Altermagnetism and chiral order in a collinear antiferromagnet (MnF₂)

In this work, we study hitherto unknown magnetic properties of a collinear antiferromagnet of widespread interest, because there is intense speculation that the compound is a good atomic altermagnet with spin split electronic bands (manganese fluoride, MnF₂). The properties in question are exposed in studies of symmetry informed diffraction patterns for two standard scattering techniques. In one, the magnetically ordered solid is illuminated by x-rays tuned in energy to a Mn atomic resonance, and the radiation in the second technique is a beam of neutrons. With resonant x-ray scattering, intensity of Bragg spots are predicted to change on reversing the handedness of helicity in the primary beam. A change in intensity on switching between left and right handed x-ray circular polarization, say, equates to a magnetic chiral signature for the established magnetic space group. The same technique reveals the order parameter for altermagnetism, which is an axial magnetic octupole (third rank tensor). A multipole with identical discrete symmetries is found in the magnetic Bragg diffraction of neutrons. Moreover, spin-flip patterns from polarized neutron diffraction depend on electronic multipoles that are zero for the nominal 3d⁵ configuration of Mn²⁺, which make them excellent tests of the actual electronic structure.