Spin-polarized electrical transport is investigated in Al_2O_3/Ni_80Fe_20/Al_2O_3 thin films for permalloy thickness between 6 and 20nm. The degree of spin-polarization of the current flowing in the plane of the film is measured through the current induced spin wave Doppler shift. We find that it decreases as the film thickness decreases, from 0.72 at 20nm to 0.46 at 6nm. This decrease is attributed to a spin depolarization induced by the film surfaces. A model is proposed which takes into

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... nt the contributions of the different sources of electron scattering (alloy disorder, phonons, thermal magnons, grain boundaries, film surfaces) to the measured spin-dependent resistivities.