Structural dimerization in the commensurate magnetic phases of NaFe(WO$_4$)$_2$ and MnWO$_4$

TL;DR

This study investigates structural distortions and magnetoelastic coupling in NaFe(WO$_4$)$_2$ and MnWO$_4$ using neutron diffraction, revealing bond-angle modulations linked to magnetic ordering and dimerization in commensurate phases.

Contribution

It provides the first detailed analysis of structural dimerization and magnetoelastic effects in these materials' commensurate magnetic phases.

Findings

Bond-angle modulation of approximately 1.15 degrees in NaFe(WO$_4$)$_2$

Structural dimerization observed in MnWO$_4$'s commensurate phase

Strong magneto-elastic coupling linked to magnetic order

Abstract

The structural distortion and magnetoelastic coupling induced through commensurate magnetism has been investigated by neutron diffraction in structurally related MnWO$_4$ and NaFe(WO$_4$)$_2$. 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(WO$_4$)$_2$ 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 $\pm 1.15(16)$ degrees. In the commensurate phase of MnWO$_4$, 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 MnWO$_4$ result from tiny commensurate fractions that can depin multiferroic domains.