Observation of field-induced transverse Neel ordering in the spin gap system TlCuCl3

Neutron elastic scattering experiments have been performed on the spin gap system TlCuCl3 in magnetic fields parallel to the b-axis. The magnetic Brag g peaks which indicate the field-induced Neel ordering were observed for th e magnetic field higher than the gap field H-g approximate to 5.5 T at Q = (h, 0, l) with odd l in the a* - c* plane. The spin structure in the ordere d phase was determined. The temperature and field dependence of the Bragg p eak intensities and the phase boundary obtained were discussed in connectio n with a recent theory which describes the field-induced Neel ordering as a Bose-Einstein condensation of magnons.