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Metallic particles formed in oxide fuels (e.g., UO2) during neutron irradiation have an adverse impact on fuel performance. A fundamental investigation of particle precipitation is needed to predict the fuel performance and potentially improve fuel designs and operations. This study reports on the precipitation of Mo-dominant β-phase particles in polycrystalline CeO2 (surrogate for UO2) films doped with Mo, Pd, Rh, Ru, and Re (surrogate for Tc). In situ heating scanning transmission electron microscopy indicates that particle precipitation starts at ∼1073 K with a limited particle growth to ∼10 nm. While particle concentration increases with increasing temperature, particle size remains largely unchanged up to 1273 K. There is a dramatic change in the microstructure following vacuum annealing at 1373 K, probably due to phase transition of reduced cerium oxide. At the high temperature, particles grow up to 75 nm or larger with distinctive facets. The particles are predominantly composed of Mo with a body-c...

Epsilon-phase alloy precipitates have been observed with varied compositions and sizes in spent nuclear fuels, such as UO2. The presence of the inclusions, along with other oxide precipitates, gas bubbles, and irradiation-induced structural defects, can significantly degrade the physical properties of the fuel. To predict fuel performance, a fundamental study of the precipitation processes is needed. This study uses ceria (CeO2) as a surrogate for UO2. Polycrystalline CeO2 films doped with Mo, Ru, Rh, Pd, and Re (surrogate for Tc) were grown at 823 K using pulsed laser deposition, irradiated at 673 K with He+ ions, and subsequently annealed at higher temperatures. A number of methods, including transmission electron microscopy and atom probe tomography, were applied to characterize the samples. The results indicate that there is a uniform distribution of the doped metals in the as-grown CeO2 film. Pd particles of ∼3 nm in size appear near the dislocation edges after He+ ion irradiation to ∼13 dpa. Thermal...

Understanding the interaction of water with compositionally tuned metal oxides is central to exploiting their unique catalytic and magnetic properties. However, processes such as hydroxylation, wetting, and resulting changes in electronic structure at ambient conditions are challenging to probe in situ. Here, we examine the hydroxylation and wetting of Fe(3–x)TixO4 (001)-oriented epitaxial films directly using ambient pressure X-ray photoelectron spectroscopy under controlled relative humidity. Fe2+ formation promoted by Ti4+ substitution for Fe3+ increases with hydroxylation, commensurate with a decrease in the surface work function or change in the surface dipole. The incorporation of small amounts of Ti (x = 0.25) as a bulk dopant dramatically impacts hydroxylation, in part due to surface segregation, leading to coverages closer to that of TiO2 than Fe3O4. However, the Fe(3–x)TixO4 compositional series shows a similar affinity for water physisorption, which begins at notably lower relative humidity tha...

Hysteretic behavior was studied in a series of Fe thin films, grown by molecular beam epitaxy, having different grain sizes and grown on different substrates. Major and minor loops and first order reversal curves (FORCs) were collected to investigate magnetization mechanisms and domain behavior under different magnetic histories. The minor loop coefficient and major loop coercivity increase with decreasing grain size due to higher defect concentration resisting domain wall movement. First order reversal curves allowed estimation of the contribution of irreversible and reversible susceptibilities and switching field distribution. The differences in shape of the major loops and first order reversal curves are described using a classical Preisach model with distributions of hysterons of different switching fields, providing a powerful visualization tool to help understand the magnetization switching behavior of Fe films as manifested in various experimental magnetization measurements.

Polarization-induced weak ferromagnetism has been predicted a few years back in compounds MTiO 3 (M = Fe, Mn, Ni) (Fennie, 2008). We set out to stabilize this metastable, distorted perovskite structure by growing NiTiO 3 epitaxially on sapphire Al 2 O 3 (001) substrate, and to control the polar and magnetic properties via strain. Epitaxial Ni 1 − x Ti 1 − y O 3 films of different Ni/Ti ratios and thicknesses were deposited on Al 2 O 3 substrates by pulsed laser deposition at different temperatures, and characterized using several techniques. The effect of film thickness, deposition temperature, and film stoichiometry on lattice strain, film structure, and physical properties was investigated. Our structural data from x-ray diffraction, electron microscopy, and x-ray absorption spectroscopy shows that substrate-induced strain has a marked effect on the structure and crystalline quality of the films. Physical property measurements reveal a dependence of the Neel transition and lattice polarization on strain, and highlight our ability to control the ferroic properties in NiTiO 3 thin films by film stoichiometry and thickness.

Mixed Mn/Ti oxides present attractive physicochemical properties such as their ability to accommodate Li for application in Li-ion batteries. In this work, atomic parameters for Mn were developed to extend an existing shell model of the Li–Ti–O system and allow simulations of pure and lithiated Mn and mixed Mn/Ti oxide polymorphs. The shell model yielded good agreement with experimentally derived structures (i.e., lattice parameters and interatomic distances) and represented an improvement over existing potential models. The shell model was employed in molecular dynamics (MD) simulations of Li diffusion in the 1 × 1 c-direction channels of LixMn1–yTiyO2 with the rutile structure, where 0 ≤ x ≤ 0.25 and 0 ≤ y ≤ 1. In the infinite dilution limit, the arrangement of Mn and Ti ions in the lattice was found to have a significant effect on the activation energy for Li diffusion in the c channels due to the destabilization of half of the interstitial octahedral sites. Anomalous diffusion was demonstrated for Li ...

Following the Drude model for dielectric constant, e(E) = (E) + i(E), the plasmonic resonance energy Eres in a semiconductor depends on four parameters: () (or ) , effective mass m*, optical mobility µopt, and optical carrier concentration nopt. By solving the Drude equation at e(Eres) = 0, we obtain a relationship between µopt and nopt at constant Eres [or wavelength res(µm) = 1.2395/Eres(eV)]. A family of µopt vs nopt curves covering a range of res values (including the limiting wavelength res = ) constitute a plasmonic resonance phase diagram (PRPD) for a semiconductor defined by only e and m*. The PRPD is a convenient instrument that allows an immediate prediction of res from Hall-effect measurements of µH and nH. Furthermore, if the µH/nH point falls outside the family of µopt vs nopt curves, it shows that no resonance at all is possible. We apply the PRPD analysis to a series of ten ZnO samples grown by pulsed laser deposition at 200 C in an ambient of 33%H2:67%Ar and annealed in 25-C steps for 10 min in air at various temperatures from 400 C to 600 C. The unannealed sample had a resistivity  = 1.67 x 10-4 -cm, nmore » = 1.39 x 1021 cm-3, and µ = 26.8 cm2/V-s, and the 600-C sample,  = 1.52 -cm, n = 5.03 x 1017 cm-3, and µ = 8.2 cm2/V-s. Each of the ten µH/nH points was plotted on the PRPD. For the samples that were annealed at 550 C or lower, the µH/nH points yielded predicted values of res that ranged from 1.07 to 2.80 µm, respectively; however, the 575-C and 600-C samples were predicted to have no resonances at all. Reflectance curves for the eight samples annealed up to 550 C de-creased slowly from 6 eV down to about 1 eV, and then increased rapidly at an energy evidently close to Eres. In contrast, there was no such increase for the 575-C and 600-C samples, and thus presumably, no resonance. Satisfactory agreement is found between the reflectance and Hall-effect-predicted values of res.« less

MTiO3 (M = Fe, Mn, Ni) compounds have received recent attention as possible candidates for multiferroic materials capable of magnetization switching by application of an electric field. In an attempt to stabilize NiTiO3 in the rhombohedral R3 structure, epitaxial Ni1 − xTi1 − yO3 films of different thickness and composition were deposited on Al2O3(0001) by pulsed laser deposition, and characterized using several techniques. Structural parameters for ilmenite-type NiTiO3 and the metastable LiNbO3-type NiTiO3 structure with the space group R3c were predicted using density functional theory calculations, and compared with the experimental results. Our structural data from X-ray diffraction and X-ray absorption spectroscopy indicate that epitaxial ilmenite-type NiTiO3 films were grown. Furthermore, lattice strain exerted by the sapphire substrate results in a distorted ilmenite structure similar to the LiNbO3-type one. While R3c NiTiO3, the desired structure based on recent theory, cannot be claimed at this point, our results demonstrate the potential of oxide heteroepitaxy to stabilize metastable multiferroic phases that may be difficult to prepare or are inaccessible in the bulk.

In situ X-ray photoelectron spectroscopy (XPS) was utilized to identify the chemical state of silver in a range of silver oxide thin films obtained by codeposition of silver and atomic oxygen. A highly oxidized silver species was observed at an unexpectedly low Ag 3d5/2 binding energy (BE) of 366.8 eV with an associated broad satellite at 368.2 eV; this species was assigned as Ag(III). It was found to be highly unstable in vacuum but could be regenerated by further exposure to atomic oxygen. Both BE shifts and intensity changes of the O 1s peak were found to correlate with changes in the silver oxidation state. The theoretical XPS spectrum of high spin Ag(III) was calculated for both an isolated cation and an embedded AgO6 cluster.

The flux of atomic oxygen generated in an electron cyclotron resonance microwave plasma source was quantified by two different methods. The commonly applied approach of monitoring the frequency change of a silver-coated quartz crystal microbalance (QCM) deposition rate monitor as the silver is oxidized was found to underestimate the atomic oxygen flux by an order of magnitude compared to a more direct deposition approach. In the mixed-phase Ag/Ag 2 O deposition method, silver films were deposited in the presence of atomic oxygen such that the films were partially oxidized to Ag 2 O; X-ray photoelectron spectroscopy was utilized for quantification of the oxidized fraction. The inaccuracy of the QCM oxidation method was tentatively attributed to efficient catalytic recombination of O atoms on the silver surface.

The question of stability against diffusional mixing at the prototypical LaAlO3/SrTiO3(001) interface is explored using a multi-faceted experimental and theoretical approach. We combine analytical methods with a range of sensitivities to elemental concentrations and spatial separations to investigate interfaces grown using on-axis pulsed laser deposition. We also employ computational modeling based on the density function theory as well as classical force fields to explore the energetic stability of a wide variety of intermixed atomic configurations relative to the idealized, atomically abrupt model. Statistical analysis of the calculated energies for the various configurations is used to elucidate the relative thermodynamic stability of intermixed and abrupt configurations. We find that on both experimental and theoretical fronts, the tendency toward intermixing is very strong. We have also measured and calculated key electronic properties such as the presence of electric fields and the value of the valence band discontinuity at the interface. We find no measurable electric field in either the LaAlO3 or SrTiO3, and that the valence band offset is near zero, partitioning the band discontinuity almost entirely to the conduction band edge. Moreover, we find that it is not possible to account for these electronic properties theoretically without including extensive intermixing in our physical model of the interface. The atomic configurations which give the greatest electrostatic stability are those that eliminate the interface dipole by intermixing, calling into question the conventional explanation for conductivity at this interface - electronic reconstruction. Rather, evidence is presented for La indiffusion and doping of the SrTiO3 below the interface as being the cause of the observed conductivity.

Copper thiocyanate (CuSCN) is a candidate as a transparent solid p-type conductor for optoelectronic and photovoltaic applications, such as solar cells. We calculate the band structure, bonding characteristics, and basic native defect configurations of hexagonal β-CuSCN. β-CuSCN is predicted to be an indirect-gap semiconductor with an unusual orbital character: although the highest valence bands have the expected character of Cu 3d levels hybridized with S 3p states, the conduction band minimum (at the K point of the hexagonal Brillouin zone) has mostly cyanide antibonding character. This quasi-molecular character results in some unusual properties, including that the electron effective masses are comparable to or even larger than the hole effective masses. Calculated results match well with the valence band spectrum of thin film CuSCN, although optical absorption measurements do not conclusively confirm the predicted indirect nature of the lowest transitions. The dominant p-type character of this materia...

Photoemission electron microscopy (PEEM) excited by x-ray and UV sources is used to investigate epitaxial anatase thin films embedded with rutile nanocrystals, a model system for the study of heterocatalysis on mixed-phase TiO2. Both excitation sources show distinct contrast between the two TiO2 phases, however, the contrast is reversed. Rutile nanocrystals appear darker than the anatase film in X-ray PEEM images but brighter in UV-PEEM images. Topography-induced contrast is dominant X-ray PEEM imaging, whereas work function contrast, dominates for UV-PEEM. Work function contrast results from the differences in work function and surface defect state densities between the two phases near the Fermi level. This assertion is confirmed by UPS data that shows the rutile work function to be 0.2 eV lower and a greater occupied valence band density-of-states in rutile (100) than in anatase (001). Since the boundaries between rutile nanocrystals and the anatase film are clearly resolved, these results indicate that PEEM studies of excited state dynamics and heterocatalysis are possible at chemically intriguing mixed-phase TiO2 interfaces and grain boundaries.

The thermally-induced martensitic phase transformation in a polycrystalline NiTiCu thin film shape memory alloy was probed by photoelectron emission microscopy (PEEM). In situ PEEM images reveal distinct changes in microstructure and photoemission intensity at the phase transition temperatures. In particular, images of the low temperature, martensite phase are brighter than that of the high temperature, austenite phase, due to the relatively lower work function of the martensite. Ultra-violet photoelectron spectroscopy shows that the effective work function changes by about 0.16 eV during thermal cycling. In situ PEEM images also show that the network of trenches observed on the room temperature film disappear suddenly during heating and reappear suddenly during subsequent cooling. These trenches are also characterized by atomic force microscopy at selected temperatures. We describe implications of these observations with respect to the spatial distribution of phases during thermal cycling in this thin film shape memory alloy.

We have investigated the heteroepitaxial growth and materials properties of pure and Co-doped TiO2 anatase on SrTiO3(001) and LaAlO3(001), grown by oxygen plasma assisted molecular beam epitaxy. This material is a promising new diluted magnetic semiconductor that shows large magnetization and a Curie temperature well above room temperature. We have found that epitaxial films with the highest crystalline quality and most uniform distribution of Co result when a rather slow growth rate (∼0.01 nm/s) is used over a substrate temperature range of 550–600 °C on LaAlO3(001). These conditions result in layer-by-layer growth of single-crystal films and a very low density of extremely small nanocrystalline inclusions. In contrast, growth at a higher rate (∼0.04 nm/s) leads to extensive formation of secondary-phase rutile nanocrystals to which Co diffuses and segregates. The rutile nanocrystals nucleate on the evolving anatase film surface in such a way that lattice strain between the two phases is minimized. Cobalt appears to substitute for Ti in the lattice and exhibits a +2 formal oxidation state. Both pure and Co-doped films can be grown as n-type semiconductors by controlled incorporation of oxygen atom vacancies. Free electrons are required to couple the Co(II) spin to a ferromagnetic state.

The growth of MoO 3 films on SrLaAlO 4 (0 0 1), a substrate lattice-matched to β-MoO 3 , by oxygen plasma assisted molecular beam epitaxy was characterized using reflection high-energy electron diffraction (RHEED), X-ray photoelectron spectroscopy, X-ray diffraction (XRD), and atomic force and scanning tunneling microscopies (AFM and STM). It was found that the flux of reactive oxygen species to the surface was not high enough to maintain the proper stoichiometry, even at the lowest measurable deposition rates. Therefore, the films were grown by depositing Mo in small increments and then allowing the Mo to oxidize. At 675 K, the films grew epitaxially but in a three-dimensional manner. XRD of films grown under these conditions revealed a tetragonal structure that has not been previously observed in bulk MoO 3 samples. Decreasing the growth temperature to 535 K led to polycrystalline α-MoO 3 preferentially aligned with the [0 1 0] and [1 0 0] directions of the grains oriented normal to the substrate. By manipulating the initial growth conditions, relatively flat, epitaxial MoO 3 films could be grown on SrLaAlO 4 (0 0 1). In this case, the equivalent of one layer of β-MoO 3 (0 0 1) was deposited at 675 K before adding several layers of MoO 3 at 550 K. Unlike growth solely at 675 K, the MoO 3 formed in this manner did not dewet the surface when reheated to 675 K and RHEED indicated that continued MoO 3 growth at this temperature proceeded epitaxially. XRD and AFM indicated that films grown in this manner contained α-MoO 3 crystallites in addition to an epitaxial phase that accounted for most of the surface area of the film. STM images of the film that revealed step heights expected for β-MoO 3 (0 0 1), along with RHEED results that revealed in-plane lattice constants consistent with β-MoO 3 , indicated that epitaxial β-MoO 3 films can be grown and that geometric matching to the substrate can stabilize the β phase at temperatures where it is not stable in the bulk.

X-ray photoelectron emission microscopy (XPEEM) provides valuable chemical information at the micron or submicron level and has been successfully applied to a wide range of materials. We have initiated a study of metal–matrix composites (MMCs) using this technique, aimed at understanding the interfacial interactions that occur between the metal matrix and the ceramic reinforcement. The experiments have been performed on Ti/SiC (fiber) MMCs and SiC fibers. We have found that the method of preparing the sample plays a critical role in understanding the results of the measurements. Samples prepared by polishing and sputtering were examined using an electric-field imaging photoelectron microscope (EFIPM), and fractured samples were studied with a magnetic-field imaging photoelectron microscope (MFIPM). While the EFIPM exhibited exellent elemental contrast and resolution, the data were flawed by artifacts introduced by both the polishing and the sputtering. Although the spatial resolution of the MFIPM was lower than the EFIPM, the ability to study fractured samples greatly facilitated interpretation of the data, which revealed the intrinsic chemical states of the sample. These results will be interpreted in terms of the impact of sample preparation on the application of XPEEM to MMCs and other complex systems.

A purely electrostatic photoemission electron microscope with a bandpass energy filter is described. The electron optics are of a hybrid design, incorporating a high-voltage and high-magnification objective and intermediate lens, coupled to a low-voltage hemispherical capacitor configured as an achromatic imaging bandpass filter. The electron optical system design is described, along with initial performance tests. The instrument is used for both imaging X-ray absorption spectroscopy and photoemission spectroscopy.

X-ray absorption microscopy is used to study the surface of a mineral, ilmenite. High-quality surface-sensitive micrographs are easily obtained without special sample preparation beyond that usually used in preparing samples for optical microscopy. The X-ray micrographs show an expected lamellar domain structure that has a very strong contrast change at the iron and titanium L-edges. High-resolution spectroscopy from these two different regions reveals fine structure from iron atoms in two different charge states, and titanium in a single phase. A theoretical analysis and curve-fitting procedure is used to determine the precise stoichiometry of the structures in the two phases detected. These results show that quantitative surface chemistry studies can be performed on natural mineral samples using X-ray photoelectron emission microscopy.

Recent neurobiophysics experiments based on synchrotron spectromicroscopy indicated that aluminium is selectively accumulated by GABAergic neurons and glial cells rather than by granule cells. Does a similar cell specificity occur in the accumulation of other metals? We provide experimental evidence to the contrary: cobalt is found in granule cells at least with equal probability as in glial cells and GABAergic neurons. This result also confirms that neurobiophysics studies based on surface physics techniques have reached a stage of maturity.

Epitaxial chromium ferrite (Fe2CrO4), prepared by state-of-the-art oxygen plasma assisted molecular beam epitaxy, is shown to exhibit unusual electronic transport properties driven by the crystallographic structure and composition of the material. Replacing 1/3 of the Fe cations with Cr converts the host ferrimagnet from a metal into a semiconductor by virtue of its fixed valence (3+); Cr substitutes for Fe at B sites in the spinel lattice. By contrast, replacing 2/3 of the Fe cations with Cr results in an insulator. Three candidate conductive paths, all involving electron hopping between Fe2+ and Fe3+, are identified in Fe2CrO4. Moreover, Fe2CrO4 is shown to be photoconductive across the visible portion of the electromagnetic spectrum. As a result, this material is of potential interest for important photo-electrochemical processes such as water splitting.

Three closely related techniques, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and X-ray fluorescence spectroscopy, have become widely accepted as important tools for the study of the chemical composition and electronic properties of surfaces, overlayers, and interfaces. There is now a major effort to push these spectroscopic techniques into a new realm of applications with very high spatial resolution, at and below 1 μm. This results in a new set of probes which can create images of chemical composition with great subtlety. The field is growing rapidly as high brightness sources of X-rays become available. The goals and methods used in electron spectromicroscopy and related X-ray microscopies are discussed, and recent applications of instruments developed in the last few years are used to illustrate the strengths of these new microscopes.

The electrical transport properties for compositionally and structurally well-defined epitaxial \ensuremath{\alpha}-(Ti${}_{x}$Fe${}_{1\ensuremath{-}x}$)${}_{2}$O${}_{3}$(0001) films have been investigated for $x$ \ensuremath{\le} 0.09. All films were grown by oxygen plasma-assisted molecular beam epitaxy using two different growth rates: 0.05--0.06 \AA{}/s and 0.22--0.24 \AA{}/s. Despite no detectable difference in cation valence and structural properties, films grown at the lower rate were highly resistive whereas those grown at the higher rate were semiconducting (\ensuremath{\rho} $=$ \ensuremath{\sim}1 \ensuremath{\Omega} \ifmmode\cdot\else\textperiodcentered\fi{} cm at 25 \ifmmode^\circ\else\textdegree\fi{}C). Hall effect measurements reveal carrier concentrations between 10${}^{19}$ and 10${}^{20}$ cm${}^{\ensuremath{-}3}$ at room temperature and mobilities in the range of 0.1 to 0.6 cm${}^{2}$/V \ifmmode\cdot\else\textperiodcentered\fi{} s for films grown at the higher rate. The conduction mechanism transitions from small-polaron hopping at higher temperatures to variable-range hopping at a transition temperature between 180 and 140 K. The absence of conductivity in the slow-grown films is attributed to donor electron compensation by cation vacancies, which may form to a greater extent at the lower rate because of higher oxygen fugacity at the growth front.

Epitaxial films along the Fe{sub 3-x}Ti{sub x}O{sub 4} (titanomagnetite) compositional series from pure end-members magnetite (Fe{sub 3}O{sub 4}) to ulvospinel (Fe{sub 2}TiO{sub 4}) were successfully grown by pulsed laser deposition on MgO(100) substrates. Spectroscopic characterization including high resolution x-ray diffraction, x-ray photoelectron spectroscopy, and synchrotron-based x-ray absorption and magnetic circular dichroism consistently shows that Ti(IV) substitutes for Fe(III) in the inverse spinel lattice with a proportional increase in lattice Fe(II) concentration. No evidence of Ti interstitials, spinodal decomposition, or secondary phases was found in the bulk of the grown films. At the uppermost few nanometers of the Ti-bearing film surfaces, evidence suggests that Fe(II) is susceptible to facile oxidation, and that an associated lower Fe/Ti ratio in this region is consistent with surface compositional incompleteness or alteration to a titanomaghemite-like composition and structure. The surface of these films nonetheless appear to remain highly ordered and commensurate with the underlying structure despite facile oxidation, a surface condition that is found to be reversible to some extent by heating in low oxygen environments.

LaAlO3 epitaxial films with La:Al cation ratios ranging from 0.9 to 1.2 were grown on TiO2-terminated SrTiO3 (001) substrates by off-axis pulsed laser deposition. Although all films are epitaxial, rocking curve and Kiessig fringe oscillation measurements show that the crystallographic quality degrades with increasing La:Al ratio. Films with La:Al ratios of 0.9, 1.0, and 1.1 were coherently strained to the substrate. However, the out-of-plane lattice parameter increases over this range, revealing a decrease in film tetragonality. Although all film surfaces exhibit hydroxylation, the extent of hydroxylation is greater for the La-rich films. Rutherford backscattering spectrometry reveals that La from the film diffuses deeply into the SrTiO3 substrate and secondary ion mass spectroscopy shows unambiguous Sr outdiffusion into the films. Intermixing, which is generally not investigated in studies of the LaAlO3/SrTiO3 interface, may have important implications for the mechanism of electrical conductivity.

Electronic absorption, magnetic circular dichroism (MCD), photoconductivity, and valence-band x-ray photoelectron (XPS) spectroscopic measurements were performed on epitaxial ${\text{Zn}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{O}$ films to investigate the origin of the mid-gap band that appears upon introduction of ${\text{Mn}}^{2+}$ into the ZnO lattice. Absorption and MCD spectroscopies reveal ${\text{Mn}}^{2+}$-related intensity at energies below the first excitonic transition of ZnO, tailing well into the visible energy region, with an onset at $\ensuremath{\sim}2.2\text{ }\text{eV}$. Photoconductivity measurements show that excitation into this visible band generates mobile charge carriers, consistent with assignment as a ${\text{Mn}}^{2+/3+}$ photoionization transition. XPS measurements reveal the presence of occupied ${\text{Mn}}^{2+}$ levels just above the valence-band edge, supporting this assignment. MCD measurements additionally show a change in sign and large increase in magnitude of the excitonic Zeeman splitting in ${\text{Zn}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{O}$ relative to ZnO, suggesting that $sp\text{\ensuremath{-}}d$ exchange in ${\text{Zn}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{O}$ is not as qualitatively different from those in other II-VI diluted magnetic semiconductors as has been suggested. The singular electronic structure feature of ${\text{Zn}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{O}$ is the presence of this ${\text{Mn}}^{2+/3+}$ ionization level within the gap, and the influence of this level on other physical properties of ${\text{Zn}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{O}$ is discussed.

Epitaxial films of ${\text{Mn}}^{2+}$-doped ZnO were deposited by pulsed laser deposition on $\ensuremath{\alpha}{\text{-Al}}_{2}{\text{O}}_{3}(0001)$ using targets created from ${\text{Mn}}^{2+}$-doped ZnO nanoparticles. Using x-ray absorption spectroscopy and x-ray magnetic circular dichroism, Mn(II) was found to substitute for Zn(II) in the w\"urtzite ZnO lattice with only a paramagnetic dichroic component from the Mn and no magnetic component from either the O or Zn. The dichroism reveals that, while substitutional, the ${\text{Mn}}^{2+}$ distribution in the ZnO lattice is not stochastic. Rather, ${\text{Mn}}^{2+}$ has a tendency to substitute with higher effective local concentrations than anticipated from a stochastic doping model.

Monte Carlo simulations of cation doping within the ZnO lattice were utilized to evaluate dopant distributions in nanoparticles and thin films. In structures with a high surface-to-volume ratio, dopant distributions deviate significantly from predictions based on probabilistic expressions for infinitely large bulk lattices. We present empirical expressions that accurately predict dopant bonding configurations as a function of film or particle size, shape, and dopant concentration for any substitutional dopant (cation or anion) within a tetrahedrally coordinated compound, including zinc-blende, wurtzite, and diamond structures.

X-ray absorption fine-structure (XAFS) measurements have been used to characterize a series of Co-doped ZnO films grown on sapphire substrates by pulsed laser deposition. The emphasis is on characterization of the fate of the Co dopant: metallic particles or substitutional $\text{Co}2+$. It is shown that analysis of both the near edge and extended fine structure can provide a measurement of the fraction of metallic Co. Any quantitative understanding of magnetism in this system needs to take into account both types of Co. Results are reported for two types of films from two different groups that show distinctly different behaviors. Films grown with high concentrations of Co show varying amounts of metallic Co that could be identified as a close-packed form of Co. Another set of films was annealed in Zn vapor to induce magnetism. These films also showed significant metallic Co, but of a different type similar to the CoZn intermetallic. The bulk forms of both metals are magnetic and should contribute to the magnetism. However, the measured room-temperature magnetic moments for some films are inconsistent with the expected moments based on the bulk magnetic values for either Co metal or CoZn. The magnetic properties of the small metal particles are likely changed by their surroundings. Low-temperature magnetic measurements for one of the samples confirmed this with an estimated blocking temperature of 50 K.

The origin of ferromagnetism is investigated in epitaxial Co:ZnO thin films which become weakly ferromagnetic after annealing in Zn vapor. Conventional characterization techniques indicate no change after treatment. However, x-ray photoelectron spectroscopy depth profiling clearly indicates the presence of Co(0) in the Zn-treated films; x-ray absorption fine structure is utilized to identify the secondary phase as ferromagnetic CoZn. This work demonstrates that the potential for ferromagnetic secondary phases must be thoroughly discounted, through painstaking materials characterization, before claims of intrinsic ferromagnetism can be made.

We explore the feasibility of growing epitaxial $\mathrm{Ti}$-doped $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ hematite in which $\mathrm{Ti}(\mathrm{IV})$ substitutes for $\mathrm{Fe}(\mathrm{III})$ preferentially in one magnetic sublattice but not the other. Such a structure has been predicted by first-principles theory to be energetically favorable, and is expected to yield interesting and useful magnetic and electronic properties. However, we find experimentally that a majority of $\mathrm{Ti}$ dopants disperse and occupy random cation sites in both magnetic sublattices. Density functional theory predicts that the magnetically ordered and magnetically random structures are nearly isoenergetic.

The materials and magnetic properties of Cr-doped anatase ${\mathrm{TiO}}_{2}$ thin films deposited on ${\mathrm{LaAlO}}_{3}(001)$ and ${\mathrm{SrTiO}}_{3}(001)$ substrates by oxygen-plasma-assisted molecular beam epitaxy have been studied in detail to elucidate the origin of ferromagnetic ordering. Cr substitution for Ti in the anatase lattice, with no evidence of Cr interstitials, segregation, or secondary phases, was independently confirmed by transmission electron microscopy with energy dispersive x-ray spectroscopy, extended x-ray absorption fine structure, and Rutherford backscattering spectrometry in the channeling geometry. Epitaxial films deposited at $\ensuremath{\sim}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}∕\mathrm{s}$ were found to have a highly defected crystalline structure, as quantified by high-resolution x-ray diffraction (XRD). These films were also ferromagnetic at room temperature with a moment of $\ensuremath{\sim}0.5{\ensuremath{\mu}}_{B}∕\mathrm{Cr}$, Curie temperatures in the range of $400\text{--}700\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, and exhibited shape and in-plane magnetocrystalline anisotropy. However, no free carrier spin polarization was observed by Hall effect measurements, raising questions about the mechanism of magnetism. Films deposited slowly $(\ensuremath{\sim}0.015\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}∕\mathrm{s})$ possessed a nearly perfect crystalline structure as characterized by XRD. Contrary to expectations, these films exhibited negligible ferromagnetism at all Cr concentrations. Annealing in vacuum to generate additional oxygen defects and free carrier electrons did not significantly increase the ferromagnetic ordering in either fast- or slow-grown films. These results contradict both oxygen-vacancy-derived free-carrier-mediated exchange and F-center-mediated bound magnetic polaron exchange mechanisms, and instead indicate the primary role of extended structural defects in mediating the ferromagnetic ordering in doped anatase films.

The authors report the magnetic and structural characteristics of epitaxial NiTiO3 films grown by pulsed laser deposition that are isostructural with acentric LiNbO3 (space group R3c). Optical second harmonic generation and magnetometry demonstrate lattice polarization at room temperature and weak ferromagnetism below 250 K, respectively. These results appear to be consistent with earlier predictions from first-principles calculations of the coexistence of ferroelectricity and weak ferromagnetism in a series of transition metal titanates crystallizing in the LiNbO3 structure. This acentric form of NiTiO3 is believed to be one of the rare examples of ferroelectrics exhibiting weak ferromagnetism generated by a Dzyaloshinskii–Moriya interaction.

The x-ray photoemission electron microscope (X-PEEM) is a type of spectromicroscope, which can make spatial maps of chemical composition. The contrast in the images can originate from a number of properties of the sample, and by controlling the way the XPEEM is used a number of different types of information about the surface can be revealed. These contrast mechanisms include topography, band-gap, work function, chemical composition, crystalline orientation, adsorbate reactants, and magnetic domains. Some recent results are described.