Oxygen-induced positive spin polarization from Fe into the vacuum barrier

Bonding at the ferromagnet-insulator interface is an important factor which influences spin polarization of the tunneling current in ferromagnetic tun nel junctions. In this article we investigate the spin-polarized electronic structure of the (001) surface of body-centered-cubic iron covered by an o xygen overlayer, as this could reflect the mechanism of bonding and spin po larization in iron/oxide tunnel junctions. The Fe/O atomic structure is opt imized using the plane-wave code CASTEP within the spin-polarized generaliz ed-gradient approximation. The electronic structure and local densities of states are calculated using the linear muffin-tin orbital method. The resul ts show hybridization of the iron 3d orbitals with the oxygen 2p orbitals, the strong exchange splitting of the former resulting in exchange-split bon ding and antibonding oxygen states. These antibonding states are partially occupied for the majority spins but are almost unoccupied for the minority spins, which leads to a positive spin polarization in the density of states of the oxygen atoms at the Fermi energy. This positive spin polarization p ropagates from oxygen into the vacuum barrier. This is opposite to what is observed for clean Fe films, where the surface Fe layer has a negative spin polarization at the Fermi energy and remains negative into the vacuum. We infer that this p-d bonding mechanism might be responsible for the experime ntally observed positive spin polarization of the tunneling current from fe rromagnetic metals through alumina. (C) 2000 American Institute of Physics. [S0021-8979(00)54808-7].