  Long-range magnetic ordering in R_2 Ba Ni O_5 (R=magnetic rare earth)
quasi-1-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_2 Ba Ni
O_5 and Nd Y Ba Ni O_5, and to previously published diffraction data for Er_2
Ba Ni O_5. From this analysis we extract the bare staggered magnetization curve
for Haldane spin chains in these compounds.
