Abstract
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.
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