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

Burger, A.

Cui, Y.

Dadson, J.B.

Hopkins, C.

Hunter, D.

Lasley, B.

Lord, K.

Pearson, N.

Pradhan, Aswini K.

Rakhimov, Rakhim R.

Roy, U.N.

Sellmyer, David J.

Wilkerson, A.L.

Williams, T.M.

Zhang, Jun

Zhang, Kai

English

UNL | Libraries

University of Nebraska - Lincoln 
DigitalCommons@University of Nebraska - Lincoln 
David Sellmyer Publications Research Papers in Physics and Astronomy 
5-30-2005 
Ferromagnetism in nanocrystalline epitaxial Co:TiO2 thin films 
Aswini K. Pradhan 
Center for Materials Research, Norfolk State University, apradhan@nsu.edu 
D. Hunter 
Center for Materials Research, Norfolk State University, Norfolk, Virginia 
J.B. Dadson 
Center for Materials Research, Norfolk State University, Norfolk, Virginia 
T.M. Williams 
Center for Materials Research, Norfolk State University, Norfolk, Virginia 
Kai Zhang 
Center for Materials Research, Norfolk State University, Norfolk, Virginia 
See next page for additional authors 
Follow this and additional works at: https://digitalcommons.unl.edu/physicssellmyer 
 Part of the Physics Commons 
Pradhan, Aswini K.; Hunter, D.; Dadson, J.B.; Williams, T.M.; Zhang, Kai; Lord, K.; Lasley, B.; Rakhimov, 
Rakhim R.; Zhang, Jun; Sellmyer, David J.; Roy, U.N.; Cui, Y.; Burger, A.; Hopkins, C.; Pearson, N.; and 
Wilkerson, A.L., "Ferromagnetism in nanocrystalline epitaxial Co:TiO2 thin films" (2005). David Sellmyer 
Publications. 10. 
https://digitalcommons.unl.edu/physicssellmyer/10 
This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at 
DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in David Sellmyer Publications 
by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. 
Authors 
Aswini K. Pradhan, D. Hunter, J.B. Dadson, T.M. Williams, Kai Zhang, K. Lord, B. Lasley, Rakhim R. 
Rakhimov, Jun Zhang, David J. Sellmyer, U.N. Roy, Y. Cui, A. Burger, C. Hopkins, N. Pearson, and A.L. 
Wilkerson 
This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/
physicssellmyer/10 
Ferromagnetism in nanocrystalline epitaxial Co:TiO 2 thin films
A. K. Pradhan,a! 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
sReceived 21 February 2005; accepted 28 April 2005; published online 25 May 2005d
We report on the observation of remarkable room-temperature ferromagnetism in nanocrystalline
epitaxial Co:TiO2 films grown on sapphires0001d 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.
fDOI: 10.1063/1.1944209g
Recent advances in the emerging field of diluted mag-
netic semiconductorsDMSd 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 ferromagnetismFMd 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 sCo:TiO2 or CTOd has received interest
8,9 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.
Several techniques have been employed to fabricate ep-
itaxial Co:TiO2 films using oxide-based substrates, such as
SrTiO3, LaAlO3 sLAOd 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-
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–Sapphires0001d epitaxial films were grown by
the PLD techniquesKrF excimer,l=248 nm, laser repetition
rate of 2–5 Hzd with a pulse energy density of 1 to 2 J/cm2
and utilizing both target and substrate rotation facilities.
High-density Ti1−xCoxO sCTOd targets were used. Stoichio-
metric amount of TiO2 and CoOsboth 99.99% purityd 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 temperatureTs=700–800°C, keeping
oxygen partial pressure PO2=1310
−5 Torr. Clean single
crystalline sapphire substrates were loaded to the chamber
and heated just after the ultimate base pressure,4
310−8 Torr is reached. The film thickness is about 57 nm.
The x-ray diffraction sXRDd of the films was done in a
Rigaku x-ray diffractometer using Cu Ka radiation. The Ra-
man spectra were recorded using a LabRam micro-Raman
spectrometer with He–Ne laser excitationswavelength: 632.8
nmd. The ferromagnetic resonancesFMRd experiments were
done on a Bruker EMX spectrometer operating at 9.6 GHz
microwave frequency. Quantum Design Superconducting
Quantum Interference DevicesSQUID- MPMSd was used to
measure the magnetization of films.
adAuthor to whom correspondence should be addressed; electronic mail:
apradhan@nsu.edu
APPLIED PHYSICS LETTERS86, 222503s2005d
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Figure 1 shows the XRD patterns of CTO–Sapphire
s0001d 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
s0001d reveal one dominating planes002d 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-maximumsFWHMd cal-
culated from x-rays200d 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 Cos002d reflec-
tion preferably oriented in the planesc axisd either in a small
cluster form or as interstitials. On the other hand, the film
grown on LAO substrate exhibits only one dominating peak
s004d, illustrating the presence of only anatase phase. No
evidence of Co clusters or Co intermetallic compounds was
observed in films deposited on LAO substrate. Thec axis
parameter of anatase as deduced from thes004d 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 thec axis strongly suggests that mostly Co is in
12 state whose atomic radiuss0.072 nmd is larger than Ti4+,
which is in agreement with a recent report.15 In view of the
above, the slight shift of rutiles002d 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.
In order to study the variation of Co density as a func-
tion of film thicknesssco-depth profiled, 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 interfaces,57 nmd where
a sharp jump in Al depth profile starts, illustrating that Co
diffuses into the substrates and probably acts as a nucleation
center for the formation Co clusters. On the other hand, it is
noted that the x-ray mapping of CTO–sapphire films over
20 mm320 mm area on the surface of the film clearly illus-
trates that Co is uniformly distributed in the TiO2 matrix.
The atomic force microscopysAFMd images of as-
grown Co:TiO2 films are shown in Figs. 2sad–2sdd. The AFM
images show remarkable nanocrystalline features in Figs.
2sad and 2scd 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. 2sbd. On the other hand, CTO-1 grown with a
slightly higher repetition rates7 Hzd shows a distinct three-
dimensionals3Dd islandlike growth feature with average par-
ticle size of 50 nm as shown in Fig. 2sdd over a 1.5mm
31.5 mm 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 sapphires0001d substrate. Besides the Raman bands at
350, 421, 433, and 583 cm−1, which are due to sapphire sub-
trates, 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-
FIG. 1. sColor onlined 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.
FIG. 2. sColor onlined AFM pictures of as-grown Co:TiO2 films from tar-
gets containingsad 6 at. %,sbd 10 at. %,scd 12 at. %, andsdd 6 at. % of Co.
All scans fromsad to scd are 2mm32 mm in dimension. The image insdd
shows the 3D image over 1.5mm31.5 mm area.
FIG. 3. sColor onlined Raman spectra of Co:TiO2 film synthesized from the
target containing different Co concentration. The spectrum for the substrate
ssapphired is also shown. The inset shows the FMR spectra of Co:TiO2 film
synthesized from the target containing 10 at. % of Co at various
temperatures.
222503-2 Pradhan et al. Appl. Phys. Lett. 86, 222503 ~2005!
Downloaded 14 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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 f 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-2sshown in the inset
in Fig. 3d at different temperatures, ranging from 100 to 200
K. The temperature independent electron paramagnetic reso-
nancesEPRd 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-cooledsFCd and zero-field-
cooledsZFCd 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 magnetizationsMHd 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 slow field to high fieldd 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 theHm values. However, this may not affect the quali-
tative effects of on magnetization results.
In view of the above, the origin of the ferromagnetism in
CTO films can be attributed to the presence of Co2+ i ns 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.
This work is supported by the NASA and NSF for Cen-
ter for Research Excellence in Science and Technology
sCRESTd Grant No. HRD-9805059. One of the authors
sR.R.Rd 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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FIG. 4. sColor onlined Magnetic field dependence of field-cooledsFCd and
zero-field-cooledsZFCd 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.
222503-3 Pradhan et al. Appl. Phys. Lett. 86, 222503 ~2005!
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Ferromagnetism in nanocrystalline epitaxial Co:TiO\u3csub\u3e2\u3c/sub\u3e thin films

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln David Sellmyer Publications Research Papers in Physics and Astronomy 5-30-2005 Ferromagnetism in nanocrystalline epitaxial Co:TiO2 thin films Aswini K. Pradhan Center for Materials Research, Norfolk State University, apradhan@nsu.edu D. Hunter Center for Materials Research, Norfolk State University, Norfolk, Virginia J.B. Dadson Center for Materials Research, Norfolk State University, Norfolk, Virginia T.M. Williams Center for Materials Research, Norfolk State University, Norfolk, Virginia Kai Zhang Center for Materials Research, Norfolk State University, Norfolk, Virginia See next page for additional authors Follow this and additional works at: https://digitalcommons.unl.edu/physicssellmyer  Part of the Physics Commons Pradhan, Aswini K.; Hunter, D.; Dadson, J.B.; Williams, T.M.; Zhang, Kai; Lord, K.; Lasley, B.; Rakhimov, Rakhim R.; Zhang, Jun; Sellmyer, David J.; Roy, U.N.; Cui, Y.; Burger, A.; Hopkins, C.; Pearson, N.; and Wilkerson, A.L., "Ferromagnetism in nanocrystalline epitaxial Co:TiO2 thin films" (2005). David Sellmyer Publications. 10. https://digitalcommons.unl.edu/physicssellmyer/10 This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in David Sellmyer Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Authors Aswini K. Pradhan, D. Hunter, J.B. Dadson, T.M. Williams, Kai Zhang, K. Lord, B. Lasley, Rakhim R. Rakhimov, Jun Zhang, David J. Sellmyer, U.N. Roy, Y. Cui, A. Burger, C. Hopkins, N. Pearson, and A.L. Wilkerson This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/physicssellmyer/10 Ferromagnetism in nanocrystalline epitaxial Co:TiO 2 thin filmsA. K. Pradhan,a! D. Hunter, J. B. Dadson, T. M. Williams, Kai Zhang, K. Lord,B. Lasley, and R. R. RakhimovCenter for Materials Research, Norfolk State University, 700 Park Avenue, Norfolk, Virginia 23504Jun Zhang and D. J. SellmyerDepartment of Physics and Astronomy and Center for Materials Research and Analysis, University ofNebraska, Lincoln, Nebraska 68588-0113U. N. Roy, Y. Cui, and A. BurgerDepartment of Physics, Fisk University, 1000, 17 Avenue North, Nashville, Tennessee 37208C. Hopkins, N. Pearson, and A. L. WilkersonCollege of William and Mary, Applied Research Center, 12050 Jefferson Ave. Suite 713, Newport News,Virgina 23606sReceived 21 February 2005; accepted 28 April 2005; published online 25 May 2005dWe report on the observation of remarkable room-temperature ferromagnetism in nanocrystallineepitaxial Co:TiO2 films grown on sapphires0001d substrates by a pulsed-laser deposition techniqueusing high-density targets. The films were characterized by x-ray measurements, atomic forcemicroscopy, micro-Raman, electron-paramagnetic resonance, and magnetization studies. The filmsexhibit three-dimensional islandlike growth that contains nanocrystalline particles. Ourexperimental results suggest that the remarkable ferromagnetism in Co:TiO2 films is controlledeither by the interstitial Co2+ ions or small clusters, which are mainly present at the interface and onthe surface of the films. Our work clearly indicates that Co interstitials and nanoclusters causeroom-temperature ferromagnetism in Co-doped TiO2. © 2005 American Institute of Physics.fDOI: 10.1063/1.1944209gRecent advances in the emerging field of diluted mag-netic semiconductorsDMSd have attracted widespread at-tention due to their potential applications in the rapidly de-veloping field of spintronics.1–4 The search for integratingthe semiconductors with the magnetic properties in the samematerial has become a prerequisite for successful fabricationof potential devices of shrinking dimension, such as high-performance read-heads, nonvolatile memories, and otherstate-of-the-art storage devices.5,6 Hence, the discovery ofDMS exhibiting room temperature ferromagnetismFMd hasestablished an important achievement for possible applica-tions of functional spintronics.7 Although there have beenmany advances in incorporating magnetic ions into hostsemiconductors, particularly wide band gap III–V and II–VIbased DMS materials, applications were possible only at lowtemperatures in such materials due to their low magnetictransition temperature,Tc. Among DMS materials, Co-dopedTiO2 sCo:TiO2 or CTOd has received interest8,9 because it isa transparent semiconductor. However, the origin of the fer-romagnetic properties, particularly a possibility arising eitherfrom the Co clusters9,10 or intrinsic phenomenon11 of Co sub-stitution in Ti lattice, and the ionic state of Co remain am-biguous.Several techniques have been employed to fabricate ep-itaxial Co:TiO2 films using oxide-based substrates, such asSrTiO3, LaAlO3 sLAOd for anatase,11,12 and Al2O3 for rutilephase stabilization,13,14 which show ferromagnetism at roomtemperature. The origin of ferromagnetism in this DMS isstill debated due to possibilities of Co clustering as describedabove, and incorporation of Co atoms in the lattice. How-ever, one of the most important issues is that Co-doped TiO2could contribute significantly to the field of spintronics as apotential DMS, if successfully synthesized, exhibiting dilutemagnetic nature. Here we report the growth of Co:TiO2 filmscontaining nanoparticles ordered to form epitaxial films onAl2O3 substrates. The films demonstrate remarkable struc-tural, microscopic, ferromagnetic, and spectroscopic proper-ties at room temperature. The results demonstrate that Co cansubstitute Ti, and the predominant valence state is Co2+.Co:TiO2–Sapphires0001d epitaxial films were grown bythe PLD techniquesKrF excimer,l=248 nm, laser repetitionrate of 2–5 Hzd with a pulse energy density of 1 to 2 J/cm2and utilizing both target and substrate rotation facilities.High-density Ti1−xCoxO sCTOd targets were used. Stoichio-metric amount of TiO2 and CoOsboth 99.99% purityd pow-ders were mixed, calcined at 700 °C for 12 h followed byisostatic pressing at 400 MPa, and finally sintered at 1000 °Cin order to make high-density target. The films were depos-ited with a substrate temperatureTs=700–800°C, keepingoxygen partial pressure PO2=1310−5 Torr. Clean singlecrystalline sapphire substrates were loaded to the chamberand heated just after the ultimate base pressure,4310−8 Torr is reached. The film thickness is about 57 nm.The x-ray diffraction sXRDd of the films was done in aRigaku x-ray diffractometer using Cu Ka radiation. The Ra-man spectra were recorded using a LabRam micro-Ramanspectrometer with He–Ne laser excitationswavelength: 632.8nmd. The ferromagnetic resonancesFMRd experiments weredone on a Bruker EMX spectrometer operating at 9.6 GHzmicrowave frequency. Quantum Design SuperconductingQuantum Interference DevicesSQUID- MPMSd was used tomeasure the magnetization of films.adAuthor to whom correspondence should be addressed; electronic mail:apradhan@nsu.eduAPPLIED PHYSICS LETTERS86, 222503s2005d0003-6951/2005/86~22!/222503/3/$22.50 © 2005 American Institute of Physics86, 222503-1Downloaded 14 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspFigure 1 shows the XRD patterns of CTO–Sapphires0001d films at a constant PO2 using three different targetswith Co concentration varying from 6 to 12 at. % designated6 at. % as CTO-1, 10 at. % as CTO-2, and 12 at. % asCTO-3. The XRD patterns for all films grown on sapphires0001d reveal one dominating planes002d of the rutile phase,illustrating the epitaxial nature of the film and rutile phasestabilization. This undoubtedly indicates the preferential ori-entation of the crystal plane in parallel with the substratesurface. The rocking curves for the epitaxial growth of thefilms show that the full-width half-maximumsFWHMd cal-culated from x-rays200d line broadening increase from 0.3°to 0.35° with increasing Co concentration in the target at thesame growth conditions, illustrating the poor crystallinequality of the film for higher concentration Co. It is interest-ing to note that the peak position for the higher concentrationof Co shifts to lower angle. However, interestingly, a smallbut discernable peak at about 44.5° was observed, particu-larly in CTO-2 film. This is certainly due to Cos002d reflec-tion preferably oriented in the planesc axisd either in a smallcluster form or as interstitials. On the other hand, the filmgrown on LAO substrate exhibits only one dominating peaks004d, illustrating the presence of only anatase phase. Noevidence of Co clusters or Co intermetallic compounds wasobserved in films deposited on LAO substrate. Thec axisparameter of anatase as deduced from thes004d reflection isabout 0.9500 nm for CTO–LAO, which suggests that Coions have been incorporated into the anatase structure substi-tuting Ti ions in the lattice. Furthermore, the unit-cell elon-gation along thec axis strongly suggests that mostly Co is in12 state whose atomic radiuss0.072 nmd is larger than Ti4+,which is in agreement with a recent report.15 In view of theabove, the slight shift of rutiles002d peak in CTO–sapphireto higher angle indicates the fact that the substitutional effectis also present in CTO–sapphire when grown using targetscontaining different concentration of Co.In order to study the variation of Co density as a func-tion of film thicknesssco-depth profiled, we carried out sec-ondary mass ion spectroscopy in CTO-2 and the Co depthprofile is presented in the inset of Fig. 1. It is clear that thedensity of Co is much higher at the surface and decreases asthe film thickness decreases. However, surprisingly, the codensity suddenly increases at the interfaces,57 nmd wherea sharp jump in Al depth profile starts, illustrating that Codiffuses into the substrates and probably acts as a nucleationcenter for the formation Co clusters. On the other hand, it isnoted that the x-ray mapping of CTO–sapphire films over20 mm320 mm area on the surface of the film clearly illus-trates that Co is uniformly distributed in the TiO2 matrix.The atomic force microscopysAFMd images of as-grown Co:TiO2 films are shown in Figs. 2sad–2sdd. The AFMimages show remarkable nanocrystalline features in Figs.2sad and 2scd for CTO-1 and 2, respectively. The particles arevery uniform in size in both films. The particle size decreasesfrom 25 nm to less than 20 nm with increasing Co concen-tration. However, it is interesting to note the self-assembly ofCTO oriented nanocrystals of size 35 nm for CTO-2 film asshown in Fig. 2sbd. On the other hand, CTO-1 grown with aslightly higher repetition rates7 Hzd shows a distinct three-dimensionals3Dd islandlike growth feature with average par-ticle size of 50 nm as shown in Fig. 2sdd over a 1.5mm31.5 mm 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 constantfor the present experiment, may also have an effect on this.In Fig. 3, we show the Raman spectra of CTO films andthe sapphires0001d substrate. Besides the Raman bands at350, 421, 433, and 583 cm−1, which are due to sapphire sub-trates, two shoulder signals appeared at 442 and 610 cm−1due to rutile phase. The Raman spectra show that the struc-ture is predominantly rutile and highly crystalline. A broadpeak observed at 680 cm−1 is believed to be additional fea-FIG. 1. sColor onlined X-ray diffraction pattern of Co:TiO2 films grown onsapphire and LaAlO3 substrates using targets containing different concentra-tion of Co. The inset shows the Co depth profile of CTO-2 from secondaryion mass spectroscopy.FIG. 2. sColor onlined AFM pictures of as-grown Co:TiO2 films from tar-gets containingsad 6 at. %,sbd 10 at. %,scd 12 at. %, andsdd 6 at. % of Co.All scans fromsad to scd are 2mm32 mm in dimension. The image insddshows the 3D image over 1.5mm31.5 mm area.FIG. 3. sColor onlined Raman spectra of Co:TiO2 film synthesized from thetarget containing different Co concentration. The spectrum for the substratessapphired is also shown. The inset shows the FMR spectra of Co:TiO2 filmsynthesized from the target containing 10 at. % of Co at varioustemperatures.222503-2 Pradhan et al. Appl. Phys. Lett. 86, 222503 ~2005!Downloaded 14 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspture in the rutile structure and may be related to the Cointerstitials or small clusters. The intensity of the broad peakis much pronounced for CTO-2. Hence, the coincidence ofthe presence of this peak and the observed peak due to Co inx-ray reflection illustrate the presence of either interstitials orsmall 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 f 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 presenceof magnetic ions. In order to infer the Co charge state, wehave studied the FMR spectra of CTO-2sshown in the insetin Fig. 3d at different temperatures, ranging from 100 to 200K. The temperature independent electron paramagnetic reso-nancesEPRd of the EPR cavity is seen at about 3.3 KG. At100 K, the FMR spectra exhibit pronounced hump at around2500 G. The hump feature, which is a strong function oftemperature, weakens as temperature is raised. This is a clearsignature of ferromagnetic Co2+ as Co3+ does not show up inFMR spectra. The decrease of the FMR signal with increas-ing temperature may be related to the presence of Co3+ ionsat higher temperature. This provides strong experimental evi-dence in favor of the presence of Co2+ ionic state which isresponsible for the ferromagnetism in CTO films.Figure 4 shows the field-cooledsFCd and zero-field-cooledsZFCd magnetization of the CTO-2 film, displaying aferromagnetic behavior. The FC and ZFC curves showanomalous jump at about 100 K, illustrating a sudden changein the spin structures, which is related to a metastable mag-netic state. In order to confirm the ferromagnetic behavior,magnetic field dependence of magnetizationsMHd curves areshown at 300 K for two representative CTO films, includingCTO-2. The CTO-2 film displays remarkable ferromagneticbehavior at 300 K. The magnetic field at which the maxi-mum in magnetization,Hm slow field to high fieldd achievedis estimated to be 2 KG at 300 K. However, the room tem-perature ferromagnetic hysteresis shrinks significantly forCTO-3, which can be attributed due to the poor crystallinequality of these films when synthesized from the target con-taining higher concentration of Co as discussed earlier. It isnoted that the background magnetic contribution due to sap-phire substrate has not been corrected. This will further in-crease theHm values. However, this may not affect the quali-tative effects of on magnetization results.In view of the above, the origin of the ferromagnetism inCTO films can be attributed to the presence of Co2+ i ns thatare either in the interstitial positions or in the form of a smallmetal clusters. This is also consistent with the recent reportof channeling experiments.16 However, the ferromagnetic ex-change mechanism between Co2+–O–Co3+, giving rise toferromagnetism15 in this system seems to be totally implau-sible in the absence of Co3+ ions. The dispersed Co clustersor interstitial site occupations show a true diluted magneticnature and can be a potentially significance to the field ofspintronics.In conclusion, we have demonstrated that the epitaxialCo: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 theferromagnetism observed in Co:TiO2 film is controlled eitherby the interstitial Co2+ ions or small clusters. The Co depthprofile clearly indicates the presence of Co clusters at theinterface and on the surface which influence the ferromag-netism significantly. Our results are significantly importantfor fabricating the Co:TiO2 films for possible spintronic ap-plications.This work is supported by the NASA and NSF for Cen-ter for Research Excellence in Science and TechnologysCRESTd Grant No. HRD-9805059. One of the authorssR.R.Rd acknowledges the support from the faculty researchprogram provided by NASA. Research at the University ofNebraska is supported by NSF-MRSEC, ONR, and CMRA.1H. Ohno, Science281, 951 s1998d.2J. K. Furdyna, J. Appl. Phys.64, R29-R64s1988d.3M. Berciu and R. N. Bhat, Phys. Rev. Lett.87, 107203s2001d.4Y. D. Park, A. T. Hanbicki, S. C. Erwin, C. S. Hellberg, J. M. Sullivan, J.E. Mattson, T. 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The insetshows the ferromagnetic hysteresis loops of CTO-2 and CTO-3 films at300 K.222503-3 Pradhan et al. Appl. Phys. Lett. 86, 222503 ~2005!Downloaded 14 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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Ferromagnetism in nanocrystalline epitaxial Co:TiO\u3csub\u3e2\u3c/sub\u3e thin films

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