We report on ferromagnetic properties of ZnMnO and Co:TiO2 films grown by pulsed laser deposition with varying growth conditions. We have demonstrated that ZnMnO films show ferromagnetic properties at room temperature. However, oxygen plays a dominant role in the occurrence of ferromagnetism. Introducing carriers into ZnMnO films did not improve the ferromagnetic properties. Our experimental results indicate that the mechanism for ferromagnetism lies, probably, within the perspective of charge transfer between Mn ions through oxygen. On the other hand, our experimental results suggest that the ferromagnetism in Co:TiO2 films is controlled by the presence of small metal Co2+ clusters in the rutile TiO2 matrix, which are mainly present at the interface and on the surface of the films

Dadson, J.B.

Hunter, D.

Lasley-Hunter, B.

Pradhan, Aswini K.

Rakhimov, Rakhim R.

Sellmyer, David J.

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 
4-15-2006 
Oxide-based dilute ferromagnetic semiconductors: ZnMnO and 
Co:TiO2 
Aswini K. Pradhan 
Center for Materials Research, Norfolk State University, apradhan@nsu.edu 
D. Hunter 
Center for Materials Research, Norfolk State University, Norfolk, Virginia 
B. Lasley-Hunter 
Center for Materials Research, Norfolk State University, Norfolk, Virginia 
J.B. Dadson 
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.; Lasley-Hunter, B.; Dadson, J.B.; Zhang, Kai; Rakhimov, Rakhim R.; Zhang, 
Jun; and Sellmyer, David J., "Oxide-based dilute ferromagnetic semiconductors: ZnMnO and Co:TiO2" 
(2006). David Sellmyer Publications. 9. 
https://digitalcommons.unl.edu/physicssellmyer/9 
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, B. Lasley-Hunter, J.B. Dadson, Kai Zhang, Rakhim R. Rakhimov, Jun Zhang, 
and David J. Sellmyer 
This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/
physicssellmyer/9 
Oxide-based dilute ferromagnetic semiconductors: ZnMnO and Co:TiO2
A. K. Pradhan,a D. Hunter, B. Lasley-Hunter, J. B. Dadson, Kai Zhang, 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
Presented on 2 November 2005; published online 20 April 2006
We report on ferromagnetic properties of ZnMnO and Co:TiO2 films grown by pulsed laser
deposition with varying growth conditions. We have demonstrated that ZnMnO films show
ferromagnetic properties at room temperature. However, oxygen plays a dominant role in the
occurrence of ferromagnetism. Introducing carriers into ZnMnO films did not improve the
ferromagnetic properties. Our experimental results indicate that the mechanism for ferromagnetism
lies, probably, within the perspective of charge transfer between Mn ions through oxygen. On the
other hand, our experimental results suggest that the ferromagnetism in Co:TiO2 films is controlled
by the presence of small metal Co2+ clusters in the rutile TiO2 matrix, which are mainly present at
the interface and on the surface of the films. © 2006 American Institute of Physics.
DOI: 10.1063/1.2165919
Recently, there has been much attention paid to dilute
magnetic semiconductors DMSs due to their potential for
the development of magneto-optic electronics in the rapidly
developing area of spintronics.1–4 While III-V and II-VI
semiconductors show convincingly carrier-mediated interac-
tion intrinsic,5 the search for magnetism in oxide semicon-
ductors, such as ZnMnO, raises several controversies regard-
ing the nature of the magnetism.6–10
Following the theoretical prediction of ferromagnetism11
in II-VI semiconductors, the recent report on the intrinsic
carrier-mediated ferromagnetism observed in ZnMnO bulk
and films12 renewed the interest in this wide-band-gap semi-
conductor. However, the experimental evidence by other
report13 suggests that the observed ferromagnetism is due to
the oxygen-vacancy-stabilized metastable state. On the other
hand, although Co-doped TiO2 Co:TiO2 or CTO has re-
ceived interest,14,15 the origin of the ferromagnetic proper-
ties, particularly a possibility arising either from both Co
clusters and Co incorporation into the TiO2 matrix in the
anatase phase14 or intrinsic phenomenon16 of Co substitution
into Ti lattice, and the ionic state of Co remain ambiguou.
In this paper, we demonstrate that the thin films fabri-
cated by the pulsed laser deposition PLD by varying an-
nealing conditions clearly bring out the fact that oxygen
plays a major role in controlling the ferromagnetism in
ZnMnO system. On the other hand, ferromagnetism in rutile-
phase-stabilized Co:TiO2 films is controlled either by the
interstitial Co2+ ions or small metal clusters.
Both ZnMnO and CTO epitaxial films were grown on
sapphire 0001 substrates by the PLD technique and synthe-
sis conditions for both systems are described elsewhere.17,18
As-doped ZnO the films were in situ annealed in nitrogen for
5 min at 200 °C. The resistivity of the p-type ZnMnO films
is in the range of 1–2  cm with hole concentrations be-
tween 4–81017 cm−3.
Figure 1a shows the XRD curves for 0002 peak of
ZnMnO/sapphire 0001 films grown at Ts=500 °C and
PO2=1 mTorr, followed by immediate in situ postannealing
at different temperatures. It is noted that the XRD patterns
for all films reveal only one strong orientation 002, illus-
trating the epitaxial nature of the film. However, it is inter-
esting to note that the XRD peak position in a film grown at
Ts=500 °C and at PO2=1 mTorr exhibit a peak with a broad
full width at half maximum FWHM. The in situ postan-
nealing at 600 °C exhibits the similar XRD peak pattern,
indicating that Mn is substituted into Zn sublattice. In p-type
ZnMnO film grown at Ts=520 °C and at PO2=1 mTorr, the
002 peak shifts to higher angle as shown in Fig. 1a. The
XRD results of high oxygen pressure growth are similar to
our previous report in which a narrow FWHM was seen with
increasing substrate temperature.17 Apparently, the 002
peaks for the as-grown and annealed films at 600 °C turn out
to be a superposition of two peaks, one from ZnO and the
other from Mn-doped ZnO. We argue that a small amount of
Mn is incorporated into the Zn site and forms a minority
phase. The Mn doping introduces a minority phase into the
pure ZnO phase, resulting in poor crystallinity that broadens
the 002 peak linewidth.
Figure 1b shows the XRD pattern of CTO films grown
on sapphire 0001 which reveals one dominating plane
002 of the rutile-phase, illustrating the epitaxial nature of
the film and rutile-phase stabilization. However, a small but
discernable peak at about 44.5° was observed. This is cer-
tainly due to Co 002 reflection preferably oriented in the
plane c axis in a small metal cluster form. In order to study
the variation of Co density as a function of film thickness,
time-of-flight secondary-ion-mass spectroscopy TOF-SIMS
aAuthor to whom correspondence should be addressed; electronic mail:
apradhan@nsu.edu
JOURNAL OF APPLIED PHYSICS 99, 08M108 2006
0021-8979/2006/998/08M108/3/$23.00 © 2006 American Institute of Physics99, 08M108-1
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result is presented in the inset of Fig. 1b. 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 57 nm which is
the thickness of the film where a sharp jump emerges in
Al-depth profile, illustrating that Co diffuses into the sub-
strates and probably acts as a nucleation center for the for-
mation Co clusters.
Figures 1c and 1d show the atomic force microscopic
images of ZnMnO films with in situ annealed in O2 at
600 °C and with As doping followed by N2 annealing
p-type ZnMnO, respectively. Although, both films show
nanocrystalline behavior with crystallite size of about 10 nm,
the ZnMnO films with in situ annealing in O2 at 600 °C
show some clusters which are dispersed in the matrix. These
clusters may be the prime suspect for the presence of the
minority phase as discussed above. The surface roughnesses
of the films are about 1.7 and 0.9 nm for ZnMnO and p-type
ZnMnO films, respectively. The Co:TiO2 film is consisted of
self-assembled nanoparticles.
Figures 2a and 2b show the magnetic-field depen-
dence of magnetization MH curves at 300 K in ZnMnO
and Co:TiO2 films, respectively. The film grown at Ts
=500 °C with PO2=1 mTorr with in situ oxygen annealing
1 mTorr at 600 °C exhibits pronounced ferromagnetic hys-
teresis loop. The field at which the maximum in magnetiza-
tion, Hm, low field to high field is achieved is about
1000 G. No Tc was noticed for this film within the measured
temperature range of 10–350 K not shown here. Further
increase in annealing temperature shrinks the hysteresis
loops drastically as shown in Fig. 2a. However, very weak
ferromagnetic loop was observed for the p-type ZnMnO
films as shown for the two films in Fig. 2a. On the other
hand, CTO film displays ferromagnetic hysteresis at room
temperature.
Figure 3 shows the FMR spectra at 300 K in the films
grown at Ts=500 °C and PO2=1 mTorr with various ex situ
annealing in either oxygen or nitrogen. The FMR curves
clearly indicate that the ferromagnetic response becomes
stronger on oxygenation, where as it weakens on nitrogen
annealing. However, subsequent annealing in oxygen recov-
ers the ferromagnetic response. On the other hand, the p-type
ZnMnO films did not exhibit any ferromagnetic response.
The peak at about 3.4 kG is the electron paramagnetic re-
FIG. 1. a XRD patterns of a ZnMnO films only rocking curves grown for different conditions and b Co:TiO2 film on sapphire substrates. The inset in
b is the TOF-SIMS of Co-depth profile. AFM images of c ZnMnO film grown at Ts=500 °C and PO2=1 mTorr with in situ oxygen annealing at 600 °C
and d p-type ZnMnO film.
08M108-2 Pradhan et al. J. Appl. Phys. 99, 08M108 2006
Downloaded 13 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
sponse from the cavity. However, the p-type ZnMnO did not
show any change in the FMR response at lower tempera-
tures.
This raises a concern about the actual mechanism of fer-
romagnetism in ZnMnO system. Our results suggest that al-
though carriers did not help in ferromagnetism in ZnMnO
system, oxygen plays a dominant role in the occurrence of
ferromagnetism in ZnMnO. It is suggested that the ferromag-
netism in this system may be coupled to the mechanism re-
lated to Mn ions through oxygen, and in which the alloying
of Mn into Zn site plays a major role for controlling the
exact oxygen concentration. The slight change in the oxygen
concentration drives the system away from the ferromagnetic
state.
In Co:TiO2 film, the FMR spectra exhibit pronounced
hump at around 2500 G at 100 K and the hump feature
weakens as temperature is raised to 200 K, as shown in Fig.
3b. This is a clear signature of ferromagnetic Co2+ as Co3+
does not show up in FMR spectra. The decrease of the FMR
signal with increasing temperature may be related to the
presence of Co3+ ions at higher temperature. This provides
strong experimental evidence in favor of the presence of
Co2+ ionic state which is responsible for the ferromagnetism
in rutile-stabilized CTO films in contrast to the intrinsic fer-
romagnetism observed in anatase phase of Co:TiO2.19,20
In conclusion, we have demonstrated that ZnMnO films
show ferromagnetic properties at room temperature. How-
ever, oxygen plays a dominant role in the occurrence of fer-
romagnetism. The carriers do not play any role for ferromag-
netism in ZnMnO films. Our experimental results indicate
that the mechanism for ferromagnetism lies, probably, within
the perspective of charge transfer between Mn ions through
oxygen. On the other hand, the ferromagnetism observed in
rutile-phase-stabilized Co:TiO2 film is due to the Co2+ ions.
This work is supported by the NASA and NSF. Research
at the University of Nebraska is supported by NSF-MRSEC,
ONR, and CMRA.
1H. Ohno, Science 281, 951 1998.
2J. K. Furdyna, J. Appl. Phys. 64, R29 1988.
3S. A. Wolf et al., Science 294, 1488 2001.
4F. Matsukura et al., Phys. Rev. Lett. 83, 3073 1999.
5B. Beschoten et al., Phys. Rev. Lett. 83, 3073 1999.
6V. A. L. Roy et al., Appl. Phys. Lett. 84, 756 2004.
7Y. W. Heo et al., Appl. Phys. Lett. 84, 2292 2004.
8X. M. Cheng and C. L. Chien, J. Appl. Phys. 93, 7876 2003.
9T. Fukumura et al., Appl. Phys. Lett. 78, 958 2001.
10S. W. Yoon et al., J. Appl. Phys. 93, 7879 2003.
11T. Dietl et al., Science 287, 1019 2000.
12P. Sharma et al., Nat. Mater. 2, 673 2003.
13D. C. Kundaliya et al., Nat. Mater. 3, 709 2004.
14S. R. Shinde et al., Phys. Rev. B 67, 115211 2003.
15D. H. Kim et al., Appl. Phys. Lett. 81, 2421 2002.
16N. J. Seong et al., Appl. Phys. Lett. 81, 4209 2002.
17A. K. Pradhan et al., Appl. Phys. Lett. 86, 152511 2005.
18A. K. Pradhan et al., Appl. Phys. Lett. 86, 222503 2005.
19K. A. Griffin et al., Phys. Rev. Lett. 94, 157204 2005.
20S. Chambers et al., Phys. Rev. B 67, 100401 2003.
FIG. 2. a Ferromagnetic hysteresis loops of ZnMnO and p-type ZnMnO
films grown at various conditions are shown for T=300 K and b Co:TiO2
film at room temperature.
FIG. 3. Ferromagnetic resonance spectra of a ZnMnO and p-type ZnMnO
films grown at various conditions at T=300 K and b Co:TiO2 film at T
=100 and 200 K.
08M108-3 Pradhan et al. J. Appl. Phys. 99, 08M108 2006
Downloaded 13 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp


Oxide-based dilute ferromagnetic semiconductors: ZnMnO and Co:TiO\u3csub\u3e2\u3c/sub\u3e

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln David Sellmyer Publications Research Papers in Physics and Astronomy 4-15-2006 Oxide-based dilute ferromagnetic semiconductors: ZnMnO and Co:TiO2 Aswini K. Pradhan Center for Materials Research, Norfolk State University, apradhan@nsu.edu D. Hunter Center for Materials Research, Norfolk State University, Norfolk, Virginia B. Lasley-Hunter Center for Materials Research, Norfolk State University, Norfolk, Virginia J.B. Dadson 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.; Lasley-Hunter, B.; Dadson, J.B.; Zhang, Kai; Rakhimov, Rakhim R.; Zhang, Jun; and Sellmyer, David J., "Oxide-based dilute ferromagnetic semiconductors: ZnMnO and Co:TiO2" (2006). David Sellmyer Publications. 9. https://digitalcommons.unl.edu/physicssellmyer/9 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, B. Lasley-Hunter, J.B. Dadson, Kai Zhang, Rakhim R. Rakhimov, Jun Zhang, and David J. Sellmyer This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/physicssellmyer/9 Oxide-based dilute ferromagnetic semiconductors: ZnMnO and Co:TiO2A. K. Pradhan,a D. Hunter, B. Lasley-Hunter, J. B. Dadson, Kai Zhang, andR. 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 of Nebraska, Lincoln, Nebraska 68588-0113Presented on 2 November 2005; published online 20 April 2006We report on ferromagnetic properties of ZnMnO and Co:TiO2 films grown by pulsed laserdeposition with varying growth conditions. We have demonstrated that ZnMnO films showferromagnetic properties at room temperature. However, oxygen plays a dominant role in theoccurrence of ferromagnetism. Introducing carriers into ZnMnO films did not improve theferromagnetic properties. Our experimental results indicate that the mechanism for ferromagnetismlies, probably, within the perspective of charge transfer between Mn ions through oxygen. On theother hand, our experimental results suggest that the ferromagnetism in Co:TiO2 films is controlledby the presence of small metal Co2+ clusters in the rutile TiO2 matrix, which are mainly present atthe interface and on the surface of the films. © 2006 American Institute of Physics.DOI: 10.1063/1.2165919Recently, there has been much attention paid to dilutemagnetic semiconductors DMSs due to their potential forthe development of magneto-optic electronics in the rapidlydeveloping area of spintronics.1–4 While III-V and II-VIsemiconductors show convincingly carrier-mediated interac-tion intrinsic,5 the search for magnetism in oxide semicon-ductors, such as ZnMnO, raises several controversies regard-ing the nature of the magnetism.6–10Following the theoretical prediction of ferromagnetism11in II-VI semiconductors, the recent report on the intrinsiccarrier-mediated ferromagnetism observed in ZnMnO bulkand films12 renewed the interest in this wide-band-gap semi-conductor. However, the experimental evidence by otherreport13 suggests that the observed ferromagnetism is due tothe oxygen-vacancy-stabilized metastable state. On the otherhand, although Co-doped TiO2 Co:TiO2 or CTO has re-ceived interest,14,15 the origin of the ferromagnetic proper-ties, particularly a possibility arising either from both Coclusters and Co incorporation into the TiO2 matrix in theanatase phase14 or intrinsic phenomenon16 of Co substitutioninto Ti lattice, and the ionic state of Co remain ambiguou.In this paper, we demonstrate that the thin films fabri-cated by the pulsed laser deposition PLD by varying an-nealing conditions clearly bring out the fact that oxygenplays a major role in controlling the ferromagnetism inZnMnO system. On the other hand, ferromagnetism in rutile-phase-stabilized Co:TiO2 films is controlled either by theinterstitial Co2+ ions or small metal clusters.Both ZnMnO and CTO epitaxial films were grown onsapphire 0001 substrates by the PLD technique and synthe-sis conditions for both systems are described elsewhere.17,18As-doped ZnO the films were in situ annealed in nitrogen for5 min at 200 °C. The resistivity of the p-type ZnMnO filmsis in the range of 1–2  cm with hole concentrations be-tween 4–81017 cm−3.Figure 1a shows the XRD curves for 0002 peak ofZnMnO/sapphire 0001 films grown at Ts=500 °C andPO2=1 mTorr, followed by immediate in situ postannealingat different temperatures. It is noted that the XRD patternsfor all films reveal only one strong orientation 002, illus-trating the epitaxial nature of the film. However, it is inter-esting to note that the XRD peak position in a film grown atTs=500 °C and at PO2=1 mTorr exhibit a peak with a broadfull width at half maximum FWHM. The in situ postan-nealing at 600 °C exhibits the similar XRD peak pattern,indicating that Mn is substituted into Zn sublattice. In p-typeZnMnO film grown at Ts=520 °C and at PO2=1 mTorr, the002 peak shifts to higher angle as shown in Fig. 1a. TheXRD results of high oxygen pressure growth are similar toour previous report in which a narrow FWHM was seen withincreasing substrate temperature.17 Apparently, the 002peaks for the as-grown and annealed films at 600 °C turn outto be a superposition of two peaks, one from ZnO and theother from Mn-doped ZnO. We argue that a small amount ofMn is incorporated into the Zn site and forms a minorityphase. The Mn doping introduces a minority phase into thepure ZnO phase, resulting in poor crystallinity that broadensthe 002 peak linewidth.Figure 1b shows the XRD pattern of CTO films grownon sapphire 0001 which reveals one dominating plane002 of the rutile-phase, illustrating the epitaxial nature ofthe film and rutile-phase stabilization. However, a small butdiscernable peak at about 44.5° was observed. This is cer-tainly due to Co 002 reflection preferably oriented in theplane c axis in a small metal cluster form. In order to studythe variation of Co density as a function of film thickness,time-of-flight secondary-ion-mass spectroscopy TOF-SIMSaAuthor to whom correspondence should be addressed; electronic mail:apradhan@nsu.eduJOURNAL OF APPLIED PHYSICS 99, 08M108 20060021-8979/2006/998/08M108/3/$23.00 © 2006 American Institute of Physics99, 08M108-1Downloaded 13 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jspresult is presented in the inset of Fig. 1b. 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 interface 57 nm which isthe thickness of the film where a sharp jump emerges inAl-depth profile, illustrating that Co diffuses into the sub-strates and probably acts as a nucleation center for the for-mation Co clusters.Figures 1c and 1d show the atomic force microscopicimages of ZnMnO films with in situ annealed in O2 at600 °C and with As doping followed by N2 annealingp-type ZnMnO, respectively. Although, both films shownanocrystalline behavior with crystallite size of about 10 nm,the ZnMnO films with in situ annealing in O2 at 600 °Cshow some clusters which are dispersed in the matrix. Theseclusters may be the prime suspect for the presence of theminority phase as discussed above. The surface roughnessesof the films are about 1.7 and 0.9 nm for ZnMnO and p-typeZnMnO films, respectively. The Co:TiO2 film is consisted ofself-assembled nanoparticles.Figures 2a and 2b show the magnetic-field depen-dence of magnetization MH curves at 300 K in ZnMnOand Co:TiO2 films, respectively. The film grown at Ts=500 °C with PO2=1 mTorr with in situ oxygen annealing1 mTorr at 600 °C exhibits pronounced ferromagnetic hys-teresis loop. The field at which the maximum in magnetiza-tion, Hm, low field to high field is achieved is about1000 G. No Tc was noticed for this film within the measuredtemperature range of 10–350 K not shown here. Furtherincrease in annealing temperature shrinks the hysteresisloops drastically as shown in Fig. 2a. However, very weakferromagnetic loop was observed for the p-type ZnMnOfilms as shown for the two films in Fig. 2a. On the otherhand, CTO film displays ferromagnetic hysteresis at roomtemperature.Figure 3 shows the FMR spectra at 300 K in the filmsgrown at Ts=500 °C and PO2=1 mTorr with various ex situannealing in either oxygen or nitrogen. The FMR curvesclearly indicate that the ferromagnetic response becomesstronger on oxygenation, where as it weakens on nitrogenannealing. However, subsequent annealing in oxygen recov-ers the ferromagnetic response. On the other hand, the p-typeZnMnO films did not exhibit any ferromagnetic response.The peak at about 3.4 kG is the electron paramagnetic re-FIG. 1. a XRD patterns of a ZnMnO films only rocking curves grown for different conditions and b Co:TiO2 film on sapphire substrates. The inset inb is the TOF-SIMS of Co-depth profile. AFM images of c ZnMnO film grown at Ts=500 °C and PO2=1 mTorr with in situ oxygen annealing at 600 °Cand d p-type ZnMnO film.08M108-2 Pradhan et al. J. Appl. Phys. 99, 08M108 2006Downloaded 13 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jspsponse from the cavity. However, the p-type ZnMnO did notshow any change in the FMR response at lower tempera-tures.This raises a concern about the actual mechanism of fer-romagnetism in ZnMnO system. Our results suggest that al-though carriers did not help in ferromagnetism in ZnMnOsystem, oxygen plays a dominant role in the occurrence offerromagnetism in ZnMnO. It is suggested that the ferromag-netism in this system may be coupled to the mechanism re-lated to Mn ions through oxygen, and in which the alloyingof Mn into Zn site plays a major role for controlling theexact oxygen concentration. The slight change in the oxygenconcentration drives the system away from the ferromagneticstate.In Co:TiO2 film, the FMR spectra exhibit pronouncedhump at around 2500 G at 100 K and the hump featureweakens as temperature is raised to 200 K, as shown in Fig.3b. This is a clear signature of ferromagnetic Co2+ as Co3+does not show up in FMR spectra. The decrease of the FMRsignal with increasing temperature may be related to thepresence of Co3+ ions at higher temperature. This providesstrong experimental evidence in favor of the presence ofCo2+ ionic state which is responsible for the ferromagnetismin rutile-stabilized CTO films in contrast to the intrinsic fer-romagnetism observed in anatase phase of Co:TiO2.19,20In conclusion, we have demonstrated that ZnMnO filmsshow ferromagnetic properties at room temperature. How-ever, oxygen plays a dominant role in the occurrence of fer-romagnetism. The carriers do not play any role for ferromag-netism in ZnMnO films. Our experimental results indicatethat the mechanism for ferromagnetism lies, probably, withinthe perspective of charge transfer between Mn ions throughoxygen. On the other hand, the ferromagnetism observed inrutile-phase-stabilized Co:TiO2 film is due to the Co2+ ions.This work is supported by the NASA and NSF. Researchat the University of Nebraska is supported by NSF-MRSEC,ONR, and CMRA.1H. Ohno, Science 281, 951 1998.2J. K. Furdyna, J. Appl. Phys. 64, R29 1988.3S. A. Wolf et al., Science 294, 1488 2001.4F. Matsukura et al., Phys. Rev. Lett. 83, 3073 1999.5B. Beschoten et al., Phys. Rev. Lett. 83, 3073 1999.6V. A. L. Roy et al., Appl. Phys. Lett. 84, 756 2004.7Y. W. Heo et al., Appl. Phys. Lett. 84, 2292 2004.8X. M. Cheng and C. L. Chien, J. Appl. Phys. 93, 7876 2003.9T. Fukumura et al., Appl. Phys. Lett. 78, 958 2001.10S. W. Yoon et al., J. Appl. Phys. 93, 7879 2003.11T. Dietl et al., Science 287, 1019 2000.12P. Sharma et al., Nat. Mater. 2, 673 2003.13D. C. Kundaliya et al., Nat. Mater. 3, 709 2004.14S. R. Shinde et al., Phys. Rev. B 67, 115211 2003.15D. H. Kim et al., Appl. Phys. Lett. 81, 2421 2002.16N. J. Seong et al., Appl. Phys. Lett. 81, 4209 2002.17A. K. Pradhan et al., Appl. Phys. Lett. 86, 152511 2005.18A. K. Pradhan et al., Appl. Phys. Lett. 86, 222503 2005.19K. A. Griffin et al., Phys. Rev. Lett. 94, 157204 2005.20S. Chambers et al., Phys. Rev. B 67, 100401 2003.FIG. 2. a Ferromagnetic hysteresis loops of ZnMnO and p-type ZnMnOfilms grown at various conditions are shown for T=300 K and b Co:TiO2film at room temperature.FIG. 3. Ferromagnetic resonance spectra of a ZnMnO and p-type ZnMnOfilms grown at various conditions at T=300 K and b Co:TiO2 film at T=100 and 200 K.08M108-3 Pradhan et al. J. Appl. Phys. 99, 08M108 2006Downloaded 13 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

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Oxide-based dilute ferromagnetic semiconductors: ZnMnO and Co:TiO\u3csub\u3e2\u3c/sub\u3e

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