Magnetic exchange hardening in polycrystalline GdN thin films.

Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, UK.
Journal of Physics Condensed Matter (Impact Factor: 2.22). 08/2010; 22(30):302003. DOI: 10.1088/0953-8984/22/30/302003
Source: PubMed

ABSTRACT We report the observation of intrinsic exchange hardening in polycrystalline GdN thin films grown at room temperature by magnetron sputtering. We find, in addition to the ferromagnetic phase, that a fraction of GdN crystallizes in a structural polymorphic form which orders antiferromagnetically. The relative fraction of these two phases was controlled by varying the relative abundance of reactive species in the sputtering plasma by means of the sputtering power and N(2) partial pressure. An exchange bias of ∼ 30 Oe was observed at 10 K. The exchange coupling between the ferromagnetic and the antiferromagnetic phases resulted in an order of magnitude enhancement in the coercive field in these films.

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    ABSTRACT: 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 (Tc) of less than ~60 K showed low coercive fields, a significant increase in the hysteretic loss was observed for samples with Tc≳ 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 Tc for GdN. We believe that the origin of the antiferromagnetic phase and the enhanced Tc 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.
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