Mixed-state parameters and vortex pinning in single-crystalline K 3 C 60 fullerene superconductors

We report on detailed magnetic investigations of big ${\mathrm{K}}_{3}{\mathrm{C}}_{60}$ single crystals with sizes of $\ensuremath{\sim}1\mathrm{mm}.$ The samples were characterized by x-ray analysis, neutron diffraction, and ac magnetization measurements. The temperature dependence of the upper and lower critical fields was obtained, the latter by the trapped magnetic-moment method. The coherence length and the penetration depth at $T=0$ and close to the transition temperature were evaluated. The magnetic field and the temperature dependence of the critical current density were studied and the irreversibility lines determined. Long-term magnetic relaxation, measured in a wide range of temperatures and magnetic fields, allowed us to obtain the temperature dependence of the relaxation rate and the flux creep activation energy at different magnetic fields. The influence of sample inhomogeneities on all these properties was investigated. The results are compared to those obtained on powder and polycrystalline samples.