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

The temperature and field dependence of the magnetization have been measure d for TlCuCl3 and KCuCl3 which have a singlet-ground state with an excitati on gap in the field range up to 70 kOe for TlCuCl3 and up to 300 kOe for KC uCl3 at helium temperature, respectively. It is found that both materials u ndergo held-induced Neel ordering. For TlCuCl3, the phase boundaries betwee n 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 di rections coincide when normalized by the g-factor. The exponent of the phas e 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. Al l rights reserved.