Metal-Oxide Interfaces in Magnetic Tunnel Junctions

Metal-oxide interfaces play an important role in spintronics—a new area of microelectronics that exploits spin of electrons in addition to the traditional charge degree of freedom to enhance the performance of existing semiconductor devices. Magnetic tunnel junctions (MTJs) consisting of spin-polarized ferromagnetic electrodes sandwiching an insulating barrier are such promising candidates of spintronic devices. The paper reviews recent results of first-principle density-functional studies of the atomic and electronic structure of metal-oxide interfaces in Co/Al2O3/Co and Co/SrTiO3/Co MTJs. The most stable interface structures, O-terminated for fcc Co (111)/α-alumina(0001) and TiO2-terminated with oxygens on top of Co atoms for fcc Co (001)/SrTiO3(001) were identified based on energetics of metal-oxide cohesion at the interface. The covalent character of bonding for both the Co/alumina and Co/SrTiO3 interface structures has been determined based on the pattern of electron distribution across the interface. The Al-terminated Co/alumina interface that corresponds to an under-oxidized MTJ exhibits a metallic character of bonding. The unusual charge transfer process coupled with exchange interactions of electrons in Co results in quenching of surface magnetism at the interface and substantial reduction of work of separation. The electronic structure of the O-terminated Co/Al2O3/Co MTJ 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. The Co/SrTiO3/Co MTJ shows an exchange coupling between the interface Co and Ti atoms mediated by oxygen, which results in an antiparallely aligned induced magnetic moment on Ti atoms. This may lead to a negative spin polarization of tunneling across the SrTiO3 barrier from the Co electrode. The results illustrate the important fact that spin-polarized tunneling in magnetic tunnel junctions is not determined entirely by bulk density of states of ferromagnet electrodes, but is also very sensitive to the nature of the insulating tunneling barrier, as well as the atomic structure and bonding at the ferromagnet/insulator interface.