  Linear-chain nickelates with the composition (Nd_x Y_{1-x})_2 Ba Ni O_5 (x=1,
x=0.75, x=0.5, and x=0.25) are studied in a series of neutron scattering
experiments. Powder diffraction is used to determine the temperature dependence
of the magnetic structure in all four systems. Single-crystal inelastic neutron
scattering is employed to investigate the temperature dependence of the
Haldane-gap excitations and low-energy spin waves in the x=1 compound Nd_2 Ba
Ni O_5. The results of these experiments are discussed in the context of the
``Haldane chain in a staggered field'' model for R_2 Ba Ni O_5 systems, and
quantitative agreement with theory is obtained.
