Orbital Torque in Rare-Earth Transition-Metal Ferrimagnets

Orbital currents have recently emerged as a promising tool to achieve electrical control of the magnetization in thin-film ferromagnets. Efficient orbital-to-spin conversion is required in order to torque the magnetization. Here, we show that the injection of an orbital current in a ferrimagnetic ${\mathrm{Gd}}_{\mathrm{y}}{\mathrm{Co}}_{100\ensuremath{-}\mathrm{y}}$ alloy generates strong orbital torques whose sign and magnitude can be tuned by changing the Gd content and temperature. The effective spin-orbital Hall angle reaches up to $\ensuremath{-}0.25$ in a ${\mathrm{Gd}}_{\mathrm{y}}{\mathrm{Co}}_{100\ensuremath{-}\mathrm{y}}/{\mathrm{CuO}}_{\mathrm{x}}$ bilayer compared to $+0.03$ in $\mathrm{Co}/{\mathrm{CuO}}_{\mathrm{x}}$ and $+0.13$ in ${\mathrm{Gd}}_{\mathrm{y}}{\mathrm{Co}}_{100\ensuremath{-}\mathrm{y}}/\mathrm{Pt}$. This behavior is attributed to the local orbital-to-spin conversion taking place at the Gd sites, which is about 5 times stronger and of the opposite sign relative to Co. Furthermore, we observe a manyfold increase in the net orbital torque at low temperature, which we attribute to the improved conversion efficiency following the magnetic ordering of the Gd and Co sublattices.