DYNAMIC MAGNETIC-PERMEABILITY OF A THIN, HIGH-T-C SUPERCONDUCTING WAFER

The field dependence of the vibrational contribution to the dynamic ma gnetic permeability mu(v)(H) is calculated for a thin (of thickness d similar to lambda) high-T-c superconducting wafer in a magnetic field parallel to the surface. The resulting curves are plotted on the basis of an exact numerical analysis of the vortex structures both for the thermodynamic-equilibrium vortex lattice and in the presence of pinnin g forces and the Bean-Livingston surface barrier. It is shown that the mu(v)(H) curves are highly sensitive to the size factor (d/lambda) an d exhibit abrupt changes corresponding to a change in the number of vo rtex rows. The equilibrium mu(v)(H) curve is found to be similar in it s general behavior and absolute value (obtained with allowance for the distribution of grain sizes and with appropriate values of lambda and x) to the experimental mu(v)(H) curve plotted at nitrogen temperature for fine-grained YBa2Cu3Ox with grain diameters < D >similar to lambd a in an increasing magnetic field. It is established that the main cau se of the experimentally observed irreversible behavior of the mu(v)(H ) curves during cyclic variation of the applied magnetic field is the existence of a surface barrier to the exit of vortices from the superc onductor. The lower limit H-min(B) of stability of the mixed state in the presence of an ideal surface barrier in a thin, high-T-c supercond ucting wafer (d similar to lambda) is determined, along with the range of the vortex state (H-max-H-min) for a fixed number of vortices in m icrometer-size grains of the investigated YBaCuO samples. (C) 1997 Ame rican Institute of Physics.