MnSb 2 O 6 is based on the structural chiral P321 space group No. 150 where the magnetic Mn 2+ moments (S = 5/2, L ≈ 0) order antiferromagnetically at T N = 12 K. Unlike the related iron based langasite (Ba 3 NbFe 3 Si 2 O 14 ) where the low-temperature magnetism is based on a proper helix characterized by a time-even pseudoscalar "magnetic" chirality, the Mn 2+ ions in MnSb 2 O 6 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 MnSb 2 O 6 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.

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We address the degeneracy of the ground state multiplet on the 5d1 Re6+ ion in double perovskite Ba2MgReO6 using a combination of specific heat measurements and density functional calculations. For Ba2MgReO6, two different ground state multiplets have previously been proposed - a quartet (with degeneracy N=4) [1] and a doublet (N=2) [2]. Here we employ two independent methods for the estimation of phonon contribution in heat capacity data to obtain the magnetic entropy Smag, which reflects the degeneracy of the ground state multiplet N through Smag = RlnN. In both cases, we obtain that in the temperature range covering 2 to 120 K the released entropy is better described by Smag = Rln2. The detailed nature of the ground state multiplet in Ba2MgReO6 remains an open question.

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