Observation of Field-Induced Transverse Néel Ordering in the Spin Gap System TlCuCl₃

Abstract

Neutron elastic scattering experiments have been performed on the spin gap system TlCuCl₃ in magnetic fields parallel to the b-axis. The magnetic Bragg peaks which indicate the field-induced Néel ordering were observed for the magnetic field higher than the gap field H_g≈ 5.5 T at Q=(h, 0, l) with odd l in the a^*-c^* plane. The spin structure in the ordered phase was determined. The temperature and field dependence of the Bragg peak intensities and the phase boundary obtained were discussed in connection with a recent theory which describes the field-induced Néel ordering as a Bose-Einstein condensation of magnons.