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 TN = 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.
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