Experimental measurement of the staggered magnetization curve for a haldane spin chain

Long-range magnetic ordering in R{sub 2}BaNiO{sub 5} (R=magnetic rare earth) quasi-one-dimensional mixed-spin antiferromagnets is described by a simple mean-field model that is based on the intrinsic staggered magnetization 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 Nd{sub 2}BaNiO {sub 5} and NdYBaNiO{sub 5} , and to previously published diffraction data for Er{sub 2}BaNiO {sub 5} . From this analysis we extract the bare staggered magnetization curve for Haldane spin chains in these compounds. {copyright} {ital 1998} {ital The American Physical Society}