COEXISTENCE OF HALDANE-GAP EXCITATIONS AND LONG-RANGE ANTIFERROMAGNETIC ORDER IN MIXED-SPIN NICKELATES R2BANIO5

The spin dynamics of the S=1 Ni chains in mixed-spin antiferromagnets Pr2BaNiO5 and NdxY2-xBaNiO5 is described in terms of a simple Ginzburg -Landau Lagrangian coupled to the sublattice of rare-earth ions. Withi n this framework we obtain a theoretical explanation for the experimen tally observed coexistence of Haldane-gap excitations and long-range m agnetic order, as well as for the increase of the Haldane-gap energy b elow the Neel point. We also predict that the degeneracy of the Haldan e triplet is lifted in the magnetically ordered phase. The growth of b oth gaps are shown to follow from the magnon repulsion. The theoretica l results are consistent with the available experimental data.