Unusual antiferromagnetism in Quasi-one-dimensional BaFe₂Se₄ and K₃Fe₂Se₄

Quasi-one-dimensional (Q-1D) chalcogenides have attracted increased attention recently due to the rich physics of exotic magnetic order, charge-density order, topological state, quantum spin hall effect, and emergent superconductivity discovered in these materials. Here we systematically synthesized and studied two ternary iron chalcogenides with the same FeSe₄ tetrahedral building motif commonly seen in high-temperature iron chalcogenide superconductors. BaFe₂Se₄ and K₃Fe₂Se₄ have similar Q-1D chain structures with edge-shared FeSe₄ tetrahedral units whereas the latter shows a distorted structure and different iron valence state. Magnetic susceptibility and neutron powder diffraction unambiguously revealed antiferromagnetic structure in both compounds with T_N ~ 310 K for BaFe₂Se₄ and T_N ~ 110 K for K₃Fe₂Se₄. However, BaFe₂Se₄ exhibits a canted antiferromagnetic structure with a net ferromagnetic moment, whereas K₃Fe₂Se₄ shows block-type antiferromagnetic ordering despite their similar Q-1D building motifs. More results, their similarity and difference, and the implications will be discussed.