Dependency of Flux Jump on Temperature and Field-Sweep Rate for Textured (Nd1/3Eu1/3Gd1/3)Ba2Cu3O7- δ Bulk Superconductor with Gd-211 Particles

Anisotropy flux jumps in the mixed state of a textured (Nd_0.33Eu_0.33Gd_0.33)Ba₂Cu₃O_7-δ bulk superconductor with Gd-211 doping particles have been studied by means of magnetization measurements in the model of the magnetic field paralleling and perpendicular to the c axis. A typical anisotropy flux jump was observed at a temperature ranging from 2.0 to 3.0 K. Under the magnetic field perpendicular to c axis, no flux jump was found at whole temperature range until the sweep rate of 200 Oe/sec. For the magnetic field paralleling to the c axis, the number of flux jumps decreased with the increase of temperature, and the third quadrant of the M–H curve is the most flux-instability quadrant. The magnetic field sweep rate dependences of flux jumps were also studied and the influence of flux creep on flux jumps was also discussed.     

Flux Jump Behaviors and Mechanism of MgB<Subscript>2</Subscript> Synthesized by the Non-special Atmosphere Synthesis

MgB₂ samples were prepared from a stoichiometry mixture of Mg and B inside stainless steel tubes. The transition temperature of the specimens was 37.5 K with a sharp transition width within 1 K. From the magnetic hysteresis measurements, flux jump was observed up to 15 K. The flux jump is believed to begin at the fluxes pinned at the defects. An over-moving flux situation formed at the places where there were moving fluxes that had been pinned at the defect and steady state ones. The flux jumps depend not on the amount of impurities and second phases, but on their distribution, pinning strength and heat capacity.     

Thermal Relaxation and Heat Transport in the Spin Ice Material Dy2Ti2O7

The thermal properties of single crystalline Dy₂Ti₂O₇ have been studied in a temperature range from 0.3 K to 30 K and magnetic fields applied along [110] direction up to 1.5 T. Based on a thermodynamic field theory various heat relaxation and thermal transport measurements were analysed. So we were able to present not only the heat capacity of Dy₂Ti₂O₇ in the whole temperature and magnetic field range, but also the different contributions of the magnetic excitations and their corresponding relaxation times in the spin ice phase. In addition, the thermal conductivity and the shortest relaxation time were determined by thermodynamic analysis of steady state heat transport measurements. Finally, we were able to reproduce the temperature profiles recorded in heat pulse experiments on the basis of the thermodynamic field theory using the previously determined heat capacity and thermal conductivity data without additional parameters. Thus, the thermodynamic field theory has been proved to be thermodynamically consistent in describing three thermal transport experiments on different time scales. The observed temperature and field dependencies of heat capacity contributions and relaxation times indicate the magnetic excitations in the spin ice material Dy₂Ti₂O₇ as thermally activated monopole-antimonopole defects.     

Increasing thermomagnetic stability of composite superconductors with additives of extremely-large-heat-capacity substances

We have studied the thermomagnetic stability (with respect to magnetic flux disturbances) of composite superconductors screened by additives of rare earth compounds possessing extremely high heat capacity at low temperatures. Three tubular composite structures have been manufactured and studied with respect to screening of the central region from variations of an external magnetic field. The effect of large-heat-capacity substances (LHCSs) was evaluated by measuring a jump in the magnetic flux in response to the rate of variation (ramp) of the external magnetic field. It is established that the adiabatic criterion of stability (magnetic-flux jump field) in the sample structures containing LHCSs significantly increases—by 20% for HoCu₂ intermetallic compound and 31% for Gd₂O₂S ceramics—as compared to the control structure free of such additives.     

Asymmetry between flux penetration and flux expulsion in Tl-2212 superconductors

dc magnetic hysteresis as well as flux penetration and flux expulsion were investigated in Tl_2–y Ba₂CaCu₂O_8–x polycrystals and monocrystals. All measurements were performed at 35 K and in the 0–5 T field range. Hysteresis measurements revealed an irreversibility field of about 2 T. Existing models predict identical field-cooled (fc) and zero-field-cooled (zfc) magnetizations and vanishing time dependence above this field. Although the identical fc and zfc magnetizations are in fact observed, the time dependence vanishes only for flux penetration after zero-field cooling; a remanence is preserved after field cooling and decays with a finite relaxation rate. Activation energies calculated on the basis of the thermal activation model display a pronounced field dependence, and arelower for flux penetration than for flux expulsion in high fields (H3 T) for all orientations. This behavior of extreme layered superconductors contradicts classical theoretical models and questions the original definition of the irreversibility line as well. All of our results are consistent with the recent theory of lock-in transition, and can be well interpreted by using those principles.     

Flux jump in the flux creep state

Taking the magnetic diffusivity of the superconductor in the flux creep state into consideration, the maximum magnetic field differences (H_fj) at which a flux jump will occur are calculated. From our calculated results, assuming the extreme limit D_m ? D_th, H_fj will have a power dependence of between $\text{?}\text{1}\text{6}$ and $\text{?}\text{1}\text{2}$ on the magnetic field sweep rates. For low magnetic field sweep rates and good cooling conditions, the values of H_fj in the flux creep state become considerably larger than those calculated by other authors in the adiabatic state.     

Magnetothermal properties of Nb-W,Nb-Ti,and Nb-Mo alloys in the mixed state

Flux jumps and magnetothermal effects in single crystals of niobium and niobium doped with tungsten, titanium, and molybdenum have been studied in the mixed state in a vacuum calorimeter. It is observed that (a) the field at which the first flux jump occurs is independent of the sweep rate at low sweep rates (1–10 Oe sec) but increases sharply at higher sweep rates (～100 Oe/sec) of the magnetic field, (b) the heat evolved during the flux jump becomes larger as the sample's temperature is reduced, (c) the temperature fluctuations of the type observed by Zebouni et al. and Levy et al. are evidently due to the imperfect adiabatic conditions of experiments, and (d) the interval between the successive flux jumps is in good agreement with that expected from the critical state model of Swartz and Bean.     

ac magnetic instability in non-ideal type-II superconductors

The theoretical approach is carried out with the combined thermal and magnetic equations which are based on the critical state model for non-ideal type-II superconductors and include controllable physical parameters. Also, the behaviour of flux jumps is studied on Nb-Ti-Zr ternary alloy wires in the temperature region 1.8–9.0 K under various heat transfer conditions. The ac field method applied here has the advantage of determining flux jump fields more exactly than the linear sweep method.     

Etude dilatometrique de la transition ferroelastique de l'orthophosphate de plomb monocristallin

Single crystals of lead phosphate Pb₃ (PO₄)₂ have been grown by Czochralski pulling. Thermal expansion has been investigated in this material by standard dilatometry and by X-ray powder diffraction, between room-temperature and 250° C, across the 180° C ferroelastic phase transition. Behaviour of the spontaneous strain, a most important quantity for ferroelastics, is deduced from thermal expansion measurements along the b and c monoclinic axes. The large room-temperature value of the strain component corresponding to a shear in the (100) plane of the crystal, 22 × 10^?3, agrees with the value derived from lattice parameters. A jump of the spontaneous strain is observed at the Curie point which discloses a first order transition. This result, confirmed by differential thermal analysis measurements, is also consistent with recent birefringence data. The estimated transition heat is 0.17 cal/g.     

Observation of magneto-thermal instabilities in (YxTm1−x)Ba2Cu3O7 single crystals in pulsed magnetic field magnetization measurements

The understanding of flux jumps in the high temperature superconductors is of importance since the occurrence of these jumps may limit the perspectives of the practical use of these materials. In this work we present the experimental study of the role of heavy ion irradiation in stabilizing the HTSC against flux jumps by comparing un-irradiated and 7.5 · 10¹⁰ Kr-ion/cm² irradiated (Y_xTm_1–x)Ba₂Cu₃O₇ single crystals. Using pulsed field magnetization measurements, we have applied broad range of field sweep rates from 0.1T/s up to 1800 T/s to investigate the behavior of the flux jumps. The observed flux jumps, which may be attributed to thermal instabilities, are incomplete and have different amplitudes. The flux jumps strongly depend on the magnetic field, on the magneto-thermal history of the sample, on the magnetic field sweep rate, on the critical current density j_c, on the temperature and on the thermal contact with the bath in which the sample is immersed.     

Specific heat jump of two-band superconductor KFe2As2 using Ginzburg-Landau theory

In this study specific heat jump using two-gap Ginzburg-Landau (GL) theory has been calculated. In contrast to the previous approaches, we have taken into account intergradient order parameters interaction in the GL free energy functional. The thermodynamic magnetic field revealed nonlinear temperature dependence due to interband interaction between order parameters and their gradients. The calculations showed that the specific heat jump in two-order parameter superconductors was smaller than that of single-order parameter superconductors. It has been shown that such a model is in good agreement with experimental data for KFe₂As₂ superconductors.     

Pulsed magnetic field losses and critical current densities of V3 Ga and Nb3Sn multifilament superconductors

Pulsed magnetic field loss measurements have been performed on special V₃Ga filament superconductors, which are known to have very high critical current densities, and on a NB₃Sn filament superconductor.The dependence of the hysteresis losses on the diffusion heat treatment parameters for the V₃Ga layer formation is reported. To analyse the results, the hysteresis losses of hollow superconducting filaments in parallel and perpendicular magnetic fields were calculated.From E.J. Kramer's theory of flux pinning we postulate that, for a given material, upper and lower intrinsic limits of the hysteresis losses are due to the flux line shearing effect. A part of the results was presented at the ICEC 7.     

Current–Voltage Measurements and Vortex Dynamics in a Bi2Sr2CaCu2O x Whisker: Observation of the Peak Effect and Calculation of the Critical Current in the Flux Flow Regime with and Without Applied Field

In this article, we report current–voltage measurements carried out on a Bi₂Sr₂CaCu₂O _x whisker in a large temperature range below the critical temperature with and without applied magnetic field. We examine the critical current peak effect and the vortex dynamics at low field. The critical current peak effect consists of the initial increase of the critical current that subsequently decreases as the applied field is increased. For current–voltage measurements, this effect is associated with a change in the current–voltage curves that are typical of the flux flow regime at low fields and resemble flux creep characteristics for higher fields. As a general rule, our observations are consistent with the models that link the peak effect to vortex phase transitions. We calculate the critical current in self-field in the flux flow regime taking into account intervortex forces. We suppose that most vortices are pinned by defects while mobile vortices move through plastic channels between the strongly pinned vortex regions. When an external field is applied, we suggest that the increase in the critical current that is observed is linked to oscillations of the pinned vortices.     

Thermodynamic and transport properties of UPt3

In the normal phase of UPt₃, the magnetoresistivity is large and positive with the striking feature of a quasi-independence of the temperature and magnetic field terms; the thermoelectric power has aT ² dependence and the susceptibility is almost constant up to 4 K. The superconducting transition is broadened and the specific heat jump is weak due to the strong anisotropy of its normal phase. FromT _c to 146 mK (the lowest experimental temperature), a largeT ² contribution is observed in the specific heat, but the thermal conductivity has the same dependence only below 150 mK. These results are compared with the predictions given for polar odd-pairing superconductivity.     

Pinning Energies and Relaxation Rates in Laser Ablated Al-Doped YBCO Superconducting Thin Films

Systematic magnetization measurements were performed on laser ablated Al-doped YBCO thin films in the temperature range from 4 to 80 K and fields up to 12 T. The magnetization as a function of logarithmic time in the temperature range from 10 to 80 K generally showed a linear behavior. This type of performance allows us to determine the temperature and field dependence of the apparent pinning potential on the basis of the Kim–Anderson model for thermal activation of flux motion. The relaxation rate values are calculated from the time-dependent magnetization curves. The relaxation rate dependence on temperature, magnetic field, and doping concentrations has been investigated for different Al concentrations and applied fields up to 5 T. A strong nonlinearity was observed for the relaxation rate dependence on both temperature and applied fields. The apparent pinning potential values, U _o, are estimated from the dependence of relaxation rates temperature curves using U _o=k _B T/S expression. The dependence of U _o on both temperature and fields is elaborated and discussed in detail. The pinning potential, U _o, increases with temperature up to 40 K, however, above 40 K, a rapid decrease of U _o has been observed. Besides, a noticeable decrease in U _o as the applied field increases up to 0.1 T for temperatures below 40 K, and it is almost field independent for temperatures exceeding 40 K.     

Magnetic and Thermal Properties of HTS Bulk Magnet in the Pulsed-Field Magnetizing Process

In order to enhance the field-trapping ability of high T _c superconducting melt-textured bulk materials which act as quasi-permanent magnets when they capture the external magnetic fields, it is important to enhance the mechanical toughness of the materials to stand the stress induced by the magnetic repulsive force and thermal expansion. We adopted a dense Dy123-based bulk material with reduced void concentration in the experiment. Since the heat generations due to the flux motions in the samples results in the degradation of J _c, the time evolutions of the trapped magnetic fields and the temperature rises during and after the pulsed-field magnetizing processes were precisely measured at the same time to evaluate the penetrating flux motions and the heat generations in the sample. A single and couple of the magnetic pulsed fields with various intensities were successively applied to the sample at 30 K. The single magnetic field application exhibited a peak effect in the trapped-field behavior and tended to decline due to the heat generation. In the iterative pulsed-field application, the behaviors of the trapped fields and the temperature changes were found to be inverse between the first and the second pulsed-field applications. This implies that the flux penetration behavior into the sample magnet at the second field application is strongly restricted by the presence of former trapped fields which were formed by the first field applications.     

Development of a Flux Stabilizer for NMR Measurements with a Hybrid Magnet

A flux stabilizer has been developed for solid-state NMR measurements with a hybrid magnet installed at the National Institute for Materials Science. The stabilizer cancels the fluctuating magnetic field at a sample position by inductive field regulation. Stability of the magnetic field was improved from 2.7 ppm_rms to 1.2 ppm_rms by the flux stabilizer. Advantages of the flux stabilizer are demonstrated by ⁷⁹Br magic-angle-spinning NMR measurements of KBr.     

Sintering Effects in Na-Substituted Bi-(2212) Superconductor Prepared by a Polymer Method

In this study, Na-substituted Bi₂Sr₂Ca_0.9Na_0.1 Cu₂O superconductor samples were prepared by a polymer solution method. Three different sintering temperatures (850, 860, and 870 °C) were used to study the effect of Na substitution. The samples have been characterized using X-ray diffraction, scanning electron microscopy (SEM), DC electrical resistivity, and DC magnetic measurements. Magnetoresistivity measurements have shown a broadening of the superconducting transition under magnetic field which is explained on the basis of the thermally activated flux flow (TAFF) model. The calculated flux pinning energies of the samples varied from 0.17 to 0.02 eV by means of increasing magnetic field 0 to 9 T. The upper critical magnetic field H _c2(0) and the coherence length ( ζ(0)) at T = 0 K were calculated using the resistivity data. H _c2(0) and ξ(0) values have been calculated as 194, 144, and 139 T and 15.5, 15.2, and 13 Å at 850, 860, and 870 °C, respectively. TAFF model has shown Bi₂Sr₂Ca_0.9Na_0.1Cu₂O_8+y flux pinning energies are 0.015 eV at 9 T in all cases, while they were 0.165, 0153, and 0.149 eV at 0 T for samples sintered at 850, 860, and 870 °C, respectively.     

An inverse heat conduction problem with heat flux measurements

Using heat flux measurements as additional information to solve inverse heat conduction problems was and is still rarely employed. Lot of disadvantages linked to heat flux measurement specificities (local disturbance, intrusive measurement, lack of knowledge and proficiency, etc.) make people prefer temperature measurements which are well documented and very widespread. Solving inverse heat conduction problems with heat flux measurements is quite different than the one which uses temperatures and need to be investigated deeply. In this work, this problem is approached through the solution of a bioengineering problem consisting in the development of a non‐invasive blood perfusion probe. The effort here is focused on the development of a methodology for the estimation of time‐dependent blood perfusion from heat flux measurements. The physical probe incorporates a thin heat flux sensor, which is placed in contact with the tissue region where the perfusion is to be measured. The sensor records the heat flux due to an imposed thermal event, which is achieved by air flow. A one‐dimensional mathematical model is used to simulate the thermal event occurring at the contact region holding between the probe and the tissue. A combined parameter and function estimation procedure is developed to estimate simultaneously time‐dependent blood perfusion and thermal contact conductance between the probe and the tissue. The robustness of the method was demonstrated through several test cases using simulated data. The presented examples include various functional changes in the time evolution of blood perfusion. Results from this study have shown the feasibility of solving inverse problems with heat flux measurements and the two unknowns are estimated with no a priori information about their functional forms. Copyright © 2006 John Wiley & Sons, Ltd.     

Structural,Electrical and Magnetic Behaviour of FeTe0.5Se0.5 Superconductor

We report the synthesis as well as structural and physical properties of the bulk polycrystalline FeTe and FeTe_0.5Se_0.5 compounds. These samples are synthesised by the solid state-reaction method via vacuum encapsulation. Both studied compounds are crystallized in a tetragonal phase with space group P4/nmm. The parent FeTe compound shows an anomaly in resistivity measurement at around 78 K, which is due to the structural change along with a magnetic phase transition. The superconductivity in the FeTe_0.5Se_0.5 sample at 13 K is confirmed by the resistivity measurements. DC magnetisation along with an isothermal (M–H) loop shows that FeTe_0.5Se_0.5 possesses bulk superconductivity. The upper critical field is estimated through resistivity ρ (T,H) measurements using Gingzburg–Landau (GL) theory and is above 50 T with 50 % resistivity drop criterion. The origin of the resistive transition broadening under magnetic field is investigated by thermally activated flux flow. The magnetic field dependence of the activation energy of the flux motion is discussed.