Neutron scattering study of magnetic ordering and excitations in the doped spin-gap systemTl(Cu1−xMgx)Cl3

Abstract

Neutron elastic and inelastic scattering measurements have been performed in order to investigate the spin structure and the magnetic excitations in the impurity-induced antiferromagnetic ordered phase of the doped spin gap system $\mathrm{Tl}({\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}){\mathrm{Cl}}_{3}$ with $x=0.03.$ The magnetic Bragg reflections indicative of the ordering were observed at $\mathbf{Q}=(h,0,l)$ with integer h and odd l below ${T}_{N}=3.45\mathrm{K}.$ It was found that the spin structure of the impurity-induced antiferromagnetic ordered phase on average in $\mathrm{Tl}({\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}){\mathrm{Cl}}_{3}$ with $x=0.03$ is the same as that of the field-induced magnetic ordered phase for $\mathbf{H}\ensuremath{\Vert}b$ in the parent compound ${\mathrm{TlCuCl}}_{3}.$ The triplet magnetic excitation was clearly observed in the ${a}^{*}\ensuremath{-}{c}^{*}$ plane and the dispersion relations of the triplet excitation were determined along four different directions. The lowest triplet excitation corresponding to the spin gap was observed at $\mathbf{Q}=(h,0,l)$ with integer h and odd l, as observed in ${\mathrm{TlCuCl}}_{3}.$ It was also found that the spin gap increases steeply below ${T}_{N}$ upon decreasing temperature. This strongly indicates that the impurity-induced antiferromagnetic ordering coexists with the spin gap state in $\mathrm{Tl}({\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}){\mathrm{Cl}}_{3}$ with $x=0.03.$