Coexistence of Haldane-gap excitations and long-range antiferromagnetic order in mixed-spin nickelates

The spin dynamics of the (Formula presented) Ni chains in mixed-spin antiferromagnets (Formula presented) and (Formula presented) is described in terms of a simple Ginzburg-Landau Lagrangian coupled to the sublattice of rare-earth ions. Within this framework we obtain a theoretical explanation for the experimentally observed coexistence of Haldane-gap excitations and long-range magnetic order, as well as for the increase of the Haldane-gap energy below the Néel point. We also predict that the degeneracy of the Haldane triplet is lifted in the magnetically ordered phase. The growth of both gaps are shown to follow from the magnon repulsion. The theoretical results are consistent with the available experimental data.