Effects of hydroxyls on the structural and room temperature ferromagnetic properties of Co doped SnO2 nanoparticles

Co doped SnO₂ nanoparticles have been prepared via a wet chemical method with different precipitation processes. The structure and morphology of Co doped SnO₂ nanoparticles demonstrate that the nanoparticles are in a rutile single phase and uniform, respectively. X-ray photoelectron spectroscopy shows that the Co dopants are in 2+ oxidation valence state and doped ∼2 atm% in SnO₂ nanoparticles. Moreover, Raman spectroscopy further confirms that Co doped SnO₂ nanoparticles have single phase crystallinity without forming any extra modes related to secondary phases. The magnetic measurements reveal that all nanoparticles exhibit room temperature ferromagnetism (RTFM) due to the presence of disorders and defects introduced by hydroxyls in the crystal structure. In addition, it has been clearly observed that the saturated magnetic moments are strongly affected by the precipitation processes which control the incorporation of hydroxyls into the lattice.     

Precision measurement of theg-factor of the 2+ γ-vibrational state of160Dy and test of theF-spin purity

The rotations of the 392-966 keV and the 299–966 keV directional correlations of¹⁶⁰Dy in the static hyperfine field of DyTb have been measured with high precision. A neutron activated Tb single crystal was used which was magnetized to saturation at 4.2 K by an external magnetic field of 4.5 T applied in the direction of theb-axis. The agreement of both rotations proves that the relaxation of the 4f-shell after the-decay takes less than 10 ps. A quantum mechanical calculation within the magnon model gave a relaxation time of the order of 1 ps. Theg-factor of the 2⁺ -vibrational state was derived as

K X-ray fluorescence cross-sections for some light elements excited by keV photons

K and K X-ray fluorescence cross-sections have been experimentally determined for the elements Cu, Se, Y, and Mo at excitation energies 23.62, 24.68, 36.82, 43.95, 48.60, and 50.20keV using an X-ray tube with a secondary exciter system as the excitation source. The X-ray tube with a secondary target arrangement was used to obtain high intensity with high degree of monochromatization. Experimental values were compared with the theoretical values using tabulated I ratios based on Hartree-Fock and Hartree-Slater theories calculated by Scofield. The experimental values for all the elements at various excitation energies are in good agreement with the theoretical values.     

The effect of contamination of dielectric target surfaces under electron irradiation

The influence of the contamination film formed under the electron bombardment of the sample surface on the conditions of experimental studies using analytical electron-probe apparatus (scanning electron microscopes, X-ray microanalyzers) is considered. The accompanying artifacts, namely the decreased effective value of the secondary electron emission coefficient and the shifted value of the second crossover energy of primary electrons are calculated.     

Photoluminescence of Anisotropically etched silicon

Visible room-temperature luminescence of Anisotropically Chemically Etched (ACE) silicon under spontaneous chemical surface modification in HNO₃:HF solution is reported. The material is investigated by SEM, AES, IR transmission and Raman scattering methods.     

Excitation power controlled luminescence switching in Yb-Tm co-doped hexagonal NaYF4 nanorods

Intense infrared-to-visible up-conversion (UC) emissions were obtained in hexagonal Yb³⁺-Tm³⁺ co-doped NaYF₄ nanorods under excitation at 980 nm. Especially, luminescent switching between different UC emission wavelengths at 800, 480 and 450 nm were observed by adjusting excitation powers. Based on power-dependent spectral analyses, it was found that the cooperative energy transfer between Yb³⁺-Yb³⁺ pairs and Tm³⁺ ions play a key role on the luminescent switching besides the saturation effect of Yb³⁺²F_5/2 and Tm³⁺¹G₄ excited states. Our results indicate that hexagonal NaYF₄ nanostructures have potential applications in miniaturized solid-state laser, optical processing sensors and fluorescent biolabels.     

Calculations of localized modes on surface and impurity layer embedded in a semi-infinite Heisenberg ferromagnet

We investigate the magnetic excitations for the magnetic problem arising from the absence of magnetic translation symmetry in one dimension due to the presence of an impurity layer embedded within a semi-infinite ferromagnet. A Heisenberg model is employed to investigate the possibility that localized modes can occur with an impurity layer implanted within a semi-infinite ferromagnet. No electronic effects are considered. The theoretical approach employs the matching procedure in the mean field approximation and determines the propagating and evanescent spin amplitude fields including the contribution due to an applied field. The results are used to calculate the energies of localized modes associated with the impurity layer and with the surface. Numerical examples of the modes are given and they are found to exhibit various effects due to the interplay between the impurity layer and surface modes. It is shown that more localized modes can occur and the modification of the spin wave spectra can be signaled by the appearance of surface and impurity modes, besides the bulk excitations. Also, the bulk spin fluctuations field, the spin waves localized on the surface as well as on impurity layer depend are shown to depend on the nature of the exchange coupling between spin sites, the values of spin sites and the position of the impurity layer from the surface.     

Temperature dependence of the photoluminescence and phosphorescence time decay of magnesia-stabilized zirconia

Temperature dependence of photoluminescence (PL) spectra and time decay ranging from 90 to 330 K are investigated in magnesia-stabilized zirconia single crystals. The emission PL spectra can be decomposed into two bands. The prominent one is centered in the blue-green region of the spectrum whereas the secondary one is centered in the yellow-orange region. The temperature dependence of these bands are analyzed in terms of the so-called configuration coordinate model. The Huang-Rhys parameter for the prominent band is found near 40 and the effective phonon at about 0.030 eV. Thermal quenching energy is determined to be 0.24 eV from the decreasing part of the I(T) curve. Luminescent decays were satisfactorily fitted by two exponentials over the whole temperature range investigated. Total lifetime temperature dependence can be accounted for by assuming a radiative decay from two metastable levels with a separation energy of 0.073 eV. Results are discussed on the basis of the major defects, oxygen vacancies and complex defects.     

Raman and photoluminescence spectroscopy of zinc tungstate powders

ZnWO₄ powders, synthesized using co-precipitation technique and annealed in air at different temperatures in the range of 80-, were studied by Raman and photoluminescence spectroscopy. ZnWO₄ single crystal was used for comparison. The interpretation of the observed variations of the Raman spectra and intrinsic photoluminescence band upon annealing is suggested.     

Dopant distribution, oxygen stoichiometry and magnetism of nanoscale Sn0.99Co0.01O2

In recent work, we have shown that chemically synthesized Sn_1−xCo_xO₂ nanoscale powders with x≤0.01 are ferromagnetic at room temperature when prepared by annealing the reaction precipitate in the narrow temperature window of 350-600 ^°C. Combined high resolution x-ray photoelectron spectroscopy (on as-prepared and Ar⁺ ion sputtered samples), x-ray diffraction and magnetometry measurements showed that the Co distribution is more uniform throughout the individual Sn_0.99Co_0.01O₂ particles when prepared at lower annealing temperatures of 350-600 ^°C and this uniform dopant distribution is essential to produce stable high temperature ferromagnetism. However, surface segregation of the dopant atoms in samples annealed at >600 ^°C destroys the room-temperature ferromagnetic behavior and reduces the Curie temperature to <300 K.