Bulk versus interface origin of spin-orbit torques
In plain words
A current in a heavy metal like platinum or tungsten can flip an adjacent magnetic layer through spin-orbit torque. Whether the torque comes mostly from spin currents generated inside the metal or from effects at the interface is disputed.
Precise statement
In heavy-metal/ferromagnet bilayers (Pt/Co, W/CoFeB, Ta/CoFeB), damping-like and field-like torque efficiencies depend on metal thickness, interface quality and temperature. Quantify the contributions of the bulk spin Hall effect, interfacial Rashba-Edelstein effect, interface spin-orbit scattering (spin memory loss) and orbital currents, e.g. by a first-principles transport calculation with interface disorder matching the thickness dependence of the damping-like efficiency in Pt/Co. An answer is a percentage partition with uncertainty for benchmark bilayers.
What would settle it
First-principles nonequilibrium transport calculations with realistic interfaces that reproduce measured torque efficiencies versus thickness and temperature in at least two bilayer systems.