Size Effects on Levitation Force Between a Magnet and a Superconducting Thin Film in the Meissner State

The levitation force between a permanent magnetic disk and a thin superconducting disk in the Meissner state is calculated using the dipole–dipole interactions model. The levitation force as a function of the magnet and superconductor radii, the levitation height, and the superconductor thickness is studied under the assumption that the radius of the magnet is much smaller than the radius of the superconductor. Results showed an increase in the levitation force as a function of the radius of the superconducting disk. However, the levitation force decreases as the radius of the permanent magnetic disk increases. Demagnetizing effects are taken into account by considering the appropriate demagnetizing factor for the suggested system.     

Levitation Force Between a Small Magnet and Superconducting Sphere

Based on the Meissner effect and the image method, we studied the interaction between a magnetic point dipole and a superconducting sphere. We obtained analytical expressions for the interaction energy and the levitation force when the dipole is in an arbitrary orientation. Our calculations show the validity of using the image method for a antisymmetrical superconducting sphere-magnet system. Results show that the energy and the force are maxima when the magnetic dipole is perpendicular to the surface of superconductor and minima when it is tangent to the surface. Furthermore, the force acting on the tip of a magnetic force microscope above a superconducting sphere was derived as a generalization of the levitation force acts on a point dipole.      

Levitation Force between a Horizontally Oriented Point Magnetic Dipole and a Superconducting Sphere

The magnetic interaction between a point dipole of horizontally oriented moment and a superconducting sphere in the complete Meissner state is investigated in detail. It is demonstrated that the levitation force for this configuration is precisely one half the value of the configuration with vertically oriented point dipole. An extension to other applied fields, still assuming the perfect flux exclusion state, is presented. The results have application to models for magnetic levitation and magnetic force microscopy.     

Magnetic Levitation With High-T c Superconducting Thin Films

The levitation force between a permanent magnet and a superconducting thin film was investigated experimentally. The configuration consisted of a cylindrical NdFeB permanent magnet placed above a circular YBa₂Cu₃O_7?δ disk with common cylinder axis. Precise measurements were made of the vertical force F _z and the magnetic stiffness κ _z as a function of the magnet–superconductor separation at 77 K. Several features contrasting the levitation force produced using bulk superconductors were observed. Thin films produced very high values for F _z and κ _z per unit volume of superconducting material. The hysteretic behavior of F _z during decreasing and increasing separation resulted in loops of nearly symmetrical shape, which also contain a peak in the repulsive force branch. The observations are analyzed and explained with good quantitative agreement using recent theories for flux penetration in thin superconductors in transverse magnetic fields.     

Magnetic Levitation With High-Tc Superconducting Thin Films

The levitation force between a permanent magnet and a superconducting thin film was investigated experimentally. The configuration consisted of a cylindrical NdFeB permanent magnet placed above a circular YBa₂Cu₃O_7– disk with common cylinder axis. Precise measurements were made of the vertical force F _z and the magnetic stiffness _z as a function of the magnet–superconductor separation at 77 K. Several features contrasting the levitation force produced using bulk superconductors were observed. Thin films produced very high values for F _z and _z per unit volume of superconducting material. The hysteretic behavior of F _z during decreasing and increasing separation resulted in loops of nearly symmetrical shape, which also contain a peak in the repulsive force branch. The observations are analyzed and explained with good quantitative agreement using recent theories for flux penetration in thin superconductors in transverse magnetic fields.     

Modeling of thermally stimulated depolarization current (TSDC) using dipole–dipole interaction concept

The study of thermally stimulated depolarization current (TSDC) using the dipole–dipole interaction model is describedin this work. The dipole–dipole interaction model (DDIM) determines the TSDC peak successfully since it gives significant peak parameters (i.e. activation energy (E{\bf \textit{E}}) and pre-exponential factor (t₀){\boldsymbol\tau}_{{\bf 0}})) in addition to the dipole–dipole interaction strength parameter (d_i){\bf \textit{d}}_{{\bf i}}). Application of this model to study the peak parameters of some polymeric systems is presented.     

Comment on ‘Resistivity Minimum Behavior and Weak Magnetic Disorder Characteristics in La<Subscript>2/3</Subscript>Ca<Subscript>1/3</Subscript>MnO<Subscript>3</Subscript> Manganites’

In a recent paper (Chen et al. in J. Supercond. Nov. Magn. 22:465, 2009) Zhenping Chen et al. reported measurements of the electrical resistivity (ρ) and magnetization versus temperature (T) and magnetic field (H) on polycrystalline manganite compound La_2/3Ca_1/3MnO₃. In particular, the ρ(T) dependence shows a shallow minimum at T _min∼20 K. Application of the external H up to 8 T leads to marked flattening of such minimum and to notable lowering of the T _min on ρ(T,H) curves. The authors attributed this low-temperature anomaly to competitive quantum Kondo effect and three-dimensional electron–electron interaction, using the results of numerical fittings of ρ versus T and H dependences in the interval of about 5–30 K. It is suggested in the comment that such a claim seems to be doubtful due to the fundamental inapplicability of this approach for analysis of low-temperature conductivity in polycrystalline manganites. It is noted that a natural cause for both the low-temperature minimum on ρ(T) curve and its evolution upon magnetic field application, is the well-known grain boundary effect. The numerous uncertainties and obscurities are also characteristic for the paper being commented upon.     

Effect of reciprocating motions around working points on levitation force of superconductor-magnet system

In order to simulate vibration around working points in practical operation of superconducting levitation system, magnet in a simple superconductor-magnet system are conducted reciprocating motions around static height in this study. Two YBCO cylindrical samples with different grain orientations are used to investigate the effect of reciprocating motions of magnet on superconducting magnetic force. The c-axis of sample S1 is perpendicular to the top surface while sample S2 is parallel to the top surface. The initial cooling processes for the superconductors include zero-field-cooled (ZFC) and filed-cooled (FC). Compared to the levitation force before reciprocating motions, the ZFC levitation force at static height becomes smaller after reciprocating while the FC force presents opposite phenomenon. It is found that levitation force at static height tends to be stable after several times of reciprocating under ZFC and FC conditions and its time-decay phenomenon is suppressed in some extent, which is meaningful for the practical application of superconducting levitation system. Based on vortex dynamic, some physical discussions are presented to the experimental results.     

Influence of Re-magnetized HTS Bulk Samples on the Levitation Performances of a Maglev System

High-T _c superconducting (HTS) bulk samples used for a maglev system can be re-magnetized after being subjected to another applied magnetic field which may influence the levitation performance of a HTS bulk over the permanent magnetic guideway (PMG). In order to clarify the relationship between the HTS bulk’s re-magnetization and its levitation performance, a YBCO bulk was re-magnetized at different values and its levitation performance studied experimentally above a permanent guideway (PMG) system. The results show that the increase of trapped magnetic flux inside the HTS bulk is closely related to the magnitude and direction of the applied field during the re-magnetization process. Furthermore, the levitation force changes on the HTS-PMG system affected by the re-magnetized HTS bulk are discussed allowing for optimization improvements to the maglev system load capabilities.     

Force Analysis of a Permanent Magnet and a Superconducting Hollow Cylinder

The interaction between a cylindrical magnet and a superconducting hollow cylinder in the Meissner state was analyzed using a dipole?Cdipole model. Analytical expression of the levitation force was derived as a function of the magnetic moment, radius of the magnet, radius, and thickness of the superconductor sample. The effect of the magnet??s dimensions on the levitation force was studied. The obtained results show that there is strong dependence of the levitation force on the magnetic dipole orientation at a small magnet?Csuperconductor distance.     

Levitation Force Between a Point Magnetic Dipole and Superconducting Sphere

The magnetic interaction between a point dipole and a superconducting sphere in the Meissner state is investigated in detail. It is demonstrated that the special case of perfect flux exclusion can be solved by the method of images when the dipole is vertically magnetized. The incorporation of a nonzero penetration depth is then made by developing the solution within London theory. The analytic solution, given in terms of an infinite series representation, is refined by exactly summing the perfectly diamagnetic response. Throughout, special cases of physical interest are discussed and applications to magnetic force microscopy and levitation are described.     

Levitation Force Density on Different Thicknesses of?YBa2Cu3O7?x Bulk Superconductor Fabricated by?FQMG?Process

A precursor YBCO in the stoichiometric ratio of 1:2:3 was prepared by the flame-quench-melt-growth (FQMG) method. Both the superconducting and nonsuperconducting regions were decided by measuring the magnetic levitation force per unit volume as a function of thickness of the superconducting sample. It was determined that superconducting forming ratio depends on where it forms in the sample, and thus a region near the top surface of the sample contains more superconducting forming ratio than the other regions. The levitation force density measured at different thicknesses of the sample ranging from 6.54 to 1.58 mm firstly increased and then separately reached a saturation point (maximum value) for both repulsive and attractive levitation force density. It was found that the maximum values of the repulsive and attractive levitation forces were respectively 125 mN/cm³ and ?23 mN/cm³. The considerably improved F _L ?z performance in FQMG sample can be translated to large-scale Y123 blocks intended for real superconducting levitation applications.     

The Maximum Levitation Force of High-T c Superconductors

In this paper we present the dependence of the maximum levitation force (F _z ^max) of a high-T _c superconductor (HTS) on the structural factors of high-T _c superconducting systems based on the Bean critical state model and Ampère’s law. A transition point of the surface magnetic field (B _s ) of a permanent magnet (PM) is found at which the relation between F _z ^max and B _s changes: while the surface magnetic field is less than the transition value the dependence is subject to a nonlinear function, otherwise it is a linear one. The two different relations are estimated to correspond to partial penetration of the shielding currents inside the superconductor below the transition point and complete penetration above it respectively. The influence of geometric properties of superconductors on the dependence is also investigated. In addition, the relation between F _z ^max and the critical current density (J _c ) of the HTS is discussed. The maximum levitation force saturates at high J _c . An optimum function of the J _c and the B _s is presented in order to achieve large F _z ^max.      

Flux Concentrator Optimization of PMG for High-Temperature Superconducting Maglev Vehicle System

The permanent magnetic guideway (PMG) composed of permanent magnet (PM) and steel is developed under flux concentration principle, which is the crucial component of high-temperature superconducting (HTS) maglev vehicle system. Optimum PMG design is an effective way to increase levitation force and associated stiffness for improving the load capability of HTS maglev vehicle. In order to realize higher vertical field component B _z in upper surface, three PMG demonstrators with three different forms of flux concentrator are fabricated with same volume of magnet. The levitation performances of onboard HTS bulks array over them are studied. The experimental results indicate that the PMG with a permanent magnet as the flux concentrator would produce biggest levitation force, levitation stiffness and trapped flux when interacting with HTS superconductor.     

Influence of the Trapped Field on the Levitation Performance of the Magnetized Bulk High-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> Superconductor

A magnetized bulk high-T _c superconductor (HTSC) magnet is a good candidate to improve the levitation performance of the high-T _c superconducting (HTS) maglev system. Compared with the unmagnetized bulk HTSC, the magnetized bulk HTSC magnet can supply stronger levitation or guidance force above a permanent magnet guideway (PMG). Different from the permanent magnet, the magnetic field of a magnetized bulk HTSC magnet is sustained by the induced superconducting current produced during the magnetizing process. Given that the induced superconducting current within the magnetized bulk HTSC magnet is very sensitive to the magnetic field, the levitation performance of the magnetized bulk HTSC magnet is directly related to its own trapped field and the magnetic field of the PMG. This article discusses the influence of trapped and external magnetic fields on the levitation performance of a magnetized bulk HTSC magnet by experiments, and the Critical State Model is used to analyze the test results. The analyses and conclusions of this article are useful for the application of magnetized bulk HTSC magnet in practical HTS maglev systems.     

The Influence of Many-Body Interactions on the Electron States in Quantum Point Contacs: Persistence of Exchange-Driven Splitting at High External Magnetic Fields

Spontaneous spin polarization of a quasi-1D electron gas in quantum point contacts (QPCs) is an important concept when analyzing conductance anomalies in the quantum limit. As suggested by recent measurements (Koop et al., J. Supercond. Nov. Magn. 20:433, 2007) there is a splitting of the subband levels in QPCs related to 0.7 conductance anomaly, both for zero- and finite in-plane magnetic fields. In the present paper we present theoretical results for spin polarization occurring in a QPC in a magnetic field as obtained from the local spin-density approximation (LSDA). Our numerical simulations are consistent with the findings of Koop et al. and support the idea that spin polarization underpins the conductance anomaly.     

Evanescent waves in the magnetic field of an electric dipole

The magnetic field of radiation emitted by an electric dipole contains travelling and evanescent waves when represented as an angular spectrum. The evanescent waves decay exponentially away from the xy-plane, and will therefore not contribute to the detectable radiation in the far field, in general. It is well known, however, that in a small region around the z-axis the evanescent waves of the electric field do end up in the far zone. We have studied the corresponding magnetic evanescent waves, and we have found that the evanescent waves of the magnetic field do not contribute to the far zone in the neighbourhood of the z-axis. When considering the neighbourhood of the xy-plane, it appears that both the electric and magnetic evanescent waves end up in the far field, and the travelling and evanescent waves contribute equally to the radiation in the far zone. Close to the dipole the radiation field diverges, and we have shown that this is entirely due to the evanescent waves.     

Investigation of YBa2Cu3O x , Bi(Pb)-HTSC and Nd(123)-Particles in a Running Magnetic Field Near the Superconducting Transition Temperature

A novel system of electromagnetic separation (EMS) employing an alternating magnetic field (RMF) of suspended HTSC-particles in liquid nitrogen is described (Broide in Supercond. Cryoelectron. Summer:33, [1998]; Broide in Superconductor Week 12, No. 21, [1998]; Broide in US Patent 5,919,737, [1999]; Broide et al. in International Conference, Barcelona, Spain, [1999]). The EMS method is realized at temperatures close to those of the HTSC transition by extracting particles of YBaCuO, BiSrCaCuO, BiPbSrCaCuO, and NdBaCuO with optimal physical properties from the powders, concentrating particles with a greater critical current at one side. This process has shown great promise in its ability to produce high quality superconducting materials, being also an effective method of testing and analyzing powders to determine if synthesis adjustments are necessary in order to achieve optimal materials properties. In addition, a correlation between the speed of YBa₂Cu₃O _x particles and the percentage of oxygen (O _x ) was discovered. Since the temperature of the superconducting (SC) transition (T _c ) practically has a linear dependence on the oxygen percentage, the knowledge of the speed of the HTSC-particles during the separation, one may immediately give (within 10 s) the percentage of the oxygen in the chemical structure of the new HTSC materials. The EMS method enables statistical improvement in the uniformity of HTSC powders by decreasing the quantity of defective zones in the separated powders, the number of nonvalid admixtures, and thus, an increase in T _c and I _c (critical current). At the same time, ΔT _c the width of the SC-transition is decreased. Three YBa₂Cu₃O _x thick films have been produced by the paint method on MgO substrates from the source material, the separated high-quality concentrate, and the low-quality remnant (tail). The films exhibit different structural and electrical properties, in particular, different I _c , and ΔT _c . For the film that was made from the high-quality concentrate, ΔT _c is decreased from 5–7 K to 1–2 K. I _c is increased by a factor of 2–5 from the I _c of the source material (before EMS) and T _c is changed from 89 K to 93 K. Thick films, which had been produced from low-quality remnant (tail) do not have SC transition down to 77 K.      

Dynamic Simulation of High Temperature Superconductors Above a Spinning Circular Permanent Magnetic Guideway

Before a high temperature superconducting (HTS) magnetic levitation (Maglev) vehicle system can be fully applied and operational, the study of its dynamic characteristics is necessary. With the developed HTS Maglev dynamic measurement system (SCML-03), with a circular permanent magnet guideway (PMG) of 1.5 m in diameter, the vehicle’s translational motion above a PMG can be effectively simulated with the PMG allowed to rotate freely. Levitation force measurements of a high temperature superconductor (HTSC) array of seven YBa₂Cu₃O_7−x bulks were carried out above regular (linear) and a simulated (circular) PMG. The levitation force above a linear PMG segment and a circular PMG segment in the static state is found to be in good agreement with each other. The levitation force in the dynamic state is found to slowly attenuate since the presence of a rotating circular PMG below the HTS array is found to be analogous to the application of an AC external magnetic field.     

Effects of the relative speed of a magnet moving to and from a single-domain YBCO bulks on their interaction force

Two single-domain YBCO bulks with different grain orientations were used to investigate the effect of relative moving speed between the magnet and YBCO bulk, on their interaction force at LN₂ temperature under zero field cooling (ZFC) state. It is found that the levitation force of the sample S1 with c-axis perpendicular to the top surface is higher than that of the sample S2 with c-axis parallel to the top surface; the maximum levitation force (F_ML) is increasing with the increase of relative moving speed (V_RMS) between the magnet and the samples at first and reaches a peak value for sample S1 and a peak zone values for sample S2, then the F_ML decreases with the increase of V_RMS; the stiffness between the samples and the magnet is decreasing with the increase of the V_RMS. This result is useful for the application of superconducting levitation.