Magnetic excitations and soft-mode transition in Pr2BaNiO5

The quasi–1-dimensional (1D) antiferromagnet Pr2BaNiO5 was studied by single-crystal inelastic neutron scattering. Magnetic gap excitations of 1D nature in the S=1 Ni chains were observed above and below the Néel temperature $T_{\rm N}=24$ K. Two temperature-dependent magnetic-exciton bands were found and associated with a crystal field transition in Pr. One of these modes was found to undergo a complete softening at the antiferromagnetic wave vector as $T \rightarrow T_{\rm N}$ from above, driving the magnetic ordering.