Preparation and photoluminescence characteristics of Eu-doped MgAl1.8Y0.2O4 nanocrystals

A simple combustion route was employed for the preparation of Eu³⁺-doped MgAl_1.8Y_0.2−xO₄ nanocrystals using metal nitrates as precursors and urea as a fuel in a preheated furnace at 500 °C. The powders thus obtained were then fired at 1000 °C for 3 h to get better luminescent properties. The incorporation of Eu³⁺ activator in these nanocrystals was checked by luminescence characteristics. These nanocrystals displayed bright red color on excitation under 254 nm UV source. The main emission peak was assigned to the transition [⁵D₀→⁷F₂] at 615 nm. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies were carried out to understand surface morphological features and the particle size. Crystal structures of the nanocrystals were investigated by the X-ray diffraction (XRD) technique. The crystallite size of the as-prepared nanocrystals was around 29 nm, which was evaluated from the broad XRD peaks. The crystallite size increased to ∼45 nm on further heat treatment at 1000 °C.     

The structural and multiferroic properties of (Bi1−xLax)(Fe0.95Co0.05)O3 ceramics

La and Co co-doped BiFeO₃ ((Bi_1−xLa_x)(Fe_0.95Co_0.05)O₃ (x=0, 0.10, 0.20, 0.30)) ceramics were prepared by tartaric acid modified sol–gel method. The X-ray diffraction patterns indicate a transition from rhombohedral structure to tetragonal structure at x=0.20, which has been confirmed by the Raman measurements. The band gap increases with increasing x to 0.20, and then decreases with further increasing x to 0.30. The structural transition has significant effects on the multiferroic properties. The remnant magnetization and saturate ferromagnetic magnetization decrease abruptly with increasing x to 0.10, and then gradually increase with further increasing x up to 0.30. The coercivity is significantly reduced with increasing La doping concentration. The ferroelectricity has been improved by La doping, and the polarization increases with increasing x to 0.10, then decreases with further increasing x up to 0.30. The simultaneous coexistence of soft ferromagnetism and ferroelectricity at room temperature in tetragonal Bi_0.70La_0.30Fe_0.95Co_0.05O₃ indicates the potential multiferroic applications.     

Structural and electrical properties of nanometric Ni-Cu ferrites synthesized by citrate precursor method

Nanometric nickel copper ferrites Ni_1−xCu_xFe₂O₄, 0≤x≤0.45 were prepared by the citrate precursor method. X-ray diffraction measurements confirm the formation of single phase cubic spinel structure. The lattice parameter (a) is increased with increasing Cu²⁺ ion substitution. The crystallite size was calculated from XRD data and compared with that obtained from TEM micrographs. A significant increase in the density is observed with increasing Cu content. The IR absorption spectra were used for the detection and confirmation of the chemical bonds in spinel ferrites. The dielectric constant ε′ and dielectric loss showed a decrease with increasing frequency for all samples. The decrease in the ac conductivity was ascribed to the increase in hopping length.     

Synthesis and characterization of CoxZn1−xFe2O4 magnetic nanoparticles via a PEG-assisted route

We present an investigation of properties of Co_xZn_1−xFe₂O₄ (x=0.0-1.0) nanoparticles synthesized by a polyethylene glycol (PEG)-assisted hydrothermal route. X-ray powder diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) and vibrating scanning magnetometry (VSM) were used to characterize the structural, morphological and magnetic properties. The particle size obtained from TEM and XRD are consistent with each other. It was observed that the lattice constant for each composition decreases with increasing Co substitution and follows Vegard's law. Magnetization measurements show that while the materials with high Zn substitution are superparamagnetic at room temperature, they are ferromagnetic at temperatures lower than the blocking temperature. The materials with less Zn substitution are ferromagnetic below room temperature. Magnetizations and the coercivities of the samples decrease with the Zn substitution. The resultant overall magnetic behavior of the superparamagnetic samples are found to be considerably different than that of conventional superparamagnetic systems due to the antiferromagnetic interactions both in intra- and inter-cluster spins, and size (effective moment) distribution of the particles.     

Synthesis of nanocrystalline Zn0.5Mn0.5Fe2O4 via in situ polymerization technique

Nanocrystalline Zn_0.5Mn_0.5Fe₂O₄ was synthesized through the pyrolysis of polyacrylate salt precursors prepared via in situ polymerization of the metal salts and acrylic acid. The pyrolysis behavior of the polymeric precursors was studied by use of thermal analysis. The as-obtained product was characterized by powder X-ray diffraction (XRD), transmission electron microscope (TEM), electron diffraction (ED) pattern, scanning electron microscopy (SEM) and electron dispersive X-ray (EDX) analysis. The results revealed that the particle size is in the range of 15–25 nm for Zn-Mn ferrites with good crystallinity. Magnetic properties of the sample at 300 K were measured using a vibrating sample magnetometer, which showed that the sample exhibited characteristics of superparamagnetism.     

High heat generation ability in AC magnetic field for nano-sized magnetic Y3Fe5O12 powder prepared by bead milling

Nano-sized magnetic Y₃Fe₅O₁₂ ferrite having a high heat generation ability in an AC magnetic field was prepared by bead milling. A commercial powder sample (non-milled sample) of ca. 2.9 μm in particle size did not show any temperature enhancement in the AC magnetic field. The heat generation ability in the AC magnetic field improved with a decrease in the average crystallite size for the bead-milled Y₃Fe₅O₁₂ ferrites. The highest heat ability in the AC magnetic field was for the fine Y₃Fe₅O₁₂ powder with a 15-nm crystallite size (the samples were milled for 4 h using 0.1 mm? beads). The heat generation ability of the excessively milled Y₃Fe₅O₁₂ samples decreased. The main reason for the high heat generation property of the milled samples was ascribed to an increase in the Néel relaxation of the superparamagnetic material. The heat generation ability was not influenced by the concentration of the ferrite powder. For the samples milled for 4 h using 0.1 mm? beads, the heat generation ability (W g⁻¹) was estimated using a 3.58×10⁻⁴ fH² frequency (f/kHz) and the magnetic field (H/kA m⁻¹), which is the highest reported value of superparamagnetic materials.     

Effect of Mg substitution on the magnetic properties of NiCuZn ferrite nanoparticles prepared through a novel method using egg white

Nanocrystalline Mg-substituted NiCuZn ferrites were successfully synthesized, for the first time, by using metal nitrates and freshly extracted egg white. The thermal decomposition process of the nitrate-egg white precursors was investigated by thermogravimetric (TG) technique. X-ray diffraction (XRD) revealed that, single-phase cubic ferrites with average particle size of 23.9-35.1 nm were directly formed after ignition at 500 °C. No noticeable variation of lattice parameters with increasing magnesium content was observed, while X-ray densities were found to decrease. This can be explained on the basis of ionic radii and atomic masses of the substituted cation. Transmission electron microscope (TEM) shows that, particles are permanently magnetized and get agglomerated. The saturation magnetization (M_s) and coercivity (H_c) as a function of Mg content were investigated using vibrating sample magnetometer (VSM). It has been found that the M_s increases firstly up to x=0.2 and then decreases, while H_c continuously decreases. Magnetic susceptibility measurements give results which agree well with those obtained by VSM. The obvious decrease in the Curie temperature (T_C) with increasing Mg indicates that the ferrimagnetic grains are widely separated and enclosed by non-magnetic magnesium ions.     

The effect of different temperature annealing on phase relation of LaFe11.5Si1.5 and the magnetocaloric effects of La0.8Ce0.2Fe11.5−xCoxSi1.5 alloys

The phase relation of LaFe_11.5Si_1.5 alloys annealed at different high-temperature from 1223 K (5 h) to 1673 K (0.5 h) has been studied. The powder X-ray diffraction (XRD) patterns show that large amount of 1:13 phase begins to form in the matrix alloy consisting of α-Fe and LaFeSi phases when the annealing temperature is 1423 K. In the temperature range from 1423  to 1523 K, α-Fe and LaFeSi phases rapidly decrease to form 1:13 phase, and LaFeSi phase is rarely observed in the XRD pattern of LaFe_11.5Si_1.5 alloy annealed at 1523 K. With annealing temperature increasing from 1573  to 1673 K, the LaFeSi phase is detected again in the LaFe_11.5Si_1.5 alloy, and there is La₅Si₃ phase when the annealing temperature reaches 1673 K. There almost is no change in the XRD patterns of LaFe_11.5Si_1.5 alloys annealed at 1523 K for 3-5 h. According to this result, the La_0.8Ce_0.2Fe_11.5−xCo_xSi_1.5 (0≤×≤0.7) alloys are annealed at 1523 K (3 h). The analysis of XRD patterns shows that La_0.8Ce_0.2Fe_11.5xCo_xSi_1.5 alloys consist of the NaZn₁₃-type main phase and α-Fe impurity phase. With the increase of Co content from x=0 to 0.7, the Curie temperature T_C increases from 180 to 266 K. Because the increase of Co content can weaken the itinerant electron metamagnetic transition, the order of the magnetic transition at T_C changes from first to second-order between x=0.3 and 0.5. Although the magnetic entropy change decreases from 34.9 to 6.8 J/kg K with increasing Co concentration at a low magnetic field of 0-2 T, the thermal and magnetic hysteresis loss reduces remarkably, which is very important for the magnetic refrigerant near room temperature.     

Structure and electrical properties of Bi5GexSe95−x thin films

Bi₅Ge_xSe_95−x (30, 35, 40 and 45 at.%) thin films of thickness 200 nm were prepared on glass substrates by the thermal evaporation technique. The influence of composition and annealing temperature, on the structural and electrical properties of Bi₅Ge_xSe_95−x films was investigated systematically using X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX). The XRD patterns showed that the as-prepared films were amorphous in nature with few tiny crystalline peaks of relatively low intensity for 30 and 45 at.% and the Bi₅Ge₄₀Se₅₅ annealed film was polycrystalline. The chemical composition of the Bi₅Ge₃₀Se₆₅ film has been checked using energy dispersive X-ray spectroscopy (EDX). The electrical conductivity was measured in the temperature range 300-430 K for the studied compositions. The effect of composition on the activation energy (ΔE) and the density of localized states at the Fermi level N(E_F) were studied, moreover the electrical conductivity was found to increase with increasing the annealing temperature and the activation energy was found to decrease with increasing the annealing temperature. The results were discussed on the basis of amorphous-crystalline transformations.     

Structural and magnetic properties of Co1−xZnxFe2O4 nanoparticles by co-precipitation method

Co_1−xZn_xFe₂O₄ nanoparticles were prepared by co-precipitation method with x varying from 0 to 1.0. The powder samples were characterized by X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and Fourier transform infrared spectroscopy (FTIR). The average crystallite sizes of the particles were determined from XRD. X-ray analysis showed that the samples were cubic spinel. The average crystallite size (D_aveXR) of the particles precipitated was found to vary from 6.92 to 12.02 nm decreasing with the increase in zinc substitution. The lattice constant (a_o) increased with the increase in zinc substitution. The specific saturation magnetization (M_S) of the particles was measured at room temperature. The magnetic parameters such as M_S, H_c, and M_r were found to decrease with the increase in zinc substitution. FTIR spectra of the Co_1−xZn_xFe₂O₄ with x varying from 0 to 1.0 in the range 400–4000 cm⁻¹ were reported. The spinel structure and the crystalline water adsorption of Co_1−xZn_xFe₂O₄ nanoparticles were studied by using FTIR.     

Reentrant spin glass behavior and large initial permeability of Co0.5−xMnxZn0.5Fe2O4

A series of polycrystalline ferrites having nominal chemical composition Co_0.50−xMn_xZn_0.5Fe₂O₄ (0<x<0.4) have been synthesized by the solid-state reaction technique. The XRD analysis confirms single phase cubic spinel structure for all compositions. Lattice constant increases from 0.84195 to 0.84429 nm with the increasing Mn content and obeys Vegard's law. The average grain size increases by increasing both Mn content and sintering temperatures. Room temperature saturation magnetization increases for x=0.1 and decreases for increasing Mn content. The coercivity decreases with increasing Mn content due to the decrease of anisotropy constant. A reentrant spin glass behavior of these samples is observed from the zero field cooled magnetization measurements. The real part of the initial permeability increases by increasing both Mn content and sintering temperatures. This is due to the homogeneous grain growth and densification of the ferrites. The highest initial permeability 137 is observed for x=0.4 sintered at 1573 K on the other hand, the highest relative quality factor (2522) is obtained for the sample Co_0.2Mn_0.3Zn_0.5Fe₂O₄ sintered at 1523 K. The Mn substituted Co_0.50−xMn_xZn_0.5Fe₂O₄ ferrites showed improved magnetic properties.     

Structural and magnetic properties of nano-crystalline Ni–Zn ferrites synthesized using egg-white precursor

Nano-crystalline nickel–zinc ferrites of different compositions; Ni_1−xZn_xFe₂O₄ (x=0.0–1.0) were prepared by a precursor method involving egg-white and metal nitrates. An appropriate mechanism for the egg-white-metal complexation was suggested. Differential thermal analysis-thermogravimetry, X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), vibrating sample magnetometer and AC-magnetic susceptibility measurements were carried out to investigate chemical, structural and magnetic aspects of Ni–Zn ferrites. XRD confirmed the formation of spinel cubic structure. The average crystallite size was calculated using line broadening in XRD patterns. Structural parameters like lattice constant, X-ray density, bond lengths and inter-cationic distance were determined from XRD data. TEM showed agglomerated particles with average size agreed well with that estimated using XRD. FT-IR spectra confirm the formation of spinel structure and further lends support to the proposed cation distribution. Zn-content was found to have a significant influence on the magnetic properties of the system. The changes in the magnetic properties can be attributed to the influence of the cationic stoichiometry and their occupancy in the specific sites.     

Structure and magnetostriction of Tb0.2Pr0.8(Fe0.4Co0.6)1.93−xCx intermetallic compounds

The structure and magnetostriction of Tb_0.2Pr_0.8(Fe_0.4Co_0.6)_1.93−xC_x intermetallic compounds were studied by X-ray diffraction and magnetic measurements. Almost a single cubic Laves phase forms in the alloys for x ≤0.20, and a small amount of C can inhibit the formation of the 1:3 phase. The lattice parameter increases when 0≤x≤0.15, while the T_c and the spontaneous magnetization decreases with increasing x. The lattice parameter decreases slowly when 0.15≤x≤0.30, while the T_c decreases evidently with increasing x. The magnetostriction λ_a (=λ_∥-λ_⊥) is improved at low magnetic fields at room temperature for the compounds with 0.05≤x≤0.10, indicating that these C-containing compounds are promising magnetostrictive materials.     

Enhancing upconversion emissions of NaTm0.02YbxY0.98−xF4 nanocrystals through increasing Yb doping

By increasing the content of Yb³⁺ ions from 20% to 98% in NaTm_0.02Yb_xY_0.98−xF₄ (x=0.2−0.98) nanocrystals with size about 10 nm, the intensities of near infrared (800 nm) and blue (470 nm) upconversion (UC) luminescence can be enhanced by orders of 45 and 49 times, respectively, under 970 nm diode laser excitation. Pump power dependence illustrated that the 800 and 470 nm radiations are still two- and three-photon processes, respectively. TEM imaging showed that the enhancement is not from the change of the crystal size. Steady-state equation and the measured lifetimes indicated that the enhanced 800 nm radiations can induce the enhancement of the 470 nm emissions, which is in good agreement with the experimental data.     

Microstructures, surface bonding states and room temperature ferromagnetisms of Zn0.95Co0.05O thin films doped with copper

Zn_0.95−xCo_0.05Cu_xO (ZCCO, where x = 0, 0.005, 0.01 and 0.015) thin films were deposited on Si (1 0 0) substrates by pulsed laser deposition technique. Crystal structures, surface morphologies, chemical compositions, bonding states and chemical valences of the corresponding elements for ZCCO films were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and X-ray photoelectron spectroscopy (XPS). XRD and FESEM results indicate that crystallite sizes of the highly (0 0 2)-oriented ZCCO films slightly decrease with increasing Cu content. When the Cu content increases from 0 to 0.015, Zn 2p_3/2, Co 2p, Cu 2p_3/2 and O 1s peaks of the ZCCO film shift towards higher or lower binding energy regions, and the reasons for these chemical shifts are investigated by fitting the corresponding XPS narrow-scan spectra. Both in-plane and out-of-plane magnetization-magnetic field hysteresis loops of the ZCCO films reveal that all the films have room temperature ferromagnetisms (RTFMs). The conceivable origin of the RTFM is ascribed to the combined effects of the local structural disorder resulted from (Co²⁺, Cu²⁺, Cu¹⁺)-cations which substitute Zn²⁺ ions in the ZnO matrices, ferromagnetic coupling between coupled dopant atoms caused by Co²⁺ (3d⁷4s⁰) and Cu²⁺ (3d⁹4s⁰) spin states, and exchange interactions between the unpaired electron spins originating from lattice defects induced by Cu doping in the Zn_0.95Co_0.05O matrices.     

Composition-dependent structural and electrical properties of PbSe1−xTex thin films

Polycrystalline PbSe_1−xTe_x ingots were prepared by solid-state microwave synthesis. Thin films of PbSe_1−xTe_x were then deposited onto clean glass substrates using vacuum evaporation technique. Their nanostructure morphologies and stoichiometric ratio were examined using scanning electron microscopy (SEM) and energy dispersive X-ray spectra (EDX). X-ray diffraction (XRD) patterns indicated that the lattice constants of PbSe_1−xTe_x powders and thin films increased with the increasing amount of Te. From the electrical property measurements, the thin films were characterized by n-type behavior.     

Investigations on structural and optical properties of Zn1−xGdxS nanoparticles

Zn_1−xGd_xS (x = 0.00, 0.02 and 0.04) nanoparticles were synthesized by facile chemical co-precipitation method using PVP as a surfactant. ZnS nanoparticles could be doped with Gd ions during synthesis without altering the XRD patterns of ZnS. Also, the pattern of the powders showed cubic zincblende structure. The particle size obtained from the XRD studies lies in the range 3-5 nm, whereas from TEM analysis it is 4 nm for x = 0.02 sample. The UV-Vis absorption spectra revealed that Zn_1−xGd_xS nanoparticles exhibit strong confinement effect as the blue shift in the absorption spectra with that of the undoped ZnS. The photoluminescence spectra showed enhanced luminescence intensity and the entry of Gd into host lattice.     

A novel UV-emitting phosphor: Li6CaB3O8.5:Pb

Pure Li₆CaB₃O_8.5 and Li₆Ca_1−xPb_xB₃O_8.5 (0.005≤x≤0.04) materials were prepared by a solution combustion synthesis method. The phase of synthesized materials was determined using the powder XRD and FTIR. The synthesized materials were investigated using spectrofluorometer at room temperature. The emission and excitation bands of the synthesized phosphors were observed at 307 and 268 nm, respectively. The dependence of the emission intensity on the Pb²⁺ concentration for the Li₆Ca_1−xPb_xB₃O_8.5 (0.005≤x≤0.04) was studied and observed that the optimum concentration of Pb²⁺ in phosphor is 0.01 mol. The Stokes shift of the synthesized phosphor was calculated to be 4740 cm^-1.     

The effects of Cr-doping on the room temperature ferromagnetism of chemically synthesized CeO2−δ nanoparticles

We report an investigation of the nature of room-temperature ferromagnetism enhancement in Ce_1−xCr_xO_2−δ nanoparticles (0.00≤x≤0.05), synthesized by a sol–gel-based method. Energy-dispersed X-ray spectrometry (EDS) analysis was used to estimate the dopant concentrations. The average crystallite sizes and particle size were estimated by X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. Both studies showed a gradual decrease in the size of the crystallites and particles for x>0.01. Cr can substitute for Ce in the crystal lattice, and the Raman measurements indicated that structural defects in the samples increased as a function of the Cr content in the CeO₂ crystal lattice. The surface topography, examined by scanning electron microscopy (SEM), showed that the undoped sample has a porous and loosely organized structure, whereas the Cr-doped samples exhibited a dense and compact structure. Magnetic measurements of the Ce_1−xCr_xO_2−δ samples at 27 ^oC showed a maximum remanent magnetization value of 0.01 emu/g for x=0.05. The nature and enhancement of room-temperature ferromagnetism was interpreted by taking into account the exchange interaction between Cr³⁺ ions and oxygen vacancies in CeO₂.     

Synthesis and magnetic properties of La-substituted M-type Sr hexaferrites

Single-phase M-type hexagonal ferrites Sr_1−xLa_xFe₁₂O₁₉ (0≤x≤1) were prepared by a ceramic route. The stability limits of the ferrite phases were determined with a combination of various microscopy techniques, electron-probe micro-analysis, powder X-ray diffraction and thermal analysis. SrFe₁₂O₁₉ (x=0) is stable up to 1420 °C, whereas LaFe₁₂O₁₉ (x=1) exists between 1360 and 1400 °C only. The lattice parameters of Sr_1−xLa_xFe₁₂O₁₉ exhibit a linear variation with x, i.e. a₀ slightly increases and c₀ decreases with x, leading to a decrease of the unit cell volume with x. The saturation magnetization at T=5 K decreases with increasing La concentration. Room temperature Mössbauer analysis shows that the Fe³⁺/Fe²⁺ valence change occurs in the 2a sites for the whole composition range.