Comparative x-ray absorption spectroscopy study of Co-doped ...

JOURNAL OF APPLIED PHYSICS VOLUME 95, NUMBER 11 1 JUNE 2004
Comparative x-ray absorption spectroscopy study of Co-doped
SnO 2 and TiO 2
A. Lussier, a) J. Dvorak, and Y. U. Idzerda
Department of Physics, Montana State University, Bozeman, Montana 59717
S. B. Ogale, S. R. Shinde, R. J. Choudary, and T. Venkatesan
Department of Physics, Center for Superconductivity Research, University of Maryland, College Park,
Maryland 20742-4111
Presented on 8 January 2004
We performed x-ray absorption spectroscopy measurements at the cobalt L 2,3 edge and the oxygen
K edge of Co-doped SnO 2 and Co-doped TiO 2 . Our measurements confirm that doped cobalt atoms
are in the same local environment in both compounds. Furthermore, the results support the idea that
cobalt atoms occupy substitutional cation sites. Additionally, the oxygen spectral shapes offer
insight into a possible cause for the observed giant magnetic moment of cobalt atoms present in
SnO 2 , but not in TiO 2 . © 2004 American Institute of Physics. DOI: 10.1063/1.1688655
Ferromagnetic semiconductors, if successfully synthesized,
could contribute significantly to the field of
spintronics. 1 Early and ongoing efforts with diluted magnetic
impurities in III–V semiconductors produced successful ferromagnetic
semiconductors at low temperatures. 2 In 2001,
Matsumoto et al. reported on room temperature ferromagnetism
in cobalt doped anatase TiO 2 . 3 Since then, this material
has been studied extensively by several groups. In particular,
x-ray absorption spectroscopy XAS measurements
on cobalt doped TiO 2 by Chambers et al. 4 shed light on the
cobalt atoms’ valence state and provide a basis for comparison
with our spectra. The origin of ferromagnetism in this
compound is still debated, with some authors claiming cobalt
clustering, 5 and others claiming full incorporation of cobalt
atoms in the lattice. 6 Additional insight into the ferromagnetism
and magnetic dopant arrangement in the host lattice
can be gained by comparing element-specific information
from systems that are structurally related. To investigate
these structural similarities, we present XAS measurement
on Co-doped SnO 2 and TiO 2 films taken at the cobalt L 2,3
edges and the oxygen K edge.
Undoped SnO 2 is considered ‘‘the’’ prototype transparent
conductor. 7 It has high metallic conductivity and is optically
transparent in the visible range. This makes SnO 2 with
its alloy with In 2 O 3 ) technologically useful as a transparent
electrical contact in such devices as flat-panel displays 8 and
solar cells. 9 The conductivity has been explained theoretically
in terms of high structural nonstoichiometry due to
small defect formation energies of interstitial tin, and oxygen
vacancies. 10 Experimental results on dilute magnetic semiconductors,
in particular Co-doped TiO 2 , by other
authors 11,12 suggest that magnetic interactions in those materials
are charge carrier mediated. In light of this, tin dioxide’s
relatively high carrier concentration 9 makes it a promising
candidate as a ferromagnetic semiconductor.
a Author to whom correspondence should be addressed; electronic mail:
lussier@physics.montana.edu
The Co x Sn 1x O 2 samples were grown by pulsed laser
ablation, with thicknesses of approximately 1500 Å, on
R-plane sapphire substrates. The cobalt doping levels were
x5% and 8%, and stands for oxygen vacancies. More
details on growth can be found elsewhere. 13 The
Co 0.07 Ti 0.93 O 2 samples were grown by pulsed laser deposition
with similar thicknesses on STO or LAO substrates.
Again, sample growth details can be found elsewhere. 6 The
XAS measurements were carried out at the MSU Materials
X-ray Characterization Facility located on beamline U4B of
the National Synchrotron Light Source.
The cobalt XAS spectra, taken in total electron yield
mode with linear polarized light with 0.4 eV energy resolution
for small concentrations of cobalt in SnO 2 , are similar
to those of cobalt in TiO 2 as can be seen in Fig. 1. This
spectral shape is known not to be that of metallic cobalt,
FIG. 1. The XAS cobalt L 2,3 edge spectra for Co-doped SnO 2 and TiO 2 .
The great similarity in features offers evidence for cobalt atoms occupying
oxygen octahedral coordinated cation sites in both compounds. Below the
5% and 8% curves are shown the difference curves, for which the anatase
7% Co-doped TiO 2 spectrum has been subtracted.
0021-8979/2004/95(11)/7190/2/$22.00 7190
© 2004 American Institute of Physics
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J. Appl. Phys., Vol. 95, No. 11, Part 2, 1 June 2004 Lussier et al.
7191
FIG. 2. The XAS oxygen K edge spectra of Co-doped SnO 2 and TiO 2 .The
t 2g band in the SnO 2 samples is completely suppressed. The t 2g directionality
is demonstrated by the feature suppression with changing photon incidence
direction.
which precludes extensive metallic clustering, at least to our
probing depth of a few hundred angstroms. The cobalt L 2,3
edge spectra of the Co-doped SnO 2 , and anatase TiO 2
samples all share the same general features with only very
slight differences in relative peak intensities. The strong
similarities in the spectra can be seen from the difference
spectra between the SnO 2 and TiO 2 spectra included on Fig.
1. The similarity we observe in XAS spectral shapes is indicative
of a similar local environment structural and chemical
and oxidation state for cobalt atoms, regardless of
whether they are doped in TiO 2 or SnO 2 . The cation substitutional
sites in both compounds have the same distorted
oxygen octahedral coordination 14,15 suggesting that cobalt atoms
sit at the substitutional site, in agreement with cobalt
K-edge XAS of Co-doped TiO 2 samples by Chambers
et al. 16 Our L-edge spectra indicate that cobalt atoms are
most likely substitutional in SnO 2 also.
A marked difference is that cobalt doped in SnO 2 leads
to giant magnetic moments of the cobalt atoms, 11 which is
not observed for cobalt in TiO 2 . Although the cobalt atoms
have analogous nearest neighbor coordination in both compounds,
we detect an important difference in their magnetic
behavior. The oxygen spectra hint to a possible explanation.
To reiterate, the ferromagnetism in these dilute compounds
is generally accepted to be charge carrier mediated.
Additionally, the charge carrier density is determined by
oxygen vacancies, 17 and therefore closely associated with
oxygen. If the electronic states of oxygen in TiO 2 and SnO 2 ,
which are revealed in the XAS spectra, are significantly different,
we might expect the magnetic properties of the compound
will also be different. As can be seen in Fig. 2, although
the cobalt spectra are similar, the oxygen spectra are
dramatically different. Note that oxygen atoms in TiO 2 or
SnO 2 have only about 5% of their cation neighbors replaced
by cobalt. The XAS spectra of oxygen are therefore virtually
unaffected by the cobalt, and the obvious spectral differences
reflect the different properties of the host lattice. The first
peak at 530.5 eV in the TiO 2 oxygen spectrum represents the
t 2g band. It is a sharp feature related to directional bonds
and is completely suppressed in SnO 2 . The bond directionality
is made evident by comparison of the spectra taken on
a Co-doped TiO 2 sample with the photon incident beam normal
to the surface, or at 60° with respect to the sample
normal. The t 2g feature is partly suppressed in the latter case.
These differences indicate differences in the host oxide’s
electronic properties, which would in turn affect the carrier
mediated magnetic interactions and possibly lead to giant
cobalt magnetic moments in SnO 2 . Among the proposed explanations
for the giant magnetic moment is unquenching of
the cobalt orbital moments 11 and our observation of the electronic
variations for oxygen could be related to quenching.
Another possible explanation lies in spin transfer on neighboring
Sn atoms, a possibility that we are currently investigating.
To summarize, our cobalt L 2,3 XAS measurements allowed
us to demonstrate that doped cobalt atoms are in the
same local environment in SnO 2 as in TiO 2 . These measurements
also corroborate other published results that show cobalt
atoms occupy substitutional titanium sites in TiO 2 . Additionally,
XAS measurements at the oxygen K edge reveal
vastly different oxygen electronic states in the two compounds.
In light of the role played by oxygen vacancies in
charge carrier generation, and by virtue of the proposed role
played by charge carriers in the magnetic behavior of these
materials, the differing oxygen states suggest an explanation
for the giant magnetic moment observed in SnO 2 but absent
in TiO 2 .
This work is supported by the National Science Foundation
and the National Synchrotron Light Source is supported
by the Department of Energy. Additionally, we would like to
acknowledge support under NSF-MRSEC Grant No. DMR
00-80008 and DARPA SpinS Grant No. N000140210962.
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