Neutron induced disorder in superconducting A-15 compounds

The effect of high energy neutron (E>1 MeV) irradiation at 60°C on the superconducting critical temperature, Tc, the upper critical field, Hc2, the lattice parameter, ao, and the degree of Long Range Order has been measured for Nb and V based A-IS superconducting compounds. Large reductions in Tc, and Hc2are observed for fluences up to 5.0 × 10¹⁹n/cm². For Nb3Al aoincreases and the degree of Long Range Order is significantly reduced as Tcis depressed. The results are discussed in terms of atomic ordering in the A-15 structure.     

Neutron irradiation of Nb3Sn and NbTi multifilamentary composites

NbTi and Nb3Sn multifilamentary composites have been irradiated with fast-neutrons at 60 ± 5°C to fluences of 1.2×10²⁰n/cm²(E > 1 MeV). The NbTi samples show only a moderate reduction of Icas a function of neutron fluence in an applied field of 40 kG. Reductions in Icwere observed for fluences greater than 3 × 10¹⁷n/cm²and saturate at 18% for fluences greater than 3-4 × 10¹⁹n/cm². The Nb3Sn composites showed large neutron radiation induced changes in Tc, Icand Hc2. Reductions in Tcwere observed for fluences greater than 7 × 10¹⁷n/cm². No measurable changes in Ic(40 kG) were observed below 10¹⁸n/cm². Between 2 and 3×10¹⁸n/cm², however, there is an apparent threshold where a very rapid reduction in Ic(40 kG) is initiated. At the threshold the decrease in Tcis 13%. Between the threshold and 1.1 × 10¹⁹n/cm², I2(40 kG) has been reduced to 4% of the unirradiated value. These changes in superconducting properties in NbTi and Nb3Sn are analyzed in terms of the radiation induced defects. The impact of the response to irradiation of both materials on their applications in fusion reactor magnets is discussed.     

Static and dynamic properties of short, narrow, variable-thickness microbridges

The electrical properties, including the Josephson-effect response to microwave radiation, have been studied for extremely small, high-resistance microbridges of Pb-In alloy and unalloyed In, with dimensions ranging from 300Å to 2000Å. The IcR product of In and Pb-In microbridges decreases smoothly as the bridge cross section is reduced, approaching the Ginzburg-Landau limit of 0.64 mV/K for the smallest bridges. The voltage range of microwave response and the temperature range of hysteresis-free operation both increase (improve) as the bridge is made narrower, in agreement with Joule heating theory. For example, an 8 ohm Pb0.9In0.1bridge with all dimensions ≤500 Å has a maximum step voltage ofV_{max} = 1.5mV and a nonhysteretic temperature range ofDeltaT_{no hyst} = 1.2K. Bridges of unalloyed In can show still better response due to a longer coherence length, and nonhysteretic operation over the full temperature range below Tcis possible.     

Electric and magnetic properties of Eu1-xGdxS films and GdS-EuS film junctions

Polycrystalline Eu1-xGdxS films (0 < x < 0.35 and x = 1; thickness is 40 to 600 nm) were prepared by electron-beam and flash evaporation onto heated substrates. The electrical and magnetic properties of these films were investigated through resistivity and hysteresis measurements at temperatures between 4 and 300 K, with a magnetic field, B, of 0.4 Tesla. After a short discussion of intrinsic differences between the electric and magnetic behavior of films and single crystals, the first results on I-V characteristics of isomorphic GdS-EuS-GdS film junctions are presented. With increasing x and/or lattice-defect concentration, the Curie temperature, TC(x), increases to about 150 K, while the magnetization Js(x), strongly decreases (extrapolating to Js=0 at x=0.5). Typical properties of the GdS-EuS-GdS junctions are the N-type I-V characteristics and the attributed current oscillations at T ≪ TC, the change to Ohmic I-V characteristics for T approaching TC, and the large negative magnetoresistance, ranging up to ΔR/R/ΔB = -10 T^-1, at small B and T > TC, where ρ(T) passes a broad maximum.     

High field properties of ternary metal-molybdenum-sulfides

Measurements of the upper critical fields are presented for ternary metal-molybdenum-sulfides as a function of temperature, T, with dc magnetic fields up to ∼215kG and pulsed fields to ∼500kG. A nearly linear variation of HC2versus T is observed up to the highest dc fields for all the compounds. The materials (nominal composition), Tc(measured);(dH_{C2}/dT)_{T=T_{c}}(measured), and HC2(4.2 K) (calculated) for a dirty type II superconductor assuming no paramagnetic limiting are respectively: (a) Pb0.9Mo5.1S6: 11.7 K, 60 kG/K, 390 kG; (b) SnAl0.5Mo4.5S6: 14.2 K, 32 kG/K; 275 kG; (c) SnMo5S6:13.4K, 37kG/K; 290 kG; (d) PbMo5.1S6: 14.4, 60kG/K, 515kG. Pulsed field measurements yield HC2(4.2 K) = 390 kG for (a), 275 kG for (b), 290 kG for (c), and ≃510kG for (d). The PbMo5.1S6has the highest value of HC2measured to date.     

Magnetostatic interactions in a duplex ferromagnetic alloy

The chill-cast alloy, Fe75Si15B10, consists of submicron rods of Fe2B, a magnetically "hard' phase with high Tc, in a matrix of Fe3Si, a "soft" phase with low Tc. Magnetic properties change markedly on cooling through 590°C, the Tcof Fe3Si, and 245°C, where the magnetocrystalline anisotropy of Fe2B changes from easy-axis to easy-plane. The unusual magnetic and thermomagnetic behavior observed is discussed in terms of the short-range and long-range magnetostatic interactions between the two phases.     

Irradiation and annealing effects of deuteron irradiated NbTi and V3Ga multifilamentary composite wires at low temperature

To study the effects of low temperature irradiation on technological type II-superconductors, NbTi and V3Ga multifilamentary composite wires, the critical current Icand the transition temperature Tcwere measured before and after irradiation with 50 MeV deuterons at 10 K and 15 K, respectively. While the irradiation effects on Icand Tcof NbTi are substantially unaffected, the V3Ga wires undergo a reduction in Icof about 50 % and Tcdecreases from 14.7 ± 0.1 K to 12.3 ± 0.1 K at a total deuteron flux of 2.7 × 10¹⁷cm^-2. Annealing experiments at room temperature and 100° C show only a small recovery of the superconducting properties up to 15 %. The field dependence of the volume pinning force densities Pvwas determined and the results are shown to be consistent with a qualitative dynamic pinning model.     

Magnetic moments and curie temperatures of (Fe, Ni)80(P, B)20amorphous alloys

The magnetic moment per transition metal atom at 0°K and the Curie temperature were obtained for a series of (Fe, Ni)80(P, B)20amorphous quenched alloy ribbons. Fe/Ni and P/B compositions were varied separately. The moment data can be fitted well by assigning 2.1 Bohr magnetons per Fe atom and 0.6 per Ni atom, with the moment being lowered by 0.3 per B atom and 1.0 per P atom. Alternatively, moments varying with composition, as shown by neutron diffraction in crystalline alloys, combined with a lowering of 1.2 per B atom and 2.1 per P atom, also fit well. For a given P/B composition, Tcshows a broad maximum at Fe:Ni of about 3:1. For a given transition metal composition, Tcincreases with increasing B content.     

Surface-micromachined pyroelectric infrared imaging array withvertically integrated signal processing circuitry

Surface-micromachining techniques have been used in the fabrication of a 64×64 element PbTiO₃ pyroelectric infrared imager. Polysilicon microbridges of 1.2 μm-thickness have been formed 0.8 μm above the surface of a silicon wafer. Each of the 4096 polysilicon microbridges measures 50×50 μm² and forms a low thermal mass support for a 30×30 μm² PbTiO₃ pyroelectric capacitor with a thickness of 0.36 μm. The air-bridge formed reduces the thermal conduction path between the detector element and substrate. An NMOS preamplifier cell is located directly beneath each microbridge element. The measured blackbody voltage responsivity at 30 Hz is 1.2×10⁴ V/W. The corresponding measured normalized detectivity (unamplified) D* is 2×10⁸ cm-Hz^1/2W at 30 Hz. The test chip fabricated measures 1×1 cm² and contains more than ten thousand transistors and 4096 micromechanical structures with integrated ferroelectric microsensors. The technique of stacking of microsensors and integrated circuits represents a new approach for achieving high-density and high-performance integrated pyroelectric microsensors through minimization of circuit to sensor interconnection with extremely small thermal crosstalk     

Phase-slip centers in superconducting indium microbridges

Measurement is made of the transition to the resistive state of nearly one-dimensional superconducting microbridges of indium. The length of the microbridges ranged from 30 to 130 µm. The experiments concentrated in particular on the onset and temperature dependence of the hysteresis of the voltage-current characteristic, the temperature dependence of the excess current, and the equivalent normal length of the phase-slip centers generated during the resistive transition. The results are in good agreement with the dynamic model of a phase-slip center proposed by Kadin, Smith, and Skocpol.     

Preparation and properties of multifilament niobium carbonitride superconductor

The preparation and characterization of a multifilament niobium carbonitride yarn is reported here for the first time. The fine diameter multifilament yarn was prepared by chemical conversion of a precursor carbon yarn. The fibers are generally composed of both NbC and NbCN phases and exhibit a Tcof 16-17.5°K and an Hc2of 110- 120kG. Icvalues at zero field of up to 130 amperes have been measured corresponding to a Jcof 10⁶A/cm². Icis proportional to the amount of superconductor phase present as judged by weight gain data. At low conversion levels an anomaly has been detected in critical current behavior of the fibers suggesting a non-continuous superconducting path. Electrical contact to the fibers by copper coating is discussed showing the importance of obtaining a low resistance contact. Jcwas measured as a function of magnetic field for both the pure material and samples doped with silicon using SiCl4. The silicon dopant was found to be effective in reducing the field dependence of the critical current.     

Influence of R1parameter {=Fe2O3/(Y2O3+ Sm2O3+ Lu2O3)} on magnetic bubble properties of (YSmLuCa)3(FeGe)5O12

The influence of R1{=Fe2O3/(Y2O3+Sm2O3+Lu2O3)} in the melt composition on film properties, and growth characteristics, has been investigated for (YSmLuCa)3(FeGe)5O12. The garnet phase is the primary phase when R1is kept between 10 and 60. The temperature coefficient for the bubble collapse filed changes from -0.29 to -0.19 %/°C, and the growth rate with 10°C supercooling changes from 0.60 to 0.15 μm/min, by increasing R1=10 to 60. Distribution coefficients, K^Y, K^Sm, K^Luand K^Geincrease, and K^Feand K^Cadecrease, with increasing R1.     

Curie temperature of METGLAS magnetic alloys measured by different techniques

A series of experiments was performed to determine the Curie temperature Tcof six METGLAS® magnetic alloys using three different techniques: thermogravimetry with magnetic field, differential scanning calorimetry (DSC), and magnetic susceptibility. The purpose of the work was to assess the sensitivity of the three methods, to establish the relationships between magnetic properties and enthalpy in the vicinity of Tc, and finally, to determine physically valid criteria for Tcwhen using the susceptibility method. The full matrix of data is given for the following METGLAS alloys: 2605S-2, 2605SC, 2605S-3A, 2705M, 2826MB, and 2605CO. Good accord between the temperature-dependence of magnetic moment, susceptibility, and enthalpy of the first five alloys is shown. A feature which can be identified with Tcis defined for each of the measurement methods. Interesting behavior was observed in the case of the METGLAS alloy 2605CO which has so small an enthalpy of the magnetic transition that DSC cannot be used. Both magnetic methods used, thermogravimetry with magnetic field and susceptibility, reveal a steplike temperature-dependence of magnetic moment and susceptibility. A strong dependence of Tcon the heating rate during measurement was observed, indicating superposition of the magnetic disordering and relaxation processes in the amorphous state. Extrapolation of the data shows that the virtual Tcfor METGLAS 2605CO in the completely amorphous state should be as high as 547°C (820 K).     

Magnetic interactions in pseudobinary rare earth-3d intermetallics

Magnetic measurements on pseudobinary intermetallic compounds of the series R(MnxFe1-x)2(R = Y, Gd, Er) R6(MnxFe1-x)23(R = Y, Gd, Tb, Dy, Ho, Er, Tm) R2(FexCo1-x)17(R = Y, Gd, Dy) R2(FexNi1-x)17(R = Y, Gd) are reported. Magnetization and susceptibility measurements have been performed at temperatures from 4 K to 1500 K. The magnetic interactions between Mn and Fe are discussed in terms of a localized-moment model. In the case of the R2(FexM1-x)17(M = Co, Ni) compounds, a band model seems to be more appropriate. Mössbauer Fe⁵⁷spectra obtained on the R2(Fe,Co)17series can be explained by a superposition of four different six-line patterns corresponding to the four crystallographically nonequivalent 3d sublattices.     

Magnetic and magnetooptical properties of sputtered Fe5Si3films

Polycrystalline films of Fe5Si3have been prepared by RF sputtering from a sintered target composed of 62.5 at% Fe and 37.5 at% Si. The Faraday rotation and optical absorption were measured for these films in the visible and near infrared. At a wavelength of 6328Å, room-temperature absorption (corrected for reflection) and specific Faraday rotation are 3.7 × 10⁵cm^-1and 1.3 × 10⁵deg/cm, respectively. The rotation decreases with rising temperature and vanishes around 120°C, the Curie temperature of the films. From polar and in-plane hysteresis loop measurements the spontaneous magnetization is found to lie in the film plane. Square hysteresis loops are observed when the external field is applied in the plane of the film, and the in-plane coercive force decreases monotonically with temperature at an average rate of -2 Oe/°C as Tcis approached.     

Magnetic properties in the system Iron-Cobalt-Aluminum

The ternary alloy system Fe-Co-Al is studied in the body-centered cubic (bcc) phase for the composition dependence of room temperature magnetization, saturation induction, and Curie temperature Tc. The system has potential for high temperature application as soft magnetic material. The results expand upon earlier measurements. The presence of a broad maximum in magnetization near the composition 70 Fe-30 Co (weight percent) in their binary does not persist beyond about 5 wt% Al in the ternary family. Previously reported Curie temperature results are confirmed including an anomalous increase of Tcwith Al dilution for a certain range of composition. This feature is explained as due to a stabilization of the α-phase to higher temperatures.     

Interaction between phase-slip centers in superconducting indium microbridges

We have studied the electric behavior of phase-slip centers in superconducting indium microbridges of 0.9 µm thickness, 3–4 µm width, and 70–130 µm length. Our measurements were performed in the temperature range 3–10 mK below the critical temperature T_c resulting in relatively large values (about 10–20 µm) of the quasiparticle diffusion length. The temperature dependent Ginzburg-Landau coherence length (T) has been determined for all samples from the measured temperature dependence of the critical current density. The fabrication of two notches in the indium bridge has been found to localize reliably the nucleation site of the first two phase-slip centers. Our measurements quantitatively confirmed the dc interaction predicted by the model of Kadin, Smith, and Skocpol. In addition, for temperatures very close toT _c , an ac interaction has been observed which disappears abruptly for decreasing temperature. No voltage coupling has been detected.     

Spatial decay of the quasiparticle current in phase-slip centers

We have studied the diffusive current of quasiparticles in long superconducting tin microbridges in two different experiments. In the first experiment, the influence of quasiparticle current generated by one phase-slip center upon the appearance of other phase-slip centers in the same microbridge is examined. In the second experiment, the spatial dependence of the chemical potential of the quasiparticles generated in a single phase-slip center is measured directly. The results of these experiments demonstrate conclusively the exponential decay of both the quasiparticle current and potential from the center of the phase-slip center, and the measured values of the characteristic diffusion length agree well with theoretical predictions, both in magnitude and temperature dependence.     

Comments on superconducting science and technology

At present a large part of scientific work, both experimental and theoretical, in superconductivty is stimulated by applications. The two most active areas are (1) nonequilibrium phenomena in superconductors, stimulated by applications based on quantum aspects and (2) research on the electron-phonon interaction and other factors that determine the transition temperature, Tc, stimulated by the search for high Tcmaterials. It is suggested that large scale applications, such as to the electric power industry, would be aided by more basic research by university scientists and engineers on the mechanical and electro-magnetic properties of currently available materials that operate at liquid helium temperatures.     

A new permanent magnet material: Modified Ca ferrite

An experiment was carried out to investigate the preparation condition of anisotropic Ca system ferrite magnets with optimum magnetic and physical properties. Compositions were chosen according to the formula (CaO.nFe2O3)100-x(La2O3)x, wherexwas varied between 0 and 4.0, and n between 5.5 and 6.25. The optimum condition of making magnets and some properties of a typical specimen are as follows. The preparation condition: composition (CaO.6Fe2O3)97(La2O3)3, semisintering condition 1250°C × 1 h in O2gas, sintering condition 1275°C × 0.5 h in O2gas. Magnetic properties:4piI_{10}k = 4200G, 4πIr = 4100 G,IHC= 2100 Oe,BHC= 2050 Oe,(BH)_{max} = 3.5MG . Oe, Ku = 3.34 × 10⁶erg/cc, HA= 20.5 kOe,sigma_{s} = 64.8emu/g, Tc = 446°C.