with a transfer of electrons to Fe 3d and Ti 3d levels. Indeed, the analysis of core-level photoemission data shows that addition of iron has the effect i to promote electron charge transfer from O 2p band to localized Fe 3d orbitals and ii to reduce titanium ions by introducing 3d 1 electronic states in the otherwise empty 3d band of pure TiO 2 . These effects, resulting from the incorporation of both transition-metal ion dopants and oxygen vacancies, increase the local magnetic moments that can contribute to long-range ferromagnetic ordering. DOI: 10.1103/PhysRevB.78.075210 PACS numbers: 75.50.Pp, 73.20.At, 79.60.-i, 81.10.Fq I. INTRODUCTION The magnetic properties of doped rutile TiO 2 still repre- sent a challenge both from a theoretical and experimental point of view. 1 While evidences of intrinsic ferromagnetism FM in these systems has been provided by several research groups, the mechanism underlying the coupling between magnetic moments and the onset of ferromagnetism are still under debate. There is an emerging evidence that FM in these systems is a complex phenomenon, which involves the interplay between oxygen vacancies, doping impurities, and their effects on the density and localization of charge carries. The preparation of suitable samples has been mainly achieved by producing thin films with pulsed laser deposi- tion, reactive sputtering, molecular-beam epitaxy, and ion implantation. 2 Depending on the thin-film growth conditions, their structural, magnetic, and transport properties 3,4 may present different behaviors also for nominally identical sto- ichiometries and doping levels. Only recently the growth of pure and transition metal TM doped TiO 2 rutile single crys- tals, which display FM at room temperature RT, has been reported. 5 This kind of growth is carried out in near equilib- rium conditions by a final cooling of the melt, which may take several days. These conditions are quite different from the out of equilibrium conditions typical of laser ablation or RF sputtering and may offer a possible alternative route to produce homogeneous and segregation-free samples. Furthermore, recent evidences of RT FM in undoped TiO 2 samples 5,6 have posed further questions about the role of 3d element doping in these oxides. The open problem is whether doping introduces FM or it just enhances the magnetism al- ready present in the oxide host. Indeed, the mechanisms re- sponsible for ferromagnetism in diluted magnetic oxides DMO are still under debate. According to a phase diagram recently proposed by Coey et al., 7 ferromagnetism in these insulating oxides should arise from magnetic polarons above the percolation threshold. 8 In n-doped systems, these po- larons localize around an oxygen vacancy, may polarize the magnetic moment of TM ions present in doped crystals and in thin films, and finally polarons may percolate to yield magnetic ordering. Mean-field theories MFT have also been considered, 8 being static MFT the most appropriate mean-field approach for strongly insulating DMO. As a gen- eral rule, for the onset of ferromagnetism it seems important to introduce defects in the lattice that yield, e.g., donor levels in n-doped oxides. Defects can already appear in the pure TiO 2 lattice but they can be increased by doping the lattice with nontetravalent TM ions. Furthermore, hybridization and charge transfer CT from oxygen 2p band to nominally empty d states of TiO 2 or to partially occupied 3d states of TM dopant are expected to play an important role to enhance the Curie temperature. While ferromagnetism develops as a long-range order, the mechanisms underlying the magnetic coupling have to be searched on a more local scale, i.e., within the length scale of a few unit cells, as was recently proposed in theoretical stud- ies based on ab initio electronic structure calculations. 9 Pho- toemission spectra of open shell TM impurities are known to be significantly influenced by intra-atomic multiplet splitting and also by interatomic CT effects, which in most cases in- volve a cluster of atoms composed of the TM atom and the nearest-neighbor ligand anions. 10 Therefore most of the analysis of core-level x-ray photoemission spectroscopy XPS spectra of open shell 3d TM oxides is carried out on a local scale represented by a distorted TMO 6 octahedral cluster, being the octahedral coordination of TM cations with oxygen anions quite often found in transition-metal monox- ides and dioxides e.g., MnO, FeO, CoO, NiO, and TiO 2 . In this context, information on CT effects can be gained in the frame of configuration interaction CI impurity cluster model analysis of the experimental spectra, as recently pro- posed for Co-doped rutile TiO 2 thin films. 11 Likewise, the ionization state of the TM or Ti ions can be properly dis- cussed upon an analysis of the core-level photoemission spectra. The aim of the present study is to investigate and discuss the effects of iron doping on the magnetic and local elec- tronic properties of FM rutile TiO 2 single crystals. While magnetization measurements show an enhancement of satu- ration magnetic moment when pure TiO 2 single crystals are doped with Fe ions, the analysis of core-level photoemission PHYSICAL REVIEW B 78, 075210 2008 1098-0121/2008/787/0752106 ©2008 The American Physical Society 075210-1