Novel three-dimensional magnetic ordering in the quantum spin system NH₄CuCl₃

Low-temperature specific heat and magnetization measurements have been performed on the quantum spin system NH₄CuCl₃ which has magnetization plateaus at one-quarter and three-quarters of the saturation magnetization. It is found that the present system undergoes three-dimensional magnetic ordering at T_N = 1.3 K. It is suggested that one-quarter of the spins are ordered below T_N, and the remaining spins are in the singlet state. The small magnetic entropy at T_N indicates that the phase transition occurs under the condition of a well-developed spin correlation around T_N. The field dependence of the three-dimensional ordering is also investigated on the basis of magnetization measurements. Based on this evidence, a magnetic phase diagram for NH₄CuCl₃ is proposed.