  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.
