Collinear (Bi-collinear) antiferromagnetic order in iron-pnictides (chalcogenides)*

By the first-principles electronic structure calculations, we find that the ground state of the tetragonal $\alpha$-FeTe is in a bi-collinear antiferromagnetic order, in which the Fe local moments ($\sim2.5\mu_B$) align ferromagnetically along a diagonal direction and antiferromagnetically along the other diagonal direction on the Fe-Fe square lattice. This novel bi- collinear order results from the interplay among the nearest, the next nearest, and the next next nearest neighbor superexchange interactions, mediated by Te $5p$-band. In contrast, the ground state of the other iron pnictides or chalogenides is in a conventional collinear antiferromagnetic order, like LaFeAsO, resulting from the interplay between the nearest and the next-nearest neighbor superexchange antiferromagnetic interactions, bridged by As atoms. This finding sheds new light on the origin of magnetic ordering in Fe-based superconductors.