Schwinger-Boson Mean-Field Theory of Mixed-Spin Antiferromagnet $L_2BaNiO_5$

Abstract

The Schwinger-boson mean-field theory is used to study the three-dimensional antiferromagnetic ordering and excitations in compounds $L_2BaNiO_5$, a large family of quasi-one-dimensional mixed-spin antiferromagnet. To investigate magnetic properties of these compounds, we introduce a three-dimensional mixed-spin antiferromagnetic Heisenberg model based on experimental results for the crystal structure of $L_2BaNiO_5$. This model can explain the experimental discovery of coexistence of Haldane gap and antiferromagnetic long-range order below Néel temperature. Properties such as the low-lying excitations, magnetizations of $Ni$ and rare-earth ions, Néel temperatures of different compounds, and the behavior of Haldane gap below the Néel temperature are investigated within this model, and the results are in good agreement with neutron scattering experiments.