Dynamical switching properties and downsize scalability in Perpendicular Shape Anisotropy MTJ

The novel Perpendicular Shape-Anisotropy Spin-Transfer-Torque Magnetic Random-Access Memory (PSA-STT-MRAM) is shown to increase the thermal stability factor ∆ of the storage layer through modulation of its shape anisotropy, surpassing the scalability limitation of conventional magnetic tunnel junctions to below sub-20 nm technological nodes. However, for large aspect-ratio, the magnetization reversal becomes non-uniform and less controllable. Thus, it is necessary to reduce the total thickness of the storage layer, while keeping an appropriate ∆. We show that, making use of micromagnetic simulations, making use of a capping layer to promote an additional surface anisotropy leads to a lower thickness requirement, compatible with lower switching voltage and faster reversal. These results are backed with experimental evidence, as STT reversal is shown without field. However, this capping layer shows less efficient for very small diameters, down to 5 nm, in the limit where ∆ is limited by the energy of the domain-wall created in the layer. It is shown that it would be possible to avoid this stability capping while maintaing a single PSA layer, with an induced uniaxial anisotropy.