Tb(sup 3+) in TbCo3B2: a singlet ground state system studied by inelastic neutron scattering

The results of inelastic neutron scattering on the hexagonal compounds TbCo3B2 and Tb0.75Y0.25Co3B2, 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 (Λ1) 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 ≠ 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)(micro)B and 3(1)(micro)B for TbCo3B2 and Tb0.75Y0.25Co3B2, respectively. These values are much smaller than the free ion value of 9(micro)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