Field-induced Neel ordering in the spin gap systems TlCuCl3 and KCuCl3

The temperature and field dependence of the magnetization have been measured for TlCuCl3 and KCuCl3 which have a singlet-ground state with an excitation gap in the field range up to 70 kOe for TlCuCl3 and up to 300 kOe for KCuCl3 at helium temperature, respectively. It is found that both materials undergo held-induced Neel ordering. For TlCuCl3, the phase boundaries between the paramagnetic and the ordered states are determined for H parallel tob and H perpendicular to (1, 0, - 2). The phase boundaries for these held directions coincide when normalized by the g-factor. The exponent of the phase boundary is estimated as 2.2(1). The 3D ordering is interpreted as a Bose-Einstein condensation of dilute magnons. (C) 2001 Elsevier Science B.V. All rights reserved.