Magnon hybridization in the kagome metal ErMn₆Sn₆

The kagome metal ErMn₆Sn₆ has a ferrimagnetic ground state that lies close to a magnetic instability towards helical magnetism. ErMn₆Sn₆ shows both colinear and noncolinear magnetic ordering as a function of temperature and magnetic field. Here, we study magnetic excitations in the easy-plane ferrimagnetic phase using inelastic neutron scattering. We find a steeply dispersive and gapped acoustic magnon mode due to Mn excitations within the kagome layers and a weakly dispersive optical magnon mode associated with magnons in the Er triangular layer. At low temperatures (T ≈ 2 K), these modes repel each other and form a hybridization gap that controls the magnon chirality. As the temperature increases towards T_Spiral ≈ 92 K, the hybridization gap closes, and the acoustic magnon becomes gapless before the helical order is reached.