Abstract
In order to combine silicon technology with the functionality of magnetic systems, a number of ferromagnetic (FM) materials have been suggested for the fabrication of metal/semiconductor heterojunctions. In this work, we present a systematic study of several candidate materials in contact with the Si surface. We employ density-functional theory calculations to address the thermodynamic stability 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 energetically preferable during epitaxy since it minimizes the energetic cost of broken bonds at the surface. Moreover, we can explain the calculated trends in thermodynamic stability 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 ultrathin MnSi films are FM with sizable spin magnetic moments at the Mn atoms, while FeSi and NiSi films are nonmagnetic. However, CoSi films display itinerant ferromagnetism. For the $M_2$MnSi films with Heusler-type structure, the MnSi termination is found to have the highest thermodynamic stability. In the FM ground state, the calculated strength of the effective coupling between the magnetic moments of Mn atoms within the same layer approximately scales with the 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) and MnSi/Si(001) films. Hence this material is of possible use in FM-Si heterojunctions and deserves further experimental investigations.
Highlights
Metal-semiconductor heterojunctions have received much attention in the context of magnetoelectronics or spintronics because they could open up the possibility to inject a spinpolarized current from a ferromagneticFMmetal into a semiconductor
We present theoretical investigations of thin films for two materials classes relevant in this context, namely, transition metalTMmonosilicides M SiM = Mn, Fe, Co, Niin the CsCl crystal structure and Heusler alloys M2MnSiM = Fe, Co, Ni
The structurally simpler monosilicides have a potential to be applied in spintronics devices: Recently, we have shown that thin MnSi films on Si001͒ possess sizable magnetic moments at the Mn atoms,[5] despite the fact that bulk MnSiin the corresponding hypothetical CsCl crystal structureis nonmagnetic
Summary
Metal-semiconductor heterojunctions have received much attention in the context of magnetoelectronics or spintronics because they could open up the possibility to inject a spinpolarized current from a ferromagneticFMmetal into a semiconductor This is a prerequisite for anticipated future electronic devices making use of spin-polarized carriers.[1] In this paper, we present theoretical investigations of thin films for two materials classes relevant in this context, namely, transition metalTMmonosilicides M SiM = Mn, Fe, Co, Niin the CsCl crystal structure and Heusler alloys M2MnSiM = Fe, Co, Ni. The two materials classes are closely related in their crystal structure. One can think of M2MnSi films as being formed by the substitution of Mn for half of the Si atoms in each Si layer of the CsCl-like M SiM = Fe, Co, Nifilms Both materials classes are of potential interest for spintronics applications. Calculations of CoSi in CsCl crystal structure find thismetastablecompound to be ferromagnetic
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