Impurity-induced antiferromagnetic ordering in the spin gap systemTlCuCl3

Magnetization measurements have been performed on the doped spin gap system ${\mathrm{TlCu}}_{1-x}{\mathrm{Mg}}_x{\mathrm{Cl}}_3$ with $x<~\mathrm{0.025.}$ The parent compound ${\mathrm{TlCuCl}}_3$ is a three-dimensional coupled spin dimer system with the excitation gap $\Delta {/k}_{\mathrm{B}}=7.7\mathrm{}\hspace{0.5em}\mathrm{K}.$ The impurity-induced antiferromagnetic ordering was clearly observed. The easy axis lies in the $\mathrm{}(0,1,0)\mathrm{}$ plane. It was found that the transition temperature increases with increasing ${\mathrm{Mg}}^{2+}$ 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 $x<~\mathrm{0.017.}$