Classical spin liquid state in the emergent honeycomb-lattice material TbBO3

Abstract: The classical spin liquid state, owing to spin frustration between classical Ising spins on triangular motifs and interplay between competing degrees of freedom, is characterized by a macroscopically degenerate ground state and topological excitations linked to emergent gauge theories. Here, by using thermodynamic and local-probe measurements down to 16 mK, we demonstrate the exotic magnetism and spin dynamics in the nearly perfect honeycomb lattice material TbBO3. The latter embodies a frustrated lattice with a superimposed triangular lattice, constituted by additional Tb3+ ions at the center of each hexagon. Thermodynamic experiments reveal the presence of dominant antiferromagnetic exchange and significant dipolar interactions. Despite sizable antiferromagnetic exchange interactions between the Tb3+ moments, muon-spin relaxation does not detect any signatures of long-range magnetic order or spin freezing down to 16 mK, corroborating the specific heat and ac susceptibility down to 45 mK. This suggests that the spin-orbit driven anisotropic exchange interaction engenders a strong frustration, crucial to induce a persistent spin dynamics. The scaling of muon relaxation rate as a function of the characteristic energy scale for several spin-liquid candidates, including TbBO3, demonstrates that a common underlying mechanism is at play. This is consistent with the NMR results on TbBO3 and reminiscent of a universal spin-liquid behavior, here attributed to dominant antiferromagnetic short-range spin correlations, confirmed by the presence of a broad magnetic diffuse scattering in the elastic and low-energy inelastic neutron scattering at Q=1.03 Ang^-1 at low temperatures. Our results demonstrate that TbBO3 hosts a classical spin liquid induced by spin-orbit driven anisotropy on a frustrated honeycomb lattice antiferromagnet.