Magnetic excitations and soft-mode transition in the quasi-one-dimensional mixed-spin antiferromagnetPr2BaNiO5

Neutron diffraction and inelastic neutron scattering were used to investigate three-dimensional (3D) antiferromagnetic ordering and magnetic excitations in ${\mathrm{Pr}}_2$BaNi${\mathrm{O}}_5$, a quasi-1D mixed-spin antiferromagnet. Three distinct features of the magnetic excitation spectrum were observed above the Néel temperature $T_N$: a strongly 1D gap excitation propagating on the Ni chains, a single-ion crystal-field transition associated with ${\mathrm{Pr}}^{3+}$, and a strongly temperature-dependent mixed Pr-Ni magnetic-exciton band. Ni-chain modes persist in the 3D ordered phase, and the energy gap increases with decreasing $T$ below $T_N$. The magnetic-exciton band undergoes a complete softening at the antiferromagnetic zone center as $T_N$ is approached from above, driving the magnetic phase transition.