Appl. Phys. Lett., Vol. 83, No. 1, 7 July 2003
Manivannan et al.
113
nealing temperature was kept relatively low to avoid any
possible secondary phase segregation even in relatively
higher doping content. Films so prepared were characterized
by XRD, XPS, and micro-Raman studies in order to establish
the solubility of Co in TiO lattice up to 10 at. % Co content.
2
These films were found to be paramagnetic at room tempera-
ture but on cooling further the magnetic susceptibility fol-
lowed a Curie–Weiss law with a very low Ӎ5 K. Only for
Tр, hysteresis loop is observed. The apparent disagree-
ment of our results in the Co:TiO films prepared by spray
2
pyrolysis with other reported results on Co/TiO films pre-
2
8
–10
pared by other methods ͑where RTF has been observed͒,
is most likely due to the different nature of incorporation of
Co into the anatase lattice.
A.M. would like to thank Saitoh for experimental assis-
tance. At WVU, this research was supported by the U.S. Air
Force Office of Scientific Research and at UPR by NASA
Grant No. NCC5-518.
FIG. 4. Magnetization vs magnetic field for 10% Co/TiO2 at 5 and 30 K.
The inset shows the data for the low field regions at 5 K. The lines connect-
ing the data are for visual aid.
1
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ϭnumber of magnetic ions/g, k ϭBoltzmann constant, and
B
ϭmagnetic moment, yield Ӎ3.0(0.1) for the Co2
ϩ
2
B
4
ϩ
ions substituting for Ti ions in the anatase unit cell for
3
1
0% Co doping, assuming the composition Ti1ϪxCo O
.
x
2Ϫx
This magnitude of is somewhat lower than the expected
4
2
ϩ
value of ϭ3.87 for 10 at. % Co with spin Sϭ3/2 but
B
5
the difference can easily be explained assuming the actual
concentration of Co in the thin film is somewhat less than the
nominal 10% value. The Curie–Weiss temperature Ӎ5 K
represents antiferromagnetic exchange interaction between
6
7
2
ϩ
the Co ions. The bifurcation of the FC and ZFC data be-
low 6 K may be a manifestation of this interaction.
The field dependence of the magnetization at Tϭ5 and
8
291, 854 ͑2001͒.
30 K ͑Fig. 4͒ shows that the hysteresis is not observed at 30
9
0
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and in the inset of Fig. 4, we have plotted the low-field
1
region of the loop showing coercivity H Х200 Oe and the
c
remanant magnetization of about 0.02 emu/g.
In conclusion, we have successfully prepared Co:TiO2
11
thin films by spray pyrolysis at relatively low temperature
1
1
2
3
͑500 °C͒. In this technique, the phase formation kinetics is
expected to be very different from that in the physical vapor
deposition techniques since in spray pyrolysis, the constitu-
ent elements are mixed at molecular level and also the an-
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