Impurity-Induced Antiferromagnetic Ordering in the Spin Gap System TlCuCl$_3$

The magnetization measurements have been performed on the doped spin gap system TlCu$_{1-x}$Mg$_x$Cl$_3$ with $x\leq 0.025$. The parent compound TlCuCl$_3$ is a three-dimensional coupled spin dimer system with the excitation gap ${\Delta}/k_{\rm B}=7.7$ K. The impurity-induced antiferromagnetic ordering was clearly observed. The easy axis lies in the $(0,1,0)$ plane. It was found that the transition temperature increases with increasing Mg$^{2+}$ concentration $x$, while the spin-flop transition field is almost independent of $x$. The magnetization curve suggests that the impurity-induced antiferromagnetic ordering coexists with the spin gap for $x \leq 0.017$.