Field-induced transition of the S = 1 antiferromagnetic chain with anisotropy

The ground-state magnetization process of the S=1 antiferromagnetic chain with the easy-axis single-ion anisotropy described by negative D is investigated. It is numerically found that a phase transition between two different gapless phases occurs at an intermediate magnetic field between the starting and saturation points of the magnetization for - 1.49<D<-0.35. The transition is similar to the spin flopping, but it is second order and not accompanied with any significant anomalous behaviors in the magnetization curve. We also present the phase diagrams in the m-D and H-D planes which reveal a possible re-entrant transition.