1 2 A pr 2 00 3 Neutron Scattering Study of Magnetic Ordering and Excitations in the Doped Spin Gap System Tl ( Cu

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 Tl(Cu 1−x Mg x)Cl 3 with x = 0.03. The magnetic Bragg reflections indicative of the ordering were observed at Q Q Q = (h, 0, l) with integer h and odd l below T N = 3.45 K. It was found that the spin structure of the impurity-induced antiferromagnetic ordered phase on average in Tl(Cu 1−x Mg x)Cl 3 with x = 0.03 is the same as that of the field-induced magnetic ordered phase for H H H b in the parent compound TlCuCl 3. The triplet magnetic excitation was clearly observed in the a *-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 Q Q Q = (h, 0, l) with integer h and odd l, as observed in TlCuCl 3. It was also found that the spin gap increases steeply below T N upon decreasing temperature. This strongly 1 indicates that the impurity-induced antiferromagnetic ordering coexists with the spin gap state in Tl(Cu 1−x Mg x)Cl 3 with x = 0.03.