Crossover from impurity-induced ordered phase to uniform antiferromagnetic phase under hydrostatic pressure in the doped spin-gap system TlCu$_{1-x}$Mg$_x$Cl$_3$

摘要

Magnetic phase transition under hydrostatic pressure inTlCu$_{0.988}$Mg$_{0.012}$Cl$_3$ was investigated by magnetizationmeasurements. The parent compound TlCuCl$_3$ is a coupled spin dimer system,which undergoes a pressure-induced quantum phase transition from a gappedground state to an antiferromagnetic state at $P_{\rm c} = 0.42$ kbar due tothe shrinkage of the gap. At ambient pressure, the present doped systemexhibits impurity-induced magnetic ordering at $T_{\rm N}=2.5$ K. Withincreasing pressure, $T_{\rm N}$ increases. This is because the effectiveexchange interaction $J_{\rm eff}$ between unpaired spins is enhanced by theshrinkage of the gap. With a further increase in pressure, the present systemundergoes a phase transition to a uniform antiferromagnetic phase due to theclosing of the triplet gap in the intact dimers. The crossover from theimpurity-induced ordered phase to the uniform antiferromagnetic phase occurs at$P \simeq 1.3$ kbar.