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 in a superconducting sample is developed. The field profile in the interior of the sample is mapped from the data on the spin probe ESR-signal shift measured at different points over the plane surface. The main attention is focused to the penetration of a perpendicular magnetic field into single crystal strips. The penetration of magnetic flux into an YBCO crystal is shown to be delayed and this gives rise to formation of the steep fronts in the profile of the field near the center of the strip. The profile observed differs considerably from the well-known Bean picture. It is highly dependent on the magnetic history. For the 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 variation of temperature. The analysis of the field distribution made it possible to determine the temperature dependence of the lower critical field.