Measurements of surface resistance of grain-aligned bulk material as a function of DC magnetic field; weak link study

The temperature and DC magnetic field dependences of the surface resistance of grain-aligned YBa/sub 2/Cu/sub 3/O/sub x/ bulk material were measured at 9.5 and 75 GHz. Experiments were conducted on a-b-plane- and a-c-plane-oriented sample surfaces. A nonlinear increase of R/sub s/(H/sub dc/) at low fields, which turns into a linear dependence on H at fields larger than H/sub c1/, was observed and explained. A model that allows a quantitative fit using as parameters the lower critical field of the weak links or the grain, the decoupling parameter, and the slope alpha was employed. Two different sets of these parameters must be used to fit all of the data, indicating the existence of two kinds of weak links. The parameter values indicate that there exist inter- and intragrain weak links that contain Josephson fluxons, and a bulk crystal region that contains Abrikosov fluxons. The field-modulated microwave absorption method was used to identify intergrain weak links with lower critical fields <10/sup -4/ T. The correlation between microstructure of the samples and the angular dependence of the field modulated microwave absorption lines is discussed.<>