Mitigation of Gilbert Damping in the CoFe/CuO_<i>x</i> Orbital Torque System

Charge-spin interconversion processes underpin the generation of spin-orbit torques in magnetic/nonmagnetic bilayers. However, efficient sources of spin currents such as 5<i>d</i> metals are also efficient spin sinks, resulting in a large increase of magnetic damping. Here we show that a partially oxidized 3<i>d</i> metal can generate a strong orbital torque without a significant increase in damping. Measurements of the torque efficiency ξ and Gilbert damping α in CoFe/CuO_<i>x</i> and CoFe/Pt indicate that ξ is comparable in the two systems. The increase in damping relative to a single CoFe layer is Δα < 0.002 in CoFe/CuOx and Δα ≈ 0.005-0.02 in CoFe/Pt, depending on CoFe thickness. We ascribe the nonreciprocal relationship between Δα and ξ in CoFe/CuO_<i>x</i> to the small orbital-spin current ratio generated by magnetic resonance in CoFe and the lack of an efficient spin sink in CuO_<i>x</i>. Our findings provide new perspectives on the efficient excitation of magnetization dynamics via the orbital torque.