Fig. 4: Characteristics of two magnon eigenmodes for a magnetic field applied along the a-axis.

a Field dependence of the eigen-frequencies for two magnon eigenmodes. Softening of the low frequency eigenmode takes place for H = H_cr. b Component of the Néel vector along a-axis as a function of field applied along easy-axis, indicating a smooth orientation of Néel vector towards the c-axis. The critical field is defined at a field where n_a is zero. c Integral contribution of all magnons into magnetization averaged over the k-space with the Bose distribution function for low-frequency and high-frequency branches. For fields along the easy-axis, the ellipticity is zero above H_cr, where \({{{{{\bf{n}}}}}}\perp {{{{{\bf{H}}}}}}\) and the magnon modes are linearly polarized.