Synthesis and anomalous magnetic properties of hexagonal CoO nanoparticles

Article abstract of DOI:10.1016/j.materresbull.2011.05.037

CoO nanoparticles in the 38-93 nm range have been prepared by thermal decomposition. The particles were characterized to be pyramid shape with a hexagonal close-packed structure. Their anomalous magnetic behavior includes: (i) vanishing of antiferromagnetic transition around 300 K; (ii) creation of hysteresis below a blocking temperature of 6-11 K; (iii) presence of relatively large moments and coercivities accompany with specific loop shifts at 5 K; and (iv) appearance of an additional small peak located in low field in the electron spin resonance spectrum. Further, the present results provide evidence for the existence of uncompensated surface spins. The coercivity and exchange bias decrease with increasing particle size, indicating a distinct size effect. These observations can be explained by the multisublattice model, in which the reduced coordination of surface spins causes a fundamental change in the magnetic order throughout the total CoO particle.

Full text of DOI:10.1016/j.materresbull.2011.05.037

Materials Research Bulletin 46 (2011) 1692–1697
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Synthesis and anomalous magnetic properties of hexagonal CoO nanoparticles
*
Xuemin He, Huigang Shi
Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Article history:
CoO nanoparticles in the 38–93 nm range have been prepared by thermal decomposition. The particles
were characterized to be pyramid shape with a hexagonal close-packed structure. Their anomalous
magnetic behavior includes: (i) vanishing of antiferromagnetic transition around 300 K; (ii) creation of
hysteresis below a blocking temperature of 6–11 K; (iii) presence of relatively large moments and
coercivities accompany with specific loop shifts at 5 K; and (iv) appearance of an additional small peak
located in low field in the electron spin resonance spectrum. Further, the present results provide
evidence for the existence of uncompensated surface spins. The coercivity and exchange bias decrease
with increasing particle size, indicating a distinct size effect. These observations can be explained by the
multisublattice model, in which the reduced coordination of surface spins causes a fundamental change
in the magnetic order throughout the total CoO particle.
Received 29 November 2010
Received in revised form 9 May 2011
Accepted 27 May 2011
Available online 23 June 2011
Keywords:
A. Oxides
B. Chemical synthesis
C. Electron microscopy
D. Magnetic properties
ß 2011 Elsevier Ltd. All rights reserved.
1. Introduction
contribute to the observed magnetic moment via uncompensated
cobalt spins at the surface. Moreover, Coey and co-workers [20]
Bulk CoO is known to be an insulating antiferromagnet with
have suggested that the wurtzite CoO thin films are paramagnetic,
which is attributed to the geometric frustration of the antiferro-
magnetic Co2+–O2ꢀ–Co2+ superexchange. Obviously, the solution
to these conflicting views has become a challenging issue.
Based on the above studies, the peculiarities of structure and
even the configuration of magnetic ordering have been analyzed
with magnetic resonance [21] or electron spin resonance [22].
Taking into account obvious similarities between CoO and NiO, a
strategy which combines resonance technique and multisublattice
model may solve the problem mentioned above. Herein we report
a facile and reproducible process for the large-scale synthesis of
hexagonal CoO nanoparticles. The size of synthesized CoO
nanoparticles with pyramid shape can be controlled by changing
the precursor concentration. In the present article we focus on the
influence of the particle size on the magnetization, coercivity and
exchange bias. We present an analysis of the anomalous magnetic
properties in terms of a multisublattice model. Finally, the
magnetic state of CoO particles was studied by electron spin
resonance.
´
rocksalt cubic structure and Neel temperature of 298 K [1]. In 1961
´
Neel suggested that small antiferromagnetic nanoparticles (AFN)
should exhibit superparamagnetism or weak ferromagnetism [2].
He attributed the permanent magnetic moment to an uncompen-
sated number of spins on two sublattices. Naturally, the magnetic
behavior of cubic CoO nanoparticles was widely studied in the next
half a century [3–6]. Indeed, large magnetic moments in AFN have
been observed [7–10]; however, it also exhibits anomalous
magnetic properties such as large coercivities and loop shifts of
up to several kOe. This behavior is difficult to understand in terms
of 2-sublattice antiferromagnetic ordering which is accepted for
bulk materials. According to Kodama and co-workers [11,12], the
number of sublattice more than two in AFN, in which these
additional sublattices disturb otherwise compensated AF bulk
structure and lead to numerous effects including hysteresis and
shift of hysteresis loops.
Recently several groups reported the preparation and relevant
studies of hexagonal CoO nanocrystals with a wurtzite structure
[13–16]. On the one hand, some electronic structure calculations
predicted that wurtzite CoO has an antiferromagnetic ground state
[17,18]. On the other hand, some experimental studies concluded
that no long range magnetic ordering is present in this structure
[13,14]. Further, it has been postulated by Dietl et al. [19] that
2. Experimental
2.1. Sample preparation
´
hexagonal CoO nanoparticles with a high Neel temperature
Hexagonal CoO nanoparticles were prepared by the pyrolysis
method. In a typical synthesis process, 0.4 g (1.12 mmol) of
cobalt(III) acetylacetonate (Co(acac)3, 98+%) was added to 18.5 ml
(56.23 mmol) of oleylamine (OAm, approximate C18-content 80–
90%) in a three-necked flask. The bottom of the three-necked flask
* Corresponding author. Tel.: +86 931 8912688; fax: +86 931 8914160.
E-mail address: shihuig@lzu.edu.cn (H. Shi).
0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2011.05.037

X. He, H. Shi0 / Materials Research Bulletin 46 (2011) 1692–1697
1693
Table 1
Particle size (D), lattice parameter (a, c), saturation magnetization (Ms), coercivity
(HC) and exchange bias (HE) for the hexagonal CoO samples synthesized at different
precursor concentrations (C).
˚
Sample
C
D
a and c (A)
Ms
HC
HE
(nm)
(emu/g)
(Oe)
(Oe)
M1
M2
M3
M4
1/200
1/150
1/100
1/50
38
49
67
93
a = 3.254(3)
c = 5.216(4)
a = 3.253(1)
c = 5.214(2)
a = 3.251(9)
c = 5.213(5)
a = 3.250(6)
c = 5.210(7)
3.80
2.94
2.22
0.99
845
620
484
357
206
150
83
50
was merged into heat conduction oil for a heating-up. The green
slurry was heated at 130 8C (oil bath temperature) for 10 min
under an argon atmosphere. Following this dissolution, the green
solution was heated up to 210 8C at a rate of 5 8C minꢀ1, and was
kept at this temperature for 1 h. Then, the reaction mixture was
cooled down to room temperature and a green colloidal solution
was formed. The green precipitate was separated upon the
addition of ethanol and hexane, centrifuged, and washed using a
mixture of ethanol and toluene. Finally, the green CoO nanopar-
ticles (sample M4) were dried in an oven at room temperature,
overnight. Under similar reaction conditions, other three CoO
samples (M1–M3) with various particle sizes could be produced by
simply changing the precursor concentration (the molar ratio of
Co(acac)3 and OAm), as shown in Table 1.
Fig. 1. XRD patterns of CoO nanoparticles formed in different precursor
concentrations: (a) 1/200; (b) 1/150; (c) 1/100; (d) 1/50.
structure. Seven obvious Bragg peaks can be assigned to scattering
from the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2)
planes of the CoO crystal lattice, respectively. The peaks are slightly
broadened with decreasing precursor concentration. By making
use of the line width, the particle size (D) can be estimated using
Scherrer formula. The lattice parameter (a, c) increases slightly
with the decrease in the particle size in the 38–93 nm range, as
shown in Table 1.
2.2. Characterization
Fig. 2 shows the representative SEM images of the as-
synthesized CoO particles obtained under different precursor
concentrations. It is clear that most CoO particles have pyramid-
like shapes. The size distributions of four samples were also shown
in Fig. 2a1–d1. Their average particle size was found to be 39.7,
49.3, 64.6 and 94.1 nm, respectively, which was in excellent
agreement with the results of XRD analysis. It is obvious that the
standard deviations of particle size are less than 10% for all
samples. Moreover, the increase of the particle size deteriorates
the uniformity of the size distribution. It is reasonable to deduce
that the different concentration of oleylamine will cause a different
density of organic functional groups on the particle surface, which
may be the direct reason for easy control over the particle size and
size distribution during the growth process.
Powder X-ray diffraction (XRD) patterns of samples were
recorded from 2
equipped with
u
a
= 10–908 on a Philips X’pert diffractometer
rotating anode and Cu radiation
Ka
(l = 0.15418 nm). Scanning electron microscope (SEM) and
transmission electron microscope (TEM) images were obtained
using a Hitachi S-4800 instrument and a FEI Tecnai G2-F30
instrument, respectively. X-band (
n
ꢁ 8.98 GHz) electron spin
resonance (ESR) measurements were carried out on powdered
samples in the temperature range of 80–300 K using a JEOL FA200
spectrometer.
2.3. Magnetic measurement
Magnetic measurements were carried out using a super-
conducting quantum interference device (SQUID) magnetometer
(Quantum Design, MPMS-XL). 10 mg of the original aliquots was
Typical TEM images for CoO nanoparticles formed at different
precursor concentrations are shown in Fig. 3A–D. It is apparent
that the as-synthesized CoO particles are of hexagonal pyramid
shaped configuration with various sizes. As the precursor
concentration reduced from 1/50 to 1/200, the mean side edge
length of CoO particles slightly decreases from 85–105 nm to about
40 nm. As shown in Fig. 3E and F, the hexagonal pyramidal shape
can be confirmed by tilting the triangle or hexagon TEM images.
The side edge (corresponds to the triangle) length is almost double
that of basal edge (corresponds to the hexagon) for the hexagonal
CoO particles. Obviously, both particle size and shape are
consistent with above SEM analysis. As shown in Fig. 3G, the
dispersed in 40
ml of polyethylene glycol (PEG) solution and then
injected into a standard gelatin capsule. In order to measure the
exchange bias field, a sample is cooled from 300 to 2 K in an applied
magnetic field of 1 T. The field-dependent magnetization is then
measured at 5 and 300 K. The saturation magnetization Ms,
coercivity HC and exchange bias HE are determined from the
hysteresis loops after subtracting the linear paramagnetic magne-
tization of PEG. The HC and HE are calculated as HC ¼ ðjHCꢀj þ
jHCþjÞ=2 and HE = ꢀ (HCꢀ + HC+)/2, where HCꢀ and HC+ are the
negative and positive coercive fields, respectively. The tempera-
ture-dependent magnetization M(T) was measured in an applied
field of 0.03 T.
˚
lattice spacing (d) of 2.60 A in the triangle image corresponds to the
interplanar separation between (0 0 2) lattice planes. While the
hexagon image in Fig. 3H shows the lattice fringe images
˚
(d = 2.80 A) from the (1 0 0) planes. The corresponding fast Fourier
3. Results and discussion
transform (FFT) images of the triangle and hexagon images are
shown in the insets of Fig. 3G and H, respectively. The regular dot
matrix composed of many bright spots further confirms the hcp
structure of CoO particles.
Fig. 4 shows 300 Oe field-cooled (FC) and zero-field-cooled
(ZFC) magnetizations as functions of temperature for the CoO
Fig. 1 shows typical XRD patterns of the solid samples obtained
under different precursor concentrations. It is shown that the four
samples are all CoO particles. All the peaks of four patterns are well
indexed as wurtzite CoO phase with hexagonal close-packed (hcp)

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X. He, H. Shi / Materials Research Bulletin 46 (2011) 1692–1697
Fig. 2. SEM images and their corresponding size distributions of CoO nanoparticles formed in different precursor concentrations: (a) 1/200; (b) 1/150; (c) 1/100 and (d) 1/50.

X. He, H. Shi0 / Materials Research Bulletin 46 (2011) 1692–1697
1695
Fig. 3. TEM images of the hexagonal pyramid-shaped CoO nanoparticles formed in different precursor concentrations: (A) 1/200; (B) 1/150; (C) 1/100; (D) 1/50. TEM images of
single trigonal (E) and hexagonal (F) nanoparticles. HRTEM lattice fringe images and their corresponding FFT images (see insets) of triangle (G) and hexagon (H) particles.
nanoparticles with various sizes. The field-cooled magnetization
MFC increases monotonically with decreasing temperature, and the
MFC(T) curves are expected to show a continued rise below the
blocking temperature TB. The zero-field-cooled magnetization
MZFC increases at first and then decreases quickly with the decrease
of temperature, and the MZFC(T) curves exhibit a maximum at TB.
When the temperature is less than TB, the magnetic moment of
individual particles is blocked along one of the anisotropy
directions and does not respond to the weak applied field,
and hence, the magnetization depends on the magnetic history.
This causes the difference in FC and ZFC magnetization. Bulk
CoO is known to show an antiferromagnetic transition around
300 K. However, we do not see such a distinct AF transition in
our CoO samples. The AF transition is wiped out as the particle
size decreases, probably due to ferromagnetic interactions in
these nanodimensions. Analogous results were also observed on
cubic CoO nanoparticles [4,5]. As can be seen from the insets in
Fig. 4, there exhibit a branching phenomenon between the
MFC(T) and MZFC(T) curves. This is attributed to the non-
compensation of surface spins. Meanwhile, the blocking
temperature of four samples was found to be ꢂ6, 9, 10 and
11 K, respectively. It is known that TB depends on interparticle
interactions and follows the relationship of TBꢂKV for diluted
samples, where K is the magnetocrystalline anisotropy constant
and V is the magnetic volume of a nanoparticle. Further, our CoO
samples have the same concentrations, so that the interparticle
interactions can be ruled out. It can be deduced that the
magnetic volume V increases, whereas the effective magneto-
crystalline anisotropy K decreases with the decrease of total
particle size [3,23]. Finally, the comprehensive influence
between the two factors leads to the slight change of the
blocking temperature from 6 to 11 K.
The insets of Fig.
5 show that the hysteresis loop is
superimposed on a large linear background, is exceptionally broad
and asymmetric around the origin. Fig. 5 shows the saturating
parts of the loops, which can be obtained by subtracting the linear
parts from the original hysteresis loops. It is obvious that the
magnetization can achieve saturation, and lead to a closed loop.
Meanwhile, it clearly showing the presence of coercivity and loop
offset. For the four CoO samples with different particle sizes, the
specific values of saturation magnetization Ms, coercivity HC and
exchange bias HE obtained from Fig. 5 are shown in Table 1.
The size dependence of Ms, HC and HE are plotted in Fig. 6.
Obviously, the Ms, HC and HE increase monotonically with
decreasing particle size, indicating an obvious size effect. A
relative large Ms (3.80 emu/g) can be observed in the 38-nm CoO
particles. This is due to the magnetization contribution of the
uncompensated surface spins. However, the relative large HC
and HE are difficult to understand in terms of 2-sublattice
antiferromagnetic ordering which is accepted for bulk materials.
As has been previously noted on NiO nanoparticles [8,11,24], the
observations of large net moments are expected, due to surface
termination of the antiferromagnetic structure. Therein the
broad and open hysteresis loop was further explained as the

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X. He, H. Shi / Materials Research Bulletin 46 (2011) 1692–1697
result of multiple sublattice formation. This shows that the
lower coordination of the surface moments affects the overall
antiferromagnetic structure of the entire nanoparticle. Accord-
ing to the multisublattice model [9,11], we expect that the
canting of the spins and the number of magnetic sublattices for
38-nm CoO particles is large, and for 93-nm CoO particles is
small. As a result, the relatively weak coupling between the
sublattices allows a variety of reversal paths for the spins upon
cycling the applied field, resulting in large coercivities and loop
shifts. In summary, the smaller the particle size, the more the
number of the sublattice, and the more helpful to enhance the
coercivity and exchange bias.
In order to confirm the exact magnetic state of the CoO
nanoparticles, we also performed an ESR spectroscopy study.
Fig. 7(a) shows the typical ESR spectra of the 38 nm CoO particles.
It is clear that all spectra exhibit an asymmetric broad resonance
line. Moreover, an additional small peak located in low field Hlow
can be observed in the total ESR line (H0 is the resonance field),
shown as solid triangles in Fig. 7(b). This additional peak can be
considered similar to a ferromagnetic mode. In other words, the
resonance observed at Hlow is attributed to the weak ferromagnetic
phase in the hexagonal CoO particles. It is well known that the ESR
resonance signal of most bulk antiferromagnets will disappear
´
below the Neel point. For our hexagonal CoO nanoparticles,
however, the ESR signal can be detected even at very low
temperatures. According to Rivadulla et al. [25], the fact that the
resonance line is observable in the AF state is an indication of the
existence of canting between multiple sublattices. Obviously, this
is in perfect agreement with the results of Kodama et al. [21,22].
Therefore, the ESR study of the CoO nanoparticles has revealed a
complicated character of the temperature evolution of the ESR
spectra which can be explained in terms of the presence of at least
two different magnetic structures. The two resonance lines at H0
and Hlow may be attributed to the contributions of intrinsic AF
Fig. 4. ZFC and FC (0.03 T) magnetization curves of hexagonal CoO nanoparticles
with particle sizes of 38 nm (a), 49 nm (b), 67 nm (c), and 93 nm (d). Insets show
greater detail of the same measurements.
Fig. 5. The saturating parts of the hysteresis loops for the hexagonal CoO nanoparticles with particle sizes of 38 nm (a), 49 nm (b), 67 nm (c), and 93 nm (d), obtained by
subtracting the linear parts from the as-measured 5 K magnetization loops (see insets).

X. He, H. Shi0 / Materials Research Bulletin 46 (2011) 1692–1697
1697
show hysteresis at 5 K. Anomalous magnetic properties such as
relative large moments, coercivities and loop shifts were observed
on hexagonal CoO nanoparticles. The saturation magnetization,
coercivity and exchange bias increase monotonically with
decreasing particle size, indicating an obvious size effect. Accord-
ing to the multisublattice model, the smaller the particle size, the
more the number of the sublattice, and the more helpful to
enhance the coercivity and exchange bias. The observed two
resonance lines in ESR spectra can be attributed to the contribu-
tions of intrinsic antiferromagnetic structure and uncompensated
surface spins.
Acknowledgements
The authors would like to express their sincere thanks to Dr.
Xiaolong Fan, Prof. Li Xi and Yong Peng for valuable discussion. This
work was supported by the National Natural Science Foundation of
China (no. 50801033).
Fig. 6. Size dependence of Ms, HC and HE for the hexagonal CoO nanoparticles.
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Hexagonal CoO nanoparticles in the size range 38–93 nm were
prepared by the thermal decomposition of cobalt acetylacetonate
in oleylamine. The hcp-structured CoO nanoparticles are of
pyramid configuration, and the particle size increases with
increasing precursor concentration. The particles do not exhibit
a distinct antiferromagnetic transition around 300 K but instead
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