Spin current propagation through ultra-thin insulating layers in multilayered ferromagnetic systems

Spin current pumping from a ferromagnet through an insulating layer into a heavy metal was studied in a CoFeB/ SiO 2/Pt system in relation to the thickness and interfacial structure of the insulating layer. The propagation of spin current from the ferromagnet into the heavy metal falls rapidly with sub-nanometer thicknesses of SiO 2 and is suppressed beyond a nominal thickness of 2 nm. Structural analysis shows that SiO 2 only forms a complete barrier layer beyond around 2 nm, indicating that the presence of a discontinuous insulating barrier, and not tunneling or diffusion, explains the main observations of spin-pumping with thin insulating layers.