Particle size effect on the magnetic properties of NiO nanoparticles prepared by a precipitation method

Article abstract of DOI:10.1016/j.jallcom.2010.09.033

Nickel oxide (NiO) nanoparticles of nominal size range 16 nm and 25 nm were obtained by controlling the calcination temperature. These particles were prepared by the precipitation method. Structural, optical and morphological characterizations were done by X-ray powder diffraction, UV-vis diffuse reflectance spectroscopy and scanning electron microscopy. Studies of magnetic measurements (up to 60 kOe) and temperature variations from 2 K to 300 K of the NiO nanoparticles were investigated. Particles of 16 nm exhibited a weak ferromagnetic component and hysteresis loop. There is a increase in coercivity H_c and the remanence M_r at 8 K accompanied by an exchange bias H_E. H_E monotonically tends to zero as the particles size varied from 16 nm to 25 nm. The hysteresis loop and the size dependent χ are interpreted with the uncompensated surface spins, whereas the transition at 30 K is suggested to be Ne?el temperature T_N of the spins in the core of the 16 nm particles. In addition, the increasing temperature cannot showed an approach to saturation in the magnetization curve, it indicates the possibility of an asperomagnetism and/or spin glass behavior of the NiO nanoparticles.

Full text of DOI:10.1016/j.jallcom.2010.09.033

Journal of Alloys and Compounds 509 (2011) 181–184
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Particle size effect on the magnetic properties of NiO nanoparticles
prepared by a precipitation method
a,∗
a
b
b
a
K. Karthik , G. Kalai Selvan , M. Kanagaraj , S. Arumugam , N. Victor Jaya
a
Department of Physics, Anna University, Chennai 600 025, India
Centre for High pressure Research, School of Physics, BARD, Tiruchirappalli 620 024, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 July 2010
Received in revised form 2 September 2010
Accepted 3 September 2010
Available online 16 September 2010
Nickel oxide (NiO) nanoparticles of nominal size range 16 nm and 25 nm were obtained by controlling the
calcination temperature. These particles were prepared by the precipitation method. Structural, optical
and morphological characterizations were done by X-ray powder diffraction, UV–vis diffuse reflectance
spectroscopy and scanning electron microscopy. Studies of magnetic measurements (up to 60 kOe) and
temperature variations from 2 K to 300 K of the NiO nanoparticles were investigated. Particles of 16 nm
exhibited a weak ferromagnetic component and hysteresis loop. There is a increase in coercivity Hc and
the remanence Mr at 8 K accompanied by an exchange bias HE. HE monotonically tends to zero as the
particles size varied from 16 nm to 25 nm. The hysteresis loop and the size dependent ꢀ are interpreted
with the uncompensated surface spins, whereas the transition at 30 K is suggested to be Néel temperature
TN of the spins in the core of the 16 nm particles. In addition, the increasing temperature cannot showed
an approach to saturation in the magnetization curve, it indicates the possibility of an asperomagnetism
and/or spin glass behavior of the NiO nanoparticles.
Keywords:
Nanoparticle
Precipitation
Magnetization
Exchange bias
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
exchange bias dominates the exchange coupling between the sur-
face spins and the antiferromagnetic core leading to a shift of the
magnetic hysteresis loop along the field axis [12]. In an antiferro-
magnetic nanoparticle, the fraction of surface to total spins depends
on the particle size and hence the magnetic moment, coercivity
and loop shift should vary with particle size [13–15]. Also, temper-
ature dependent magnetic behavior of the surface spins leads to
the enhanced magnetic moments, superparamagnetism, exchange
bias and interparticle interaction [16–18].
In this paper, we report the synthesis of NiO nanoparticles by
the novel precipitation method. Detailed magnetic investigations
in the temperature range of 2–300 K and in magnetic fields up
to 60 kOe were reported. Whereas, at lower temperature region
(8 K) the particles exhibit a weak ferromagnetic component and
hysteresis loop, it significantly showed a asperomagnetic behavior
at 150 K and 300 K. For the 16 nm particle showed the significant
exchange bias HE (below at TN ≈ 30 K) with an enhanced coercivity
value (Hc = 770 Oe) and it tends weakly towards zero as tempera-
ture increases to 300 K.
Finite size effect of magnetism has become a very active area
of research because of the unique properties of nanoscaled materi-
als and their technological applications [1–3]. In nanoparticles, the
surface spins dominate the magnetization due to their lower coor-
dination and uncompensated exchange couplings. This is in turn
leads to the changes in the magnetic properties [4–6]. However,
the nanostructured oxides of antiferromagnet have very different
magnetic natures as compared to the corresponding antiferro-
magnetic bulk materials [7]. Understanding the size dependent
magnetization is not only important for practical applications; but
also it is of great interest to identify the unprecedented nature
of the antiferromagnetic oxide nanostructures between the struc-
ture and properties. Among many antiferromagnetic oxides, NiO
has attracted much attention because of its high Néel tempera-
ture (523 K). Nanostructured NiO has been extensively studied in
recent years because of its high magnetization and hysteresis loop
shift after field cooling through the Néel temperature caused by
the surface effect [8–10]. Tiwari and Rajeev proposed the detailed
study on the NiO nanoparticles and stated the spin glass behav-
ior originates by the surface spin disorder [11]. In addition, the
2. Experimental
2
.1. Preparation of NiO nanoparticles
NiO nanoparticles were prepared through
a novel precipitation method.
Nickel(II) nitrate was used as a precursor for nickel. Aqueous oxalic acid was used
as a solvent and polyethylene glycol was used for surfactant. The molar ratio of
the precursor to the solvent ratio is (1:0.4) mol. The mixture was stirred at 2 h in
∗ Corresponding author. Tel.: +91 44 22358700; fax: +91 44 22358700.
E-mail address: karthik kkdi@yahoo.com (K. Karthik).
0
925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.09.033

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K. Karthik et al. / Journal of Alloys and Compounds 509 (2011) 181–184
nanoparticles are smaller at lower calcination temperature and we
also found that, all diffraction peaks in Fig. 1 for the cubic structured
NiO had a small but systematic shift toward higher angles with
decreasing particle size was determined. The lattice parameters
´
´
estimated from the XRD patterns are 4.167 A˚ and 4.163 A˚ respec-
tively for the 25 nm and 16 nm particles, reflecting a slight decrease
in the lattice parameters with the decrease in particle size. A lattice
contraction with decreasing particle size seems reasonable from a
thermodynamic point of view due to the higher surface curvature
as is observed in many nanocrystalline materials [20]. It is apparent
that the surface stresses and surface defect dipoles are the two phe-
nomena governing the changes in the lattice volume as the particle
size is reduced [21,22].
Typical SEM images of NiO nanoparticles are shown in Fig. 2.
These images reveal a systematic evaluation of morphology of the
prepared sample. It is observed from that the sample calcined at
◦
5
00 C shows homogeneous spherical shaped particles. The magni-
Fig. 1. X-ray powder diffraction plots of NiO nanoparticles.
fied image reveals that the average size of the particles is less than
2
5 nm.
room temperature. After stirring, the green precipitates were found. The precipi-
tates were collected and washed several times with ethanol and water separately.
The final product was maintained in an oven at 60 C for the removal of hydroxyl
3.2. Optical properties
◦
radicals. The resultant NiO nanoparticle was further treated with an appropriate
calcination temperature to obtain the better quality NiO nanocrystallites.
UV–vis absorption spectroscopy is one of the important method
to reveal the energy structures and optical properties of semi-
conductor nanocrystals. In our measurements, the sample was
analyzed through the diffuse reflectance arrangement. The optical
2.2.. Characterization and measurements
◦
absorption spectrum of the NiO nanoparticles calcined at 500 C is
The structures of the samples were identified by X-ray powder diffraction (XRD)
at room temperature on a PANalytical X’pert PRO X-ray diffractometer using Cu K␣
radiation (ꢁ = 1.5406 A˚ ) as the X-ray source. The morphology and particle size of the
prepared sample was examined by scanning electron microscopy (Hitachi S-3400).
UV–vis absorption spectra of the prepared samples were recorded using UV–vis
spectrophotometer (Shimadzu UV2401 PC). The magnetic properties of the NiO
nanoparticles were studied by using a 9T Physical Property Measurement System
shown in Fig. 3. It is observed from that the strong absorption peak
with broader shoulder could be detected in the wavelength range
of 350–450 nm. The extrapolated value (the straight line to the
x axis) gives the absorption energy corresponding to Eg = 2.93 eV.
This increase in the band gap of the NiO nanoparticles is indicative
of the quantum confinement effect arising from the tiny crystal-
lites.
(
PPMS – Vibrating Sample Magnetometer of Quantum Design) in the temperature
range of 2–300 K.
3
. Results and discussion
3.3. Magnetic properties
3.1. Structural and morphological characteristics
The temperature variation of magnetic susceptibility ꢀ = M/H
(H = 1.25 T) is plotted in Fig. 4 for 16 nm and 25 nm particles.
The X-ray powder diffraction patterns of the calcined samples
Fig. 1) show only peaks due to NiO. Samples can be clearly indexed
to a cubic structure (JCPDS # 73-1519), Fm-3m space group,
without any observable peaks of impurity phases. Employing the
Scherrer ˇı s relation [19], the average particle size is determined
from the major peaks of the reflections. The average particles size
Comparison of the behavior of ꢀ versus T for the larger particle
gives enhanced magnetic susceptibility plot as compared to the
smaller particles. The small rise in ꢀ seen at lower temperature
region (below at 30 K) is observed and there is no evidence of any
peaks. When a stronger magnetic field is applied (H = 1.25 T), the
magnetization direction of the nanomaterials becomes easier and
the blocking temperature TB systematically decreases results the
absence of peaks. However, the TB should increase with increasing
(
(
d) increases monotonously from 16 nm to 25 nm as the calcination
◦
◦
temperature increases from 500 C to 800 C. It shows that the NiO
◦
Fig. 2. SEM images of NiO nanoparticles calcined at 500 C.

K. Karthik et al. / Journal of Alloys and Compounds 509 (2011) 181–184
183
Fig. 4. Temperature variation of the magnetic susceptibility ꢀ for different particle
sizes.
Fig. 3. UV–vis optical absorption spectrum of 16 nm NiO particles.
Fig. 5. (a) Measured magnetization of the 16 nm NiO nanoparticles as a function of applied field H up to 60 kOe at selected temperatures between 8 and 300 K and (b) the
separate M versus H plot of 16 nm particles at 8 K and the inset shows the representation of HEB.
Fig. 6. The magnetization curves of the 25 nm NiO nanoparticles with the filed up to 60 kOe at selected temperatures between 8 and 300 K and the inset shows the M versus
plot at 8 K.

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K. Karthik et al. / Journal of Alloys and Compounds 509 (2011) 181–184
Hc value for the 25 nm particle is 130 Oe and it infers that the Hc
value decrease toward the increase in particle size. The magneti-
zation curves for the particles at 150 K and 300 K show the linear
dependence of the field. So the main conclusions from these investi-
gations are that at T ≈ 30 K and represents a transition to a magnetic
state for which exchange bias is present.
4. Conclusions
The NiO nanocrystallites with different sizes were successfully
synthesized by the precipitation method. We found that the par-
ticle size have strong influence on the magnetic properties of
NiO nanoparticles. The measured magnetic susceptibility of NiO
nanoparticles shows an approach to rise in behavior at lower
fields (below at 30 K). The M versus H plot exhibits a weak fer-
romagnetic component at 8 K accompanied by an uncompensated
surface spins. For the 16 nm particles, TN ≈ 30 K for the spins in
the core is observed. Below this value (TN), the exchange bias and
the enhanced coercivity are observed. For particles greater than
Fig. 7. Comparison of the hysteresis loop for the NiO nanoparticles as a function of
particle size at 8 K.
1
6 nm, the weak ferromagnetic hysteresis loop shift to the posi-
tive direction expected the systematic shift in exchange bias. With
an increase in temperature (T > 8 K), the magnetization curve can-
not show an approach to weak ferromagnetic hysteresis and the
magnetic ordering is essentially similar to the antiferromagnetic
ordering. It indicates the possibility of an asperomagnetism and/or
spin glass behavior of the NiO nanoparticles.
particle size because the energy barrier separating the low energy
states is proportional to the volume of the particles.
Uncompensated surface spins leads to the changes in the mag-
netic moment for H → 0, which in turn greatly depends on the
particle size, crystal structure and morphology of the particle. The
measured magnetization under the maximum field of 60 kOe at
temperatures ranges from 2 to 300 K are shown in Fig. 5a and b
for the 16 nm particles. The plot shows the weak ferromagnetic
component at 8 K in the lower field and it progressively exhibit a
linear behavior up to 60 kOe. There is a dramatic increase in both
Mr and Hc at 8 K for the 16 nm particles, mirroring the increase in ꢀ
observed in Fig. 4 below at 30 K. Both the Mr and Hc decreases slowly
with the rise in temperature and at room temperature the magne-
tization value increases linearly with the applied field without the
presence of hysteresis behavior. There is no sign of demagnetization
plateau in the 16 nm particles for the magnetization curves at T > 8 K
do not show an approach to saturation, it implies that the possibil-
ity of an asperomagnetism and/or spin glass behavior of the sample
Acknowledgement
We acknowledge the DST Govt of India for providing PPMS-VSM
at Bharathidasan University, Tiruchirappalli by which magnetic
measurements were carried out.
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