Enhanced Optical Band Gap in Ultrathin Epitaxial Ferromagnetic Insulator GdN Films*

Full understanding of the optical and magnetic properties of insulating ferromagnetic GdN epitaxial ultrathin films would enable proximity coupling it to Graphene and 2D layer systems. Moreover, exchange split band of GdN acting as a spin-filter would offer the ideal means for spin polarized current injection in the above systems as well as into superconductors. With this objective we investigated the intrinsic magnetic and optical behavior of GdN films reactively rf sputtered in a UHV system under various conditions to optimize the high quality of films and reduce their defects. This in turn can enable GdN to achieve high spin filtering and the needed effective interfacial exchange interaction for 2D systems. In the range of film thickness (2 to 300nm, protected by AlN film) the dependence of direct and indirect band-gap energies as well as the ferromagnetic ordering temperature and magnetic moment was studied: optical gap increased while the magnetic moment reduced with film thickness reduction.