Strong magnetoresistance modulation by Ir insertion in a Ta/Ir/CoFeB trilayer

The spin-orbit torque (SOT) switching of trilayers with two heavy-metal layers on the same side of the ferromagnetic metal layer was studied, realizing tunable spin Hall angle, domain-wall motion, and Dzyaloshinskii-Moriya interaction. However, systematic research on the magnetoresistance in such structures is still lacking. In this work, we investigate the anisotropic magnetoresistance (AMR) and the spin Hall magnetoresistance (SMR) by inserting an ultrathin Ir layer (${t}_{\mathrm{Ir}}\ensuremath{\le}1.4\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$) into Ta/CoFeB. The Ir layer with the thickness larger than 0.4 nm can transform the AMR from positive to negative, which is attributed to the electronic structure of Ir. This process realizes the interfacial modulation of AMR, which is generally considered as a bulk property. The SMR ratio decreases first and then increases with increasing Ir thickness, producing the minimum and maximum at ${t}_{\mathrm{Ir}}=0.3\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ and ${t}_{\mathrm{Ir}}=0.9\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, respectively, which reflects the ultrasmall spin-diffusion length (0.5 nm) and strong spin-memory loss in Ir. Further analyses combined with the SOT switching measurements unravel the existence of the anomalous Hall magnetoresistance, implying the non-negligible spin accumulation due to the anomalous Hall effect of the ferromagnetic metal. The combination of a large negative AMR and comparatively smaller SMR results in a negative planar Hall resistance. Our findings enrich the understanding of the magnetoresistances of heavy-metal/ferromagnetic metal trilayer systems.