M. Zwierzycki

Graphite and graphene as perfect spin filters

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) hcp Ni or Co. The spin filtering is quite insensitive to roughness and disorder. The formation of a chemical bond between graphite and the open d-shell transition metals that might complicate or even prevent spin injection into a single graphene sheet can be simply prevented by dusting Ni or Co with one or a few monolayers of Cu while still preserving the ideal spin-injection property.

Theoretical prediction of perfect spin filtering at interfaces between close-packed surfaces of Ni or Co and graphite or graphene

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 spin-filtering 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.

First-principles study of magnetization relaxation enhancement and spin transfer in thin magnetic films

The interface-induced magnetization damping of thin ferromagnetic films in contact with normal-metal layers is calculated from first principles for clean and disordered Fe/Au and Co/Cu interfaces. Interference effects arising from coherent scattering turn out to be very small, consistent with a very small magnetic coherence length. Because the mixing conductances which govern the spin transfer are to a good approximation real-valued, the spin pumping can be described by an increased Gilbert damping factor but an unmodified gyromagnetic ratio. The results also confirm that the spin-current-induced magnetization torque is an interface effect.

Conductance calculations for quantum wires and interfaces: Mode matching and Green's functions

Landauers formula relates the conductance of a quantum wire or interface to transmission probabilities. Total transmission probabilities are frequently calculated using Greens-function techniques and an expression derived by C. Caroli et al. [J. Phys. C 4, 916 (1971)]. Alternatively, partial transmission probabilities can be calculated from the scattering wave functions that are obtained by matching the wave functions in the scattering region to the Bloch modes of ideal bulk leads. An elegant technique for doing this, formulated by Ando [Phys. Rev. B 44, 8017 (1991)], is here generalized to any Hamiltonian that can be represented in tight-binding form. A more compact expression for the transmission matrix elements is derived, and it is shown how all the Greens function results can be derived from the mode-matching technique. We illustrate this for a simple model that can be studied analytically, and for an Fe|vacuum|Fe tunnel junction that we study using first-principles calculations.

Spin injection through an Fe/InAs interface

The spin dependence of the interface resistance between ferromagnetic Fe and InAs is calculated from first principles for specular and disordered (001) interfaces. Because of the symmetry mismatch in the minority-spin channel, the specular interface acts as an efficient spin filter with a transmitted current polarization between 98% and 89%. The resistance of a specular interface in the diffusive regime is comparable to the resistance of a few microns of bulk InAs. Symmetry breaking arising from interface disorder reduces the spin asymmetry substantially, and we conclude that efficient spin injection from Fe into InAs can only be realized using high-quality epitaxial interfaces.

Orientation-Dependent Transparency of Metallic Interfaces

As devices are reduced in size, interfaces start to dominate electrical transport, making it essential to be able to describe reliably how they transmit and reflect electrons. For a number of nearly perfectly lattice-matched materials, we calculate from first principles the dependence of the interface transparency on the crystal orientation. Quite remarkably, the largest anisotropy is predicted for interfaces between the prototype free-electron materials silver and aluminum, for which a massive factor of 2 difference between (111) and (001) interfaces is found.

Dynamic ferromagnetic proximity effect in photoexcited semiconductors

The spin dynamics of photoexcited carriers in semiconductors in contact with a ferromagnet is treated theoretically and compared with time-dependent Faraday rotation experiments. The long-time response of the system is found to be governed by the first tens of picoseconds in which the excited plasma interacts strongly with the intrinsic interface between the semiconductor and the ferromagnet in spite of the existence of a Schottky barrier in equilibrium.

Spin accumulation and decay in magnetic Schottky barriers

The theory of charge and spin transport in forward-biased Schottky barriers reveals characteristic and experimentally relevant features. The conductivity mismatch is found to enhance the current-induced spin imbalance in the semiconductor. The GaAs?MnAs interface resistance is obtained from an analysis of the magnetic-field-dependent Kerr rotation experiments by Stephens et al. and compared with first-principles calculations for intrinsic interfaces. With increasing current bias, the interface transparency grows toward the theoretical values, reflecting increasingly efficient Schottky barrier screening.