  Spin-orbit torques (SOTs) have been widely understood as an interfacial
transfer of spin that is independent of the bulk properties of the magnetic
layer. Here, we report that SOTs acting on ferrimagnetic FexTb1-x layers
decrease and vanish upon approaching the magnetic compensation point because
the rate of spin transfer to the magnetization becomes slower than the rate of
spin relaxation into the crystal lattice due to spin-orbit scattering. These
results indicate that the relative rates of competing spin relaxation processes
within magnetic layers play a critical role in determining the strength of
SOTs, which provides a unified understanding for the diverse and even seemingly
puzzling SOT phenomena in ferromagnetic and compensated systems. Our work
indicates that spin-orbit scattering within the magnet should be minimized for
efficient SOT devices. We also find that the interfacial spin-mixing
conductance of interfaces of ferrimagnetic alloys (such as FexTb1-x) is as
large as that of 3d ferromagnets and insensitive to the degree of magnetic
compensation.
