Tuning perpendicular magnetic anisotropy in the MgO/CoFeB/Ta thin films

The characteristic of magnetic anisotropy is one of the keys to determine the applications of the ferromagnetic thin films for decades. The perpendicular magnetic anisotropy (PMA) in the ferromagnetic thin film has been found in the conventional ferromagnetic metal/non-magnetic metal (FM/NM) multilayers, such as Co/Pd multilayers [1], and in the annealed ferromagnetic metal/ oxide thin films, such as Pt/Co/AlO_x and Ta/CoFeB/MgO thin films [2,3]. The strength of PMA can be tuned not only by varying the thickness of FM layer, but also by changing the oxidation status at the interface of FM/MO_x. Since it is of great interest in both experimental and theoretical views to investigate the mechanism of PMA, the MgO/CoFeB interface have received continuous attentions due to their potential use in highly sensitive magnetic field sensors or in spin-transfer-torque magnetic random access memories. In this paper, we propose a new approach to manipulate the strength of PMA at the MgO/CoFeB interface. It is found that the strength of PMA dramatically decreases with the increasing of MgO thickness in the MgO/CoFeB/Ta thin films, due to the onset of crystalline MgO forming.