Flux Creep Process Research Articles

On the force relaxation in the magnetic levitation system with a high-Tc superconductor

The effect of magnetic flux creep on the lift force in a magnet/superconductor system was studied. It was shown experimentally that in the case of real levitation (when a levitating object bears only on a magnetic field) the suspension height and consequently the lift force did not change over a long period of time. When the levitating object is fixed for some time (i.e. a rigid constraint is imposed on it), the levitation height decreases after removal of the external constraint. It is assumed that free oscillations of the levitating object slow down the flux creep process, which is activated when these oscillations are suppressed.

Effect of External Nonuniform Magnetic Field on Flux Creep Process in Superconductor

The effect of an external nonuniform magnetic field on the flux creep rate in a high-temperature superconductor with trapped magnetic flux was studied. The magnetic relaxation was suppressed when the superconductor was put into a field of permanent magnets or when it approached a ferromagnet. The effect arises when the field sources (being magnetized, the ferromagnet produces its own field) are placed near the superconductor surface, where the flux line ends are located. For these cases, we carried out the calculations of vortex and current density distributions, which demonstrate that reverse currents flow in the near-surface regions of the sample. This verifies the hypothesis suggested earlier about the influence of counter Lorentz forces retarding the creep of the vortices. In the interpretation of the results, we also take into consideration the magnetic force acting on the vortex ends in the external nonuniform magnetic field that allows us to explain the experimental results, in which the current structure in the sample is unipolar.

Slowing-down of the vortex motion in the flux creep process by counter forces exerted on the vortex ends

The effect of the suppression of flux creep in a high-temperature superconductor by counter forces exerted on the vortex ends was studied. These forces, acting in the opposite direction to the process of vortex diffusion, arise when the magnetized sample is put into a nonuniform magnetic field. The source of the field should be placed near the surface of the superconductor where the flux line ends are located. The critical states in a disk sample placed near a ferromagnet or permanent magnets were calculated. The results of the calculations verify the bipolar current structure and, therefore, the existence of counter Lorentz forces in the near-surface region. It was shown that in addition to the Lorentz force it is necessary to take into account the magnetic force acting on the vortex ends in the external nonuniform magnetic field. Depending on the direction of this force, the magnetic relaxation slows down or accelerates.

Influence of the Flux Pinning on Critical Current in HTc Superconductors with Nano-Sized Defects

Performed analysis of the flux pinning on nano-sized centers, allowed to receive new results on the critical current problems in the high temperature oxide superconductors. A new model of the interaction pancake vortices with the nano-sized centers is proposed and comparison of obtained results with others pinning forces approaches is presented. The energy balance approach is applied, which leads to the appearance of the potential barrier determining the pancake vortices movement in the flux creep process. Various initial positions of the captured vortex have been analyzed and variation of the screening currents for these configurations has been considered. Initial calculations of the influence of this effect on the current-voltage characteristics have been performed. The dependence of the critical current on the pinning centers parameters has been determined, as well as comparison with the experimental data presented. Pinning force has been calculated and its temperature dependence, which is in qualitative agreement with experiments. The critical current determines the magnetic induction profiles and therefore also flux trapping, very important parameter from the point of view of application HTc superconductors as permanent magnets. The influence of the critical current and granular structure parameters on the flux trapping in ceramic superconductors is regarded. The influence of the critical current on a.c. losses in second generation HTc tapes with magnetic substrate is briefly discussed too.

CRITICAL STATE MODELS FOR GRANULAR MAGNETIC SUPERCONDUCTOR RuSr2GdCu2O8

A systematic investigation of the ac susceptibility of polycrystalline RuSr 2 GdCu 2 O 8 was carried out with different field amplitudes and various frequencies. The study covered a wide range of frequencies between 0.020 to 12.5 kHz and temperatures from 7 to 50 K. The first derivative of susceptibility curves indicated the presence of a superconducting intragranular transition coupled by weak links distribution of intergranular transitions. The intragranular-, χg, and intergranular-, χi contribution to susceptibility were estimated and found to be consistent with the prediction of the critical state model. Variation of the peak temperature of the imaginary part with frequency was employed to determine the vortex activation energy. The derived low activation energy values provide clear evidence of the thermally activated flux creep process in the superconducting state. Qualitative analysis of the χ′′ - χ′ parametric relation was carried out.

The effect of a pulse magnetic field on the microwave properties of a YBCO thin filmstrip-line resonator

The effect of a single pulse magnetic field on the resonant frequency and quality factor of anYBa2Cu3O7−δ thin film strip-line resonator was studied at different temperatures and input power levels.It was shown that: (i) the induced magnetic flux and screening dc current, resulting fromthe pulse action, noticeably change both the resonant frequency and the quality factor; and(ii) relaxation of the trapped flux and induced current, caused by the flux creep process (atrather high temperatures), lead to a time evolution of these resonator characteristics.This relaxation proceeds in two stages: quick relaxation during the first 1–2 s (atT = 78 K) and long term (logarithmic) relaxation. The rate of long term relaxation increases withtemperature increase; on the other hand it does not depend on the input power level.

MAGNETORESISTIVITY PROPERTIES AND EFFECTIVE ACTIVATION ENERGY IN YBa2Cu3O7-ρ SUPERCONDUCTING THIN FILM

The resistive properties and activation energy of YBa 2 Cu 3 O 7-ρ ( YBCO ) superconducting materials change in magnetic field. It is explained that magnetoresistive behavior in terms of the presence of two-dimensional vortices being pinned effectively when they are perpendicular to the CuO 2 planes and an exponential behavior of the activation energy versus the applied field was obtained. The resulting activation energies ranging from 1 to 5 Tesla were attributed to inter-granular flux creep process.

Flux creep in type-II superconductors: The self-organized criticality approach

We consider the current density distribution function of a flux creep regime in type-II superconductors by mapping the flux creep process to the dynamics of a model with a self-organized criticality. We use an extremal Robin Hood type model which evolves to Been's type critical state to treat magnetic flux penetration into a superconductor and derive an analog of the current-voltage characteristics in the flux creep region.

Accelerated magnetic relaxation of transient disordered vortex states in Bi 2Sr 2CaCu 2O 8+ δ

Time relaxation of the local persistent current in a Bi 2Sr 2CaCu 2O 8+ δ crystal was magneto-optically recorded after suddenly exposing the crystal to a constant magnetic field between 140 and 840 G. A remarkable behavior of the relaxation is revealed for fields below the order–disorder vortex phase transition: The relaxation starts at a slow rate, it then accelerates for a short period of time, after which a slow relaxation rate is resumed. We show that this acceleration marks a local transition of a transient disordered vortex state to a thermodynamically favored quasi-ordered vortex phase. The slow relaxation rates, before and after the transition, reflect ordinary thermally activated flux creep process in the disordered and quasi-ordered vortex phases, respectively.

Relaxation of the remanent state in thin superconducting samples

We report the importance of the induction planar component B x in the relaxation of the remanent state in thin superconducting samples. Analysis based on the one-dimensional rate equation for thermally activated flux motion, which considers only the normal component B z , yields unphysical divergence of the average flux line velocity as the neutral line is approached. Two-dimensional analysis resolves this problem and yields a modified scenario for the flux creep process in which vortex bending and the neutral line play a major role. These results are demonstrated in analysis of the local relaxation data obtained from a thin Nd 1.85Ce 0.15CuO 4− δ crystal in the remanent state.

Simulation on local flux creep in type-II superconductors

Using C programming language, we have simulated the flux creep process in nonideal type-II superconductors. Global and local magnetization curves are calculated and the logarithmic time dependence of local magnetic induction B under a constant external field is examined. The effects of nonuniform pinning potential and self-organized criticality (SOC) model on the simulations are discussed. The results show that the main feature of flux creep is the relaxation effect. The form of hysteresis loops is dependent on the magnetic field sweep rate. SOC can account for the occurrence of fluctuation to a certain extent and nonuniform pinning potential can enhance the fluctuation.

Vortex-induced voltage instabilities in a superconducting BSCCO thin film

Flux-induced voltage switching events assuming the form of random telegraph signals have been investigated in a current biased Bi/sub 2/Sr/sub 2/Ca/sub 1/Cu/sub 2/O/sub 8+x/ (BSCCO) thin film, at weak magnetic fields (<10 G) and at temperatures ranging below 7 K. The observed phenomena cannot be framed within traditional theoretical models. Rather, they strongly indicate two distinct vortex dynamical states at the basis of the switching events. A novel interpretation is proposed, relying on the random decay of a flux-creep process in the superconductor.

Hall-array gradiometer for measurement of the magnetic induction vector in superconductors

An experimental technique for measuring the distribution of the normal and planar components of the magnetic induction near the surface of a superconducting sample is described. This technique utilizes a design of a double-layered Hall sensor array fabricated from a GaAs/AlGaAs heterostructure containing two parallel layers of a two-dimensional electron gas. Applications of this technique are demonstrated in measuring the current density distribution and in characterizing the flux creep process in a thin YBa2Cu3O7 crystal.

Voltage-current characteristics in c-axis-oriented Bi1.8Pb0.4Sr2Ca2Cu3O/Ag tapes

Detailed measurements of the voltage-current (V-I) characteristics for c-axis-oriented Bi1.8Pb0.4Sr2Ca2Cu3Oy (Bi(2:2:2:3))/Ag tapes have been performed in a low-magnetic field range. The results show that the V-I curves are determined by a thermally activated flux creep process and possess a power-law behaviour. The activation energy UJ derived from the V-I curves was found to follow a (1-T/Tc)n/H0.15 functional relationship, n being 0.37 for H//ab and 0.57 for H//c. In contrast with the (1-T/Tc)32//H theoretical prediction, this result shows very weak field and temperature dependences of UJ in the low-field region. A small negative curvature of log V versus log I plots near the temperature Tm at which the V-I curves change from being strictly linear to being nonlinear can also be found. This observation was tested using vortex glass (VG) or collective pinning (CP) models. The results showed that the VG or CP descriptions fail probably because of the high current density used in the measurement. We think that the negative curvature can be interpreted from the possible existence of the crossover from flux creep to flux flow.

Field-induced granularity in a well-textured (Bi,Pb)2Sr2Ca2Cu3Oy tape.

The magnetoresistivity of a well-textured (Bi,Pb${)}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{y}}$ tape is studied experimentally for a temperature range very close to ${\mathit{T}}_{\mathit{c}}$ (T/${\mathit{T}}_{\mathit{c}}$&gt;0.93) in a very low magnetic-field range (H0.1 T). In this H-T region, the dissipation associated with magnetoresistance curves seems to arise from the granularity induced by the field. As the temperature decreases, related Josephson junctions may well be coupled or even become proximity-effect superconductors, thus the dissipation is dominated by the conventional flux-creep process described by Anderson and Kim. This observation was further confirmed by the temperature dependence of the critical current density ${\mathit{J}}_{\mathit{c}}$(T) at various magnetic fields. The ${\mathit{J}}_{\mathit{c}}$(T) dependence seems to support the hypothesis that the Josephson coupling in the present sample is dominated by superconductor-normal-superconductor proximity-effect junctions.

The resistive transition of a Sm 1.85Ce 0.15CuO 4−y single crystal

Current-voltage V( I) characteristics and the magnetoresistivity ϱ( T, H) have been measured for a Sm 1.85 Ce 0.15 CuO 4− y single crystal in magnetic fields parallel to the c-axis. We have found that a well defined line H ∗( T) exists in the H-T plane which separates ohmic from non-ohmic behavior. Below H ∗( T), the V( I) curves are described by V/ I≁exp(- U( T, H, I)/ k B T) where U( T, H, I)≁ U( T, H)ln( I 0/ I). From the analysis of the V( I) characteristics, the effective flux creep energy barriers at any temperature and magnetic field are extracted. These data are used to calculate the magnetoresistivity curves ϱ( T, H) which compare very favorably with the experimental data over ⩽80%ϱ n of the resistive transition. The U( I)≁-ln I dependence, indicating the elastic nature of the flux creep process, is predicted by some recent theories: collective creep and vortex glass. We have not found evidence of the phase transition predicted in the latter case. The upper part of the resistive transition can be described in terms of the field enhanced 2D fluctuations. However, the significance of this analysis is questioned.

Angle dependent broadening of the resistivity transition in single crystalline YBa2Cu3O7

Angle dependent resistivity measurements have been performed on single crystalline YBa2Cu3O7 at various temperatures for field and current directions within the ab-plane. The angular modulation in the resistivity at higher temperatures is interpreted in terms of flux-flow. At lower temperatures, a nucleation-type of flux-creep process is described for the movement of flux-lines that are intrinsically pinned by the layered structure.

Flux Creep in Type-II Superconductors

We have made measurements of the evanescent decay of the irreversible magnetization induced by magnetic cycling of solid superconducting cylinders in order to elucidate the mechanisms of Anderson's thermally activated flux-creep process. A superconducting quantum interferometer device coupled to the creep specimen by a superconducting flux transformer made possible observations of flux changes with a resolution of one part in ${10}^{9}$. The general applicability of Anderson's theory of flux creep was confirmed and the results were analyzed to show that: (1) The total flux in the specimen changed logarithmically in time, i.e., $\ensuremath{\Delta}\ensuremath{\varphi}\ensuremath{\propto}\frac{\mathrm{ln}t}{{t}_{0}}$. (2) The logarithmic creep rate $\frac{d\ensuremath{\varphi}}{d\mathrm{ln}t}$ is proportional to the critical current density ${J}_{c}$ and to the cube of the specimen radius. (3) The logarithmic creep rate appears to be only weakly temperature-dependent because a proportionality to $T$ is nearly compensated by the proportionality to ${J}_{c}$, which decreases as $T$ increases. (4) The creep process is a bulk process that is not surface-limited (in this case). (5) Flux enters and leaves the surface in discrete events containing from about one flux quantum up to at least ${10}^{3}$ flux quanta. (6) On departing from the critical state to a subcritical condition, the creep process tends to remain logarithmic in time, but the rate is decreased exponentially by decreasing $T$ and is decreased extremely rapidly by backing off of the applied field from the critical state. (7) At magnetic fields $H&lt;{H}_{c1}$ on the initial magnetization curve, no flux creep was observed, but the logarithmic creep rate showed a modest increase above ${H}_{c1}$ and a broad rise as $H$ approached ${H}_{c2}$. The creep process is characterized by a dimension parameter $\mathrm{VX}$ consisting of a flux bundle volume $V$ and pinning length $X$, and by an energy ${U}_{0}$, both of which are supposed to be material-sensitive parameters characteristic of the irreversible processes. These parameters were determined from the experiments. Bundle volumes $V\ensuremath{\approx}{10}^{\ensuremath{-}12}$ ${\mathrm{cm}}^{3}$ and energies ${U}_{0}\ensuremath{\approx}1$ eV were found, indicating that groups of fluxoids must be pinned and must move cooperatively. The results are found compatible with a recent model for flux pinning that includes these cooperative effects.