Structurally-driven magnetic state transition of biatomic Fe chains on Ir(001)

Using first-principles calculations, we demonstrate that the magnetic exchange interaction and the magnetocrystalline anisotropy of biatomic Fe chains grown in the trenches of the 5x1 reconstructed Ir(001) surface depend sensitively on the atomic arrangement of the Fe atoms. Two structural configurations have been considered which are suggested from recent experiments. They differ by the local symmetry and the spacing between the two strands of the biatomic Fe chain. Since both configurations are very close in total energy they may coexist in experiment. We have investigated collinear ferro- and antiferromagnetic solutions as well as a collinear state with two moments in one direction and one in the opposite direction (up-down-up-state). For the structure with a small interchain spacing, there is a strong exchange interaction between the strands and the ferromagnetic state is energetically favorable. In the structure with larger spacing, the two strands are magnetically nearly decoupled and exhibit antiferromagnetic order along the chain. In both cases, due to hybridization with the Ir substrate the exchange interaction along the chain axis is relatively small compared to freestanding biatomic iron chains. The easy magnetization axis of the Fe chains also switches with the structural configuration and is out-of-plane for the ferromagnetic chains with small spacing and along the chain axis for the antiferromagnetic chains with large spacing between the two strands. Calculated scanning tunneling microscopy images and spectra suggest the possibility to experimentally distinguish between the two structural and magnetic configurations.Comment: Accepted for publication in Physical Review

Extracting convergent surface energies from slab calculations

The formation energy of a solid surface can be extracted from slab calculations if the bulk energy per atom is known. It has been pointed out previously that the resulting surface energy will diverge with slab thickness if the bulk energy is in error, in the context of calculations which used different methods to study the bulk and slab systems. We show here that this result is equally relevant for state-of-the-art computational methods which carefully treat bulk and slab systems in the same way. Here we compare different approaches, and present a solution to the problem that eliminates the divergence and leads to rapidly convergent and accurate surface energies.Comment: 3 revtex pages, 1 figure, in print on J. Phys. Cond. Mat

Intrinsic spin Hall effect in platinum metal

Spin Hall effect in metallic Pt is studied with first-principles relativistic band calculations. It is found that intrinsic spin Hall conductivity (SHC) is as large as $\sim 2000 (\hbar/e)(\Omega {\rm cm})^{-1}$ at low temperature, and decreases down to $\sim 200 (\hbar/e)(\Omega {\rm cm})^{-1}$ at room temperature. It is due to the resonant contribution from the spin-orbit splitting of the doubly degenerated $d$-bands at high-symmetry $L$ and $X$ points near the Fermi level. By modeling these near degeneracies by effective Hamiltonian, we show that SHC has a peak near the Fermi energy and that the vertex correction due to impurity scattering vanishes. We therefore argue that the large spin Hall effect observed experimentally in platinum is of intrinsic nature.Comment: Accepted for publication in Phys. Rev. Let

Non-collinear Korringa-Kohn-Rostoker Green function method: Application to 3d nanostructures on Ni(001)

Magnetic nanostructures on non-magnetic or magnetic substrates have attracted strong attention due to the development of new experimental methods with atomic resolution. Motivated by this progress we have extended the full-potential Korringa-Kohn-Rostoker (KKR) Green function method to treat non-collinear magnetic nanostructures on surfaces. We focus on magnetic 3d impurity nanoclusters, sitting as adatoms on or in the first surface layer on Ni(001), and investigate the size and orientation of the local moments and moreover the stabilization of non-collinear magnetic solutions. While clusters of Fe, Co, Ni atoms are magnetically collinear, non-collinear magnetic coupling is expected for Cr and Mn clusters on surfaces of elemental ferromagnets. The origin of frustration is the competition of the antiferromagnetic exchange coupling among the Cr or Mn atoms with the antiferromagnetic (for Cr) or ferromagnetic (for Mn) exchange coupling between the impurities and the substrate. We find that Cr and Mn first-neighbouring dimers and a Mn trimer on Ni(001) show non-collinear behavior nearly degenerate with the most stable collinear configuration. Increasing the distance between the dimer atoms leads to a collinear behavior, similar to the one of the single impurities. Finally, we compare some of the non-collinear {\it ab-initio} results to those obtained within a classical Heisenberg model, where the exchange constants are fitted to total energies of the collinear states; the agreement is surprisingly good.Comment: 11 page

Orbital magnetism in the half-metallic Heusler alloys

Using the fully-relativistic screened Korringa-Kohn-Rostoker method I study the orbital magnetism in the half-metallic Heusler alloys. Orbital moments are almost completely quenched and they are negligible with respect to the spin moments. The change in the atomic-resolved orbital moments can be easily explained in terms of the spin-orbit strength and hybridization effects. Finally I discuss the orbital and spin moments derived from X-ray magnetic circular dichroism experiments

Quantum interference in deformed carbon nanotube waveguides

Quantum interference (QI) in two types of deformed carbon nanotubes (CNTs), i.e., axially stretched and AFM tip-deformed CNTs, has been investigated by the pi-electron only and four-orbital tight-binding (TB) method. It is found that the rapid conductance oscillation (RCO) period is very sensitive to the applied strains, and decreases in an inverse proportion to the deformation degree, which could be used as a powerful experimental tool to detect precisely the deformation degree of the deformed CNTs. Also, the sigma-pi coupling effect is found to be negligible under axially stretched strain, while it works on the transport properties of the tip-deformed CNTs.Comment: 14 pages and 5 figure

Surface state scattering by adatoms on noble metals

When surface state electrons scatter at perturbations, such as magnetic or nonmagnetic adatoms or clusters on surfaces, an electronic resonance, localized at the adatom site, can develop below the bottom of the surface state band for both spin channels. In the case of adatoms, these states have been found very recently in scanning tunneling spectroscopy experiments\cite{limot,olsson} for the Cu(111) and Ag(111) surfaces. Motivated by these experiments, we carried out a systematic theoretical investigation of the electronic structure of these surface states in the presence of magnetic and non-magnetic atoms on Cu(111). We found that Ca and all 3$d$ adatoms lead to a split-off state at the bottom of the surface band which is, however, not seen for the $sp$ elements Ga and Ge. The situation is completely reversed if the impurities are embedded in the surface: Ga and Ge are able to produce a split-off state whereas the 3$d$ impurities do not. The resonance arises from the s-state of the impurities and is explained in terms of strength and interaction nature (attraction or repulsion) of the perturbing potential.Comment: 6 pages, 5 figure

Ferromagnetism in Nitrogen-doped MgO

The magnetic state of Nitrogen-doped MgO, with N substituting O at concentrations between 1% and the concentrated limit, is calculated with density-functional methods. The N atoms are found to be magnetic with a moment of 1 Bohr magneton per Nitrogen atom and to interact ferromagnetically via the double exchange mechanism. The long-range magnetic order is established above a finite concentration of about 1.5% when the percolation threshold is reached. The Curie temperature increases linearly with the concentration, and is found to be about 30 K for 10% concentration. Besides the substitution of single Nitrogen atoms, also interstitial Nitrogen atoms, clusters of Nitrogen atoms and their structural relaxation on the magnetism are discussed. Possible scenarios of engineering a higher Curie temperature are analyzed, with the conclusion that an increase of the Curie temperature is difficult to achieve, requiring a particular attention to the choice of chemistry

Hysteretic resistance spikes in quantum Hall ferromagnets without domains

We use spin-density-functional theory to study recently reported hysteretic magnetoresistance \rho_{xx} spikes in Mn-based 2D electron gases [Jaroszy\'{n}ski et al. Phys. Rev. Lett. (2002)]. We find hysteresis loops in our calculated Landau fan diagrams and total energies signaling quantum-Hall-ferromagnet phase transitions. Spin-dependent exchange-correlation effects are crucial to stabilize the relevant magnetic phases arising from distinct symmetry-broken excited- and ground-state solutions of the Kohn-Sham equations. Besides hysteretic spikes in \rho_{xx}, we predict hysteretic dips in the Hall resistance \rho_{xy}. Our theory, without domain walls, satisfactorily explains the recent data.Comment: 4 pages, 4 figures, published version (some changes to the text; same figures as in v1

Interface properties of the NiMnSb/InP and NiMnSb/GaAs contacts

We study the electronic and magnetic properties of the interfaces between the half-metallic Heusler alloy NiMnSb and the binary semiconductors InP and GaAs using two different state-of-the-art full-potential \textit{ab-initio} electronic structure methods. Although in the case of most NiMnSb/InP(001) contacts the half-metallicity is lost, it is possible to keep a high degree of spin-polarization when the interface is made up by Ni and P layers. In the case of the GaAs semiconductor the larger hybridization between the Ni-$d$ and As-$p$ orbitals with respect to the hybridization between the Ni-$d$ and P-$p$ orbitals destroys this polarization. The (111) interfaces present strong interface states but also in this case there are few interfaces presenting a high spin-polarization at the Fermi level which can reach values up to 74%.Comment: 9 pages, 9 figure