Antiferromagnetic long-range spin ordering in Fe- and NiFe2-doped BaTiO3 multiferroic layers

We report on the Fe doping and on the comparative Ni-Fe codoping with composition close to NiFe2 of fully oxidized BaTiO3 layers (similar to 20 nm) elaborated by atomic oxygen plasma assisted molecular beam epitaxy; specifically any role of oxygen vacancies can be excluded in our films. Additionally to the classical in situ laboratory tools, the films were thoroughly characterized by synchrotron radiation x-ray diffraction and x-ray absorption spectroscopy. For purely Fe-doped layers, the native tetragonal perovskite structure evolves rapidly toward cubiclike up to 5% doping level above which the crystalline order disappears. On the contrary, low codoping levels (similar to 5% NiFe2) fairly improve the thin film crystalline structure and surface smoothness; high levels (similar to 27%) lead to more crystallographically disordered films, although the tetragonal structure is preserved. Synchrotron radiation magnetic dichroic measurements reveal that metal clustering does not occur, that the Fe valence evolves from Fe2+ for low Fe doping levels to Fe3+ for high doping levels, and that the introduction of Ni favors the occurrence of the Fe2+ valence in the films. For the lower codoping levels it seems that Fe2+ substitutes Ba2+, whereas Ni2+ always substitutes Ti4+. Ferromagnetic long-range ordering can be excluded with great sensitivity in all samples as deduced from our x-ray magnetic absorption circular dichroic measurements. On the contrary, our linear dichroic x-ray absorption results support antiferromagnetic long-range ordering while piezoforce microscopy gives evidence of a robust ferroelectric long-range ordering showing that our films are excellent candidates for magnetic exchange coupled multiferroic applications.