| Recently, magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) had attracted widely attentions for its potential applications in MRAMs and magnetic sensors. Investigation on perpendicular magnetic anisotropy of CoFeB thin films is of great importance as for its key role in MTJs. In this article, we concentrated on fabrication and optimization of CoFeB thin films with PMA. In general, the main contents include the effects of preparation conditions, structures, thickness and annealing temperature of thin films on PMA of Ta\CoFeB\MgO system. Vibrating Sample Magnetometer(VSM), Anomalous Hall Effect and ESR was used to characterize the properties of different samples. Finally, we investigated the influence of different layer’s thickness and repeats number on PMA in [Ta\CoFeB\MgO]N system. The important conclusions are as follows:a) For the films deposited at 5mTorr, Ta\CoFeB\MgO(1.0nm)\Ta thin films show good PMA with CoFeB layer thickness varying between 0.75 and 1.1 nm, while the change of Ta buffer layer thickness within the range of 1-20 nm has no obvious effects on perpendicular anisotropy for Ta\CoFeB(0.98nm)\MgO(1.0nm)\Ta(1.0nm) films. Proper annealing can improve the PMA of Ta\CoFeB\MgO\Ta films. It is found that the Ta\CoFeB(0.98nm)\MgO\Ta films show maximum PMA after annealing at 250 ℃, with an effective anisotropy field up to 0.9 T and the large saturation magnetization of 1528 emu\cc. However, the films with CoFeB thickness of 1.5 nm show perpendicularly anisotropy after 300 and 350℃ annealing. For thinner CoFeB, field annealing could improve PMA of Ta\CoFeB\MgO\Ta further when annealing temperature is lower than 300 ℃。b) The films with a structure of [Ta(5nm)\CoFeB(0.6-1.5nm)\MgO(1.0nm)]×6 exhibit good PMA when the thickness of CoFeB layer is 0.9-1.1nm. When CoFeB layer is thicker than 1.1 nm, PMA disappears. The films show superparamagnetic behavior with the thickness of CoFeB layer smaller than 0.9nm. Magnetic hysteresis loops of [Ta(x)\CoFeB(1.1 nm)\MgO(1.0nm)]×N structure show bow-tie shape when N is larger than 2, whiel maze domain is observed in these samples. Nucleation field (Hn) inceases with the repeats number N or decreasing of buffer layer Ta thickness. On the contrary, magnetic domain width W shows an opposite change trends. The magnetization process of the multilayer was fitted by a strip domain model. It is found that the calculated nucleation field and domain width agree well with the experimental results. It proved that the decrease of domain size mainly originates from the magnetostatic interactions between different magnetic layers. |
