Chalcogen-Height Dependent Magnetic Interactions and Magnetic Order Switching in FeSexTe1-x

Magnetic properties of iron chalcogenide superconducting materials are investigated using density-functional calculations. We find that the stability of magnetic phases is very sensitive to the height of chalcogen species from the Fe plane: while FeTe with optimized Te height has the double-stripe (pi, 0) magnetic ordering, the single-stripe (pi, pi) ordering becomes the ground state when Te is lowered below a critical height by, e. g., Se doping. This behavior is understood by opposite Te-height dependences of the superexchange interaction and a longer range magnetic interaction mediated by itinerant electrons. We also demonstrate a linear temperature dependence of the macroscopic magnetic susceptibility in the single-stripe phase in contrast with the constant behavior in the double-stripe phase. Our findings provide a comprehensive and unified view on the magnetism in FeSexTe1-x and iron pnictide superconductors.