Magnetic Flux Pinning Research Articles

A geometrical rationalization of the special properties of the 14° [001] grain boundary in YBa2Cu3O7−δ

14° [001] tilt grain boundaries in YBa2Cu3O7−δ have been found to exhibit anomalously good superconducting critical currents, associated with magnetic flux pinning, although no special interfacial structure has heretofore been associated with this misorientation. We present an analysis of the geometry of the grain boundary in question and show that it can correspond to a special interface, within the definition of constrained lattice coincidence, which may lead to new understanding of the superconducting behavior of grain boundaries. Our analysis shows that the tetragonal-to-orthorhombic phase transformation may have important effects upon grain boundary structure and properties, indicating new approaches to producing high-current superconductors.

Flux dynamics in high-temperature superconductors

Magnetic flux penetration, flux pinning and depinning are examined. The induced electric field and the resistivity due to the thermally activated hopping of the flux lines are analysed. Depending on the relative magnitude of the transport or the induced current density, J, relative to the critical current density, Jc,, three flux regimes are identified: (i) the thermally activated flux flow (or TAFF) for J << Jc; (ii) the flux creep for J approximately= Jc; and (iii) the flux flow when J >> Jc. The critical state and its relation to flux creep is defined. The flux creep, TAFF and flux flow are contrasted in terms of the current-voltage characteristics and in terms of the vortex lattice. A phase diagram, in the magnetic field versus temperature plane, is identified for a typical high-temperature superconductor. Vortex fluid (TAFF) and vortex solid (flux creep) phases are defined. Flux relaxation and the flux profile inside a superconducting sample are examined. The characteristic time frame for the flux hopping is contrasted with a characteristic time of measurement in an AC field situation and frequency effects are explained. Finally, a basic outline of pinning and the methods which have been attempted to increase pinning are discussed.

Magnetic properties of oxygen-depleted YBa2Cu3O7-y single crystals.

Magnetic properties and flux pinning of oxygen-deficient ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{y}}$ single crystals have been studied. The superconducting transition temperature and inductive critical current density decrease as the level of oxygen depletion increases. No obvious enhancement in the pinning strength was found in the oxygen-depleted ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{y}}$ system for the applied field along the c axis. We attribute this absence of enhanced pinning to the mismatch of the oxygen vacancy size with the superconducting coherence length over the entire superconducting regime.

The temperature- and field-dependent activation energy of a proton irradiated YBCO single crystal

We have investigated the separate temperature and field dependences of the activation energy U ( T, H, J ) for pinning of magnetic flux in a high- J c , proton irradiated Y 1 Ba 2 Cu 3 O 7−δ single crystal. The work achieves two objectives: (1) to clarify arguments concerning the “anomalous” rise of U with increasing temperature and (2) to resolve differences in magnitude of U found when comparing magnetic relaxation and resistivity results. The temperature and field dependences of U for fixed J were first obtained by extrapolating U ( J ) as observed in magnetic relaxation (flux creep) experiments to lower J . For fixed current density J , peaks in U were observed with respect to both temperature and magnetic field. For H > 2T, U ( H ) varied as ∼ H −α , with α depending on temperature and current. The attempt time, treated as a fitting parameter, lies near 10 −3 ∼10 −2 s in most cases. The magnitude of U agrees well with values reported from resistivity studies on high- J c epitaxial films.

Magnetic torque and flux pinning in high- Tc superconductor YBa 2Cu 3O 7 prepared by a melt-textured growth method

Highly oriented superconductors YBa 2Cu 3O 7 including Y 2BaCuO 5 phase were prepared by a melt-textured growth method. The temperature and magnetic field dependences of the magnetic torque were measured, and the critical current density, which is evaluated using a critical state model, is improved by the presence of (211) inclusions as the pinning centers.

Design, construction and development of an apparatus for the transport-current characterization of high-temperature superconductors at a range of temperatures and magnetic fields

In this paper we describe the development of an apparatus based on an original design for conventional superconductors by Hudson et al. 1 and improved by Hampshire and Jones 2. The latter is capable of measuring critical currents up to 500 A in fields up to 16 T at a range of temperatures from 2.2 K to around 20 K. This is a preliminary effort at extending the temperature range to enable the characterization of high-temperature superconductors across the full J- T plane and in fields up to 18 T. Full thermodynamic consideration is given, along with a treatment of magnitude and sources of error. Silver-clad superconducting wire in short sample coil format is investigated, with comments on the limitations of the apparatus thus observed. Representative results are discussed with reference to the disputed ‘flux-lattice melting’ effect occasionally observed in bulk samples of BiSCO on inorganic substrates 3, and in short (<3 cm) lengths of silver-clad BiSCCO 4,5. It is speculated that the use of silver-clad BiSCCO in lengths between 10 and 100 cm. requiring significant transport currents, may help clarify the process of current transport and magnetic flux pinning.

Magnetic relaxation and flux pinning in YBa 2Cu 3O 7− δ high- T c oxide irradiated by 5.3 GeV Pb ions

Critical current densities and pinning energies have been respectively inferred from hysteresis loops and magnetic relaxation measurements for YBa 2Cu 3O 7− δ polycrystalline samples irradiated by 5.3 GeV Pb ions. Pb ions latent tracks induce a strong anisotropic effect on flux pinning and critical current density which exhibits a twentyfold increase at 77 K for a fluence of 3×10 11 cm −2 and a magnetic field of 2T oriented parallel to the tracks. Large increases of the pinning energies are also observed but contrary to J c's they do not depend on the magnetic field orientation with respect to the direction of the tracks. Pinning energies of 0.5 eV are thus inferred at 77 K for samples irradiated at φt=10 11 Pb ions/cm 2.

Irradiation-induced pinning in single-crystal and thin-film Tl 2Ca 2Ba 2Cu 3O y superconductors

Magnetization relaxation measurements were used to directly measure the effects of ion irradiation on magnetic flux pinning. The pinning potential U in a Tl 2Ca 2Ba 2Cu 3O y single crystal irradiated with 4.5 MeV protons, to a damage level of 4 × 10 −4 dpa, increased by ≈ 25% at 25 K. In contrast, U for a highly oriented polycrystalline (pc) thin film irradiated with 300 keV He ions to a damage level of 2 × 10 −4 dpa increased by only 5% at 25 K (and less at higher temperatures). Enhancement of the magnetization critical current density J cm is determined as a function of temperature and damage in the single crystal and pc thin film. High-energy proton irradiation increased J cm in the single crystal by an order of magnitude at 25 K but had only a small effect on J cm in the pc thin film.

Effect of density, magnetic field modulation and magnetic field penetration depth on the microwave absorption properties of YBa2Cu3O7 –δsuperconductors

Magnetic field sweep hysteresis of the low-field microwave absorption (LFMA) in YBa2Cu3O7 –δ is stronger for low magnetic field modulation, low temperatures and low microwave power. The field-sweep hysteresis is related to the pinning of magnetic fluxons. Under conditions favouring substantial fluxon pinning, noise-like structures are superimposed on the LFMA signal. The LFMA signal amplitude depends on magnetic flux pinning, the relative contributions of intergranular and intragranular currents, the distribution of Josephson junctions and the variation in the magnetic field penetration depth. The hysteresis behaviour is similar for samples with different densities at low and high modulation amplitudes. However, the LFMA amplitude decreases with increasing density. The effective magnetic field penetration depth is measured by electron paramagnetic resonance line broadening of spin probes adsorbed on the sample surface. The results are discussed in terms of several current models for the LFMA effect.

Magnetic relaxation and flux pinning in zone melt textured YBa 2Cu 3O 6+x

The zero field cooled magnetic relaxation and the remnant magnetization as a function of applied magnetic field were studied for zero melt textured YBa 2Cu 3O 6+ x . The pinning potential was calculated from the flux creep rate of the zero field cooled magnetization. It was found that there is a strong correlation between the field dependency of the pinning potential and the critical current which essentially supports the line pinning model of Smith et al. [1].

Magnetic flux pinning and irreversibility in YBa 2Cu 3O 7 superconducting crystal

The irreversibility behaviour of superconducting YBa2Cu3O7 crystal in external magnetic fields is studied by ac magnetic permeability. A flux creep model with a single-relaxator form is used in the analysis. The irreversibility line B*(T*) is found to scale as (1 - T*/Tc)n, with n ≃ 1.5 for Bdc>1.5 T, and n ≃ 1 for lower fields. The frequency dependence of T* is for the first time carefully studied in the low frequency region from 10 Hz to 105Hz and it is found to be logarithmic. The peak in the imaginary part of the permeability is successfully fitted to the predictions of the flux creep model for different Bdc. A dynamic screening length is neeeded to account for the observed frequency dependence of the irreversibility temperature.

Magnetic field response and flux pinning in HTS r.f and microwave squids

A disc of ceramic superconductor has been shown to operate as a radio frequency and microwave SQUID at temperatures up to 77K, possessing in general a single well-defined flux-sensitive loop. We report the temperature variation of the active loop area and present a simple model for this behaviour.

Magnetic field response and flux pinning in a high Tc squid

A disc of ceramic superconductor is shown to operate as a ratio frequency SQUID at temperatures up to 77 K, possessing in general a single well defined flux sensitive loop. We report the temperature variation of the active loop area and present a simple model for this behaviour. In addition results are presented of flux pinning in the sintered disc.

Computer modeling of magnetic flux pinning by grain boundaries in high-field superconductors

We report here some results for the critical current density of a high-field superconductor such as Nb 3 Sn calculated with a simple two-dimensional model using computer simulation methods similar to those of Brandt[1] and utilizing an expression for the magnetic flux line-grain boundary interaction energy derived by Welch[2] based on the electron-scattering mechanism.

Mechanical effects in flux motion

Abstract The motion and pinning of magnetic flux in superconducting indium and indium-thallium have been studied by observing the damping of the oscillations of a torsional pendulum. The forces acting on moving and pinned flux were deduced, and their variation with field and temperature observed. Viscous forces, frictional forces, and non-linear friction were observed and analysed.

Measurements of the static pinning of magnetic flux in superconductors

Results of measurements on the static pinning of magnetic flux in superconducting sheets of low κ value (κ ≲ 2) are reported in this paper. The pinning force F is measured with a torsion balance. For all specimens the simple temperature dependence F = F 0(1 − T/ T c ) 2 was found regardless of the ratio in which surface pinning and bulk pinning contribute to F. A detailed separation of the contributions to F arising from the different pinning mechanisms appeared not to be possible, due to insufficient knowledge of the metallurgical conditions of the pure and well-annealed, but polycrystalline specimens.

Magnetic Flux Flow and Fluctuation Effects in Thin-Film Superconductors

Resistance measurements have been used to investigate magnetic flux pinning, flux motion, and fluctuation effects in thin films of superconducting indium, thallium, and aluminum which were condensed onto cold substrates and maintained below 20 \ifmmode^\circ\else\textdegree\fi{}K throughout the measurements. The depinning threshold of vortex motion was studied as a function of transport-current density, applied perpendicular magnetic field, and temperature. The absence of the peak effect in films is explained in terms of Pearl's theory of long-range electromagnetic interactions between vortices in films. Our critical-state data do not satisfy the empirical equation of Kim et al. which relates the current density to the critical depinning field in bulk samples. Vortex guiding has been observed, and Hall-effect measurements are also reported. Measurements on superimposed films of indium and thallium suggest that the surfaces of these films play an important role in flux pinning. Measurements of the resistive transition in most of our samples showed a peak in resistance near the transition temperature, similar to resistance peaks noticed by some other investigators. This peak disappeared as a perpendicular magnetic field was applied. An explanation of the peak is suggested; it involves small-angle electron scattering from regions in which superconducting fluctuations are occuring.