PINNING-FORCE DENSITY AND CRITICAL-CURRENT DENSITY IN SUPERCONDUCTINGRB3C60 FULLERENE

The magnetic critical-current density J(c) and the volume-pinning-forc e density F-p in a well-characterized Rb3C60 fullerene with T-c = 30.5 K have been measured as functions of magnetic field and temperature. They were determined by de magnetic hysteresis loops at temperatures b etween 5 and 30 K in an applied field up to +/-5.5 T. J(c) appears to be a monotonically decreasing function of T and H in the temperature a nd field regions measured. At T = 5 K, F-p initially increases rapidly with increasing field, and it seems to level off when H > 0.5 T, and finally it reaches a saturation value of about 10(10) dyne/cm(3) at H similar to 1.4 T. For T > 5 K, however, F-p(H) appears to peak at a ma ximum value, F-p,F-max at field H-p that occurs between 0 and the irre versibility field (H-irr). Both H-p and F-p,F-max are temperature-depe ndent and they seem to be proportional to (1-T/T-c)(n).