We report calorimetric, magnetic, and neutron scattering studies on an $S=5/2$, nearly Heisenberg triangular-lattice antiferromagnet ${\mathrm{K}}_2\mathrm{Mn}{\left({\mathrm{SeO}}_3\right)}_2$ with weak XXZ easy-axis anisotropy. Multiple magnetic phases are identified, including a noncollinear Y phase in zero field, a field-induced collinear $m=1/3$ magnetization plateau, and a high-field V phase. In the Y phase, the magnetic excitation spectrum exhibits both single-magnon excitations and an extended high-energy continuum. Both features are well described by nonlinear spin wave theory. In the field-induced phases, complex effects of the spectrum renormalization even for large $S=5/2$ material are clearly detectable. These results underscore the essential role of magnon-magnon interactions in the dynamics of large-$S$ Heisenberg spin systems on a triangular lattice.

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• Magnetic order
• Magnetic phase transitions
• Spin dynamics
• Magnetic insulators
• Noncollinear magnets
• Triangular lattice
• Inelastic neutron scattering
• Magnetization measurements
• Neutron diffraction
• Specific heat measurements
• Spin wave theory