The
(In1–x
Co
x
)2O3 films with x =
(0.055, 0.08, 0.10, 0.15) have been prepared by a radio frequency
magnetron sputtering technique and investigated by X-ray diffraction,
X-ray photoelectron spectroscopy, X-ray absorption fine structure,
Hall effect, and room-temperature magnetic measurements. The detailed
structural analyses and full multiple-scattering ab initio calculations
indicate that most Co2+ ions substitute for In3+ sites of In2O3 lattice and form CoIn2+
+ VO complexes with the O vacancy in the
nearest coordination shell, whereas a portion of the Co atoms form
the precipitate of Co metal clusters for all the (In1–x
Co
x
)2O3 films. Despite the formation of Co clusters, magnetic characterizations
show that the saturated magnetization M
s of films first increases and then decreases with the increase of
Co concentration, suggesting that the small Co clusters are superparamagnetic.
The electronic conducting mechanism is dominated by Mott variable
range hopping behavior for all the films. The strong localization
of carriers suggests the bound magnetic polarons scenario. It can
be concluded that the observed room-temperature ferromagnetism in
the (In1–x
Co
x
)2O3 films is intrinsic and originates
from electrons bound in defect states associated with oxygen vacancies.
There exists an optimal localization radius ξ of variable range
hopping for achieving the largest M
s in
the (In1–x
Co
x
)2O3 films.
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