Magnetoresistance hysteresis of granular YBa2Cu3O7−δ high-temperature superconductor in weak magnetic fields

The variation of the transverse magnetoresistance of YBa₂Cu₃O_～6.95 high-temperature superconducting ceramic with external magnetic field intensity H _ext first increasing from zero to ～500 Oe (Δρ⁺ / ρ_{273 K}) and then decreasing from about 500 Oe to zero (Δρ^? / ρ_{273 K}) is studied for transport current densities varying from j/j _c ≈ 0.01 to ≈0.99 (where j _c is the critical current density in the absence of the magnetic field) at 77.3 K. It is found that the field dependence of the magnetoresistance of YBa₂Cu₃O_～6.95 high-temperature superconductor is characterized by pronounced hysteresis, the difference Δρ⁺ / ρ_{273 K} ? Δρ^? / ρ_{273 K} increasing with j/j _c. As j/j _c grows, the effective critical fields of Josephson weak links, H _c2J , and the lower critical fields of superconducting grains, H _c1A , decline. When field H _ext rises, the critical fields are lower than when H _ext diminishes: H _c2J ⁺ < H _c2J ^? and H _c1A ⁺ < H _c1A ^? .     

Magnetic hyperfine fields in Y2Fe17−x Si x compounds

Y₂Fe_17–x Si _x compounds withx=0, 0.5, 1.0, 1.5, 2.0, 2.5 and3.0 were investigated by magnetic measurement andNMR. It is found that with increasing Si content the Curie temperatureT _C increases while the average Fe magnetic moment _Fe decreases. NMR study indicates that Si preferentially substitute the Fe atoms at 4f sites, which is responsible for the increase ofT _C.     

Internal magnetic fields in the high-temperature superconductor YBa2Cu3O7−δ from muon spin rotation experiments

Muon spin rotation (SR) measurements of the internal magnetic fields in sintered YBa₂Cu₃O_7– samples are reported. Pronounced differences between field-cooled (FC) and zero-field-cooled (ZFC) SR signals, hysteresis effects and flux trapping are observed. The SR results are compared with the results of macroscopic experiments (susceptibility, magnetization, resistivity) and with predictions obtained from models of weakly-coupled superconducting regions.     

Study of magnetic properties of melt textured YBa2Cu3O7−δ with Pt,Ce and Ag dopants

Bulk YBa₂Cu₃O_7−δ samples, doped with PtO₂, Ag₂O, CeO₂ or with a starting mixture of nanosized Y₂O₃, have been prepared by the top seeded melt texturing process. Magnetic measurements have been performed on these samples using VSM and SQUID magnetometer. Critical current densities as a function of the applied magnetic field J_c(B), temperature and field dependent normalized relaxation rates S(T) and S(B), as well as E(J) relationships have been derived. The effects of different dopants on these results have been investigated.     

Hyperfine fields and magnetic anisotropy in Fe1000−x Gd x thin films and Fe80−x GdxB20(0≤x≤24) thin films and ribbons

The Mössbauer measurements performed on Fe_100–x Gd _x thin films and on Fe_80–x Gd _x B₂₀ both as thin films and ribbons show a dependence of the spins orientation and H_hyp versus temperature, Gd content and preparation conditions. Increasing the Gd content, the initial low anisotropy disappears and H_hyp decreases. A sharp increase of the anisotopy with temperature in ribbons with low Gd concentration is evidenced.     

Dimensionality crossover regimes due to microcrystalline anisotropy in the resistivity of granular Y1Ba2Cu3O7−y in weak magnetic fields at low temperature

Precise measurements of the electrical resitivity of Y₁Ba₂Cu₃O_7–y have been taken in the temperature range between the percolation transition temperatureT _R and the superconducting onset temperatureT ₀ in presence of low magnetic fields (0–4 kG). Data analysis shows markedly different resistivity regimes from which superconductivity characteristic temperature ranges are defined. Exponential, linear, exponential, power law, and linear regimes are successively seen starting from low temperature. They are explained in terms of dimensionality effects inherent to the anisotropic microcrystalline structures. Experimental laws for the temperature and the field dependence are given, e.g. T-B ^0.3 between the applied field and the temperature shift of the knee onR(T).     

Magnetoresistance of superconducting grains in ceramic YBa2Cu3O7 − δ high-temperature superconductors in weak magnetic fields

The current-voltage characteristics $E(j)_{H_{treat} } = const$ of ceramic (granular) YBa₂Cu₃O_6.95 samples preliminarily magnetized in different transverse magnetic fields H _treat have been measured in a zero field (H _ext = 0) at T = 77.3 K for elucidating specific features of dissipation in superconducting grains of high-temperature superconductors (HTSCs). The current-voltage curves measured in the range 0 ≤ H _trap ≤ H _c2J (where H _trap is the magnetic field trapped as a result of the pretreatment in H _treat and H _c2J is the upper critical field of the Josephson weak links) have been used to construct the field dependences of the magnetoresistance ρ_A(H _treat) _{j = const} of superconducting grains. It has been established that the magnetoresistance ρ_A of the superconducting grains is significantly lower than the magnetoresistance ρ_J for the Josephson medium. The dependence of ρ_A on H _treat and on the transport current density j has been investigated. It has been shown that the dependences ρ_A(H _treat) _{j = const} exhibit a clearly pronounced tendency to saturation, ρ_satur, and the value of ρ_satur increases with increasing j. It has been found that the lower critical field H _c1A of the superconducting grains strongly depends on the transport current density.     

Magnetotransport study of in situ magnetic field textured Dy1Ba2Cu3O7−δ superconductors

The electrical resistivity, thermoelectric power and the Nernst effect have been studied as a function of temperature and magnetic field for a typical superconductor DyBa₂Cu₃O_7– magnetically textured in situ at 1035°C. The three transport coefficients show hybrid microscopic features. In particular, we show the anisotropy with respect to the field direction (H//a,H//c) in all transport coefficients. We verify that Tinkham's law is obeyed for the broadening of the resistive transition in a magnetic field. Similarly we obtainanisotropic broadening exponents for each integrated excess property. From Arrhenius plots we obtain orders of magnitude for the activation energies characterizing each property. They are markedly different from each other.     

Electromagnetic wave absorption properties of ε-Fe3N/Y2O3 nanocomposites derived from Y2Fe17 intermetallic compound

The electromagnetic wave absorption properties of ε-Fe₃N/Y₂O₃ nanocomposites were characterized in a frequency range of 0.05–20.05 GHz. The imaginary part of relative permeability μ_r″ exhibited “twin peak” dispersion and μ_r″ value retained high over a 0.5–10 GHz range. The real part (ε_r′) and imaginary part (ε_r″) of relative permittivity almost kept a low constant in a region of 0.5–10 GHz, respectively. As a result, the resin composites with 51 vol% ε-Fe₃N/Y₂O₃ powders exhibited excellent electromagnetic wave absorption properties (RL<−20 dB) in a frequency range of 0.6–4.4 GHz, with a thickness of 3.3–19.3 mm. A minimum reflection loss of −55 dB was observed at 1.8 GHz with an absorber thickness of 7.05 mm.     

A two-frequency He−Ne laser at the 3S 2→3P 4 transition in mutually orthogonal transverse magnetic fields

By taking as an example a He−Ne laser that operates at the 3S₂−3P₄ transition (λ=3.39 μm), we studied the specific features of nonlinear interaction of the waves of linear orthogonal polarizations in a transverse magnetic field. Using mutually orthogonal transverse magnetic fields on the neighboring portions of an active medium, we implemented a regime of two-frequency generation, which was investigated theoretically and experimentally. To whom correspondence should be addressed. B. I. Stepanov Institute of Physics, National Academy of Sciences of Belarus, 68, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 4, pp. 568–573, July–August, 1999.     

157 T internal magnetic field in Fe[C(SiMe3)3]2 compound at 20 K

Fe[C(SiMe₃)₃]₂ compound, in which iron is coordinated by two carbons, was prepared using the reaction of FeCl₂ with (Me₃Si)₃CLi, and investigated by XRD, ⁵⁷Fe Mössbauer spectroscopy and DFT calculations. 157.5 T hyperfine magnetic field was found at the site of the iron nucleus of this compound at 20 K. DFT calculations predict the quintet states to be clearly favored energetically over the lower spin states. The population analysis reveals considerable 4s as well as large unpaired 3d electron contributions, which can be responsible for the extremely high hyperfine field.     

Transient fields for Ag and Pd ions IN Fe and Co;g-Factor measurements of the lowest 3/2− and 5/2− states in107,109 Ag

Transient field precessions of the 2 ₊ ¹ state of¹⁰⁸Pd were measured simultaneously with those of the 3/2 _– ¹ and 5/2 _– ¹ states of¹⁰⁷Ag and¹⁰⁹Ag as their ions traversed thin Co and Fe foils. Precessions of the 2 ₊ ¹ states of^{104,106,108,110}Pd were also measured using a natural Pd target evaporated onto 0.9 m thick Co. The results of these studies are in accord with prior predictions that the AgFe transient field might be diminished relative to that for PdFe at ion velocities appropriate for molecular orbital vacancy sharing. Transient field strengths for PdCo and PdFe were found to be consistent with scaling according to bulk magnetization of the ferromagnetic host. Gyromagnetic ratios were inferred for the first 3/2^– and 5/2^– states of^107,109Ag relative to theg factor of the 2 ₊ ¹ state of¹⁰⁸Pd. Values obtained usingg(2 ₊ ¹ ;¹⁰⁸Pd)=0.36± 0.03 are:g(3/2;¹⁰⁷Ag) =0.63±0.09,g(5/2;¹⁰⁷Ag)=0.37±0.06,g(3/2;¹⁰⁹Ag)=0.77 ±0.10,g(5/2;¹⁰⁹Ag)=0.36±0.05.     

Spin crossover in the magnetic phase transition in YBa2(Cu1−x Fex)3O7±δ superconductors

All iron ions in the Cu1 and Cu2 local lattice sites of the YBa₂(Cu_0.9 ⁵⁷Fe_0.1)₃O_7.01 superconductor with T _c=31 K experienced magnetic ordering below T _m=22 K. Therefore, at T < T _m, magnetic ordering coexisted with superconductivity. According to the Mössbauer spectroscopy data, iron ions in Cu2 (Fe2) sites were in the low-spin state at T < T _m(S= 3/2 or 1/2), whereas an equal number of iron ions in Cu1 (Fe1) sites were in the high-spin Fe³⁺ state (S=5/2). The magnetic transition near T _m changed iron ion spin states-low-spin ions turned into high-spin ions, and vice versa. This preserved the spin balance between iron ions in the Cu1 and Cu2 layers. Control measurements on other samples of the YBa₂(Cu_{1? x} Fe_x)₃O_7±δ series substantiated these conclusions.     

Investigation of Ce0.9Sr0.1Cr0.5Co0.5O3−δ as the anode material for solid oxide fuel cells fueled with H2S

Ce_0.9Sr_0.1Cr_0.5Co_0.5O_3?δ (CSCrCo) as an anode catalyst was studied in a solid oxide fuel cell (SOFC), where hydrogen sulfide (H₂S) was used as fuel. The conductivities were evaluated with a four-probe DC technique in 3 % H₂-N₂ and 5 % H₂S-N₂ at 570–800 °C, respectively. X-ray diffraction (XRD) patterns show that CSCrCo powders are fluorite structure which is similar to that of CeO₂ parent (JCPDS card no. 34-0394). Meanwhile, CSCrCo anode material has good chemical compatibility with electrolyte (Ce_0.8Sm_0.2O_1.9 (SDC)) in N₂. Through the analysis of XRD and Fourier transform infrared patterns, no other new phase is detected after treatment in 5 % H₂S-N₂ at 800 °C for 5 h, which indicate that the material has a good sulfur tolerance. H₂ temperature-programmed reduction and Tafel curves indicate that the temperature of the best catalytic activity is 600 °C. The electrochemical properties of the cell comprising CSCrCo-SDC/SDC/Ag are measured in 5 % H₂S-N₂ at low temperatures (500 and 600 °C). The maximal open circuit voltage is 1.04 V, the maximal power density is 12.55 mW cm^?2, and the maximal current density is 40 mA cm^?2 at 500 °C. While at 600 °C, the corresponding values are 0.95 V, 14.21 mW cm^?2, and 90.01 mA cm^?2, respectively. After SOFC operating in 5 % H₂S, X-ray photoelectron spectroscopy is used to compare the fresh sample with the H₂S-treated one.     

Epitaxial growth of Ce2Y2O7 buffer layers for YBa2Cu3O7−δ coated conductors using reel-to-reel DC reactive sputtering

Biaxially textured Ce₂Y₂O₇ (CYO) films were deposited on Ni–5at.%W (Ni–5W) tapes by a DC reactive sputtering technique in a reel-to-reel system. Subsequent YBa₂Cu₃O_7?δ (YBCO) films were prepared using pulsed laser deposition leading to a simplified coated conductor architecture of YBCO/CYO/Ni–5W. X-ray diffraction measurements revealed an epitaxial growth of the CYO buffer layer with a texture spread down to 2.2° and 4.7° for the out-of-plane and in-plane alignment, respectively. Microstructural investigations showed a dense, smooth and crack-free surface morphology for CYO film up to a thickness of 350 nm, implying an effective suppression of cracks due to the incorporation of Y in CeO₂. The superconducting transition temperature T_c of about 90 K with a narrow transition of 0.8 K and the inductively measured critical current density J_c of about 0.7 MA/cm² indicate the potential of the single CYO buffer layer.     

The coexistence of magnetic ordering and superconductivity in YBa2(Cu0.94Fe0.06)3O9−σ

We have made superconducting samples of the Y-Ba-Cu-O system with 6% Fe substituted for the Cu. The sample has been verified by X-ray diffraction to be a single-phased 1-2-3 compound. The 6% Fe substitution reduces T_c from 94 to 80 K. The room temperature Mössbauer spectrum shows two pairs of doublets, indicating there are two distinct Cu sites in the sample. The most surprising result is that magnetic ordering of the Fe coexists with superconductivity of the material at 4.2 K.     

Micromechanical model for self-organized secondary phase oxide nanorod arrays in epitaxial YBa2Cu3O7−δ films

A micromechanical model based on the theory of elasticity has been developed to study the configuration of self-assembled secondary phase oxide nanostructures in high-temperature superconducting YBa₂Cu₃O_7?δ films. With the calculated equilibrium strain and elastic energy of the impurity doped film, a phase diagram of lattice mismatches vs. elastic constants of the dopant was obtained that predicts the energetically preferred orientation of secondary phase nanorods. The calculation of the nanorod orientation and the film lattice deformation has yielded excellent agreement with experimental measurements.     

The origin of the enhancement of magnetic properties in Sm2Fe17Nx (0

《Journal of magnetism and magnetic materials》2002,247(1):42-54