2, Yunlin University of Science and Technology, Douliu, , Taiwan
The effect of the annealing temperature on the perpendicular magnetic anisotropy (PMA) of composite structures was studied. We merged a MgO/CoFeB bilayer  and an ultra-thin [Co/Pd]6 multilayer separated by a non-magnetic Ta spacer  of variable thickness tTa. Composite magnetic structures with PMA are technologically relevant in developing high-density memory devices.
The stacks were deposited using magnetron sputtering at room temperature in the absence of an external magnetic field. Thermally oxidized (100) Si wafers were used as substrates. After deposition, samples were heat-treated for 2 hrs. at 250, 300, or 350 C in a high-vacuum magnetic annealing oven with a field of 5 kOe perpendicular to the film plane. Magnetic properties were studied by using an alternating gradient magnetometer (AGM) where magnetization vs. applied
magnetic field (M-H) hysteresis loops were obtained with either out-of-plane or in-plane magnetic fields.
Hysteresis loops show sharp switching characteristics indicating ferromagnetic coupling between the MgO/CoFeB bilayer and the Co/Pd multilayers. Structures lacking a Ta layer show not PMA regardless of whether or not they were magnetically annealed. PMA is obtained after inserting a Ta layer and is observed even in the as-prepared state .
Our results show that Ta layer is essential for obtaining perpendicular axis in the composite MgO/CoFeB/Ta/[Co/Pd]6 structure. Composite structures retain PMA upon magnetic annealing up to 350 C. The ferromagnetic exchange was strong enough to switch MgO/CoFeB and [Co/Pd]6 together, with the magnetic moment either lying along the film plane when tTa = 0, or pulled out of the film plane when tTa ≠ 0. No antiparallel magnetic coupling was observed within the Ta thickness interval explored (0 ≤ tTa ≤ 0.7 nm). Perpendicular composite structures with sharp magnetization reversal and annealing stability are relevant in perpendicular CoFeB-based magnetic tunnel junctions for the development of gigabit-scale nonvolatile memory.