| AVS 64th International Symposium & Exhibition | |
| Magnetic Interfaces and Nanostructures Division | Tuesday Sessions |
| Session MI+2D+AC+NS-TuA |
| Session: | Spin-Orbit Phenomena at Surfaces and Interfaces |
| Presenter: | Kevin Fitzell, University of California at Los Angeles (UCLA) |
| Authors: | K. Fitzell, University of California at Los Angeles (UCLA) X. Li, University of California at Los Angeles (UCLA) C.T. Karaba, University of California at Los Angeles (UCLA) A. Buditama, University of California at Los Angeles (UCLA) G. Yu, University of California at Los Angeles (UCLA) K. Wong, University of California at Los Angeles (UCLA) D. Wu, UCLA; Fudan University, Republic of China N. Altieri, UCLA C. Grezes, UCLA N. Kioussis, CSU, Northridge S.H. Tolbert, UCLA Z. Zhang, Fudan University, Republic of China J.P. Chang, UCLA P.K. Amiri, UCLA K.L. Wang, UCLA |
| Correspondent: | Click to Email |
Paramount to the continued scaling of MRAM devices is a comprehensive understanding and control of the factors affecting the interfacial phenomena that occur at the CoFeB|MgO interface, from which the perpendicular magnetic anisotropy (PMA) of the CoFeB originates. Efficient manipulation of this PMA using an applied voltage, known as the voltage-controlled magnetic anisotropy (VCMA) effect, offers significant energy savings over electric-current-controlled alternatives such as STT-RAM. Ab initio studies in the literature on Fe/MgO interfaces revealed a dependence of the VCMA effect on the oxidation state of interfacial Fe atoms1 and on the addition of various heavy metal insertion layers2 at the CoFeB/MgO interface. While this effect of metallic insertion layers at the CoFeB/MgO interface has not been extensively studied experimentally, inserting a thin Mg layer at the CoFeB/MgO interface has been shown in the literature to improve the (001) texture of the MgO, the tunneling magnetoresistance (TMR) ratio of the MTJ, and the thermal stress stability of the CoFeB layer’s PMA.3,4 What is lacking in the literature, however, is experimental work studying the dependence on the VCMA effect of Mg insertion layers at the CoFeB/MgO interface.
In this work, the impact of several types of metallic insertion layers (Ta, Pt, and Mg) at the CoFeB|MgO interface on the VCMA characteristics and other magnetic properties is studied. For the case of Mg insertion, four different regimes of materials properties were observed, corresponding to the oxidation state at the CoFeB|MgO interface. Inserting an ultrathin Mg layer of 0.1–0.3 nm yielded a VCMA coefficient of ~100 fJ/V×m, representing more than a factor of 3 improvement over average values of ~30 fJ/V×m reported in Ta|CoFeB|MgO-based structures. Ultrathin Ta and Pt insertion layers also showed a small improvement, yielding VCMA coefficients around 40 fJ/V×m. Electrical, magnetic, and synchrotron-based X-ray diffraction results reveal that a 1.1–1.3 nm Mg insertion layer gives rise to the highest perpendicular magnetic anisotropy and saturation magnetization, as well as to the best CoFe and MgO crystallinity; Mg insertion layers thicker or thinner than this give rise to either under- or over-oxidation of the CoFeB|MgO interface. These results demonstrate that precise control over the oxidation level at the CoFeB|MgO interface is crucial for the development of electric-field-controlled perpendicular magnetic tunnel junctions with low write voltage.