Magnetic excitations of the exchange-dipole quasi-one-dimensional XXZ antiferromagnet are studied in the ordered phase. We observe a transformation of the electron spin resonance (ESR) spectrum when crossing the Néel temperature near 0.2 K. The single-mode ESR of a correlated XXZ chain transforms in the multimode spectrum in the ordered phase. The multimode spectrum consists mainly of the intensive mode of a single correlated chain, which is surrounded and/or indented by numerous weak satellites. The number of securely fixed modes is eight at $\mathbf{H}\parallel b$ and twelve at $\mathbf{H}\parallel a$. In addition to the multimode resonance observed at the transverse polarization of the microwave and static magnetic fields, we reveal a wide band of absorption by ($k$,–$k$) pairs of quasiparticles at the longitudinal polarization. This kind of absorption of microwaves occurs both in the ordered and specific spin-liquid phases, revealing the presence of quasiparticles in the specific spin-liquid phase.

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• Magnetic order
• Magnetism
• Magnons
• Quasiparticles & collective excitations
• Spin dynamics
• Spinon
• 1-dimensional spin chains
• Antiferromagnets
• Quantum many-body systems
• Electron spin resonance