Density-functional theory calculations are employed to investigate both the epitaxial growth and the magnetic properties of thin Mn and MnSi films on Si(001). For single Mn adatoms, we find a preference for the second-layer interstitial site. While a monolayer Mn film is energetically unfavorable, a cappingSi layer significantly enhances the thermodynamic stability and induces a change from antiferromagnetic to ferromagnetic order. For higher Mn coverage, a sandwiched Si-Mn thin film (with CsCl-like crystal structure) is found to be the most stable epitaxial structure. We attribute the strong ferromagnetic intralayer coupling in these films to Mn 3d-Si 3s3p exchange. DOI: 10.1103/PhysRevLett.92.237202 PACS numbers: 75.70.-i, 75.50.Pp A possible route to spintronics devices involves the injection of spin-polarized current from a ferromagnetic (FM) metal into a semiconductor [1,2]. In this context, FM-metal-semiconductor heterojunctions have attracted considerable attention [3][4][5][6][7]. From the viewpoint of applications, it would be highly desirable to grow welldefined FM films on the most common semiconductor, silicon, in particular on the technologically relevant Si(001) surface. However, common FM metals (Fe, Co) react strongly with Si, and the resultant silicide films are weakly magnetic or even nonmagnetic (NM), and, thus, unsuitable for spintronics devices. We note that Mn atoms, in suitable chemical environments, possess sizable magnetic moments. Moreover, both fcc-Mn and some Mn-Si compounds are closely lattice matched with the Si(001) surface. We therefore investigated the suitability of thin films of manganese or its intermetallic Mn-Si compounds for growing FM films on Si(001). Thus far, there are only a few experimental studies on Mn=Si111 [8-10], and even fewer on Mn=Si001 [11]. Theoretical investigations are lacking.In this Letter, we present first-principles calculations for pseudomorphic Mn and Mn-Si films on Si(001), focusing on their atomic structure, stability, and magnetic properties. On the basis of our calculations, we propose that metastable FM MnSi films on Si(001) can be grown that lend themselves to applications in spintronics: First, we find Mn atoms to prefer a second-layer interstitial site to sites at the surface or in the third layer. This opens the possibility to grow Si-Mn layered structures with a welldefined, atomically sharp interface on Si(001) required to obtain high spin injection efficiency [6]. Second, we predict a two-layer Si-Mn sandwich structure to display the desired FM metallic behavior.The calculations are performed using densityfunctional theory, employing the all-electron fullpotential augmented plane-wave plus local-orbital method (APW LO) [12]. The generalized gradient approximation (GGA) [13] is adopted for the exchangecorrelation potential, since it was demonstrated [14] and confirmed by our test calculations that GGA gives a much better description for bulk Mn than the local-spin-density approximation. Our GGA calculations for the fcc-Mn give the corr...

Ultrathin films of manganese silicides on silicon are of relevance as a possible material system for building spintronics devices with silicon technology. In order to achieve insight into epitaxial growth of such films on Si͑001͒, total-energy calculations are presented using density-functional theory and the full-potential augmented plane wave plus local orbital method. For adsorption of a single Mn atom on Si͑001͒, we find that binding at the subsurface sites below the Si surface dimers is ϳ0.9 eV stronger than on-surface adsorption. There is an energy barrier of only 0.3 eV for adsorbed Mn to go subsurface, and an energy barrier of 1.2 eV for the reverse process. From the calculated potential-energy surface for the Mn adatom, we conclude that the most stable site on the surface corresponds to the hollow site where Mn is placed between two Si surface dimers. For on-surface diffusion, both along and perpendicular to the Si dimer rows, the Mn atoms have to overcome energy barriers of 0.65 eV. For deposition of 0.5 monolayers ͑ML͒ or more, we find that the Si dimers of the Si͑001͒ surface are broken up, and a mixed MnSi layer becomes the energetically most favorable structure. For coverages above 1 ML, the lowest-energy structure changes to a full Mn subsurface layer, capped by a layer of Si adatoms. We identify this transition with the onset of Mn-silicide formation in an epitaxially stabilized CsCl-like crystal structure. Such MnSi films are found to have sizable magnetic moments at the Mn atoms near the surface and interface, and ferromagnetic coupling of the Mn magnetic moments within the layers. Layer-resolved electronic densities of state are presented that show a high degree of spin polarization at the Fermi level, up to 45% and 27% for films with two or three Si-Mn layers, respectively.

Recent theoretical work [H. Wu et al., Phys. Rev. Lett. 92, 237202 (2004); M. Hortamani et al., Phys. Rev. B 74, 205305 (2006); M. Hortamani, Ph.D. thesis, Freie Universität, Berlin, 2006] predicted ferromagnetism at zero temperature in thin MnSi films of B2-type crystal structure on Si(100). The relevance of this finding for finite-temperature experiments needs to be clarified by further investigations, since bulk MnSi is a weak ferromagnet with an experimentally measured Curie temperature of only T_c=30 K, and T_c is generally expected to be lower in thin films than in bulk materials. Here, we estimate Tc of such MnSi films using a multiple-sublattice Heisenberg model with first- and second-nearest-neighbor interactions determined from density functional theory calculations for various collinear spin configurations. The Curie temperature is calculated either in the mean-field approximation (MFA) or in the random-phase approximation (RPA). In the latter case we find a weak logarithmic dependence of T_c on the magnetic anisotropy parameter, which was calculated to be 0.4 meV for this system. In stark contrast to the above mentioned rule, large Curie temperatures of above 200 K for a monolayer (ML) MnSi film and above 300 K for a two ML MnSi film with B2-type structure on Si(100) are obtained within the RPA, and even higher values in MFA. Complementary calculations of MnSi bulk structures and thin unsupported MnSi films are performed in order to analyze these findings. We find that bulk MnSi in the cubic B2 structure is paramagnetic, in contrast to MnSi in the B20 ground-state structure in agreement with the Stoner criterion. In a tetragonally distorted B2 structure, the Mn atoms gradually develop a spin magnetic moment, passing through a low-spin and a high-spin state. However, the ferromagnetism of the MnSi/Si (100) films cannot be explained by tetragonal distortions alone, since the distorted B2 bulk structure is found to order antiferromagnetically. Comparison of the calculations of supported and unsupported films suggests that the reduced coordination of Mn atoms near surfaces and interfaces is crucial for the ferromagnetic ground state of the films. The coordination number of the Mn atoms in B2-type MnSi films on Si(100) constitutes a borderline case, where the spin magnetic moments of Mn are still large despite their sixfold coordination to Si, but the sp-d hybridization with Si states gives rise to a sizable ferromagnetic coupling of the Mn spins. We conclude that the Curie temperatures predicted from the Heisenberg Hamiltonian make thin MnSi films an interesting subject for further experimental investigation of spintronics materials

The deposition of Mn atoms onto the Si͑001͒-͑2 ϫ 1͒ reconstructed surface has been studied using scanning tunneling microscopy ͑STM͒ and first-principles electronic structure calculations. Room-temperature deposition of 0.1 ML ͑monolayer͒ of Mn gives rise to a disordered surface structure. After in situ annealing between 300 and 700°C, most of the Mn is incorporated into three-dimensional manganese silicide islands, and Si dimer rows reappear in the STM images on most of the substrate surface. At the same time, rowlike structures are visible in the atomic-scale STM images. A comparison with calculated STM images provides evidence that Mn atoms are incorporated into the row structures in subsurface interstitial sites, which are the lowest-energy position for Mn on Si͑001͒. The subsurface Mn alters the height and local density of states of the Si dimer atoms, causing them to appear 0.6 Å higher than a neighboring Si dimer with no Mn below. This height difference that allows the detection the subsurface Mn results from a subtle interplay of geometrical and electronic effects.

In contradiction to the nature of the spin-orbit driven Rashba splitting of surface states which increases with atomic number, Shikin et al. [Phys. Rev. Lett. 100, 057601 (2008)] have observed that the size of the splitting in Au overlayers on W(110) is smaller than for Ag overlayers. In the framework of first-principle density functional theory, we have studied the origin of the Rashba splitting at Au/Ag overlayers on the W(110) surface. We show how the asymmetric behavior of the wave function in the vicinity of the surface atom nucleus, in addition to the strength of the nuclear potential gradient, plays a crucial role for the size of the splitting. The influence of the electronic structure and spin dependent hybridization on the Rashba splitting is discussed. The asymmetric behavior of the surface wave function originates from the surface-interface sp-d hybridization. We find that a spin dependent hybridization in the Ag overlayer influences strongly the size of the Rashba splitting.The generation of a dissipationless spin current in a twodimensional (2D) electron gas without applying an external field can be achieved by exploiting the spin-orbit coupling (SOC) at surfaces and interfaces. Due to the breaking of the spatial inversion symmetry, surface electrons are subject to an electric field perpendicular to the surface, thereby experiencing an effective magnetic field. The coupling of such a magnetic field to the electron spin causes a k-dependent spin splitting of the surface state known as the Rashba (RB) effect. 1 It was exploited in the concept of spin field effect transistor. 2 This effect was observed first by LaShell et al. for the Shockley surface state of Au(111) 3 and has been extended to several surfaces. [4][5][6][7][8][9][10][11] The SOC Hamiltonian can be divided into two terms according to the momentum components, parallel (k ) and perpendicular (k ⊥ ) to the surface. The k part leads to the RB term H R , while the k ⊥ should be irrelevant for the spin splitting behavior of the in-plane 2D band structure. The RB Hamiltonian iswhereẑ is the surface normal direction, s is the direction of the spin, and α is the RB parameter. The RB energy can be calculated from the integral of the potential gradient along the surface normal direction times the surface wave function squared 12 :Many experimental studies show a large RB splitting due to the SOC at metal surfaces. For example, a large RB splitting was observed for the surface state of hydrogen as well as lithium terminated W(110), bare Bi(100), Au(111), Sb(111), 3,13-16 and also for surface binary alloys like Bi/Ag(111), Pb/Ag(111), and Au/Ge(111). [17][18][19] Since the strength of SOC in a 2D electron gas depends on the atomic number and the potential gradient, it is logical to tailor the size of the splitting by replacing light elements with a mixture of light and heavy elements. 20-22 Such hybrid structure could have potential applications in spintronics devices. It has been observed that the RB splitting of the W(110) surface is enhanced when ...

The stability of thin films and of small crystallites of Mn monosilicide (MnSi) on the Si(111) surface is investigated by density-functional theory calculations. Extending previous studies of MnSi/Si(001), our calculations indicate that MnSi films on Si(111) have similar electronic and magnetic properties, i.e., large magnetic moments at the Mn atoms near the surfaces and interfaces and a high degree of spin polarization at the Fermi level. Hence, such MnSi films could be interesting as a spintronics material compatible with silicon. Moreover, from our calculated total energies we conclude that the Si(111) substrate should be more suitable to grow MnSi layers than the Si(001) substrate. This result is obtained by analyzing the conditions for the formation of three-dimensional (3D) MnSi islands, either in the B20 crystal structure or as pseudomorphic islands in the B2 structure: On Si(001), 3D islands, even if they are just a few lattice constants wide, are found to be already more stable than a homogeneous MnSi film. A bipyramidal “iceberg” island consisting of MnSi in the B20 structure on the Si(001) substrate is found to be most stable among the structures investigated. For MnSi on Si(111), however, our calculations show that the nucleus for forming a 3D island is larger. Therefore, Mn deposition initially leads to the formation of flat 2D islands. On Si(111), the lowest-energy structure for such islands is found to be similar to the B20 structure of bulk MnSi, whereas on Si(001) this structure is incompatible with the substrate lattice. Our results are in agreement with the experimental observations, formation of an almost closed film with (√3x√3) structure on Si(111), and 3D island formation on Si(001)

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