Channel decomposition and spin injection

The conductance of a piece of material is usually measured by attaching leads on either side, applying a voltage difference and measuring the current. In the Landauer-Bűttiker formulation of electronic transport, the conductance is expressed in terms of the probabilities that electronic states in one lead are transmitted ("scattered") into states in the other lead. In a tight-binding muffin-tin-orbital (TB-MTO) implementation of this formalism, the lead states ("channels") are calculated explicitly which makes possible the analysis ("channel decomposition") of the scattering induced by specular interfaces, interface disorder etc. The calculation of the transmission matrices scales linearly with the number of layers L in the scattering region and as the cube of the number of atoms N in lateral supercells used to model disorder. The efficient TB-MTO basis set allows scattering regions containing N x L ~ 104 atoms to be handled in practice. To analyse the interface scattering at disordered Co/Cu and Fe/Cr interfaces, calculations could be carried out for lateral supercells containing 20x20 atoms.

We recently applied this method to the problem of spin injection from an itinerant metal ferromagnet (Fe) into a semiconductor. We chose to study the semiconductor InAs because it forms an ohmic contact with Fe and because of the interest in using it to study spin transport in the intrinsic two dimensional electron gas formed on its surface. For a clean ordered (001) interface, transmission through the interface from Fe into InAs depends strongly on the spin. The calculated values of the spin-dependent interface resistances suggest that spin-injection should be possible. However, Fig. 1 shows that this result is very sensitive to the quality of the interface and a small amount of disorder can quench the spin injection.

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Figure 1. Interface resistances for a) In- and b) As-terminated interfaces as a function of the fraction of interfacial In or As atoms substituted by Fe for majority (O) and minority (Δ) spins. The inset in (a) shows the same data with a blown-up ordinate scale. For both terminations the symmetry-induced spin asymmetry is strongly reduced by disorder.