CRITICAL-STATE OF AN YBA2CU3OX STRIP IN A PERPENDICULAR MAGNETIC-FIELD AS REVEALED BY A SCANNING ESR PROBE

The distribution of a magnetic field in a superconducting sample is st udied by a scanning ESR microprobe. The main attention is focused to t he penetration of a perpendicular magnetic field into an YBaCuO single crystal strip. The field profile in the interior of the sample is map ped from the data on the ESR-signal shift of the spin probe measured a t different points over the plane surface. The penetration of the magn etic flux is shown to be delayed and this gives rise to the formation of steep fronts in the profile of the field near the center of the str ip. The profile observed is highly dependent on the magnetic history. The demagnetizing effects and the distribution of the current are disc ussed. The analysis of the experimental data obtained gives evidence t hat the critical state picture differs considerably from the well know n Bean model.