Spin-Torque-Triggered Magnetization Reversal in Magnetic Tunnel Junctions with Perpendicular Shape Anisotropy
Nuno Caçoilo, Steven Lequeux, B. M. S. Teixeira, B. Diény, R. C. Sousa, Н. А. Соболев, Olivier Fruchart, Lucian Prejbeanu, L. D. Buda-Prejbeanu
Abstract
Recently, the concept of perpendicular shape anisotropy spin-transfer-torque magnetic random-access memory (PSA STT MRAM) was proposed to enhance the thermal stability factor, $\mathrm{\ensuremath{\Delta}}$, of the storage layer through its shape anisotropy, thus enabling the downsizing scalability of conventional STT MRAM beyond sub-20-nm technological nodes. However, this is expected to negatively impact the writing current and switching time, calling for the search for the best compromise. Here, we report a micromagnetic study of the STT-driven magnetization reversal of sub-20-nm PSA STT MRAM cells, with a particular emphasis on the technologically relevant case of $\mathrm{\ensuremath{\Delta}}\ensuremath{\simeq}80$, thus providing guidelines for their practical design. For an aspect ratio ($\ensuremath{\eta}$) of the storage layer of $\ensuremath{\eta}\ensuremath{\le}1$, magnetization reversal obeys a macrospinlike mechanism, while for $\ensuremath{\eta}>1$ a noncoherent reversal is observed, which is characterized by buckling or nucleation of a transverse domain wall at the ferromagnet-insulator interface, which then propagates along the vertical axis. Decreasing the lateral size while maintaining a constant value of $\mathrm{\ensuremath{\Delta}}$ implies a larger height, which is found to lead to an increase of the switching voltage. In all cases, the inverse of the switching time scales linearly with the applied voltage.