Structural dimerization in the commensurate magnetic phases of Na Fe ( WO 4 ) 2 and Mn WO 4

International audienceThe structural distortion and magnetoelastic coupling induced through commensurate magnetism has been investigated by neutron diffraction in structurally related MnWO4 and NaFe(WO4)2. Both systems exhibit a competition of incommensurate spiral and commensurate spin up-up-down-down ordering along the magnetic chains. In the latter commensurate phases, the alternatingly parallel and antiparallel arrangement of Fe 3+ respectively Mn 2+ moments leads to sizeable bond-angle modulation and thus to magnetic dimerization. For NaFe(WO4)2 this structural distortion has been determined to be strongest for the low-field up-up-down-down arrangement, and the structural refinement yields a bond-angle modulation of ±1.15(16) degrees. In the commensurate phase of MnWO4, superstructure reflections signal a comparable structural dimerization and thus strong magneto-elastic coupling different to that driving the multiferroic order. Pronounced anharmonic second-and third-order reflections in the incommensurate and multiferroic phase of MnWO4 result from tiny commensurate fractions that can depin multiferroic domains