Correlation between the direction of exchange bias and spin transfer torque in metallic antiferromagnets/Permalloy bilayers

Recent work shows that magnetic precession in a ferromagnet (FM) can be excited by spin transfer torque arising from spin Hall effect in metallic antiferromagnets (AF) [1]. However, in these measurements the AF was separated from the FM by a thin copper layer to avoid direct exchange coupling. Here we investigate spin transfer torques in exchange biased systems where a hysteresis loop is shifted relative to the zero-field position due to the exchange interaction between AF and FM [2]. We use spin torque ferromagnetic resonance measurement on AFs/Py (Ni$_{\mathrm{80}}$Fe$_{\mathrm{20}})$ bilayers in order to investigate the effect of exchange bias (EB) on spin transfer torques. For that purpose, we perform field-cooling experiments with an applied field parallel and perpendicular to the sample plane and compare the resultant spin Hall conductivities. Interestingly, we observed a unidirectional behavior, where the current induced torques are inequivalent for opposite field directions. [1] W.~Zhang \textit{et al}., Phys. Rev. B \textbf{92}, 144405 (2015). [2] J.~Sklenar \textit{et al}., AIP Adv. \textbf{6}, 055603 (2016).