Spin-wave analysis of the spin-1 Heisenberg antiferromagnet with uniaxial single-ion anisotropy in a field

Spin-wave approaches are explored for the spin-1 Heisenberg antiferromagnet with uniaxial single-ion anisotropy in a field. In the spin-flop region a standard 1/S expansion with canted quantization axes gives good results for small anisotropies or near the saturated ferromagnetic boundary. It is shown analytically that the formulation respects the Goldstone theorem through second order in 1/S. For the square lattice case, we find that in the quantum paramagnetic phase a two-boson formulation gives good results at large anisotropies. Near the boundary of the quantum paramagnetic phase, neither approach gives good results and a more generalized approach is needed.