Bulk Pinning Research Articles

Peculiarities of Pinning and Microwave Absorption Hysteresis in Thin Superconducting Films

The results of experimental and theoretical studies of the hysteretic microwave absorption (MWA) in the superconducting Bi2Sr2CaCu2O8 thin films are presented. It has been found experimentally that the hysteresis loop manifests some unusual features such as the non-monotone hysteresis variation and the change of a hysteresis sign. We have shown that such unusual behavior is due to the special nature of the bulk pinning in a superconducting film with a thickness comparable with the field penetration depth. The theoretical model of the MWA hysteresis has been developed taking into account the inhomogeneous distribution of centers with different symmetry of a pinning potential and their variation with the magnetic field value.

Surface barrier and bulk pinning in MTG YBaCuO

The magnetization curves are numerically studied based on nonlinear flux creep model simultaneously considering the effects of surface barrier and bulk pinning. The numerical results show that there exists a maximum of magnetization at an intermediate field (“fishtail effect” (FE)) in the M–H curves, which originates from bulk pinning. These simulation results also give a prediction that the FE in M–H curves cannot be simultaneously observed on a single M–T curve under zero-field-cooled (ZFC) process. The experimental results of melt-textured growth YBCO materials are similar with the numerical M–H curves, which suggest that the behaviors of M–H curves are the cooperation effects of the surface barrier and the bulking pinning force. Moreover, it is suggested that the surface barrier is more important at high temperatures, and the bulk pinning force plays a more important role at low temperatures.

Surface barrier and bulk pinning in anSmBa2Cu3O7−y single crystal

Based on a flux creep simulation, magnetization curves are assumed to be caused simultaneously bysurface barrier and bulk pinning. The key point of our assumption is that the bulk pinning force(Fb) changes monotonically with temperature but reaches a maximum at a middle reduced field. Thenumerical results show that a ‘fishtail effect’ (FE) appears only on a single isothermal hysteresis loop(M–H curve) but not on a single isofield magnetization versus temperature curve(M–T curve) under a zero-field-cooled (ZFC) process. Measurements of isothermalM–H andisofield ZFC M–T curves arecarried out on a SmBa2Cu3O7−y single crystal, and the experimental results are in agreement withthe simulation results. The results suggest that the FE in theSmBa2Cu3O7−y single crystal originates from the bulk pinning. Moreover, the surface barrier is moreimportant at high temperatures, whereas the bulk pinning is dominant at lowtemperatures.

Anisotropic pancake vortex transport in the crossing lattices regime ofBi2Sr2CaCu2O8+δsingle crystals

We have used micro-Hall probe magnetometry to investigate how the out-of-plane ``local'' magnetization of a superconducting ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ single crystal depends on the strength and direction of an in-plane magnetic field, ${H}_{\ensuremath{\Vert}}$, in the crossing vortex lattices regime. The ${H}_{z}=0$ remanent magnetization exhibits a pronounced anisotropy, being largest with ${H}_{\ensuremath{\Vert}}$ parallel to the crystalline $a$-axis, and smallest when it is parallel to the orthogonal $b$-axis. We attribute this to the presence of strongly pinning linear defects (LDs), which are known to lie close to the $a$-axis in these crystals. With ${H}_{\ensuremath{\Vert}}$ parallel to the $a$-axis Josephson vortex (JV) stacks become indirectly pinned along LDs, and channel pancake vortices (PVs) into these regions of high disorder, increasing the measured irreversibility. With ${H}_{\ensuremath{\Vert}}$ along the $b$-axis PVs are efficiently channeled into the sample centre, readily cutting across LDs, and the irreversibility is low. At high temperatures the remanent magnetisation as a function of the in-plane field angle exhibits a pronounced shoulder which appears to be related to the critical accommodation angle for the indirect trapping of JVs by LDs, above which JVs snap free. At low temperatures random bulk pinning is increasingly in competition with LDs, and the measured anisotropy becomes much weaker.

Irreversible properties of superconductingHoNi2B2Csingle crystals

We report on magnetic measurements on a $\mathrm{Ho}{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ single crystal. The magnetic and the superconducting phase diagrams were determined in different crystal directions and the superconductive properties evaluated as a function of temperature and applied field. The results on the critical current densities reveal bulk pinning in those regions of the phase diagram, where superconductivity is not suppressed by metamagnetically ordered structures of the Ho $4f$ moments.

Effective demagnetization factor of quasi-single-crystal and granular YBa2Cu3O7−x thin disks

The effective demagnetization factor neff of quasi-single-crystal and granular YBa2Cu3O7−x thin disks with different values of the edge barrier height, bulk pinning, and demagnetization is measured by a Hall probe with the use of a trapped magnetic flux as a test field. As the trapped magnetic flux increases, neff reaches a maximum as a function of the external field; the qualitative behavior of this maximum is independent of the morphology of a sample. It is shown that, when the densities of intragranular and intergranular critical currents differ by about one and a half orders of magnitude, the demagnetization factor neff is mainly determined by the geometry of a sample rather than by the energy of Josephson coupling between grains. The experimental results are in agreement with the results of calculations performed within the proposed model of isolated crystallites (grains).

Superconducting Nanocomposites: Enhancement of Bulk Pinning and Improvement of Intergrain Coupling

Heterogeneous sonochemical method was applied for synthesis of novel superconducting nanocomposites consisting of magnetic (and/or nonmagnetic) nanoparticles embedded into the bulk of ceramic superconductors. In addition to in-situ production of the efficient pinning centers, this synthesis method considerably improves the interbrain coupling. Significant enhancement of the magnetic irreversibility is reported for Fe2O3 nanoparticles embedded into the bulk of MgB2 superconductor. Nonmagnetic Mo2O5 nanoparticles also increase pinning strength, but less than magnetic Fe2O3. Detailed magnetization and electron microscopy characterization is presented. Theory of bulk magnetic pinning due to ferromagnetic nanoparticles of finite size embedded into the superconducting matrix is developed.

Single vortices observed as they enter NbSe 2

We observe single vortices as they penetrate the edge of a superconductor using a high-sensitivity magneto-optical microscope. The vortices leap across a gap near the edge, a distance that decreases with increasing applied field and sample thickness. This behaviour can be explained by the combined effect of the geometrical barrier and bulk pinning.

Simulations of Dynamical Ordering in Pinned Vortex Systems

We model a vortex system in a sample with bulk pinning and superficial pinning generated by a magnetic decoration. We perform a sequence of finite temperature numerical experiments in which external forces are applied to obtain, a dynamically ordered vortex lattice. We analyze the final structures and the behavior of the total energy of the system.

Effects of substituting calcium for yttrium on the superconducting properties of YBa2Cu3Oz bulk samples

We report systematic studies of AC magnetic susceptibility and transport properties of Y1−xCaxBa2Cu3Oz bulk samples with 0⩽x⩽0.4. Single phase materials, reduction of carrier concentration and decrease of Tc to 83 K were obtained at doping levels up to 20%. For Y0.7Ca0.3Ba2Cu3Oz sample the improvement of grains boundary transport and screening capabilities has been observed as a result of the optimal ratio between carrier concentration and impurity phase BaCuO2 presence. The appearance of bulk pinning and nonlinear effects starting at the highest temperature were detected also.

Peak effect at the weak to strong pinning crossover

In type-II superconductors, the magnetic field enters in the form of vortices; their flow under application of a current introduces dissipation and thus destroys the defining property of a superconductor. Vortices get immobilized by pinning through material defects, thus resurrecting the supercurrent. In weak collective pinning, defects compete and only fluctuations in the defect density produce pinning. On the contrary, strong pins deform the lattice and induce metastabilities. Here, we focus on the crossover from weak to strong bulk pinning, which is triggered either by increasing the strength f p of the defect potential or by decreasing the effective elasticity of the lattice (which is parametrized by the Labusch force f Lab). With an appropriate Landau expansion of the free energy we obtain a peak effect with a sharp rise in the critical current density j c∼ j 0( a 0 ξ 2 n p)( ξ 2/ a 0 2)( f p/ f Lab−1) 2.

Effects of substituting calcium for yttrium on the superconducting properties of YBa 2Cu 3O z bulk samples

We report systematic studies of AC magnetic susceptibility and transport properties of Y 1− x Ca x Ba 2Cu 3O z bulk samples with 0⩽ x⩽0.4. Single phase materials, reduction of carrier concentration and decrease of T c to 83 K were obtained at doping levels up to 20%. For Y 0.7Ca 0.3Ba 2Cu 3O z sample the improvement of grains boundary transport and screening capabilities has been observed as a result of the optimal ratio between carrier concentration and impurity phase BaCuO 2 presence. The appearance of bulk pinning and nonlinear effects starting at the highest temperature were detected also.

Geometrical edge barriers and magnetization in superconducting strips with slits

We theoretically investigate the magnetic-field and current distributions for coplanar superconducting strips with slits in an applied magnetic field H_a. We consider ideal strips with no bulk pinning and calculate the hysteretic behavior of the magnetic moment m_y as a function of H_a due solely to geometrical edge barriers. We find that the m_y-H_a curves are strongly affected by the slits. In an ascending field, the m_y-H_a curves exhibit kink or peak structures, because the slits prevent penetration of magnetic flux. In a descending field, m_y becomes positive, because magnetic flux is trapped in the slits, in contrast to the behavior of a single strip without slits, for which m_y =0.

Vortex edge barriers and critical current density in granular superconductors

AbstractType‐II superconductors, even without bulk pinning and in the absence of a macroscopic Bean–Livingston surface barrier for vortex penetration, can exhibit barriers for flux penetration. We investigated some new edge barriers based on the laminar model, that are governed by the anisotropy ratio, grain coupling strength, and grain size. Expressions describing the pinning potential Up and critical current density Jc are derived for magnetic fields near Hc1. It is shown that edge barrier effects play a substantial role in a realistic description of the critical current density for both MgB2 and high‐Tc superconductors.

Erratum: Field-dependent critical current in type-II superconducting strips: Combined effect of bulk pinning and geometrical edge barrier [Phys. Rev. B66, 220506 (2002)

Received 14 January 2003DOI:https://doi.org/10.1103/PhysRevB.67.099901©2003 American Physical Society

Transport and magnetic properties in MgB2 single crystals

We report measurements of the transport and the magnetic properties of high-quality MgB 2 single crystals with clear hexagonal-plate shapes. The Debye temperature of Θ D ∼ 1100 K, obtained from the zero-field resistance curve, suggests that the normal-state transport properties are dominated by electron-phonon interactions. The resistivity ratio between 40 K and 300 K is about 5. The superconducting anisotropy, γ, increases from a value around 2 near T c to about 3.3 at 26 K. The low-field magnetization and the magnetic hysteresis curves show the bulk pinning of these crystals to be very weak.

Field-dependent critical current in type-II superconducting strips: Combined effect of bulk pinning and geometrical edge barrier

Recent theoretical and experimental research on low-bulk-pinning superconducting strips has revealed striking dome-like magnetic-field distributions due to geometrical edge barriers. The observed magnetic-flux profiles differ strongly from those in strips in which bulk pinning is dominant. In this paper we theoretically describe the current and field distributions of a superconducting strip under the combined influence of both a geometrical edge barrier and bulk pinning at the strip's critical current Ic, where a longitudinal voltage first appears. We calculate Ic and find its dependence upon a perpendicular applied magnetic field Ha. The behavior is governed by a parameter p, defined as the ratio of the bulk-pinning critical current Ip to the geometrical-barrier critical current Is0. We find that when p > 2/pi and Ip is field-independent, Ic vs Ha exhibits a plateau for small Ha, followed by the dependence Ic-Ip ~ 1/Ha in higher magnetic fields.

Origin of the irreversible microwave absorption versus the state of vortex matter in Bi 2Sr 2CaCu 2O x single crystals

We have investigated properties of hysteretic microwave absorption (MWA) by the Bi 2Sr 2CaCu 2O x single crystals in the temperature range from 10 K to T c. It was found that the shape of the MWA hysteresis loop and its dependence on the magnetic field modulation amplitude undergo essential changes with temperature. The analysis has shown that near 50 K, the origin of the signal detected by electron spin resonance spectrometer with magnetic field modulation changes. It is due to crossover to another pinning type. At T<50 K where the bulk pinning is the most effective pinning mechanism, the signal registered by a spectrometer is the magnetically modulated MWA. At high temperatures, 50 K< T< T c, when vortices are pinned by the surface or the edge barrier, the signal is the derivative of the absorption power with respect to field d P/d H. The shape of the MWA hysteresis loop and its modulation enable one to determine unambiguously the pinning type (bulk or surface) controlling the irreversible properties of a superconductor.

AC response of clean type-II superconductors in the mixed state

The ac response of a slab of a clean type-II superconductor in the mixed state is theoretically investigated taking into account the bulk and surface pinning. In extremely clean superconductors sharp resonances of the ac response are predicted. Pinning-induced resonance is found only for an asymmetric ac magnetic field h( ω) pattern with the direction of h( ω) being opposite on opposite sides of the superconducting slab. The contribution of the surface pinning to the resonance frequency depends on the tilt modulus of the flux line lattice. A resonance at the cyclotronic frequency is found only in samples thinner than the London penetration depth. In the universal flux-flow regime, predicted for d-wave superconductors, wide maxima are found in the frequency dependence of the ac absorption.

Hysteresis curves and self-field alternating-current losses in superconducting tapes with transport current: Edge barrier effect

The combined influence of both bulk pinning and edge barrier on zero-field transport properties and alternating-current (ac) losses in superconducting tapes and films is considered. The equilibrium distributions of the magnetic flux density n(y) and of the sheet current density i(y) are found analytically for the tapes with an oscillating transport current. The critical current is calculated taking into account both bulk and edge pinning of the magnetic flux, and the dependence of the dissipation loss power upon the ac amplitude I0 is found. The crossover from barrier- to pinning-controlled ac losses is predicted with the increase of current oscillation amplitude I0.