EXPERIMENTAL-MEASUREMENT OF THE STAGGERED MAGNETIZATION CURVE FOR A HALDANE SPIN CHAIN

Long-range magnetic ordering in R2BaNiO5 (R = magnetic rare earth) qua si-one-dimensional mixed-spin antiferromagnets is described by a simpl e mean-field model that is based on the intrinsic staggered magnetizat ion function of isolated Haldane spin chains for the Ni subsystem, and single-ion magnetization functions for the rare earth ions. The model is applied to new experimental results obtained in powder diffraction experiments on Nd2BaNiO5 and NdYBaNiO5, and to previously published d iffraction data for Er2BaNiO5. From this analysis we extract the bare staggered magnetization curve for Haldane spin chains in these compoun ds.