Structural evolution and competing magnetic orders in polycrystalline GdN films

We report a study of the structure and magnetic behavior of polycrystalline GdN films grown at room temperature by reactive magnetron sputtering. By controlling the relative fraction of reactive species during film growth, we observe a continuous crossover from soft ferromagnetic films into relatively hard ferromagnetic films. While samples with a Curie temperature (T-c) of less than similar to 60 K showed low coercive fields, a significant increase in the hysteretic loss was observed for samples with T-c greater than or similar to 60 K. Accompanying the change in the magnetic behavior of the films, signatures of a secondary phase of GdN (GdN-II) were observed in x-ray diffraction measurements. Such dual-phase samples (with GdN and GdN-II) showed an exchange bias effect, which confirmed that the GdN-II phase was antiferromagnetic. The Curie temperatures of the dual-phase samples were found to be much higher than the reported value of T-c for GdN. We believe that the origin of the antiferromagnetic phase and the enhanced T-c of ferromagnetic GdN can be closely related to nitrogen vacancies in the samples. While the local strain induced by nitrogen vacancies can strengthen antiferromagnetic ordering in GdN-II, the change in carrier concentration due to the nitrogen vacancies strengthens the ferromagnetic ordering in the GdN phase. Hall effect measurements showed that transport properties of polycrystalline GdN films can be tuned from almost-insulating to semimetallic behavior by varying the amount of nitrogen in the samples. Amid a continuing debate on the origin of ferromagnetism in GdN, our data show considerable support for a carrier-mediated mechanism of ferromagnetism.