Spin-wave dynamics and exchange interactions in multiferroic NdFe3(BO3)4 explored by inelastic neutron scattering

Magnetic excitations and exchange interactions in multiferroic NdFe3(BO3)4 were studied by inelastic neutron scattering in the phase with commensurate antiferromagnetic structure. The observed spectra were analyzed in the frame of the linear spin-wave theory. It was shown that only the model, which includes the exchange interactions within eight coordination spheres, describes satisfactorily all observed dispersion curves. The calculation showed that the spin-wave dynamics is governed by the strongest antiferromagnetic intra-chain interaction and three almost the same inter-chain interactions. Other interactions, including ferromagnetic exchange, appeared to be insignificant. The overall energy balance of the antiferromagnetic inter-chain exchange interactions, which couple the moments from the adjacent ferromagnetic layers as well as within a layer, stabilizes ferromagnetic arrangement in the latter. It demonstrates that the pathway geometry plays a crucial role in forming of the magnetic structure.