Highly efficient spin-orbit torque in a perpendicular synthetic ferrimagnet

Abstract

Perpendicular synthetic ferrimagnet (pSFi) due to the low stray field and net magnetization is expected to replace single ferromagnet as a free layer to optimize the memory devices for high storage density and low-energy consumption. Here, we investigate the dependence of exchange-coupling strength on the thickness of the spacer Ru and current-induced magnetization reversal due to spin-orbit torque in $\mathrm{Ta}/\mathrm{Pt}/{[\mathrm{Pt}/\mathrm{Co}]}_{2}/\mathrm{Ru}/{[\mathrm{Co}/\mathrm{Pt}]}_{4}/\mathrm{Pt}$ structure with a perpendicular magnetic anisotropy. An oscillating interlayer exchange coupling as a function of Ru spacer layer thickness with a period of 1.16 nm is determined by combining the anomalous Hall effect and the polar magnetic-optical Kerr effect. Furthermore, current-controllable magnetization reversal experiments reveal that the magnetizations of top and bottom layers with antiferromagnetic coupling switch simultaneously due to the combination of spin-orbit torque generated from the two adjacent heavy-metal layers and the interlayer exchange torque. The SOT efficiency $\ensuremath{\chi}\ensuremath{\sim}57.52\phantom{\rule{0.16em}{0ex}}\mathrm{Oe}/({10}^{6}\phantom{\rule{0.16em}{0ex}}\mathrm{A}/\mathrm{c}{\mathrm{m}}^{2})$, corresponding to an effective spin Hall angle $\phantom{\rule{4pt}{0ex}}{\ensuremath{\xi}}_{DL}\ensuremath{\sim}0.68$, is estimated by analyzing current-dependent anomalous Hall resistance with our proposed quasistatic balance equation of magnetic moments. In addition to the advantage of minimizing stray field acting on the storage layer, the observed low switching current density and high spin-orbit torque efficiency suggest that the pSFi structure with a high thermal stability factor has great potential to realize the high-density and low-power consumption of nonvolatile magnetic memory devices.