  The concept of perpendicular shape anisotropy spin-transfer torque magnetic
random-access memory (PSA-STT-MRAM) consists in increasing the storage layer
thickness to values comparable to the cell diameter, to induce a perpendicular
shape anisotropy in the magnetic storage layer. Making use of that
contribution, the downsize scalability of the STT-MRAM may be extended towards
sub-20 nm technological nodes, thanks to a reinforcement of the thermal
stability factor $\Delta$. Although the larger storage layer thickness improves
$\Delta$, it is expected to negatively impact the writing current and switching
time. Hence, optimization of the cell dimensions (diameter, thickness) is of
utmost importance for attaining a sufficiently high $\Delta$ while keeping a
moderate writing current. Micromagnetic simulations were carried out for
different pillar thicknesses of fixed lateral size 20 nm. The switching time
and the reversal mechanism were analysed as a function of the applied voltage
and aspect-ratio (AR) of the storage layer. For AR $<$ 1, the magnetization
reversal resembles a macrospin-like mechanism, while for AR $>$ 1 a
non-coherent reversal is observed, characterized by the nucleation of a
transverse domain wall at the ferromagnet/insulator interface which then
propagates along the vertical axis of the pillar. It was further observed that
the 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 layer
increases as the lateral size is reduced, accompanied with a transition from
macrospin-reversal to a buckling-like reversal at high aspect-ratios.
