Structural defects-mediated room-temperature ferromagnetism in Co-doped SnO2 insulating films

Co-doped SnO₂ insulating films with different crystal quality were fabricated using magnetron sputtering. X-ray photoelectron spectroscopy and X-ray absorption near-edge structure spectrum reveal a solid solution of Co dopants in SnO₂ lattice, where Co is in 2+ oxidation state and substitutes for Sn⁴⁺. The as-grown films with different crystallinity exhibit clearly room-temperature ferromagnetism, and the films with more structural defects show higher saturated magnetic moment. With the increase of annealing time, the crystallinity of the films is improved and the defects decrease, leading to the decrease or disappearance of ferromagnetism in the films. The results show that the ferromagnetism in Co-doped SnO₂ films is intrinsic and can be considerably influenced by the concentration of structural defects.     

Silica fracture

In part I of this series, a ring contraction model was proposed as the basic mechanism of slow crack growth in silica glass. AM1 molecular orbital theory running on a CAChe workstation was used to find the transition state for the contraction of a 4-fold ring into a 3-fold ring. Using the same AM1 method, the predicted transition state has been found for the contraction of a 5-fold ring into a 4-fold ring. The activation barrier to fracture for this contraction is E _f = +7.9 Kcal mol^–1 using Unrestricted Hartree Fock (UHF) theory. As would be expected, the barrier calculated for Restricted Hartree Fock (RHF) was a little higher at E _f = +14.8 Kcal mol^–1. This confirms our initial hypothesis that ring contraction can lead to much lower fracture energies than expected from simple Si-O bond breaking. Several different schemes of ring contractions are possible for both 5-fold and 6-fold ring structures. All contraction paths have different intermediate structures that lead to the same end point of slow crack growth. The various barriers to fracture range from +8 to +52 Kcal mol^–1 for the 5-fold ring contractions and from +9 to +41 Kcal mol^–1 for 6-fold ring contractions.     

Effect of mixed valence impurities on superconductivity

The phase transition temperatureT _c , the specific heat discontinuity C, and the critical magnetic field of a superconductor containing intermediate valence impurities are calculated. The impurities are described by the Anderson model with orbital degeneracy in theU limit when two active ionic configurations, corresponding to4f ⁰ and4f ¹ (Ce impurities), are present. The properties of the superconducting alloy are expressed in terms of thet-matrix for the scattering off the impurities. It is assumed that no correlation between the impurities exists. The transition temperature decreases approximately exponentially with the impurity concentration as for the spin-fluctuation limit and the strong coupling Kondo limit. The critical field deviation functionD(T/T _c ) is the same as for the BCS theory. This indicates pair weakening rather than pair breaking, which is consistent with the picture that the mixed valence problem is driven by charge fluctuations, while spin fluctuations play only a secondary role.     

Vacancy-mediated ferromagnetism in Co-implanted ZnO studied using a slow positron beam

Co\(^+\) ions with multiple energies from 50 to 380 keV were implanted into ZnO single crystals up to a total dose of \(1.25\times 10^{17}\,\hbox {cm}^2\). The implanted samples were annealed in open air for 30 min between 200 and 1100 \(^{\circ }\)C. All the samples before and after implantation and annealing were characterized by X-ray diffraction (XRD), Raman scattering and positron annihilation measurements. XRD and Raman scattering measurements indicate that Co implantation induces severe lattice damage, and after annealing the damage recovers gradually. No Co clusters or Co-related second phase was observed in the implanted samples. Doppler broadening of positron annihilation radiation measurements using a slow positron beam reveals a large number of vacancy clusters introduced by Co implantation. After annealing up to 1000 \(^{\circ }\)C, almost all the defects induced by implantation are removed. The implanted samples show clear ferromagnetism measured at 5 K. It shows very slight decrease after annealing at 700 \(^{\circ }\)C and becomes much weaker after annealing at 1000 \(^{\circ }\)C. The origin of ferromagnetism is most probably due to substitution of Co\(^+\) ions at Zn lattice sites. However, it is apparent that the decrease in magnetization after annealing is consistent with the vacancy recovery process, indicating that the ferromagnetism in Co-implanted ZnO is mediated by defects such as Zn vacancy (V\(_{Zn}\)) or vacancy clusters. First principles calculations also support that Zn-related monovacancies and vacancy clusters can enhance the ferromagnetism in Co-doped ZnO.     

Robustly chain transitive sets with orbital shadowing diffeomorphisms

Let f be a diffeomorphism of a closed C ^∞ manifold M, and let Λ???M be a closed f-invariant set. We show that if f?|_Λ is robustly chain transitive with orbital shadowing property, then Λ is a basic set.     

Changes in the self-diffusion coefficient during the crystal-melt phase transition

It is shown that the self-diffusion coefficients (D _f) of metals and semiconductors acquire a universal value of D _f(S−L) × 10⁻⁵ cm²/s during the solid-liquid (crystal-melt) phase transition (PT). Attaining this value can be considered a necessary condition for the onset of PT, while D _f < 2.5 × 10⁻⁷ cm²/s and D _f > 10⁻⁴ cm²/s are the sufficient conditions for the termination of a PT to the solid and liquid state, respectively. These values are applicable both to substances that exhibit normal melting and to those melting with a decrease in the specific volume upon the liquid phase formation. It is pointed out that the melting is cased by a sharp delocalization of a certain fraction of atoms in a crystal.     

Luminescence properties and electronic structure of Sm<Superscript>3+</Superscript>-doped YAl<Subscript>3</Subscript>B<Subscript>4</Subscript>O<Subscript>12</Subscript>

The luminescence properties of Sm³⁺ ions in YAl₃B₄O₁₂ were studied upon synchrotron excitation in the 3.8–11 eV region. In addition to the 4f → 4f excitation bands, the excitation spectra of the Sm³⁺ emission contain broad bands at 6.1 and ~7.0 eV. These bands are attributed to charge transfer transition in Sm³⁺–O²⁻ complexes and 4f → 5d transition of Sm³⁺ ions, respectively. The optical absorption edge of YAl₃B₄O₁₂ was determined at 7.3 eV. A comparison with the results of electronic structure calculations on YAl₃B₄O₁₂ is also made.     

Variation of structural,optical, dielectric and magnetic properties of SnO<Subscript>2</Subscript> nanoparticles

We report effect of oxygen vacancies on band gap narrowing, enhancement in electrical conductivity and room temperature ferromagnetism of SnO₂ nanoparticles synthesized by simple chemical precipitation approach. As the calcination temperature is elevated from 400 to 800 °C, the average particle size increases from 12.26 to 34.43 nm, with enhanced grain growth and crystalline quality. At low temperatures, these nanoparticles are in a rather oxygen-poor state revealing the presence of many O vacancies and Sn interstitials in SnO₂ nanoparticles as in this case Sn⁺² is not oxidized completely to Sn⁺⁴ and small sized nano particles have more specific surface area, hence defects are more prominent. The oxygen content increases steadily with increasing temperature, with the Sn:O atomic ratio very near to the stoichiometric value of 1:2 at high temperatures suggesting the low density of defects. The optical band gap energies of all SnO₂ nanoparticles are in the visible light region, decreasing from 2.89 to 1.35 eV, while room temperature ferromagnetism and electrical conductivity are enhanced with reduced temperatures. The dielectric constant (ε_r) exhibited dispersion behaviour and the Debye’s relaxation peaks were observed in tanδ. The variation of dielectric properties and ac conductivity revealed that the dispersion is due to Maxwell–Wagner interfacial polarization and hopping of charge carriers between Sn⁺²/Sn⁺⁴. The narrowed band gap energies and enhanced ferromagnetism are mainly attributed to the increase of defects density (e.g., oxygen vacancies). The presence of oxygen vacancies is confirmed by EDX, Raman, PL, XPS, and UV–Vis spectra. The band gap of 1.35 eV is the smallest value for SnO₂ reported so far. This rather small band gap, enhanced conductivity and room temperature ferromagnetism demonstrate that SnO₂ nanoparticles are very promising in the visible light photo catalysis and optoelectronic devices.     

Influence of annealing temperature on the structural and optical properties of highly-oriented Al and Er co-doped ZnO films

High-quality c-axis oriented 7 mol% Al and 1.5 mol% Er co-doped ZnO films (ZEAO) were prepared on the quartz glass substrates by using sol–gel method. The influence of the annealing temperature on the crystal orientation, microstructure and optical properties of the ZEAO thin films were studied by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and UV–vis transmittance spectrophotometer, respectively. XRD results revealed that all the samples were polycrystalline with hexagonal structure and exhibited (002) preferential orientation. With increasing the annealing temperature, the grain size and orientation extent increased. The optical studies showed each ZEAO film had a relatively high transmittance above 85 %. The transmittance as high as 95 % was obtained at the annealing temperature of 800 °C, and the corresponding average grain size was about 50 nm. The cathodoluminescence (CL) spectra of these films were also used to characterize the luminescence properties. Strong UV emission centered at 380 nm was observed in the CL spectra taken for the pure ZnO. For the ZEAO sample, the blue-green emission is related to the 4f shell transition in the Er³⁺ ions of ZnO matrices, corresponding to a transition from the excited states (⁴F_5/2).     

Preparation, characterization and photocatalytic performance of Nd-doped titania nanoparticles with mesostructure

Nd³⁺-doped titania nanoparticles with mesostructures were synthesized via hydrothermal process by using cetyltrimethylammonium bromide (CTAB) as directing and pore-forming agent. The obtained materials were characterized by XRD, nitrogen adsorption-desorption, TEM and DRS. The existence of neodymium ion affect significantly the phase transition of the amorphous to anatase, and the band-gap energy was reduced because of the defect energy level induced by the 4f atomic orbital of Nd³⁺ with the optimal content of 1.5 at.% Nd. Density functional theory calculations can explain the band-gap narrowing. The maximum photocatalytic activity corresponds to the 0.5 at.% Nd³⁺-doped anatase nanopowders with mesostructures, which is higher than that of undoped samples.     

Synthesis and luminescence properties of Eu<Superscript>2+</Superscript>-doped Li<Subscript>2</Subscript>SrSiO<Subscript>4</Subscript>

We have studied the luminescence spectra of Li₂Sr_{1 − x} Eu _x SiO₄ (x = 0.0001–0.01) solid solutions prepared by solid-state reactions and a sol-gel process in a reducing atmosphere. The spectra show a broad band in the range 500–700 nm, centered at 578 nm, which is due to the 4f ⁶5d → 4f ⁷ transition. The luminescence excitation spectrum shows, in addition to bands due to Eu²⁺ 4f ⁷ → 4f ⁶5d transitions, a strong band centered at 174 nm, attributable to absorption in the SiO₄⁴⁻ group.     

Ferromagnetism in Electrospun Co-doped SrTiO3 Nanofibers

By employing electrospinning technique, subsequent calcination in air and annealing process in hydrogen, uniform Co-doped SrTiO₃ nanofibers with concentrations of Co between 0 and 0.20 were successfully produced. Their morphologies and detailed structures were characterized by scanning electron microscopy, transmission electron microscopy, and X-ray powder diffraction. And, the chemical states of Co were determined by X-ray photoelectron spectroscopy. It was shown that, after calcination, Co²⁺ was well incorporated into the perovskite structure of SrTiO₃, and the nanofibers possessed smooth surface with diameters of 50–100 nm. Magnetic properties of the hydrogen-annealed and -unannealed nanofibers were both measured by physical property measurement system from 50 to 300 K. It was explored that the Co addition and the hydrogen annealing process were both very important to the generation of the observed ferromagnetism in SrTi_1−x Co _x O₃:H₂ nanofibers. In hydrogen-annealed SrTi_0.80Co_0.20O₃:H₂ nanofibers, a saturation magnetization of 0.74 emu/g and an average moment of 0.122 μ_B/Co were obtained. The origins of the enhanced ferromagnetism in SrTi_1−x Co _x O₃:H₂ nanofibers were analyzed according to the chemical state of Co and the mediation of oxygen vacancies.     

1/f Noise properties of swift heavy ion irradiated epitaxial thin films of YBCO

Effect of 250 MeV¹⁰⁷Ag ion irradiation induced columnar defects on the noise properties of the YBCO superconductor in the normal and superconducting state have been investigated. Magnitude of the spectral density of the noise is found to scale inversely with the frequency and exhibit a quadratic dependence on the bias current confirming that the noise arises due to the resistance fluctuations. The magnitude ofS _v has been found to decrease with decrease in temperature and shows a noise peak in the transition region. The noise performance of these materials in the vicinity of the superconducting transition as well as in the normal state is found to improve by an order of magnitude after irradiation with 250 MeV¹⁰⁷Ag ions. The decrease in the magnitude of 1/f noise peak is due the irradiation induced enhanced flux pinning of the material which suppresses the flux motion induced noise in the vicinity ofT _c.     

Recovery of compressive strain in atactic polystyrene

Atactic polystyrene rods were compressed in an Instron universal testing machine to about 35% strain and then annealed near the glass transition temperature, T _g, (100° C) in a thermal mechanical analyser. The change of length during annealing was recorded and the compressive strain was found to recover obeying second-order kinetics at least during the later part of recovery. The activation enthalpy obtained from the temperature dependence of the second-order rate constant varied from 126 to 260 kcal mol^–1 as the annealing temperature decreased from 112 to 94° C. These activation enthalpies are attributed to the diffusion of positive or negative defects or configurations which annihilate during recovery. While these activation enthalpies agree with the findings of Andrews on retraction of hot-stretched filaments, they differ considerably from the spectrum obtained by Kimmel and Uhlmann using the data from Andrews' work. The reason for such differences is discussed.     

Nature of the Chemical Bond in Uranium Dioxide UO<Subscript>2</Subscript>

Fine structure of the X-ray photoelectron and conversion spectra of low-energy (0–40 eV) electrons of uranium dioxide UO₂ was analyzed based on the electronic structure calculations for the UO ₈ ¹²⁻ cluster with O _h symmetry, simulating the nearest surrounding of uranium in UO₂, by the relativistic X _α discrete variation method. It was predicted theoretically and validated experimentally that, in UO₂, the U5f electrons (∼1 U5f electron) can directly participate in chemical bonding: ∼2 U5f electrons weakly contributing to chemical bonding are localized at −1.9 eV; ∼1 U5f electron participating in chemical bonding is delocalized in the range of outer valence molecular orbital energies from −4 to −9 eV; and unfilled U5f states are localized mostly at low (from 0 to 5 eV above zero) energies. It was shown experimentally that the U6p electrons actively participate in formation of not only inner valence but also outer valence (0.6 U6p electron) molecular orbitals. The density of the U6p states in UO₂ was estimated experimentally. The composition and sequence of the inner valence molecular orbitals at energies within 13–40 eV were also elucidated.__________Translated from Radiokhimiya, Vol. 47, No. 3, 2005, pp. 193–202.Original Russian Text Copyright © 2005 by Yu. Teterin, Maslakov, Ryzhkov, Traparic, Vukcevic, A. Teterin, Panov.     

VUV spectroscopy of wide bandgap materials

In this paper we will present VUV spectroscopy experiments performed at the Superlumi station of Hasylab, DESY, Hamburg, on samples of BaF₂ crystals activated with Ce and BaF₂, (Ba,La)F₂ crystals activated with Er. The results of these experiments include time resolved luminescence and luminescence excitation spectra obtained under wavelength selective VUV and UV excitation by pulsed synchrotron radiation.We will reveal the information provided by the VUV/UV excitation spectra of the Ce³⁺ 5d → 4f as well as Er³⁺ 4fⁿ⁻¹5d → 4fⁿ and 4fⁿ → 4fⁿ emissions on energy transfer mechanisms from the fluoride host to the rare earth ion. We will demonstrate that the fast energy transfer channels involve bound excitons while the generation of free electrons and holes leads to slower processes dependant on hole and/or electron trapping.We will demonstrate that differences between the excitation spectra of the 5d → 4f emission in Ce and 4f¹⁰5d → 4f¹¹ emission in Er activated BaF₂ are generated by the coupling of the 4f → 5d transition to the 4f¹⁰ core of the Er³⁺ ion. We will also identify the additional band, absent for Ce, which is due to the exchange split high spin (HS) state of the 4f¹⁰5d configuration responsible for the slow decay of the excited Er³⁺ ions in BaF₂ and (Ba,La)F₂.Finally we will provide evidence and explain why the dominant VUV 4f¹⁰5d → 4f¹¹ Er³⁺ emission in BaF₂ is spin-forbidden and slow while in the mixed (Ba,La)F₂ crystals it is spin-allowed and fast.     

Red luminescence from hydrothermally synthesized Eu-doped ZnO nanoparticles under visible excitation

Eu-doped ZnO nanoparticles were synthesized by hydrothermal method. The Eu-dopant concentration has been varied by varying the amount of Eu-dopant concentration. These nanoparticles were structurally characterized by X-ray diffraction, transmission electron microscopy and selected area electron diffraction and it confirms the formation of nanoparticles having standard wurtzite structure. Photoluminescence studies show that these nanoparticles exhibit a sharp red luminescence due to the intra-4f transitions of Eu³⁺ ions at an excitation of 397 nm and 466 nm. Luminescence quenching is observed in the nanoparticles as the Eu-dopant concentration increases. Incorporation of Eu in the nanoparticles was confirmed by the energy dispersive X-ray studies.     

Preparation and properties of fluorozirconate glasses containing divalent europium

The glass forming region of the (ZrF₄BaF₂ThF₄)EuF₂ system has been studied. EuF₂ was found to replace up to about 85% of the BaF₂ in a glass with a composition .615 ZrF₄, .32BaF₂, .07ThF₄. The glass transition temperature is seen to increase with the EuF₂ concentration. The 4f⁷→ 4f⁶5d¹ transition of the Eu⁺² ion in the glass matrix was studied by optical absorption and compared with data taken on crystalline compounds containing Eu⁺² in different environments. It is concluded from these results that the Eu⁺² ion - and also the Ba⁺² ion - is coordinated by 12 fluorides in the glass.     

Impact of Nitrogen Pressure on the Structural, Morphologic and Magnetic Properties of the GaMnN Thin Films

GaMnN thin films were fabricated by using pulsed laser deposition followed by annealing. In our experiments, the impact of the background nitrogen pressure on the crystal quality, morphology and the ferromagnetism of the GaMnN thin films were studied systematically. It is found that the samples deposited at 0.75 Pa have the best crystal quality. All the annealed samples showed ferromagnetic behavior, and the Curie temperature was estimated up to 340 K. We found the magnetization of our samples changes depending on the concentration of Mn³⁺ ions, and the origin of the room temperature ferromagnetism of our samples can be explained by bound magnetic polaron theory. The nitrogen vacancies may cause the ferromagnetism to be weakened.     

Studies on the relaxor behavior of sol-gel derived Ba(Zr x Ti1− x )O3 (0.30≤x≤0.70) thin films

We have studied the relaxor behavior of sol-gel derived Ba(Zr _x Ti_{1− x} )O₃ (0.30≤ x≤0.70) thin films. The plausible mechanism of the relaxor behavior has been analyzed from the dielectric data and micro-Raman spectra. Substitution of Zr⁺⁴ for Ti⁺⁴ in BaTiO₃ lattice reduces its long-range polarization order yielding a diffused paraelectric to ferroelectric phase transition. The solid solution system is visualized as a mixture of Ti⁺⁴ rich polar region and Zr⁺⁴ rich regions and with the increase in Zr contents the volume fraction of the polar regions are progressively reduced. At about 25.0 at% Zr contents the polar regions exhibit typical relaxor behavior. The degree of relaxation increases with Zr content and maximizes at 40.0 at% Zr doped film. The frequency dependence of the polar regions follows Vogel-Fulcher relation with a characteristic cooperative freezing at freezing temperature (T _f). Below T_f, a long range polarization ordering was ascertained from the polarization hysteresis measurement.