Kagome Metals*

  

The RMn₆Sn₆ (R166, R = rare earth) hexagonal kagome metals show intriguing topological and magnetic properties which depend on the choice of R. Here, we present inelastic neutron scattering (INS) studies of R166 for R = Tb, Ho and Er. All three compounds exhibit ferrimagnetic order at low-temperature, but Er166 has an intermediate temperature triple-spiral phase with an incommensurate antiferromagnetic (AFM) propagation vector. Our INS data show that all three compounds have strong ferromagnetic (FM) interactions within individual Mn layers but differing temperature-dependent magnetic anisotropies. Strong FM interactions within a Mn layer determines the overall magnetic bandwidth of ∼ 230 meV. The low-energy magnetic excitations are characterized by strong FM Mn-Mn and AFM Mn-R nearest-neighbor interlayer exchange, as well as weaker and competing longer range FM and AFM Mn-Mn interlayer exchange. Competition between the interlayer interactions and also between the Mn easy-plane and R uniaxial anisotropies reveals itself with changing temperature as an energy shift of the R magnon band associated with the thermal population of R crystalline-electric-field levels. Our data confirm density-functional-theory calculations, which find that competing Mn-Mn interlayer magnetic interactions occur in all R166 compounds with R = Y, Gd-Lu, which in turn results in magnetic instabilities and tunability when Mn-R interactions are weak.