Magnetic excitations and exchange interactions in the substituted multiferroics (Nd,Tb)Fe3(BO3)4 revealed by inelastic neutron scattering

Inelastic neutron scattering spectra in the antiferromagnetic ferroborates Nd1-xTbxFe3(BO3)(4) (x = 0, 0.1, 0.2, and 1) reveal various magnetic excitations of the interacting iron and rare-earth subsystems. We observe an evolution of the magnetic system from easy-plane, in the Tb-free (x = 0) case, to easy-axis anisotropy for samples substituted with Tb. The spectra show hybridized Fe and Nd branches, which are determined by the Fe-Nd exchange splitting of the ground-state Nd3+ doublet. In the easy-plane configuration, near the Brillouin zone center, there are two different pairs of anticrossing quasiacoustic Fe and Nd modes in contrast to the easy-axis state, where the two corresponding pairs of the branches are degenerated. The high-energy (exchange) branches are similar in both spin configurations. The Ising-type anisotropy of the Tb ion prevents the magnetic moment from precession. The increasing of the Tb content changes the effective magnetic anisotropy and stabilizes the easy-axis state. The spin-wave dispersion in the substituted and pure TbFe3(BO3)(4) compounds, which have the same, easy-axis magnetic structure, but different crystal symmetry, strongly differ. The observed spectra were analysed in the frame of linear spin-wave theory and the exchange parameters were determined.

Automated summary

Inelastic neutron scattering spectra of antiferromagnetic ferroborates Nd1-xTbxFe3(BO3)(4) show magnetic excitations of iron and rare-earth subsystems. The magnetic system evolves from easy-plane to easy-axis anisotropy with Tb substitution. The hybridized Fe and Nd branches are influenced by Fe-Nd exchange splitting and the Tb Ising-type anisotropy.