Low current density induced spin-transfer torque switching in CoFeB-MgO magnetic tunnel junctions with perpendicular anisotropy

We present the thickness effects of CoFeB free layer on tunnelling magnetoresistive (TMR), perpendicular magnetic anisotropy (PMA) and spin-transfer torque (STT) in CoFeB-MgO based magnetic tunnel junctions (MTJs). It is found that a post-annealing process could significantly improve both TMR and PMA of the MTJ systems. When the free layer thickness is reduced from 1.3 nm to 1 nm, TMR continuously decays from 80% to 20%. On the other hand, PMA is maximized for a 1.28 nm free layer, above which demagnetization becomes stronger and results in lower PMA. If the free layer thickness is very small, dead layer effect could damage interfacial perpendicular anisotropy and PMA is reduced as a consequence. For STT-induced magnetization switching, the lowest intrinsic critical switching current density (J_c0) of 2.1 MA cm^-2 is achieved at a free layer thickness of 1.16 nm, accompanied by a TMR of 52% and product of resistance and area (RA) of 16 Ω νm². Further increasing the free layer thickness will first enhance J_c0 and then reduce it due to the balance between PMA and the total free layer volume. STT studies suggest that the CoFeB free layer thickness should be optimized to make a trade-off among large PMA, high TMR and low switching current density in perpendicular CoFeB-MgO MTJ systems.