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 Neel temperature T-N = 24 K. Two temperature-dependent magnetic-excito n bands were found and associated with a crystal field transition in P r. One of these modes was found to undergo a complete softening at the antiferromagnetic wave vector as T --> T-N from above, driving the ma gnetic ordering.