Long term magnetic relaxation in single crystalline K3C60 superconductors

Long-term magnetic relaxation in the fullerene superconductor K3C60 was measured on single crystals in a wide range of temperatures and magnetic fields. A logarithmic M(t) dependence is observed on samples with 100% shielding fraction. The relaxation rate at different magnetic fields increases progressively with temperature. The flux creep activation energy is found to be in the range from 10 to 80 meV with a peak in its temperature dependence. From measurements on samples with non-perfect stoichiometry we show that inhomogeneities strongly affect the relaxation process and may mask a logarithmic behaviour.