Flux Motion Research Articles

Temperature rise in melt-textured large grain superconducting bulk magnets during their magnetizing operations

The temperature rises in the melt-processed RE–Ba–Cu–O (RE = Y, Sm, Gd) bulk superconductors were evaluated when the magnetizing operations were performed by the field cooling (FC), the zero field cooling (ZFC) and the pulsed field magnetization (PFM) processes. It is known that the field-trapping ability of the superconducting bulk magnets is restricted by the heat generation due to the flux motion inside the sample not only in the PFM but also even in the FC process. In this paper, the authors discuss the mechanism of heat generation and the flux motions by means of the temperature and resultant trapped field measurement during three magnetizing processes. The sample was cooled in a 5 T static field and then the field was removed with various descending rates (2.53–11.3 mT/s) in FC method. The fields were also applied and then removed with the same rates after setting the sample in the superconducting state in ZFC. These data were compared with the results obtained in the PFM. The temperature evolutions have shown apparent peaks that correspond to the heat generation in their magnetizing processes. The maximum temperature rises have reached 5.9 K in FC and 7.6 K in ZFC operated at 57.6 K and 51.5 K, respectively. The temperature rise exhibited 2 K even when the slowest rate of 2.53 mT/s was applied in FC mode. These data suggest it is crucial to take the heat generation into account in estimating the field-trapping ability of the high performance bulk magnets.

Magnetoresistance behavior in the (BiPb) 2Sr 2Ca 2Cu 3O x thin films

High quality epitaxial c-axis oriented (BiPb) 2Sr 2Ca 2Cu 3O x thin films have been fabricated on the single crystal MgO (001) substrates using radio frequency sputtering technique. The resistive transition of the films has been measured under applied magnetic field, parallel to the c-axis, up to 6 T. The results obtained showed that dissipative resistivity can be well fitted by thermally activated flux motion below critical temperature, T p, under a magnetic field. In the thermally activated flux flow region the temperature and field dependence of the activation energy has been investigated. The m value from 1.38 to 1.45 and α value 0.301 to 0.659 were calculated. The results obtained suggested that all samples show 3D-like behavior and the thermally activated flux flow is the dominant mechanism for the films prepared in this work.

Transport of magnetic vortices by surface acoustic waves

In a thin film of superconducting Y Ba 2Cu 3O 7 the impact of surface acoustic waves (SAWs) traveling on the piezoelectric substrate is investigated. A pronounced interaction between the ultrasonic waves and the vortex system in the type II superconductor is observed. The occurrence of a SAW-induced dc voltage perpendicular to the sound path is interpreted as dragging of vortices by the piezoacoustic SAW, which acts as a conveyor for the flux quanta. The antisymmetry of this voltage with respect to the magnetic field directly evidences the induced, directed flux motion. This dynamic manipulation of vortices can be seen as an important step towards flux-based electronic devices.

Dynamics of vortex shells in mesoscopic superconducting Corbino disks

In mesoscopic superconducting disks vortices form shell structures as recently observed in Nb disks. We study the dynamics of such vortices, driven by an external current ${I}_{0}$, in a Corbino setup. At very low ${I}_{0}$, the system exhibits rigid-body rotation while at some critical current ${I}_{c,i}$ vortex shells rotate separately with angular velocities ${\ensuremath{\omega}}_{i}$. This critical current ${I}_{c,i}$ has a remarkable nonmonotonous dependence on the applied magnetic field which is due to a dynamically induced structural transition with a rearrangement of vortices over the shells similar to the Coster-Kronig transition in hollow atoms. Thermally activated externally driven flux motion in a disk with pinning centers explains experimentally observed ${\ensuremath{\omega}}_{i}$ as a function of ${I}_{0}$ and $T$ and the dynamically induced melting transition.

Magnetoresistance studies of Tl-1212 phase substituted by scandium

Superconducting samples of TlBa2CaCu2−xScxO7−δ with x = 0.0,0.025,0.05,0.1,0.15 and 0.2 have been prepared via the solid-state reaction technique. The effect ofweak magnetic fields up to 4.4 kG on the electrical resistivity of the preparedsamples has been studied to investigate the flux motion for this phase. Theresults reveal a slight shift in the superconducting transition temperatureTc and an increase in the superconducting transition widthΔT with increasing magnetic field. The magnetic field has a small effect on the electricalresistivity in the normal state and in the first stage of transition, whereasthe change in the second stage of superconducting transition is very large.The experimental data, in the second stage of superconducting transition, fitwell with the thermally activated flux creep model and the activation energyU(B) shows a power law dependence on magnetic field asB−β. Also, the transition width is related to the magnetic field according to the relationΔTαBn. Thevalues of β and n are strongly dependent on the scandium content. The magneticfield and temperature dependence of the activation energyU(B,T) is found tobe U(B,T)∼ΔTB−η, whereη = β+n. The critical currentdensity Jc(0) and theupper critical field Bc2(0) are calculated, from the above measurements, as a function ofscandium content. Finally, the electronic thermal conductivityκe, estimated from the Wiedermann–Franz law, is reported at different applied magneticfields for the prepared samples.

Magnetovoltage Measurements and Field Sweep Rate Dependence of V-H curves in Superconducting Polycrystalline Y1Ba2Cu3O7-x

Magnetovoltage measurements (V-H curves) were carried out in superconducting polycrystalline bulk Y1Ba2Cu3O7-x (YBCO) material as a function of current (I), temperature (T), field sweep rate (dH/dt) and field orientation with respect to the transport current. A relative decrease in the dissipation measured in V-H curves was observed as dH/dt is increased, which implies that the time spent to plot the whole cycle has an importance on the evolution of the V-H curves. Thus, it could be possible to observe the relaxation effects in magnetovoltage measurements. In addition, the several significant steps and plateaus in V-H curves evolve depending on the magnitude of the transport current and also dH/dt. These observations were attributed to locking of the flux lines to decrease or increase in size of the easy motion flow channels. The strong hysteresis effects in V-H curves were discussed mainly by means of the flux trapping within the granularity of sample and the different degree of the inhomogeneous flux motion with respect to the sweeping of the external magnetic field up and down.

Recovery of trapped field distribution around a growth sector in a Gd–Ba–Cu–O HTS bulkwith pulsed-field magnetization

Pulsed-field magnetization is a useful magnetization technique for high-temperaturesuperconductivity bulk crystals for practical applications. However, the trappedmagnetic flux is reduced due to the temperature increase from the mobileflux, and the distribution of the flux density is considerably distorted in thea–b crystallographic axes plane. We present two proposals to improve the trappedmagnetic flux and its distribution upon pulsed-field magnetization. Firstly, split-typevortex pulsed copper coils with diameters smaller than that of the bulk disk.Secondly, two HTS bulks are stacked layered with misalignment of their crystala–b axes to reduce the flux motion in the four-fold growth sectors, since the critical current densityJc in the growth sectors is lower. Combining these techniques, the decayed trapped fielddistribution was recovered and the total flux was increased up to 30% in comparison with aconventional pulsed-field magnetization.

Influence of bicrystal microstructural defects on high-transition-temperature direct-current superconducting quantum interference device

Using atomic force microscopy and scanning electron microscopy (SEM), we investigate the correlations between the microstructural defects and the electrical characteristics of the bicrystal grain-boundary Josephson junctions and dc superconducting quantum inference devices (SQUIDs). The structural defects are shown to correlate qualitatively with the characteristics of grain-boundary Josephson junctions patterned on the YBa2Cu3O7−x film. SEM images show that these defects grown on the grain boundary were a few submicron depth of the groove. The low flux noise characteristics were observed when the groove depth was smaller than 18nm in the junctions of the SQUID. The existence of these defects is expected to affect the supercurrent and the motion of the magnetic flux in the films, which dominate the excess noise in the SQUID with bicrystal junctions.

Scaling analysis of vortex phase transition in HgBa 2Ca 2Cu 3O 8+ x superconductors

Using the vibrating-reed technique, the internal friction (IF) Q −1 as a function of temperature is measured for sing-phase HgBa 2Ca 2Cu 3O 8+ x superconductor at low applied magnetic field up to 0.5 T. An IF peak associated with flux motion can be found below T C . The IF peak becomes higher and shifts towards lower temperature with increasing magnetic field. By using scaling theory we have demonstrated that the internal friction around the transition temperature can be collapsed into a single curve, indicating that the IF peak is originated from a phase transition associated with a vortex glass transition.

Anomalous flux dynamics in magnesium diboride films

The flux dynamics in magnesium diboride films becomes altered by scattering resulting in an unprecedented quenching of flux motion that is beyond conventional vortex pinning. At currents approaching the pair-breaking value and magnetic fields approaching the upper-critical field, the conductance diverges and the current-resistance and temperature–conductance curves become parallel, showing mainly a shift in the transition due to pair-breaking with minimal signs of flux motion.

Scaling analysis of phase transition in Hg-based superconductor

With the vibrating-reed technique, the internal friction (IF) Q −1 is measured for sing-phase (Hg 0.66Pb 0.34)Ba 2Ca 2Cu 3O 8+ x superconductor as a function of temperature at low applied magnetic field up to 0.5 T and as a function of frequency at normal state temperatures. An IF peak associated with flux motion can be found below T C. The IF peak becomes higher and shifts towards lower temperature with increasing magnetic field. In addition an IF peak is found near 200 K. By scaling analysis we have demonstrated that the internal friction around the peak temperature can be collapsed into a single curve, indicating that the IF peak below T C is originated from a phase transition associated with a vortex glass transition and a structural phase transition occurs at around 200 K in the superconductor.

Anomalous Hall effect studied via guided vortex motion

The anomalous Hall effect is examined using guided vortex motion in patterned high-Tc superconducting thin films. Rows of micrometer holes (antidots) affect the direction of flux motion in Hall arrangements. This allows to distinguish between different contributions to the Hall signal. We demonstrate, that the anomalous Hall effect is not caused by normal charge carriers, but rather by vortex motion antiparallel to the applied current. This is demonstrated by the suppression of the anomalous Hall effect in the case of guidance of vortices perfectly perpendicular to the applied current. It is concluded that the Magnus force being a force on the vortices that would contain a transverse component might be responsible for the anomalous Hall effect.

Non-power-law I–V characteristics inCa-doped polycrystalline Y1−xCaxBa2Cu3O7−δ

Experiments reveal that there is a non-ohmic dissipation behaviour in the Ca-doped polycrystallineY1−xCaxBa2Cu3O7−δ even attemperature T = 20 K. This non-ohmic dissipation seems to come from the vortexexcitation along the grain boundaries and appears in anomalousI–V characteristics with a non-power-law behaviour at temperatures below60 K, and then with a power-law behaviour as the temperature approachesTc. This abnormal behaviour cannot be explained by the thermal flux creepmodel. We suppose that a giant flux motion along the grain boundaries mightbe responsible for this anomalous non-ohmic dissipation behaviour. A modifiedI–V relation was worked out based on Larkin’s collective pinning model, and the analyticalI–V curves were found to be coincident with the experimental curves quite well. This confirmedthat the vortex excitation process along the grain boundaries in Ca-doped polycrystallineY1−xCaxBa2Cu3O7−δ might be described with a modified collective pinning model.

Determination of pinning energy U0 for melt textured YBCO in accord with collective pinning theory

In collective pinning theory, vortex pinning energy, U, is a key parameter in describing thermally activated flux motion, but it is a function of the critical current density Jc, glassy exponent μ and pinning energy U0, under the base of Griessen’s and our group’s methods of obtaining Jc and μ, respectively. Determining U0 is a troublesome problem. In this paper, we introduce a new way of determining pinning energy U0, thus, the complex problem of getting U has been solved successfully.

2-D Model for Calculating Current Density Distribution and Flux-Flow Resistivity of MCP BSCCO-2212 Rod during Quenching Process in Self Field

This paper presents a 2-D model for calculating the current density distribution and the flux-flow resistivity of a Melt Cast Process BSCCO 2212 rod during the quenching process in self field with large current density. Based on the forces analysis of the flux-line lattice, the equilibrium equation for the 2-D viscous flux motion is derived from the model. With this equation, the current density distribution and the flux density distribution are obtained in not only the critical state but also the flux-flow state. Subsequently, the average flux-flow resistivity is calculated with the knowledge of the 2-D field distribution. The calculation results are in accordance with the experimental results. Finally, the applications of the 2-D model are extended to the superconducting tube and the low-Tc superconductor.

The effect of inhomogeneous flux penetration into bulk superconductor by pulsed fieldmagnetization

Pulsed field magnetization (PFM) is a practical method for magnetizing bulksuperconductors, but the amount of trapped magnetic flux is usually smaller than thatmagnetized by the field cooling (FC) method due to the heat generation by the flux motionduring pulsed field application. In this study, we magnetized a Y–Ba–Cu–O bulksuperconductor grown from a seed which was placed at deviating positions from the centreof the bulk to investigate the effect of inhomogeneous penetration of the flux into the bulksuperconductor during the PFM. The maximum trapped field by the PFM was almostthe same value as by the FC method. This suggests that the inhomogeneity ofthe bulk superconductor affects the trapped field on the PFM. The magneticflux can penetrate into the centre of the bulk even if the applied field is small,which contributes to reducing the extra heat generation due to the flux motion.

Thermally activated flux motion in MgCNi3

The time decay of the non-equilibrium magnetization of superconductor MgCNi3 is studied. It is found that the superconducting diamagnetic moment is logarithmically dependent on time. The nearly linear dependence of the normalized magnetization relaxation rate S on T suggests that the flux motion in MgCNi3 is described by the thermally activated mechanism. The relation between activation energy and current density is obtained by using Maley’s technique.

Fabrication of<tex>$a/c$</tex>Axes Oriented Grain Boundaries in YBCO Films by Selective Growth

The a/c axes grain boundaries of YBCO films are very interesting from the viewpoints of Josephson junction technology and the magnetic flux pinning. In this paper, we intend to flow superconducting current along the grain boundary and to prevent the magnetic flux motion from passing across the grain boundary. The a/c axes grain boundaries were fabricated on SrLaGaO/sub 4/ (100) substrates by pulsed laser deposition on Gd/sub 2/CuO/sub 4/ (GCO) buffer layers. The preferred orientation and in-plane orientation of the films were characterized by X-ray /spl theta//2/spl theta/ diffraction and /spl phi/-scan spectroscopy. 4-micrometer pitch GCO patterns were fabricated by a lithography technology. Subsequently, the YBCO films were deposited on the pattern. A TEM image showed that a-axis grains grew over c-axis grains.

Thermally assisted flux flow in MgB2: strong magnetic field dependence of the activation energy

The origin of the resistive transition broadening of superconducting MgB2 thin films is investigated. Thermally activated flux flow is found to be responsible for the resistivity in the vicinity of the critical temperature. The activation energy for flux motion is observed to have an extraordinary strong dependence on the magnetic field. The results are discussed.

Macroscopic Magnetic Flux Motion in Y-Ba-Cu-O Bulk Superconductor During Pulsed Field Magnetization

Pulsed Field Magnetization (PFM) is one of the key technologies for utilizing RE-Ba-Cu-O bulk superconductors as a strong flux source for various applications. However, the performance of PFM is dependent on the conditions of applied magnetic field, cooling method, etc. and therefore, efficient and effective magnetization is an essential issue for practical use of PFM for high-performance bulk superconductors. In this work, macroscopic magnetic flux motion in Y-Ba-Cu-O bulk superconductor during PFM was observed using five co-axial pick-up coils.