Surface alloying and iron selenide formation in ${\text{Fe/Bi}}_2{\mathrm{Se}}_3\left(0001\right)$ observed by x-ray absorption fine structure experiments

The atomic structure of ultrathin iron films deposited on the (0001) surface of the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is analyzed by surface x-ray absorption spectroscopy. Iron atoms deposited on a ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ (0001) surface kept at 160 K substitute bismuth atoms within the first quintuple layer. Iron atoms are neighbored by six selenium atoms at a distance in the 2.4 Å range indicating substantial atomic relaxations. Mild annealing up to 520 K leads to the formation of $\alpha$-FeSe, characterized by a local order extending up to the sixth shell (5.80 Å). Ab initio calculations predict a noncollinear magnetic ordering with a transition temperature of 3.5–10 K depending on the iron concentration and the number of the layers in which Fe is located.

Figure 1

X-ray absorption spectra for approximately 0.3 ML iron on ${\mathrm{Bi}}_2{\mathrm{Se}}_3$(0001) collected using $E_{\parallel }$ (red) and $E_{\perp }$ (blue) polarizations. Spectra are normalized to the edge jump and shifted for clarity. The inset shows the corresponding $k^2$-weighted $\chi \left(k\right)$ functions.

Figure 2

Schematic view of adsorption sites for iron on ${\mathrm{Bi}}_2{\mathrm{Se}}_3$(0001). Bismuth and selenium atoms are shown as large blue and small yellow spheres, respectively. Iron is represented by light gray balls. High symmetry adsorption sites of iron are indicated.

Figure 3

Experimental (symbols) and calculated (lines) magnitude of the Fourier transforms of the $k^2$-weighted $\chi \left(k\right)$ functions for $E_{\parallel }$ (a) and $E_{\perp }$ (b). The inset shows the corresponding $k^2$-weighted $\chi \left(k\right)$ functions. No polarization dependence is observable.

Figure 4

Fe-K-edge XANES spectra of 0.3 ML ${\text{Fe/Bi}}_2{\mathrm{Se}}_3$ collected in $E_{\parallel }$ geometry (1) compared with that of a bulk $\alpha$-FeSe powder (2) and of the 520 K annealed sample (6). Spectra plotted by dashed lines (3, 4, 5) are reproduced from Ref. [49] emphasizing the pressure dependence of the XANES.

Figure 5

(a) Fe-K EXAFS measured at 160 K on the 0.3 ML ${\text{Fe/Bi}}_2{\mathrm{Se}}_3$ sample after annealing at 520 K. The inset shows the $k^2$-weighted $\chi \left(k\right)$ function (symbols: experiment; fit: lines). (b) Corresponding magnitude of the FT. The inset shows a side view of one Se-Fe-Se triple layer, where labels “1” and “2” indicate the first (Se) and second (Fe) neighbor shell corresponding to the first and second FT maximum, respectively. (c) Structure model for one Se-Fe-Se triple layer. Labels $i$ with $i=1,2,\cdots ,6$ indicate neighbor shells taken into account in the fitting.

Figure 6

Calculated exchange interaction in ${\mathrm{Bi}}_{1.9}{\mathrm{Fe}}_{0.1}{\mathrm{Se}}_3$ (Fe is only in the subsurface Bi layer) and ${\mathrm{Bi}}_{1.8}{\mathrm{Fe}}_{0.2}{\mathrm{Se}}_3$ (Fe is in both Bi layers of the first QL). The inset shows the structure of the first quintuple layer with the corresponding exchange parameters indicated.