Ordered states and structural transitions in a system of Abrikosov vortices with periodic pinning

A system of Abrikosov vortices in a quasi-two-dimensional HTSC plate is considered for various periodic lattices of pinning centers. The magnetization and equilibrium configurations of the vortex density for various values of external magnetic field and temperature are calculated using the Monte Carlo method. It is found that the interaction of the vortex system with the periodic lattice of pinning centers leads to the formation of various ordered vortex states through which the vortex system passes upon an increase or a decrease in the magnetic field. It is shown that ordered vortex states, as well as magnetic field screening processes, are responsible for the emergence of clearly manifested peaks on the magnetization curves. Extended pinning centers and the effect of multiple trapping of vortices on the behavior of magnetization are considered. Melting and crystallization of the vortex system under the periodic pinning conditions are investigated. It is found that the vortex system can crystallize upon heating in the case of periodic pinning.     

Features of magnetization reversal in LCMO/YBCO heterostructures

The change in the magnetic domain structure due to the proximity of a superconductor has been experimentally investigated for the first time. The complex character of magnetization reversal at temperatures below critical, caused by the mutual long-range effect of a superconductor and a magnet, has been shown. In particular, it is found that even magnetization reversal of the heterostructure by an in-plane field leads to the formation of Abrikosov vortices in the superconductor, carrying a flux perpendicularly to the film plane. It is shown that this is a consequence of the transformation of narrow domain walls into wide stripes due to the interaction with scattering fields from the superconductor. In turn, after penetration of the magnetic flux into the superconductor at some depth, the scattering fields cause backward magnetization reversal of the external film edge, as a result of which vortices with oppositely directed fluxes enter the crystal and propagate in the superconductor bulk in the form of chains along twins, as in the case of magnetization by a perpendicular magnetic field. Thus, at longitudinal magnetization, the flux enters the superconducting film in the form of wide stripes with alternating perpendicular induction, which is explained by the long-range interaction of the scattering fields of the superconductor with the manganite magnetization.     

Viscous flow of magnetic vortices and rapid relaxation of magnetization in YBaCuO ceramics

The viscous flow of magnetic vortices in granular YBaCuO ceramics of various granule sizes is investigated using the microwave absorption method (f=80 MHz) in the range of varying magnetic fields H=±1500 Oe and of temperatures T=77 to 95 K. The conditions for the formation and melting of the vortex lattice, as well as the transition of the high-temperature superconductor (HTSC) state from the irreversibility region to the region of the viscous flow of magnetic vortices, are considered. The rapid magnetic relaxation times in the range τ=72 to 111 ms are determined from the magnetization relaxation in the HTSC materials under investigation depending on the granule size d, which varies from 1 to 30 μm. The experimental results on the variation of the relaxation time τ from 77 to 120 ms in a ceramic exposed to fast neutrons with various fluences (Φ=10¹⁶ to 10¹⁹ cm⁻²) are considered. The depinning process is analyzed; it occurs in the form of a thermally assisted flux flow in the range of low activation energies. __________ Translated from Fizika Tverdogo Tela, Vol. 43, No. 6, 2001, pp. 968–973. Original Russian Text Copyright ? 2001 by Chashchin.     

Electronic thermal properties of the interface between a normal metal and a high-temperature superconducting material

The electron thermal flux and electron thermal conductivity of the interface between a normal metal and a high-temperature superconductor (HTSC) are calculated using quasiclassical equations. Calculations are made for various values of the interface transparency and various orientations of the axes of a HTSC crystal. It was shown that compared with an interface between a normal metal and an “ordinary” superconductor (s-type symmetry, isotropic order parameter), the thermal conductivity of an HTSC-normal-metal interface is substantially higher and has a nonactivation dependence. The thermal properties were calculated for various interface models, including mirror and diffuse, and also for various potential barrier profiles. An analysis is made of the possibility of using devices based on normal-metal-HTSC interfaces for bolometric and microrefrigerator applications.     

高温超导磁悬浮测试过程对测试结果的交叉影响作用

在高温超导磁悬浮系统中,超导体和应用外磁场之间的电磁作用比较复杂。通常,任何测试过程对高温超导磁悬浮系统中相互作用力(悬浮力和导向力)结果都有影响。为了能够得到准确的测试结果,文中研究了测试过程对导向力和悬浮力测试结果的影响,发现高温超导磁悬浮系统中存在的磁历史效应导致了这种影响的存在是必然的,而且是交叉影响作用。实验数据进一步指出,在实际的测试过程中必须根据具体的运动路径来选择悬浮力和导向力的测试过程和顺序,以尽可能地减小两者之间的负面影响。根据不同的测试目的,文中也推荐了对应合理的测试方法。     

Magnetization reversal processes in layered high-temperature superconductors with ferromagnetic impurities

The self-consistent interaction of a vortex system of a high-temperature superconductor and ferromagnetic impurities, including single impurities and their clusters, has been considered in the model of a layered high-temperature superconductor. For different temperatures and concentrations of ferromagnetic impurities, the magnetization reversal loops have been calculated by the Monte Carlo method taking into account an ensemble of ferromagnetic particles with different orientations of their easy magnetization axes with respect to the direction of an external magnetic field and for different magnetic anisotropy energies. It has been demonstrated that there is a nonlinear interaction of the high-temperature superconductor with ferromagnetic impurities, in which the initially thermodynamically reversible character of the magnetization reversal of the ferromagnetic ensemble can become irreversible. For a periodic lattice of clusters of ferromagnetic impurities, the magnetization curves of the high-temperature superconductor have been calculated for different sizes and configurations of the clusters. It has been revealed that, when extended defects are oriented parallel to the direction of the entrance of vortices in the sample, the length of the defects does not affect the remanent magnetization. It has been shown that the inclusion of the interaction between the magnetic moments inside the impurity cluster leads to a decrease in the magnetization reversal loop, the coercivity, and, accordingly, the energy loss due to magnetization reversal.     

Elastic properties of HTSC ceramics

Elastic properties of ceramics of various high-temperature superconductor (HTSC) types are considered based on experimental data. The effect of porosity of HTSC ceramics is analyzed.     

Simulation of Magnetic Levitation Systems Based on Superconducting Rings

A comprehensive model of the HTSC ring magnet is developed. Magnetic and transport characteristics of commercialHTSCtapes are used in calculations, features of their layered structure are taken into account. The magnetic system parameters in external magnetic fields of various configurations are calculated. Magnetic fields, currents, magnetization, and levitation force are calculated for various geometrical parameters of magnetized rings.     

Longitudinal critical current in a ferrite-type-II superconductor structure

The effect of the interaction of Abrikosov vortices with the magnetization on the longitudinal vortical instability in a layered ferromagnet-type-II superconductor structure is analyzed. It is shown that in the vicinity of the orientational phase transition in the magnet, where the transverse magnetic susceptibility is large, the magnitude of the longitudinal critical current in the structure can be almost 1.5 times smaller than in the isolated superconductor. The reason for this is compensation of stray field sources outside the superconductor by “magnetic charges” arising from a jump in the transverse magnetization on the surface of the magnet. A structure is considered in which the thickness of the superconductor significantly exceeds the London penetration depth of the magnetic field and the wavelength of the critical mode. For this reason (in light of the absence of high-quality bulk high-temperature superconductors), to experimentally study the described phenomenon it is necessary to use conventional low-temperature superconductors. Fiz. Tverd. Tela (St. Petersburg) 39, 231–235 (February 1997)     

Conceptual design of a novel insertion device using bulk superconducting magnet

An undulator or a wiggler with a strong magnetic field will play an important role in future synchrotron light sources, free electron lasers, and linear colliders. We proposed the bulk high critical temperature superconductor staggered array undulator (Bulk HTSC SAU) in order to generate a strong periodic field. The Bulk HTSC SAU consists of stacked bulk high-Tc superconductors (HTSs) and a solenoid magnet which is used to magnetize the bulk HTSs. A periodic magnetic field was produced and controlled using a prototype of the Bulk HTSC SAU using 11 pairs of REBaCuO bulk HTSs at 77 K. The expected performance at low temperatures around 20 K is calculated using a loop current model.     

Inverse crystallization of a system of Abrikosov vortices with periodic pinning

A system of vortices in a quasi-two-dimensional HTSC plate with periodic pinning is considered. The magnetization curves are calculated by the Monte Carlo method for different values of an external magnetic field and different temperatures. It is shown that the vortex system with periodic pinning may crystallize with an increase in temperature.     

Levitation performance of the magnetized bulk high-Tc superconducting magnet with different trapped fields

To a high-T_c superconducting (HTS) maglev system which needs large levitation force density, the magnetized bulk high-T_c superconductor (HTSC) magnet is a good candidate because it can supply additional repulsive or attractive force above a permanent magnet guideway (PMG). Because the induced supercurrent within a magnetized bulk HTSC is the key parameter for the levitation performance, and it is sensitive to the magnetizing process and field, so the magnetized bulk HTSC magnets with different magnetizing processes had various levitation performances, not only the force magnitude, but also its force relaxation characteristics. Furthermore, the distribution and configuration of the induced supercurrent are also important factor to decide the levitation performance, especially the force relaxation characteristics. This article experimentally investigates the influences of different magnetizing processes and trapped fields on the levitation performance of a magnetized bulk HTSC magnet with smaller size than the magnetic inter-pole distance of PMG, and the obtained results are qualitatively analyzed by the Critical State Model. The test results and analyses of this article are useful for the suitable choice and optimal design of magnetized bulk HTSC magnets.     

Magnetization of two-dimensional superconductors with defects

A new method is developed for numerical simulation of the magnetization of layered superconductors with defects that is based on the Monte Carlo algorithm. The minimization of the free energy functional of a two-dimensional vortex system enables one to obtain equilibrium configurations of vortex density and calculate the magnetization of a superconductor with arbitrary distribution of defects in a wide temperature range. Magnetization curves are obtained for the first time for a defective superconductor under conditions of cyclic variation of the external magnetic field for different temperatures. The magnetic induction profiles and the magnetic flux distribution inside a superconductor are calculated, which support the validity of Bean’s model. It is demonstrated that the process of magnetization reversal is accompanied by the emergence of an annihilation wave, i.e., the motion of a zone with zero magnetic induction at the leading front of the incoming magnetic flux.     

The influence of measurement and relaxation time on flux jumps in high temperature superconductors

The influence of the magnetization and relaxation time on flux jumps in high temperature superconductors (HTSC) under varying magnetic field is studied using the fundamental electromagnetic field equations and the thermal diffusion equation; temperature variety corresponding to flux jump is also discussed. We find that for a low sweep rate of the applied magnetic field, the measurement and relaxation times can reduce flux jump and to constrain the number of flux jumps, even stabilizing the HTSC, since much heat produced by the motion of magnetic flux can transfer into coolant during the measurement and relaxation times. As high temperature superconductors are subjected to a high sweep rate or a strong pulsed magnetic field, magnetization undergoes from stability or oscillation to jump for different pause times. And the period of temperature oscillation is equal to the measurement and relaxation time.     

Pinning of vortices by the domain structure in a two-layered type II superconductor-ferromagnet system

The effectiveness of magnetic pinning of vortices in a layered system formed by a uniaxial ferromagnet and type II superconductor is considered. It is shown that, irrespective of the saturation magnetization of the ferromagnet, the energy of pinning at the domain structure does not exceed, in order of magnitude, the energy of artificial pinning at a column-type defect. The limitation of pinning energy is caused by the interaction of external vortices with a spontaneous vortex lattice formed in the superconducting film when the magnetization of the ferromagnetic film exceeds the critical value.     

Demagnetizing fields of crystallites and a method for measuring the thermodynamic fields of quasi-single-crystal and polycrystalline thin YBa2Cu3O7 − x disks

The role of the demagnetizing fields of crystallites in HTSC samples is studied. An increase in the crystallite size is shown to suppress the intra-and intercrystalline critical currents of the sample in lower fields. The demagnetizing fields of crystallites are shown to be one of the main causes of the fact that the Bean model is invalid for HTSC samples. A method is proposed to measure the thermodynamic field of a superconductor; this method allows the first thermodynamic critical magnetic fields of the sample and its crystallites and “subcrystallites” to be measured with a high accuracy. The first thermodynamic critical magnetic fields are used to estimate the critical current density J _c of the sample, crystallites, and subcrystallites.     

Bragg interaction of surface magnetostatic waves with a periodic granular HTSC structure

The Bragg interaction of surface magnetostatic waves with periodic granular HTSC structure has been investigated. The dispersion equation for the coupled waves has been obtained. Resonant absorption of waves near the critical temperature involving the granular structure of the superconductor has been found. The possibility of using the observed effect for making frequency-selective structures and high-speed bolometric photodetectors is shown. Tomsk University. Radio Electronic and Control Systems. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 94–98, October, 1997.     

Magnetoresistance of porous polycrystalline HTSC: Effect of the transport current on magnetic flux compression in intergranular medium

The hysteretic dependences of the magnetoresistance of porous (38% of the theoretical density) granular high-temperature superconductor (HTSC) Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x have been analyzed in the model of the effective intergranular field. This effective field has been defined by the superposition of the external field and the field induced by magnetic moments of superconducting grains. The magnetic flux compression in an intergranular medium, characterized by the effective field, controls the hysteretic behavior of the magnetoresistance. It has been found that the magnetoresistance hysteresis width for the studied porous HTSC depends on the transport current, in contrast to the superconductor of the same composition with high physical density (more than 90% of the theoretical value). For a porous superconductor, a significant current concentration occurs in the region of the grain boundaries, which is caused by features of its microstructure. A current-induced increase in the effective boundary length results in a decrease in the flux compression, a decrease in the effective field in the intergranular medium, and a magnetoresistance hysteresis narrowing with increasing current.     

Effect of a constant magnetic field on echo signals in high-temperature superconductor powders

The generation of acoustic and vortex oscillations in high-temperature superconductor (HTSC) powders excited by radiofrequency (rf) pulses was analyzed in detail in our earlier publications. The rf magnetic field stimulates oscillations of magnetic vortices on the surface of an HTSC grain, which are transformed into lattice vibrations via the pinning centers at the surface, thus inducing a propagating acoustic wave. The allowance for second-order nonlinearity in the gradient of deviation of the crystal lattice from its equilibrium position in the equation for the acoustic wave leads to a dependence of the natural frequency of crystal lattice vibrations on the amplitude and duration of pulses exciting these vibrations. Such a dependence is responsible for echo signals that can be detected experimentally. The proposed model makes it possible to interpret most experimental results for BiPbSrCaCuO superconducting samples. We consider the effect of a constant magnetic field on the amplitude and the echo signal decay time. We observed a clearly manifested peak that was not described by other authors. The model proposed here provides an obvious explanation for this peak.     

Equilibrium states and quasi-static magnetization switching of a multilayer structure

The equilibrium orientations of magnetic moments that correspond to various values and directions of the biasing field are found in a set of magnetic films with cubic crystalline anisotropy and uniaxial induced anisotropy. The films are coupled by exchange interaction of the antiferromagnetic type. Field intervals are established where noncollinear and bistability states causing orientational phase transitions and hysteresis exist. Ninety degree magnetization switching (per switching cycle) of the magnetic moments of the films, as well as an orientational phase transition of bifurcation character, is discovered. Hysteresis loops for 180° in-plane magnetization switching are constructed.