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Studies the effects of oxygen partial pressure PO2 during low temperature postheat treatments on the superconducting properties of silver-sheathed Bi2Sr2CaCu2Oy tapes. PO2's impact on the critical temperature Tc of the resulting tape; Characteristics of heat-treated tapes.

Soft X-ray absorption spectroscopy (XAS), using fluorescence yield, was used to study the boron K near edge in MgB2 superconductor. The sample consists of MgB2 crystallites randomly oriented in a magnesium matrix. Abrasion of the sample surface in vacuum provides a surface relatively free of boron-containing impurities. The intrinsic boron K near edge spectrum of the sample at a temperature of 295 K is identified. This spectrum is then compared in detail with a spectrum calculated using the full potential linearized augmented plane wave method. Features predicted by the theory appear near the expected energies, with qualitative agreement regarding shape and intensity. [ABSTRACT FROM AUTHOR]

Abstract: The perovskites Sr2VMoO6 and Ca2VMoO6 have been synthesized by liquid-mix technique in citrate melts, and their electronic, magnetic and structural properties have been investigated. No signs of V/Mo ordering are seen by synchrotron X-ray powder diffraction, but despite the chemical disorder both oxides are highly conductive and Pauli paramagnetic. Electrical conductivities of these solid solutions are comparable or higher than those reported for polycrystalline AMoO3 end members. It is suggested that the delocalized metallic conductivity of these compounds with two different transition-metal atoms implies valence equilibrium between the degenerate oxidation-state couples V4+Mo4+ and V3+Mo5+. [Copyright &y& Elsevier]

A series of multifilament composite conductors (tapes), (Bi,Pb)2Sr2Ca2Cu3Oy (Bi-2223)/Ag, was studied by transmission synchrotron X-ray diffraction, including c-axis texture studies, and by critical current measurements. The diffraction measurements indicate that the tapes typically consist of ∼90% Bi-2223 phase. Rocking curve studies of the Bi-2223 (2 0 0) peak reveal full widths at half maximum (FWHMs) in the range 13–19°. A weak correlation between rocking curve FWHMs and critical current densities suggests that the critical current values in these tapes may not be limited by the Bi-2223 c-axis orientation. [Copyright &y& Elsevier]

Rapidly solidified MnxBi(1-x) (x=0.05 Mn5, x=0.10 Mn10) alloys form as NiAs-type low-temperature phase MnBi nanoparticles in a Bi matrix. Microstructural differences underlie diverse magnetic behavior: Mn5 has well-separated nanorods along the basal plane axial directions of Bi, while Mn10 consists of MnBi nanorods and submicron-sized MnBi grains. The Curie transition of Mn10 is TC=630 K with a second-order thermodynamic character, while Mn5 nanorods exhibit a reduced and hysteretic TC=520 K. Magnetic field annealing fosters (00n) alignment of the MnBi phase. Mn10 also shows alignment of the Bi matrix by a proposed templating mechanism. [ABSTRACT FROM AUTHOR]

Isolated NiAs-type MnBi nanoparticles of 10 nm diameter, embedded in a Bi matrix, were fabricated by melt-spinning and then annealing at 528 K for 1 h. A large coercivity, over 16 kOe at 300 K, with a positive temperature coefficient is observed. In agreement with bulk results, ac susceptibility measurements show a spin reorientation around 90 K coincident with an abrupt decrease in the coercivity with decreasing temperature. The high coercivity at 300 K is attributed to a mixture of nucleation and pinning mechanisms for magnetic reversal. [ABSTRACT FROM AUTHOR]

A large positive magnetic entropy change ΔS[SUBM] is observed in the metallic antiperovskite Mn[SUB3]GaC near its first-order metamagnetic transition temperature 159 K where the stoichiometric compound transforms from an antiferromagnetic to a canted ferromagnetic state accompanied by a discontinuous volume change of 20.46% without change of symmetry. The unusual field dependence of the ΔS[SUBM] of Mn[SUB3]GaC shows a very rapid linear increase from zero to a saturation value within a field interval smaller than 1 T. The broadening of the peak of ΔS[SUBM] to low temperature with increasing field change creates an ΔS[SUBM] plateau in the temperature dependence of the MCE, which is of significance for practical application of these materials in the Ericsson-cycle magnetic refrigerator. [ABSTRACT FROM AUTHOR]

Abstract: The character of the magnetic phase transitions in the metallic antiperovskite Mn3GaC1− δ (, 0.22) was analyzed to better evaluate the associated magnetic entropy changes under various field and temperature conditions. The stoichiometric Mn3GaC compound exhibits an abrupt magnetostructural transition at that produces a large entropy change with a plateau-like temperature dependence. A unique spike in of Mn3GaC at , associated with the abrupt magnetic transformation, is attributed to the interplay between the temperature- and field-induced transitions. The peak magnitude of this entropy change spike approaches 33J/kgK, over two times the averaged . It was observed that the field development of mimics the analogous magnetization behavior. An analytical phenomenological model based on the temperature and field dependence of the magnetization of this type of magnetic transition was developed to account for these results. [Copyright &y& Elsevier]

Magnetization measurements have been made on several high Jc Nb3Sn strands fabricated by different internal-Sn designs. In general these conductors have high magnetization at low fields, often exhibiting flux-jumps that are characteristic of large superconductor diameter. The effective filament size doff is approximately the size of the sub-element because the filament pack within each sub-element is fully coupled. Dividing the filament pack of the sub-element by adding Ta is effective for reducing deff and magnetization instability. But, some residual coupling across the dividers seems to remain below 6 K, perhaps due to Ta3Sn. Implications for accelerator magnets are discussed. [ABSTRACT FROM AUTHOR]

Magnetization measurements show that modern high current-density Nb3Sn strands made for HEP programs exhibit flux-jump instabilities at low fields, due to their having large effective filament diameters. Such instabilities might be problematic because they can initiate a quench in low-field regions of magnets. We explored magnetization and transport measurements of the most recent high Jc Nb3Sn strands and cables to probe the instability behavior. In the regime where flux jumps are seen by magnetization measurements, transport current measurements show a threshold for stability. This threshold is significantly lower than the critical current at higher fields, and above this threshold, quenching in the strand could be initiated by ramping the magnetic field. The threshold current depends on the wire size and internal filament design, and Is consistent with stability criteria. In cables, quench currents were nearly independent of field after training, and were far below the expected critical currents. Details of these measuresments and their implications for testing and use in magnets are discussed. [ABSTRACT FROM AUTHOR]

Nb3Sn was prepared by milling Nb and Sn powder mixtures followed by limited reactions to restrict disorder recovery. Although disorder reduced the superconducting critical temperature Tc, the concomitant electron scattering increased the upper critical field μ0Hc2 to as high as 35 T at 0 K, as determined by the Werthamer-Helfand-Hohenberg equation. Hc2 was higher for longer milling times and lower annealing temperatures. Substitution of 2% Ti for Nb did not appreciably enhance Hc2, suggesting that alloying mitigates the benefits of disorder. Since alloyed Nb3Sn wires have μ0Hc2(0)≈29 T, wires based on heavily milled powders could extend the field range for applications if they can be made with high current density. [ABSTRACT FROM AUTHOR]

A Mn5Bi95 alloy was rapidly solidified into a mixture of nanocrystalline Bi and metastable Bi(Mn). Heating the ribbons to temperature T=525 K in a dc magnetic field causes formation and c-axis alignment of low-temperature phase (LTP) MnBi nanorods along the applied field direction. Nanorod alignment increases with increased magnetic field, with a calculated alignment half-angle of 47° for a sample heated to 520 K at 50 kOe. In situ magnetization changes suggest that nanorod alignment is achieved by rotation of MnBi particles. Particle alignment enables the measurement of the MnBi nanorod spin reorientation temperature of 100 K, the same as its bulk counterpart. [ABSTRACT FROM AUTHOR]

Rapidly solidified, annealed ribbons (20 μm thick) of elemental Bi show a room-temperature ordinary magnetoresistive effect of 250% at 5 T with the field applied perpendicular to the ribbon surface. The effect increases to 10000% at 5 K and 5 T. These values are intermediate to those obtained for single-crystal Bi films and sputtered or evaporated polycrystalline Bi films of comparable thicknesses. The large magnetoresistance of the ribbons is attributed to a very good crystallinity and partial c-axis texture of the ribbon achieved during solidification. Rapid solidification by melt spinning is a promising technique for synthesis of Bi with potential application in magnetoelectric devices. [ABSTRACT FROM AUTHOR]

The magnetic and transport properties of a nanostructured Mn-Bi eutectic composition (∼Mn5Bi95) produced by melt spinning and low-temperature/short time vacuum annealing were studied. A hysteretic magnetostructural transformation from low-temperature phase to high-temperature phase MnBi is confirmed at 520 K. The fact that the transition temperature is lower than that reported for bulk MnBi (633 K), is tentatively attributed to interfacial strain between MnBi and the Bi matrix. A positive temperature coefficient of coercivity is confirmed in the nanocomposites, with a maximum coercivity value of 36 kOe at 500 K. Magnetic field annealing the as-spun composites (525 K at 10 kOe) produces nanoparticle alignment. Annealed MnBi/Bi composites have a very large ordinary magnetoresistance (MR) ratio normal to the ribbon at 5 T, 275% at room temperature and 10 000% at 5 K. [ABSTRACT FROM AUTHOR]

The magnetic character of Pd40FexNi40-xP20, a metallic glass where x=17.5, is experimentally studied over a temperature range of 10–375 K. Hysteresis and temperature-dependent magnetization measurements derived from superconducting quantum interference device magnetometry on the amorphous alloy provide insight into the role of applied magnetic field on magnetic phase formation during the nanocrystallization process. Specifically, the microstructure of the melt-spun form of Pd40(Fe,Ni)40P20 is discussed in correlation to the observed superparamagnetic and spin-glass behavior. Despite the extremely high quenching rate used to synthesize the alloy, this composition shows evidence of chemical segregation on the nanoscale in the as-solidified state. [ABSTRACT FROM AUTHOR]

The perovskites Sr2MnRuO6 and LaSrMnRuO6 have been investigated and their properties compared with those of the broader family of A2MRuO6 double perovskites (A=Sr, La; M=Cr, Mn, Fe, Co, Ni). Neutron powder diffraction shows that both phases lack long-range Mn/Ru order. Sr2MnRuO6 exhibits a cooperative Jahn–Teller distortion, conductivity that proceeds via variable-range hopping, and antiferromagnetic (AFM) order (C-type structure, TN≅200K, 2.05(1) μB per transition metal). LaSrMnRuO6 exhibits nearly undistorted octahedra, conductivity that proceeds via diffusion-assisted small-polaron hopping, and ferromagnetic (FM) order ( TC≅220K, 1.60(4) μB transition metal). Band structure calculations show that AFM Mn–O–Ru coupling optimizes polarization of the Ru t2 g orbitals, while FM coupling favors delocalization of the Ru t2 g electrons. The transition from antiferromagnetism to ferromagnetism is linked to a loss of orbital order and stabilized over competing glassy states by delocalization of Ru t2 g electrons. [ABSTRACT FROM AUTHOR]

As-prepared, single-crystalline bismuth ferrite nanoparticles show strong size-dependent magnetic properties that correlate with:  (a) increased suppression of the known spiral spin structure (period length of ∼62 nm) with decreasing nanoparticle size and (b) uncompensated spins and strain anisotropies at the surface. Zero-field-cooled and field-cooled magnetization curves exhibit spin-glass freezing behavior due to a complex interplay between finite size effects, interparticle interactions, and a random distribution of anisotropy axes in our nanoparticle assemblies. [ABSTRACT FROM AUTHOR]

Abstract: The magnetic properties of the new metal oxide bi-layer hydrate Na0.3NiO2·1.3H2O were investigated. DC and AC magnetic susceptibility measurements revealed the compound to be a spin glass at low temperatures with a freezing point of T f∼3.5K. Using the Vogel–Fulcher relationship an activation energy of E a/k∼71.6K, a Vogel–Fulcher temperature of T o∼1.9K and a frequency dependence of the freezing temperature consistent with a frustrated magnetic spin-glass rather than a RKKY spin glass were determined. Ni K-edge X-ray absorption spectroscopy (XAS) show an absorption edge shift toward higher energies in Na0.3NiO2·1.3H2O compared to Na0.7NiO2 and suggests that Ni occurs in two possible electronic configurations: a combination of 3d7 (S=1/2) and 3d6 (S=0) or 3d7L states, with L representing a hole in the oxygen p-states. [Copyright &y& Elsevier]

The complex interplay between superconducting and magnetic phases remains poorly understood. Here, we report on the phase separation of doped holes into separate magnetic and superconducting regions in superoxygenated La2−xSrxCuO4+y, with various Sr contents. Irrespective of Sr-doping, excess oxygen raises the superconducting onset to 40 K with a coexisting magnetic spin-density wave that also orders near 40 K in each of our samples. The magnetic region is closely related to the anomalous, 1/8-hole-doped magnetic versions of La2CuO4, whereas the superconducting region is optimally doped. The two phases are probably the only truly stable ground states in this region of the phase diagram. This simple two-component system is a candidate for electronic phase separation in cuprate superconductors, and a key to understanding seemingly conflicting experimental observations. [ABSTRACT FROM AUTHOR]

Abstract: We have discovered an electronically driven phase separation in super-oxygenated La2− x Sr x CuO4+ y , with various Sr contents. The samples have been characterized by muon spin rotation and bulk magnetization measurements. For all Sr levels with x<0.125, excess oxygen produces samples with a superconducting phase with T C near 40K and a separate magnetic phase, a spin density wave that also orders near 40K. The observed magnetic region is closely related to the anomalous, 1/8th hole doped, magnetic versions of La2CuO4, and the superconducting region is optimally doped. The two phases are likely to be the only stable ground states in this region of the phase diagram. This simple two-component system may represent the most direct observation of electronic phase separation in cuprate superconductors. Thus La2CuO4, doped with substitutional Sr and interstitial O introduces an interesting new material system for the field of high-temperature superconductivity. [Copyright &y& Elsevier]

Abstract: The effects of growth conditions and composition on the crystal growth of La-2126-phase superconducting materials have been studied by the floating-zone method. The composition of La-2126-phase single crystals was (La1− x Sr x )2CaCu2O6+ δ (). The crystals were grown under oxygen pressures from 1bar to 11bar. A smooth planar solid–liquid growth interface of (La1− x Sr x )2CaCu2O6+ δ () tended to break down into a cellular interface when the growth velocity was higher than 0.5mm/h, or the Sr concentration . When the planar solid–liquid growth interface broke down into a cellular interface, the single-crystal size in an as-grown rod decreased abruptly and the as-grown rod was not of single phase. Various small sizes of CaO second phases were found inside the as-grown single crystals of (La1− x Sr x )2CaCu2O6+ δ (x ⩾0.125). Based on the occurrence of these CaO second phases inside the as-grown crystals, the estimated solubility of as-grown single crystals is for (La1− x Sr x )2CaCu2O6+ δ . The melting point of the La-2126 materials increases with increasing oxygen pressure. Cubic-centimeter-size single crystals were successfully grown for (La1− x Sr x )2CaCu2O6+ δ (). The superconducting transition temperature of the as-grown (La1− x Sr x )2CaCu2O6+ δ single crystals increases from to 46K with increasing the Sr concentration x from 0.075 to 0.25 at 11bar oxygen pressure. [Copyright &y& Elsevier]

Abstract: The effects of growth conditions and composition on the crystal growth of (La1−x Ca x )2CaCu2O6+δ (x=0.05–0.15) superconducting materials have been studied by using the floating zone method. A smooth solid-liquid growth interface of (La1−x Ca x )2CaCu2O6+δ (x=0.05–0.06) tended to break down into a cellular interface when the growth velocity was higher than 0.5mm/h. When the solid–liquid interface became cellular, the single crystal size in an as-grown rod decreased abruptly and the as-grown rod was not single phase. Various sized CaO inclusions were found inside the as-grown single crystals of (La1−x Ca x )2CaCu2O6+δ (x≥0.075). Cubic centimeter size single crystals were successfully grown for both (La0.95Ca0.05)2CaCu2O6+δ and (La0.94Ca0.06)2CaCu2O6+δ . The as-grown single crystals of (La1−x Ca x )2CaCu2O6+y under 1bar pressure of oxygen were not superconducting. The superconducting temperature of the (La1−x Ca x )2CaCu2O6+y crystals grown at 11bar oxygen is 28K. The application of high pressure oxygen during crystal growth is an effective method to get bulk superconductivity in the crystals. [Copyright &y& Elsevier]

Abstract: We have prepared the oxyhydrate Na0.3RhO2·0.6H2O by extracting Na+ cations from NaRhO2 and intercalating water molecules using an aqueous solution of Na2S2O8. Rietveld refinement, thermogravimetric analysis (TGA), and energy-dispersive X-ray analysis (EDX) reveal that a non-stoichiometric Na0.3(H2O)0.6 network separates layers of edge-sharing RhO6 octahedra containing Rh3+(4d6, S=0) and Rh4+ (4d5, S=1/2). The resistivities of NaRhO2 and Na0.3RhO2·0.6H2O (T&#60;300) reveal insulating and semi-conducting behavior with activation gaps of 134 and 7.8meV, respectively. Both Na0.3RhO2·0.6H2O and NaRhO2 show paramagnetism at room temperature, however, the sodium-deficient sample exhibits simultaneously a weak but experimentally reproducible ferromagnetic component. Both samples exhibit a temperature-independent Pauli paramagnetism, for NaRhO2 at T>50K and for Na0.3RhO2·0.6H2O at T>25K. The relative magnitudes of the temperature-independent magnetic susceptibilities, that of the oxide sample being half that of the oxyhydrate, is consistent with a higher density of thermally accessible electron states at the Fermi level in the hydrated sample. At low temperatures the magnetic moments rise sharply, providing evidence of localized and weakly-ordered electronic spins with effective moment per formula unit values of 2.0×10−1 μB for NaRhO2 and 0.8×10−1 μB for Na0.3RhO2·0.6H2O. [Copyright &y& Elsevier]

We present our studies of superconducting and microstructural properties of bulk MgB[sub 2]/Mg nano-composites and press-sintered MgB[sub 2]. TEM investigation revealed that the composites are very dense, consisting of nano-sized MgB[sub 2] grains connected by the well-defined clean grain boundaries (GB). Both amorphous and structural intact GB's were observed in the press-sintered samples. Magnetization and transport measurements showed that the composite carried over 1 MA/cm² critical current density (J[sub c]) at 5 K and self-field, while the pressed sintered MgB[sub 2] has J[sub c] ∼ 0.25 MA/cm² at 5 K and self-field. The origin of substantially higher J[sub c] found in the composites is mostly due to their nano-sized grains and clean grain boundaries. [ABSTRACT FROM AUTHOR]

Fe ions of dose 5×1016 cm-2 were implanted at 200 keV into a-plane ZnO epitaxial films. The epitaxial quality of the postannealed samples was verified by x-ray diffraction ω-rocking curves and φ scans, whereas x-ray absorption spectroscopy identified the presence of both Fe2+ and Fe3+ ions, as well as changes in their relative concentration during postannealing. Superconducting quantum interference device measurements show that the as-implanted and postannealed films are ferromagnetic at room temperature. The saturation magnetization reduces during annealing possibly due to the decrease in the number of oxygen vacancies. [ABSTRACT FROM AUTHOR]

Using an electron diffraction technique, the thicknesses of the (nonorthorhombic) twin boundary layers were measured for pure and alloyed YBa2(Cu1-xMx)3O7-δ. Boundary thickness varied from ∼0.7 nm for M=Ni, x=0.02, and δ[bar_over_tilde:_approx._equal_to]0 to ∼2.6 nm for M=Al, x=0.02, and δ[bar_over_tilde:_approx._equal_to]0, while it was ∼1.0 nm for a pure YBa2Cu3O7. High-resolution transmission electron imaging of similar specimens supports the existence of such twin boundary layers. [ABSTRACT FROM AUTHOR]