Magnetocrystalline anisotropy of Fe and Co slabs and clusters on SrTiO_3\_3 by first-principles

In this work, we present a detailed theoretical investigation of the electronic and magnetic properties of ferromagnetic slabs and clusters deposited on SrTiO$\_3$ via first-principles, with a particular emphasis on the magneto-crystalline anisotropy (MCA). We found that in the case of Fe films deposited on SrTiO$\_3$ the effect of the interface is to quench the MCA whereas for Cobalt we observe a change of sign of the MCA from in-plane to out-of-plane as compared to the free surface. We also find a strong enhancement of MCA for small clusters upon deposition on a SrTiO$\_3$ substrate. The hybridization between the substrate and the $d$-orbitals of the cluster extending in-plane for Fe and out-of-plane for Co is at the origin of this enhancement of MCA. As a consequence, we predict that the Fe nanocrystals (even rather small) should be magnetically stable and are thus good potential candidates for magnetic storage devices.Comment: Physical ReviewB, 201

Ballistic conductance of magnetic Co and Ni nanowires with ultrasoft pseudo-potentials

The scattering-based approach for calculating the ballistic conductance of open quantum systems is generalized to deal with magnetic transition metals as described by ultrasoft pseudo-potentials. As an application we present quantum-mechanical conductance calculations for monatomic Co and Ni nanowires with a magnetization reversal. We find that in both Co and Ni nanowires, at the Fermi energy, the conductance of $d$ electrons is blocked by a magnetization reversal, while the $s$ states (one per spin) are perfectly transmitted. $d$ electrons have a non-vanishing transmission in a small energy window below the Fermi level. Here, transmission is larger in Ni than in Co.Comment: 9 pages, 6 figures, to appear in PR

Interaction of CO with an Au monatomic chain at different strains: electronic structure and ballistic transport

We study the energetics, the electronic structure, and the ballistic transport of an infinite Au monatomic chain with an adsorbed CO molecule. We find that the bridge adsorption site is energetically favored with respect to the atop site, both at the equilibrium Au-Au spacing of the chain and at larger spacings. Instead, a substitutional configuration requires a very elongated Au-Au bond, well above the rupture distance of the pristine Au chain. The electronic structure properties can be described by the Blyholder model, which involves the formation of bonding/antibonding pairs of 5{\sigma} and 2{\pi}* states through the hybridization between molecular levels of CO and metallic states of the chain. In the atop geometry, we find an almost vanishing conductance due to the 5{\sigma} antibonding states giving rise to a Fano-like destructive interference close to the Fermi energy. In the bridge geometry, instead, the same states are shifted to higher energies and the conductance reduction with respect to pristine Au chain is much smaller. We also examine the effects of strain on the ballistic transport, finding opposite behaviors for the atop and bridge conductances. Only the bridge geometry shows a strain dependence compatible with the experimental conductance traces

Magnetic and orbital blocking in Ni nanocontacts

We address the fundamental question of whether magneto-resistance (MR) of atomic-sized contacts of Nickel is very large because of the formation of a domain wall (DW) at the neck. Using {\em ab initio} transport calculations we find that, as in the case of non-magnetic electrodes, transport in Ni nanocontacts depends very much on the orbital nature of the electrons. Our results are in agreement with several experiments in the average value of the conductance. On the other hand, contrary to existing claims, DW scattering does {\em not} account for large MR in Ni nanocontacts.Comment: 5 pages, 3 Figure

Ballistic conductance of Ni nanowire with a magnetization reversal

The approach proposed by Choi and Ihm for calculating the ballistic conductance of open quantum systems is generalized to deal with magnetic transition metals. The method has been implemented with ultrasoft pseudopotentials and plane wave basis set in a DFT-LSDA ab-initio scheme. We present the quantum-mechanical conductance calculations for monatomic Ni nanowire with a single spin reversal. We find that a spin reversal blocks the conductance of $d$ electrons at the Fermi energy of the Ni nanowire. On the other hand, two $s$ electrons (one per each spin) are perfectly transmitted in the whole energy window giving $2G_0$ for the total conductance. The relevance of these results in connection with recent experimental data is discussed.Comment: 4 pages, 1 figure, to be published in Surface Scienc

Effect of electron correlations in Pd, Ni, and Co monowires

We investigated the effect of mean-field electron correlations on the band electronic structure of Co, Ni, and Pd ultra-thin monatomic nanowires, at the breaking point, by means of density-functional calculations in the self-interaction corrected LDA approach (LDA+SIC) and alternatively by the LDA+$U$ scheme. We find that adding static electron correlations increases the magnetic moment in Pd monowires, but has negligible effect on the magnetic moment in Co and Ni. Furthermore, the number of $d$-dominated conductance channels decreases somewhat compared to the LDA value, but the number of $s$-dominated channels is unaffected, and remains equal to one per spin.Comment: to appear in PR

Interaction of a CO molecule with a Pt monatomic wire: electronic structure and ballistic conductance

We carry out a first-principles density functional study of the interaction between a monatomic Pt wire and a CO molecule, comparing the energy of different adsorption configurations (bridge, on top, substitutional, and tilted bridge) and discussing the effects of spin-orbit (SO) coupling on the electronic structure and on the ballistic conductance of two of these systems (bridge and substitutional). We find that, when the wire is unstrained, the bridge configuration is energetically favored, while the substitutional geometry becomes possible only after the breaking of the Pt-Pt bond next to CO. The interaction can be described by a donation/back-donation process similar to that occurring when CO adsorbs on transition-metal surfaces, a picture which remains valid also in presence of SO coupling. The ballistic conductance of the (tipless) nanowire is not much reduced by the adsorption of the molecule on the bridge and on-top sites, but shows a significant drop in the substitutional case. The differences in the electronic structure due to the SO coupling influence the transmission only at energies far away from the Fermi level so that fully- and scalar-relativistic conductances do not differ significantly.Comment: 12 pages, 12 figures; figure misplacement and minor syntax issues fixed, some references updated and correcte

Effect of stretching on the ballistic conductance of Au nanocontacts in presence of CO: a density functional study

CO adsorption on an Au monatomic chain is studied within density functional theory in nanocontact geometries as a function of the contact stretching. We compare the bridge and atop adsorption sites of CO, finding that the bridge site is energetically favored at all strains studied here. Atop adsorption gives rise to an almost complete suppression of the ballistic conductance of the nanocontact, while adsorption at the bridge site results in a conductance value close to 0.6 G0, in agreement with previous experimental data. We show that only the bridge site can qualitatively account for the evolution of the conductance as a function of the contact stretching observed in the experimental conductance traces. The numerical discrepancy between the theoretical and experimental conductance slopes is rationalized through a simple model for the elastic response of the metallic leads. We also verify that our conductance values are not affected by the specific choice of the nanocontact geometry by comparing two different atomistic models for the tips

Magnetocrystalline anisotropy energy of Fe(001)(001), Fe(110)(110) slabs and nanoclusters: a detailed local analysis within a tight-binding model

We report tight-binding (TB) calculations of magnetocrystalline anisotropy energy (MAE) of Iron slabs and nanoclusters with a particuler focus on local analysis. After clarifying various concepts and formulations for the determination of MAE, we apply our realistic TB model to the analysis of the magnetic anisotropy of Fe$(001)$, Fe$(110)$ slabs and of two large Fe clusters with $(001)$ and $(110)$ facets only: a truncated pyramid and a truncated bipyramid containg 620 and 1096 atoms, respectively. It is shown that the MAE of slabs originates mainly from outer layers, a small contribution from the bulk gives rise, however, to an oscillatory behavior for large thicknesses. Interestingly, the MAE of the nanoclusters considered is almost solely due to $(001)$ facets and the base perimeter of the pyramid. We believe that this fact could be used to efficiently control the anisotropy of Iron nanoparticles and could also have consequences on their spin dynamics

Orbital eigenchannel analysis for ab-initio quantum transport calculations

We show how to extract the orbital contribution to the transport eigenchannels from a first-principles quantum transport calculation in a nanoscopic conductor. This is achieved by calculating and diagonalizing the first-principles transmission matrix reduced to selected scattering cross-sections. As an example, the orbital nature of the eigenchannels in the case of Ni nanocontacts is explored, stressing the difficulties inherent to the use of non-orthogonal basis sets and first-principles Hamiltonians.Comment: 5 pages, 5 figurs; replaced with final version, introduction revised; to be published in PR