Long spin coherence length and bulk-like spin-orbit torque in ferrimagnetic multilayers

Spintronics relies on magnetization switching through current-induced spin torques. However, because spin transfer torque for ferromagnets is a surface torque, a large switching current is required for a thick, thermally stable ferromagnetic cell, and this remains a fundamental obstacle for high-density non-volatile applications with ferromagnets. Here, we report a long spin coherence length and associated bulk-like torque characteristics in an antiferromagnetically coupled ferrimagnetic multilayer. We find that a transverse spin current can pass through >10-nm-thick ferrimagnetic Co/Tb multilayers, whereas it is entirely absorbed by a 1-nm-thick ferromagnetic Co/Ni multilayer. We also find that the switching efficiency of Co/Tb multilayers partially reflects a bulk-like torque characteristic, as it increases with ferrimagnet thickness up to 8 nm and then decreases, in clear contrast to the 1/thickness dependence of ferromagnetic Co/Ni multilayers. Our results on antiferromagnetically coupled systems will invigorate research towards the development of energy-efficient spintronics.