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

Abstract

The critical current density Jc0 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, αMStF. 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 tF while minimizing α and maximizing MS, ultimately leading to lower Jc0 while maintaining high thermal stability. Bottom-pinned MRAM cells with a device diameter in the range of 55–130 nm were fabricated, and Jc0 is the lowest for the thinnest (1.2 nm) FLs, down to 4MA/cm2 for 65 nm devices, ∼30% lower than 1.7 nm FLs. The thermal stability factor Δdw, as high as 150 for the smallest device size, was determined using a domain wall reversal model from field-switching probability measurements. With high Δdw and the lowest Jc0, the thinnest FLs have the highest spin-transfer torque efficiency.