FLUX DYNAMICS AND LOW-FIELD MAGNETIC-PROPERTIES OF THE HEAVY-FERMION SUPERCONDUCTOR CECU2SI2

Strong flux creep effects are observed in CeCu2Si2 at low magnetic inductions and temperatures in the range 6 mK < T < T(c). Measurements of flux creep in single crystal as well as polycrystalline specimens show that the magnetization in CeCu2Si2 decays following a power law time dependence of the type M(t) is-proportional-to t(-alpha) for 1 < t < 10(5) sec, where the exponent a ranges between 0.015 and 0.06. The activation energy values obtained from the relaxation rates are around 2-3 me V. The flux creep data give clear evidence of the existence of different types of pinning centers in CeCu2Si2: a large number of shallow potential barriers that characterize the vortex dynamics at short times and few strong pinning centers responsible for the long-time relaxation behavior. Quantum tunneling of flux lines through weak pinning centers probably gives rise to the non-thermally activated relaxation observed at the lowest temperatures. The physical origin of weak and strong pinning centers in CeCu2Si2 is discussed. From isothermal dc magnetization measurements we estimate the lower critical field H(c1) as well as the critical current density J(c). The H(c1) versus T curve shows an anomalous positive curvature for temperatures close to T(c).