Field-induced transition of the Haldane antiferromagnets

The field-induced long-range antiferromagnetic order in the quasi-one-dimensional S = 1 antiferromagnet is investigated with the exact diagonalization of finite chains and the mean field approximation for the interchain interaction. In the presence of the single-ion anisotropy D, we present the phase diagrams in the plane of the magnetic field H and the temperature T for H parallel toD and H perpendicular toD. It is found that the two boundary curves of the ordered phase for H parallel toD and H perpendicular toD have two intersections in the case of D >0 (XY like), while no intersection in the case of D <0 (Ising like). (C) 2001 American Institute of Physics.