First-principles study of thin magnetic transition-metal silicide films on Si(001)

摘要

In order to combine silicon technology with the functionality of magneticsystems, a number of ferromagnetic (FM) materials have been suggested for thefabrication of metal/semiconductor heterojunctions. In this work, we present asystematic study of several candidate materials in contact with the Si surface.We employ density-functional theory calculations to address the thermodynamicstability and magnetism of both pseudomorphic CsCl-like $M$Si ($M$=Mn, Fe, Co,Ni) thin films and Heusler alloy $M_2$MnSi ($M$=Fe, Co, Ni) films on Si(001).Our calculations show that Si-termination of the $M$Si films is energeticallypreferable during epitaxy since it minimizes the energetic cost of broken bondsat the surface. Moreover, we can explain the calculated trends in thermodynamicstability of the $M$Si thin films in terms of the $M$-Si bond-strength and the$M$ 3d orbital occupation. From our calculations, we predict that ultrathinMnSi films are FM with sizable spin magnetic moments at the Mn atoms, whileFeSi and NiSi films are nonmagnetic. However, CoSi films display itinerantferromagnetism. For the $M_2$MnSi films with Heusler-type structure, the MnSitermination is found to have the highest thermodynamic stability. In the FMground state, the calculated strength of the effective coupling between themagnetic moments of Mn atoms within the same layer approximately scales withthe measured Curie temperatures of the bulk $M_2$MnSi compounds. In particular,the Co$_2$MnSi/Si(001) thin film has a robust FM ground state as in the bulk,and is found to be stable against a phase separation into CoSi/Si(001) andMnSi/Si(001) films. Hence this material is of possible use in FM-Siheterojunctions and deserves further experimental investigations.