Article

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.

0 Bookmarks
 · 
78 Views
  • [Show abstract] [Hide abstract]
    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.
    Physical review. B, Condensed matter 01/2011; 83(1). · 3.77 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report an interplay between magnetism and charge transport in the ferromagnetic semiconductor GdN, pointing to the formation of magnetic polarons centred on nitrogen vacancies. The scenario goes some way to resolving a long-standing disagreement between the measured and predicted Curie temperature in GdN. It further constitutes an extension of concepts that relate closely to the behaviour of ferromagnetic semiconductors generally, and EuO in particular.
    Physical review. B, Condensed matter 10/2012; 87(3). · 3.77 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: AlN/GdN/AlN double heterostructures were grown on c-sapphire substrates using a reactive rf sputtering method under high vacuum conditions. The optical absorption spectrum of the GdN shows a clear fundamental band edge of GdN around 800 nm; this transition is attributed to the minority spin band energy of GdN at the X point. Nitrogen vacancy centers cause a blue-shift of the optical band edge of GdN, which could be ascribed to both the band filling, and the electron-hole interactions resulting from the free carriers generated by nitrogen vacancies. Temperature-dependent magnetization measurements demonstrate a clear change in the magnetization values of GdN with respect to the N2 partial pressure. Nitrogen vacancy centers in the thin GdN film raise the Curie temperature from 31 K to 39 K, which has been accurately measured by the Arrott plots.
    Physics of Condensed Matter 86(2). · 1.28 Impact Factor

Full-text (2 Sources)

Download
24 Downloads
Available from
Jun 6, 2014