Schwinger-boson mean-field theory of the mixed-spin antiferromagnet L2BaNiO5

The Schwinger-boson mean-field theory is used to study the three-dimensional antiferromagnetic (AF) ordering and excitations in the compounds L2BaNiO5, a large family of quasi-one-dimensional mixed-spin antiferromagnets. To investigate the magnetic properties of these compounds, we introduce a three-dimensional mixed-spin AF Heisenberg model based on experimental results for the crystal structure of L2BaNiO5. This model can explain the experimental finding of coexistence of the Haldane gap and AF long-range order below the Neel temperature. Properties such as the low-lying excitations, magnetizations of Ni and rare-earth ions, Neel temperatures of different compounds, and the behaviour of the Haldane gap below the Neel temperature are investigated within this model, and the results are in good agreement with neutron scattering experiments.