= Antiferromagnetic spin wave
{wiki}
An <antiferromagnetic spin wave> is a small collective variation of the ordered spin pattern in an antiferromagnet. In a bipartite <Néel state>, neighbouring <spins> point oppositely; the slowly varying <Néel order parameter> describes their staggered orientation. A long-wavelength disturbance changes that orientation smoothly rather than independently flipping each site.
For an isotropic ordered antiferromagnet in the usual long-wavelength regime, the low-energy <dispersion relation> is linear, $\omega\simeq c|\mathbf k|$, with <spin-wave velocity> $c$. This differs from the quadratic low-<wavenumber> <ferromagnetic magnon> in a conventional isotropic ferromagnet. <Anisotropy> or an applied <magnetic field> can alter the branches or introduce gaps. The starting ordered state also needs justification: strong <quantum fluctuations>, especially in low dimension, can invalidate a naive <linear spin-wave approximation>. A computed divergence is then a warning about the approximation, not a negative physical <magnetization>.
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