Flux Creep Rate Research Articles

Critical currents and flux creep in low T c Pb/Ge multilayers

Magnetization studies of critical currents J c (H⊥,T) and of the flux creep rate dlnM/dlnt have been carried out on low T c Pb/Ge multilayers, which can be used to model the superconducting properties of the layered high T c cuprates. By taking a proper Ge separator thickness a crossover between two different pinning regimes can be observed. This transition leads to an anomalous dependence of critical current and the flux creep rate on perpendicular magnetic field H ⊥ in Pb/Ge multilayers, while a single Pb film demonstrates a monotonous suppression of J c by H ⊥. These magnetization data fully confirm the anomalous behaviour of J c (H ⊥) previously found in Pb/Ge multilayers from measurements of transport critical current.

Effect of vortex confinement on the collapse of the flux creep rate in Bi 2Sr 2CaCu 2O x single crystals

We report on detailed measurements of the flux creep rate S(T,H) in Bi 2Sr 2CaCu 2O y single crystals over a wide range of fields and temperatures. A sharp anomaly in S(T) is observed at the crossower temperature T=T CR. The confinement of the flux lines by the well-defined columnar amorphous tracks in Kr- or Ar-irradiated samples leads to the increase of T CR with the irradiation dose.

Magnetization relaxation in YBa2Cu3O7−δ

The time and field sweep rate dependencies of magnetization in high temperature superconductors are derived from a combined view based on the thermally activated flux creep. The τ0 in the magnetization relaxation expression M=M0+M1 ln(1+t/τ0) is a macroscopic quantity related primarily to the sample geometry and other experimental conditions via the effective activation energy U(J0). This activation energy provides an alternative interpretation to the nearly inverse proportionality of τ0 to the field sweep rate. The values of τ0 in the magnetization relaxation of single crystal YBa2Cu3O7−δ are consistent with the calculated ones. As the nonlinear U(J) is taken into account, the curvature in the ‖M‖-ln t plot may change from negative to positive over a sufficiently long time window, and the magnetization hysteresis M(H) depends on the field sweep rate Ḣ by const+A ln Ḣ+B ln2 Ḣ, where A is slightly larger than M1 and B≳0. A discussion of the temperature dependency of the normalized flux creep rate is at the conclusion of the article.

Flux creep through the surface barrier and columnar defects in HTSC

The rate of flux creep over the surface barrier in clean HTSC samples is found. This rate proves to be significantly different for the cases of vortex entry and escape (relaxation “in” and “out”, respectively). Since a flat surface is topologically similar to the artificial columnar defects, their comparison is useful. The vortex creep over the surface is equivalent to that over the cylinder defect of radius r > λ, whereas the radii of real defects obtained so far are of order ξ ▪ λ . We discuss the effect of the smallness of defect radius on the creep process, which results in very small activation energies U at large critical current J c. The importance of the increase of the defect radius is predicted.

Time scales of the flux creep in superconductors.

We have studied both theoretically and experimentally flux-creep dynamics in superconductors. A theoretical analysis of nonlinear flux diffusion shows that the relaxation of the electric field proves to be similar for different models of thermally activated flux creep, whereas the long-time decay of the magnetic moment M(t) can be essentially model dependent. A proposed scaling analysis indicates that the short-time decay of M(t) in the subcritical region j${\mathit{j}}_{\mathit{c}}$ is universal and consists of two stages. The initial nonlogarithmic stage is due to a transient redistribution of magnetic flux over the sample cross section, the duration of this stage ${\mathrm{\ensuremath{\tau}}}_{0}$ being entirely determined by macroscopic quantities, such as sample sizes, flux creep rate ${\mathit{M}}_{1}$(T,B)=dM/d lnt, and magnetic ramp rate B${\mathrm{\ifmmode \dot{}\else \.{}\fi{}}}_{\mathit{e}}$=dB/dt. The second stage corresponds to the approximately logarithmic relaxation M(t)=${\mathit{M}}_{\mathit{c}}$-${\mathit{M}}_{1}$ln(t/${\mathit{t}}_{0}$), with ${\mathit{t}}_{0}$ being a macroscopic time constant that also depends on sample sizes, ${\mathit{M}}_{1}$(T,B), and the voltage criterion ${\mathit{E}}_{\mathit{c}}$ at which the critical current density ${\mathit{j}}_{\mathit{c}}$ is defined.We consider different models of flux dynamics with nonlinear flux-creep-activation barriers U(j) and obtain explicit formulas for ${\mathrm{\ensuremath{\tau}}}_{0}$ and ${\mathit{t}}_{0}$ for the exponential V-I curve and the vortex-glass model. We have also performed an experimental study of magnetic relaxation in grain-oriented ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ in which the time constant ${\mathrm{\ensuremath{\tau}}}_{0}$ has been measured directly at different temperatures 4.2 KT88 K, magnetic fields 0B8 T, and ramp rates 5 \ensuremath{\mu}T/sB${\mathrm{\ifmmode \dot{}\else \.{}\fi{}}}_{\mathit{e}}$10 mT/s. We have observed the inverse dependence of ${\mathrm{\ensuremath{\tau}}}_{0}$ upon B${\mathrm{\ifmmode \dot{}\else \.{}\fi{}}}_{\mathit{e}}$, with ${\mathrm{\ensuremath{\tau}}}_{0}$ ranging from 1 s to ${10}^{3}$ s and reaching 5000 s at B${\mathrm{\ifmmode \dot{}\else \.{}\fi{}}}_{\mathit{e}}$=5 \ensuremath{\mu}T/s, B=6 T, and T=20 K. It is shown that, in accordance with our model, the dependences of ${\mathrm{\ensuremath{\tau}}}_{0}$ upon T and B coincide with those for the flux-creep rate ${\mathit{M}}_{1}$(T,B)=dM/d lnt measured on the logarithmic stage of the flux creep. We have also measured the dependences of the initial magnetic moment M(0) on T, B, and B${\mathrm{\ifmmode \dot{}\else \.{}\fi{}}}_{\mathit{e}}$. Manifestations of the obtained results in magnetic and relaxation measurements on high-${\mathit{T}}_{\mathit{c}}$ superconductors are discussed.

Transport and magnetic properties of YBCO/Ag composites

The transport and magnetic properties of pure Y-Ba-Cu-O and Y-Ba-Cu-O/Ag composites prepared by the co-precipitation method have been studied. The normal state resistivity decreases with addition of silver. The magnetic properties reveal that addition of silver to the YBCO sample degrades the superconducting properties. However, the increase in transport critical current density both at 77 and 4.2 K suggests the presence of proximity junctions (SNS) at the intergranular regions. The magnetic critical current density is reduced and the magnetization hysteresis loops measured at both 77 and 4.2 K shrink with addition of silver, indicating changes in the inter- and intragranular properties. The flux creep rate increases and the activation energy calculated from the relaxation data (5.5 T) at 4.2 K decreases with increasing silver concentration, showing a reduction in the number of pinning centres.

Low-temperature magnetic and microwave studies of Gd0.95Ba2Cu3.05O7??: Agx composites

Gd[sub 0.95]Ba[sub 2]Cu[sub 3.05]O[sub 7-[delta]]: Ag[sub x] (x = 0.0, 0.02, 0.04, 0.06) ceramic superconducting samples were prepared by a co-precipation technique using organic carbonates in the presence of stable polymers. The room-temperature normal-state resistivity decreases with Ag addition. However, T[sub c] and crystallographic parameters were unaffected. Low-temperature (4.2 K) magnetic and electrical transport studies in high magnetic fields (5 T) reveal that the addition of Ag into the Gd[sub 0.95]Ba[sub 2]Cu[sub 3.05]O[sub 7-[delta]] (GBCO) enhances the magnetic critical current density (J[sub mc]), volume pinning force (F[sub p]), flux explosion, and transport critical current density (J[sub tc]). Microwave-induced dc voltage studies show the reduction in the total number of weak links between superconducting GBCO grains with increase in Ag concentration. The flux creep rate was also increased with increase in Ag concentration in GBCO resulting in stronger pinning potential. The increase in J[sub mc], J[sub tc], and, hence, F[sub p] with Ag addition in the GBCO suggests the creation of an SNS-type proximity junction at the intergrannular region and stronger Josephson current paths between the superconducting intergrains, which may be due to the physical densification and reduction of the total number of weak links by Ag addition into the GBCOmore » superconducting system. 26 refs., 6 figs., 1 tab.« less

Effect of flux creep on the temperature dependence of the current density in Y-Ba-Cu-O crystals.

Temperature-dependent critical current density and normalized flux-creep rate, together with the current dependence of the vortex pinning energy, were studied in as-grown and proton-irradiated Y-Ba-Cu-O crystals. A consistent interpretation is found within the collective-pinning or vortex-glass models.

Exact treatments of activation energy, quantum tunneling and flux-creep rate in high- Tc superconductors

Within the framework of both thermally activated and quantum-tunneling flux-creep models, an exact treatment of the activation energy, the quantum tunneling and the flux-creep rate in high- T c superconductors is presented. For a typical quartic pinning potential in a various medium, the parameter-dependent activation energy and quantum tunneling of the system are studied in detail. Reliable determinations of the magnetic relaxation rate for a given strong type-II high- T c superconductor will enable a check of the validity of the theoretical results.

Assessment of thick film YBCO for flux transformer and magnetic screening applications

Measurements on the flux creep properties of a series of thick-film YBCO tubes and rings that have been processed at a range of different temperatures are reported, and their potential for use in large-area flux transformers and shields for superconducting quantum interference device (SQUID) magnetometers is considered. It is found that films showing flux creep rates adequate for flux transformer or low-field ( approximately 100- mu T) magnetic shielding applications can be made without recourse to special doping or heat treatment techniques. Application of flux creep theory to the data and observations of the microstructure in similar films implies that the scale over which the supercurrents are flowing is on the order of tens of mm and that this would place a lower limit on fabrication accuracy. For screening the earth's magnetic field, the films reported here would be adequate; for higher fields, thicker or multiple shields may by required. >

Methods for reduction of flux creep in high and low Tc type II superconductors

This paper reports the experimental investigations into two different methods of reducing flux creep and magnetic relaxation in high T c YBa 2Cu 3O 7 and low T c NbTi. For temperatures T ≤ 4.2 K temperature reduction leads to a considerable reduction of the flux creep rate. The second method for reducing relaxation is the choice of appropriate magnetic field cycling prior to measurements being taken. As already found for YBa 2Cu 3O 7, NbTi also shows a strong dependence on magnetic history. These results are of interest for various practical applications.

Critical current, magnetic irreversibility line and relaxation in a single TlO layer 1223 superconductor

We have carried out systematic studies on the critical current, magnetic irreversibility line and magnetic relaxation for a c-axis orientated powdered (Tl 0.5Pb 0.5)Sr 2Ca 2Cu 3O 9 (Tl-1223) sample. This single TlO layer compound, in comparison with the double TlO layer compound Tl 2Ba 2Ca 2Cu 3O 10 (Tl-2223), exhibited a high irreversibility line, less temperature and magnetic field dependent critical currents and a smaller flux creep rate. The substantially greater flux pinning ability of Tl-1223 can be understood in terms of its structural characteristics; in Tl-1223, there is only one insulating TlO layer between superconducting CuO 2 blocks while there are two in Tl-2223. The increase in the number (and distance) of insulating layers weakens the Josephson coupling between CuO 2 blocks along the c-axis and the results in a decrease in the value of J c. This decoupling between blocks causes the vortex lattice to break into pancake-like vortices which are easily thermally activated.

Effects of field-sweep rate on the magnetization of melt-textured YBa2Cu3O7- delta.

The effects of field-sweep rate K=\ensuremath{\partial}H/\ensuremath{\partial}t on magnetization hysteresis loops M(H) and on flux-creep studies M(t) in high-temperature superconductors have been investigated both theoretically and experimentally. We find the basic relation between M and K is, to first order, the following: M=const-{[dM/d ln(t)] ln(K)}-[${\mathit{Kt}}_{\mathit{e}\mathit{f}\mathit{f}}$/10], where dM/d ln(t)=aC/30 is the flux-creep rate in a cylindrical sample of radius a, and ${\mathit{t}}_{\mathit{e}\mathit{f}\mathit{f}}$ is an effective attempt time for vortex hopping. The largest possible M, which corresponds to the critical current density ${\mathit{J}}_{\mathit{c}0}$ in the absence of thermal activation, develops when K\ensuremath{\ge}${\mathit{K}}_{\mathrm{max}}$=aC/[(1+a\ensuremath{\alpha})${\mathit{t}}_{\mathit{e}\mathit{f}\mathit{f}}$] with \ensuremath{\alpha}=\ensuremath{\partial}J/\ensuremath{\partial}H. The time origin of flux creep, which is essential in studying the initial stages of relaxation, is given by ${\mathit{t}}^{\mathrm{*}}$=aC/K(1+a\ensuremath{\alpha}). The model agrees well with experiments on a melt-textured-growth sample of ${\mathrm{Y}}_{1}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$, yielding ${\mathit{t}}_{\mathit{e}\mathit{f}\mathit{f}}$\ensuremath{\sim}0.24\ifmmode\pm\else\textpm\fi{}0.03 s at 27 K. By incorporating the calculated time origin into flux-creep studies of M(t), we obtain a very good description in terms of the interpolation formula from vortex-glass--collective pinning theory.

Flux-creep studies of vortex pinning in an aligned YBa2Cu3O7- delta superconductor with oxygen deficiencies delta

Flux-creep measurements were carried out in magnetically aligned, sintered samples of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ with various oxygen deficiencies \ensuremath{\delta} (in the range 0\ensuremath{\le}\ensuremath{\delta}\ensuremath{\le}0.2), for temperatures T in the range from 5 to 75 K, with an applied magnetic field H\ensuremath{\parallel}c axis of 1 T. The normalized flux-creep rate S=d lnM/d lnt was determined as a function of T and \ensuremath{\delta}. The effective vortex-pinning potential ${\mathit{U}}_{\mathrm{eff}}$(J) and the pinning energy ${\mathit{U}}_{0}$ were calculated both in the linear Anderson-Kim approximation as well as in the nonlinear formalism of vortex-glass and collective-pinning theory. The applicability of the nonlinear formalism was tested and discussed. There is a close correspondence between the energy scaling parameter ${\mathit{U}}_{0}$ of the nonlinear model and the pinning energy of a pointlike pinning center. A comparison of the experimental results with predictions of collective-pinning theory reveals a consistent picture of vortex motion in this family of superconductors.

The irreversibility line and flux creep: A comparative study of Ba − xK xBiO 3−δ and YBa 2Cu 3O 7−δ

The relative flux creep rate S( t)≡d ln M/d ln t of a single crystal of Ba 1− x K x BiO 3−δ(BKBO) was measured up to a few degrees Kelvin below the irreversibility line. It is nearly constant at low temperatures but rises sharply after a threshold temperature T t . Above T t , the creep changes from logarithmic to non-logarithmic decay. All these are very similar to those observed in Y 1Ba 2Cu 3O 7−δ (YBCO) samples and suggest a flux-lattice phase sub-boundary. The ratio between the boundary field B t and the irreversibility field B i falls between 0.15 and 0.25 for both BKBO and YBCO at various temperatures, implying a possible close relationship between the boundary and the irreversibility line.

Flux creep in the melt-processed Bi-based oxide superconductors including normal particles

The Bi-based (2212) single grains including a large number of normal particles, (Ca,Sr)2Cu1O3, were synthesized by a melt process. To characterize the flux creep behavior in such a superconductor, magnetization measurements were performed at various temperatures and magnetic fields. The flux creep rate exhibited a maximum at 20 K in both magnetic fields parallel and normal to c axis. The magnetic field dependence of the pinning potential, U0, evaluated from the residual magnetization was not represented by the expression, B−α, in Y1Ba2Cu3O7 superconductors. The relation between U0 and the critical current density is discussed.

Flux creep characteristics after the increase in temperature or external field

Abstract The parameters of power-law magnetic relaxation; M s , R and β, have been studied on a laser-ablated YBCO thin film. It was found that at given T and H, M s was almost independent on measuring process. R and β obtained from fixed H and step-increased T process were respectively larger than those from fixed T and step-increased H process. These demonstrated that the rate of flux creep is more influenced by Lorentz force than by thermal activation factor.

High field trapping and flux creep of large QMG(Quench and Melt Growth) crystal

Abstract Magnetic flux density trapped in large QMG (Quench and Melt Growth) crystal was measured at 77K and 63K. QMG crystals which had no high-angle grain boundaries were enlarged by seeding. Distribution of magnetic flux density was measured on the surface of the QMG crystal using several Hall probes. Field cooling was performed to investigate trapping of the magnetic field. According to the critical state model, it is expected that trapped flux density increases with increasing sample size. A QMG crystal which size was 45mm in diameter 15mm thick trapped 0.72T at the center of the surface. Double piled QMG crystals trapped 1.35T at the center at 77K. The overall critical current density which was estimated by Bean type critical state model was about 7×10 3 A/cm 2 . Flux creep rate (normalized decay rate) was about 2.5×10 −2 at 77K. Moreover, a QMG crystal trapped 1.07T in triplet point of nitrogen (63K). Flux jump was not observed during these measurements. From these results, it is found that QMG crystal could stably trap high magnetic field without stabilization at 77K and 63K unlike conventional metal superconductors at 4.2K.

Flux creep in YBa2Cu3O7 films

Abstract Flux creep phenomenon in the diamagnetic magnetization of high-J c YBa 2 Cu 3 O 7 films prepared by CVD has been measured in magnetic fields up tp 50kOe. The activation energy U 0 was estimated from the flux creep rate as a function of temperature at various magnetic fields. The U 0 value increases almost linearly with increasing temperature and after taking a broad maximum at about 60K decreases both for H⊥c-axis and H//c-axis. The U 0 value near the maximum is suppressed by increasing magnetic field, although it is almost independent of the magnetic field at low temperature. Moreover, we observed that the anisotropy in the U 0 value was very small.

Low-temperature magnetic and electromagnetic studies of Y-Ba-Cu-O: Agx Ceramic superconductor

Magnetization and magnetic relaxation measurements have been studied at 4.2 K on YBa2Cu3O7−δ:Ag x (x=0 to 1.2) superconducting samples prepared by a coprecipritation technique. Remanent magnetization and hysteresis loops were found to be increased on Ag addition. Transport current density (J c) was enhanced with the addition of Ag concentration. Flux creep rate was also increased with increase in Ag addition resulting in stronger pinning potential. Microwave-induced d.c. voltage measurements show a reduction of weak links with increase in Ag concentration. The enhancement of transportT c is attributed to the stronger Josephson current paths due to the reduction of the total number of weak links after Ag addition.