Anomalous spin-orbit torque switching in synthetic antiferromagnets

We report that synthetic antiferromagnets (SAFs) can be efficiently switched by spin-orbit torques (SOTs) and the switching scheme does not obey the usual SOT switching rule. We show that both the positive and negative spin Hall angle (SHA)-like switching can be observed in Pt/SAF structures with only positive SHA, depending on the strength of applied in-plane fields. A switching mechanism directly arising from the asymmetric domain expansion is proposed to explain the anomalous switching behaviors. Contrary to the macrospin-based switching model where the SOT switching direction is determined by the sign of SHA, this switching mechanism suggests that the SOT switching direction is dominated by the field-modulated domain wall motion and can be reversed even with the same sign of SHA. The presence of this switching mechanism is further confirmed by the domain wall motion measurements. The anomalous switching behaviors provide important insights for understanding SOT switching mechanisms and also offer novel features for applications.