Mössbauer study of rare-earth ferroborate NdFe₃(BO₃)₄

An ⁵⁷Fe Mössbauer spectroscopic study has been conducted on the single-crystal NdFe₃(BO₃)₄ to investigate its magnetic structure and multiferroicity. In the multiferroic state below the Néel temperature T_N = 29 K, the magnitude of the quadrupole coupling constant e²qQ=2 becomes smaller than that in the paramagnetic phase. The decrease is explained by the spin-orbit coupling due to the simultaneous order of Fe³⁺ and Nd³⁺ moments, which intensifies the 4 f-3d hybridization. This is consistent with the d-p hybridization mechanism as the origin of the multiferroicity. Although the commensurate antiferromagnetic order along the a-axis is confirmed at 25 K, the pronounced incommensurate order is not recognized at 4.2 K. In the latter, it is probable that the Fe³⁺ moments are distributed only around the a-axis, such as within the ±10° region, or that they are fluctuating or frustrating, within the c-plane dynamically with the relaxation time of, for example, 10⁻⁹ s.