Optimisation of perpendicular magnetic tunnel junction structures using scanning transmission electron microscopy

Here, we report work on sputter deposited CoFeB/MgO/CoFeB magnetic tunnel junction (MTJ) stacks with a CoFeB thickness ≤1.5 nm to obtain perpendicular magnetic anisotropy (PMA) and thin continuous layers, which are essential for good quality MTJs. Informed by scanning transmission electron microscopy (STEM) analysis, improved MTJ structures have been grown at room temperature. These structures have improved characteristics through optimisation of the Ta seed layer deposition conditions, as reducing the roughness of the Ta layer results in higher-quality CoFeB and MgO layers. The thicknesses of the MTJ layers were determined using X-ray reflectometry (XRR), and the magnetic properties were investigated using vibrating sample magnetometry (VSM). We demonstrate tunnelling within the MTJ structures by performing current-in-plane-tunnelling (CIPT) measurements on as-deposited (unannealed) MTJ structures, producing a peak tunnelling-magnetoresistance (TMR) ratio of 4.3%. Furthermore, after annealing at 100 °C, the MTJ structure exhibits a strong interfacial PMA, demonstrating the importance of optimising the layer quality when growing such structures.