Spin injection from a half-metallic electrode in the presence of thermal spin
disorder is analyzed using a combination of random matrix theory,
spin-diffusion theory, and explicit simulations for the tight-binding s-d
model. It is shown that efficient spin injection from a half-metal is possible
as long as the effective resistance of the normal metal does not exceed a
characteristic value, which does not depend on the resistance of the
half-metallic electrode, but is rather controlled by spin-flip scattering at
the interface. This condition can be formulated as \alpha<(l/L)/T, where \alpha
is the relative deviation of the magnetization from saturation, l and L the
mean-free path and the spin-diffusion length in the non-magnetic channel, and T
the transparency of the tunnel barrier at the interface (if present). The
general conclusions are confirmed by tight-binding s-d model calculations. A
rough estimate suggests that efficient spin injection from true half-metallic
ferromagnets into silicon or copper may be possible at room temperature across
a transparent interface.Comment: 9 pages, 4 figures, revtex4-1; expanded introduction, added
references, additional comments in Section V, fixed typo
