Conventional Flux Creep Research Articles

Metal frame as local protection of superconducting films from thermomagnetic avalanches

Thermomagnetic avalanches in superconducting films propagating extremely fast while forming unpredictable patterns, represent a serious threat for the performance of devices based on such materials. It is shown here that a normal-metal frame surrounding a selected region inside the film area can provide efficient protection from the avalanches during their propagation stage. Protective behavior is confirmed by magneto-optical imaging experiments on NbN films equipped with Cu and Al frames, and also by performing numerical simulations. Experimentally, it is found that while conventional flux creep is not affected by the frames, the dendritic avalanches are partially or fully screened by them. The level of screening depends on the ratio of the sheet conductance of the metal and the superconductor in the resistive state, and for ratios much larger than unity the screening is very efficient.

Flux creep properties in Dy–Ba–Cu–O bulk superconductors

We have investigated how the oxygen-annealing temperature and RE211 addition influence the flux creep at 77 K in melt-processed DyBa 2Cu 3O 7− δ (Dy123). By making conventional flux creep experiments, we found that Dy211 contents suppress the flux creep. It was also found that the flux creep is more suppressed by the annealing treatment in the lower temperature. A suppression in the flux creep was observed for samples with the secondary peak effect.

Dissipative effects near the transition temperature of high Tc superconductor

The complex magnetic susceptibility of oxide superconductors near the transition temperature has been computed as a function of temperature and frequency by numerical solution of the non linear, flux diffusion equation. For field values much lower than the upper critical field, a description of dissipative effects in terms of the conventional flux creep and flux flow pictures is not able to reproduce experimental data.

Transport critical current in c-axis-oriented Bi1.8Pb0.4Sr2Ca2Cu3Oy/Ag tapes.

The transport critical current density ${\mathit{J}}_{\mathit{c}}$ of c-axis-oriented Bi-based silver sheathed tapes was studied as a function of temperature T and magnetic field H in the case of the field applied parallel (H\ensuremath{\parallel}ab) and perpendicular (H\ensuremath{\parallel}c) to the tape surface. It can be concluded that the ${\mathit{J}}_{\mathit{c}}$ dependences of the temperature ${\mathit{J}}_{\mathit{c}}$(T) at various magnetic fields are dominated by the weak-link effect at temperatures very close to ${\mathit{T}}_{\mathit{c}}$ (above 90 K). For the temperatures well below ${\mathit{T}}_{\mathit{c}}$ (77--90 K), ${\mathit{J}}_{\mathit{c}}$(T) is primarily controlled by the conventional flux creep model of Anderson-Kim in the low-magnetic-field regime. This observation was further confirmed by the field dependence of ${\mathit{J}}_{\mathit{c}}$. The anisotropy in ${\mathit{J}}_{\mathit{c}}$ can be well described by using the effective-mass model and the overall ${\mathit{J}}_{\mathit{c}}$ of the tapes is determined by the small-angle grain boundaries. In the high-field range, an exponential behavior of ${\mathit{J}}_{\mathit{c}}$(H) at higher temperatures and a linear ${\mathit{J}}_{\mathit{c}}$(H) dependence at lower temperatures were clearly observed and can be well accounted for on the basis of collective pinning theory and a strong intrinsic pinning effect, respectively, in this anisotropic material.

Current-voltage characteristics of two-dimensional vortex-glass models.

We have performed Monte Carlo simulations to determine current-voltage characteristics of two vortex-glass models in two dimensions. Our results confirm earlier studies which concluded that there is a zero-temperature transition. Additionally we find that, as the temperature approaches zero, the linear resistance vanishes exponentially, and the current scale ${\mathit{J}}_{\mathit{n}\mathit{l}}$ where nonlinearities appear in the current-voltage characteristics, varies roughly as ${\mathit{T}}^{3}$. This result is quite different from the prediction of conventional flux creep theory in which ${\mathit{J}}_{\mathit{n}\mathit{l}}$\ensuremath{\sim}T. The results for the two models agree quite well with each other, and also agree fairly well with recent experiments on very thin films of Y-Ba-Cu-O.

Anti-Meissner effect and low field magnetic relaxation in sintered Bi-2212

A detailed study of the low field magnetic properties of some sintered Bi-2212 samples has been performed. For sufficiently low applied magnetic fields, typically for fields H < 0.2 Oe, the field-cooled magnetisation becomes positive at some temperature below T c (anti-Meissner effect), contrary to the usual Meissner effect. The field dependence of this effect is comparable to the predicted behaviour for a system in an orbital glass state. Additionally, the relaxation of the zero-field-cooled magnetisation displays an unexpected field dependence, the relaxation rate S = ∂M/ ∂ log( t) increases as H α with α ≈ 0.9 at very low fields. In contrast to the behaviour found in sintered YBCO, this result is not accounted for in conventional flux creep models, where the prediction is α ≥ 2.

Flux creep in YBa2Cu3O7−y thin films induced by self-field of the transport current

Self-field induced flux creep has been studied in YBa 2 Cu 3 O 7− y thin films in zero external field at temperatures below T c . Self-field induced flux creep model, which is the extension of Anderson and Kim conventional flux creep model has been introduced. The verification of the model has been done by performing voltagecurrent measurements on patterned YBa 2 Cu 3 O 7− y thin films. The variation of pinning potential, u 0 , as function of temperature is obtained.

Study of magnetic relaxation processes in ceramic Tl-2223 high- Tc superconductors

The magnetic relaxation of polycrystalline Tl 2Ba 2Ca 2Cu 3O χ was studied in the temperature range 4–100 K. The results were compared with the predictions of the conventional flux creep theory, collective pinning model and vortex glass-vortex liquid phase transition theory. In the chosen time window ( t⩽2500 s) the magnetization decay at low ( T⩽10 K) and high temperatures ( T⩾40 K) can excellently be described by a simple logarithmic relation M ≈ In( t). Deviations from the In( t)-decay occuring in the intermediate temperature range can be explained by the collective pinning model as well as by the vortex glass picture. Considering the conventional flux creep model as a first approximation to our measurement data, the distributions of the pinning potentials were obtained with peaks at U 0≊60 meV. This low value of most of the activation barriers might be responsible for the drastic decrease of the remanent magnetization and critical current density at temperatures T⩾30 K.

Evidence for collective flux pinning from magnetic relaxation combined with I– V characteristics in YBCO thin films

Magnetic relaxation and current-voltage ( I– V) characteristic measurements have been performed on YBa 2Cu 3O 7− x thin films. The magnetic relaxation M( t) was measured on samples patterned into a well-defined ring geometry allowing I– V data to be calculated from the M( t) curves. Whereas the direct transport measurements give a typical electric field sensitivity of 10 −6 V/cm, the M( t) data lead to a sensitivity of better than 10 −14V/cm. Although the conventional flux creep model might serve as a first approximation to the two sets of data separately, it is unable to describe the data combined to span ten decades in an electric field. Rather, the measurements are well described by the relation E= E 0exp[− U 0/ kT( J 0/ J) 1/ σ ], as predicted by the collective pinning theory and in the vortex glass model.

Reduction of flux creep in spite of a j c decrease in YBa2Cu3O7−δ after neutron irradiation

YBa2Cu3O7−δ single crystals with Tc=90.5 K and different transition width have been irradiated with fast neutrons (E≳0.1 MeV) and a fluence of Φ⋅t=2×1017 cm−2. The flux-creep dM/dlnt at B=0.5 T is reduced at any temperature up to T=60 K by a factor of about 2. Correspondingly, the effective pinning potential Ueff is enhanced up to 100kBT at low temperatures and 50kBT at T=77 K, independent of an increase or decrease of the magnetization current density after irradiation. Therefore, the enhancement of Ueff may not in every case be related to a jc increase, which is also determined by the number of pinning centers or a certain defect structure, which may not necessarily show deep pinning potentials. The results can be explained within the conventional flux creep theory and are in a good agreement with recent considerations of the role of electronic anisotropy in the dissipative behavior of YBa2Cu3O7−δ.

Non-logarithmic magnetic relaxation in Bi 2.2Sr 1.7CaCu 2O 8 single crystals; evidence for collective flux pinning

High precision magnetic relaxation measurements have been performed on BiSrCaCuO single crystals with the field H applied parallel to the c-axis. At temperatures T/ T c > 0.5, the data exhibit two different field regimes. One for H< H∗ ( H∗ corresponds to the field where full flux penetration of the sample is achieved) where the ZFC and RM magnetization relaxes logarithmically with time in agreement with the conventional flux creep model; a second one for H > H ∗ where the relaxation follows a [ln( t/ t 0] -α dependence. This relaxation law is predicted in the collective flux creep theory for J ⪡ J c as well as in the vortex-glass model. In the temperature range 0.25 < T/ T c < 0.4, there is a change in character of the measured decay of the magnetization. This change in character is interpreted to arise from thermal fluctuations of the flux-line lattice giving rise to a considerable reduction in the pinning strength above the depinning temperature.

Non-logarithmic magnetic relaxation in Bi-Sr-Ca-Cu-O single crystals

The time dependence of the low field susceptibility of a Bi2.2Sr1.7CaCu2O8 single crystal has been investigated in the field range 0.1-300 gauss and at temperatures 0.25<or=T/Tc<or=0.9. The relaxation of the magnetization is found to change from a logarithmic to a non-logarithmic form as the applied field is increased, implying that a conventional flux creep model cannot sufficiently describe the behaviour.

Comparison between conventional flux creep in constant magnetic field and the effect of creep on the shape of magnetic hysteresis loops in high-Tc superconductors

The effect of the magnetic induction sweep rate B= delta Bext/ delta t on the shape of magnetic hysteresis loops, in particular on the induced magnetic moment m ( implies/implied by critical current jc) was calculated directly from the basic mechanism of thermally activated flux creep introduced by Anderson taking jc constant through the sample. It was found (a) that m changes linearly with ln mod B mod , and (b) that delta m/ delta ln mod B mod is equal to - delta m/ delta ln t in conventional relaxation at Bext=constant. Similar results were obtained recently from a phenomenological model using differential susceptibility chi to characterize the effect of sweeping Bext.

Non-logarithmic magnetic relaxation in PbMo 6S 8

The time dependence of the zero-field-cooled magnetization of the extreme type-II superconductor PbMo6S8 has been measured at temperatures 0.7≤T/Tc≤0.95 and for fields 1G≤H≤300G. The results show that the decay changes from a logarithmic to a non-logarithmic form as the field is increased, indicating that the conventional flux creep model is not sufficient to describe the phenomena.

Time dependent magnetic response in a GdBa 2Cu 3O 7−δ superconductor: Flux creep or superconducting glass state?

Time dependent magnetic response has been examined for a GdBa 2Cu 3O 7−δ superconductor. Logarithmic time dependence of the magnetic relaxation is observed over several decades in 1n(t). A temperature and field dependent prefactor A(T,H) = dM/d1n(t) is observed for these materials for relaxation from the zero field cooled (ZFC) state and from the remanent state. The Meissner state (field cooled (FC)) is characterized by zero relaxation. This magnetic relaxation is discussed in terms of a conventional flux creep model and in light of the superconducting glass state model. It is concluded that since the relaxation coefficient A disappears at the glass transition temperature, and not at T c, the superconducting glass state model is most descriptive of these results.