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Absence of spin polarization in the thermocurrent emitted by a cesiated ferromagnetic iron surface.

A simple explanation is proposed for the recently observed absence of spin polarization of the thermocurrent emitted by cesiated ferromagnetic surfaces of Fe and Ni. First, the spin-dependent flux across the surface is computed, starting from a self-consistent spin-polarized band-structure calculation. This yields the correct result for Ni, but a large (-34%) negative polarization for Fe. It is then argued that most minority-spin electrons arise from narrow d bands and will not contribute to the current due to localization effects and wave function and velocity mismatch at the surface. A quantitative estimate is given in terms of a lower cutoff in the band-velocity distribution.