Exchange coupling torque in ferrimagnetic Co/Gd bilayer maximized near angular momentum compensation temperature

Highly efficient current-induced motion of chiral domain walls was recently demonstrated in synthetic antiferromagnetic (SAF) structures due to an exchange coupling torque (ECT). The ECT derives from the antiferromagnetic exchange coupling through a ruthenium spacer layer between the two perpendicularly magnetized layers that comprise the SAF. Here we report that the same ECT mechanism applies to ferrimagnetic bi-layers formed from adjacent Co and Gd layers. In particular, we show that the ECT is maximized at the temperature T_A where the Co and Gd angular momenta balance each other, rather than at their magnetization compensation temperature T_M. The current induced velocity of the domain walls is highly sensitive to longitudinal magnetic fields but we show that this not the case near T_A. Our studies provide new insight into the ECT mechanism for ferrimagnetic systems. The high efficiency of the ECT makes it important for advanced domain wall based spintronic devices.