H. Bratsberg

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.

Shape Distortion by Irreversible Flux-Pinning-Induced Magnetostriction

Exact analytical results are obtained for the flux-pinning-induced magnetostriction in cylindrical type-II superconductors placed in parallel magnetic fields. New modes of irreversible deformation are found: In contrast to the circular cylinder where shape is conserved, it is shown that a square cross section deforms with considerable distortion. During a field cycle, concave, convex, and even more complicated distortions are predicted. Strong implications for dilatometric measurements on crystals are emphasized. The main results are valid for any critical-state model,

Theory for lateral stability and magnetic stiffness in a high-Tc superconductor-magnet levitation system

{j}_{c}{\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}j}_{c}(B)

Magneto-optical study of magnetic-flux penetration into a current-carrying high-temperature-superconductor strip

.

LATERAL FORCE ON A MAGNET PLACED ABOVE A PLANAR YBA2CU3OX SUPERCONDUCTOR

A quantitative first‐order theory for the lateral force between a permanent magnet and a type‐II superconductor is presented. The levitation configuration discussed is that of a long rectangular bar magnet placed above a finite‐sized rectangular superconductor. The central issues of stability and stiffness (elastic spring constant) associated with lateral vibrations are discussed. Closed‐form expressions for both the force and stiffness are derived, thus bringing out clearly the significance of both geometrical dimensions and the magnetic response of the superconductor. It is assumed that the superconductor is either a sintered granular or consists of grains embedded in a nonactive matrix (composite) so that only intragranular shielding currents are important. The predicted behavior as a function of levitation height agrees very well with existing experimental results.

Vibrations of a magnet levitated over a flat superconductor

The magnetic-flux distribution across a high-temperature superconductor strip is measured using magneto-optical imaging at 15 K. Both the current-carrying state and the remanent state after transport current are studied up to currents of

CRITICAL-STATE MAGNETIZATION OF TYPE-II SUPERCONDUCTORS IN RECTANGULAR SLAB AND CYLINDER GEOMETRIES

{0.97I}_{c}

Investigation of the lateral magnetic force and stiffness between a high‐Tc superconductor and magnet of rectangular shapes

where

Flux-pinning-induced magnetostriction in cylindrical superconductors

{I}_{c}

Bending of silver-sheathed (Bi,Pb)-2223 tapes investigated by magneto-optical flux visualization

is the critical current. To avoid overheating of the sample current pulses with a duration of 50 ms were employed. The results are compared with predictions of the Bean model for the thin strip geometry. In the current-carrying state, reasonable agreement is found. However, there is a systematic deviation\char22{}the flux penetration is deeper than theoretically predicted. A much better agreement is achieved by accounting for flux creep as shown by our computer simulations. In the remanent state, the Bean model fails to explain the experimental results. The results for the currents