Tb3+inTbCo3B2: A singlet ground state system studied by inelastic neutron scattering

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The results of inelastic neutron scattering on the hexagonal compounds ${\text{TbCo}}_{3}{\text{B}}_{2}$ and ${\text{Tb}}_{0.75}{\text{Y}}_{0.25}{\text{Co}}_{3}{\text{B}}_{2}$, at several temperatures, are reported. The crystal-field level scheme of ${\text{Tb}}^{3+}$ ions in the paramagnetic phase is determined. This scheme contains a nonmagnetic singlet $({\ensuremath{\Gamma}}_{1})$ 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 $⟨{J}_{x}⟩\ensuremath{\ne}0$. 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 $5.6(3){\ensuremath{\mu}}_{B}$ and $3(1){\ensuremath{\mu}}_{B}$ for ${\text{TbCo}}_{3}{\text{B}}_{2}$ and ${\text{Tb}}_{0.75}{\text{Y}}_{0.25}{\text{Co}}_{3}{\text{B}}_{2}$, respectively. These values are much smaller than the free ion value of $9{\ensuremath{\mu}}_{B}$ 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.