FexNi100−x nanometric films deposited by laser ablation on SiO2/Si substrates

Fe_xNi_100−x nanometric films were deposited on SiO₂/Si substrates at room temperature using the pulsed laser deposition technique. The targets were Fe-Ni amorphous magnetic foils with composition Fe₅₀Ni₅₀, Fe₃₅Ni₆₅ and Fe₂₂Ni₇₈. Morphological and structural properties of the deposited films were investigated using scanning electron microscopy, Rutherford backscattering spectrometry, grazing incidence X-ray diffraction, and X-ray reflectivity. Electrical and magnetic characteristics of the films were investigated by using the four-point probe and the magneto-optic Kerr effect techniques, respectively. The film properties are strictly dependent on the Fe-Ni compositional ratio.     

Electrical properties and spin fluctuations studies of Y(Co1−xNix)2 compounds

Synthesis by arc melting, the structural and the electric properties of Y(Co_1−xNi_x)₂ alloys were studied by X-ray diffraction (XRD) and four probe dc electrical measurements. XRD analysis (300 K) shows that all samples crystallize in a cubic MgCu₂-type structure. The lattice parameters linearly decrease with Ni content. Electrical resistivity for the Y(Co_1−xNi_x)₂ intermetallic series was measured in a temperature range of 15-1100 K. The parameters involved in the dependence of resistivity on temperature were determined. Residual, phonon and spin fluctuations resistivity were separated from electrical resistivity using both the Matthiesen formula and the Bloch-Gruneisen formula. The spin fluctuations resistivity of the Y(Co_1−xNi_x)₂ series are compared to the mean square amplitudes of spin fluctuations previously calculated by the Linear Muffin Tin Orbital-Tight Binding Approach method for these series in the literature. The contribution of spin fluctuations to total resistivity ρ_sf is proportional to T² at low temperatures. The proportionality parameter strongly reduces across the Y(Co_1−xNi_x)₂ series.     

The structure and luminescence properties of long afterglow phosphor Y3−xMnxAl5−xSixO12

A series of phosphors with the composition Y_3−xMn_xAl_5−xSi_xO₁₂ (x=0, 0.025, 0.050, 0.075, 0.150, 0.225, 0.300) were prepared with solid state reactions. The X-ray powder diffraction analysis of samples shows that the substitution of Mn²⁺ and Si⁴⁺ does not change the garnet structure of phosphors, but makes the interplanar distance decrease to a certain extent. The emission spectra show that Mn²⁺ in Y₃Al₅O₁₂ emits yellow-orange light in a broad band. With the increment of substitution content, the emission intensity of the phosphors increases firstly then decreases subsequently, and the emission peak moves to longer wavelength. Afterglow spectra and decay curves show that all the Mn²⁺ and Si⁴⁺ co-doped samples emit yellow-orange light with long afterglow after the irradiation of ultraviolet light. The longest afterglow time is 18 min. Thermoluminescence measurement shows that there exist two kinds of traps with different depth of energy level and their depth decreases with the increment of substitution content.     

X-ray photoemission and magnetometric studies of valence changes in Ce(Cu1−xNix)4Ga

The compounds Ce(Cu_1−xNi_x)₄Ga crystallize in the hexagonal CaCu₅-type structure for the whole doping range 0≤x≤1. The border compounds CeCu₄Ga and CeNi₄Ga represent a heavy fermion and fluctuating valence systems, respectively. We report on the studies of the valence evolution in Ce(Cu_1−xNi_x)₄Ga employing the X-ray photoemission spectroscopy (XPS) and magnetic susceptibility measurements. The photoemission of the Ce 3d peaks shows a gradual decrease of the occupation of the f states with Ni content. Simultaneously, the hybridization strength and the low temperature magnetic susceptibility are reduced. Within the valence band spectrum a transition from the dominance of the Cu 3d to the dominance of the Ni 3d states is well visible with the traces of the Ce 4f¹ states for up to x=0.5.     

Magnetic properties of GdxY1−xNi5 alloys

Magnetic measurements have been performed on single crystals of Yni₅ and GdNi₅ and on polycrystalline samples of Gd_xY_1?xNi₅. YNi₅ is a Pauli paramagnet, the susceptibility of which is enhanced by exchange and slightly temperature dependent (χ = 23 × 10^?4 emu/mole at 4.2 K). In GdNi₅, where the anisotropy is very weak, a polarization of the d band opposite to the Gd magnetization is observed. At 4.2 K, this polarization is 0.16 ± 0.02μ_B/Ni. The decrease of this polarisation with decreasing magnetic interactions has been studied in Gd_xY_1?xNi₅-type alloys. It is concluded that the polarization is more homogeneous than that of cobalt observed in Gd_xY_1?xCo₂.     

Characterization of Cd1−xFexS diluted magnetic semiconductors grown at near phase conversion temperature

Fe-based cadmium sulfide alloy thin films have been grown on c-plane sapphire substrates by a low-pressure metalorganic chemical vapor deposition technique at different growth temperatures. From X-ray diffraction and absorption spectra of the samples, the evolutions with growth temperature show an inflexion at the growth temperature of 300 ^°C. This was attributed to the phase transformation from zinc-blende to wurtzite. With increasing growth temperature from 270 ^°C to 360 ^°C, Fe concentration in the films increases monotonously. The electronic states of Cd_1−xFe_xS were investigated by X-ray photoelectron spectroscopy. Magnetic measurement shows Van Vleck paramagnetism of the Cd_1−xFe_xS thin film in the temperature region below 7 K.     

The dependence of the structural and optical properties on the Te mole fraction in ZnS1−xTex/GaAs heterostructures

Lattice-mismatched ZnS_1−xTe_x epilayers with various Te mole fractions on GaAs (100) substrates were grown by double well temperature gradient vapor deposition. X-ray diffraction patterns showed that the grown ZnS_1−xTe_x layers were epitaxial films. The photoluminescence spectra showed that the peak position of the acceptor-bound exciton (A⁰, X) varied dramatically with changing the Te mole fraction and that the behavior of the (A⁰, X) peak position of the ZnS_1−xTe_x epilayers with a small amount of the Te mole fraction was attributed to a bowing effect. The reflectivity and ellipsometry spectra showed that the absorption energy peak was significantly affected due to the Stoke's effect. These results provide important information on the structural and optical properties of ZnS_1−xTe_x/GaAs heterostructures for improving optoelectronic device efficiencies operating in the spectral range between near ultraviolet and visible regions.     

Magnetic,electronic and structural properties of ZnxFe3−xO4

A series of samples Zn_xFe_3−xO₄ have been prepared by the chemical coprecipitation technique and characterized by X-ray diffraction (XRD), vibrating sample magnetometry (VSM) and X-ray photoelectron spectroscopy (XPS). XRD demonstrates all the samples of Zn_xFe_3−xO₄ have a spinel structure same as Fe₃O₄. The magnetic hysteresis loops of Zn_xFe_3−xO₄ obtained from VSM indicate that the saturation magnetization has a maximum when x is ∼1/3. The chemical states of Fe atoms and Zn atoms in zinc ferrites have been measured using XPS and Auger electron spectroscopy (AES). The Fe 2p core-level XPS spectra and Zn L₃M₄₅M₄₅ Auger peaks have been analyzed and the results have been discussed in correlation with the samples’ magnetic properties. These results suggest most of Zn atoms occupy the tetrahedral sites and a small amount of them occupy the octahedral sites.     

Surface composition and electronic structure of Ni2+xMn1−xGa studied by X-ray photoelectron spectroscopy

The surface of Ni_2+xMn_1−xGa have been studied using X-ray photoelectron spectroscopy under different conditions of surface preparation. We find that the surface becomes Ni rich by argon ion sputtering. However, the change in surface composition due to sputtering can be reverted by annealing at appropriate temperature so that the bulk composition is restored at the surface. The origin of a satellite feature observed both in the valence band and the Ni 2p core-level spectra is discussed. The magnetic microstructures at the surface have been observed by using magnetic force microscopy.     

Mössbauer studies and magnetic studies of Ni doped orthoferrites PrFe1−xNixO3 (x≤0.3)

The magnetic properties of polycrystalline PrFe_1−xNi_xO₃ (x≤0.3) system were studied using Mössbauer spectroscopy and magnetization measurements. The Mossbauer spectra exhibit six line spectra which loses its sharpness as the Ni substitution increases within the system. As the Ni concentration in the system increases, the hyperfine field and isomer shift shows decrease, which is vivid from the sluggish nature of the sextets. The small value of quadrupole splitting confirms the octahedral environment of the Fe⁺³ ions. The magnetization curves show the reversible behavior and represent the fall in negative molecular field leading to AFM frustration. From these results, we conclude that sagging in the spectra reveals the change from antiferromagnetic state to ferromagnetic state, which can be attributed to mixed state of Fe⁺³ ions i.e. high spin (HS) and low spin (LS) which is a consequence of progressive collapse of Hund’s rule due to HS→LS transition. These results confirm the weak ferromagnetic component due to canted-AFM spin arrangement of Fe³⁺ magnetic moments.     

Study of the quasi two-dimensional CoxNi1-xTa2O6 compounds by X-ray diffraction and magnetic susceptibility measurements

We report results on the structural and magnetic properties of the Co_xNi_1−xTa₂O₆ series of compounds by X-ray powder diffraction, magnetic susceptibility and magnetization measurements. X-ray refinements carried out by the Rietveld method show that these compounds crystallize in a P4₂/mnm tetragonal structure. Magnetic susceptibility curves show a broadened maximum witnessing that these compounds exhibit two-dimensional antiferromagnetic behaviors. All the Co_xNi_1−xTa₂O₆ compounds order below 10 K and present a large ion anisotropy. The magnetic properties have been determined in both the paramagnetic and antiferromagnetic state. In the hypothesis of two dimensional AF ordering, the near neighbor exchange constants (J₁) and the next near neighbor exchange constants for two different paths (J₂ and J'₂) were determined. The composition dependence of the magnetic properties including ordering temperature, exchange constants and anisotropy factors are discussed. The drastic reduction of the ordering temperature for x=0.20 for Co_xNi_1−xTa₂O₆, suggest the hypothesis of a peculiar magnetic behavior for this composition.     

Effect of Li substitution on the magnetic properties of LixMg0.40Ni0.60−2xFe2+xO4 ferrites

Structural and magnetic properties of a series of polycrystalline spinel type ferrites with the nominal chemical composition Li_xMg_0.40Ni_0.60−2xFe_2+xO₄, where x=0.0-0.3 in steps of 0.05, were investigated thoroughly. The formation of spinel structure was confirmed by X-ray diffraction analysis. Lattice constants and average grain diameters increased with increase in Li content. The real part of the initial permeability (μ^/_i) is found to increase not only with increase in Li content up to x=0.25 but also with the increase in sintering temperature up to 1100 °C. However, it decreases for further increase in sintering temperature except for x=0 and 0.05 compositions. The grain size dependent μ^/_i is also observed clearly in this research. From the magnetization as a function of applied magnetic field plots, it is clear that all samples exhibit ferrimagnetic state at room temperature and have a low saturation field. The magnetization obtained is explained with the help of redistribution of cations in the tetrahedral and octahedral sites and spin canting due to weakening of exchange interaction.     

Magnetic transitions in Ni1−xMox and Ni1−xWx disordered alloys

We present a mean-field study of the magnetic phase diagram of Ni_1−xMo_x and Ni_1−xW_x alloys. The pair energies that enter the internal energy part of the free energy are obtained from a first-principles calculation. We try to understand why spin-glass phase is not observed in these alloys.     

Optical properties and deep levels in annealed Si1−xMnx bulk materials

The optical properties and the deep levels in bulk Si_1−xMn_x formed by using an implantation and annealing method were investigated. Transmission electron microscopy, X-ray diffraction, and Hall-effect measurements showed that the annealed bulk Si_1−xMn_x samples were p-type crystalline semiconductors. The photoluminescence spectra for the annealed bulk Si_1−xMn_x material showed luminescence peaks corresponding to excitons bound to neutral acceptors and related to dislocations due to the existence of Mn impurities. Deep-level transient spectroscopy results for the annealed bulk Si_1−xMn_x showed deep levels related to the interstitial and substitutial sites of the Mn⁺ ions. These results can help improve understanding of the optical properties and the deep levels in annealed bulk Si_1−xMn_x material.     

Raman scattering from GaSe1−xTex

The long-wavelength optical phonons of the layer GaSe_1?xTe_x have been investigated at room temperature by means of Raman scattering spectroscopy. The spectra of the Bridgman grown crystals were excited with the 1,06 μm line of the continuously operated YAG:Nd³⁺ laser. Detailed study of the Raman spectra of GaSe_1?xTe_x solid solutions showed that there is an abrupt change in the frequency-composition dependences for all observed modes. It is shown, that a phase transition from hexagonal ?-GaSe to monoclinic GaTe in GaSe_1?xTe_x solid solutions takes place in the composition range 0.27 ? × ? 0.72. Only one mode behaviour of the optical phonons was observed in GaSe_1?xTe_x system.     

EPR and magnetic properties of vapour phase grown Zn1−xCrxTe crystals

The EPR and magnetic properties were investigated on vapour phase grown Zn_1−xCr_xTe (0.001?x?0.005) crystal samples at room temperature. The EPR spectra were observed for samples with x=0.001, 0.002 only. The simulations of the spectra confirm Cr³⁺ charge state of the dopant ion at tetrahedral symmetry. The magnetic behaviour of the samples with x=0.001 and 0.002 is neither that of Brillouin paramagnets nor Van Vleck systems while the samples with compositions x=0.003, 0.004 and 0.005 exhibited hysteresis behaviour.     

NMR and magnetic susceptibility of CeCu5−xNix intermetallic compounds

The ⁶³Cu Knight shift and magnetic susceptibility of the CeCu_5?xNi_x intermetallic system have been investigated as a function of temperature. The compounds studied were all paramagnets in the temperature range 90–700K with θ_p < 0. The results obtained reveal a reversal s-f coupling with positive J_sf and J_sd coupling constants.     

Thermal dependence of impurity induced fluorescence in the system RbMgxMn1-xF3

The absorption spectrum of RbMnF₃ and the excitation spectra of the system RbMg_xMn_1-xF₃ at 10 K as well as the fluorescence spectra and lifetimes of Mn²⁺ in the systems RbMg_xMn_1-xF₃ and KMg_xMn_1-xF₃ in the region 10–300 K were measured. The lifetime and fluorescence temperature dependence suggest that the origin of the fluorescence occurs at Mn²⁺ sites slightly perturbed by impurity ions and that a non-radiative energy transfer mechanism is responsible for the observed thermal quenching. By using different Mn²⁺ concentrations in the above systems the dependence of the energy transfer on the Mn²⁺ concentration is shown. Finally, a preliminary observation on laser stimulated Mn²⁺ luminescence in the system RbMg_xMn_1-xF₃ is reported.     

Ab initio calculation of Ni50−xFexTi50

The structure, lattice dynamics and electronic band structure of Ni_43.75Fe_6.25Ti₅₀ were obtained using ab initio calculations. The phonon dispersion relations and phonon density of states were calculated using the direct method. The stability of Ni_50−xFe_xTi₅₀ structure for x=0.0, 6.25, 12.5, 25 has been investigated and shown that the orthorhombic structure is the most stable phase for x=25.