EFFECT OF REVERSIBLE FLUXOID MOTION ON AC SUSCEPTIBILITY OF HIGH-TEMPERATURE SUPERCONDUCTORS

It has been found in measurements of AC susceptibility versus temperature for high temperature superconductors that, with decreasing AC magnetic field amplitude, the peak value of the imaginary part, chi", decreases remarkably accompanied by a shift of its position to higher temperatures, while the height of the transition of the real part, chi', does not change appreciably. This behavior contradicts the prediction by the critical state model in which the flux pinning phenomenon is assumed to be completely irreversible. It is shown that this anomalous magnetic behavior originates from reversible fluxoid motion in pinning potentials. The AC susceptibility is calculated on the basis of the Campbell model for the reversible fluxoid motion. The result obtained warns us against incorrect use of the critical state model for analysis of experimental data.