The effect of an alternating magnetic field on the stability of hard levitation of high-temperature superconductors

The stability of the magnetic suspension of some HTSC samples (single crystals and melt-texture ceramics) in an inhomogeneous field of a permanent magnet was studied subject to an alternating magnetic field. The effect of a resonance swinging and break-off of the samples from the suspension level was found. The critical amplitude versus the field frequency and the critical frequency and amplitude of the field versus the distance to the magnet were measured. The temperature dependences were determined. The theory of break-off is discussed. The magnetic levitation is shown to lose stability owing to the break-off of vortices from pinning centers. Certain expressions for the dynamic and static forces of levitation, the natural frequency of oscillations, the break-off amplitude of the field and the lower and upper break-off frequencies, which bound the critical frequency interval, were derived. The criteria, which determine stability of levitation in an alternating magnetic field, were defined. The results of the calculations agree with the experimental data.