Search

Identification of kinetic and thermodynamic factors that control crystal nucleation and growth represents a central challenge in materials synthesis. Here we report that apparently defect-free growth of ${\mathrm{La}}_{2}{\mathrm{MnNiO}}_{6}$ (LMNO) thin films supported on ${\mathrm{SrTiO}}_{3}$ (STO) proceeds up to 1--5 nm, after which it is disrupted by precipitation of NiO phases. Local geometric phase analysis and ensemble-averaged x-ray reciprocal space mapping show no change in the film strain away from the interface, indicating that mechanisms other than strain relaxation induce the formation of the NiO phases. Ab initio simulations suggest that the electrostatic potential build-up associated with the polarity mismatch at the film-substrate interface promotes the formation of oxygen vacancies with increasing thickness. In turn, oxygen deficiency promotes the formation of Ni-rich regions, which points to the built-in potential as an additional factor that contributes to the NiO precipitation mechanisms. These results suggest that the precipitate-free region could be extended further by either incorporating dopants that suppress the built-in potential or by increasing the oxygen fugacity in order to suppress the formation of oxygen vacancies.

We have measured high-resolution core-level and valence-band x-ray photoemission spectra for single-crystal $\mathrm{T}{\mathrm{i}}_{2}{\mathrm{O}}_{3}$ cleaved anoxically. The Ti(III) spectra for this lattice are considerably more complex than those measured for Ti(IV)-based oxides due to the presence of a single unpaired electron in the conduction band. This open-shell electron configuration leads to ligand-field split and frequently unresolved multiplets. The Ti $2p$ and $3p$ spectra have been calculated using relativistic Dirac-Hartree-Fock (DHF) theory with the sudden approximation for the intensities. Agreement between theory and experiment is excellent for the $3p$ spectrum, and very good for the $2p$ spectrum, the primary deficiency being a pair of features not captured by theory for the latter. The spectral line shapes are driven by final-state effects associated with angular momentum coupling of the unpaired valence electron with the core hole, one- and two-electron ligand-to-metal charge-transfer (shake) processes accompanying core photoionization, and core-hole screening by conduction-band electrons. The first two of these are accurately predicted by DHF theory with a small embedded cluster containing a single Ti cation and six oxygen ligands. The third effect is not predicted using this cluster in which screening of the core hole from electrons associated with more distant atoms is not possible.

We use angle-resolved photoemission under ultraviolet laser excitation to demonstrate that the electron emission properties of Ag(001) can be markedly enhanced and redirected along the surface normal by the deposition of a few monolayers of epitaxial MgO. We observe new low-binding energy states with small spreads in their surface parallel momenta as a result of $\text{MgO}/\text{Ag}(001)$ interface formation. Under 4.66 eV laser excitation, the quantum efficiency of $\text{MgO}/\text{Ag}(001)$ is a factor of 7 greater than that of clean Ag(001), revealing the utility of such heterojunctions as advanced photocathodes.

Ultrathin films of nickel exhibit an unusual sequence of transitions from in-plane to perpendicular magnetization as a function of film thickness. A sharp transition from in-plane to perpendicular magnetization is found near 7 ML thickness, followed by a gradual transition back to in-plane magnetization beginning at 37 ML. This sequence of transitions cannot be explained by the surface or shape anisotropies, both of which favor in-plane magnetization in the thickness range where perpendicular anisotropy is found. We have measured the thickness dependence of these transitions for nickel film wedges, and films capped by nonmagnetic and magnetic overlayers, to experimentally determine the surface, interface, and magnetoelastic anisotropies. We find that both the surface and interface anisotropy constants are negative (favoring in-plane magnetization), with the magnitude of the surface term being larger than that of the interface. A correlation is found between the critical thickness for misfit dislocation formation in the nickel film and a sharp transition in the coercive field. This transition is used to accurately determine the onset of a thickness dependence in the bulk magnetoelastic energy, which causes the magnetization to rotate back into the film plane. This model gives a complete description of the mechanism for the easy-axis changes at both the 7 and 37 ML thicknesses. \textcopyright{} 1996 The American Physical Society.

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.

Core-level and valence-band x-ray photoemission spectra measured for molecular-beam-epitaxy-grown LaCrO(3)/SrTiO(3)(001) yield band offsets and potential gradients within the LaCrO(3) sufficient to trigger an electronic reconstruction to alleviate the polarity mismatch. Yet, the interface is insulating. Based on first principles calculations, we attribute this unexpected result to interfacial cation mixing combined with charge redistribution within CrO(2) layers, enabled by low-lying d states within LaCrO(3), which suppresses an electronic reconstruction.

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.

We reexamine the mechanism of ferromagnetism in doped TiO(2) anatase, using epitaxial Cr:TiO(2) with excellent structural quality as a model system. In contrast to highly oriented but defective Cr:TiO(2) (approximately 0.5 micro(b)/Cr), these structurally superior single crystal films exhibit negligible ferromagnetism. Similar results were obtained for Co:TiO(2). We show for the first time that charge-compensating oxygen vacancies alone, as predicted by F-center mediated exchange, are not sufficient to activate ferromagnetism. Instead, the onset of ferromagnetism correlates with the presence of structural defects.

The quest for diluted magnetic semiconductors (DMS) which retain their magnetism at and above room temperature is spanning several classes of materials. Such materials are critically important in the development of spintronics as spin injectors for semiconductor heterostructures that can operate without cryogenic cooling. Group IV, III-V, and II-VI DMS materials typically exhibit Curie temperatures (Tc) well below ambient due to weak interaction of the magnetic impurities. Calculations based on the Zener model of magnetism suggest that the strongest interaction is that mediated by holes, and experimental studies carried out to date have borne out this prediciton. One notable exception is that of Mn-doped GaN, which grows n-type by gas-source molecular beam epitaxy under certain conditions, and appears to be ferromagnetic at room temperature. In addition, it has recently been shown that at least one oxide semiconductor - Co-doped TiO₂ anatase or CoxTi₁-xO₂-x - is ferromagnetic well above room temperature when doped n-type by oxygen vacancies for x < ~0.1, but the mechanism of magnetism remains unknown.

We have measured high-energy-resolution Fe 2p photoelectron spectra for α-Fe₂O₃/α-Al₂O₃(0001) grown by oxygen-plasma-assisted molecular beam epitaxy at emission angles chosen to enhance bulk and surface contributions. We quantitatively reproduce the subtle lineshape differences in these spectra by taking into account the break in symmetry and change in crystal field splitting generated by the presence of the surface.

Metal and oxygen core-level Auger and photoemission spectra have been measured for ultrathin epitaxial films of $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ and $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ on Pt(111). The binding-energy shifts of monolayer films relative to thicker films suggest a much smaller extent of screening by the metal substrate than that reported earlier for MgO/ Ag(001) [S. Altieri, L. H. Tjeng, F. C. Voogt, T. Hibma, and G. A. Sawatsky, Phys. Rev. B 59, R2517 (1999)]. This finding suggests that the higher degree of bond covalency in $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ and $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ relative to MgO results in more effective intrinsic screening of charge-transfer processes, rendering the metal much less effective for extrinsic screening. As a result, the metal is expected to have a negligible effect on the optical, electronic, and magnetic properties of the oxide.