Flux dynamics and vortex phase diagram of the new superconductor MgB2

Magnetic relaxation, critical current density and transport properties have been investigated on MgB2 bulks from 1.6 K to T-c at magnetic fields up to 8 T. A vortex phase diagram is depicted based on these measurements. Two phase boundaries H-irr(bulk) (T) and H-irr(g)(T) characterizing different irreversible flux motions are found. The H-irr(bulk)(T) is characterized by the appearance of the linear resistivity. A large separation between the bulk irreversibility field at 0 K and the upper critical field H-c2(0) has been found, it is interpreted as either due to a quantum vortex liquid induced by strong quantum fluctuation of vortices at 0 K, or flux flow through weak-link channels. It is further found that the magnetic relaxation rate is weakly dependent on temperature but strongly dependent on field indicating a trivial influence of thermal fluctuation on the vortex depinning process. Therefore the phase line H-irr(bulk) (T) may be attributed to quantum vortex melting in the rather clean system at a finite temperature. The second boundary H-irr(g)(T) reflects the irreversible flux motion in some local regions due to either very strong pinning or the surface barrier on the tiny grains. (C) 2001 Elsevier Science B.V. All rights reserved.