Field-induced three-dimensional magnetic ordering in the spin-gap system

Abstract

The temperature and field variations of the magnetization have been measured for which has a singlet ground state with an excitation gap. It is found that undergoes three-dimensional ordering in magnetic fields. The phase boundaries between the paramagnetic and ordered states are determined for and . The phase boundaries for these field directions coincide when normalized by the g-factor. The excitation gap at zero temperature is re-evaluated as .