MIXED-STATE STABILITY RANGE IN A YBACUO SINGLE-CRYSTAL

The range of applied magnetic field H corresponding to the stability o f mixed state with a fixed magnetic induction B is determined for a hi gh-T-c superconducting YBa2Cu3Ox single crystal for H perpendicular to c-axis. For this purpose, the field dependences of the contribution m u(upsilon) to the dynamic magnetic permeability mu(ac)' ac are analyze d for the hysteresis loops mu(upsilon) (H) in the temperature range fr om 70 to 84 K. It is shown that the mu(upsilon) (H) hysteresis can be interpreted in terms of the interaction between the vortex lattice and the surface and is associated with the B (H) hysteresis as a manifest ation of the Bean-Livingston surface barrier. As a result, the mu(upsi lon) (H) hysteresis loops corresponding to different temperatures can be described by a universal curve in dimensionless coordinates. The ob tained estimates and the available experimental data reveal a signific ant suppression of the surface barrier in actual YBaCuO single crystal s as compared to the barrier predicted for a perfect surface. The lowe r branch of the mu(upsilon) (H) hysteresis loop, which corresponds to increasing H, is found to be close to the equilibrium mu(upsilon) (H) curve, and the surface barrier appreciably affects the behavior of mu( upsilon) (H) only for decreasing field, i.e., the emergence of magneti c vortices from the sample surface. (C) 1998 American Institute of Phy sics. [S1063-777X(98)00209-6].