Ultrathin perpendicular free layers for lowering the switching current in STT-MRAM

The critical current density J c 0 required for switching the magnetization of the free layer (FL) in a spin-transfer torque magnetic random access memory (MRAM) cell is proportional to the product of the damping parameter, saturation magnetization, and thickness of the free layer, α M S t F. Conventional FLs have the structure CoFeB/nonmagnetic spacer/CoFeB. By reducing the spacer thickness, W in our case, and also splitting the single W layer into two layers of a sub-monolayer thickness, we have reduced t F while minimizing α and maximizing M S, ultimately leading to lower J c 0 while maintaining high thermal stability. Bottom-pinned MRAM cells with a device diameter in the range of 55–130 nm were fabricated, and J c 0 is the lowest for the thinnest (1.2 nm) FLs, down to 4 MA / cm 2 for 65 nm devices, ∼ 30 % lower than 1.7 nm FLs. The thermal stability factor Δ dw, as high as 150 for the smallest device size, was determined using a domain wall reversal model from field-switching probability measurements. With high Δ dw and the lowest J c 0, the thinnest FLs have the highest spin-transfer torque efficiency.