STUDIES OF MAGNETIC CHARACTERIZATION AND CRITICAL-CURRENT DENSITY IN RB3C60

Temperature-dependent magnetization and magnetic-hysteresis curves for the zero-field cooled and the field-cooled processes of Rb3C60 superc onducting fullerene with T-c of 30.5 +/- 0.1 K were measured between 5 and 30 K in magnetic fields up to 1 T. Small field-cooled diamagnetis m and flux jumps were observed at low temperatures and in low fields. Both results strongly suggest the existence of a rather large pinning strength in our sample. This suggestion is further confirmed by the fi eld-cooled hysteresis curve and high critical current densities, J(c)( H, T), at low temperatures. Based on the major hysteresis loops and th e Bean critical-state model, the J(c) at T = 5 K are 2.1 x 10(6) A/cm( 2) for H = 0 T and 5.2 x 10(5) A/cm(2) for H = 0.9 T. These J(c) value s are larger than other powdered superconductors with comparable sampl e sizes, except for YBa2Cu3Oy. For T > 10 K, J(c) versus H agrees well with the expression: J(c)(H, T)= k(1)/(1 + H/H-01)(n). On the other h and, for T < 10 K, J(c)(H, T) may be expressed as J(c)(H, T) = k(2) ex p(-H/H-02). Here k(1), k(2), H-01, H-02 and n are temperature-dependen t parameters. The dependence of J(c) on temperature thus obeys the emp irical scaling relation, J(c)(H, T) = J(c0)(H)(1 - T/T-c)(m), where J( c0) and m are field-dependent parameters. For 0.4 T < H < 1 T, m equal s about 2.