Magnetic tunnel junctions (MTJs) have attracted considerable interest due to their applications in magnetic random access memories and magnetic field sensors. A MTJ consists of a thin insulating layer separating two ferromagnetic electrodes. The electrical resistance of a MTJ depends on the relative magnetization orientation of the electrodes, the phenomenon known as tunneling magnetoresistance. In addition, MTJs exhibit the interlayer exchange coupling, affecting the alignment of the magnetization of the electrodes. These properties of MTJs are affected by defects which are always present in industrially-important tunnel junctions, such as Fe/MgO/Fe.
Recent work in collaboration with scientists from National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan have demonstrated, both experimentally and theoretically, that O vacancies in the MgO barrier layer play a significant role in Fe/MgO/Fe(001) MTJs [Katayama et al., Appl. Phys. Lett. 89, 112503 (2006); Velev et al., IEEE Trans. Magn. 22, 2770 (2007)]. It was found that for thin MgO barriers resonant coupling through O vacancies makes the interlayer exchange coupling antiferromagnetic. With increasing MgO thickness, however, the resonant contribution is reduced, and the coupling becomes ferromagnetic typical for perfect barriers. Also O vacancies strongly affect spin-dependent conductance of the MTJs by scattering tunneling electrons which causes a substantial reduction of tunneling magnetoresistance. These results show that improving the quality of the MgO barrier by reducing O vacancy concentration would further enhance tunneling magnetoresistance and reduce the interlayer exchange coupling which are beneficial for applications of MTJs.
This research is supported by the National Science Foundation, Division of Materials Research, Materials Research Science and Engineering Program, Grant 0213808.
Charge density plot of an Fe/MgO/Fe(001) magnetic tunnel junction containing O vacancy in the middle of 5 monolayer thick MgO barrier.