Neutron Diffraction Study of the Pressure-Induced Magnetic Ordering in the Spin Gap System TlCuCl$_3$

Neutron elastic scattering measurements have been performed under the hydrostatic pressure in order to investigate the spin structure of the pressure-induced magnetic ordering in the spin gap system TlCuCl$_3$. Below the ordering temperature $T_{\rm N}=16.9$ K for the hydrostatic pressure $P=1.48$ GPa, magnetic Bragg reflections were observed at the reciprocal lattice points ${\mib Q}=(h, 0, l)$ with integer $h$ and odd $l$, which are equivalent to those points with the lowest magnetic excitation energy at ambient pressure. This indicates that the spin gap closes due to the applied pressure. The spin structure of the pressure-induced magnetic ordered state for $P=1.48$ GPa was determined.