Structural and magnetic properties of chemically synthesized Fe doped ZnO
Shalendra Kumar, Y. J. Kim, B. H. Koo, S. K. Sharma, J. M. Vargas, M. Knobel, S. Gautam, K. H. Chae, D. K.
Kim, Y. K. Kim, and C. G. Lee
Citation: Journal of Applied Physics 105, 07C520 (2009); doi: 10.1063/1.3073933
View online: http://dx.doi.org/10.1063/1.3073933
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/105/7?ver=pdfcov
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JOURNAL OF APPLIED PHYSICS 105, 07C520 共2009兲
Structural and magnetic properties of chemically synthesized Fe doped
ZnO
Shalendra Kumar,1,a兲 Y. J. Kim,1 B. H. Koo,1 S. K. Sharma,2 J. M. Vargas,2 M. Knobel,2
S. Gautam,3 K. H. Chae,3 D. K. Kim,4 Y. K. Kim,4 and C. G. Lee1,b兲
1
School of Nano and Advanced Materials Engineering, Changwon National University, 9 Sarim dong,
Changwon-641-773, Republic of Korea
2
Instituto de Fisica Gleb Wataghin, Universidade Estadual de Campinas (UNICAMP), Campinas 13,
083-970 Sao Paulo, Brazil
3
Materials Science and Technology Research Division, KIST, Seoul 136-791, Republic of Korea
4
Department of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of Korea
共Presented 13 November 2008; received 8 September 2008; accepted 2 December 2008;
published online 25 March 2009兲
We report on the synthesis of Fe-doped ZnO with nominal composition of Zn0.99Fe0.01O by using a
coprecipitation method. X-ray diffraction and selective area electron diffraction studies reveal a
single phase wurtzite crystal structure without any secondary phase. Field emission transmission
electron microscopy measurements infer that Zn0.99Fe0.01O have nanorod-type microstructures.
Magnetic hysteresis measurement performed at different temperatures show that Zn0.99Fe0.01O
exhibits a weak ferromagnetic behavior at room temperature. A detailed investigation of the
electronic and local structure using O K-, Fe L3,2 near edge x-ray absorption fine structure suggests
that Fe is substituting Zn in ZnO matrix and is in Fe3+ state. © 2009 American Institute of Physics.
关DOI: 10.1063/1.3073933兴
I. INTRODUCTION
In recent years, diluted magnetic semiconductors
共DMSs兲 have become the subject of an intensive research
because of the possibility of manipulating charge and spin
degree of freedom in a single material.1–3 These materials are
the potential candidates for the technological applications in
spintronics, optoelectronics, and microwave devices. Investigations on DMSs were originally inspired by the discovery
of low temperature ferromagnetism 共FM兲 in Mn doped GaAs
with Curie temperature 共TC兲 of about 110 K.4 Theoretical
studies5 showed that transition metal 共TM兲 doped wide band
gap semiconductors are potential candidates for the room
temperature ferromagnetism 共RTFM兲. In the quest for materials with high TC, TM doped ZnO has emerged as a promising candidate based on the theoretical5 and experimental
studies.6 This has been also supported by ab initio calculations based on the local density approximation on the ZnO
based ferromagnetic semiconductors.7 In fact, RTFM has
been observed in TM doped ZnO.8–11 However, the results
remain controversial and some reports showed a very low
magnetic ordering temperature in TM doped ZnO 共Ref. 12兲
or even the absence of FM in these samples prepared using
different techniques. Some reports claim that FM arises from
clustering or impurity while others claim that it is of intrinsic
origin. These controversial results give an indication that
RTFM in DMSs is very sensitive to preparation methods and
hence of preparation conditions. Furthermore, even magnetic
properties of TM doped samples prepared by same method
for same concentration of dopant show the lack of reproducibility.
a兲
Author to whom correspondence should be addressed. Electronic mail:
[email protected].
b兲
Electronic mail: [email protected]. Tel.: ⫹82-55-213-3703. FAX:
⫹82-55-261-7017.
0021-8979/2009/105共7兲/07C520/3/$25.00
In this work, a single phase polycrystalline Zn0.99Fe0.01O
sample was synthesized by coprecipitation method and a
weak FM has been observed at room temperature. Near edge
x-ray absorption fine structure 共NEXAFS兲 measurements
were performed to get information about the valence state
and site symmetry of Fe ions. NEXAFS spectra measured at
Fe L3,2 and O K edge reflect that Fe exists in 3+ valence
states.
II. EXPERIMENTAL
Compounds with nominal composition Zn0.99Fe0.01O
was synthesized by using a coprecipitation method. Analytical grade metal nitrates 关Zn共NO3兲2 · 6H2O , Fe共NO3兲3 · 9H2O兴
were dissolved in de-ionized water to get a final concentration of 0.6M. This solution was kept at 25 ° C for 1 h with
constant stirring. In this solution, 5M of NH4OH solution
was added drop wise until the final pH of solution reached to
9 and then the solution was further stirred for 3 h at room
temperature and filtered. The mixture was dried at 80 ° C for
15 h. The dried mixture was ground and annealed at 500 ° C
for 3 h. A detailed characterization of the samples was carried out using XRD, transmission electron microscopy
共TEM兲, dc magnetization, and NEXAFS measurements.
XRD measurement was carried out using Phillips X’pert
共MPD 3040兲 x-ray diffractometer with Cu K␣ radiations 共␭
= 1.5406 Å兲 operated at voltage of 36 kV and current of 30
mA. Microstructural analysis of Zn0.99Fe0.01O sample was
carried using field emission transmission electron microscope 共JEM 2100F兲. Magnetization measurements were performed using a commercial Quantum Design physical property measurement system. NEXAFS measurement of
Zn0.99Fe0.01O sample along with the reference compounds of
Fe2O3, Fe3O4, and FeO at O K- and Fe L3,2-edges were performed in the soft x-ray beam line 7B1 XAS KIST of the
105, 07C520-1
© 2009 American Institute of Physics
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07C520-2
Kumar et al.
J. Appl. Phys. 105, 07C520 共2009兲
FIG. 2. Magnetization hysteresis curves for Zn0.99Co0.01O recorded 300 K.
Inset 1 shows the magnetization hysteresis curve recorded at 5 K. Inset 2
displays zoom part of magnetization hysteresis curve taken in low fields at
room temperature.
FIG. 1. 共Color online兲 XRD pattern of Zn1−xFexO 共x = 0.0 and 0.01兲. The
inset shows 共a兲 共002兲 peak of pure ZnO and Zn0.99Co0.01O, 共b兲 TEM micrograph of Zn0.99Co0.01O, and 共c兲 SAED pattern for Zn0.99Co0.01O along
共共21̄1̄0兲兲 zone axis.
Pohang Accelerator Laboratory 共PAL兲, operating at 2.5 GeV
with a maximum storage current of 200 mA. The spectra
were normalized to incident photon flux and the energy resolution was better than 0.2 eV. The data are normalized and
processed using Athena 0.8.056/IFEFFIT 1.2.11.
III. RESULTS AND DISCUSSION
Powder XRD pattern obtained from Zn1−xFexO 共0.0ⱕ x
ⱕ 0.01兲 is shown in Fig. 1. XRD pattern was indexed using
POWDER X 共Ref. 13兲 software and it clearly indicates that the
samples having a single phase polycrystalline behavior with
wurtzite lattice and rules out the possibility of any secondary
phase. However, a careful analysis of peak positions 关see
inset 共a兲 in Fig. 1兴 suggestive of a small shifting in its value
toward a lower 2␪ value with Fe doping 共see inset in Fig. 1兲.
The values of lattice parameters refined using POWDER X
software were a = b = 3.251 Å and c = 5.201 Å for pure ZnO.
However, the corresponding values for Zn0.99Fe0.01O were
a = b = 3.252 Å and c = 5.209 Å. The increasing trend of lattice parameters clearly indicates that Fe ions are substituting
Zn in ZnO matrix. Moreover, our results are in an excellent
agreement with those reported earlier by Karmakar et al.14
Inset 共b and c兲 in Fig. 1 shows the TEM micrograph and
SAED pattern for the Zn0.99Fe0.01O sample. The mean diameter and length of the rods estimated from the TEM micrograph using standard software 共IMAGE J兲 are found to be ⬃82
and ⬃235 nm, respectively. SAED is obtained by focusing
the beam on the Zn0.99Fe0.01O rod. The EDS results 共for
brevity of paper spectra are not shown here兲 collected from
different parts of doped nanorods reflect that these rods are
made of Zn, O, and Fe elements only. SAED pattern 关see
inset 共c兲 in Fig. 1兴 clearly indicates the crystalline nature of
each rod and demonstrates that Zn0.99Fe0.01O is indeed in
single phase with wurtzite structure.
Magnetization 共M兲 versus field 共H兲 curves for
Zn0.99Fe0.01O sample measured at two different temperatures
共5 and 300 K兲 are displayed in Fig. 2. It can be seen that
magnetization curve measured at 5 and 300 K 共see inset 1
and 2 of Fig. 2兲 is ferromagnetic in nature. Further, the magnetization value increases rapidly at lower fields which indicate its ferromagnetic behavior with a coercive field 共HC兲 of
27 Oe 共see inset 2 in Fig. 3兲. The magnetic moment per Fe
ion as calculated from the magnetization data was found to
decrease from 0.05␮B / Fe at 5 K to 0.004␮b / Fe at room temperature. This value of magnetic moment per Fe is far below
the full magnetic moment due to Fe3+ ion. Therefore, it
clearly indicates that only a small fraction of the substituted
Fe is contributing in the long range ferromagnetic order.
Some groups have also reported that the small value of magnetic moments in DMSs may originate due to the presence of
additional antiferromagnetic-type coupling between some
neighboring ions. As a result, this antiferromagnetic coupling
may lead to the canting on the spin. Therefore, the observed
weak FM in the present case is an intrinsic property and is
not due to the Fe clusters as supported by our XRD and TEM
results. In fact, oxidation states of Fe ions in ZnO are critical
to the observed magnetic properties. It is well known that the
existence of Fe3+ state at the Zn site will introduce a hole in
the system. Dietl et al.5 and Sato and Katayama-Yoshida15
suggested that the FM in such type of system may originate
due to hole-mediated exchange interactions. However, the
origin of FM in Fe doped ZnO is still not clear. Kumar et
al.16 explained that Fe–ZnO introduce holes which lead a
transition from antiferromagnetic state to ferromagnetic
state, whereas Parra-Palomino et al.17 explained that the FM
in Fe doped ZnO is due to generation of lattice defects in
ZnO or lattice distortion. However, in the present system, it
is clear from NEXAFS that the oxidation state of Fe ion is in
3+. So, the substitution of Zn by Fe3+ ion will induce a hole
in the system which might be responsible for the observed
RTFM in Zn0.99Fe0.01O.
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07C520-3
J. Appl. Phys. 105, 07C520 共2009兲
Kumar et al.
existing literature and band structure calculations,19 four
spectral features 共A2 − D2兲 are identified. The first two spectral features 共A2 ⬃ 529 eV and B2 ⬃ 534 eV兲, which form
the bottom of the conduction band, are attributed to transitions from 1s core state to oxygen 2p states hybridized with
Fe 3d orbitals. The continuum absorption peak D2 after 542
eV is assigned due to the O 2p hybridization with extended
Fe 4sp orbitals. Formation of spectral feature A2 and enhancement of feature C2 indicate the concentration of Fe3+
ions in the compound. These spectral features are also seen
clearly in the difference spectra plotted in the inset 2 of Fig.
3.
IV. CONCLUSIONS
In summary, we have successfully synthesized single
phase polycrystalline Fe doped ZnO nonorods by coprecipitation technique. Weak RTFM has been observed for
Zn0.99Fe0.01O sample in the present case as observed from
magnetization data. NEXAFS study indicates that Fe is in
Fe+3 states.
ACKNOWLEDGMENTS
FIG. 3. 共Color online兲 Normalized O K-edge spectra along with reference
spectra for the Fe3O4, Fe2O3, and FeO. Inset 1 shows the normalized
Fe L3,2-edge spectra of Zn0.99Fe0.01O samples along with Fe2O3, FeO, and
Fe3O4 as reference compounds. Inset 2 shows the difference spectra of normalized E for Zn0.99Fe0.01O subtracting from ZnO with Fe2O3 spectra.
In order to confirm the valence state of Fe, we have
performed NEXAFS measurement at Fe L3,2 and O K edges
along with the reference compounds Fe2O3, Fe3O4, and FeO.
Inset 1 in Fig. 3 shows the normalized spectra of
Fe L3,2-edge for the Zn0.99Fe0.01O along with reference compounds. One can clearly see an intense doublet peak at L3
edge and a weaker one at L2 edge. These spectral features are
primarily assigned due to the Fe 2p → 3d hybridization and
are strongly influenced by the core-hole potentials. The intensity of these peaks can be regarded as a measure of the
total unoccupied Fe 3d states. The two broad multiple structures L3 and L2 are well known for reference compounds,
viz., Fe2O3, FeO, and Fe3O4. The L3 feature of Fe2O3 is
characterized by a well developed doublet, a small intensity
peak marked as A1 and a main peak marked as B1, while in
FeO; first peak becomes a shoulder of the main peak.18 These
two spectral features in the L3 region were assigned to Fe t2g
and eg sub-bands, respectively. It is clearly evident from the
spectra that the observed features in Zn0.99Fe0.01O are very
similar to Fe2O3, which indicates that Fe is primarily in Fe+3
state.
Figure 3 shows normalized O K-edge spectra of Fe
doped ZnO along with Fe2O3, Fe3O4, and FeO as reference
compounds. According to dipole selection rules, these spectra represent the orbital character of spectral features of the
O 2p unoccupied states in the conduction band and its hybridization with different Zn and Fe orbitals. Based on the
This work was supported by the Korean Research Foundation Grant funded by the Korea Government 共MOEHRD兲
共Grant No. KRF–2008–005–J02703兲. Authors 共S.K.S. and
M.K.兲 are very grateful to FAPESP and CNPq 共Brazil兲 for
providing financial support 共Grant No. 06/06792-2兲. Authors
共S.G. and K.H. Chae兲 are also thankful to KIST financial
support 共Grant No. 2V01320兲. This work is also supported in
part by the National Research Laboratory Program 共Grant
No. M10500000105-05J000-10510兲.
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2
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