Photo-induced reversible modification of the Curie-Weiss temperature in paramagnetic gadolinium compounds

Gadolinium oxyhydride GdHO is a photochromic material that darkens under illumination and bleaches back by thermal relaxation. As an inorganic photochromic material that can be easily deposited by magnetron sputtering, GdHO has very interesting potential applications as a functional material, specially for smart glazing applications. However, the underlying reasons behind the photochromic mechanism-which can be instrumental for the correct optimisation of GdOH for different applications-are not completely understood. In this paper, we rely on the well-stablished magnetic properties of Gd$^{3+}$ to shed light on this matter. GdOH thin films present paramagnetic behaviour similar to other Gd$^{3+}$ compounds such as Gd$_2$O$_3$. Illumination of the films result in a reversible increase of the Curie-Weiss temperature pointing to RKKY interactions, which is consistent with the resistivity decrease observed in the photo-darkened films.