  Magnetization maesurements under hydrostatic pressure were performed on an
S=1/2 coupled spin system TlCuCl3 with a gapped ground state under magnetic
field H parallel to the [2,0,1] direction. With increasing applied pressure P,
the gap decreases and closes completely at Pc=0.42 kbar. For P>Pc, TlCuCl3
undergoes antiferromagnetic ordering. A spin-flop transition was observed at
Hsf=0.7T. The spin-flop field is approximately independent of pressure,
although the sublattice magnetization increases with pressure. The gap and Neel
temperature are presented as function is attributed to to the relative
enhancement of the interdimer exchange interactions compared with the
intradimer exchange interaction.
