Based upon the observations (i) that their in-plane lattice constants match almost perfectly and (ii) that their electronic structures overlap in reciprocal space for one spin direction only, we predict perfect spin filtering for interfaces between graphite and (111) fcc or (0001) The observation [1,2] of giant magnetoresistance in systems where the transmission through interfaces between normal and ferromagnetic metals (FM) is spin dependent has driven a major effort to study spin filtering effects in other systems and extend applications from field sensing to storage [3], reprogrammable logic [4], and quantum computing [5]. An ideal spin filter would allow all carriers with one spin through but none with the other spin. Interfaces with half-metallic ferromagnets (HMFs) [6] should have this property but progress in exploiting it has been slow because of the difficulty of making stoichiometric HMFs with the theoretically predicted bulk properties and then making devices maintaining these properties at interfaces [7].If the nonmagnetic metal is replaced by an insulator (I) or semiconductor (SC), spin filtering still occurs giving rise to tunneling magnetoresistance (TMR) in FMjIjFM magnetic tunnel junctions and spin-injection at FMjSC interfaces. If the spin polarization of the ferromagnet is not complete, then the conductivity mismatch between metals and semiconductors or insulators has been identified as a serious obstacle to efficient spin injection [8]. It can be overcome if there is a large spin-dependent interface resistance but this is very sensitive to the detailed atomic structure and chemical composition of the interface. Knowledge of the interface structure is a necessary preliminary to analyzing spin filtering theoretically and progress has been severely hampered by the difficulty of experimentally characterizing FMjI and FMjSC interfaces.The situation improved with the confirmation of large values of TMR in tunnel barriers based upon crystalline MgO [9,10] which had been predicted by detailed electronic structure calculations [11,12]. While the record values of TMR-in excess of 500% at low temperatures [13]-are undoubtedly correlated with the crystallinity of MgO, the nature of this relationship is not trivial [14]. The sensitivity of TMR (and spin injection) to details of the interface structure [15,16] make it difficult to close the quantitative gap between theory and experiment. In view of the reactivity of the open-shell transition metal (TM) ferromagnets Fe, Co, and Ni with typical semiconductors and insulators, preparing interfaces where disorder does not dominate the spin filtering properties remains a challenge. With this in mind, we wish to draw attention to a quite different material system which should be intrinsically ordered, for which an unambiguous theoretical prediction of perfect spin filtering can be made in the absence of disorder, and which is much less sensitive to interface roughness and alloy disorder than TMR or spin injection.

The enhancement of Gilbert damping observed for Ni80Fe20 (Py) films in contact with the nonmagnetic metals Cu, Pd, Ta and Pt, is quantitatively reproduced using first-principles scattering calculations. The "spin-pumping" theory that qualitatively explains its dependence on the Py thickness is generalized to include a number of extra factors known to be important for spin transport through interfaces. Determining the parameters in this theory from first-principles shows that interface spin-flipping makes an essential contribution to the damping enhancement. Without it, a much shorter spin-flip diffusion length for Pt would be needed than the value we calculate independently.PACS numbers: 72.25.Mk, 76.50.+g, 75.70.Tj Introduction.-Magnetization dissipation, expressed in terms of the Gilbert damping parameter α, is a key factor determining the performance of magnetic materials in a host of applications. Of particular interest for magnetic memory devices based upon ultrathin magnetic layers [1][2][3] is the enhancement of the damping of ferromagnetic (FM) materials in contact with non-magnetic (NM) metals [4] that can pave the way to tailoring α for particular materials and applications. A "spin pumping" theory has been developed that describes this interface enhancement in terms of a transverse spin current generated by the magnetization dynamics that is pumped into and absorbed by the adjacent NM metal [5,6]. Spin pumping subsequently evolved into a technique to generate pure spin currents that is extensively applied in spintronics experiments [7][8][9].A fundamental limitation of the spin-pumping theory is that it assumes spin conservation at interfaces. This limitation does not apply to a scattering theoretical formulation of the Gilbert damping that is based upon energy conservation, equating the energy lost by the spin system through damping to that parametrically pumped out of the scattering region by the precessing spins [10]. In this Letter, we apply a fully relativistic density functional theory implementation [11][12][13] of this scattering formalism to the Gilbert damping enhancement in those NM|Py|NM structures studied experimentally in Ref. 4. Our calculated values of α as a function of the Py thickness d are compared to the experimental results in Fig. 1. Without introducing any adjustable parameters, we quantitatively reproduce the characteristic 1/d dependence as well as the dependence of the damping on the NM metal.

Using a formulation of first-principles scattering theory that includes disorder and spin-orbit coupling on an equal footing, we calculate the resistivity ρ, spin-flip diffusion length l(sf), and Gilbert damping parameter α for Ni(1-x)Fe(x) substitutional alloys as a function of x. For the technologically important Ni(80)Fe(20) alloy, Permalloy, we calculate values of ρ = 3.5 ± 0.15 μΩ cm, l(sf) = 5.5 ± 0.3 nm, and α = 0.0046 ± 0.0001 compared to experimental low-temperature values in the range 4.2-4.8 μΩ cm for ρ, 5.0-6.0 nm for l(sf), and 0.004-0.013 for α, indicating that the theoretical formalism captures the most important contributions to these parameters.

The in-plane lattice constants of close-packed planes of fcc and hcp Ni and Co match that of graphite almost perfectly so that they share a common two dimensional reciprocal space. Their electronic structures are such that they overlap in this reciprocal space for one spin direction only allowing us to predict perfect spin filtering for interfaces between graphite and (111) fcc or (0001) hcp Ni or Co. First-principles calculations of the scattering matrix show that the spin filtering is quite insensitive to amounts of interface roughness and disorder which drastically influence the spinfiltering properties of conventional magnetic tunnel junctions or interfaces between transition metals and semiconductors. When a single graphene sheet is adsorbed on these open d-shell transition metal surfaces, its characteristic electronic structure, with topological singularities at the K points in the two dimensional Brillouin zone, is destroyed by the chemical bonding. Because graphene bonds only weakly to Cu which has no states at the Fermi energy at the K point for either spin, the electronic structure of graphene can be restored by dusting Ni or Co with one or a few monolayers of Cu while still preserving the ideal spin injection property.

To understand the band bending caused by metal contacts, we study the potential and charge density induced in graphene in response to contact with a metal strip. We find that the screening is weak by comparison with a normal metal as a consequence of the ultra-relativistic nature of the electron spectrum near the Fermi energy. The induced potential decays with the distance from the metal contact as x −1/2 and x −1 for undoped and doped graphene, respectively, breaking its spatial homogeneity. In the contact region the metal contact can give rise to the formation of a p-p , n-n , p-n junction (or with additional gating or impurity doping, even a p-n-p junction) that contributes to the overall resistance of the graphene sample, destroying its electron-hole symmetry. Using the work functions of metal-covered graphene recently calculated by Khomyakov et al. [Phys. Rev. B 79, 195425 (2009)] we predict the boundary potential and junction type for different metal contacts.