FeMoO4 Revisited: Crosslike 90 degrees Noncollinear Antiferromagnetic Structure Caused by Dzyaloshinskii-Moriya Interaction

The ground state of Fe2+ (S = 2) in alpha- and beta-FeMoO4 is investigated by experiments including X-ray diffraction, Raman scattering, and Fe-57-Mossbauer 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 alpha- and beta-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 V-ii. Contrary to the alpha-phase, a degenerate set of V-zz and V-yy for both iron sites in beta-FeMoO4 is extracted from density functional theory calculations. In the vicinity of the phase transition (beta -> alpha), the degeneracy of the beta-phase is lifted. Correspondingly, we observe a softening of the nu(Mo-O) phonon modes. Detailed Mossbauer spectra confirm the crosslike 90 degrees antiferromagnetic structure for both modifications and solve the origin of the longstanding issue of disparate quadrupole splittings in alpha- and beta-FeMoO4.