Tb³⁺ in TbCo₃B₂: A singlet ground state system studied by inelastic neutron scattering

The results of inelastic neutron scattering on the hexagonal compounds TbCo₃B₂ and Tb_0.75Y_0.25Co ₃B₂, at several temperatures, are reported. The crystal-field level scheme of Tb³⁺ ions in the paramagnetic phase is determined. This scheme contains a nonmagnetic singlet (Γ₁) as the ground state. Inelastic neutron scattering at low temperature (10 K) leads to a different energy-level scheme, where the singlet ground state is ferromagnetic with 〈Jx〉 ≠0. This is a "self-induced" ferromagnetism on the Tb sublattice, resulting from the admixture of higher crystal-field levels into the singlet ground state by the exchange field. The resulting magnitudes of these ground state magnetic moments are 5.6 (3) μ_B and 3 (1) μ_B for Tb₃B₂ and Tb_0.75Y_0.25Co₃B₂, respectively. These values are much smaller than the free ion value of 9 μ_B and are in agreement with previously observed values. Such large reductions are characteristic of the "self-induced" ferromagnetism. The temperature dependences of the magnetic moment, magnetic anisotropy, Tb sublattice dilution, and magnetic susceptibility are discussed.