Noncollinear Magnetic Structures in the Chiral Antiperovskite β-Fe₂SeO

We present the magnetic properties of the chiral, polar, and possibly magnetoelectric antiperovskite β-Fe₂SeO as derived from magnetization and specific-heat measurements as well as from powder neutron diffraction and Mössbauer experiments. Our macroscopic data unambiguously reveal two magnetic phase transitions at T_N1 ≈ 103 K and T_N2 ≈ 78 K, while Rietveld analysis of neutron powder diffraction data reveals a noncollinear antiferromagnetic structure featuring magnetic moments in the a-b plane of the trigonal structure and a ferromagnetic moment along c. The latter is allowed by symmetry between T_N1 and T_N2, weakly visible in the magnetization data yet unresolvable microscopically. While the intermediate phase can be expressed in the trigonal magnetic space group P3₁, the magnetic ground state is modulated by a propagation vector q = (1/2 1/2 0) resulting in triclinic symmetry and an even more complex low-temperature spin arrangement which is also reflected in the Mössbauer hyperfine patterns indicating additional splitting of Fe sites below T_N2. The complex noncollinear spin arrangements suggest interesting magnetoelectric properties of this polar magnet.