Spin-transfer torque efficiency in ferro- and antiferromagnetically coupled synthetic free layers studied with superparamagnetic magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) controlled by spin-transfer torque (STT) are key elements in nonvolatile memories and have recently been explored as stochastic devices in unconventional computing. The use of in-plane ferromagnetically coupled (FC) or antiferromagnetically coupled (AFC) synthetic free layers has been proposed for these applications, as it enables tuning of device performance and reliability. Until now, the effect of STT on the dynamics of FC and AFC free layers has remained controversial, yet clarifying this issue is increasingly important as STT-MTJ technologies evolve. In this study, we show an unambiguous comparison of the STT efficiency in FC and AFC synthetic free layers. This was enabled by a single measurement of a time-averaged response of superparamagnetic MTJs as a function of the applied current, unlike previous studies that measure the energy barrier and critical switching current independently. Despite the distinct magnetic configurations, we find indistinguishable STT efficiency between the two structures. This result indicates that interlayer mutual torque provides a negligible contribution within the experimentally accessible parameter range, while the torque from the reference layer dominates the STT effect on the synthetic free layers.