Negligible magnetism in excellent structural quality Cr(x)Ti(1-x)O(2) anatase: contrast with high-T(C) ferromagnetism in structurally defective Cr(x)Ti(1-x)O(2)

Electronic band structure of iridates

In this review, an attempt has been made to compare the electronic structures of various 5d iridates (iridium oxides), with an effort to note the common features and differences. Both experimental studies, especially angle-resolved photoemission spectroscopy (ARPES) results, and first-principles band structure calculations have been discussed. This brings to focus the fact that the electronic structures and magnetic properties of the high-Z 5d transition iridates depend on the intricate interplay of strong electron correlation, strong (relativistic) spin-orbit coupling, lattice distortion, and the dimensionality of the system. For example, in the thin film limit, SrIrO₃ exhibits a metal-insulator transition that corresponds to the dimensionality crossover, with the band structure resembling that of bulk Sr₂IrO₄.

Ultrathin Chromia on a Hexagonally-Ordered d0 Ferromagnet: Evidence of Interfacial Exchange Bias at the Cr2O3/TiO2-x Interface

Heterostructures consisting of 10 Å thick chromia films and 50 Å thick titania films display significant exchange bias at and above room temperature. Chromia films ∼10 Å thick were deposited by molecular beam epitaxy (MBE) of Cr at room temperature in ultrahigh vacuum on 50 Å thick TiO_2-x(111) films (x < 0.3) also deposited epitaxially by MBE on Al₂O₃(0001). Cr deposition yields increased Ti(III) formation in the titania substrate and the formation of a Cr₂O₃ overlayer, without Cr/Ti interfacial mixing, as determined by in situ photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS). In situ low-energy electron diffraction (LEED) and XPS data indicate that the chromia overlayer is hexagonally ordered and ∼10 Å thick. Longitudinal and polar magneto-optic Kerr effect (MOKE) measurements at 285-315 K provide evidence of strong exchange bias between the boundary layer magnetization of chromia and the ferromagnetic substrate. These data demonstrate the robust room-temperature interaction of the boundary layer magnetization of a multiferroic antiferromagnet with a d⁰ ferromagnetic substrate.

Room temperature ferromagnetism in metallic Ti1-x V x O2 thin films

Transition metal doped TiO₂ diluted magnetic semiconductors have attracted considerable interest due to their room temperature ferromagnetism. However, most TiO₂ films are highly insulating, and thus the magnetic properties can not be controlled by tuning the carrier concentration. This will limit their application in controlling magnetization via electrical gating. Here, we deposit rutile Ti_1−xV_xO₂ (x = 0.03 and 0.05) films with the thickness between 30 and 245 nm by the pulsed laser deposition technique, and observe an obvious room temperature ferromagnetic behavior in all films. The high resolution X-ray photoelectron spectroscopy results indicate that V substituting Ti⁴⁺ ions in the TiO₂ lattice, with the +3 valence state having two unpaired d electrons, is responsible for the local spin. More importantly, the systemic investigations of transport properties for Ti_1−xV_xO₂ films reveal that the films are n-type and have metallic conductivity with a carrier density of about 10²⁰/cm³. Further studies suggest that the oxygen vacancies play a dual role of contributing to the metallic conductivity of the Ti_1−xV_xO₂ films, and also providing the free electrons to mediate the long-range ferromagnetic coupling between two magnetic polarons. These findings may offer promise for gate-tunable ferromagnetism in future semiconductor spintronics.

Transition metal doped TiO₂ diluted magnetic semiconductors have attracted considerable interest due to their room temperature ferromagnetism.

Understanding the effects of Cr doping in rutile TiO2 by DFT calculations and X-ray spectroscopy

The effects of Cr on local environment and electronic structure of rutile TiO₂ are studied combining theoretical and experimental approaches. Neutral and negatively charged substitutional Cr impurities Cr_Ti⁰ and Cr_Ti¹⁻ as well as Cr-oxygen vacancy complex 2Cr_Ti + V_O are studied by the density functional theory (DFT) within the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) functional. Experimental results based on X-Ray absorption spectroscopy (XAS) and X-Ray photoelectron spectroscopy (XPS) performed on Cr doped TiO₂ at the Synchrotron facility were compared to the theoretical results. It is shown that the electrons of the oxygen vacancy tend to be localized at the t_2g states of the Cr ions in order to reach the stable oxidation state of Cr³⁺. Effects of Cr on crystal field (CF) and structural distortions in the rutile TiO₂ cell were analyzed by the DFT calculations and XAS spectra revealing that the CF and tetragonal distortions in TiO₂ are very sensitive to the concentration of Cr.

Tuning Band Alignment Using Interface Dipoles at the Pt/Anatase TiO₂ Interface

The Schottky barrier heights at the Pt/TiO2 (001) junctions are modulated over 0.8 eV by inserting <1 nm of LaAlO3. The large electric field in the LaAlO3 is stabilized by preserving the continuity of in-plane lattice symmetry at the oxide interface. These results greatly expand the application of dipole engineering to versatile polycrystalline metal/binary oxide functional interfaces.

Investigating the development of spurious magnetism in single crystalline BaTi₀.₉₅Fe₀.₀₅O₃-δ with high δ by local structural probes

BaTi₀.₉₅Fe₀.₀₅O₃-δ has been suggested to be an intrinsic dilute magnetic oxide with a clear dependence of magnetism on oxygen vacancy concentration. However, it has also been shown that the dopant Fe ions distribute themselves rather inhomogeneously within the lattice, though without disrupting the crystal phase of the parent BaTiO₃. With the help of x-ray absorption spectroscopy (XAS), here we find that the incorporation of a larger amount of anion vacancy pushes this inhomogeneity to the extreme, leading to the precipitation of Fe metal clusters. It is also observed that the residual solid, without the Fe-metal cluster, undergoes massive structural and compositional reorganization.

Origins of electronic band gap reduction in Cr/N codoped TiO2

Recent studies indicated that noncompensated cation-anion codoping of wide-band-gap oxide semiconductors such as anatase TiO2 significantly reduces the optical band gap and thus strongly enhances the absorption of visible light [W. Zhu et al., Phys. Rev. Lett. 103, 226401 (2009)]. We used soft x-ray spectroscopy to fully determine the location and nature of the impurity levels responsible for the extraordinarily large (∼1 eV) band gap reduction of noncompensated codoped rutile TiO2. It is shown that Cr/N codoping strongly enhances the substitutional N content, compared to single element doping. The band gap reduction is due to the formation of Cr 3d3 levels in the lower half of the gap while the conduction band minimum is comprised of localized Cr 3d and delocalized N 2p states. Band gap reduction and carrier delocalization are critical elements for efficient light-to-current conversion in oxide semiconductors. These findings thus raise the prospect of using codoped oxide semiconductors with specifically engineered electronic properties in a variety of photovoltaic and photocatalytic applications.

Microscopic distribution of metal dopants and anion vacancies in Fe-doped BaTiO(3-δ) single crystals

A detailed microscopic structural study on two single crystalline dilute magnetic oxides, BaTi0.95Fe0.05O(3-δ) with and without perceptible δ, has been carried out. Although it has been reported earlier that varying δ significantly affects high temperature ferromagnetism, the real distribution/redistribution of vacancies and dopant Fe ions inside the 6H hexagonal structure was never probed. This study reveals that oxygen vacancies reduce the dopant Fe(3+) ions to Fe(2+) and mostly accumulate around these Fe(2+) ions. Another distinct trend is the tendency of the dopant Fe ions to get closer instead of being distributed randomly, thereby creating Fe2(2+)O(9-δ') like dimers within the 6H hexagonal unit cell. This experimental observation definitively confirms previous hypotheses based on theoretical models.

Origin of magnetism and quasiparticles properties in Cr-doped TiO2

Combining the local spin density approximation (LSDA)+U and an analysis of superexchange interactions beyond density functional theory, we describe the magnetic ground state of Cr-doped TiO2, an intensively studied and debated dilute magnetic oxide. In parallel, we correct our LSDA+U (+ superexchange) ground state through GW corrections (GW@LSDA+U) that reproduce the position of the impurity states and the band gaps in satisfying agreement with experiments. Because of the different topological coordinations of Cr-Cr bonds in the ground states of rutile and anatase, superexchange interactions induce either ferromagnetic or antiferromagnetic couplings of Cr ions. In Cr-doped anatase, this interaction leads to a new mechanism which stabilizes a (nonrobust) ferromagnetic ground state, in keeping with experimental evidence, without the need to invoke F-center exchange. The interplay between structural defects and vacancies in contributing to the superexchange is also unveiled.

Cationic-vacancy-induced room-temperature ferromagnetism in transparent, conducting anatase Ti1-xTaxO2 (x~0.05) thin films

We report room-temperature ferromagnetism (FM) in highly conducting, transparent anatase Ti(1-x)Ta(x)O(2) (x∼0.05) thin films grown by pulsed laser deposition on LaAlO(3) substrates. Rutherford backscattering spectrometry (RBS), X-ray diffraction, proton-induced X-ray emission, X-ray absorption spectroscopy (XAS) and time-of-flight secondary-ion mass spectrometry indicated negligible magnetic contaminants in the films. The presence of FM with concomitant large carrier densities was determined by a combination of superconducting quantum interference device magnetometry, electrical transport measurements, soft X-ray magnetic circular dichroism (SXMCD), XAS and optical magnetic circular dichroism, and was supported by first-principles calculations. SXMCD and XAS measurements revealed a 90 per cent contribution to FM from the Ti ions, and a 10 per cent contribution from the O ions. RBS/channelling measurements show complete Ta substitution in the Ti sites, though carrier activation was only 50 per cent at 5 per cent Ta concentration, implying compensation by cationic defects. The role of the Ti vacancy (V(Ti)) and Ti(3+) was studied via XAS and X-ray photoemission spectroscopy, respectively. It was found that, in films with strong FM, the V(Ti) signal was strong while the Ti(3+) signal was absent. We propose (in the absence of any obvious exchange mechanisms) that the localized magnetic moments, V(Ti) sites, are ferromagnetically ordered by itinerant carriers. Cationic-defect-induced magnetism is an alternative route to FM in wide-band-gap semiconducting oxides without any magnetic elements.

Defect-driven magnetism in luminescent n/p-type pristine and Gd-substituted SnO2 nanocrystalline thin films

The effects of rare-earth-element Gd doping on the intrinsic magnetic ordering, photoluminescence, and electrical-conducting properties of the pristine SnO(2) nanocrystalline thin films fabricated by radio-frequency (RF) sputtering are investigated. The pristine SnO(2) thin film exhibits significant ferromagnetism while Gd doping results in an absence of intrinsic ferromagnetism. The presence of large amounts of singly ionized oxygen vacancies (V(O)(+)) is traced by photoluminescence spectroscopic analysis and they are found to be responsible for the observed ferromagnetism in pristine SnO(2) thin films. A significant reduction of oxygen vacancies is observed after Gd doping, and that might be insufficient to mediate long-range ferromagnetic ordering between V(O)(+) defects in a Gd-doped SnO(2) system. Although the associated magnetic moment is increased by 1 order of magnitude, because of the insertion of Gd(3+) ions, which have localized f-shell paramagnetic moment, there is no intrinsic FM ordering. Hall measurement reveals that the pure SnO(2) exhibits n-type behavior whereas Gd-doped SnO(2) films show the p-type conductivity with higher resistivity. The studies demonstrate that only structural defects such as V(O)(+) defects, not magnetic ions such as Gd(3+), are responsible for inducing ferromagnetism in SnO(2) thin films.

Dilute doping, defects, and ferromagnetism in metal oxide systems

Over the past decade intensive research efforts have been carried out by researchers around the globe on exploring the effects of dilute doping of magnetic impurities on the physical properties of functional non-magnetic metal oxides such as TiO(2) and ZnO. This effort is aimed at inducing spin functionality (magnetism, spin polarization) and thereby novel magneto-transport and magneto-optic effects in such oxides. After an early excitement and in spite of some very promising results reported in the literature, this field of diluted magnetic semiconducting oxides (DMSO) has continued to be dogged by concerns regarding uniformity of dopant incorporation, the possibilities of secondary ferromagnetic phases, and contamination issues. The rather sensitive dependence of magnetism of the DMSO systems on growth methods and conditions has led to interesting questions regarding the specific role played by defects in the attendant phenomena. Indeed, it has also led to the rapid re-emergence of the field of defect ferromagnetism. Many theoretical studies have contributed to the analysis of diverse experimental observations in this field and in some cases to the predictions of new systems and scenarios. In this review an attempt is made to capture the scope and spirit of this effort highlighting the successes, concerns, and questions.

Size-driven magnetic transitions in monodisperse MnO nanocrystals

We report the observation of weak ferromagnetism up to T(C) approximately 250 K and a spin-glass-like behavior at temperatures below T(SG) approximately 30 K in nanoscale MnO particles. T(SG) is considerably lower and T(C) is much higher than the Néel temperature (T(N)=122 K) of bulk MnO. While the dominant low temperature behavior (below 30 K) may be attributed to the effects studied in this system before, such as uncompensated surface spins in antiferromagnetic particles, no manganese oxides have been observed with the Curie temperature as high as 250 K. We explain the magnetic ordering below T(C) as due to indirect exchange of the type observed in dilute magnetic oxides, which can be mediated by crystal defects andor surfaces.

Synthesis and surface control of colloidal Cr3+-doped SnO2 transparent magnetic semiconductor nanocrystals

The synthesis of colloidal Cr(3+)-doped SnO(2) nanocrystals prepared under mild conditions via a hydrolysis method is described. We show by means of nanocrystal surface ligand exchange that even under mild reaction conditions a significant fraction of the dopant ions reside on the nanocrystal surfaces. Two different approaches aimed at achieving internal dopant incorporation-surface-bound dopant complexation and isocrystalline shell growth-are described and compared. While free-standing nanocrystals are paramagnetic, the films prepared from the same nanocrystals exhibit ferromagnetic ordering at room temperature. The measured magnetization is associated with structural defects formed at the interfaces of nanocrystals in their films, and discussed in terms of the defect-related itinerant-electron-mediated mechanism. The observed ferromagnetism is compared to ferromagnetism in Cr(3+)-doped In(2)O(3) nanocrystalline films. These results demonstrate the possibility of controlling surface structure and composition of doped oxide nanocrystals using different approaches. Furthermore, this work emphasizes the importance of surface structure and composition in tailoring properties of doped multifunctional transparent conducting oxide nanostructures.

Magnetism of nanocrystalline (La(0.5)Sr(0.5))TiO3 doped with 3d cations

Nanocrystallites of nominal composition (La(0.5)Sr(0.5))TiO(3) 12-15 nm in diameter exhibit a diamagnetic susceptibility that is greater by a factor of three than that of bulk ceramic material, due to a much-reduced Pauli paramagnetic contribution associated with oxidation (cation deficiency) of the material. Doping the nanocrystallites with 1.5 or 2.0% of every transition metal from V-Ni adds a Curie term to the susceptibility. Exchange coupling between these paramagnetic ions is very weak. In the cases of Fe, Co and Ni there is an additional hysteretic ferromagnetic magnetization, with a moment equivalent to a few tenths of a Bohr magneton per dopant atom, which is attributed to a secondary ferromagnetic impurity phase. Mössbauer analysis of samples prepared with (57)Fe reveals the presence of some metallic iron. Metallic nickel is detected by x-ray diffraction, but no direct evidence of metallic cobalt was found in the ferromagnetic Co-doped material. The possibility of high temperature defect-related ferromagnetism in a metallic oxide is discussed.

Epitaxial growth and properties of doped transition metal and complex oxide films

The detailed science and technology of crystalline oxide film growth using vacuum methods is reviewed and discussed with an eye toward gaining fundamental insights into the relationships between growth process and parameters, film and interface structure and composition, and electronic, magnetic and photochemical properties. The topic is approached first from a comparative point of view based on the most widely used growth methods, and then on the basis of specific material systems that have generated very high levels of interest. Emphasis is placed on the wide diversity of structural, electronic, optical and magnetic properties exhibited by oxides, and the fascinating results that this diversity of properties can produce when combined with the degrees of freedom afforded by heteroepitaxy.

Molecular beam epitaxial growth of doped oxide semiconductors

Molecular beam epitaxy coupled with the use of activated oxygen is shown to be a powerful tool for the growth of well-defined, structurally excellent oxide semiconductor films. The basics of the methodology are discussed. Several case studies are presented to illustrate some of the physical phenomena that can be investigated; these include Cr- and Co-doped TiO(2) anatase, Ti-doped α-Fe(2)O(3) hematite, and N-doped TiO(2) rutile.

Absence of intrinsic ferromagnetic interactions of isolated and paired Co dopant atoms in Zn1-xCoxO with high structural perfection

We report element specific structural and magnetic investigations on Zn(1-x)Co(x)O epitaxial films using synchrotron radiation. Co dopants exclusively occupy Zn sites as revealed by x-ray linear dichroism having an unprecedented degree of structural perfection. Comparative magnetic field dependent measurements by x-ray magnetic circular dichroism and conventional magnetometry consistently show purely paramagnetic behavior for isolated Co dopant atoms with a magnetic moment of 4.8 (mu B). However, the total magnetization is reduced by approximately 30%, demonstrating that Co-O-Co pairs are antiferromagnetically coupled. We find no sign of intrinsic ferromagnetic interactions for isolated or paired Co dopant atoms in Co:ZnO films.

Evidence of structural defect enhanced room-temperature ferromagnetism in Co-doped ZnO

We report experimentally that room-temperature ferromagnetism of Co-doped ZnO is strongly correlated with structural defects, and that carriers involved in carrier-mediated exchange are by nature by-products of the creation of the defects. On the other hand, the carrier concentration has pronounced influences on the absorption edge in optical transmission spectra of epitaxial Co:ZnO films, in which both a blue-shift and a red-shift are observed. Furthermore, high-temperature annealing results in evolution of the local Co structure from Co(2+) replacing Zn(2+) to a Co(3)O(4)-based phase, which most likely accounts for the transition from ferromagnetism to paramagnetism in the Co:ZnO films.

Probing the local coordination environment for transition metal dopants in zinc oxide nanowires

It is hypothesized that a highly ordered, relatively defect-free dilute magnetic semiconductor system should act as a weak ferromagnet. Transition-metal-doped ZnO nanowires, being single crystalline, single domain, and single phase, are used here as a model system for probing the local dopant coordination environments using X-ray absorption spectroscopy and diffraction. Our X-ray spectroscopic data clearly show that the dopant resides in a uniform environment, and that the doping does not induce a large degree of disorder in the nanowires. This homogeneous nature of the doping inside the oxide matrix correlates well with observed weakly ferromagnetic behavior of the nanowires.