Origin of the absent damping-like torque sign reversal

Determine the physical mechanism responsible for the unexplained lack of inversion symmetry that causes the damping-like torque in Cr(10)/Gd(t_Gd)/Co(3) and Co(3)/Gd(t_Gd)/Cr(10) heterostructures not to reverse sign when the stack order is inverted.

Background

The paper reports that, unlike the field-like torque, the damping-like torque does not reverse sign when the Cr/Gd/Co stack is inverted to Co/Gd/Cr. The authors rule out a large self-torque from the Co/Gd pair and note that the injected orbital polarization reverses under stack inversion. They therefore identify the absence of the expected damping-like torque sign reversal as an unexplained symmetry property of the heterostructures. A possible explanation involving an interface-normal-dependent orbital-to-spin conversion coefficient is proposed, but the mechanism is not established.