Synthesis and magnetic properties of Gd doped EuS nanocrystals with enhanced Curie temperatures

EuS nanocrystals (NCs) were doped with Gd resulting in an enhancement of their magnetic properties. New EuS and GdS single source precursors (SSPs) were synthesized, characterized, and employed to synthesize Eu(1-x)Gd(x)S NCs by decomposition in oleylamine and trioctylphosphine at 290 °C. The doped NCs were characterized using X-ray diffraction, transmission electron microscopy, and scanning transmission electron microscopy, which support the uniform distribution of Gd dopants through electron energy loss spectroscopy (EELS) mapping. X-ray absorption spectroscopy (XAS) revealed the dopant ions in Eu(1-x)Gd(x)S NCs to be predominantly Gd(3+). NCs with a variety of doping ratios of Gd (0 ≤ x < 1) were systematically studied using vibrating sample magnetometry and the observed magnetic properties were correlated with the Gd doping levels (x) as quantified with ICP-AES. Enhancement of the Curie temperature (T(C)) was observed for samples with low Gd concentrations (x ≤ 10%) with a maximum T(C) of 29.4 K observed for NCs containing 5.3% Gd. Overall, the observed T(C), Weiss temperature (θ), and hysteretic behavior correspond directly to the doping level in Eu(1-x)Gd(x)S NCs and the trends qualitatively follow those previously reported for bulk and thin film samples.     

Microstructure,dielectric and piezoelectric properties of (Pb1−xSrx)Nb1.96Ti0.05O6 ceramics

(Pb_1−xSr_x)Nb_1.96Ti_0.05O₆ with 2 wt% excess PbO (x = 0, 0.02, 0.04, 0.06, 0.08) piezoelectric ceramics with high Curie temperature were fabricated via the conventional solid state reaction method. Effects of Sr²⁺ amount on crystallite structure, microstructure, dielectric and piezoelectric properties were studied. The substitution of Sr²⁺ ions for Pb²⁺ ions is effective to lower sintering temperatures. X-ray diffraction patterns indicate that all ceramics form the single orthorhombic ferroelectric phase. The doping of Sr²⁺ ions facilitates improving densification of the ceramics. Grain size and lattice parameters of the ceramics vary with the change of the Sr²⁺ contents. Both Curie temperature and maximum dielectric constant change with increasing the Sr²⁺ amounts. The dielectric constant data were also studied using the Curie–Weiss law and modified Curie–Weiss law. The ceramic with x = 0.04 possesses excellent piezoelectric and dielectric properties, presenting a high potential to be used in high-temperature applications as piezoelectric transducers.     

Magnetic properties of lanthanide chalcogenide semiconducting nanoparticles

To understand the importance of the band gap to the magnetic ordering in magnetic semiconductors, we have studied the effect of particle size on the ferromagnetic Curie temperature in semiconducting EuS. We have synthesized capped approximately 20 nm EuS nanoparticles using a single-source precursor, [Eu(S(2)CN(i)()Bu(2))(3)Phen] decomposed in trioctylphosphine. The nanoparticles have been characterized by X-ray powder diffraction, TEM, and magnetic susceptibility measurements as a function of temperature and field. The Curie temperature, based on Arrott plots, is depressed by 1-2 K from the bulk value.     

Structure,photoluminescence and electrical properties of Eu–Nd codoped CaBi4Ti4O15 synthesized by sol–gel method

Eu–Nd codoped CaBi_3.75−x Eu _x Nd_0.25Ti₄O₁₅ (CBENT) powders were synthesized by a simple sol–gel method. Their structures were examined by X-ray diffraction, Raman spectrum, respectively. The coexistence of orthorhombic and tetragonal phase was found when the Eu³⁺ doping content varied from 0.3 to 0.6. The photoluminescence properties of the CBENT powders show that the intense emission bands centered at around 574, 590 and 612 nm are due to the transitions of ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ of the Eu³⁺ ions, respectively. The spectra of the temperature dependence of dielectric constant indicate that the Curie temperature T _C gradually decreases from 785 to 765 °C. The peaks of dielectric constant are restrained with increasing Eu³⁺ doping concentration, suggesting the weakness of the ferroelectricity. Combining the photoluminescence with the ferroelectric property in the CBENT, the optimal Eu³⁺ doping concentration x is 0.2.     

Dielectric properties of barium zirconate titanate (BZT) ceramics tailored by different donors for high voltage applications

The dielectric properties of BZT ceramics with different Zr contents and donor types are studied. It is shown that the large enhancement of the dielectric permittivity is achieved accompanying with the remarkable shift of Curie temperature in the samples doped with donors such as La, Nd, Sm, Eu, Dy, and Yb, whereas the results are different in them doped with other type of donors such as Ce and Y. The shift of Curie temperature is related to the ionic radii of the doped donors. A smaller ion results in a greater shift. The dielectric properties of BZT with different doping levels of Yb are also studied and showed that there is a preferable doping content as Yb ∼0.1% to make the material with the highest permittivity, ɛ ∼25,800, withstand voltage >∼3.5 kV/mm.     

Structure distortion and magnetism in double perovskites Ca2−xLaxFeReO6 (0≤x≤0.8)

The crystal structure and magnetism of Ca_2−xLa_xFeReO₆ (0≤x≤0.8) double perovskites have been investigated. The samples with low doping (x≤0.4) are found to crystallize with the monoclinic P2₁/n superstructure, while those in the high doping ones (x≥0.6) have orthorhombic Pbnm superstructure. With the increase of an La doping, the anti-site defects increases, giving rise to highly disordered samples at the Fe and Re positions. At the low doping region (x≤0.4), the compounds undergo a simultaneous structural and magnetic transition accompanying a slight increase of the Curie temperature. The increase of Curie temperature is discussed in terms of the structural change with doping.     

Room temperature ferromagnetism in undoped and Fe doped ZnO nanorods: Microwave-assisted synthesis

One-dimensional (1D) undoped and Fe doped ZnO nanorods of average length ∼1 μm and diameter ∼50 nm have been obtained using a microwave-assisted synthesis. The magnetization (M) and coercivity (H_c) value obtained for undoped ZnO nanorods at room temperature is ∼5×10⁻³ emu/g and ∼150 Oe, respectively. The Fe doped ZnO samples show significant changes in M -H loop with increasing doping concentration. Both undoped and Fe doped ZnO nanorods exhibit a Curie transition temperature (T_c) above 390 K. Electron spin resonance and Mössbauer spectra indicate the presence of ferric ions. The origin of ferromagnetism in undoped ZnO nanorods is attributed to localized electron spin moments resulting from surface defects/vacancies, where as in Fe doped samples is explained by F center exchange mechanism.     

Influence de divers types de substitutions cationiques sur les propriétés diélectriques de niobates de structure “bronzes oxygénés de tungstène quadratiques”

The value of the ferroelectric Curie temperature in compounds with the “tetragonal tungsten bronze structure” is related to the steric effect of large alkali or alkaline-earth cations in the lattice tunnels, to the covalency of the transition element-oxygen bonds, and to the introduction of Li⁺ ions, which due to their small size are able to occupy the nine coordination sites of the structure anisotropically.     

Effect of hole-doping on the valence state and magnetic property in S = 1/2 spin-chain system LiCuVO4

The valence state and magnetic properties of hole-doped LiCuVO₄ are investigated. By analyzing the Cu 2p core-level photoemission spectra, the holes are suggested to be introduced into Cu²⁺ site in the formation of Cu³⁺ ion. The calculation of effective moment also confirms the presence of nonmagnetic Cu³⁺ ion which should be responsible for the decrease of high-temperature susceptibility after hole doping. At low temperature, the antiferromagnetic transition at 26 K and 2.3 K disappears due to the enhancement of Curie term. Magnetic hysteresis at 2 K shows that there exists a small ferromagnetic moment of 0.15 emu/g in Li_0.9CuVO₄.     

Effect of strontium substitution on the composition and microstructure of sol–gel derived calcium phosphates

Sr-doped calcium phosphates have been prepared by sol–gel chemistry. All samples exhibit two phases: hydroxyapatite (HAp) and tricalcium phosphate (β-TCP). With respect to undoped sample, the Sr-doped samples exhibit higher proportion of β-TCP phase but the quantity appears to be quite independent of the doping level. To explain the mismatch with the nominal stoichiometry, the presence of amorphous CaO and SrO compounds have been postulated and their proportions evaluated. The insertion of Sr²⁺ ions in the two crystalline phases HAp and β-TCP is almost total for low doping levels but quite incomplete for the highest doping level. The majority of the inserted Sr²⁺ ions are in the β-TCP phase. Considering the acknowledged beneficial effect of Sr²⁺ on the bone regeneration process, the effective partial substitution of Sr in biphasic calcium phosphate makes these materials very interesting for clinical applications. The Sr-substituted HAp and β-TCP cell parameters agree fairly well with the Vegard’s law and Sr²⁺ ions substitute preferentially for Ca²⁺ in the Ca2 site for hydroxyapatite and in the Ca4 site for β-TCP. The microstructural parameters confirm the previous observation and give a new evidence of clear stabilizing effect of Sr²⁺ ions towards the β-TCP structure.     

Heavy doping of H ion in 12CaO·7Al2O3

12CaO·7Al₂O₃ (C12A7, mayenite), which has a nanoscale porous structure that can accommodate extraframework species such as hydride (H⁻), oxide (O²⁻), hydroxide (OH⁻) ions, and electrons, has been doped with H⁻ ions to investigate its effects as dominant extraframework species. Chemical doping with CaH₂ enables the concentration of H⁻ ions to reach almost the theoretical maximum. The concentration of H⁻ ions is characterized by optical absorption intensity ascribed to photoionization of H⁻ ions, and ¹H magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. Persistent electron generation, which is accompanied by the formation of an F⁺ absorption band and electrical conductivity, by irradiation with ultraviolet light at room temperature increases as the H⁻ ion doping increases until it reaches half the theoretical maximum and then decreases as the H⁻ ion concentration increases further. This dependence indicates that both H⁻ and O²⁻ ions are necessary for the generation of persistent electrons.     

Magnetotransport properties of Mo substituted La0.7Ca0.3Mn1−xMoxO3 perovskites

We studied the effects of Mo substitution on the structural, transport, and magnetic properties of the La_0.7Ca_0.3Mn_1−xMo_xO₃ (x ≤ 0.1) samples. Powder X-ray diffraction analysis reveals that the samples studied crystallize in the orthorhombic structure with space group Pbnm. Both particle size and morphology change significantly as the Mo content x varies. The metal-insulator transition temperature (T_MI) and Curie temperature (T_C) decrease monotonically as x increases. Magnetization data reveal that long-range FM ordering persists in all samples and the saturation moment decreases linearly as x increases. The smaller depression rate of dT_C/dx observed is mainly ascribed to the increased amount of Mn²⁺ ions with Mo doping, which opens the FM coupling between Mn²⁺–O–Mn³⁺ in the samples.     

Mild hydrothermal synthesis,isomorphous substitution,crystal structure characterization and magnetic properties of BaMP2O7 (M = Mn,Cu)

At mild hydrothermal conditions triclinic modifications of BaMP₂O₇ (M = Mn and/or Cu) have been succeeded. The method offers cheap, one step, impurity free and chemical flexible fabrication of family of metal phosphates that are potential low-dimensional quantum magnets. Partial isomorphous substitution of the Mn²⁺ by Cu²⁺ resulted into mixed-metal solid that has been structurally and magnetically characterized. Rietveld refinement study confirmed the structures and revealed the influence of transition metal substitution. The temperature-dependent magnetic measurements revealed that the system is paramagnetic in almost all temperature range and an apparent antiferromagnetic phase transition occurs around 5 K. Using the Curie–Weiss law, a Curie–Weiss temperature, θ_P = −11.0 K, and a Curie constant C = 3.39 emu K mol⁻¹ was obtained. The small negative θ_P value and the χT behavior as a function of temperature reveal a weak antiferromagnetic interaction between the Cu²⁺/Mn²⁺magnetic centers.     

Microstructural,magnetic and crystal field investigations of nanocrystalline Dy3+ doped zinc oxide

Dy³⁺ doped zinc oxide was prepared by co-precipitation method. The as-prepared samples were annealed at different temperatures to obtain the samples with different particle sizes. The crystallographic phases of all the samples were confirmed by X-ray diffraction (XRD) patterns. Rietveld analysis of the XRD pattern of the sample annealed at 80 °C showed that most of the Dy³⁺ ions were substituted in the Zn²⁺ site of the hexagonal ZnO lattice. But in case of samples annealed at higher temperatures, a fraction of Dy³⁺ ions comes out from the ZnO lattice and this fraction increases with the increase of annealing temperature. The sizes of nanoparticles and the lattice strains of all the samples were obtained from the Hall–Williamson plot. High resolution transmission electron microscopy showed that ZnO nanoparticles are more or less spherical. Magnetic susceptibilities (χ) of some selected samples measured in the temperature range of 300–14 K indicate that the samples are paramagnetic. Values of χ were successfully fitted by Curie–Weiss law. A good theoretical simulation of χ of the sample annealed at 80 °C has been achieved using the one-electron crystal field interaction of the Dy³⁺ ions with its diamagnetic neighbors in the hexagonal single crystal.     

Research on synthesis and conductivity of ferrocenyl Schiff base and its salt

Ferrocenyl Schiff base was synthesized through the condensation of ferrocenecarboxaldehyde and p‐phenylenediamine under neutral conditions, and then a new interesting category of organometallic charge transfer complex was obtained by the doping of ferrocenyl Schiff base with Fe³⁺, Al³⁺ and Ti³⁺ salts. The effects of the dosage of doping agent and doping temperature on the room‐temperature electric conductivity of samples were discussed; in addition, the temperature dependence of the electric conductivity of samples was studied, their structures and compositions were characterized by ¹H‐NMR spectra, infrared spectra, ultraviolet spectra and an electron probe X‐ray microanalyser. The results showed that the electric conductivity of sample can increase 4–5 orders of magnitude after doping with a metallic salt, and the electric conductivity has a positive temperature coefficient effect. The electrical activation energies of the complexes in the range 0.09–1.54 eV were calculated from Arrhenius plots, indicating their favourable semiconducting behaviour. Copyright © 2006 John Wiley & Sons, Ltd.     

Size-controlled aggregation of cube-shaped EuS nanocrystals with magneto-optic properties in solution phase

Size-controlled formation of colloidal aggregates composed of cube-shaped EuS nanocrystals, EuS NCs, in solution phase are reported and their optical and magneto-optical properties are studied. The average size of the colloidal particles of the EuS NCs-aggregates formed in 1-pentanol, 1-hexanol, and 1-octanol were ca. 800, 500, and 100 nm, respectively. Self-organized cubic-type superlattice structure was evaluated in the colloidal aggregates by means of the small-angle X-ray diffraction measurements, which is similar to those in the macroscopic 3D superlattice structures. The distances between NCs in the EuS NCs-aggregates are dependent on alkyl chain length of the solvent alcohol. Magneto-optic properties of EuS NCs-aggregates and the monomeric EuS nanocrystals in liquid media are characterized with magnetic circular dichroism spectra. The active wavelength of EuS NCs-aggregates is considerably longer than that of the monomeric EuS nanocrystals.     

Electron doping effect on structural and magnetic phase transitions in Sr2−xNdxFeMoO6 double perovskites

Polycrystalline Sr_2−xNd_xFeMoO₆ (x=0.0, 0.1, 0.2, 0.4) materials have been synthesized by a citrate co-precipitation method and studied by neutron powder diffraction (NPD) and magnetization measurements. Rietveld analysis of the temperature-dependent NPD data shows that the compounds (x=0.0, 0.1, 0.2) crystallize in the tetragonal symmetry in the range 10-400 K and converts to cubic symmetry above 450 K. The unit cell volume increases with increasing Nd³⁺ concentration, which is an electronic effect in order to change the valence state of the B-site cations. Antisite defects at the Fe-Mo sublattice increases with the Nd³⁺ doping. The Curie temperature was increased from 430 K for x=0 to 443 K for x=0.4. The magnetic moment of the Fe-site decreases while the Mo-site moment increases with electron doping. The antiferromagnetic arrangement causes the system to show a net ferrimagnetic moment.     

Synthesis and characterization of highly preferred orientation polycrystalline Co-doped ZnO thin films prepared by improved sol–gel method

Highly preferred orientation polycrystalline Zn_1?xCo_xO (x = 0, 0.03, 0.06, 0.09) thin films were prepared by improved sol–gel method on quartz glass substrates. The structural, optical and magnetic properties were investigated. The X-ray diffraction patterns show that all the samples have the same structure with one highly oriented c-axis (002) peak. None of the samples showed any signal of impurity phases. The c-axis lattice constant increased linearly with the increase in Co doping content, indicating that the doping of Co ions into the host lattice did not change the wurtzite structure of ZnO. UV–Vis transmittance spectroscopy showed that the average optical transmittance of the films is about 90 % in visible wavelength range. The optical band gap (Eg) decreased with increasing Co content. Also, the results of vibration sample magnetization ascertained the ferromagnetic behavior of Co-doped ZnO, having a Curie temperature higher than room temperature.     

Remarkable magneto-optical properties of europium selenide nanoparticles with wide energy gaps

The enhanced magneto-optical properties of nanoscaled lanthanide chalogenide semiconductors which have a wide energy gap were observed at around 500 nm for the first time. The nanoscaled semiconductors, Eu(1-x)Se nanoparticles 1 (cubic shapes) and 2 (spherical shapes), were synthesized by the thermal reduction of Eu(III) ion with organic ligands containing Se atoms. The resulting Eu(1-x)Se nanoparticles were characterized by X-ray diffraction, high-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, superconducting quantum interference devices magnetometer, and microwave induced plasma atomic emission spectroscopy measurements. The particle grain sizes of 1 and 2 were estimated to be 11 and 20 nm, respectively. The concentration-normalized Veldet constants (the magnitude of the Faraday effect) of Eu(1-x)Se nanoparticles were much larger than those of corresponding bulk EuSe and EuS nanoparticles.     

Theoretical insights on the electron doping and Curie temperature in La-doped Sr2CrWO6

The structure and electronic and magnetic properties of La(x)Sr(2-x)CrWO(6) (x = 0.0, 0.5, 1.0, 1.5, 2.0) were investigated by using the density functional theory. With the increase of La doping, the extra electrons are injected into W 5d orbitals, which makes the spin moments of W increase. In addition, the calculated Curie temperature and total magnetic moments decrease with the increase of the electron doping, in agreement with the experimental observation. This also means that the decrease of Curie temperature with the electron doping is intrinsic. Half metallic properties are obtained for x = 0.0, 0.5, 1.5, and 2.0, whereas for x = 1.0, the compound is semiconducting.