Ch. Rüegg, M. Oettli, J. Schefer, O. Zaharko, A. Furrer, H. Tanaka, K. W. Krämer, H.-U. Güdel, P. Vorderwisch, K. Habicht, T. Polinski and M. Meissner
Neutron Scattering Study of the Field-Dependent Ground State and the Spin Dynamics in Spin-One-Half NH₄CuCl₃
Phys. Rev. Lett. 93, 037207/1-4 (2004)                 

Abstract: Elastic and inelastic neutron scattering experiments have been performed on the dimer spin system NH₄CuCl₃, which shows plateaus in the magnetization curve at m = 1/4 and m = 3/4 of the saturation value. Two structural phase transitions at T₁ 156 K and at T₂ = 70 K lead to a doubling of the crystallographic unit cell along the b direction and as a consequence a segregation into different dimer subsystems. Long-range magnetic ordering is reported below T_N = 1.3 K. The magnetic field dependence of the excitation spectrum identifies successive quantum phase transitions of the dimer subsystems as the driving mechanism for the unconventional magnetization process in agreement with a recent theoretical model.