Magnetism in BiFe<Subscript>1−x</Subscript>Ni<Subscript>x</Subscript>O<Subscript>3</Subscript>: studied through electron spin resonance spectroscopy

The effect of Ni doping in BiFe_1?xNi_xO₃ (BFNO) multiferroics are studied by X-ray diffraction (XRD), Fourier transmission infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), hysteresis loop (M–H), temperature dependent magnetization (FC-ZFC) measurements and electron spin resonance (ESR) techniques. The XRD and FTIR studies indicate that the BFNO compounds remain in rhombohedral (R3c) phase without appearance of any structural transformation due to Ni doping. The XPS studies show the oxidation states of Fe ions as 3⁺, whereas Bi is found to be in a mixed valence state of 2⁺ and 3⁺ in BFNO. The Ni ion doping enhances the saturation magnetization from 0.179 emu/g (x?=?0.025) to 2.38 emu/g (x?=?0.20), which is higher than the reported values found in literature. The FC-ZFC magnetization studies suggest the presence of a magnetic phase transition from a weak ferromagnetic to a spin glass state at low temperature. The ESR studies confirm the ferromagnetic state of BFNO samples.     

Microscopic and macroscopic magnetic properties of MgAl<Subscript>x</Subscript>Cr<Subscript>x</Subscript>Fe<Subscript>2 − 2x</Subscript>O<Subscript>4</Subscript> spinel ferrite system

In order to study the effect of co-substitution of Al^{3 +} and Cr^{3 +} for Fe³⁺ in MgFe₂O₄ on its structural and magnetic properties, the spinel system MgAl _x Cr_xFe_{2 ? 2x}O₄ (x = 0.0–0.8) has been characterized by X-ray diffraction, high field magnetization, low field a.c. susceptibility and ⁵⁷Fe Mössbauer spectroscopy measurements. Contrary to the earlier reports, about 50% of Al^{3 +} is found to occupy the tetrahedral sites. The system exhibits canted spin structure and a central paramagnetic doublet was found superimposed on the magnetic sextet in the Mössbauer spectra (0.5 > x > 0.2). Thermal variation of a.c. susceptibility exhibits normal ferrimagnetic behaviour.     

Laser and electromagnetic loss properties of Perovskite SmNi<Subscript>x</Subscript>Fe<Subscript>1−x</Subscript>O<Subscript>3</Subscript>

SmNi_xFe_1?xO₃ (0?≤?x?≤?0.5) with perovskite-type structure has been successfully prepared by conventional solid-state reaction as a microwave and laser multi-functional material. The optimized synthesis temperature and the effects of Ni doping on the reflectivity, electromagnetic loss properties were investigated in details. XRD results shown that synthesis temperature did not change the perovskite structure of SmFeO₃. The reflectivity at 1.06 μm was about 0.33% at 1200–1300?°C. Doping Ni did not cause the change of perovskite structure. The incorporation of Ni in SmFeO₃ contributed to the decrease of reflectivity at a wider wavelength, SmNi_0.3Fe_0.7O₃ possessed the lowest reflectivity at 1.06 μm. Moreover, electromagnetic property was very sensitive to Ni content. The real and imaginary parts of complex permeability were enhanced remarkably at a certain frequency. The changes in magnetic performance provided possibility of choosing specific frequency of magnetic loss. The difference in electric and magnetic losses caused by Ni concentration could result in microwave absorption at different frequency. In a word, SmNi_xFe_1?xO₃ could be a promising candidate for a multi-functional material with compatible camouflage capability for radar and laser waveband.     

Effect of annealing on exchange bias in Mg<Subscript>0.2</Subscript>Fe<Subscript>x</Subscript>Cr<Subscript>1.8−x</Subscript>O<Subscript>3</Subscript> nano oxides

Magnesium-iron chromium oxides (Mg_0.2Cr_1.8?x Fe _x O₃ with x varying from 0.3 to 0.9) produced by hydrothermal process in a stirred pressure reactor from pure metal chlorides have been annealed at 700 °C. Single phase corundum structure and nanophase structure of the as-synthesized samples were confirmed by X-ray diffraction (XRD). Instead of the correlation between H _EB and D _XRD observed at T _A = 600 °C, we find significant changes. The H _EB increases with decreasing particle size reaches a maximum at ~43 nm (x = 0.5) then decreases.     

Structural Stability of SrCo<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>x</Subscript>O<Subscript>3 − δ</Subscript> Solid Solutions

SrCo_{1 − x} Fe_xO_{3 − δ} solid solutions with 0.3 ≤ x ≤ 0.9 are shown to have the cubic perovskite structure. The unit-cell parameter and volume of the solid solutions are nonmonotonic functions of Fe content, with a minimum at x = 0.4. Dilatometric data are used to determine the thermal expansion coefficients of the solid solutions. At low oxygen partial pressures ( ≤ 40 Pa), the high-temperature, disordered perovskite phase exists between 850 and 1000°C, which is the optimal temperature range for the effective use of SrCo_{1 − x} F_xO_{3 − δ} ceramics as oxygen membranes in oxygen partial pressure gradients of 10⁴–10⁵/10–100 Pa.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 8, 2005, pp. 998–1004.Original Russian Text Copyright © 2005 by Kokhanovskii, Zonov, Ol’shevskaya, Pan’kov.     

The optical and electrical transport studies of Zn<Subscript>x</Subscript>Co<Subscript>1−x</Subscript>S thin films

In an attempt to design and fabricate a suitable II–VI group material of variable optical gap, we have synthesized a series of Zn_xCo_1?xS (0 ≤ x ≤ 0.4) thin films via a facile chemical solution growth technique. To gain insight of the materials properties we have opted for different characterization techniques and are reporting our observations pertaining to the elemental analysis, magneto-topography, optical and electrical transport studies. Excellent agreement of binding energy values for Co2p, Zn2p and S2p levels in elemental analysis concluded the oxidation states as Co²⁺, Zn²⁺ and S^2?. Magnetic force microscopy confirmed the existence of randomly distributed magnetic domains mimicking the surface topography. The optical studies determined the high absorption coefficient (α ≈ 10⁴ to 10⁵ cm^?1) in the as-grown thin films. The optical band gap is found to be increased non-linearly from 1.59 to 2.50 eV as the composition parameter (x) is increased. The D.C. electrical conductivity measurements showed decrease in conductivity with increased composition parameter (x). The thermoelectric studies confirmed degenerative nature of the as-deposited thin films with n-type conduction.     

Y<Subscript>3−x</Subscript>Gd<Subscript>x</Subscript>Fe<Subscript>5</Subscript>O<Subscript>12</Subscript>: controlled synthesis,characterization and investigation of its magnetic properties

Gadolinium substituted yttrium iron garnet (Gd: YIG: Y_3?xGd_xFe₅O₁₂ where x?=?0, 0.25, 0.5, 0.75, 1, 1.25, 1.5) nanopowders were synthesized by microwave hydrothermal method. Phase structure of synthesized powders was examined using fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) which revealed a cubic garnet structure. Spherical-like morphology of synthesized powders was confirmed by field emission scanning electron microscopy (FESEM) studies. The synthesized powders were sintered at 1100?°C for 60 min using microwave furnace and characterized by using XRD and FESEM. Magnetic properties such as saturation magnetization and Curie temperature were measured using vibration sample magnetometer (VSM) and differential scanning calorimetry (DSC) studies respectively. Temperature stability of magnetization was measured using pulse magnetometer and complex permeability was measured over a frequency range of 100 kHz–1.8 GHz. The obtained results showed that the saturation magnetization as well as permeability decrease while curie temperature and temperature stability increase with Gd concentration. It is concluded that substitution of Gd makes the YIG better microwave magnetic material, which may be used in high power non-reciprocal microwave devices in the microwave region.     

Coexistence of Superconductivity and Ferromagnetism in Ni-Doped Bi<Subscript>4 −x</Subscript>Ni<Subscript>x</Subscript>O<Subscript>4</Subscript>S<Subscript>3</Subscript> (0.075 ≤<Emphasis Type="Italic">x</Emphasis>≤ 0.150)

The effects of Ni doping on the physical properties of the newly discovered layered superconductor Bi₄O₄S₃ are studied. X-ray diffraction data indicates that the lattice constants a and c decrease with the increasing Ni doping. From resistivity-temperature curves, the superconducting transition temperature (\(T_{c}^{\text {onset}})\) is suppressed by only 0.5 K with the increase of Ni doping from 0.075 to 0.15; the \(T_{c}^{\text {zero}}\) is almost the same constant at different Ni ions’ doping level. The magnetic susceptibility results suggest the coexistence of superconductivity and ferromagnetism in this system. A possible superconductivity transition is observed around ?14 K from M-T (FC) curves in x = 0.125, 0.15 samples, which may result from the doped magnetic Ni ions.     

Dielectric and impedance studies of (Pb<Subscript>1−x</Subscript>Ca<Subscript>x</Subscript>)(Fe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>)O<Subscript>3</Subscript> dielectric ceramics

Ceramic samples of (Pb_1?xCa_x)(Fe_0.5Nb_0.5)O₃ with x = 0.20, 0.40, 0.45, 0.50, 0.55 and 0.60 were obtained by columbite precursor method. All the synthesized samples have perovskite structure with pseudo-cubic symmetry. Dielectric properties of all the samples were measured as a function of frequency from room temperature up to 573 K. Two dielectric anomalies were observed in ε_r–T plots at about 400 and 500 K. The impedance analysis depicts a single relaxation process. Activation energies obtained from temperature dependence of relaxation frequency, f₀ and grain resistance, R_g were found to be more or less comparable. The observed relaxation in all the samples seems to be due to electron relaxation associated with oxygen vacancies.     

Cation distribution in nanocrystalline Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> spinel ferrites

We have studied the cation distribution over the tetrahedral and octahedral sites in the spinel structure of nanocrystalline Ni _x Zn_{1 ? x} Fe₂O₄ ferrites prepared by spray pyrolysis. ⁵⁷Fe Mössbauer spectroscopy data for the ferrites demonstrate that, depending on the composition of the materials, the tetrahedral site may accommodate only Fe³⁺ (inverse spinel, x ≥ 0.4) or both Fe³⁺ and Zn²⁺ cations (mixed spinel, x = 0 and 0.2), which accounts for the fact that the composition dependence of the unit-cell parameter for the ferrites deviates from Vegard’s law.     

Effects on dielectric and magnetic properties of Cr-doped Bi<Subscript>0.9</Subscript>Ba<Subscript>0.1</Subscript>Fe<Subscript>1−x</Subscript>Cr<Subscript>x</Subscript>O<Subscript>3</Subscript> ceramic

In this paper, Cr-doped Bi_0.9Ba_0.1Fe_1?xCr_xO₃ (x = 0.00, 0.05, 0.10, 0.15, 0.20) ceramic materials were prepared by traditional state solid synthesized method, and the effects of Cr³⁺ ion on magnetic and dielectric properties were investigated. All samples showed BiFeO₃ phase formation were successful synthesized. The SEM images showed the shape of samples changed from regular to irregular shape. With increasing of Cr²⁺ ions, Saturation magnetization (M _s) increased from 5.24 to 8.6 emu/g, and then decreased to 7.31 emu/g, and coercivity (H _c) increased from 110.66 to 256.49 Oe. All the samples showed high dielectric constants at low frequency and the values of dielectric constants decreased slightly with frequency increasing. Delectric loss (tanδ) values kept a steady in a wide range frequency of 10–600 MHz. They ranged in tanδ from 0.01 to 0.07, which was a low dielectric loss in Bi_0.9Ba_0.1Fe_1?xCr_xO₃ ceramics.     

Chemical,morphological, structural,optical, and magnetic properties of Zn<Subscript>1−x</Subscript>Nd<Subscript>x</Subscript>O nanoparticles

In the present investigation, we made an endeavor to fabricate the ZnO nanoparticles and achieved the tunable properties with Nd doping. The Nd-doped ZnO nanoparticles were characterized via X-ray diffraction (XRD), Raman, and X-ray photoelectron spectroscopy (XPS) studies that confirmed the successful doping of Nd ions in the ZnO crystal lattice without amending its hexagonal phase. The particle morphology revealed nearly spherical particles with uniform size distribution. The band gap of these samples was determined using diffuse-reflectance spectra (DRS) and was found to vary from 3.17 to 3.21 eV with increasing Nd concentration. A broad and intense emission band at 1083 nm for Nd doped ZnO nanoparticles is observed and is assigned to corresponding emission transition ⁴F_3/2?→?⁴I_11/2 of Nd³⁺ ions. Furthermore, the magnetic studies indicate that the Nd doping altered the magnetic behavior of nanocrystalline ZnO particles from diamagnetic to ferromagnetic at 300 K and that the magnetization of these samples decreased with increasing Nd concentration. The tunable optical band gap as well as room-temperature ferromagnetism of these samples may find applications in both optoelectronics and spintronics.     

Structural and electrical studies of Cd<Subscript>1−x</Subscript>Zn<Subscript>x</Subscript> Te thin film by vacuum evaporation technique

Cd_1−xZn_xTe (where x = 0.02, 0.04, 0.06, 0.08) thin film have been deposited on glass substrate at room temperature by thermal evaporation technique in a vacuum at 2 × 10⁻⁵ torr. The structural analysis of the films has been investigated using X-ray diffraction technique. The scanning electron microscopy has been employed to know the morphology behaviour of the thin films. The temperature dependence of DC electrical conductivity has been studied. In low temperature range the thermal activation energy corresponding to the grain boundary—limited conduction are found to be in the range of 38–48 μeV, but in the high temperature range the activation energy varies between 86 and 1.01 meV. The built in voltage, the width of the depletion region and the operating conduction mechanism have been determined from dark current voltage (I–V) and capacitor-voltage (C–V) characteristics of Cd_1−xZn_xTe thin films.     

Characterization and magnetic studies of Sr<Subscript>2</Subscript>Fe<Subscript>x</Subscript>Mo<Subscript>2−x</Subscript>O<Subscript>6</Subscript> (0.8 ≤ x ≤ 1.5) double perovskites prepared by sonochemical method

Synthesis of Sr₂Fe_xMo_2?xO₆ (x?=?0.8 to 1.5 with a step increment of 0.1) (SFMO) double perovskite oxides were done by sonochemical method at relatively low preparation temperature (950 °C). Purity and existence of all elements (Sr, Fe, Mo, and O) in SFMO compound have been analyzed by energy dispersive X-ray spectroscopy (EDS). Lattice parameters and unit cell volume were studied by X-ray powder diffraction (XRD). The structural transition from tetragonal to cubic occurs around composition x?=?1.1 has been noticed from XRD data. Scanning electron microscopy (SEM) confirmed grain sizes of SFMO series, which are found to be in the range of 75–185 nm. Perovskite formation has been confirmed by Fourier transform infrared spectroscopy (FTIR) spectrum analysis. Investigations of Magnetic properties have been carried out using Vibrating Sample Magnetometer (VSM) at applied magnetic field from ??20 to +?20 kOe. Saturation magnetization is found to increase from x?=?0.8 to 1 and then decreases from x?=?1 to 1.5, the behavior is correlated with degree of Fe/Mo ordering.     

La<Subscript>1−x</Subscript>Ag<Subscript>x</Subscript>FeO<Subscript>3</Subscript>/halloysites nanocomposite with enhanced visible light photocatalytic performance

La_1?xAg_xFeO₃/halloysites nanotubes (HNTs) nanocomposite was synthesized by sol–gel method. It was characterized by X-ray diffraction, transmission electron microscope, Fourier transform infrared spectroscopy and UV–visible diffused reflectance spectroscopy measurements. The photo-activity of the La_1?xAg_xFeO₃/HNTs nanocomposite was evaluated via degradation of methylene blue (MB) under visible-light irradiation. The results showed that the HNTs with unique pore structure favored the adsorption of organic molecules. Adequate Ag⁺ doping improved the absorption ability for visible light. The La_0.95Ag_0.05FeO₃/HNTs demonstrated the best photocatalytic performance, which achieved as high as 99 % for MB degradation exposed 2 h irradiation. However,further increasing of Ag⁺ doping gradually reduced the photocatalytic activity. The nanocomposite catalyst showed outstanding recyclability after eight cycles which still remained up to 90 %.     

Microstructure and properties of the Ba<Subscript>0.75−x</Subscript>Ca<Subscript>x</Subscript>La<Subscript>0.25</Subscript>Fe<Subscript>11.6</Subscript>Co<Subscript>0.25</Subscript>Ti<Subscript>0.15</Subscript>O<Subscript>19</Subscript> hexaferrites

In the case of Ti⁴⁺ remain unchanged, the Ca²⁺ substituted Ba_0.75?xCa_xLa_0.25Fe_11.6Co_0.25Ti_0.15O₁₉ (0?≤?x?≤?0.05) were prepared by conventional solid-state reaction method at temperature of 1280 °C. A ball-to-power weight ratio of 10:1. Their crystal structure and magnetic properties were mainly investigated. The results show that the single magnetoplumbite phase structure transformed into the multiphase structure. Meanwhile, the small amount of α-Fe₂O₃ phase existed in M-type phase. The micrographs were observed by a field emission scanning electron microscopy (SEM). Vibrating sample magnetometer (VSM) was used to analyze the magnetic properties. The saturation magnetization (M _s ) first increases then decreases when x from 0 to 0.03. But, when x from 0.03 to 0.05, the saturation magnetization (M _s ) first increases then decreases too. The maximum value is at x?=?0.04 (M _s ?=?70.73 emu/g). The value of coercivity (H _c ) first increases then decreases when x from 0 to 0.04. But, the value increased when x from 0.04 to 0.05. The maximum value is at x?=?0.02 (H _c ?=?1691 Oe).     

Spectral and current-voltage characteristics of Si-Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>x</Subscript> heterostructures grown by liquid phase epitaxy

The spectral and current-voltage (I–V) characteristics of Si-Si_1?xGe_x heterostructures grown by liquid phase epitaxy on silicon substrates were studied. The dependence of the longwave photosensitivity boundary of these structures on the variband solid solution composition was determined. It is shown that these variband solid solutions can serve as transition buffer layers between silicon substrates and a structure based on a different semiconductor. These structures can be employed in elements converting a part of the IR solar radiation in cascade solar cells and in photodetectors for the optical fiber communication lines transmitting signals with the wavelengths λ=1.33 and 1.5 μm.     

Influence of Dy-doping on ferroelectric and dielectric properties in Bi<Subscript>1.05−x</Subscript>Dy<Subscript>x</Subscript>FeO<Subscript>3</Subscript> ceramics

Bi_1.05−xDy_xFeO₃ (BDFO) (x = 0−0.2) ceramics were synthesized by solid-state reaction method. The influence of Dy dopant on crystal structural, dielectric and ferroelectric properties was investigated. The lattice parameter and the Curie temperature of BDFO were degraded continuously with increasing contents of Dy³⁺ cations. Leakage current density, ferroelectric polarization and dielectric loss were improved by appropriate Dy doping. When x = 0.1, BDFO showed the best electric properties. At applied electric field of 53 kV/cm, the remnant polarization (2P _r ) was 12.2 μC/cm².These improvements in electric properties in BDFO ceramics could have resulted from the relatively low oxygen vacancy concentration and structural distortion.     

Fluorine doping in dilute magnetic semiconductor Sn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2</Subscript>

Recent studies have reported room-temperature ferromagnetism (FM) in Fe doped SnO₂. The FM in semiconductors due to transition metal doping has been argued to be carrier mediated. Fluorine (F) doping in pure SnO₂ has been reported to significantly increase the carrier concentration. In this work, we investigated the role of F doping in the range from 0% to 0.79% on the FM of chemically synthesized single phase Sn_1?x Fe _x O₂ using X-ray diffraction, UV–Vis spectrophotometry, particle-induced X-ray emission, particle-induced gamma ray emission and magnetometry. The saturation magnetization M _s (0.03 emu/g) increased by a factor of 2.5 and the lattice volume and band gap energy decreased by 0.35 Å³ and 0.2 eV, respectively, with 0.67% F doping (F/Sn atom %) compared to the sample without any fluorine.     

Wide range magnetoresistance and high temperature coefficient of resistance in La<Subscript>0.7</Subscript>Sr<Subscript>0.3−x</Subscript>Ag<Subscript>x</Subscript>MnO<Subscript>3</Subscript> system

We report observation of composition and process parameter dependant wide range magnetoresistance and high temperature coefficient of resistance in La_0.7Sr_0.3−xAg_xMnO₃ (0 ≤ x ≤ 0.3) system. The polycrystalline samples synthesized through solid state reaction method exhibited rhombohedral structure with systematic change in lattice parameters. The samples with x ≤ 0.2 showed metallic behavior below room temperature. A broad metal-insulator transition is observed for x = 0.3, which becomes sharp for the samples sintered at high temperature. The broad transition resulted in magnetoresistance (MR) value around 35% over a wide temperature range whereas the sharp transition results in MR value as high as 85%. The temperature coefficient of resistance value of 11% is seen as a consequence of sharp transition. The magnetic transition was sharp for the samples sintered at high temperature.