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\nsystem TlCuCl$_3$ in magnetic fields parallel to the $b$-axis. The magnetic\nBragg peaks which indicate the field-induced N\\'{e}el ordering were observed\nfor magnetic field higher than the gap field $H_{\\rm g}\\approx 5.5$ T at $Q=(h,\n0, l)$ with odd $l$ in the $a^*-c^*$ plane. The spin structure in the ordered\nphase was determined. The temperature and field dependence of the Bragg peak\nintensities and the phase boundary obtained were discussed in connection with a\nrecent theory which describes the field-induced N\\'{e}el ordering as a\nBose-Einstein condensation of magnons.\n