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The concept of perpendicular shape anisotropy spin-transfer torque magneticrandom-access memory (PSA-STT-MRAM) consists in increasing the storage layerthickness to values comparable to the cell diameter, to induce a perpendicularshape anisotropy in the magnetic storage layer. Making use of thatcontribution, the downsize scalability of the STT-MRAM may be extended towardssub-20 nm technological nodes, thanks to a reinforcement of the thermalstability factor $\Delta$. Although the larger storage layer thickness improves$\Delta$, it is expected to negatively impact the writing current and switchingtime. Hence, optimization of the cell dimensions (diameter, thickness) is ofutmost importance for attaining a sufficiently high $\Delta$ while keeping amoderate writing current. Micromagnetic simulations were carried out fordifferent pillar thicknesses of fixed lateral size 20 nm. The switching timeand the reversal mechanism were analysed as a function of the applied voltageand aspect-ratio (AR) of the storage layer. For AR $<$ 1, the magnetizationreversal resembles a macrospin-like mechanism, while for AR $>$ 1 anon-coherent reversal is observed, characterized by the nucleation of atransverse domain wall at the ferromagnet/insulator interface which thenpropagates along the vertical axis of the pillar. It was further observed thatthe inverse of the switching time is linearly dependent on the applied voltage.This study was extended to sub-20 nm width with a value of $\Delta$ around 80.It was observed that the voltage necessary to reverse the magnetic layerincreases as the lateral size is reduced, accompanied with a transition frommacrospin-reversal to a buckling-like reversal at high aspect-ratios.

Magnetization reversal driven by spin-transfer-torque in perpendicular shape anisotropy magnetic tunnel junctions