Chalcogen-Height Dependent Magnetic Interactions and Magnetic Order Switching in ${\mathrm{FeSe}}_x{\mathrm{Te}}_{1-x}$

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 ${\mathrm{FeSe}}_x{\mathrm{Te}}_{1-x}$ and iron pnictide superconductors.

Figure 1

(a) Atomic structure of FeTe where $z_{\mathrm{Te}}$ is defined with respect to the Fe plane marked by a shaded square. Spin densities of (b) AFM1, (c) AFM2, and (d) AFM3 phases are displayed on the $z=0$ plane, and the supercell is denoted by a black square in each figure.

Figure 2

(a) Total energy versus $z_{\mathrm{Te}}$ calculated for FM, AFM1, AFM2, and AFM3 phases. (b) Exchange parameters $J$s [multiplied by $(2.95{\mu }_B{)}^2$] versus $z_{\mathrm{Te}}$ evaluated with fixed Fe magnetic moment of $2.95{\mu }_B$. Vertical arrows, which are slightly different in (a) and (b) due to different treatment of local magnetic moments (see text), denote the critical $z_{\mathrm{Te}}$s where the AFM2-AFM3 switch takes place.

Figure 3

Magnetic susceptibility ${\chi }_s(r,r^{\prime }=0;\omega =0)$ of the NM phase is shown for (a) $z_{\mathrm{Te}}=1.81$ and (b) $z_{\mathrm{Te}}=1.63\AA$. Deltalike magnetic field is applied at the center of the figure. PDOS in the NM phase is displayed for (c) Fe $3d$ and (d) Te $5p$ states.

Figure 4

(a) Macroscopic magnetic susceptibility ${\chi }_M$ versus electron temperature is shown for two AFM orderings. (b) DOS of FeTe for AFM2 and AFM3 phases.