  Ferromagnetic spintronics has been a main focus as it offers non-volatile
memory and logic applications through current-induced spin-transfer torques.
Enabling wider applications of such magnetic devices requires a lower switching
current for a smaller cell while keeping the thermal stability of magnetic
cells for non-volatility. As the cell size reduces, however, it becomes
extremely difficult to meet this requirement with ferromagnets because
spin-transfer torque for ferromagnets is a surface torque due to rapid spin
dephasing, leading to the 1/ferromagnet-thickness dependence of the spin-torque
efficiency. Requirement of a larger switching current for a thicker and thus
more thermally stable ferromagnetic cell is the fundamental obstacle for
high-density non-volatile applications with ferromagnets. Theories predicted
that antiferromagnets have a long spin coherence length due to the staggered
spin order on an atomic scale, thereby resolving the above fundamental
limitation. Despite several spin-torque experiments on antiferromagnets and
ferrimagnetic alloys, this prediction has remained unexplored. Here we report a
long spin coherence length and associated bulk-like-torque characteristic 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 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 the
ferrimagnet-thickness up to 8 nm and then decreases, in clear contrast to
1/thickness-dependence of Co/Ni multilayers. Our results on
antiferromagnetically coupled systems will invigorate researches towards
energy-efficient spintronic technologies.
