FeMoO₄ Revisited: Crosslike 90° Noncollinear Antiferromagnetic Structure Caused by Dzyaloshinskii–Moriya Interaction

Abstract

The ground state of Fe2+ (S = 2) in α- and β-FeMoO4 is investigated by experiments including X-ray diffraction, Raman scattering, and 57Fe–Mössbauer spectroscopy below 300 K and evaluated by theoretical modeling. Both modifications crystallize in the space group C2/m with the same set of Wyckoff positions. The structural feature of α- and β-FeMoO4 is a tetramer of the so-called butterfly motif. Two iron-sites (Fe2) form an antiferromagnetically coupled dimer whereas two Fe1 establish an antiferromagnetic intertetramer coupling. The effective magnetic exchange of the two magnetic sublattices is based on dominating Dzyaloshinskii–Moriya interaction due to the rare situation of canceling Heisenberg exchange interactions. According to our investigations, the ground states of the two polymorphs differ in terms of their Fe-site specific electric field gradients Vii. Contrary to the α-phase, a degenerate set of Vzz and Vyy for both iron sites in β-FeMoO4 is extracted from density functional theory calculations. In the vicinity of the phase transition (β → α), the degeneracy of the β-phase is lifted. Correspondingly, we observe a softening of the ν(Mo–O) phonon modes. Detailed Mössbauer spectra confirm the crosslike 90° antiferromagnetic structure for both modifications and solve the origin of the longstanding issue of disparate quadrupole splittings in α- and β-FeMoO4.