High-field magnetization in the quantum spin magnet NiCl24SC(NH2)2

The field-induced antiferromagnetic phase transitions in NiCl24SC(NH2)2 have been determined in fields up to 18 T and temperatures down to 0.016 K. This is an S=1 system with a singlet ground state separated from an excited doublet by D/kB. In zero field this spin-gap system remains nonmagnetic down to T=0, because the small antiferromagnetic exchange interaction (J) is “subcritical” (2zJmagnons. The phase-boundaries B1(T) and B2(T) determined follow power-law dependence, Bi(T)−Bi(0)∝Tφ, with φ=2.6±0.2 for both.