Axial and polar magnetism in hexagonal YMnO3

Abstract: Newly published diffraction data on hexagonal YMnO3 at a temperature of 10 K are shown to be consistent with a trusted expression of the magnetic symmetry, although the data alone are not definitive [M. Ramakrishnan et al., Phys. Rev. Research 5, 013203 (2023)]. Howard et al. conclude from an exhaustive review of experimental data that the symmetry of the antiferromagnetic motif of Mn ions is most likely P63'cm' [C. J. Howard et al., Acta Crystallogr. B 69, 534 (2013)]. The data reported by Ramakrishnan et al. does not eliminate symmetry P63' from our calculated diffraction patterns, because the studied reflection vector is parallel to the common chiral vector associated with each triangle of Mn axial dipole moments. Proposed diffraction patterns will give decisive statements about the magnetic symmetry in future investigations using resonant x-ray and magnetic neutron diffraction. To this end, both axial and polar magnetism in the multiferroic are essential in the analysis of diffraction patterns. The account by Ramakrishnan et al. of polar magnetism in resonant x-ray diffraction uses a magnetic symmetry of hexagonal YMnO3 not yet justified, e.g., ferromagnetism and a linear magnetoelectric effect allowed by their chosen symmetry have not been observed. We study polar magnetism in symmetry P63'cm' using Dirac multipoles, including Mn anapoles. They also feature in amplitudes for magnetic neutron diffraction together with Dirac quadrupoles, previously shown to account for diffraction by pseudo-gap phases of cuprate superconductors.