We have investigated the electric field effects in low resistance perpendicular magnetic tunnel junction (MTJ) devices and found that the electric field can effectively reduce the coercivity (Hc) of free layer (FL) by 30% for a bias voltage Vb = −0.2 V. In addition, the bias field (Hb) on free layer is almost linearly dependent on Vb yet independent on the device size. The demonstrated Vb dependences of Hc and Hb in low resistance MTJ devices present the potential to extend the scalability of the electric field assisted spin transfer torque magnetic random access memory and improve its access speed.

Annealing temperature (Ta) and free layer thickness dependencies of magnetic properties and spin-transfer torque switching were investigated in CoFeB-MgO based magnetic tunnel junctions with perpendicular magnetic anisotropy (PMA). Annealing process was found to be critical to buildup PMA. As Ta increases, switching field of free layer and reference layer is enhanced first then drops, corresponding to the improvement and collapse of PMA in both layers. However, it should be noted that PMA of free layer and the tunneling magnetoresistive (TMR) are maximized at different Ta zones. Spin transfer torque study pointed out that switching current density (Jc) depends on the combined effects from PMA, spin polarization, and saturation magnetization, which all depend on Ta values. Thickness dependence study revealed that Jc relies on the competing results of the thickness and PMA. The lowest critical switching current density achieved is 2.1 MA/cm2, accompanied with a TMR around 52% at room temperature.

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 µm2. 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.