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.     

Preparation of tetragonal ZrO2-Gd2O3 powders

ZrO₂-Gd₂O₃ alloys containing 2,3,5 and 8 mol.% Gd₂O₃ have been prepared by mixed oxide (MO), hybrid sol-gel (SG), and co-precipitation (CP) routes. No tetragonal (t) phase is retained in the MO method, while 100% t phase is obtained in the calcined CP samples; the SG method leads to only partial stabilization of the t phase. Washing of the CP powders with propan-2-ol leads to unagglomerated powders with increased specific surface area (145 versus 89 m²g^–1) and sintered density (98% versus 79%). Cubic and t phase also appear on sintering the samples with >2 mol.% Gd₂O₃.     

Influence of Y(Gd) and Ni(Fe) on the principal structural characteristics of Al87Y5? x GdxNi8? y Fe ( x = 0?5; y = 0?4) amorphous metallic alloys

The X-ray diffraction method is used to study the short-range ordering of Al₈₇Y₅Ni₈, Al₈₇Gd₅Ni₈, Al₈₇Y₄Gd₁Ni₈, Al₈₇Y₄Gd₁Ni₄Fe₄, and Al₈₇Gd₅Ni₄Fe₄ amorphous alloys. The structural factors are obtained and the pair correlation functions are computed. The principal structural parameters are analyzed. It is shown that the investigated amorphous alloys are characterized by the topological short-range order based on the close-packed structure of aluminum and Al₃Y(Gd)-and Al₃Ni(Fe)-type chemical short-range ordering. It is discovered that the replacement of Y atoms with Gd atoms and Ni atoms with Fe atoms weakly affects the topology of the atomic distribution of the aluminum matrix and leads to changes in the character of chemical ordering. This affects the degree of short-range microhomogeneity of the short-range ordering and the dimensions of structural units. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 4, pp. 75–79, July–August, 2008.     

Synthesis and photoluminescence properties of (Y,Gd)BO3 : Eu phosphor prepared by ultrasonic spray

The mixed solution of acetate of Y, Gd, Eu and boric acid diluted in water was used as the precursor for the ultrasonic spray for the synthesis of (Y,Gd)BO₃ : Eu phosphor. It was found that (Y,Gd)BO₃ : Eu phosphor made by ultrasonic spray pyrolysis had a spherical shape and a narrow size distribution having a mean size of 1.3 m, while it had irregular, coarse and non-uniform size distribution for the phosphor formed by solid-state reaction. The as-sprayed particles was amorphous but they converted into the same polycrystalline phase of solid state reaction after post heat treatment at 1100 °C for 2 hr. The emitting intensity under 147 nm VUV excitations for the spray-formed (Y,Gd)BO₃ : Eu phosphor, however, was inferior to the later one. It was found that the optimum concentrations of Gd and Eu were 30% and 5%, respectively in (Y_1–x Gd _x )_1–y BO₃ : Eu _y phosphors prepared both by spray and solid state reaction.     

Controlled processing of (Gd,Ln)2O3:Eu (Ln = Y,Lu) red phosphor particles and compositional effects on photoluminescence

Synthesis of (Gd_0.95−xLn_xEu_0.05)₂O₃ (Ln = Y and Lu, x = 0–0.95) powders via ammonium hydrogen carbonate (AHC) precipitation has been systematically studied. The best synthesis parameters are found to be an AHC/total cation molar ratio of 4.5 and an ageing time of 3 h. The effects of Y³⁺ and Lu³⁺ substitution for Gd³⁺, on the nucleation kinetics of the precursors and structural features and optical properties of the oxides, have been investigated. The results show that (i) different nucleation kinetics exist in the Gd–Y–Eu and Gd–Lu–Eu ternary systems, which lead to various morphologies and particle sizes of the precipitated precursors. The (Gd,Y)₂O₃:Eu precursors display spherical particle morphologies and the particle sizes increase along with more Y³⁺ addition. The (Gd,Lu)₂O₃:Eu precursors, on the other hand, are hollow spheres and the particle sizes increase with increasing Lu³⁺ incorporation, (ii) the resultant oxide powders are ultrafine, narrow in size distribution, well dispersed and rounded in particle shape, (iii) lattice parameters of the two kinds of oxide solid solutions linearly decrease at a higher Y³⁺ or Lu³⁺ content. Their theoretical densities linearly decrease with increasing Y³⁺ incorporation, but increase along with more Lu³⁺ addition and (iv) the two kinds of phosphors exhibit typical red emissions at ∼613 nm and their charge-transfer bands blue shift at a higher Y³⁺ or Lu³⁺ content. Photoluminescence/photoluminescence excitation intensities and external quantum efficiency are found to decrease with increasing value of x, and the fluorescence lifetime mainly depends on the specific surface areas of the powders.     

Study of dielectric, magnetic and ferroelectric properties in Bi1 − xGdxFeO3

Gd-doped BiFeO₃ polycrystalline ceramics were synthesized by solid-state reaction method and their dielectric and magnetic properties were investigated. X-ray diffraction pattern showed that Bi_1 − xGd_xFeO₃ (x = 0, 0.05 and 0.1) ceramics were rhombohedral. The Gd substitution has suppressed the usual impurity peaks present in the parent compound and we obtained single phase Bi_0.9Gd_0.1FeO₃ ceramic. Gd substitution reduced the antiferromagnetic Néel temperature (T_N) in Bi_1 − xGd_x FeO_3. An anomaly in the dielectric constant(ε) and dielectric loss(tan (δ)) in the vicinity of the antiferromagnetic Néel temperature (T_N) was observed. Ferroelectric and magnetic hysteresis loops measured at room temperature indicated the coexistence of ferroelectricity and magnetism. The room temperature magnetic hysteresis loops were not saturated, but the magnetic moment was found to increase with increase in Gd concentration.     

Effect of novel (Gd,Cu) substitution on the electrical properties and magnetoelectric coupling of bismuth ferrite ceramics

The influence of Gd dopant and (Gd, Cu) dopants on the ferroelectric, dielectric and magnetoelectric properties of single phase BiFeO₃ (BFO) were investigated. Nanoparticles of undoped BiFeO₃, Bi_0.95Gd_0.05FeO₃ and Bi_1?xGd_xFe_0.98Cu_0.02O₃ (x?=?1, 2, 3, 4 and 5%) were prepared by sol–gel method. X-ray diffraction reveals that all the samples crystallize in rhombohedral phase. The simultaneous Gd and Cu doping at BFO lattice has significantly enhanced the ferroelectric properties of BFO compared to that of BFO. Substitution of Gd alone at the Bi site, gave rise to attractively enhanced remnant polarization. Though the (Gd, Cu) doped BFO samples exhibit relatively less enhancement, their values of remnant polarization are appreciable. Doping of (Gd, Cu) in the BFO lattice leads to an appreciable dielectric properties. An effective magnetoelectric coupling has been recorded for doped BFO when compared to BFO.     

YAG and Y2O3 laser ceramics from nonagglomerated nanopowders

Undoped and Nd- or Yb-doped laser-grade yttrium aluminum garnet and (Y,La)₂O₃ ceramics with a transmittance above 80% in the 1-μm lasing region have been prepared by solid-state reactions using nonagglomerated Y₂O₃ and Al₂O₃ nanopowders. The Y₂O₃ nanopowders were prepared via laser evaporation, chemical precipitation from urea solutions, and grinding of commercially available Y₂O₃ in a purposedesigned laboratory-scale attritor at stirrer rotation rates of up to 1500 rpm. The YAG ceramics were prepared using commercially available Al₂O₃. After grinding, all of the powders had a particle size on the order of a hundred nanometers. Green compacts produced from the nanopowders were sintered in a vacuum furnace between 1615 and 1750°C to give highly transparent ceramic samples.     

Saturation magnetic properties of Y3?xRexFe5O12(Re: Gd, Dy, Nd, Sm and La) nanoparticles grown by a sol–gel method

Re³⁺-substituted garnet nanoparticles Y_3−x Re _x Fe₅O₁₂ (Re = Gd, Dy, Nd, Sm and La) were fabricated by a sol–gel method .The XRD patterns of all samples have only peaks of the garnet structure and the sizes of nanoparticles rang from 34 to 69 nm. Results of VSM show that the saturation magnetization of Y_3−x Gd _x Fe₅O₁₂, Y_3−x Dy _x Fe₅O₁₂ and Y_3−x Sm _x Fe₅O₁₂ nanoparticles decrease evidently as the Re³⁺ concentration (x) is increased. The saturation magnetization of Y_3−x Nd _x Fe₅O₁₂ and Y_3−x La _x Fe₅O₁₂ increases firstly and then decreases as the Re³⁺ concentration (x) is increased. In the meanwhile, it is observed that may be due to the enhancement of the surface spin effects, the saturation magnetization rises as the nanoparticle size is increased.     

Preparation and Electrical Conductivity of BaZr1 – xRxO3 – α(R = Sc, Y, Ho, Dy, Gd, In)

The influence of process parameters on the density of BaZr_0.9Y_0.1O₃proton-conducting ceramics was investigated. Dense ceramics (94–96% of theoretical density) were obtained by synthesis from coprecipitated zirconium and yttrium hydroxides and barium carbonate, followed by sintering at 1700°C. The conductivity of BaZr_0.93R_0.07O_{3 –} (R = Sc, Y, Ho, Dy, Gd, In) was measured as a function of temperature and oxygen partial pressure and was found to decrease in the order Y > Ho > Sc > Dy, In > Gd. In air, these materials are hole–ion conductors. At 800°C, BaZr_0.93Y_0.07O_{3 –} has an ionic transference number of 0.16 and ionic conductivity of 3.6 × 10^–4S/cm.     

Crystal growth and characterization of Tm doped mixed rare-earth aluminum perovskite

In this work, we present results of structural characterization and optical properties including radio luminescence of (Lu_xGd_yY_0.99?x?yTm_0.01)AP single crystal scintillators for (x, y) = (0.30, 0.19), (0, 0.19) and (0, 0) grown by the micro-pulling-down (μ-PD) method. The grown crystals were single phase materials with perovskite structure (Pbnm) as confirmed by XRD and had a good crystallinity. The distribution of the crystal constituents in growth direction was evaluated, and significant segregation of Lu and Gd was detected in (Lu_0.30Gd_0.19Y_0.50Tm_0.01)AP sample. The crystals demonstrated 70% transmittance in visible wavelength range and some absorption bands due to Tm³⁺, Gd³⁺ and color centers were exhibited in 190–900 nm. The radioluminescence measurement under X-ray irradiation demonstrated several emission peaks ascribed to 4f–4f transitions of Tm³⁺ and Gd³⁺. The ratio of emission intensity in longer wavelength range was increased when Y was replaced by Lu or Gd.     

Densification and microstructure development in the reaction sintering process of yttrium iron garnet

Factors affecting the densification and microstructure development in the reaction sintering process (RSP) of yttrium iron garnet were investigated. Three different powder mixtures were used: Fe₂O₃/Y₂O₃, Fe₂O₃/YFeO₃ (1100 ° C calcined), and Fe₂O₃/YFeO₃ (1200 ° C calcined). The conventionally prepared garnet powder was also adopted as a reference material. It was found that the RSP using Fe₂O₃-YFeO₃ systems has a beneficial effect on densification from the dilatation occurring along with the reaction of garnet formation. On the other hand, it has a detrimental effect due to the local contraction induced by the reaction in the Fe₂O₃-Y₂O₃ system. The densification rate and ultimate density achievable are also affected by the YFeO₃ powder adopted in RSP. A high grain-growth rate was obtained for garnet when the 1200 ° C calcined YFeO₃ powder was used. This leads to a high densification rate at low temperature. However, the densification ability deteriorates at temperatures above 1425 ° C due to the trap of pores in the fast-grown grains. Conversely, the grain-growth rate in RSP with 1100 ° C-calcined YFeO₃ was moderate, and although it gives a slower densification rate at low temperature, the ultimate density can be raised to 99% theoretical density at 1450 ° C.     

Magnetic behaviour and microstructural properties of amorphous and crystallized Y3−x GdxFe5O12 prepared by the amorphous citrate process (x=0, 0.5 and 3)

The systems Y_3–x GdxFe₅O₁₂ withx=0, 0.5 and 3 have been synthesized by the amorphous citrate process. Their structural and magnetic properties have been studied for various heat-treatment schedules. The systems withx=0, 0.5 and 3 are amorphous when heat treated up to 650, 600 and 500° C, respectively, and the results of amorphous, as well as crystallized, samples are discussed. The volume fraction of the garnet phase is found to increase with higher heat-treatment temperature. Besides the garnet phase, two other phases appear to grow, the volume fraction of the spinel phase increasing with increase in gadolinium concentration. Microstructral properties have been further correlated with the magnetic properties.     

Bi-substituted iron garnet films for one-dimensional magneto-photonic crystals: Synthesis and properties

The crystallization processes in Bi_2.8Y_0.2Fe₅O₁₂, Bi_2.5Gd_0.5Fe_3.8Al_1.2O₁₂, Bi_1.5Gd_1.5Fe_4.5Al_0.5O₁₂ and Bi_1.0Y_0.5Gd_1.5Fe_4.2Al_0.8O₁₂ garnet films deposited by reactive ion beam sputtering on (1 1 1) gadolinium–gallium garnet substrates, optical glass-ceramic and SiO₂ films have been studied. Films were annealed at low pressure in oxygen atmosphere and in the air. The possibility of preparation of crystalline garnet films with high concentration of bismuth on the SiO₂ films using a buffer layer with low concentration of Bi has been shown. This allows to produce one-dimensional magneto-photonic crystals with high effective Faraday rotation (several tens of°/μm for the visible optical spectrum).     

Fabrication and characterization of compositionally graded Bi1−x Gd x FeO3 thin films

An undoped BiFeO₃ thin film, Gd doped Bi_0.95Gd_0.05FeO₃ thin film with a constant composition, Gd up-graded doped Bi_1?x Gd _x FeO₃ and Gd down-graded doped Bi_1?x Gd _x FeO₃ thin films were successfully grown on Pt (111)/Ti/SiO₂/Si (100) substrates using a sol-gel and spin coating technique. The crystal structure, ferroelectric and dielectric characteristics as well as the leakage currents of these samples were thoroughly investigated. The XRD (X-Ray Diffraction) patterns indicate that all these thin films consist of solely perovskite phase with polycrystalline structure. No other secondary phases have been detected. Clear polarization-electric field (P-E) hysteresis loops of all these thin films demonstrate that the incorporation of Gd³⁺ into the Bi site of BFO thin film have enhanced the ferroelectric performance of pure BiFeO₃ thin film, and the Gd down-graded doped Bi_1?x Gd _x FeO₃ thin film has the best ferroelectric properties. Compared to other thin films, the optimal ferroelectric behavior of the Gd down-graded doped Bi_1?x Gd _x FeO₃ thin film results from its large dielectric constant, low dissipation factor and low leakage current.     

Hot-sprayed yttrium iron garnet

Hot spraying (evaporative decomposition of solution) of sulfates of yttrium and iron at 1000°C and above produced powders which could be sintered to 99% X-ray density with homogeneous micro-structure and controlled grain size. The powder obtained from the hot-spray reactor was well gelated. However, no detectable YIG phase was obtained in the powder. The only constituents were YFeO₃ and Fe₂O₃. This is attributed to the fast nucleation of YFeO₃ phase in the reactor. The calcination studies of these powders showed that the conversion to YIG phase began above 900°C. The calcined powder with more than 90% conversion to YIG phase was still very fine and reactive. It could be sintered to high density product with average grain size ranging from 2 μ upwards.     

Shaped single crystal growth and scintillating application of Yb:(Gd,Lu)3(Ga,Al)5O12 solid solutions

Shaped single crystals of (Yb_0.05Lu_xGd_0.95−x)Ga₅O₁₂ (0.0x0.9) and Yb_0.15Gd_0.15Lu_2.7(Al_xGa_1−x)O₁₂ (0.0x1.0) were grown by the modified micro-pulling-down method. Continuous solid solutions with garnet structure and a linear compositional dependency of crystal lattice parameter in the system Yb:(Gd,Lu)₃(Ga,Al)₅O₁₂ are formed. Measured optical absorption spectra of the samples show 4f–4f transitions related to Gd³⁺ ion at 275 and 310 nm, and also an onset of charge transfer transitions from oxygen ligands to Gd³⁺ or Yb³⁺ cations below 240 nm. A complete absence of Yb³⁺ charge transfer luminescence under X-ray excitation in any of the investigated samples was explained by the overlapping of charge transfer absorption of Yb³⁺ by that of Gd³⁺ ions. For specific composition of Lu_1.5Gd_1.5Ga₅O₁₂ an intense defect––host lattice-related emission, which achieve of about 40% integrated intensity compared with Bi₄Ge₃O₁₂, was found.     

Reactively hot-pressed yttrium iron garnets

Reactive hot-pressing of co-precipitated oxalates of Yttrium and Iron, under 6000–8000 psi, to a maximum temperature of 1100°C, produced high density specimens. Only the stooichiometric mixture (Fe/Y=1.66) produced some garnet phase. Off-stoichiometric specimens were mainly composed of orthoferrites. On annealing in air or oxygen at 1400°C, the orthoferrite phase was converted to the garnet phase (with excess Fe₂O₃ or Y₂O₃ in the lattice) with an accompanying increase in magnetic moment.     

Sintering studies on submicrometre-sized Y-Ba-Cu-oxide powder

The isothermal sintering behaviour of submicrometre-sized (<50 nm) powders of single-phase YBa₂Cu₃O _x (123) and unreacted stoichiometric mixture of submicrometre-sized (<50 nm) powders of BaCO₃, Y₂O₃ and CuO (which on calcination at 1173 K gives YBa₂Cu₃O _x ) was investigated through dilatometry under different sintering atmospheres. The sintering rate of the powder compacts was impeded by the presence of oxygen. The activation energies,Q, of sintering were determined to be 1218 kJ mol^–1 in argon, 1593 kJ mor^–1 in air and 2142 kJ mol^–1 in oxygen. A decrease in the apparent sintered density with increasing oxygen partial pressure was also observed. X-ray diffraction and thermal analyses (thermogravimetry and differential thermal analysis) showed no reaction during sintering of the single-phase product. Pellets fabricated from uncalcined powder exhibit two stages of sintering, one between 1073 and 1173 K having an activation energyQ=627kJ mol^–1, and a second one above 1173 K withQ=383.7 kJ mol^–1. A.c. susceptibility, resistivity and critical current density were determined as a function of the temperature of the sintered samples.     

Investigation on Electrical and Magnetic Properties of Gd-doped BiFeO<Subscript>3</Subscript>

Multiferroic ceramic samples of Bi_1−x Gd _x FeO₃ (x=0, 0.05, 0.1 and 0.15) have been prepared by rapid liquid-phase sintering technique. The effect of Gd substitution on ferroelectric and magnetic properties of Bi_1−x Gd _x FeO₃ ceramics has been investigated. The results of X-ray diffraction (XRD) patterns show that the single-phase BiFeO₃ sample has a rhombohedral structure and Gd³⁺ substitution for Bi³⁺ has not affected its structure. Experimental results suggest that for Bi_1−x Gd _x FeO₃ system, the ferroelectric and magnetic properties of BiFeO₃ are improved by Gd doping and the loop area increases with the Gd content. When x=0.15, saturated ferroelectric hysteresis loop is observed at room temperature with the maximal 2Pr=1.62 μC/cm², which is about 578.6% higher than that of BiFeO₃.