Ferromagnetism in nanocrystalline epitaxial Co: TiO2 thin films

Article abstract of DOI:10.1063/1.1944209

We report on the observation of remarkable room-temperature ferromagnetism in nanocrystalline epitaxial Co: TiO2 films grown on sapphire (0001) substrates by a pulsed-laser deposition technique using high-density targets. The films were characterized by x-ray measurements, atomic force microscopy, micro-Raman, electron-paramagnetic resonance, and magnetization studies. The films exhibit three-dimensional islandlike growth that contains nanocrystalline particles. Our experimental results suggest that the remarkable ferromagnetism in Co: TiO2 films is controlled either by the interstitial Co2+ ions or small clusters, which are mainly present at the interface and on the surface of the films. Our work clearly indicates that Co interstitials and nanoclusters cause room-temperature ferromagnetism in Co-doped TiO2.

Full text of DOI:10.1063/1.1944209

Ferromagnetism in nanocrystalline epitaxial Co: TiO 2 thin films
A. K. Pradhan, D. Hunter, J. B. Dadson, T. M. Williams, Kai Zhang, K. Lord, B. Lasley, R. R. Rakhimov, Jun
Zhang, D. J. Sellmyer, U. N. Roy, Y. Cui, A. Burger, C. Hopkins, N. Pearson, and A. L. Wilkerson
Citation: Applied Physics Letters 86, 222503 (2005); doi: 10.1063/1.1944209
View online: http://dx.doi.org/10.1063/1.1944209
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APPLIED PHYSICS LETTERS 86, 222503 ͑2005͒
Ferromagnetism in nanocrystalline epitaxial Co:TiO2 thin films
A. K. Pradhan,a͒ D. Hunter, J. B. Dadson, T. M. Williams, Kai Zhang, K. Lord,
B. Lasley, and R. R. Rakhimov
Center for Materials Research, Norfolk State University, 700 Park Avenue, Norfolk, Virginia 23504
Jun Zhang and D. J. Sellmyer
Department of Physics and Astronomy and Center for Materials Research and Analysis, University of
Nebraska, Lincoln, Nebraska 68588-0113
U. N. Roy, Y. Cui, and A. Burger
Department of Physics, Fisk University, 1000, 17 Avenue North, Nashville, Tennessee 37208
C. Hopkins, N. Pearson, and A. L. Wilkerson
College of William and Mary, Applied Research Center, 12050 Jefferson Ave. Suite 713, Newport News,
Virgina 23606
͑Received 21 February 2005; accepted 28 April 2005; published online 25 May 2005͒
We report on the observation of remarkable room-temperature ferromagnetism in nanocrystalline
epitaxial Co:TiO2 films grown on sapphire ͑0001͒ substrates by a pulsed-laser deposition technique
using high-density targets. The films were characterized by x-ray measurements, atomic force
microscopy, micro-Raman, electron-paramagnetic resonance, and magnetization studies. The films
exhibit three-dimensional islandlike growth that contains nanocrystalline particles. Our
experimental results suggest that the remarkable ferromagnetism in Co:TiO2 films is controlled
either by the interstitial Co2+ ions or small clusters, which are mainly present at the interface and on
the surface of the films. Our work clearly indicates that Co interstitials and nanoclusters cause
room-temperature ferromagnetism in Co-doped TiO2. © 2005 American Institute of Physics.
͓DOI: 10.1063/1.1944209͔
Recent advances in the emerging field of diluted mag-
netic semiconductors ͑DMS͒ have attracted widespread at-
tention due to their potential applications in the rapidly de-
veloping field of spintronics.1–4 The search for integrating
the semiconductors with the magnetic properties in the same
material has become a prerequisite for successful fabrication
of potential devices of shrinking dimension, such as high-
performance read-heads, nonvolatile memories, and other
state-of-the-art storage devices.5,6 Hence, the discovery of
DMS exhibiting room temperature ferromagnetism ͑FM͒ has
established an important achievement for possible applica-
tions of functional spintronics.7 Although there have been
many advances in incorporating magnetic ions into host
semiconductors, particularly wide band gap III–V and II–VI
based DMS materials, applications were possible only at low
temperatures in such materials due to their low magnetic
transition temperature, Tc. Among DMS materials, Co-doped
TiO2 ͑Co:TiO2 or CTO͒ has received interest8,9 because it is
a transparent semiconductor. However, the origin of the fer-
romagnetic properties, particularly a possibility arising either
from the Co clusters9,10 or intrinsic phenomenon11 of Co sub-
stitution in Ti lattice, and the ionic state of Co remain am-
biguous.
ever, one of the most important issues is that Co-doped TiO2
could contribute significantly to the field of spintronics as a
potential DMS, if successfully synthesized, exhibiting dilute
magnetic nature. Here we report the growth of Co:TiO2 films
containing nanoparticles ordered to form epitaxial films on
Al2O3 substrates. The films demonstrate remarkable struc-
tural, microscopic, ferromagnetic, and spectroscopic proper-
ties at room temperature. The results demonstrate that Co can
substitute Ti, and the predominant valence state is Co2+.
Co:TiO2–Sapphire͑0001͒ epitaxial films were grown by
the PLD technique ͑KrF excimer, ␭=248 nm, laser repetition
rate of 2–5 Hz͒ with a pulse energy density of 1 to 2 J/cm2
and utilizing both target and substrate rotation facilities.
High-density Ti1−xCoxO ͑CTO͒ targets were used. Stoichio-
metric amount of TiO2 and CoO ͑both 99.99% purity͒ pow-
ders were mixed, calcined at 700 °C for 12 h followed by
isostatic pressing at 400 MPa, and finally sintered at 1000 °C
in order to make high-density target. The films were depos-
ited with a substrate temperature Ts=700–800 °C, keeping
oxygen partial pressure PO2=1ϫ10−5 Torr. Clean single
crystalline sapphire substrates were loaded to the chamber
and heated just after the ultimate base pressure Ͻ4
ϫ10−8 Torr is reached. The film thickness is about 57 nm.
The x-ray diffraction ͑XRD͒ of the films was done in a
Rigaku x-ray diffractometer using Cu K␣ radiation. The Ra-
man spectra were recorded using a LabRam micro-Raman
spectrometer with He–Ne laser excitation ͑wavelength: 632.8
nm͒. The ferromagnetic resonance ͑FMR͒ experiments were
done on a Bruker EMX spectrometer operating at 9.6 GHz
microwave frequency. Quantum Design Superconducting
Quantum Interference Device ͑SQUID- MPMS͒ was used to
measure the magnetization of films.
Several techniques have been employed to fabricate ep-
itaxial Co:TiO2 films using oxide-based substrates, such as
SrTiO3, LaAlO3 ͑LAO͒ for anatase,11,12 and Al2O3 for rutile
phase stabilization,13,14 which show ferromagnetism at room
temperature. The origin of ferromagnetism in this DMS is
still debated due to possibilities of Co clustering as described
above, and incorporation of Co atoms in the lattice. How-
a͒
Author to whom correspondence should be addressed; electronic mail:
apradhan@nsu.edu
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0003-6951/2005/86͑22͒/222503/3/$22.50 86, 222503-1 © 2005 American Institute of Physics
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222503-2
Pradhan et al.
Appl. Phys. Lett. 86, 222503 ͑2005͒
FIG. 2. ͑Color online͒ AFM pictures of as-grown Co:TiO2 films from tar-
gets containing ͑a͒ 6 at. %, ͑b͒ 10 at. %, ͑c͒ 12 at. %, and ͑d͒ 6 at. % of Co.
All scans from ͑a͒ to ͑c͒ are 2 ␮mϫ2 ␮m in dimension. The image in ͑d͒
shows the 3D image over 1.5 ␮mϫ1.5 ␮m area.
FIG. 1. ͑Color online͒ X-ray diffraction pattern of Co:TiO2 films grown on
sapphire and LaAlO3 substrates using targets containing different concentra-
tion of Co. The inset shows the Co depth profile of CTO-2 from secondary
ion mass spectroscopy.
Figure 1 shows the XRD patterns of CTO–Sapphire
͑0001͒ films at a constant PO2 using three different targets
with Co concentration varying from 6 to 12 at. % designated
6 at. % as CTO-1, 10 at. % as CTO-2, and 12 at. % as
CTO-3. The XRD patterns for all films grown on sapphire
͑0001͒ reveal one dominating plane ͑002͒ of the rutile phase,
illustrating the epitaxial nature of the film and rutile phase
stabilization. This undoubtedly indicates the preferential ori-
entation of the crystal plane in parallel with the substrate
surface. The rocking curves for the epitaxial growth of the
films show that the full-width half-maximum ͑FWHM͒ cal-
culated from x-ray ͑200͒ line broadening increase from 0.3°
to 0.35° with increasing Co concentration in the target at the
same growth conditions, illustrating the poor crystalline
quality of the film for higher concentration Co. It is interest-
ing to note that the peak position for the higher concentration
of Co shifts to lower angle. However, interestingly, a small
but discernable peak at about 44.5° was observed, particu-
larly in CTO-2 film. This is certainly due to Co ͑002͒ reflec-
tion preferably oriented in the plane ͑c axis͒ either in a small
cluster form or as interstitials. On the other hand, the film
grown on LAO substrate exhibits only one dominating peak
͑004͒, illustrating the presence of only anatase phase. No
evidence of Co clusters or Co intermetallic compounds was
observed in films deposited on LAO substrate. The c axis
parameter of anatase as deduced from the ͑004͒ reflection is
about 0.9500 nm for CTO–LAO, which suggests that Co
ions have been incorporated into the anatase structure substi-
tuting Ti ions in the lattice. Furthermore, the unit-cell elon-
gation along the c axis strongly suggests that mostly Co is in
ϩ2 state whose atomic radius ͑0.072 nm͒ is larger than Ti4+,
which is in agreement with a recent report.15 In view of the
above, the slight shift of rutile ͑002͒ peak in CTO–sapphire
to higher angle indicates the fact that the substitutional effect
is also present in CTO–sapphire when grown using targets
containing different concentration of Co.
center for the formation Co clusters. On the other hand, it is
noted that the x-ray mapping of CTO–sapphire films over
20 ␮mϫ20 ␮m area on the surface of the film clearly illus-
trates that Co is uniformly distributed in the TiO2 matrix.
The atomic force microscopy ͑AFM͒ images of as-
grown Co:TiO2 films are shown in Figs. 2͑a͒–2͑d͒. The AFM
images show remarkable nanocrystalline features in Figs.
2͑a͒ and 2͑c͒ for CTO-1 and 2, respectively. The particles are
very uniform in size in both films. The particle size decreases
from 25 nm to less than 20 nm with increasing Co concen-
tration. However, it is interesting to note the self-assembly of
CTO oriented nanocrystals of size 35 nm for CTO-2 film as
shown in Fig. 2͑b͒. On the other hand, CTO-1 grown with a
slightly higher repetition rate ͑7 Hz͒ shows a distinct three-
dimensional ͑3D͒ islandlike growth feature with average par-
ticle size of 50 nm as shown in Fig. 2͑d͒ over a 1.5 ␮m
ϫ1.5 ␮m area. The above results show that the Co concen-
tration in the target affects the microstructure of the film,
although other growth parameters, which are kept constant
for the present experiment, may also have an effect on this.
In Fig. 3, we show the Raman spectra of CTO films and
the sapphire ͑0001͒ substrate. Besides the Raman bands at
350, 421, 433, and 583 cm−1, which are due to sapphire sub-
strates, two shoulder signals appeared at 442 and 610 cm−1
due to rutile phase. The Raman spectra show that the struc-
ture is predominantly rutile and highly crystalline. A broad
peak observed at 680 cm−1 is believed to be additional fea-
In order to study the variation of Co density as a func-
tion of film thickness ͑co-depth profile͒, we carried out sec-
ondary mass ion spectroscopy in CTO-2 and the Co depth
profile is presented in the inset of Fig. 1. It is clear that the
density of Co is much higher at the surface and decreases as
the film thickness decreases. However, surprisingly, the co
density suddenly increases at the interface ͑ϳ57 nm͒ where
a sharp jump in Al depth profile starts, illustrating that Co
FIG. 3. ͑Color online͒ Raman spectra of Co:TiO2 film synthesized from the
target containing different Co concentration. The spectrum for the substrate
͑sapphire͒ is also shown. The inset shows the FMR spectra of Co:TiO2 film
synthesized from the target containing 10 at. % of Co at various
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diffuses into the substrates and probably acts as a nucleation
temperatures.
130.216.129.208 On: Mon, 08 Dec 2014 21:10:19

222503-3
Pradhan et al.
Appl. Phys. Lett. 86, 222503 ͑2005͒
In view of the above, the origin of the ferromagnetism in
CTO films can be attributed to the presence of Co2+ ions that
are either in the interstitial positions or in the form of a small
metal clusters. This is also consistent with the recent report
of channeling experiments.16 However, the ferromagnetic ex-
change mechanism between Co2+–O–Co3+, giving rise to
ferromagnetism15 in this system seems to be totally implau-
sible in the absence of Co3+ ions. The dispersed Co clusters
or interstitial site occupations show a true diluted magnetic
nature and can be a potentially significance to the field of
spintronics.
In conclusion, we have demonstrated that the epitaxial
Co:TiO2 films grown on sapphire substrates show remark-
able surface morphology comprising of nanocrystalline par-
ticles from a 3D islandlike growth. The studies from x-ray,
magnetic and spectroscopic measurements suggest that the
ferromagnetism observed in Co:TiO2 film is controlled either
by the interstitial Co2+ ions or small clusters. The Co depth
profile clearly indicates the presence of Co clusters at the
interface and on the surface which influence the ferromag-
netism significantly. Our results are significantly important
for fabricating the Co:TiO2 films for possible spintronic ap-
plications.
FIG. 4. ͑Color online͒ Magnetic field dependence of field-cooled ͑FC͒ and
zero-field-cooled ͑ZFC͒ curves of CTO-2 in a field of 200 G. The inset
shows the ferromagnetic hysteresis loops of CTO-2 and CTO-3 films at
300 K.
ture in the rutile structure and may be related to the Co
interstitials or small clusters. The intensity of the broad peak
is much pronounced for CTO-2. Hence, the coincidence of
the presence of this peak and the observed peak due to Co in
x-ray reflection illustrate the presence of either interstitials or
small clusters due to Co. The most probable presence is ei-
ther Co interstitials or very small Co clusters of nm scale,
which is consistent with the recent observation9 of Co nano-
clusters of about 10 nm size in the anatase phase from trans-
mission electron microscopy studies.
FMR is one of the sensitive tools to study the presence
of magnetic ions. In order to infer the Co charge state, we
have studied the FMR spectra of CTO-2 ͑shown in the inset
in Fig. 3͒ at different temperatures, ranging from 100 to 200
K. The temperature independent electron paramagnetic reso-
nance ͑EPR͒ of the EPR cavity is seen at about 3.3 KG. At
100 K, the FMR spectra exhibit pronounced hump at around
2500 G. The hump feature, which is a strong function of
temperature, weakens as temperature is raised. This is a clear
signature of ferromagnetic Co2+ as Co3+ does not show up in
FMR spectra. The decrease of the FMR signal with increas-
ing temperature may be related to the presence of Co3+ ions
at higher temperature. This provides strong experimental evi-
dence in favor of the presence of Co2+ ionic state which is
responsible for the ferromagnetism in CTO films.
Figure 4 shows the field-cooled ͑FC͒ and zero-field-
cooled ͑ZFC͒ magnetization of the CTO-2 film, displaying a
ferromagnetic behavior. The FC and ZFC curves show
anomalous jump at about 100 K, illustrating a sudden change
in the spin structures, which is related to a metastable mag-
netic state. In order to confirm the ferromagnetic behavior,
magnetic field dependence of magnetization ͑MH͒ curves are
shown at 300 K for two representative CTO films, including
CTO-2. The CTO-2 film displays remarkable ferromagnetic
behavior at 300 K. The magnetic field at which the maxi-
mum in magnetization, Hm ͑low field to high field͒ achieved
is estimated to be 2 KG at 300 K. However, the room tem-
perature ferromagnetic hysteresis shrinks significantly for
CTO-3, which can be attributed due to the poor crystalline
quality of these films when synthesized from the target con-
taining higher concentration of Co as discussed earlier. It is
noted that the background magnetic contribution due to sap-
phire substrate has not been corrected. This will further in-
crease the Hm values. However, this may not affect the quali-
This work is supported by the NASA and NSF for Cen-
ter for Research Excellence in Science and Technology
͑CREST͒ Grant No. HRD-9805059. One of the authors
͑R.R.R͒ acknowledges the support from the faculty research
program provided by NASA. Research at the University of
Nebraska is supported by NSF-MRSEC, ONR, and CMRA.
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