Z.J. Yang

Logarithmic relaxation in the levitation force in a magnet‐high Tc superconductor system

The dynamic response of the levitation force between a permanent magnet and a bulk YBCO superconductor when subjected to a sudden motion‐induced step in the magnetic field has been measured. It is found that the levitation force relaxes logarithmically with time. Assuming that force and magnetization behave similarly, the rate of decay can be interpreted in terms of a thermally activated flux creep with a depinning activation energy of U0=0.20 eV. A replot of previously published results for melt‐quenched YBCO also demonstrates that also in this material the force relaxation is characterized by a logarithmic decay.

LATERAL FORCE ON A MAGNET PLACED ABOVE A PLANAR YBA2CU3OX SUPERCONDUCTOR

The lateral force interaction between a permanent bar magnet and a large slab of high Tc superconductor has been investigated at 77 K, under conditions of a constant vertical separation of 2 mm. The restoring force as a function of lateral displacement rises very steeply, and reaches 90% of its saturation value, 5.1 mN, after 1.8 mm. The profile of the force‐displacement curve is in qualitative agreement with the existing theory. A significant quantitative discrepancy is interpreted as due to a theoretical penetration depth exceeding the sample thickness. The lateral magnetic stiffness or spring constant, is found to be independent of displacement away from lateral equilibrium.

Vibrations of a magnet levitated over a flat superconductor

Abstract Based on a dipole-dipole interaction model, we discuss the levitation force and related vibration problems to understand the effects of flux pinning in type-II superconductors, and the applications of levitation in superconducting systems.

Lifting forces acting on a cylindrical magnet above a superconducting plane

Abstract Based on the London theory, we have calculated and measured the lifting force acting on a cylindrical magnet placed above a superconductor. Comparison between the theoretical expectation and experimental data shows a good agreement.

Transition from elastic to dissipative motion in a magnet–high‐Tc superconductor system

Using quasistatic harmonic excitations, we have measured the lateral force between a permanent magnet and a Bi‐based ceramic high‐Tc superconductor (HTSC) for lateral displacement amplitudes down to 1 μm. We find clear evidence for a transition from elastic (nondissipative) to inelastic interaction, and attribute the effect to flux pinning. The crossover amplitude can easily reach several micrometers, with the consequence that the lateral disturbance of a levitated magnet will decay to this amplitude, whereas further damping will be extremely slow. For applications of HTSCs in magnetic bearing systems this can be a very relevant aspect of the interaction, and it can set the limit for precision positioning of levitated objects.

Potential and force between a magnet and a bulk Y1Ba2Cu3O7-δ superconductor studied by a mechanical pendulum

The interaction between a magnet and a high Tc superconductor (HTSC) is investigated by a novel method. A permanent magnet mounted at one end of a stiff rod hanging by a knifes edge forms a mechanical pendulum. When a HTSC is placed below the magnet the frequency v and damping factor Gamma will be changed due to the interaction between the magnet and the HTSC. The experiments show that, as the separation between the magnet and the HTSC decreases, the flux lines start to penetrate the sample via intergranules and grain boundaries, which results in a horizontal restoring force even for the field range B<<Bc1. The present experimental results are consistent with the theoretical model of weak links for bulk HTSC. The power-law-like potential Veff, and force, Feff, are introduced to describe the interaction. The behaviour of Veff and Feff obtained from the present measurements is in disagreement with the dipole-image model even for the field range B<<Bc1.

Comment on ‘‘Lateral restoring force on a magnet levitated above a superconductor’’ [J. Appl. Phys. 67, 2631 (1990)]

Based on Davis’ model for the lateral restoring force on a magnet levitated above a superconductor [J. Appl. Phys. 67, 2631 (1990)], we calculate the restoring force as a function of lateral displacement for a more realistic magnetic field profile.

Investigation of the interaction between a magnet and a type-II superconductor by vibration methods

Abstract Vibration methods are used to investigate the interaction between a magnet and a type-II superconductor on the basis of a dipole-dipole interaction model. For this purpose, a compound pendulum was constructed for studying the interaction between a magnet and a high- T c superconductor. The experiments show that the damping of the pendulum is much larger when the motion is parallel to the direction of the magnetic moment than when it is perpendicular. This novel result is discussed in relation to the model and movements of flux lines.

A pendulum feedback system to measure the lateral force on a magnet placed above a high‐Tc superconductor

Based on a simple mechanical pendulum design, we developed a sensor for measurement of the horizontal force on a permanent magnet placed at any distance above a high‐Tc superconductor. In this fully computerized system, which employs closed‐loop feedback control, the lateral interaction with its restoring and hysteretic nature can be studied under conditions of constant vertical separation. Results are presented for the case of a small Nd‐Fe‐B magnet at a distance of 1 mm above a large planar disk of a YBa2Cu3Ox superconductor.

Quasistatic responses of fluxoids in an Y1Ba2Cu3O7−° film

Abstract The quasistatic magntic field responses (typically 0.8 Hz) of fluxoids in a c -axis oriented Y 1 Ba 2 Cu 3 O 7−° (Y123) film have been studied using a computerized mechanical pendulum at 77K. The resonant frequency of the pendulum is significantly shifted to higher values due to the interaction between the Y123 film and a magnet mounted on the pendulum. The contribution of the magnet-superconductor interaction to the shift of the resonant frequency squared shows, in a certain range, a logarithmic dependence on the driving force (proportional to the amplitude). The present results are consistent with the previous studies on the bulk Y123 materials for a different configuration.