Tuning the bottleneck effect in Ag metallic host doped with magnetic Gd and non-magnetic Sb ions

The Ag metallic host doped with Gd and Sb is an excellent model system to study the bottleneck effect associated to the conduction-electron (c-e) spin-flip scattering mechanism. Electron spin resonance of Gd³⁺ in both, Ag-(Gd doped)- and Ag-(Gd and Sb doped)-systems, reveal the presence of bottleneck which can be tuned by the amount of Gd and Sb impurities. The increase of the Gd concentration leads to a c-e spin-flip relaxation rate to the magnetic Gd³⁺ ions larger than that to the lattice, favoring the bottleneck regime. Whereas the effect of the non-magnetic impurities (Sb ions) is to increase, via spin–orbit scattering, the spin-flip relaxation rate of the c-e to the lattice, weakening the bottleneck regime.     

Enhancement of critical current density in Gd123 bulk superconductor doped with magnetic powder

We studied the materials processing to grow high performance magnet with doping of magnetic powder as candidature of pinning centers. The magnetic powder was doped into Gd123 matrix and grew into single domain bulk in air. With doping magnetic powder in Gd123 bulk superconductors, the flux pinning was enhanced and the underlying mechanism was discussed. The experimental results showed that suitable amount of magnetic powder inclusion can be effect as pinning center to enhance the J_c of the bulk both in low magnetic field and high magnetic field, which was different from the bulk doped with paramagnetic nano-scaled particles. The T_c of the bulk was suppressed obviously with the content of the magnetic powder increasing, form 93.0 K without doping decrease to 90.5 K with 0.12 wt.% doping. This is consistent with that the magnetic impurities generally suppress superconductivity. We report the detailed results from the viewpoint of practical application to increase the integrated flux trapped inside the bulk.     

Incorporation of Ce3+ in crystalline Gd-silicate nanoclusters formed in silica

Structural and optical properties of sol–gel silica based glassceramics doped with 0.1 mol% Ce and codoped with Gd at high concentrations, from 5 mol% up to 40 mol%, are investigated and compared to those of analogous samples doped only with Ce. Raman scattering, transmission electron microscopy, and x-ray diffraction reveal the formation of Gd apatite-like silicate (Gd_4.67O(SiO₄)₃) and of Gd pyrosilicate (Gd₂Si₂O₇) nanophases whose morphology and crystallinity depend on the Gd concentration and thermal treatments. Optical absorption measurements demonstrate the role of the densification atmosphere in modifying the charge state of Ce ions. The incorporation of Ce³⁺ ions in the nanophases is put in evidence by photo- and radio-luminescence results.     

A comparative analysis of infra-red luminescence spectra of ZnSe crystals doped with Yb,Gd or Cr impurities

Infra-red (IR) photoluminescence (PL) spectra of ZnSe crystals doped with Yb, Gd rare-earth impurities and Cr impurity are investigated. The influence of stoichiometric deviation on the spectra is studied and the structure of complex IR PL bands is analysed. The good coincidence between the structures of IR PL spectra of the samples doped with Yb, Gd, and Cr is shown. Correlation between the component parts of the bands at 1 and 2 μm is found and possibility to control the composition of IR PL spectra by enrichment of the samples with Zn or Se is discussed. The models that explain the formation of complexes based on rare-earth and background Cr and Cu impurities, responsible for IR PL bands, are proposed. Keywords: IR luminescence, ZnSe, Rare-earth impurities, Cr impurity.     

Radiolysis of LaF<Subscript>3</Subscript> crystals with rare-earth impurities

The absorption spectra of LaF₃ crystals, both pure and doped with rare-earth fluorides (YF₃, CeF₃, NdF₃, PrF₃, SmF₃, EuF₃, GdF₃, TbF₃, DyF₃, HoF₃, ErF₃, TmF₃, YbF₃, and LuF₃) have been investigated. All these impurities can be separated into two groups with respect to the shape of the absorption spectra of irradiated crystals. The spectra of the crystals doped with Nd, Sm, Tm, and Yb exhibit, along with 200-nm hole band F₃^-, weak bands due to RE²⁺-anion vacancy centers. The spectra of LaF₃ crystals with Y, Ce, Pr, Gd, Tb, Dy, Ho, Er, and Lu impurities exhibit, along with the hole-center bands (F₃^- at 200 nm and V_kA at 320 nm), bands of comparable intensity, which can be attributed to RE³⁺–F centers. This conclusion is confirmed by preliminary quantum-chemical calculations and the estimation of the levels location in the energy-band diagram.     

The magnetic hyperfine field and the electric field gradient at181Ta impurities in Gd metal

The magnetic hyperfine fieldH _hf and the electric field gradientV _zz at¹⁸¹Ta impurties in metallic Gd were determined by time differential perturbed angular correlation measurements with the 133 keV K-conversion electron 482 keV -cascade of¹⁸¹Ta. The sources for these measurements were prepared by implantation of radioactive¹⁸¹Hf ions into Gd. The results are: |H _hf(TaGd; 77 K)|=285(14)kG, and |V _zz(TaGd; 330 K)|=5.32(15)·10¹⁷V/cm². The value ofH _hf fits well into the systematics for 5d impurities in Gd and indicates a positive core polarisation contribution, which is expected if the conduction electrons of Gd have to a large extent d-character. The electric field gradients of the 5d impurities in Gd are not consistent with a proportionality between the ionic and the electronic contribution.     

Effect of Gd doping on structural, electrical and magnetic properties of BiFeO3 electroceramic

Room temperature multiferroic electroceramics of Gd doped BiFeO₃ monophasic materials have been synthesized adopting a slow step sintering schedule. Incorporation of Gd nucleates the development of orthorhombic grain growth habit without the appearance of any significant impurity phases with respect to original rhombohedral (R3c) phase of un-doped BiFeO₃. It is observed that, the materials showed room temperature enhanced electric polarization as well as ferromagnetism when rare earth ions like Gd doping is critically optimized (x=0.15) in the composition formula of Bi_1+2xGd_2x/2Fe_1−2xO₃. We believe that magnetic moment of Gd⁺³ ions in Gd doped BiFeO₃ tends to align in the same direction with respect to ferromagnetic component associated with the iron sub lattice. The dielectric constant as well as loss factor shows strong dispersion at lower frequencies and the value of leakage current is greatly suppressed with the increase in concentration of x in the above composition. Addition of excess bismuth and Gd (x=0.1 and 0.15) caused structural transformation as well as compensated bismuth loss during high temperature sintering. Doping of Gd in BiFeO₃ also suppresses spiral spin modulation structure, which can change Fe-O-Fe bond angle or spin order resulting in enhanced ferromagnetic property.     

EXAFS studies of ceramic proton conductors

Abstract

We report the results of EXAFS studies of rare earth doped SrCeO₃ and BaCeO₃, two systems that are important proton-conducting oxides. Using Gd doped SrCeO₃ as an example we outline the strategy of the measurements and the way they are compared with parallel atomistic simulations.     

EPR conduction-electron relaxation by cerium and gadolinium impurities in LaAl2

The conduction-electron relaxation by cerium impurities is computed by use of the effective exchange Hamiltonian which takes into account both combined spin and orbit exchange scattering and crystalline field effects, which finally can account for recent EPR measurements of Gd in LaAl₂ with cerium impurities.     

Distribution of oxygen vacancies and gadolinium dopants in ZrO2–CeO2 multi-layer films grown on α-Al2O3

Gdolinia doped ZrO₂ and CeO₂ multi-layer films were deposited on α-Al₂O₃ (0001) using oxygen-plasma-assisted molecular-beam epitaxy. Oxygen vacancies and Gd dopant distributions were investigated in these multi-layer films using X-ray diffraction (XRD), conventional and high-resolution transmission electron microscopy (HRTEM), annular dark-field imaging in scanning transmission electron microscopy (STEM), X-ray energy dispersive spectroscopy (EDS) elemental mapping and X-ray photoelectron spectroscopy (XPS) depth profiling. EDS and XPS depth profiling reveal that the Gd concentration in the ZrO₂ layer is lower than that in the CeO₂ layer. As a result, a higher oxygen vacancy concentration exists in the CeO₂ layers compared to that in the ZrO₂ layers. In addition, Gd is found to segregate only at the interfaces formed during the deposition of CeO₂ layers on ZrO₂ layers. On the other hand, the interfaces formed during the deposition of ZrO₂ layers on CeO₂ layers did not show any Gd segregation. The Gd segregation behavior at every other interface is believed to be associated with the low solubility of Gd in ZrO₂.     

Impurities in Magnetic Insulators Studied by Low-Temperature Nuclear Orientation

Although the effects of impurities in metallic magnets have been extensively studied there have been far fewer experiments on impurities in insulating magnets, and, in an effort to understand this problem, we have investigated the effect of impurities in various systems. In MnCl₂⋅4H₂O samples doped with 1.6, 4.0 and 6.2% Co, the ⁵⁴Mn NMRON line is significantly broadened compared to the line in the pure crystal and shows structure on the low frequency side. The nuclear spin-lattice relaxation time is also lower by an order of magnitude. ⁵⁴Mn has also been doped into FeCl₂⋅4H₂O for which a 4-sublattice structure has been proposed and some details of this structure have been obtained. This revised version was published online in September 2006 with corrections to the Cover Date.     

Mössbauer study of the magnetic and electric hyperfine interaction of dilute57Fe impurities in rare earth metal hosts

The hyperfine interaction of dilute⁵⁷Fe in the rare earth (RE)metals Gd to Lu was investigated by Mössbauer measurements with⁵⁷Co doped RE sources. In all hosts well split, 2-lines spectra were observed at room temperature, with slight asymmetries of the line intensities in some cases. The quadrupole splitting eQV_zz/2 increases from 0.29 mm/sec for Gd to 0.50 mm/sec for Tb, and decreases by less than 10 % between Tb and Lu. Only about 10 % of the corresponding electric fieldgradient (EFG) can be accounted for by the ionic EFG on a substitutional RE site. The temperature dependence of the EFG was measured in the case of Tb. No variation within 3 percent was found between 300 K and 700 K. Measurements of the magnetic hyperfine interaction at low temperatures were carried out in Tb. The saturation field of⁵⁷Fe in this host is H_hf(FeTb;4.2 K)=25(2) KOe. The temperature dependence of the magnetic hyperfine field does not follow the host magnetization (T_c=220K) but vanishes at about 80 K. Similar anomalies of H_hf(T) have previously been observed for other transition element impurities in the RE ferromagnets.     

Effects of copper(II) and iron(II) doping on the bound electronic states in the “one-dimensional” semiconductor Magnus' green salt

EPR and conductivity measurements on the “one-dimensional” semiconductor Magnus' Green Salt doped with Fe or Cu impurities are described. It is found that the introduction of these impurities causes significantly higher localization of the previously identified d_z2 holes.     

Specific heat of the (La,Gd) Al2 system in the superconducting and normal state

Specific heat measurements between 0.5 and 4.2°K are reported for the system ($\text{La}$, Gd) Al₂ in both the superconducting and normal state. The observed specific heat jump at the superconducting transition temperature T_c is in excellent agreement with the Abrikosov-Gor'kov (AG) theory. This is in accordance with the previously reported close correspondence of the T_c vs. Gd concentration curve with the AG theory. Two very interesting features occur in the normal state specific heat. First, the Gd impurities cause a surprisingly strong enhancement of the electronic specific heat coefficient. Second, there is a large magnetic field dependent Schottky-like anomaly at low temperatures. This anomaly persists even in the superconducting state.     

Changes in trap parameters in various mixed oxide garnets

Shallow and deep electronic traps in various mixed oxide garnet ceramics (Y,Lu,Gd)₃(Al,Ga)₅O₁₂ have been studied by thermally stimulated luminescence spectroscopy in the 80–550 K temperature range. It is shown that the substitution of Al ions by Ga and Y by Gd or Lu in YAG:Ce affects the properties of the traps. It is established that the studied ceramics contain residual impurities of chromium and ytterbium. On the base of the obtained and literature data, a table of the Cr-related trap position relative to the bottom of the conduction band in RE₃(Ga_x,Al_5-x)₅O₁₂:Ce garnets (RE = Lu, Y, Gd and combinations thereof) has been constructed.     

Magnetic susceptibility of the rare earth element impurities in the IV-VI semiconductors and Curie-Weiss law

Peculiarities of applying the Curie-Weiss law to the analysis of the magnetic susceptibility of the impurities of the rare earth elements in the IV-VI crystals are examined. It is shown that the traditional approach to the determination of the paramagnetic Curie temperature of ferromagnets and antiferromagnets applied to the impurities of ferromagnetic and antiferromagnetic elements in the doped crystal may lead to fundamental mistakes. The results of this examination are used to analyze the high temperature magnetic susceptibility of Eu and Gd impurities in the PbTe doped crystals grown from the melt using the Bridgman method. It is established that the doping of PbTe with Eu leads to the formation of ferromagnetic inclusions and the same doping with Gd leads to the formation of antiferromagnetic inclusions in the crystal. It is shown that these inclusions are most likely the complexes based on the europium or gadolinium oxides EuO and Gd₂O₃, respectively.     

Structure and thermal conduction in solid O2 with nonmagnetic impurities

Thermal conductivities of solid oxygen doped with nonmagnetic impurities of nitrogen and argon were measured in the 1–35 K. The antiferromagnetic α-O₂ doped with this impurities exhibits a strong thermal conductivity anomaly, which is assumed to be a result of supperpression of the magnon contribution to the heat transport by impurities. At the same time, the jump in the thermal conductivity at the α-β transition and th thermal conductivity in the β-O₂ are insensitive to the doping effect, which we ascribe to strong magnetostriction effects. The halfwidth of the x-ray reflexes indicates that doping with Ar and Kr enhanced magnetostriction effects while doping with N₂ weakens. This research was made possible by the financial support of the International Science Foundation Grants No U9R000 and U9R200.     

Rare-Earth Doped Sol-Gel Silicate Glasses for Scintillator Applications

Radio-, photo- and thermally stimulated—luminescence (RL, PL, TSL) measurements have been performed on SiO₂ sol-gel glasses doped by 0.1 mol% Ce and 3 mol% Gd, and on (0.1 mol% Ce, 3 mol% Gd) co-doped samples. Ce^3? 5d-4f emission peaking at about 2.7 eV has been observed in the RL of SiO₂: 0.1 mol% Ce, while the typical ⁶P-⁸S emission of Gd^3? centred at 3.97 eV has been detected in SiO₂: 3 mol% Gd. The co-doped sample displays both 5d-4f Ce^3? and ⁶P-⁸S Gd^3? emissions with reduced intensities with respect to those observed in the singly doped glasses. Moreover, in co-doped glasses the PL time decay patterns of both rare earth ions show a non exponential dependence and are significantly shortened. To explain such an effect non radiative de-excitation of both RE ions excited states involving energy transfers to defect levels is suggested. Bidirectional Gd^3? ? Ce^3? energy transfers could also occur. Complementary TSL measurements put in evidence the existence of broad glow peaks at about 100 K and 220 K. The TSL spectra feature the RE ions emissions.     

Surfactant effect on the upconversion emission and decay time of ZrO2:Yb-Er nanocrystals

Luminescent properties of doped ZrO₂:Er³⁺ and codoped ZrO₂:Yb³⁺-Er³⁺ nanocrystals with average size ∼54 nm were analyzed as a function of non-ionic surfactant (Pluronic F-127) concentration. Surfactant and non-surfactant samples were prepared by the sol-gel micelle process with hydrothermal aging and annealed at 1000 °C for 5 h. The introduction of the surfactant reduces the presence of impurities such as OH and CO₂ on both samples, and increments the tetragonal phase for codoped nanocrystals. It induces an increment larger than 90% and 70% for doped and codoped, respectively, for an optimum molar ratio of 0.0082. The observed enlargement of fluorescence decay time is partly the result of the nanosize effect but is dominated by the reduction of impurities attached on the nanocrystalline surface.     

Confirmation of selective site occupation of dilute Co impurities in Fe3Si using the Mössbauer effect technique

The distribution of a dilute solution of Co impurities in Fe₃Si is investigated by means of a Mössbauer source experiment. The results suggest that all Co impurities selectively replace Fe_II site atoms in the lattice, in agreement with NMR data of Burch et al.,⁷ whereas neutron diffraction experiments in more heavily doped material do not preclude the possibility of a lesser degree of preferential substitution.