Block-Type Antiferromagnetism in Single Chain Quasi-One-Dimensional K_{3}Fe_{2}Se_{4}

One-dimensional (1D) structures provide a unique platform to study the correlated quantum interactions and phase transitions such as unconventional magnetism and superconducting states. Here, we report that iron chalcogenide K_{3}Fe_{2}Se_{4} exhibits an unusual block-type canted antiferromagnetic (AFM) order with a clear single chain quasi-1D structure, which is structurally different from the two-leg ladder BaFe_{2}Se_{3}, through both experimental measurements and density matrix renormalization group (DMRG) calculations. The narrow bandgap semiconductor K_{3}Fe_{2}Se_{4} has a quasi-1D edge-shared FeSe_{4} tetrahedra chain structure and orders antiferromagnetically below 110 K. The magnetic moments couple antiferromagnetically along the quasi-1D chain direction of the b axis and form an up-down-down-up (↑-↓-↓-↑)-like spin structure with a commensurate propagation vector k=(0,0,0), where block-type spin ↑-↑ or ↓-↓ coupling are between the longer Fe-Fe bonds of the quasi-1D chain. DMRG results show that block antiferromagnetic state is stable in K_{3}Fe_{2}Se_{4} and reveal that the block-ordered arrangement of Fe^{2.5+} ions spins arise from the competition between ferromagnetic and AFM interaction in the presence of strong electronic correlation. Our research results not only report the discovery of a clear block-type canted antiferromagnetic structure in a real quasi-1D chain material but also provide a theoretical approach to understand the block-type antiferromagnetism in quasi-1D iron chalcogenides.