Block magnetism coupled with local distortion in the iron-based spin-ladder compound BaFe 2 Se 3

Magnetism in the insulating BaFe${}_{2}$Se${}_{3}$ was examined through susceptibility, specific heat, resistivity, and neutron diffraction measurements. After formation of a short-range magnetic correlation, a long-range ordering was observed below ${T}_{\mathrm{N}}\ensuremath{\sim}255$ K. The transition is obscured by bulk properties. Magnetic moments ($\ensuremath{\parallel}a$) are arranged to form a Fe${}_{4}$ ferromagnetic unit, and each Fe${}_{4}$ stacks antiferromagnetically. This block magnetism is of the third type among magnetic structures of ferrous materials. The magnetic ordering drives unusually large distortion via magnetoelastic coupling.