The effect of HfO2 on the magnetic anisotropy, electrical structure and microstructure of CoFeB/MgO films

Metal oxides are used in magnetic tunnel junctions to improve the magnetic properties and thermostability of materials in high-performance applications. In this study, an ultrathin Hf film is deposited on a Ta buffer layer. Moreover, subsequent annealing at 350 °C leads to the oxidation of Hf into HfO2, as confirmed by X-ray photoelectron spectrometry (XPS). The effect of HfO2 on magnetic anisotropy is evaluated in Ta/Hf/CoFeB/MgO/Ta multilayer films. Easy-axis magnetisation transition from in-plane to out-of-plane direction is observed at an annealing temperature of 200 °C. Moreover, the sample annealed at a temperature of 350 °C exhibits clear perpendicular magnetic anisotropy, which satisfies industry requirements in terms of annealing temperature for magnetic random access memory applications. XPS, positron annihilation spectroscopy (PAS), X-ray reflectivity (XRR) and atomic force microscopy (AFM) are used to study film composition, defects, interface width and surface roughness in Ta/Hf/CoFeB/MgO/Ta multilayers. XPS results demonstrate partial suppression of Ta atom diffusion and enhanced formation of Fe (Co)oxides in CoFeB/MgO. The results of PAS analysis suggest that the density of film defects decreases after annealing at 350 °C. XRR and AFM results reveal that annealing at 350 °C produces a smoother CoFeB/MgO interface. Together, these factors lead to improved magnetic properties and thermostability in a Ta/Hf/CoFeB/MgO/Ta film.