Materials Sciences and Applications, 2013, 4, 528-537
http://dx.doi.org/10.4236/msa.2013.49065 Published Online September 2013 (http://www.scirp.org/journal/msa)
Chromium-Doped ZnO Nanoparticles Synthesized by
Co-Precipitation: Chromium Effects
Santi Septiani Sartiman, Nadia Febiana Djaja, Rosari Saleh
Departement Fisika, FMIPA-Universitas Indonesia, Depok, Indonesia.
Received November 3rd, 2012; revised June 8th, 2013; accepted June 30th, 2013
Copyright © 2013 Santi Septiani Sartiman, et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
The influence of chromium doping on the physical properties of ZnO nanoparticles synthesized using a low temperature
co-precipitation technique is presented. In particular, we have studied the correlation between the structural and the
magnetic properties as a function of chromium concentrations. In order to investigate the magnetic properties, vibrating
sample magnetometry and electron spin resonance were employed. X-ray diffraction, energy-dispersive X-ray spec-
troscopy, Fourier transform infrared spectroscopy and UV-Vis spectroscopy were used. X-ray diffraction patterns of all
the samples showed peaks consistent with a hexagonal wurzite structure. The structure and composition analyses re-
vealed that chromium is incorporated into the lattice structure, forming a solid solution instead of precipitates. All of
the samples in this study exhibit ferromagnetic behavior. The implications of the effects of chromium are also dis-
cussed.
Keywords: Nanocrystalline ZnO Particles; Chromium; Room-Temperature-Ferromagnetic; ESR; Co-Precipitation
1. Introduction
The possibility to achieve both semiconducting and mag-
netic properties within a single material system by dop-
ing semiconducting materials with small concentrations
of transition metals has spawned a new field of elec-
tronics known as spintronics [1-4]. The discovery of fer-
romagnetism at temperatures above 100 K in III-V-based
ferromagnetic semiconductors such as Mn-doped GaAs
has made it possible to fabricate spin injecting structures
as well as structures for electrical or optical control of
ferromagnetism [5-8]. However, most of the III-V-based
ferromagnetic semiconductors have a highest Currie tem-
perature (TC) of 110 K, still far from room temperature,
which is required for practical device applications [9-11].
Since Dietl et al. [12] predicted that p-type ZnO would
be a promising candidate for high-TC ferromagnetic
semiconductors, there has been considerable research and
progress on transition metal-doped ZnO for the
realization of a room temperature TC. Thereafter, there
has been considerable research and progress on transition
metal doped ZnO for the realization of a TC at or above
room temperature. Experimentally, room temperature
ferromagnetism has been observed in Cr-, Mn-, Fe-, Co-,
Ni-, and Cu-doped ZnO [13-18]. However, the results are
inconsistent and controversial, both in experiments and
in theory. This inconsistency suggests that room tem-
perature ferromagnetism in transition metal-doped ZnO
is highly sensitive to the synthesis procedures and con-
ditions. Another suspicion is that ferromagnetism might
be caused by a secondary phase, such as the doping ele-
ments or their oxides, rather than by the substitution of
transition metal ions into Zn sites [19-21].
There are several theoretical explanations of the origin
of room temperature ferromagnetism in transition metal-
doped ZnO. Dietl et al. [12] first predicted that a high TC
for transition metal doped ZnO would require a large
density of mobile holes to induce the ferromagnetic
exchange interaction. However, a high concentration of
hole is difficult to achieve due to the compensation of the
wide band gap ZnO. Additionally, such a theory cannot
explain the RT ferromagnetism in n-type TM-doped ZnO.
Therefore, Sato and Katayama-Yoshida [22] suggested a
theory based on a combination of the Korringa-Kohn-
Rostoker (KKR) method and the coherent potential ap-
proximation (CPA) to explain the origin of ferromag-
netism in n-type TM-doped ZnO. Recently, Coey et al.
[23] have argued that a carrier mediated exchange inter-
action cannot produce long-range magnetic order when

Chromium-Doped ZnO Nanoparticles Synthesized by Co-Precipitation: Chromium Effects 529
the doping level of magnetic ions is only a few percent.
They proposed a spin-split impurity band theory derived
from the bound magnetic polarons (BMPs). In this model,
the ferromagnetic exchange interaction is mediated by
shallow donor electrons, which are treated in two-sub-
lattice mean field approximation. The only remaining
option to obtain a higher TC is to increase the donor elec-
trons in the impurity band that will delocalize onto the
TM ions.
Despite a number of reports available, confusion still
persists on the existence and origin of room temperature
ferromagnetism in transition metal doped ZnO. Hence,
an attempt was made to reinvestigate Cr-doped ZnO. For
synthesis, a co-precipitation method was chosen because
it is cost effective, it requires low temperatures for pro-
cessing, and it has a higher degree of solubility. The
powder samples were characterized with regard to their
composition, structure and phase (EDX, XRD, and UV-
Vis), and magnetic properties (VSM and ESR).
2. Experimental
The following starting materials were used without fur-
ther purification: zinc (II) sulfate (ZnSO4.7H2O, 99%,
Merck)and chromium (III) chloride (CrCl3.6H2O, 99%
Merck). To obtain the desired degree of doping of Cr,
ZnSO4·7H2O was mixed in distilled water with
CrCl3·6H2O. This solutions were designated as solution
A. Solution A was placed in an ultrasonic cleaner oper-
ating at 57 kHz for 2 h. Solution B was obtained by add-
ing 44 mmol of NaOH to 440 mL of de-ionized water.
After sonication, solution A was mixed with a magnetic
stirrer at room temperature, and solution B was added to
solution A until a pH of 12 was reached. The resulting
solution was magnetically stirred for 0.5 h, and then it
was allowed to stand at room temperature for 18 h. Sub-
sequently, the solution was centrifuged and washed sev-
eral times with ethanol and distilled water to remove re-
sidual and unwanted impurities. The final product was
dried in a vacuum oven at 200˚C for 1 h to yieldCr-doped
ZnO powder.
Elemental analyses of the samples were performed by
energy dispersive X-ray (EDX) spectroscopy using a
scanning electron microscope. Magnetic measurements
were performed at room temperature using an Oxford
Type 1.2 T vibrating sample magnetometer (VSM). The
powder samples were tightly packed in a clear plastic
drinking straw. Magnetization data were recorded as a
function of an applied magnetic field, 0 ± 1T. The data
reported in this study were corrected because background
signalswere introduced from the sample holder.
To evaluate the phase purity of the samples, X-ray
diffraction (XRD) measurements were performed using a
Philips PW 1710 and monochromatic Cu-Ka (l = 1.54060
Å) radiation operated at 40 kV and 20 mA in the range of
10˚ to 80˚.The instrumental broadening, including the
instrumental symmetry, was calibrated using a Si pow-
der standard. The X-ray diffraction patterns were ana-
lyzed by means of the MAUD program using Rietveld
whole profile fitting to determine the crystal structure
and lattice parameters.
To study the electronic interactions near the optical
band gap resulting from the addition of dopant atoms,
diffuse reflectance UV-Vis measurements were performed
using a Shimadzu UV-Vis spectrophotometer with an
integrating sphere and a spectral reflectance standard
over a wavelength range of 250 - 800 nm. The diffuse re-
flectance, R, of the sample is related to the Kubelka-
Munk function, F(R), according to the following equa-
tion:
 
2
12
F
RR R (1)
The energy band gap of the samples was calculated
from the diffuse reflectance spectra by plotting the F(R)2
as a function of energy and extrapolating to F(R)2 = 0.
To obtain information regarding the oxidation state
and site occupancy of the Cr ions in the ZnO matrix,
electron spin resonance (ESR) was performed using an
X-band JEOL JES-RE1X at room temperature and an
X-band spectrometer equipped with a 9.1 GHz field mo-
dulation unit. The resonance was optimized for the mo-
dulation amplitude, receiver gain, time-constant and scan
time. The amount of powder used in all measurements
was the same. DPPH was used as the standard. The shape
and area of the ESR spectra were analyzed using stan-
dard numerical methods.
3. Results and Discussion
3.1. Chemical Analysis
EDX measurement were used to determine the chemical
composition of the Cr-doped ZnO samples. Figure 1
shows EDX spectra of the Cr-doped ZnO samples. The
content of different elements in the sample can be ob-
served in the spectra, confirming the incorporation of Cr
atoms into the nanocrystalline ZnO particles. Calculat-
ing the area of the corresponding spectral K lines en-
abled quantitative characterization of the Cr/Zn ratios.
The amounts of Cr in the nanocrystalline ZnO parti-
cles were determined to vary between 3 and 16 at.%. The
results were obtained by averaging four values from dif-
ferent regions of a sample. The inset of Figure 1 illus-
trates how the Cr incorporated into the nanocrystalline
ZnO particles varied as a function of the initial cations
ratio used in the synthesis.The EDX results indicate that
the amounts of Cr incorporated in the samples were
slightly lower than the amounts of Cr introduced in the
synthesis.

Chromium-Doped ZnO Nanoparticles Synthesized by Co-Precipitation: Chromium Effects
530
3.2. Structural Characterization and Phase
Evaluation
Typical XRD patterns of ZnO nanoparticles doped with
Cr at different concentrations are shown in Figure 2. A
pure wurtzite, single phase structure of ZnO can be ob-
tained for all samples doped with Cr concentrations up to
16 at.%. No impurity peaks are detected, such as Cr2O3
and ZnCr2O4.This confirms that Cr dopant does not alter
the crystal structure. However, the results of these data
do not mean there is an absence of Cr clusters because
we do not exclude the possibility of cluster formation
below the detection limit of the X-ray diffractometer.
TheXRD data were further analyzed by the Rietveld
technique, using the program MAUD [24].
The evolution of lattice parameter a, and c and the unit
cell volume, V, with doping concentrations are plotted in
Figure 1. EDX spectra of Cr-doped nanocrystalline ZnO
particles for various doping concentrations. For clarity, the
spectra are shifted vertically.The inset shows the Cr incor-
poration in the nanoparticles as a function of the initial
cation ratio in the starting solution.
Figure 2. XRD patterns of Cr-doped nanocrystalline ZnO
particles synthesized with different concentrations of Cr .
Figure 3. For the Cr-doped samples, the lattice constant
decreaseds with increasing Cr concentration. Such a de-
crease of the lattice parameters in Cr-doped ZnO is quite
obvious as the ionic radii of Cr ions are smaller than
those of Zn.
3.3. Microstrain and Stress Analysis
It is known that the breadth of the XRD peak can be
linked to the average crystallite size, microstrain and
defects or dislocations. The average crystallite size using
XRD measurements is not generally the same as the par-
ticle size due to powder aggregates. The average crystal-
lite size as related to the line broadening can be calcu-
lated using Scherrer’s equation, as given in Equation (2):

cos
K
D


 (2)
where <D> = volume weighted crystallite size,  = shape
factor (close to unity in our work, it was set to 0.9),
 =
wavelength of Cu-Ka, 12
22
measuredinstrumental hkl hkl
bb b



= instrumental corrected integral breadth of the reflection
located at 2
 , and  = angle of reflection. The average
crystallite size for the Cr-doped ZnO calculated using
Figure 3. The lattice parameters a, c, and cell volume V of
Cr-doped nanocrystalline ZnO particles as a function of
doping concentration.

Chromium-Doped ZnO Nanoparticles Synthesized by Co-Precipitation: Chromium Effects
531
Scherrer’s equation is presented in Table 1. By plotting the value of 4sin  along the x-axis and

hklcos
 along the y-axis, the lattice strain and the aver-
age crystallite size can be calculated through a linear fit.
The UDM for Cr-doped ZnO are shown in Figure 4. It is
clear that there is a decrease in the lattice strain with
increasing doping concentrations. However, the UDM
becomes less valid because the material properties vary
with crystallographic directions. The other modified
forms of the WH plot, that consider the anisotropic na-
ture of the crystals are the uniform stress deformation
model (USDM) and the uniform deformation energy
density model (UDEDM). USDM and UDEDM give an
idea of the stress-strain relation and the strain as a func-
tion of energy density, respectively. In USDM the ani-
sotropic strain direction

hkl can be related with the de-
formation stress using Young’s modulus:
The strain arising from crystal imperfections and dis-
tortions, such as vacancies and lattice deformations, can
also induce a broadening in the XRD profile. Therefore,
the line broadening is a combination of crystallite size
and strain. Although the separation of these two phe-
nomena is not straightforward, the contribution of each
type of broadening can be determined by constructing a
modified form of William and Hall (W-H) plot [25],
namely UDM. In this form the strain was assumed to be
uniform in all crystallographic direction, thus the mate-
rial is considered to be isotropic. Strain-induced broad-
ening is calculated by the equation:
4tan
hkl

   (3)
where  = the lattice strain.
The UDM approach considers the case of both the
domain effect and lattice deformation operating simulta-
neously, and their combined effects give the final line
broadening, which is the sum of Equation (2) and Equa-
tion (3):
hklhkl hkl
E



(6)
where Ehkl is Young’s modulus in the direction perpen-
dicular to the (hkl) plane and

hkl is the anisotropic lattice
strain. In this model the WH equation becomes:
4sin
cos
hkl hkl
K
DE


  (7)

4tan
cos
hkl K
D



  (4)
The deformation stress can be estimated from the
slope of the linear fit of the graph which is plotted be-
tween 
hklcos
 and 4sin  /Ehkl, and the crystallite size
<D> from the intercept. For samples with a hexagonal
crystal phase, Young’s modulus Ehkl is related to:
Rearranging the equation, the following equation can
be obtained:
cos4 sin
hkl K
D

   (5)




22
2
2
22
42
22
113313 44
2
3
22
2
33
hkl
hk al
hc
Ehk hk
al al
ShSS Sh
cc








 

 
  
 
 
 
 
where a and c are lattice parameters and S11, S33, S13 and
S44 are the elastic compliance of ZnO with values of 7.858
× 10−12, −2.206 × 10−12, 6.940 × 10−12 and 23.57 × 10−12
m2N−1, respectively [26]. The USDMs for Cr-doped ZnO
are also plotted in Figure 4. Using the linear fits of Figure
4, the strain in anisotropic hexagonal crystals is calculated
from the peak position, which is different from the USDM
and UDM, their values are tabulated in Table 1.
Table 1. The average crystallite size, Young’s modulus strain, stress, and anisotropic energy density of Cr-doped na
nocrystalline ZnO particles calculated using William-Hall approximation.
Debye Scherrer UDM USDM UDEDM
Sample at.%
<D> nm <D> nm e (10−4)<D> nm
 (MPa)e (10−4) <D> nmu (kJm−3)  (MPa)e (10−4)
3 22 18 0.0007 17 71 0.0009 17 16 45 0.0005
7 20 15 0.0001 16 64 0.0008 16 13 41 0.0004
12 18 15 0.0001 15 11 0.0002 15 1 13 0.0001
Cr doped ZnO
16 17 14 0.0003 13 9 0.0001 14 4 23 0.0002

Current Distortion Evaluation in Traction 4Q Constant Switching Frequency Converters 532
Figure 4. William-Hall plot for each doping concentration
of Cr-doped nanocrystalline ZnO particles with UDM (up)
and USDM (down) method.
In UDM, we have considered that the crystal is homo-
geneous isotropic, whereas in USDM, the assumption of
homogeneity and isotropy is not fulfilled. Additionally,
all of the constants proportionality associated with the
stress-strain relation are no longer independent when the
strain energy density, u, is considered. According to
Hook’s law the energy density is connected to the Young’s
modulus through the equation:
2
2
hkl
E
u

 (9)
Therefore Equation (7) should be modified to the
form:
1/2
2
cos 4sin
hkl hkl
K
DE
 

 

u
(10)
The anisotropic energy density, u, can be estimated
from the slope of the curve

hklcos
 as a function of

12
4sin2 hkl E 
. The average crystallite size can be
obtained from the intercept and the strain from s/Ehkl. The
variation of average crystallite size, strain lattice, stress
and anisotropic energy density along with the Cr concen-
trations is shown in Table 1. The crystallite size calcu-
lated using Scherrer’s formula differs slightly from that
obtained from the UDM, USDM and UDEDM. It was
observed that the strain and stress values decreased with
decreasing average crystallite size as the dopant concen-
tration was increased.
3.4. Optical Properties
To study the electronic interactions near the optical band
gap region due to the presence of dopants diffuse-reflec-
tance measurements were performed on the samples in
the UV-Vis region at room temperature.
All spectra were obtained in the range of 200 - 800 nm.
Figure 5 shows the diffuse-reflectance spectra, R, as a
function of the wavelength for the samples shown in
Figure 2. The bandgap energy of the doped ZnO samples
was calculated from the diffuse-reflectance spectra by
plotting the square of the Kubelka-Munk function [27]
F(R)2 vs. the energy in electron volts. The linear part of
the curve was extrapolated to F(R)2 = 0 to calculate the
direct bandgap energy. The inset of Figure 5 shows the
bandgap as a function of the doping concentrations. The
absorption edge shifts to lower energies/longer wave-
lengths. A similar shift in the absorption edge and band
gap energy upon TM doping was reported in Co-, Ni-,
and Mn-doped ZnO nanoparticles [28-30] as well as in
other TM-doped semiconductor nanoparticle systems
[31-33].
Additionally to the reduction in the band gap energy, a
decrease in the diffuse reflectance was also observed
with increasing dopant concentration. A change in the
band gap can be explained by the variation in the lattice
parameters as a result of doping. The systematic variation
of the diffuse-reflectance near the band edge for the
doped samples in comparison with the undoped sample is
a further confirmation of Cr ion incorporation in the ZnO
lattice. The presence of Cr in the ZnO lattice is also
consistent with our EDX results (Figure 1). Although the
orresponding results were explained in terms of changes c

Chromium-Doped ZnO Nanoparticles Synthesized by Co-Precipitation: Chromium Effects
533
Figure 5. Diffuse-reflectance spectra of the Cr-doped nanocrystalline ZnO particles synthesized at various doping concen-
trations. The inset shows the correlated optical band gap of the Cr-doped nanocrystalline ZnO particles as a function of the
doping concentrations.
in the lattice parameters, other groups [34-36] argued that
this factor is not likely to significantly influence the
bandgap energy. They reported that the red shift in the
bandgap energy of transitionmetal-doped II-VI semi-
conductors is attributed to the sp-d spin exchange inter-
action between the band electrons and the localized d-
electrons of the transition metal ion that substitutes the
cation.
‘borrowed’ electron from the valence band to result in a
negatively charged center to which the hole is bound; 3)
Cr can also have a 3d3 configuration, where the Cr3+ ion
can incorporate one 4s and one 3d electron into the bond-
ing scheme, while another 3d is donated to the conduc-
tion band (making Cr a single donor), leaving the Cr cen-
ter positively charged and able to bind the free electron.
Considering the earlier works on ESR spectroscopy of
II-VI compounds [38,39], one usually relates the ESR
spectra detected in the Cr-doped II-VI semiconductors to
the Cr2+ valance state. Therefore, it is also expected that
the Cr2+ ion in the Cr-doped ZnO will substitute for the
Zn ion. However, our g-value and the peak-to-peak line
width are in contradiction with the reported values of the
ESR spectra of Cr2+ in II-VI compounds. Vallin et al. [40]
showed that the Cr2+ ESR spectra in II-VI compounds
have peak-to-peak line widths of 500 G at 20 K.
Therefore, the expectation is that at room temperature,
ESR signal of Cr2+ will be broader and unable to ob-
served [41]. Additionally, a study of Cr doped GaAs [42]
revealed that the Cr3+ valence state is predominant if the
chromium concentration significantly exceeds the concen-
tration of shallow donors in the crystal.
The exchange constant that constantly involves the
s-like states decreases the energy of the bottom conduc-
tion band, the exchange factor involving the p-like states
increases the energy of the top valence band by a cons-
tant factor that is independent of temperature.
3.5. Electron Spin Resonance Study
The ESR spectra for synthesized Cr-doped ZnO at
various composition observed at room temperature are
shown in Figure 6. The spectra for all samples exhibited
a very broad linewidth. The line width and intensity
increased with increasing Cr content. All spectra had a
symmetrical Lorentzian shape and have a similar line-
shape. A stable electronic configuration of a free Cr atom
is 3d5 4s1, rather than 3d4 4s2. As a dopant of the ZnO
matrix, Cr can be incorporated in several ways [37]: 1)
the electronic configuration of Cr can be 3d4 4s2, with the
two 4s electrons incorporated in the tetrahedral bonding
scheme, resulting in Cr2+ as an isoelectronic center; 2) a
3d5 4s2 configuration can be formed by borrowing an
electron from the valence band of the host with only one
4s electron of Cr being incorporated in the tetrahedral
bonding scheme, resulting in Cr+ and causing the other
Therefore, we believed that our ESR signal arose from
the Cr3+ centers instead of the Cr2+ ions. A similar result
was also obtained by Krishnaiah et al. [43] in ZnCrTe
crystals.All spectra lines were centered at approximately
550 G, corresponding to an effective g-value of 1.989
which is attributed to the typical value of Cr3+ ions
[42,45]. All of the g-values of the spectrum observed in
Cr-doped ZnO correspond not only to the Cr3+ ions

Chromium-Doped ZnO Nanoparticles Synthesized by Co-Precipitation: Chromium Effects
534
observed in Cr-doped II-VI compounds, but also to an
unpaired electron trapped at an oxygen vacancy site. It is
possible that the broad line of the Cr3+ ions overlap with
the resonance line of an unpaired electron trapped on an
oxygen vacancy site. However, the deconvolution would
be problematic because an accurate estimate of the inten-
sity under the broad resonance line is difficult.
3.6. Magnetic Characterization
The magnetic properties of the Cr-doped ZnO samples
were investigated using VSM in the magnetic field range
of 0 to ±1 T. The magnetization (M) versus the magnetic
field (H) measured at room temperature is illustrated in
Figure 7. The magnetization is plotted as a function of
magnetic field for different doping concentrations. All of
the Cr-doped samples in this study exhibit ferromagnetic
behavior.
An interesting feature is observed when we analyze
the magnetization data of Cr-doped samples studied in
this work. The 7, 12 and 16 at.% Cr-doped samples
showed the a remnant magnetizations of 0.35, 0.35, and
0.34 (×10−2) emu/g and coercive field of 482, 322 and
Figure 6. Experimental ESR spectra of the Cr-doped nano-
crystalline ZnO particles prepared at various Cr concen-
trations.
Figure 7. Room temperature M-H curves for Cr-doped
nanocrystalline ZnO particles sy nthe sized with various dop-
ing concentrations.
164 Oe, respectively. The magnetization increased for
samples doped with higher Cr concentrations. The 7 at.%,
12 at.% and 16 at.% Cr-doped samples possessed net
moments of 0.060, 0.077 and 0.144 µB/Cr, respectively.
The Cr-doped samples also had net moments far below
the theoretical values. However, only a small fraction of
Cr atoms contribute to ferromagnetism. The low mag-
netic moment values might be a result of the competition
between the ferromagnetic and antiferromagnetic coupling
that occurs at short distances between a pair of Cr ions.
Similar to ZnO doped with other transition metals, the
secondary phase in our samples is thought to be the source
of the spurious magnetic signal. In principle, as the Cr
concentration increases, a number of antiferromagnetic
phases may occur, such as ZnCr2O4 (TN 11K), Cr metal
clusters, Cr2O3 and Cr3O4 [46-48]. However, none of the
phases can be detected via XRD. Even, if these phases
are present in small quantities, none of them to ferro-
magnetism, except the CrO2 phase (TC ~386 K) [49].
Therefore, room temperature ferromagnetism in our Cr-
doped sample cannot be explained by the secondary
phases. Xu et al. [50] observed ferromagnetism in Cr-
doped ZnOnanorods. They observed a uniform incorpo-
ration of Cr into Zn with no secondary phases and sug-
gested that the combined effects of structural defects and
exchange interactions of the Cr ions substituting for Zn
in the ZnO matrix were responsible for the magnetization
in their Cr-doped ZnO samples.
The existence of compressive stress/and/or strain due
to the oxygen vacancies may also be a factor that to in-
fluences the shift of the XRD peaks position. Defects are
known to play a primary role in ferromagnetic ordering
in transition metal-doped semiconductors such as ZnO
[23,46,51]. An increasing number of studies have dem-
onstrated that the formation of oxygen vacancies and
zinc interstitial could lead to room temperature ferro-
magnetism or enhanced ferromagnetism [52-54]. Ac-
cording to the donor impurity band exchange model, the
ferromagnetic-exchange between transition-metal ions
and semiconductors is mediated by shallow donor elec-
trons that form bound magnetic polarons which overlap
to create a spin-split impurity band.
With regard to the origin of the ferromagnetism ob-
served in our samples and considering the above meas-
urements, we conclude that ferromagnetism in our Cr-
doped samples is an intrinsic property of Cr-doped ZnO.
Our data support the hypothesis that the magnetic be-
havior observed is related to the presence of intrinsic
defects. This can be understood via the BMP model dis-
cussed above.
4. Summary
In summary, a series of Cr-doped ZnO nanoparticle sam-
ples with different doping concentrations were success-

Chromium-Doped ZnO Nanoparticles Synthesized by Co-Precipitation: Chromium Effects 535
fully synthesized and characterized by EDX, XRD,
UV-Vis, ESR and VSM. The XRD patterns suggested
the formation of a wurtzite structure in theCr-doped ZnO.
The XRD results also indicated that Cr-ions were incur-
porated into the ZnO matrix. The XRD line broadening
due to the combination of the coherent scattering of X-
rays from a particular lattice plane and the random dis-
placement of atoms from their original positions, which
generates strain was analyzed by Scherrer’s formula and
by a modified form of W-H analysis. The three modified
W-H analyses were helpful in approximating the strain,
stress and energy density values. The results show that
the strain, stress and energy density values decreased with
increasing doping concentrations.
The magnetic measurements show that the Cr-doped
ZnO samples are ferromagnetic in nature with a well-
defined hysteresis at room temperature and the coercive
field (HC) and the remnant magnetization increase with
increasing doping concentrations. The analysis shows
that the room temperature ferromagnetic in Cr-doped ZnO
might be explained by BMP model.
REFERENCES
[1] G. A. Prinz, “Magnetoelectronics,” Science, Vol. 282, No.
5394, 1998, pp. 1660-1663.
doi:10.1126/science.282.5394.1660
[2] A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M.
Daughton, S. von Molnár, M. L. Roukes, A. Y. Chtchel-
kanova and D. M. Treger, “Spintronics: A Spin-Based
Electronics Vision for the Future,” Science, Vol. 294, No.
5546, 2001, pp. 1488-1495. doi:10.1126/science.1065389
[3] S. J. Pearton, C. R. Abernathy, M. E. Overberg, G. T.
Thaler, D. P. Norton, N. Theodoropoulou, A. F. Hebard,
Y. D. Park, F. Ren, J. Kim and L. A. Boatner, “Wide
Band Gap Ferromagnetic Semiconductors and Oxides,”
Journal of Applied Physics, Vol. 93, No. 1, 2003, 13 p.
doi:10.1063/1.1517164
[4] K. Ueda, H. Tabata and T. Kawai, “Magnetic and Electric
Properties of Transition-Metal-Doped ZnO Films,” Ap-
plied Physics Letters, Vol. 79, No. 7, 2001, pp. 988-990.
doi:10.1063/1.1384478
[5] H. Ohno, “Making Nonmagnetic Semiconductors Ferro-
magnetic,” Science, Vol. 281, No. 5379, 1998, pp. 951-
956. doi:10.1126/science.281.5379.951
[6] H. Ohno, H. Munekata, T. Penney, S. Von Molnár and L.
L. Chang, “Magnetotransport Properties of p-Type (In,
Mn)As Diluted Magnetic III-V Semiconductors,” Physi-
cal Review Letters, Vol. 68, No. 17, 1992, pp. 2664-2667.
doi:10.1103/PhysRevLett.68.2664
[7] H. Ohno, A. Shen, F. Matsukura, A. Oiwa, A. Endo, S.
Katsumoto and Y. Iye, “(Ga,Mn)As: A New Diluted
Magnetic Semiconductor Based on GaAs,” Applied Phys-
ics Letters, Vol. 69, No. 3, 1996, pp. 363-365.
doi:10.1063/1.118061
[8] F. Matsukura, H. Ohno, A. Shen and Y. Sugawara, “Tran-
sport Properties and Origin of Ferromagnetism in (Ga,Mn)
As,” Physical Review B, Vol. 57, No. 4, 1998, pp. R2037-
R2040. doi:10.1103/PhysRevB.57.R2037
[9] Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T.
Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Ko-
shihara and H. Koinuma, “Room-Temperature Ferromag-
netic in Transparent Transition Metal-Doped Titanium
Dioxide,” Science, Vol. 291, No. 5505, 2001, pp. 854-856.
doi:10.1126/science.1056186
[10] T. Wakano, N. Fujimura, Y. Morinaga, N. Abe, A. Ashida
and T. Ito, “Magnetic and Magneto-Transport Properties
of ZnO:Ni Films,” Physica E: Low-Dimensional Systems
and Nanostructures, Vol. 10, No. 1-3, 2001, pp. 260-264.
doi:10.1016/S1386-9477(01)00095-9
[11] P. Sharma, A. Gupta, K. V. Rao, F. J. Owens, R. Sharma,
R. Ahuja, J. M. Osorio, B. Johansson and G. A. Gehring,
“Ferromagnetism above Room Temperature in Bulk and
Transparent Thin Films of Mn-Doped ZnO,” Nature
Matter, Vol. 2 , No. 10, 2003, pp. 673-677.
doi:10.1038/nmat984
[12] T. Dietl, H. Ohno, F. Matsukura, J. Cibert and D. Ferrand,
“Zener Model Description of Ferromagnetism in Zinc-
Blende Magnetic Semiconductors,” Science, Vol. 287, No.
5455, 2000, pp. 1019-1022.
doi:10.1126/science.287.5455.1019
[13] R. Saleh, S. P. Prakoso and A. Fishli, “The Influence of
Fe Doping on the Structural, Magnetic and Optical Prop-
erties of Nanocrystalline Zno Particles,” Journal of Mag-
netism and Magnetic Materials, Vol. 324, No. 5, 2012, pp.
665-670. doi:10.1016/j.jmmm.2011.07.059
[14] G. Glaspell, P. Dutta and A. A Manivanna, “A Room-
Temperature and Microwave Synthesis of M-Doped ZnO
(M = Co, Cr, Fe, Mn & Ni),” Journal of Cluster Science,
Vol. 16, No. 4, 2005, pp. 523-536.
doi:10.1007/s10876-005-0024-y
[15] O. D. Jayakumar, H. G. Salunke, R.M. Kadam, M. Mo-
hapatra, G. Yaswant and S. K. Kulshreshtha, “Magnetism
in Mn-doped ZnO Nanoparticles Prepared by A Co-Pre-
cipitation Method,” Nanotechnology, Vol. 17, No. 5,
2006, p. 1278. doi:10.1088/0957-4484/17/5/020
[16] R. Elilaressi and G. Chandrasekaran, “Structural, Optical,
and Magnetic Properties of Nanoparticles of ZnO:Ni—
DMS Prepared by Sol-Gel Method,” Materials Chemistry
Physics, Vol. 123, No. 2-3, 2010, pp. 450-455.
doi:10.1016/j.matchemphys.2010.04.039
[17] D. B. Buchholz, R. P. H. Chang, J.-Y. Song and J. B.
Ketterson, “Room-Temperature Ferromagnetism in Cu-
Doped ZnO Thin Films,” Applied Physics Letters, Vol. 87,
No. 8, 2005, Article ID: 082504. doi:10.1063/1.2032588
[18] L.-H. Ye, A. J. Freeman and B. Delley, “Half-Metallic
Ferromagnetism in Cu-Doped ZnO: Density Functional
Calculations,” Physical Review B, Vol. 73, No. 3, 2006,
Article ID: 033203. doi:10.110x3/PhysRevB.73.033203
[19] K. P. Bhatti, S. Chaudhary, D. K. Pandya and S. C. Ka-
shyap, “Intrinsic and Extrinsic Origin of Room Tempera-
ture Ferromagnetism in ZnO:Co (5 at. %),” Journal of
Applied Physics, Vol. 101, No. 10, 2007, Article ID:
103919. doi:10.1063/1.2740343
[20] J. Zhang, R. Skomski and D. J. Sellmyer, “Sample
Preparation and Annealing Effects on the Ferromagnetism

Chromium-Doped ZnO Nanoparticles Synthesized by Co-Precipitation: Chromium Effects
536
in Mn-Doped ZnO,” Journal of Applied Physics, Vol. 97,
No. 10, 2005, Article ID: 10D303.
[21] D. P. Norton, M. E. Overberg, S. J. Pearton, K. Pruessner,
J. D. Budai, L. A. Boatner, M. F. Chisholm, J. S. Lee, Z.
G. Khim, Y. D. Park and R. G. Wilson, “Ferromagnetism
in Cobalt-Implanted ZnO,” Applied Physics Letters, Vol.
83, No. 26, 2003, p. 5488. doi:10.1063/1.1637719
[22] K. Sato and H. Katayama-Yoshida, “Ferromagnetism in a
Transition Metal Atom Doped ZnO,” Physica E: Low-
Dimensional Systems and Nanostructures, Vol. 10, No.1,
2001, pp. 251-255. doi:10.1016/S1386-9477(01)00093-5
[23] J. M. Coey, M. Venkatesan and C. B. Fitzgerald, “Donor
Impurity Band Exchange in Dilute Ferromagnetic Ox-
ides,” Nature Matter, Vol. 4, No. 2, 2005, pp. 173-179.
doi:10.1038/nmat1310
[24] L. Lutterotti, “MAUD (Material Analysis Using Diffrac-
tion) version 2.33,” 2010.
http://www.ing.unitn.it/~luttero/maud
[25] G. K. Williamson and W. H. Hall, “X-Ray Line Broad-
ening from Filled Aluminium and Wolfram,” Acta Met-
allurgica, Vol. 1, No. 1, 1953, pp. 22-31.
doi:10.1016/0001-6160(53)90006-6
[26] J. F. Nye, “Physical Properties of Crystals: Their Repre-
sentation by Tensors and Matrices,” Oxford University
Press, Oxford, New York, 1985.
[27] B. Hapke, “Theory of Reflectance and Emittance Spec-
troscopy,” University Press, Cambridge, 1993.
[28] P. Koidl, “Optical Absorption of Co2+ in ZnO,” Physical
Review B, Vol. 15, No. 5, 1977, pp. 2493-2499.
doi:10.1103/PhysRevB.15.2493
[29] J. Anghel, A. Thurber, D. A. Tenne, C. B. Hanna and A.
Punnoose, “Correlation Between Saturation Magnetiza-
tion, Bandgap, and Lattice Volume of Transition Metal
(M = Cr, Mn, Fe, Co, or Ni) doped Zn1-XMXO Nanopar-
ticles,” Journal of Applied Physics, Vol. 107, No. 9, 2010,
Article ID: 09E314. doi:10.1063/1.3360189
[30] Z. H. Wang, D. Y. Geng and Z. D. Zhang, “Room-Tem-
perature Ferromagnetism and Optical Properties of Zn1-
XMnXO Nanoparticles,” Solid State Communication, Vol.
149, No. 17-18, 2009, pp. 682-684.
doi:10.1016/j.ssc.2009.02.016
[31] M. Bouloudenine, N. Viart, S. Colis and A. Dinia, “Bulk
Zn1-XCoXO MAGNETIC Semiconductors Prepared by
Hydrothermal Technique,” Chemical Physics Letters, Vol.
397, No. 1-3, 2004, pp. 73-76.
doi:10.1016/j.cplett.2004.08.064
[32] K. J. Kim and Y. R. Park, “Spectroscopic Ellipsometry
Study of Optical Transitions in Zn1-XCoXO Alloys,”
Applied Physics Letters, Vol. 81, No. 8, 2002, p. 1420.
doi:10.1063/1.1501765
[33] A. J. Hays, A. Punnoose, R. Baldner, M. H. Engelhard, J.
Peloquin, K. M. Reddy, “Relationship between the
Structural and Magnetic Properties of Co-Doped SnO2
Nano-particles,” Physical Review B, Vol. 72, No. 7, 2005,
Article ID: 075203. doi:10.1103/PhysRevB.72.075203
[34] Z. W. Zhao, B. K. Tay, J. S. Chen, J. F. Hu, B. C. Lim
and G. P. Li, “Large Magnetic Moment Observed in
Co-Doped ZnO Nanoclusters-Assembled Thin Films at
Room Temperature,” Applied Physics Letters, Vol. 90,
No. 15, 2007, Article ID: 152502. doi:10.1063/1.2721140
[35] A. Parra-Palomino, O. Perales-Perez, R. Singhal, M. To-
mar, J. Hwang, P. M. Voyles, “Structural, Optical, and
Magnetic Characterization of Monodisperse Fe-Doped
ZnO Nanocrystals,” Journal of Applied Physics, Vol. 103,
No. 7, 2008, Article ID: 07D121. doi:10.1063/1.2834705
[36] Z. H. Wang, D. Y. Geng, and Z. D. Zhang, “Room-Tem-
perature Ferromagnetism and Optical Properties of Zn1-X
MnXO Nanoparticles,” Solid State Communications, Vol.
149, No. 17-18, 2009, pp. 682-684.
doi:10.1016/j.ssc.2009.02.016
[37] X. Lu, S. Tsoi, I. Miotkowski, S. Rodriguez, H. Alawadhi
and A. K. Ramdas, “Raman Electron Paramagnetic Reso-
nance in Zn1−xCrxTe and Cd1−xCrxTe,” Physical Review B,
Vol. 75, No. 15, 2007, Article ID: 155206.
doi:10.1103/PhysRevB.75.155206
[38] J. T. Vallin and G. D. Watkins, “The Spin Hamiltonian
for Cr2+ in CdS,” Physics Letters A, Vol. 37, No. 4, 1971,
pp. 297-298. doi:10.1016/0375-9601(71)90678-5
[39] T. L. Estle and W. C. Holton, “Electron-Paramagnetic-
Resonance Investigation of the Superhyperfine Structure
of Iron-Group Impurities in II-VI Compounds,” Physical
Review, Vol. 150, No. 1, 1966, pp. 159-167.
doi:10.1103/PhysRev.150.159
[40] J. T. Vallin and G. D. Watkins, “EPR of Cr2+ in II-VI
Lattices,” Physical Review B, Vol. 9, No. 5, 1974, pp.
2051-2072. doi:10.1103/PhysRevB.9.2051
[41] M. E. J. Boonman, W. Mac, A. Twardowski, A. Wittlin,
P. J. M. van Bentum, J. C. Maan and M. Demianiuk,
“High-Magnetic-Field EPR of Cr-Based Diluted Mag-
netic Semiconductors,” Physical Review B, Vol. 61, No. 8,
2000, pp. 5358-5368. doi:10.1103/PhysRevB.61.5358
[42] J. J. Kerbs and G. H. Strauss, “EPR of Cr(3d3) in
GaAs—Evidence for Strong Jahn-Teller Effects,” Physi-
cal Review B, Vol. 15, No. 1, 1977, pp. 17-22.
doi:10.1103/PhysRevB.15.17
[43] G. Krishnaiah, N. MadhusudhanaRao, D. R. Reddy, B. K.
Reddy and P. S. Reddy, “Growth and Structural Proper-
ties of Zn1−xCrxTe Crystals,” Journal of Crystal Growth,
Vol. 310, No. 1, 2008, pp. 26-30.
doi:10.1016/j.jcrysgro.2007.10.013
[44] A. Franco Jr. and R. C. Santana, “Electron Paramagnetic
Resonance (EPR) of Antiferromagnetic Nanoparticles of
La1−xSrxCrO3 (0.000 ≤ x ≤ 0.020) Synthesized by
Combustion Reaction,” Materials Chemistry and Physics,
Vol. 120, No. 1, 2010, pp. 225-228.
doi:10.1016/j.matchemphys.2009.10.052
[45] G. Krishnaiah, N. MadhusudhanaRao, B. K. Reddy, D. R.
Reddy, T. M. Babu, S. Sambasivam and P. S. Reddy,
“EPR and Magnetic Properties of Vapour Phase Grown
Zn1−xCrxTe Crystals,” Physics Letters A, Vol. 372, No. 42,
2008, pp. 6429-6433. doi:10.1016/j.physleta.2008.08.039
[46] H. Liu, X. Zhang, L. Li, Y. X. Wang, K. H. Gao, Z. Q. Li,
R. K. Zheng, S. P. Ringer, B. Zhang and X. X. Zhang,
“Role of Point Defects in Room-Temperature Ferromag-
netism of Cr-Doped ZnO,” Applied Physics Letters, Vol.
91, No. 7, 2007, Article ID: 072511.

Chromium-Doped ZnO Nanoparticles Synthesized by Co-Precipitation: Chromium Effects
537
doi:10.1063/1.2772176
[47] X. H. Chen, H. T. Zhang, C. H. Wang, X. G. Luo and P.
H. Li, “Effect of Particle Size on Magnetic Properties of
Zinc Chromite Synthesized by Sol-Gel Method,” Applied
Physics Letters, Vol. 81, No. 23, 2002, p. 4419.
doi:10.1063/1.1526921
[48] B. K. Roberts, A. B. Pakhomov and K. M. Krishnan,
“Effect of Hydrogen Codoping on Magnetic Ordering and
Conductivity in Cr:ZnO Thin Films,” Journal of Applied
Physics, Vol. 103, No. 7, 2008, Article ID: 07D133.
[49] R. Bhargava, P. K Sharma, A. K Chawla, S. Kumar, R.
Chandra, A. C Pandey and N. Kumar, “Variation in
Structural, Optical and Magnetic Properties of Zn1-XCrXO
(x= 0.0, 0.10, 0.15, and 0.20) Nanoparticles: Role of Do-
pant Concentration on Non-Saturation of Magnetiza-
tion,” Materials Chemistry and Physics, Vol. 125, No. 3,
2011, pp. 664-671.
doi:10.1016/j.matchemphys.2010.09.075
[50] C. Xu, K. Yang, Y. Liu, L. Huang, H. Lee, J. Cho and H.
Wang, “Buckling and Ferromagnetism of Aligned Cr-
Doped ZnO Nanorods,” The Journal of Physical Chemis-
try C, Vol. 112, No. 49, 2008, pp. 19236-19241.
doi:10.1021/jp806164b
[51] C. Song, K. W. Geng, F. Zeng, X. B. Wang, Y. X. Shen,
F. Pan, Y. N. Xie, T. Liu, H. T. Zhou and Z. Fan, “Giant
Magnetic Moment in an Anomalous Ferromagnetic Insu-
lator: Co-Doped ZnO,” Physical Review B, Vol. 73, No. 2,
2006, Article ID: 024405.
doi:10.1103/PhysRevB.73.024405
[52] J. L. MacManus-Driscoll, N. Khare, Y. Liu and M. E.
Vickers, “Structural Evidence for Zn Intersititials in Fer-
romagnetic Zn1–xCoxO Films,” Advanced Materials, Vol.
19, No. 19, 2007, pp. 2925-2929.
[53] H. S. Hsu, J. C. A. Huang, S. F. Chen and C. P. Liu,
“Role of Grain Boundary and Grain Defects on Ferro-
magnetism in Co:ZnO Films,” Applied Physical Letters,
Vol. 90, No. 10, 2007, Article ID: 102506.
doi:10.1063/1.2711763
[54] J. M. D. Coey, “Dilute Magnetic Oxide,” Current Opin-
ion in Solid State and Materials Science, Vol. 10, No. 2,
2006, pp. 83-92. doi:10.1016/j.cossms.2006.12.002.