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 p
hase boundaries H-irr(bulk) (T) and H-irr(g)(T) characterizing different ir
reversible flux motions are found. The H-irr(bulk)(T) is characterized by t
he appearance of the linear resistivity. A large separation between the bul
k irreversibility field at 0 K and the upper critical field H-c2(0) has bee
n 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 wea
k-link channels. It is further found that the magnetic relaxation rate is w
eakly 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 vorte
x melting in the rather clean system at a finite temperature. The second bo
undary H-irr(g)(T) reflects the irreversible flux motion in some local regi
ons due to either very strong pinning or the surface barrier on the tiny gr
ains. (C) 2001 Elsevier Science B.V. All rights reserved.