Neutron diffraction in MnSb₂O₆: Magnetic and structural domains in a helicoidal polar magnet with coupled chiralities

MnSb₂O₆ is based on the structural chiral P321 space group No. 150 where the magnetic Mn²⁺ moments (S=5/2, L≈0) order antiferromagnetically at TN=12K. Unlike the related iron based langasite (Ba₃NbFe₃Si₂O₁₄) where the low-temperature magnetism is based on a proper helix characterized by a time-even pseudoscalar “magnetic” chirality, the Mn²⁺ ions in MnSb₂O₆ order with a cycloidal structure at low temperatures, described instead by a time-even vector “magnetic” polarity. A tilted cycloidal structure has been found [M. Kinoshita et al., Phys. Rev. Lett. 117, 047201 (2016)] to facilitate ferroelectric switching under an applied magnetic field. In this work, we apply polarized and unpolarized neutron diffraction analyzing the magnetic and nuclear structures in MnSb₂O₆ with the aim of understanding this magnetoelectric coupling. We find no evidence for a helicoidal magnetic structure with one of the spin envelope axes tilted away from the cycloidal c axis. However, on the application of a magnetic field ∥c the spin rotation plane can be tilted, giving rise to a cycloid—helix admixture that evolves towards a distorted helix (zero cycloidal component) for fields great than ≈2 T. We propose a mechanism for the previously reported ferroelectric switching based on coupled structural and magnetic chiralities requiring only an imbalance of structural chiral domains.