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Abstract

Magnetic excitations of the exchange-dipole quasi 1D XXZ antiferromagnet are studied in the ordered phase. We observe a transformation of the electron spin resonance (ESR) spectrum when crossing the N\'{e}el temperature near 0.2 K. The single-mode ESR of a correlated XXZ chain transforms in the multi-mode spectrum in the ordered phase. The multi-mode 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 magnetic field parallel b-axis and twelve at magnetic field parallel a-axis. Besides of the multi-mode 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.