Exchange engineering in a ferromagnetic semiconductor

The rare-earth nitrides are ferromagnetic semiconductors whose electronic properties can be tuned by nitrogen vacancy doping, while magnetic properties are selectable by the choice of lanthanide cation. Here, we demonstrate continuous control of the exchange energy in epitaxial thin films of ${\mathrm{Gd}}_{x}{\mathrm{Lu}}_{1\ensuremath{-}x}\mathrm{N}$ via cation substitution. Magnetometry shows that the Curie temperature and the effective exchange decrease with increasing Lu content, yet remain robust enough to facilitate ferromagnetic alignment down to compositions as low as $x=0.2$. First-principles calculations combined with Monte-Carlo simulations reproduce the composition dependence of the Curie temperature, and further indicate two exchange channels: an intrinsic mechanism present in stoichiometric films and an additional carrier-mediated contribution activated by nitrogen vacancies. These results establish (Gd,Lu)N as a material with a finely tunable exchange energy and, as such, a weak ferromagnetic semiconductor, compatible with thin-film growth, relevant for application in high-resistance magnetic Josephson junctions.