SCANNING ESR MICROPROBE STUDIES OF THE FLUX DISTRIBUTION IN TYPE-II SUPERCONDUCTORS

A new method for the study of the distribution of the magnetic field i n a superconducting sample is developed. The field profile in the inte rior of the sample is mapped from the data on the spin probe ESR-signa l shift measured at different points over the plane surface. The main attention is focused to the penetration of a perpendicular magnetic fi eld into single crystal strips. The penetration of magnetic flux into an YBCO crystal is shown to be delayed and this gives rise to formatio n of the steep fronts in the profile of the field near the center of t he strip. The profile observed differs considerably from the well-know n Bean picture. It is highly dependent on the magnetic history. For th e case of a Bi:2212 rectangular crystal, the crossover of the magnetic field profile from a dome shape, typical for a geometrical barrier (T > 30 K), to a bulk pinning picture (T < 25 K) was observed upon varia tion of temperature. The analysis of the field distribution made it po ssible to determine the temperature dependence of the lower critical f ield.