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We have studied the stability, the electronic, and the magnetic properties of Co2MnSi001 thin films for 15 different terminations using density functional theory calculations. The phase diagram obtained by ab initio atomistic thermodynamics shows that in practice the MnSi, pure Mn, or pure Si terminated surfaces can be stabilized under suitable conditions. Analyzing the surface band structure, we find that the pure Mn termination, due to its strong surface-subsurface coupling, preserves the half-metallicity of the system, while surface states appear for the other terminations. Heusler alloys are promising materials to be applied as spin injectors in the rapidly developing field of spin elec- tronics (spintronics) because they belong to the materials class of magnetic half-metals. As first evidenced by first- principles calculations (1), these systems are characterized by a metallic density of electronic states at the Fermi level ( F) for one spin channel, while the states for the other spin channel display a gap atF, leading to 100% spin-polarized metallic conductivity. Among this group, the intermetallic compound Co2MnSi, a so-called full-Heusler alloy, offers special advantages: it has the highest Curie temperature amongst the known Heusler alloys (Tc 985 K) (2) and a rather wide gap in the minority spin channel (3). However, the desired half-metallic properties can be lost near sur- faces or interfaces. While for spin injection the electronic structure of the half-metal-semiconductor interface is cru- cial (4), surfaces are more easily accessible experimentally than interfaces, and hence lend themselves to a systematic study of the factors governing the possible loss of half- metallicity. Moreover, recent interest in spin-polarized tunneling diodes and scanning tunneling microscopy (5) gives relevance to half-metallic surfaces as a research topic in its own right. Here, we present a comprehensive theoretical investiga- tion of the (001) surface of Co2MnSi. This surface is important for envisaged epitaxial growth of Co2MnSi on Ge(001) or GaAs(001), materials to which it is very closely lattice matched. So far, ab initio calculations have been performed only for the standard Co and MnSi terminations of Co2MnSi001 thin films (6). In these calculations, which did not take into account any structural relaxation, it was found that the (001) surface is no longer half- metallic, and that the spin polarization at the Fermi level is negligible. Experimental work on Co2MnSi thin films has so far mostly focused on preparation techniques and the crystallinity and magnetic properties of the films (7-9). Very recently two experimental groups reported a spin polarization of about 60% at low temperatures for Co2MnSi thin films (10,11). In this Letter, we present density functional theory (DFT) calculations demonstrating that it is possible to retain the half-metallicity of Co2MnSi by appropriate ter- mination at the (001) surface. Our results have been ob- tained by using the WIEN2K code, a full-potential linearized augmented-plane-wave code (12). In order to validate the appropriateness of the approximation to the exchange- correlation energy, we have calculated the bulk properties of Co2MnSi both in the local-density approximation (LDA) and in the generalized gradient approximation (GGA). We found that the theoretical lattice constant of the GGA (5.63 A ˚ ) agrees well with the experimental value

physical review letters

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Preserving the Half-Metallicity at the Heusler AlloyCo2MnSi(001