Thickness dependence of degree of spin polarization of electrical current in permalloy thin films

Abstract

Spin-polarized electrical transport is investigated in ${\text{Al}}_{2}{\text{O}}_{3}/{\text{Ni}}_{80}{\text{Fe}}_{20}/{\text{Al}}_{2}{\text{O}}_{3}$ thin films for permalloy thickness between 6 and 20 nm. 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 0.63 at 20 nm to 0.42 at 6 nm. This decrease is attributed to a spin depolarization induced by the film surfaces. A model is proposed which takes into account the contributions of the different sources of electron scattering (alloy disorder, phonons, thermal magnons, grain boundaries, film surfaces) to the measured spin-dependent resistivities.