Mössbauer and magnetic studies in nickel ferrite nanoparticles: Effect of size distribution

The magnetic properties of nickel ferrite nanoparticles in the form of powders, prepared by the sol-gel process and subjected to different annealing temperatures, were investigated using both static and dynamic measurements namely hysteresis, zero field cooled-field cooled magnetization (ZFC-FC) measurements and Mössbauer spectroscopy. The Transmission Electron Microscopy (TEM) studies reveal particle sizes for the as-prepared particles which increases upto 52 nm with annealing. A bimodal distribution, upto an annealing temperature of was observed. ZFC-FC measurements for the as-prepared samples reveal twin peaks, indicative of the bimodal size distribution. ZFC-FC measurements performed for fields varying from 100 Oe to 3 kOe show a superparamagnetic phase with blocking temperatures between 320 and . Numerical simulations for the ZFC-FC studies indicate that the signature of the bimodal size distribution can be seen only at very low fields. The variation of coercivity with particle size, as determined from the hysteresis measurements, shows a transition from a single domain to a multi domain state for particle sizes larger than 35 nm. Mössbauer measurements performed at room temperature for the as-prepared sample shows a six finger pattern for the samples with higher particle size and a doublet pattern for the samples with smaller particle size, which is indicative of their superparamagnetic nature.     

Problems with power content beam parameter product

The M² beam propagation factor is widely used to characterize the quality of laser radiation and its propagation. When M² is defined by the second-moments, M² ? 1 holds in the paraxial approach. For many applications it is more convenient to use the power content values (normally η = 86.5%), also proposed by ISO. For the corresponding power content factor, it is often assumed that also holds. We have demonstrated previously that for a superposition of two coherent Gauss-Laguerre modes with radial symmetry, the 86.5% value of [6]. In recent years, has also been presented experimentally for a superposition of axially shifted Gaussian beams [7]. The problems with power content for axial superposition of Gaussian beams are discussed. In this paper it is shown that the 86.5% power content value can not be smaller than one for a coherent superposition of axially shifted Gaussian beams with radial symmetry presented in Ref. [7]. A superposition of two Gaussian beams with different waists and without shift is also discussed, and the corresponding of such beam can be smaller than one, depending strongly on the power content value η. For low power content values η and a large (or very small) ratio of the two different waists approaches zero. These investigations demonstrate that is not a suitable parameter to characterize laser radiation.     

Magnetic properties of ErN films

We report a magnetization study of stoichiometric ErN nanocrystalline films grown on Si and protected by a GaN passivating layer. According to the temperature dependence of the resistivity the films are heavily doped semiconductors. Above 100 K the magnetization data fit well to a Curie-Weiss behavior with a moment expected within the free-ion Er³⁺ multiplet. Below 50 K the Curie-Weiss plot steepens to an effective moment corresponding to that in the crystal-field determined quartet ground state, and develops a clear paramagnetic Curie-Weiss temperature of about 4.5 K. Zero-field- and field-cooled magnetization curves and the AC susceptibility firmly establish a ferromagnetic ground state within that multiplet below a Curie temperature of . Due to the (1 1 1) texture of the film the comparison between the magnetization behavior, when the field is applied parallel and perpendicular to the film plane, gives new information about the magnetic structure. An arrangement of the moments according to the model derived from neutron diffraction for bulk HoN is strongly suggested.     

Surface reconstruction of clean bcc-Fe{1 1 0}: A quasi-hexagonal top-layer with periodic height modulation

Hexagonal-pillar shaped pure Fe single crystal whiskers with six {1 1 0} side planes were obtained by means of chemical vapor deposition. Atomically resolved scanning tunneling microscopy images obtained on the {1 1 0} surface showed a quasi-hexagonal atomic array with mesoscopic-range periodic height modulation of about 1/3 of an atomic step. This height modulation was found to be a result of an interference between the quasi-hexagonal top-layer and the sub-surface bcc-Fe{1 1 0} layer. Unit vectors of the mesoscopic-range modulation turned out to be expressed as , where and are the primitive vectors of the two-dimensional atomic array in the top-layer and those in the sub-surface layer, respectively. The two-dimensional density of atoms in the top-layer is slightly higher by 0.46% than that in the sub-surface layer.     

The 2ν4 overtone bands of FClO3

The high resolution infrared spectrum of the mono-isotopic species F³⁵Cl¹⁶O₃ has been studied in the region of the 2ν₄ overtone, from 2560 to 2680 cm⁻¹. The perpendicular component is strong and clearly observed while the parallel component is very weak and almost completely hidden by the perpendicular one. Their origins differ by 12.6 cm⁻¹, the being located at higher wavenumbers. The band is perturbed by the anharmonic interaction between the v₄ = 2, l₄ = ?2 and v₂ = v₄ = v₅ = 1, l₄ = l₅ = ±1 excited states, both of E symmetry. In total 3157 transitions have been assigned, 83% of these to , 12% to , and 5% to . The three bands have been analyzed simultaneously, taking into account the Fermi resonance effective between the excited states of E symmetry. The ro-vibration parameters of the excited states have been obtained, including the deperturbed band origins of and , at 2628.5890(4) and 2619.3342(5) cm⁻¹, respectively. The W₂₄₅ anharmonic constant is equal to 4.0161(4) cm⁻¹. The x₄₄+g₄₄ and x₂₄+x₄₅+g₄₅ anharmonicity constants have been derived from the obtained band origins and those of ν₄ and ν₂ + ν₅.     

Absorption spectra of AsH2 radical in 435-510 nm by cavity ringdown spectroscopy

The absorption spectra of jet-cooled AsH₂ radicals were recorded in the wavelength range of 435-510 nm by cavity ringdown spectroscopy. The AsH₂ radicals were produced by pulsed DC discharge in a molecular beam of a mixture of AsH₃, SF₆, and argon. Seven vibronic bands with fine rotational structures have been identified and assigned as the , , and (n = 1-3) bands of the electronic transition. Based on the previous studies of AsH₂ radical, rotational assignments and rotational term values for each band were obtained, and the molecular parameters including vibrational constants, rotational constants, centrifugal distortion constants, and spin-rotation interaction constants were also determined.     

Stretch-bend combination polyads in the ÃAu state of acetylene, C2H2

Rotational analyses are reported for a number of newly-discovered vibrational levels of the S₁-trans (ùA_u) state of C₂H₂. These levels are combinations where the Franck-Condon active and vibrational modes are excited together with the low-lying bending vibrations, and . The structures of the bands are complicated by strong a- and b-axis Coriolis coupling, as well as Darling-Dennison resonance for those bands that involve overtones of the bending vibrations. The most interesting result is the strong anharmonicity in the combinations of (trans bend, a_g) and (in-plane cis bend, b_u). This anharmonicity presumably represents the approach of the molecule to the trans-cis isomerization barrier, where ab initio results have predicted the transition state to be half-linear, corresponding to simultaneous excitation of and . The anharmonicity also causes difficulty in the least squares fitting of some of the polyads, because the simple model of Coriolis coupling and Darling-Dennison resonance starts to break down. The effective Darling-Dennison parameter, K₄₄₆₆, is found to increase rapidly with excitation of , while many small centrifugal distortion terms have had to be included in the least squares fits in order to reproduce the rotational structure correctly. Fermi resonances become important where the K-structures of different polyads overlap, as happens with the 2¹3¹B¹ and 3¹B³ polyads (B = 4 or 6). The aim of this work is to establish the detailed vibrational level structure of the S₁-trans state in order to search for possible S₁-cis (¹A₂) levels. This work, along with results from other workers, identifies at least one K sub-level of every single vibrational level expected up to a vibrational energy of 3500 cm⁻¹.     

Repulsive and bound parts of the interatomic potentials of the lowest singlet electronic energy states of the MeRg complexes (Me = Zn, Cd; Rg = He, Ne, Ar, Kr, Xe)

Laser-induced fluorescence excitation spectra of MeRg (Me = Zn, Cd; Rg = He, Ne, Ar, Kr, Xe) complexes were recorded using the D¹ ← X¹ free ← bound transition. The complexes were produced in their ground state in a free-jet expansion beam and excited with a dye-laser beam directly to the excited state. Analysis of free ← bound unstructured profiles provided a shape of the repulsive part of the D¹-state potentials. Valence ab initio calculations of the ZnRg and CdRg ground- and excited-state potentials and electronic transition dipole moments for the studied transition were performed, taking scalar relativistic and spin-orbit effects into account. Results of the calculations show regularities and correlations in the repulsive branches and bound wells of the X¹- and D¹-state potentials as well as provide information on the bonding character in both electronic energy states. The trends were compared with available experimental results for ZnRg and CdRg as well as for MgRg and HgRg.     

Observation of room temperature photoluminescence in proton irradiated SrTiO3 single crystal

We have irradiated SrTiO₃ single crystal with 3 MeV-proton (H⁺) beam and found that blue -, green - and infrared - frequency photoluminescence (PL) are induced simultaneously at room temperature. TEM and EELS analyses show that an oxygen-deficient amorphous layer is formed at the crystal surface by the proton irradiation. Possible origin of the PL-effect is discussed.     

Low field magnetocaloric effect and heat capacity of A-site ordered NdBaMn2O6 manganite

The magnetocaloric effect (MCE) in the A-site ordered manganite NdBaMn₂O₆ is studied. The MCE in this compound has an anomalous behavior. In low magnetic fields, the abrupt transitions between the direct and inverse magnetocaloric effects are observed. In a relatively strong magnetic field , the direct and inverse effects are observed only at cooling, while the heating mode reveals only an inverse MCE. The value of the MCE ( and for  kOe) does not reach high values, but the proximity of the effects occurring at room temperatures expects the use of both effects in the magnetic cooling technology.     

Equilibrium structure in the presence of internal rotation: A case study of cis-methyl formate

The Born-Oppenheimer (BO) equilibrium molecular structure () of cis-methyl formate has been determined at the CCSD(T) level of electronic structure theory using Gaussian basis sets of at least quadruple-ζ quality and a core correlation correction. The quadratic, cubic and semi-diagonal quartic force field in normal coordinates has also been computed at the MP2 level employing a basis set of triple-ζ quality. A semi-experimental equilibrium structure () has been derived from experimental ground-state rotational constants and the lowest-order rovibrational interaction parameters calculated from the ab initio cubic force field. To determine structures, it is important to start from accurate ground-state rotational constants. Different spectroscopic methods, applicable in the presence of internal rotation and used in the literature to obtain “unperturbed” rotational constants from the analysis and fitting of the spectrum, are reviewed and compared. They are shown to be compatible though their precision may be different. The and structures are in good agreement showing that, in the particular case of cis-methyl formate, the methyl torsion can still be treated as a small-amplitude vibration. The best equilibrium structure obtained for cis-methyl formate is: r(C_m-O) = 1.434 Å, r(O-C_c) = 1.335 Å, r(C_m-H_s) = 1.083 Å, r(C_m-H_a) = 1.087 Å, r(C_c-H) = 1.093 Å, r(CO) = 1.201 Å, ∠(COC) = 114.4°, ∠(CCH_s) = 105.6°, ∠(CCH_a) = 110.2°, ∠(OCH) = 109.6°, ∠(OCO) = 125.5°, and τ(H_aCOC) = 60.3°. The accuracy is believed to be about 0.001 Å for the bond lengths and 0.1° for the angles.     

A theoretical-spectroscopy, ab initio-based study of the electronic ground state of SbH3

For the stibine isotopologue , we report improved theoretical calculations of the vibrational energies below 8000 cm⁻¹ and simulations of the rovibrational spectrum in the 0-8000 cm⁻¹ region. The calculations are based on a refined ab initio potential energy surface and on a new dipole moment surface obtained at the coupled cluster CCSD(T) level. The theoretical results are compared with the available experimental data in order to validate the ab initio surfaces and the TROVE computational method [Yurchenko SN, Thiel W, Jensen P. J Mol Spectrosc 2007;245:126-40] for calculating rovibrational energies and simulating rovibrational spectra of arbitrary molecules in isolated electronic states. A number of predicted vibrational energies of are provided in order to stimulate new experimental investigations of stibine. The local-mode character of the vibrations in stibine is demonstrated through an analysis of the results in terms of local-mode theory.     

Strong correlation effects and new phase transition at high pressure-low temperature in La0.5Ba0.5FeO3

The physical properties of La_0.5Ba_0.5FeO₃ perovskite have been investigated. The resistivity and magnetism change at around the charge disproportionation temperature. The ferromagnetic cluster-glass state appears when . Under 5 kbar pressure a new phase transition at 5.5 K is found in the magnetization measurement. The transition temperature increases with the increase of the applied pressure. It is suggested that this transition is induced by the spin state transition from to with the pressure increase.     

Effect of high temperature treatments on defect centers and impurities in hydrothermally grown ZnO

Deep level transient spectroscopy (DLTS) has been employed to study electron traps in hydrothermally grown n-type ZnO samples after thermal treatments up to 1500 °C. Schottky barrier contacts were formed by e-beam evaporation of Pd, followed by DLTS and secondary ion mass spectrometry (SIMS) measurements in order to investigate possible correlations between electron traps in the upper part of the band gap and the concentration of the most prominent impurities. The DLTS results show three different levels having energy positions of , , and (E_c denotes the conduction band edge). The SIMS results showed that the most pronounced impurities were Li, Al, Si, Mg, Fe, Mn, and Ni with concentrations up to . A decrease in the level is observed after temperature treatments above 1300 °C, and in the same temperature range the Li concentration drops from ∼10¹⁷ to . However, based on absolute concentration values an association between Li and the level can be ruled out. In contrast, the level, which is not stable above 1300 °C, may be associated with Li but further experimental data are needed to substantiate this assignment. The level occurred in selected samples and is presumably impurity-related but no correlation was found with the main impurities detected by SIMS. Except for Li, the concentration of all the impurities remained essentially constant as a function of heat treatment temperature.     

One-dimensional defects in iodine adlayers on Pt(1 0 0)

One-dimensional defect structures of closed-packed adlayers of iodine on Pt(1 0 0) were studied with scanning tunneling microscopy (STM). On the terraces of the Pt(1 0 0) surface we observed rotational domains with line defects running in [0 1 0] directions, in coexistence with nearly defect-free domains. In addition to these prevailing line defects (A-defects) with a local coverage lower than that of a defect-free surface, we report on much less frequently observed line defects with higher local coverages (B- and C-defects). The strong dependence of the concentration of these defects on the adsorption temperature is governed by the decrease of the overall iodine coverage with increasing temperature. Iodine adsorption at ∼1100 K leads to self-organization of A-defects in quasi-periodic arrangements. The relevance of these defects as important structural elements of commensurate superstructures of iodine on Pt(1 0 0) is stressed.     

Temperature-dependent infrared absorption cross sections of methyl cyanide (acetonitrile)

Pressure broadened (1 atm. N₂) absorption cross sections and integrated band intensities have been derived from laboratory spectra of CH₃CN, recorded at 276, 298, and 323 K, covering 600-. The spectra were recorded at a resolution of using a commercial Fourier transform spectrometer and a custom flowing sample delivery system. We report integrated absorption cross sections for intervals corresponding to the most prominent bands, compare the results with previously reported values, and discuss error sources, which are estimated as ∼7% with systematic error the largest error source.     

Preparation of CaWO4:Ln@SiO2 (Ln=Tb, Dy and Ho) nanoparticles by a combustion reaction and their optical properties

The CaWO₄:Ln³⁺@SiO₂ (Ln=Tb, Dy and Ho) nanoparticles were synthesized via a combustion process at 800 °C, using citric acid as chelating agent and fuel, ammonium nitrate as fuel, boric acid as flux material and silica as supports. The persistent phosphor nanoparticles were characterized by X-ray diffraction (XRD), reflectance UV-vis and fluorescence spectroscopy (PL) and transmission electron microscopy (TEM) techniques. XRD patterns indicated that crystalline calcium tungstate with scheelite structure was produced. The reflectance UV-vis spectra showed the broad absorption band of groups and the PL spectra showed the wide excitation band, broad emission band of and characteristic emissions of Ln³⁺ ions. The average particle sizes were determined by TEM, which are about 50 nm.