The vortex phase diagram in single crystalline La1.85Sr0.075CuO4 has b
een studied using an AC-susceptibility technique. A peak in the out-of
-phase (chi '') component of the susceptibility indicates a transition
from a pinned flux lattice (FLL) to an unpinned one. This peak is fre
quency dependent for all the values of the magnetic field measured (0.
01 to 4 T), and this, as well as the general behavior found in the cup
rates, has prompted us to interpret our data as evidence for a vortex-
glass to liquid transition in the FLL. The activation energies obtaine
d can be fitted to a theory developed by Vinokur et al. Measurements w
ith the magnetic field at an angle with the Cu-O planes may also be un
derstood qualitatively within this framework.