Anatomy of Type- x Spin-Orbit-Torque Switching

Abstract

Using a type-x spin-orbit-torque (SOT) switching scheme, in which the easy axis (EA) of the ferromagnetic (FM) layer and the charge-current flow direction are collinear, it is possible to realize a lower-power-consumption, higher-density, and better-performance SOT magnetoresistive random-access memory (SOT MRAM) compared with the conventional type-y design. Here, we systematically investigate type-x SOT switching properties through both macrospin and micromagnetic simulations. The out-of-plane external field and anisotropic field dependence of the switching-current density (${J}_{\mathrm{sw}}$) is first examined in the ideal type-x configuration. Next, we study the FM-layer canting-angle ($\ensuremath{\varphi}$) dependence of ${J}_{\mathrm{sw}}$ through macrospin simulations and experiments, which show the transformation of switching dynamics from type x to type y with increasing $\ensuremath{\varphi}$. By further integrating fieldlike torque (FLT) into the simulated system, we find that a positive FLT can assist type-x SOT switching, while a negative one brings about complex dynamics. More crucially, with the existence of a sizable FLT, type-x switching mode results in a lower critical switching current than that of type y at a current pulse width less than about 10 ns, indicating the advantage of employing the type-x design for ultrafast switching using material systems with FLT. Our work provides a thorough examination of the type-x SOT scheme with various device and materials parameters, which can be informative for designing next-generation SOT MRAM.