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arXiv · 2508.03899

Longitudinal magnons in large-$S$ easy-axis magnets

A. El Mendili·T. Ziman·M. E. Zhitomirsky
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Abstract

Longitudinal magnons are a distinct type of multipolar excitations in magnetic materials with large spins $S\ge 1$ and strong easy-axis anisotropy. These excitations have angular momentum $S^z = \pm 2S$ and can be viewed as a propagating full spin reversal. We study longitudinal magnons for the nearest-neighbor Heisenberg ferromagnet and antiferromagnet on a square lattice with large single-ion anisotropy. In the strong-coupling limit, we derive an effective spin-1/2 model including two leading contributions in $J/D$. The effective model provides a simple description of the longitudinal magnon dynamics. For $S=1$, we compare results from several theoretical approaches that include the effective spin-1/2 model, the linked-cluster expansion, the multiboson spin-wave theory, and, for a ferromagnet, an exact two-particle solution. Among these approaches, the multiboson spin-wave theory provides the decay rate of longitudinal magnons and describes evolution of the excitation spectra from strong to weak anisotropy.

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A. El Mendili, T. Ziman, M. E. Zhitomirsky. 2025-08-05. Longitudinal magnons in large-$S$ easy-axis magnets. https://doi.org/10.1103/gy3k-c1w3

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