  In this work, we report the broad-band ferromagnetic resonance spectroscopy
of EuSn$_2$As$_2$ single crystals at different temperatures in combination with
magnetization measurements and structural characterization. We observe
conventional collective acoustic resonance mode of the A-type antiferromagnetic
spin-flop phase in the Eu sub-lattice, and its transition to the paramagnetic
resonance above the ordering temperature. Furthermore, we observe reproducibly
additional well-defined spectral line. The origin of the additional line
remains unclear. However, its temperature dependence attributes it to magnetism
in the Eu sub-lattice.
