Diffusion equation and spin drag in spin-polarized transport

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We study the role of electron-electron interactions for spin polarized transport using the Boltzmann equation and derive a set of coupled transport equations. For spin polarized transport the electron-electron interactions are important, because they tend to equilibrate the momentum of the two spin species. This ``spin drag'' effect enhances the resistivity of the system. The enhancement is stronger the lower the dimension and should be measurable in for example a two dimensional electron gas with ferromagnetic contacts. We also include spin flip scattering which has two effects: it equilibrates the spin density imbalance and, provided it has a non s-wave component, also the current imbalance.