  Magnetic phase transitions under hydrostatic pressures in spin gap systems
TlCu$_{0.988}$Mg$_{0.012}$Cl$_3$ and KCu$_{0.973}$Mg$_{0.027}$Cl$_3$ were
investigated by magnetization measurements. The present doped systems exhibit
impurity-induced magnetic orderings. With increasing pressure, ordering
temperature $T_{\rm N}$ increases. With a further increase in pressure, the
present systems undergo phase transitions to uniform antiferromagnetic phases
due to the closing of the triplet gap in the intact dimers. The crossover from
the impurity-induced ordered phase to the uniform antiferromagnetic phase
occurs at $P \simeq 1.3$ kbar for TlCu$_{0.988}$Mg$_{0.012}$Cl$_3$.
