Formation of Heusler alloy Co$_2$FeSi thin films on the surface of single-crystal silicon

The initial stages of Heusler alloy (Co$_2$FeSi) thin film growth by reactive epitaxy on the Si(100)2 $\times$ 1 surface are studied, and formation conditions for this alloy are found. At a substrate temperature of lower than, or equal to, 180$^\circ$C, an island film of ternary Co–Fe–Si film grows on the surface. The silicon content in this film is lower than in the compound to be synthesized. The film becomes continuous when its thickness exceeds 1.2 nm. It is shown that post-growth annealing at 240$^\circ$C can raise the silicon content in the film and be conducive to obtaining Heusler alloy of a desired composition. In situ measurements of the films show that ferromagnetic ordering in them has a threshold and shows up at the coalescence growth stage of the Co–Fe–Si island alloy.