Searching for Si-based spintronics by first principles calculations

Density functional theory (DFT) calculations are used to study the epitaxial growth and the magnetic properties of thin films of MnSi on the Si(001) surface. For adsorption of a single Mn atom, we find that binding at the subsurface site below the Si surface dimers is the most stable adsorption site. There is an energy barrier of only 0.3 eV for adsorbed Mn to go subsurface, and an energy barrier of 1.3 eV for penetration to deeper layers. 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. Despite Si(001) geometrically being an anisotropic surface, the on-surface diffusion for both directions along and perpendicular to the Si dimer rows has almost the same diffusion barrier of 0.65 eV. For coverage above 1 ML, the lowest energy structure is a pure Mn subsurface layer, capped by a layer of Si adatoms. We conclude that the Mn-silicide films stabilize in an epitaxially CsCl-like (B2) 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 clarify within the layers. Layer-resolved electronic densities-of-states are presented that show a high degree of spin polarization at the Fermi level, up to 30 and 50% for films with one or two MnSi films, respectively. In order to clarify the stability of ferromagnetism at finite temperatures we estimate the Curie temperature (Tc) of MnSi films using a multiple-sublattice Heisenberg model with first- and second-nearest neighbor interactions determined from DFT calculations for various collinear spin configurations. The Curie temperature is calculated both in the mean-field approximation (MFA) and in the random-phase approximation (RPA). In the latter case, we find a weak logarithmic dependence of Tc on the magnetic anisotropy parameter, which was calculated to be 0.4 meV. Large Curie temperatures of above 200K for a monolayer MnSi film, and above 300K for a 2ML MnSi film are obtained within the RPA, and even higher values in MFA. Complementary calculations are performed for non-collinear spin structures to study the limitations of the mapping of the system onto a Heisenberg model. We demonstrate that biquadratic interatomic exchange interactions and longitudinal fluctuations of atomic moments give important contributions to the energetics of the system

Electronic structure changes of Si(001)-(2x1) from subsurface Mn observed by STM

The deposition of Mn atoms onto the Si(001)-(2x1) reconstructed surface has been studied using scanning tunneling microscopy (STM) and first-principles electronic structure calculations. Room-temperature deposition of 0.1 ML (monolayer) 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 Si(001). 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

Density-functional study of Mn monosilicide on the Si(111) surface: Film formation versus island nucleation

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)

Transition-metal silicides as materials for magnet-semiconductor heterostructures

The injection of a spin-polarized current into a semiconductor, one of the key requirements for spintronics, poses the challenge for computational materials science to possibly screen suitable materials. In a number of theoretical investigations, we have put forward magnetic intermetallic compounds grown epitaxially on Si as promising candidates. We employed density functional theory calculations with the GGA-PBE exchange-correlation functional and the full-potential augmented plane wave plus local orbital (FP-APW+lo) method, as implemented in the WIEN2k package. In the spirit of computational materials science, we investigated the stability and magnetic properties of thin films of the Heusler alloy Co2MnSi, as well as of binary late transition metal monosilicides, in contact with the Si surface. For the Heusler alloy Co2MnSi, we could show that the (001) surface retains the half-metallic character of the bulk if a fully Mn-terminated surface is prepared. At interfaces with Si, a finite density of states at the Fermi energy was found for both spin channels, but the half-metallic behavior recovers only a few layers away from the interface. For the monosilicides of the late 3d-transition metals (Mn, Fe, Co, Ni), we predict a CsCl-like structure that has not yet been observed as bulk compound but may be stabilized epitaxially on Si(001). For very thin films of CoSi and MnSi grown in this structure, our calculations find a ferromagnetic ground state. Recently, we identified the atomic structure of MnSi films on Si(111) which is close to the natural crystal structure of bulk MnSi (B20), and also shows large magnetic moments of the Mn atoms at the surface and interface. All MnSi films have a high degree of spin polarization (between 30% and 50%, depending on film thickness) at the Fermi level, and are thus promising materials for fabricating electrical contacts for spin injection into Si

Efficacy of allogeneic cord blood platelet gel on wounds of dystrophic epidermolysis bullosa patients after pseudosyndactyly surgery

Epidermolysis bullosa (EB) is a rare genetic disorder characterized by the formation of blisters and wounds in skin and mucous membranes; it is classified into four types and has various methods of treatment. Management of previous wounds and prevention of formation of new lesions are the most important strategies in the course of therapy to improve patient's quality of life; lack of wound management can lead to further complications such as infection. The current study investigated the therapeutic effects of allogeneic platelet gel (prepared from umbilical cord blood) in a group of children diagnosed with dystrophic epidermolysis bullosa (DEB) eligible for surgical correction of pseudosyndactyly in the hand. The post-surgical clinical outcome in this group was compared with the clinical outcomes of DEB patients receiving the standard treatment (paraffin gauze wound dressing and topical antibiotics) after corrective surgery. The current study results showed an increase in the rate of recovery and promotion of tissue granulation, complete wound healing, and a decrease in pain level and treatment period. The application of cord blood platelet gel topical dressing was not a conventional method of treatment in patients with DEB wounds and blisters. However, the current study results demonstrated that this gel dressing could effectively accelerate epithelialization and healing of the wounds and decrease patients' pain and post-surgical recovery period, which altogether leads to improvements in patients' overall quality of life. © 2020 by the Wound Healing Societ