Iridium spacer induced spin-orbit torque switching in synthetic antiferromagnets

The capability of an Ir spacer to modulate interlayer exchange coupling (IEC) and perpendicular magnetic anisotropy in a synthetic antiferromagnetic (SAF) is critical for practical applications. Here, we present a comprehensive analysis on the dual roles of Ir for the modulation of IEC and the generation of spin current in SAF structures with symmetric and asymmetric perpendicular ferromagnetic layers. The results reveal that the Ir thickness and symmetry of the ferromagnetic layer are pivotal in tuning the IEC strength. Strikingly, a critical Ir thickness of 1.4 nm is identified: Beyond this threshold, full magnetization switching is achieved, whereas a thinner Ir layer results in partial or nonswitching due to insufficient spin current. This switching behavior is mapped using a phase diagram correlating Ir thickness with IEC states, which is further corroborated by polar magneto-optical Kerr effect microscopy. The dampinglike spin-orbit torque efficiency $[1.16\ensuremath{-}1.36\phantom{\rule{0.16em}{0ex}}\text{Oe}/({10}^{6}\phantom{\rule{0.28em}{0ex}}\mathrm{A}/{\text{cm}}^{2})]$ and the spin Hall angle $(\ensuremath{\sim}0.03)$ are quantitatively characterized, which is close to the spin Hall angle of Ir. Notably, the symmetric SAF structure exhibits superior switching performance compared to the asymmetric structure, a result attributed to the elimination of spin current cancellation between Pt and Ir layers. This paper significantly advances our understanding of SAF systems and provides essential guidelines for developing high-efficiency spintronic devices.