Flux Creep Rate Research Articles

Flux relaxation phenomena in the presence of weak AC magnetic fields

Relaxation of flux profile and magnetisation due to non-linear vortex diffusion in a superconducting slab settled in a parallel-to-the-surface DC and superimposed weak AC magnetic fields is studied for several kinds of the non-linear vortex diffusivity, corresponding to different possible shapes of the current–voltage characteristics of the superconductor. The evolution of the dynamic vortex response on applied weak AC field due to flux relaxation process is studied and relaxation characteristics of the AC magnetic susceptibility are calculated. The flux creep rate and magnetisation decay are shown to be enhanced significantly in the case of strongly non-linear regime of vortex diffusion if even rather weak AC magnetic field is applied. The possibility of `dynamical melting' of the vortex lattice occurring at rather high levels of the induced current density ( j> j c) is also demonstrated.

Magnetization dynamics in crossed AC and DC fields

We have measured the DC magnetization of melt textured and single crystal YBCO samples in crossed AC and DC fields, as a function of sweep rate, time and orientation ( h AC‖ c, h AC‖ ab plane). We observe that besides decreasing with h AC amplitude, the DC magnetization in swept fields ( Ḣ>0) also decreases as the log of AC field frequency. The rate of decrease 1/ M 0(Δ M/Δ ln f) is observed to be close to the flux creep rate. Magnetic relaxation both in zero and crossed fields are found to obey logarithmic time variation, with time constants decreasing with increasing f. The short time relaxation clearly shows the effects of frequency and enables an interpretation of the ln f dependence in terms of flux creep effects. AC magnetization measurements confirm both the homogenizing of the DC flux distribution due to the AC fields and the effect of field sweep in competing with the depinning due to the AC field.

Anomalous behaviour of vortex creep in twinned YBCO crystals

We present magnetic flux creep measurements on a twinned YBCO crystal taken using a Vibrating Sample Magnetometer, in which the sample orientation may be varied with respect to the applied field. The measured flux creep rate is sometimes of negative sign, opposite to that which is usually observed. The data are discussed in terms of the inferred electrodynamics of the sample.

Flux pinning in YBa2Cu3O7−δ through nanometer-sized MgO particles

The magnetization hysteresis loop and magnetic relaxation of YBa2Cu3O7−δ samples with x=0 and 0.1 were systematically measured over a wide range of temperature (10–80K) and magnetic field (0–7T). It is found that the addition of nanometer-sized MgO particles will enhance the critical current density and the fishtail effect in low- and intermediate-temperature region, lower the critical current density in high-temperature region, and reduce the flux-creep rate.

Flux creep and relaxation phenomena in the presence of weak AC magnetic field

Evolution of the dynamic vortex response during the relaxation process In a superconducting slab situated in parallel DC magnetic field H/sub 0/ and superimposed weak AC field h/sub AC//spl sim/ exp(i/spl omega/t) is studied theoretically for the nonlinear vortex diffusion coefficient corresponding to the vortex liquid state. The phenomenon of dynamical melting of the vortex lattice at high induced current levels and its possible manifestation in relaxation characteristics of the dynamic vortex response are discussed. The flux creep rate and the irreversible magnetisation decay are shown to be enhanced by application of weak AC magnetic field h/sub AC/.

Ac response of thin superconductors in the flux-creep regime

We calculate both analytically and numerically the ac susceptibility {chi}({omega}) and the nonlinear electromagnetic response of thin superconductor strips and disks of constant thickness in a perpendicular time-dependent magnetic field B{sub a}(t)=B{sub 0}cos{omega}t, taking account of the strong nonlinearity of the voltage-current characteristics below the irreversibility line. We consider integral equations of nonlinear nonlocal flux diffusion for a wide class of thermally activated creep models. It is shown that thin superconductors, despite being fully in the critical state, exhibit a universal Meissner-like electromagnetic response in the dissipative flux-creep regime. The expression for the linear ac susceptibility during flux creep appears to be similar to the susceptibility of Ohmic conductors, but with the relaxation time constant replaced by the time t elapsed after flux creep has started. This result is independent of any material parameter or temperature or dc field. For {omega}t{gt}1, we obtain {chi}({omega}){approx}{minus}1+pln(qi{omega}t)/(i{omega}t), where p and q are constants. Above a critical ac amplitude B{sub 0}=B{sub l}, the local response of the electric field becomes nonlinear, and there are two distinctive nonlinear regimes at B{sub 0}{gt}B{sub l}, where B{sub l}{approximately}s(d/a){sup 1/2}B{sub p}, B{sub p} is a characteristic field of full flux penetration, s(T,B)={vert_bar}dlnj/dlnt{vert_bar} is the dimensionless flux-creep rate andmore » d and a are the sample thickness and width, respectively. For B{sub l}{lt}B{sub 0}{lt}B{sub h}({omega}) the response of the electric field is strongly nonlinear but nonhysteretic, since the ac field B{sub a}(t) does not cause a periodic inversion of the critical state. As a result, the magnetic moment exhibits a Meissner-like {ital nondissipative} response, in stark contrast to the Bean model. (Abstract Truncated)« less

Experimental assessment of the current-limiting mechanisms in BSCCO/Ag high-temperature superconducting tapes

The current - voltage characteristics and the magnetic field dependence of the critical current of a range of mono-core BSCCO 2223 tapes are presented, illustrating the complementary use of transport and magnetization experiments in determining and analysing the current-limiting dissipation processes in these HTS conductors. Below a magnetic cross-over field the samples resemble a Josephson-linked current network, with the dominant dissipation at the weakest grain boundaries. In this regime, increasing field leads to a gradual fragmentation of the network. The network homogeneity and connectivity can be inferred from screening current length-scale measurements and comparison of transport and magnetization measurements. Above the cross-over field , flux motion within the surviving strongly linked backbone dominates the dissipation. The details of the dependences of the critical current and flux creep rate on magnetic field can be used to examine the intragranular pinning potential in the tapes. We show that, despite the wide range of transport critical-current values of the samples examined, the intragranular pinning proves to be remarkably sample independent. We conclude that while at low fields may be increased by improved processing that yields better intergrain connectivity, the high-field can be enhanced only by strengthening the pinning within the BSCCO 2223 crystallites themselves.

Pinning potential estimated from high-temperature magnetization

The potential barrierU estimated from the creep rate is generally known to increase with temperatureT at lowT range. This peculiar feature is considered to result from the model which assumes the linearity of the potentialU onJ. By cooling a superconductor magnetized at higher temperatureTo+ΔT to an operrating temperatureTo(High-Temperature Magnetization HTM), the flux distribution atTo+ΔT is transferred to that atTo. Then the pinning potential located at an arbirary magnetization currentJ will be realized artificially and the current dependent barrier energyU(J) can be estimated from the flux creep rate after the HTM. Measurement of the HTM method in superconducting YBaCuO samples was carried out. The nonlinear barrier potentialU(J) is analyzed and the temperature dependence of the potential depthUo(=U(0)) is discussed.

Influence of weak visible light on flux creep in YBCO film at low temperatures

We present an experimental study of flux creep in YBCO film. A ring-shaped film was used to directly obtain voltage-current characteristics of the film in a flux creep regime. It is shown that the flux creep can be described in terms of quantum tunneling of vortices up to temperatures of 10–15 K. We have also investigated the influence of weak illumination on flux creep rate. It is shown that visible light with intensity less than 1 mW/cm 2 drastically accelerates the flux creep at liquid helium temperatures. The influence of light is completely different to that caused by the temperature rise. We propose a simple qualitative model to explain the high sensitivity of the flux creep to weak illumination.

Flux creep in the presence of twin boundaries

The flux-creep rate associated with twin boundary pinning is calculated in the framework of a Brownian motion model. Systematic elimination of the velocity degrees of freedom exposes the relevant small parameter and validates the strong-friction limit for a “typical” high-temperature superconductor. The corresponding expression for the vortex-escape probability is obtained in a closed form. For a piecewise linear pinning potential approximation this expression yields an analytic result. The ensuing critical current strongly depends not only on the pinning-potential maximum depth, but also on the existence of deep, steep features in its shape. For a “realistic” twin boundary pinning potential, the calculated vortex-escape probability is compared to the Anderson-Kim theory. The comparison shows that the Anderson-Kim expression greatly overestimates the vortex-escape probability. Inter-defect diffusion is found to be particularly important in the “weak” current, low-temperature regime.

Magnetisation and flux dynamics in oxygen deficient TmBa/sub 2/Cu/sub 3/O/sub 7-δ/

A series of TmBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// crystals has been used to examine the influence of oxygen content on the irreversible magnetisation and the flux creep rate as a function of temperature T and applied magnetic field B. For 0.1 >

Influence of oxygen stoichiometry on the irreversible magnetization and flux creep in RBa2Cu3O7- delta (R=Y,Tm) single crystals.

The influence of the oxygen stoichiometry on magnetization and relaxation of R${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (R=Y,Tm) single crystals was studied for 0\ensuremath{\le}\ensuremath{\delta}\ensuremath{\le}0.55. The field dependences of the shielding currents ${\mathit{j}}_{\mathit{s}}$(H) and flux creep rates S(H) were analyzed. Three different kinds of ${\mathit{j}}_{\mathit{s}}$(H) and S(H) behavior can be seen in highly oxygenated samples (\ensuremath{\delta}0.1). (i) At high temperatures, ${\mathit{j}}_{\mathit{s}}$(H) shows a common fishtail peak, which is present for all oxygen contents. The field position and height of the ${\mathit{j}}_{\mathit{s}}$ peak increase rapidly with decreasing \ensuremath{\delta}. Approaching the stoichiometric state \ensuremath{\delta}=0, the value of the current starts to decrease with lower oxygen deficiency, in contrast to the still increasing irreversibility field ${\mathit{B}}_{\mathrm{irr}}$. This points to the importance of oxygen disorder for the pinning and stresses the melting nature of the irreversibility line. The peak position ${\mathit{B}}_{\mathrm{max}}$ is found to correlate with ${\mathit{B}}_{\mathrm{irr}}$. A possible relation of the fishtail with a synchronization effect and with the vanishing of ${\mathrm{C}}_{66}$ at the melting transition is proposed. (ii) In the intermediate-temperature region, some of the samples showed a new second peak with weak temperature and \ensuremath{\delta} dependence. The probable origin of this feature is matching with twin structure. This peak disappears for \ensuremath{\delta}g0.1, when the fishtail position shifts below the matching field. (iii) The low-temperature region is characterized by a monotonic decrease of ${\mathit{j}}_{\mathit{s}}$(B). This region becomes narrower with larger \ensuremath{\delta} and disappears for \ensuremath{\delta}g0.3. Thus in highly deoxygenated samples, the fishtail feature is observed for practically all temperatures. The possible connection of the low-temperature region with the pinning of the small vortex bundles or with the slipping of vortices along the twin planes is considered.

Composite flux-line lattices stabilized in superconducting films by a regular array of artificial defects.

Flux creep rate and critical current have been measured in superconducting Pb $/$Ge multilayers with a square lattice of submicron holes. The new composite flux-line lattices consisting of single or multiquanta vortices at holes and single cortices at interstitial positions have been identified. The ``metallic'' behavior of the flux lines at the interstices at temperatures close to the critical temperature leads to a first-order phase transition analogous to the Mott insulator-metal transition for flux lines, introduced by Nelson and Vinokur.

Vortex line pinning and Bose-glass dynamics in heavy-ion irradiated Bi2Sr2CaCu2O8+ delta single crystals.

The angular dependence of the ac screening of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ single crystals irradiated with 5.8 GeV Pb ions is used to demonstrate the importance of interactions between vortex segments in different Cu${\mathrm{O}}_{2}$ layers. The field tilt angle below which pinning by the columnar defects is effective is very large: ${\ensuremath{\Theta}}_{a}\ensuremath{\approx}70\ifmmode^\circ\else\textdegree\fi{}$. The magnitude and frequency dependence of the shielding current as well as the frequency dependence of the irreversibility line are self-consistently described by the Bose-glass theory. The irreversibility line is shown to be determined by the flux creep rate.

Flux creep, shielding current and irreversibility field in Kr irradiated Bi 2Sr 2CaCu 2O x single crystals

The influence of high energy Kr irradiation on the shielding current, irreversibility field and flux creep rate in Bi 2Sr 2CaCu 2O x single crystals has been studied. For B « B Ψ = φ 0·Ψ the pinning and creep behavior is approximately independent of fluence but significantly different from the virgin state. For high magnetic fields three temperature regions were distinguished. Small changes of the irreversibility field spite of a large increase of the shielding current with fluence point that plastic deformations dominate in the low temperature regime. The high T region is determined by the single vortex-single track interaction, whereas at intermediate temperatures the pinning of several vortices by one track becomes important.

Static and dynamic mechanisms of the anomalous field dependence of magnetization in Bi-Sr-Ca-Cu-O and Bi-Pb-Sr-Ca-Cu-O single crystals.

We have measured and analyzed both the static and dynamic contribution to the anomalous nonmonotonic dependence of the irreversible magnetization, M(B), for underdoped ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathit{y}}$ and overdoped (${\mathrm{Bi}}_{1.6}$${\mathrm{Pb}}_{0.4}$)${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathit{y}}$ single crystals. We extracted the ``unrelaxed'' magnetization ${\mathit{M}}_{\mathit{c}}$(B) and traced the time evolution of M(B,t) from short to long time scales using combined magnetization and flux-creep measurements. Despite their large differences in anisotropy, pinning force, and flux-creep rate s(B)=dln M/dln t, both samples showed the anomaly over a wide range of T, the anomaly being most pronounced in the ``unrelaxed'' state. This suggests that the primary cause of the anomaly lies in the flux-pinning defect structure of the material. However, M(B,E) also essentially depends on the induced electric field E in the sample, so the shape of the M(B) curve can strongly change with time due to the relaxation of E(t) and the nonmonotonic dependence of s(B).

Study on the properties of Sn-added YBCO prepared by the powder melting process

Magnetization and magnetic relaxations have been measured for the YBa2Cu2.985Sn0.015Oy and YBa2Cu3Oy superconductors, which were prepared by the powder melting process method. It is found that the Sn-added sample exhibits much higher Jc value and lower flux creep rate than pure YBCO. The results indicate that the melting temperature can be reduced and the pinning potential can be enhanced by substituting Sn for Cu. The Jc value estimated by Bean’s model drops with the magnetic field according to a power law. The dependence of Jc on the reduced temperature and the flux pinning force density behavior is discussed. The dependence on temperature of the maximum demagnetization field Hp and the remanent magnetization Mrem is studied. Finally, we also investigate the microstructure characteristic.

Anomalous field dependence of the critical current and flux creep rate in Pb/Ge multilayers

Critical current densities J c and normalized flux creep rate S(≡ dln M/dln t) have been measured as a function of the perpendicular field H ⊥ in 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 field-induced crossover between two different pinning regimes can be observed. This transition leads to an anomalous behavior of J c( H ⊥) and S( H ⊥) in Pb/Ge multilayers, while a sing le Pb film demonstrates a monotonic suppression of J c by H ⊥. These data have been analyzed in the framework of the collective pinning (CP) theory of magnetically coupled superconducting planes: the J c( H ⊥) anomaly is interpreted as a crossover from three-dimensional (3D) flux lines in low fields to two-dimensional (2D) pancake vortices in higher fields. This interpretation has also been confirmed by the suppression of the J c( H ⊥) anomaly in the Pb/Ge multilayers with a square lattice of holes, forming additional defects for the confinement of the flux lines and for the expansion of the 3D regime.

The electric field within high-temperature superconductors: mapping the E-J-B surface

At a single temperature, the macroscopic irreversible magnetic behaviour of a superconductor can be described fully by the relationship between the electric field E, the current density J and the magnetic induction B. We show that this relationship can be readily visualized as a three-dimensional E-J-B surface. The utility of this surface is that it is model and parameter independent, and so obviates the need to define the 'critical current density', and the 'irreversibility' field by arbitrarily imposed criteria. Furthermore, the flux creep rate corresponds directly to a gradient of this surface. Data from magnetic studies can be used to generate the surface, but some care with experimentation is needed to avoid distortion by artefacts, such as self-field effects and anisotropy. Transport studies map the surface too, but in a rather different (and limited) region of E-J-B space. The shape of the surface and its dependence on temperature reflect the underlying physics of vortex behaviour, and we discuss briefly the limitations on theoretical models imposed by its form.

Effect of the dimensional crossover on critical currents and flux creep in YBa 2Cu 3O 7/PrBa 2Cu 3O 7 multilayers

Critical currents j c(T,H) and the flux creep rate dlnM/dlnt of YBa 2Cu 3O 7/PrBa 2Cu 3O 7 multilayers have been measured in a wide range of temperatures T. By taking different thicknesses of the PrBa 2Cu 3O 7 separator layer the coupling strength between the CuO 2 planes is controlled, thus making possible the crossover from the 3D to the 2D regime, when the superconducting properties and the dissipation processes are strongly influenced by fluctuations. The characteristic critical temperature T ∗ c of the Larkin's theory for a quasi 2D superconducting layer determined from the flux creep data is in a good agreement with the T ∗ c value found from the magnetoresistance measurements.