Structural and Electronic Properties of Cobalt/Alumina Tunnel Junctions from First Principles

Authors

Ivan I. Oleinik, University of Oxford, Oxford, UKFollow
Evgeny Y. Tsymbal, University of Nebraska at LincolnFollow
David G. Pettifor, University of OxfordFollow

Date of this Version

3-2000

Comments

Published in Physical Review B 62, 3952-3959 (2000). Copyright © 2000 The American Physical Society. Used by permission.

Abstract

A detailed first-principles study of the atomic and electronic structure of the Co/Al₂O₃/Co magnetic tunnel junction has been performed in order to elucidate the key features determining the spin-dependent tunneling. The atomic structure of the multilayer with the O- and Al-terminated interfaces between fcc Co(111) and crystalline α-Al₂O₃(0001) has been optimized using self-consistent spin-polarized calculations within densityfunctional theory and the generalized gradient approximation. We found that the relaxed atomic structure of the O-terminated interface is characterized by a rippling of the Co interfacial plane, the average Co-O bond length being 2.04 Å which is within 5% of that in bulk CoO. The corresponding electronic structure is influenced by the covalent bonding between the O 2p and Co 3d orbitals resulting in exchange-split bonding and antibonding states and an induced magnetic moment of 0.07 µ_B on the interfacial oxygen atoms. The Al-terminated interface contains Co-Al bonds with an average bond length of 2.49 Å compared to 2.48 Å in bulk CoAl. Due to charge transfer and screening effects the Co interfacial layer acquires a negative charge which results in a reduced magnetic moment of 1.15 µ_B per Co atom. We found that the electronic structure of the O-terminated Co/Al₂O₃/Co tunnel junction exhibits negative spin polarization at the Fermi energy within the first few monolayers of alumina but it eventually becomes positive for distances beyond 10 Å.