The nature of interface bonding essentially determines the spin polarization of tunneling currents through ferromagnet/insulator interfaces. We focus on the influence of ultrathin off-stoichiometric MgO coverages on the spin-polarized electronic structure of the (001) surface of bcc-Fe. By applying a combination of low-energy spin-resolved and core-level photoelectron spectroscopy, we show that the MgO stoichiometry has a direct impact on the interfacial spin polarization. The results underline the importance of controlling the chemical composition and bonding at ferromagnet/oxide interfaces to further optimize spin-dependent tunneling devices.

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