Neutron Scattering Study of Magnetic Ordering and Excitations in the Doped Spin Gap System Tl(Cu${}_{1-x}$Mg${}_x$)Cl${}_3$

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 Tl(Cu${}_{1-x}$Mg${}_x$)Cl${}_3$ with $x=0.03$. The magnetic Bragg reflections indicative of the ordering were observed at $Q=(h,0,l)$ with integer $h$ and odd $l$ below $T_{\mathrm{N}}=3.45$ 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 $H\parallel b$ in the parent compound TlCuCl${}_3$. The triplet magnetic excitation was clearly observed in the $a^{\ast }$-$c^{\ast }$ 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 $Q=(h,0,l)$ with integer $h$ and odd $l$, as observed in TlCuCl${}_3$. It was also found that the spin gap increases steeply below $T_{\mathrm{N}}$ upon decreasing temperature. This strongly indicates that the impurity-induced antiferromagnetic ordering coexists with the spin gap state in Tl(Cu${}_{1-x}$Mg${}_x$)Cl${}_3$ with $x=0.03$.

arXiv comment24 pages, 7 figures, 11 eps files, revtex style, will appear in Phys. Rev. B

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