Magnetoelectric properties of the layered room-temperature antiferromagnets BaMn2P2 and BaMn2As2

Properties of two ThCr2Si2-type materials are discussed within the context of their established structural and magnetic symmetries. Both materials develop collinear, G-type antiferromagnetic order above room temperature, and magnetic ions occupy acentric sites in centrosymmetric structures. We refute a previous conjecture that BaMn2As2 is an example of a magnetoelectric material with hexadecapole order by exposing flaws in supporting arguments, principally, an omission of discrete symmetries enforced by the symmetry of sites used by Mn ions and, also, improper classifications of the primary and secondary order-parameters. Implications for future experiments designed to improve our understanding of BaMn2P2 and BaMn2As2 magnetoelectric properties, using neutron and x-ray diffraction, are examined. Patterns of Bragg spots caused by conventional magnetic dipoles and magnetoelectric (Dirac) multipoles are predicted to be distinct, which raises the intriguing possibility of a unique and comprehensive examination of the magnetoelectric state by diffraction. A roto-inversion operation in Mn site symmetry is ultimately responsible for the distinguishing features.