Metal/oxide and metal/organic interfaces in magnetic tunnel junctions

Metal-oxide and metal-organic molecule interfaces in magnetic tunnel junctions play a key role in determining the fundamental physics of spin-polarized tunneling. Magnetic tunnel junctions (MTJs) consisting of spin-polarized ferromagnetic electrodes sandwiching an insulating barrier are promising candidates of spintronic devices for applications such as magnetic random access memory (MRAM). Numerous experimental and theoretical investigations show that the spin-polarization of tunneling current that determines the tunneling magnetoresistance of MTJs is not an intrinsic property of the ferromagnet alone but depends on structural and electronic properties of the insulator and the ferromagnet-insulator interface. We investigate the atomic and electronic structure of ferromagnet-oxide and ferromagnet-organic interfaces in magnetic tunnel junctions using first-principles density functional theory. Our goals are:

1. To investigate the atomic structure of MTJs as a function of interface termination, the interfacial energetics and to identify the most stable structures.
2. To investigate the electronic and magnetic properties of MTJs for the particular types of interfaces obtained from structural studies, to calculate the charges and magnetic moments on atoms.
3. To study the character of the chemical bonding between ferromagnet and either an oxide or an organic molecule as a function of interface termination.

We performed first-principle density-functional studies of the atomic and electronic structure of metal-oxide interfaces in Co/Al₂O₃/Co, Ni/Al₂O₃/Ni, and Co/SrTiO₃/Co ferromagnet-oxide MTJs. We found that the most stable interface structures are the those with an O-terminated alumina layer for both fcc Co/Al₂O₃/Co, and Ni/Al₂O₃/Ni MTJs. For Co/SrTiO₃/Co MTJ, a TiO₂-terminated interface with oxygens on top of Co atoms is the most stable. These conclusions were made based on calculated energetics of metal-oxide cohesion at the interface. The covalent character of bonding for the Co/alumina, Ni/alumina and Co/SrTiO₃ interface structures has been determined based on the pattern of electron distribution across the interface. The Al-terminated {Co, Ni}/alumina interfaces that correspond to under-oxidized MTJs exhibit a metallic character of bonding. The unusual charge transfer process coupled with exchange interactions of electrons in the ferromagnetic electrode results in quenching of surface magnetism at the interface and substantial reduction of work of separation.

Ferromagnet/organic interfaces

Recent experiments unambiguously demonstrated successful spin injection from spin-polarized ferromagnetic electrodes into p-conjugated organic molecules. Therefore, we applied first-principles density functional theory to investigate the atomic, electronic and spin-polarized properties of organic MTJs consisting of a polythiophene monolayer sandwiched by ferromagnetic Ni electrodes.

Selected publications:

1. E. Y. Tsymbal, K. D. Belashchenko, J. P. Velev, S. S. Jaswal, M. van Schilfgaarde, I. I. Oleynik, and D. A. Stewart, “Interface effects in spin-dependent tunneling”, Progress in Materials Science 51, xxx (2006) – in press (review paper).
2. I.I. Oleynik and E.Yu. Tsymbal, “Metal-oxide interfaces in magnetic tunnel junctions”, Interface Science 12, 105 (2004) (review paper).
3. I.I. Oleinik and E.Yu. Tsymbal, “Atomic, Electronic and Magnetic Properties of Magnetic Tunnel Junctions”, Journal of Applied Physics 93, 6429 (2003).
4. I.I. Oleinik, E.Yu. Tsymbal, and D.G. Pettifor, “Atomic and electronic structure of Co/SrTiO₃/Co magnetic tunnel junctions”, Phys. Rev. B 65, 020401 (2002);