Room-temperature structural ordering of a Heusler compound Fe${}_3$Si

$D{0}_3$-ordered Fe${}_3$Si, which is one of the ferromagnetic Heusler compounds, has so far been formed by high-temperature heat treatments. Here, we demonstrate room-temperature $D{0}_3$ ordering of Fe${}_3$Si films grown on Ge(111) by using a molecular beam epitaxy (MBE) technique. In our MBE conditions, higher growth temperatures are not effective to obtain highly ordered $D{0}_3$ structures because of the interfacial reactions between Fe${}_3$Si and Ge. Even for the room-temperature growth, the degree of the $D{0}_3$ ordering can be improved with increasing film thickness. Considering the experimental data and the calculated results based on molecular dynamics, we can understand that the main structural disorder is derived from a specific Fe site in the Fe${}_3$Si film near the interface. We also discuss the room-temperature $D{0}_3$ ordering in terms of the local stoichiometry of the supplied atoms in MBE conditions.

Figure 1

(a) High-resolution cross-sectional TEM image near the Fe${}_3$Si/Ge(111) interface grown at $T_{\mathrm{G}}=$ RT. (b) The intensity profile of the HAADF image from the areas I and II enclosed in Fig. 1a. (c) HAADF image of the Fe${}_3$Si layer near the interface. The axis of the incident electron beam is parallel to the [1$\overline{1}$0] direction. (d) The intensity profile of the HAADF image from the area enclosed in Fig. 1c.

Figure 2

The saturation magnetic moments ($M_{\mathrm{S}}$) at room temperature (solid circles) and the degree of $D{0}_3$ ordering (opened squares) of the Fe${}_3$Si(25 nm) films grown at RT $\le$ $T_{\mathrm{G}}$ $\le$ 400 ${}^{\circ }$C. The gray dotted line shows $M_{\mathrm{S}}$ of the bulk sample at 6.5 K in Ref. 10. The inset displays the ideal crystal structure of $D{0}_3$-type Fe${}_3$Si.

Figure 3

(a) Room-temperature ${}^{57}$Fe Mössbauer spectrum (solid circles) of 10-nm-thick Fe${}_3$Si films grown at $T_{\mathrm{G}}=$ RT, together with seven fitting curves as denoted in the text. (b) Fitted area versus site number for Fe${}_3$Si films with various thicknesses, estimated by CEMS measurements and best fitting. The sites 1 and 2 correspond to Fe(I) and Fe(II), shown in the inset of Fig. 2, for the ideal $D{0}_3$-ordered Fe${}_3$Si. The inset shows the estimated $D{0}_3$ ordering (%) as a function of film thickness (opened squares), together with $M_{\mathrm{S}}$ at room temperature (solid circles).

Figure 4

Schematic diagrams of atomic arrangements of Fe${}_3$Si(111)/Ge(111) interface for (a) model A and (b) model B. Dashed lines denote interfaces between Fe${}_3$Si and Ge. Left and right show initial and final atomic arrangements before and after molecular dynamics calculations, respectively, for the case of $...$-Ge/Si-Fe-Fe-Fe-$...$ stacking in models A and B.

Figure 5

Schematic diagrams of the initial crystal structure of (a) $D{0}_3$ and (b) Fe+$B2$ for the first-principles molecular dynamics calculation.