Hydration of gadolinium oxide (GdOx) and its effect on voltage-induced Co oxidation in a Pt/Co/GdOx/Au heterostructure

Magneto-ionic control of magnetism has garnered great interest in recent years due to the large magnetic changes that can be induced using a relatively small voltage. One model structure for this is $\mathrm{Pt}/\mathrm{Co}/\mathrm{Gd}{\mathrm{O}}_{x}/\mathrm{Au}$, where Co is the magnetic layer and $\mathrm{Gd}{\mathrm{O}}_{x}$ is the ionic conductor, with the magnetic properties dependent on the oxidation state of Co. While this structure is commonly used, there is limited understanding of the effect of $\mathrm{Gd}{\mathrm{O}}_{x}$ properties on voltage-induced magnetic changes. In this work, we show that hydration of $\mathrm{G}{\mathrm{d}}_{2}{\mathrm{O}}_{3}$ to form $\mathrm{Gd}{(\mathrm{OH})}_{3}$ is crucial for voltage-induced Co oxidation in a $\mathrm{Pt}/\mathrm{Co}/\mathrm{Gd}{\mathrm{O}}_{x}/\mathrm{Au}$ device. By examining the rate of Co oxidation in nonhydrated and hydrated devices, we conclude that ${\mathrm{H}}_{2}\mathrm{O}$ in the $\mathrm{Gd}{\mathrm{O}}_{x}$ layer acts as an oxidant during the voltage-induced Co oxidation process. Co oxidation through this interfacial reaction process is confirmed by in situ x-ray absorption spectroscopy.