TbPt₆Al₃: A rare-earth based g-wave altermagnet with a honeycomb structure

The magnetic properties of the Tb-honeycomb lattice compound TbPt₆Al₃, which crystallizes in the NdPt₆Al₃type trigonal structure, have been studied by the measurements of electrical resistivity, magnetization M(T, B), and specific heat on single-crystalline samples. The magnetic susceptibility, M(T)/B, for B || c = 0.1 T shows a cusp at T_N = 3.5 K, in which temperature decreases with increasing the magnitude of B || c, while M(T)/B for B || a = 0.1 T remains constant with decreasing temperature below T_N. This anisotropic behavior suggests a collinear antiferromagnetic (AFM) order of the Tb³⁺ moments pointing along the c axis. The data of M(T)/B for T > 10 K on the single crystal and that of inelastic neutron scattering from powdered samples have been simultaneously analyzed using the crystal field model. The analysis reveals the non-Kramers doublet ground state for the Tb³⁺ ion under the trigonal crystal field. The neutron powder diffraction measurement shows that the collinear AFM structure with a magnetic propagation vector k = [0, 0, 0] is associated with moments of 5.1 μ_B/Tb pointing along the c axis. Comparison of the magnetic point group with the nontrivial spin Laue group indicates that TbPt₆Al₃ is classified into bulk g-wave altermagnets.