COMPLICATIONS IN THE MEASUREMENT OF THE MAGNETIC-FIELD PENETRATION DEPTH IN YBA2CU3OX AND BI2SR2CACU2OX SUPERCONDUCTORS BY ELECTRON-SPIN-RESONANCE LINE BROADENING OF SURFACE PARAMAGNETIC PROBES

The electron spin resonance line broadening of a paramagnetic probe on an oxide superconductor below the superconducting critical transition temperature is shown to depend on the parallel or perpendicular orientation of the probe to the applied magnetic field. This effect must be considered when using the line broadening associated with the magnetic flux lattice to measure the magnetic field penetration depth. The appropriate values at 0 K of the magnetic field penetration depth in YBa2Cu3Ox and Bi2Sr2CaCu2Ox sintered superconductors are 1400 and 2850 angstrom, respectively. A strong effect of silver doping in YBaCuO superconductors on the magnetic field penetration depth is also shown. It is shown that the temperature below the superconducting critical transition temperature at which significant line broadening from the flux lattice occurs is better identified as the magnetic flux lattice melting temperature. This is particularly clear in the BiSrCaCuO superconductor in which the flux lattice melting temperature lies significantly below the superconducting critical transition temperature. This also supports that line broadening associated with the magnetic flux lattice can be isolated from other sources of broadening.