Synthesis and magnetic properties of antiferromagnetic Co3O4 nanoparticles

Article abstract of DOI:10.1016/j.physb.2008.03.024

Antiferromagnetic Co₃O₄ nanoparticles with diameter around 30 nm have been synthesized by a solution-based method. The phase identification by the wide-angle X-ray powder diffraction indicates that the Co₃O₄ nanoparticle has a cubic spinel structure with a lattice constant of 0.80843(2) nm. The image of field emission scanning electron microscope shows that the nanoparticles are assembled together to form nanorods. The magnetic properties of Co₃O₄ fine particles have been measured by a superconducting quantum interference device magnetometer. A deviation of the Néel temperature from the bulk is observed, which can be well described by the theory of finite-size scaling. An enhanced coercivity as well as a loop shift are observed in the field-cooled hysteresis loop. The exchange bias field decreases with increasing temperature and diminishes at the Néel temperature. The training effect and the opening of the loop reveal the existence of the spin-glass-like surface spins.

Full text of DOI:10.1016/j.physb.2008.03.024

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Physica B 403 (2008) 3141– 3145
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Physica B
journal homepage: www.elsevier.com/locate/physb
Synthesis and magnetic properties of antiferromagnetic Co3O4 nanoparticles
Ã
H.T. Zhu a,b, J. Luo a, , J.K. Liang a,c, G.H. Rao a, J.B. Li a, J.Y. Zhang a, Z.M. Du b
a Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
b Department of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
c International Center for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Antiferromagnetic Co3O4 nanoparticles with diameter around 30 nm have been synthesized by a
solution-based method. The phase identification by the wide-angle X-ray powder diffraction indicates
that the Co3O4 nanoparticle has a cubic spinel structure with a lattice constant of 0.80843(2) nm. The
image of field emission scanning electron microscope shows that the nanoparticles are assembled
together to form nanorods. The magnetic properties of Co3O4 fine particles have been measured by a
superconducting quantum interference device magnetometer. A deviation of the Ne´el temperature from
the bulk is observed, which can be well described by the theory of finite-size scaling. An enhanced
coercivity as well as a loop shift are observed in the field-cooled hysteresis loop. The exchange bias field
decreases with increasing temperature and diminishes at the Ne´el temperature. The training effect and
the opening of the loop reveal the existence of the spin-glass-like surface spins.
Received 5 January 2008
Received in revised form
4 March 2008
Accepted 26 March 2008
PACS:
75.50.Tt
75.50.Ee
75.75.+a
Keywords:
& 2008 Elsevier B.V. All rights reserved.
Nanoparticles
Antiferromagnetism
Finite size effects
1. Introduction
involved in many advanced chemical and physical applications
such as catalysis, sensors, magnetic materials, and energy storage
[12,15,17]. In particular, many researchers have focused on the
Magnetism of nanostructured materials has attracted consid-
erable attention by virtue of their unique magnetic properties as
well as their prominent application potentials [1–7]. Normally, the
antiferromagnetic (AFM) materials have only limited applications
due to the complete canceling of the antiparallelly aligned
magnetic moment. However, increasing interest has been gener-
ated for the AFM nanoparticles since the discovery of their
potentials for exhibiting magnetization reversal by quantum
´
weak ferromagnetism below the Neel temperature and exchange
anisotropy of the AFM Co3O4 nanoparticles [12–14,18–20]. The
´
bulk Co3O4 compound is antiferromagnetic with a Neel tempera-
ture about 40 K, and has a normal cubic spinel structure with a
lattice constant of 0.808 nm [21]. The Co3O4 unit cell contains 16
Co3+ octahedral ions exhibiting diamagnetism and eight Co2+
tetrahedral ions ordered antiferromagnetically [21]. However, the
´
Neel temperature of the Co3O4 nanoparticle is size dependent. In
´
tunneling [8,9], their uncompensated magnetization below Neel
addition, the hysteresis loop and loop shift of the Co3O4
nanoparticle have been observed at low temperature [13,18,19].
In this paper, we present the magnetic properties of AFM Co3O4
nanoparticles with an average diameter around 30 nm.
temperature [10–14], and their applications in spin–valve sys-
tems, etc. [15]. As for the magnetic properties of AFM nanopar-
´
ticles, the small size and large surface play a significant role. Neel
[16] suggested that fine particles of AFM materials should exhibit
superparamagnetism or weak ferromagnetism, which was as-
cribed to the uncompensated spins on two sublattices. Kodama et
al. [11] proposed a multisublattice configuration, indicating a new
finite size effect, in which the reduced coordination of the surface
spins led to a fundamental change in the magnetic order of the
whole particle.
2. Experimental details
The AFM Co3O4 nanoparticles were synthesized by a solution-
based method. All the reagents used in our experiments were
analytically pure and were purchased from Beijng Chemical
Recently, the research on synthesis and magnetic properties of
AFM Co3O4 nanoparticles has become significant because it is
Reagent Company.
A
mixture of 2.5 mmol CoCl2 ꢀ 6H2O and
10 mmol (NH2)2CO was dissolved into the deionized water to
form the precursor solution. Then the precursor solution was
transferred into a Teflon-lined stainless steel autoclave of 65 ml
capacity. The autoclave was sealed and then heated for 10 h at
Ã
Corresponding author. Tel.: +8610 82648119; fax: +8610 82649531.
E-mail address: jluo@aphy.iphy.ac.cn (J. Luo).
0921-4526/$ - see front matter & 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2008.03.024

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Fig. 2. XRD pattern of the Co3O4 nanoparticles.
Fig. 1. TGA curves of the as-prepared precipitates.
100 1C. The precipitates were collected by centrifuging and
washed several times by deionized water to remove any
impurities after the autoclave cooled to room temperature
naturally. The washed precipitates were then dried at 60 1C for
12 h. According to the wide-angle powder X-ray diffraction (XRD)
pattern, the intermediate product was cobalt hydroxide carbonate
with the chemical formula Co(OH)x(CO3)0.5(2ꢁx) ꢀ nH2O, and the
corresponding chemical reaction in the autoclave could be
expressed as [22]
Co2þ þ xOHꢁ þ 0:5ð2 ꢁ xÞCO32ꢁ þ nH2O
! CoðOHÞxðCO3Þ0:5ð2ꢁxÞ ꢀ nH2O
(1)
The calcination temperature was set according to our TGA
measurement in dry air by TA-Q600. Fig. 1 shows that the
decomposition temperature of the product is around 320 1C.
Thereby, the product was calcinated at 380 1C for 4 h. Another
decomposition reaction at 920 1C associated with the decomposi-
tion of Co3O4 into CoO and O2 is also presented on the TGA curve,
which can be confirmed by the weight loss percentage. As shown
in Fig. 1, this reaction is reversible since the weight loss is
compensated when the sample is cooled down to 880 1C.
Phase identification and structure analysis of the sample have
been carried out by XRD using a Rigaku D/max 2500 diffract-
ometer with Cu-Ka radiation (50 kV ꢂ 250 mA) and a graphitic
monochromator. XRD data were collected by a step-scan mode
with a step width of 2y ¼ 0.021 and a sampling time of 1 s. A field
emission scanning electron microscope (FESEM, FEI-Sirion) was
used to characterize the morphology of the AFM Co3O4 nanopar-
ticles. The magnetic properties of Co3O4 nanoparticles were
measured using a superconducting quantum interference device
(SQUID) magnetometer.
Fig. 3. Diameter distribution of the Co3O4 nanoparticles.
Co3O4 nanoparticles has also been determined from the FESEM
image (Fig. 3). The average diameter of the nanoparticles
is around 30.6 nm (standard deviation 4.15), which agrees well
with the calculated result based on the peak analysis of XRD
pattern.
The morphology of AFM Co3O4 has been investigated by
FESEM. As shown in Fig. 4a, the sample consists of Co3O4
nanorods with the diameter of a few hundred nanometers
(averagely 400 nm) and lengths up to tens of microns. However,
high-resolution FESEM images show these Co3O4 nanorods are not
single crystals but assembled by nanoparticles (see Fig. 4b, the
sample in this SEM image is broken due to the ultrasonic
treatment in ethanol). According to a previous report [22], the
homogeneous precipitation method using urea generated cobalt
compounds in the shape of nanorods or needles in a closed
system. The authors of that report found that the as-prepared
precipitates were indeed single-crystal nanorods and the dia-
meter of the nanorod was controlled mainly by the synthesis
temperature. However, after calcination the cobalt-hydroxide-
carbonate nanorods were transformed into strings of Co3O4
nanoparticles interconnected along the original longitudinal
directions of the rods because the elimination of hydroxyl and
carbonate groups and the formation of new phase during the
3. Results and discussion
Fig. 2 shows the XRD pattern of the resultant Co3O4 product.
All the peak can be exclusively indexed to the cubic spinel Co3O4
¯
structure (space group Fd3m), and no trace of other impurities is
observed. The lattice constant calculated from the XRD pattern is
0.80843(2) nm. The grain size, which is in the range of 29–31 nm,
has been estimated by the Scherrer formula. Particularly, the
instrument influence has been eliminated by subtracting the peak
width of the standard materials LaB6. The size distribution of

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Fig. 4. (a) FESEM image of the Co3O4 sample, (b) high-resolution FESEM image of the Co3O4 nanorod.
Fig. 5. Temperature dependence of the ZFC and FC magnetization curves in an
applied field of 100 Oe. The inset is the ZFC curve exhibiting a broad peak around
37 K.
Fig. 6. Virgin magnetization curves at 5 K under ZFC and FC measurements. The
inset shows the amplified curve at low field.
thermal decomposition caused the disruption of the original
rod-like morphology and led to the one-dimensional beadlike
assemblies. Our experimental result agrees well with the above
growth mechanism proposed by Xu and Zeng [22].
The temperature dependence of the magnetization in a weak
applied field of 100 Oe is shown in Fig. 5. In the field-cooled (FC)
temperatures of ferromagnetic, ferrimagnetic and antiferromag-
netic nanostructured materials [10,14,24]. According to the theory
of finite-size scaling, the deviation of the transition temperature
(TN) from that of the bulk should depend on the dimension (d) of
the nanostructured material as follows [10,23]:
l
´
measurement, the sample is cooled from 100 K (above its Neel
½TNðNÞ ꢁ TNðdÞꢃ=TNðNÞ ¼ ðd0=dÞ
(2)
temperature) to 5 K in an external field of 50 kOe, and then the
magnetization is recorded, on warming, in a field of 100 Oe. As for
the zero-field-cooled (ZFC) measurement, the procedure is almost
the same, except that there is no external field during cooling. A
remarkable feature of the ZFC curve is the broad peak around 37 K
´
In our case, TN(N) and TN(d) are the Neel temperatures of the
bulk and nanostructured Co3O4, respectively. d0 is the extra-
polated correlation length at 0 K, and l is the shift exponent for
the finite-size scaling. In our calculation, we choose the proposed
value for the three-dimensional Heisenberg model (l ¼ 1.42),
which has been evaluated both theoretically and experimentally
[10,25]. With the result of our experiment (TN(d) ¼ 37 K for
d ¼ 3071 nm) and those of the latest reports (TN(d) ¼ 30 K for
d ¼ 8 nm and TN(d) ¼ 1572 K for d ¼ 4.34 nm) [13,14], the best fit
values of TN(N) ¼ 38.670.5 K and d0 ¼ 2.8770.05 nm are ob-
tained. Considering the system error during measurement and
different methods to determine the transition temperature,
38.670.5 K is a pretty reasonable value of TN(N). The value of
d0 is roughly 3.5 times of the lattice constant of Co3O4, which
fulfills its identification as a microscopic length scale. Therefore,
we conclude that the shift of TN can be well described by the effect
of finite-size scaling.
´
(see the inset of Fig. 5), which corresponds to the Neel transition.
A weak ferromagnetic (FM) behavior is indicated apparently by
the strong bifurcation of the ZFC and FC curves at low temperature
and the decreasing of the ZFC magnetization below 11 K. At 5 K,
the FC magnetization is about five times of the ZFC magnetiza-
tion; then the FC magnetization decreases dramatically with
´
increasing temperature. Above the Neel temperature, the ZFC and
FC curves become almost identical.
´
The Neel temperature of the AFM Co3O4 nanoparticles is
slightly lower than the bulk Co3O4, which has also been reported
´
by other research groups [13,14,18,19,23]. This lower Neel
temperature can be attributed to the finite size effect, which has
succeeded in explaining the shift of the critical ordering

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The ZFC and FC virgin magnetization curves at 5 K are shown in
Fig. 6, which are recorded after cooling the sample from 100 to 5 K
in an applied magnetic field of 0 and 50 kOe, respectively. The
magnetization is linearly dependent on the field and a shift
toward large magnetization is observed for the FC virgin
magnetization. The considerable magnetization indicates that
the Co3O4 nanoparticles are weak ferromagnetically ordered. For
the bulk AFM materials, the net magnetization is zero due to the
complete compensation of sublattice magnetizations. Hence, the
origin of this weak ferromagnetism in our Co3O4 nanoparticles
can only be ascribed to the finite size effect. Different models have
been proposed to explain the weak ferromagnetism in the
antiferromagnetic nanostructured materials, such as CoO thin
layers [10], NiO nanoparticles [11], Co3O4 mesoporous [12], Co3O4
nanoparticles [13,14,18,19,23,26], etc. Mainly, the weak ferromag-
netism has been attributed to the uncompensated surface spins
[10,11,16] or the partly inverted spinel structure probably by local
electron hopping [19].
In order to gain further insight into the origin of the weak
ferromagnetism in AFM Co3O4 nanoparticles, the ZFC and FC
hysteresis loops are measured with the same settings as those of
virgin magnetization curves (shown in Fig. 7). As shown in the
inset (b), the loop is open up to 40 kOe and the magnetization is
almost linear with the field. For a system with an open loop up to
a high field, it is believed that a high surface anisotropy and a
spin-glass-like surface layer exist [11,13,26]. Fig. 8 shows the
central portions of the ZFC and FC loops at 5 K. The ZFC hysteresis
loop is symmetric about the origin with a coercivity (HC) of about
75 Oe, indicating a weak ferromagnetism. However, the FC loop is
slightly broadened and shifted towards negative applied field,
which exhibits the typical feature of an exchange bias system with
an exchange bias field Heb ¼ (HFCC1+HCFC2)/2 around ꢁ390 Oe. The
exchange bias is an interfacial effect by the exchange coupling
between AFM and FM layers, which induce an unidirectional
anisotropy of the FM layer. Both the loop shift and exchange bias
field are measures of the unidirectional exchange anisotropy.
The observation of the loop shift, enhanced coercivity, as well as
the exchange bias field indicate the existence of the AFM core and
FM surface spin in our Co3O4 nanoparticles, which can be
attributed to the uncompensated surface spin due to the
reduction of the coordination number at the surface of the AFM
Co3O4 nanoparticles.
Fig. 8. Central portions of ZFC and FC hysteresis loop at low field showing an
obvious loop shift.
Fig. 9. Dependence of the exchange bias field on the number of cycles at 5 K.
As for the opening of the loop, a spin-glass-like surface system
with multiple spin configurations is believed to be its origin [13].
According to Salaba et al. [13], the surface spins can be
separated into two parts. One part is the frozen-in uncompen-
sated spins, which do not reverse during the field cycling.
The other part is the free spins, which can be aligned by an
applied field. Thus, in one cycle of loop measurement, part
of the magnetization (the frozen-in spins) is lost, leading to the
opening of the loop. The existence of the spin-glass-like surface
phase can be further confirmed by the training effect of the
exchange bias field. To measure the training effect by a SQUID,
the Co3O4 nanoparticles were cooled from room temperature
to 5 K in an applied field of 50 kOe, and then the hysteresis loop
was recorded at 5 K. As shown in Fig. 9, the exchange bias field
|Heb| decreases with the number of cycles due to the loss of
frozen-in spins.
The temperature dependence of the exchange field and
coercivity is shown in Fig. 10. The hysteresis loop was recorded
after the Co3O4 nanoparticles were cooled from room temperature
to the measure temperature in an external field of 50 kOe. Below
´
the Neel temperature, both |Heb| and HC decrease with increasing
´
temperature. |Heb| and HC approach the minimum around the Neel
temperature due to the change of magnetic order from anti-
ferromagnetic to paramagnetic.
Fig. 7. ZFC hysteresis loop measured at 5 K. Insets are the FC hysteresis loop (a) and
the high-field ZFC hysteresis loop opening up to 40 kOe (b).

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References
[1] S.H. Sun, C.B. Murray, D. Weller, L. Folks, A. Moster, Science 287 (2000) 1989.
[2] P. Gambardella, S. Rusponi, M. Veronese, S.S. Dhesi, C. Grazioli, A. Dallmeyer,
I. Cabria, R. Zeller, P.H. Dederichs, K. Kern, C. Carbone, H. Brune, Science 300
(2003) 1130.
[3] V. Skumryev, S. Stoyanov, Y. Zhang, G. Hadjipanayis, D. Givord, J. Nogues
J, Nature 423 (2003) 850.
[4] F. Dumestre, B. Chaudret, C. Amiens, P. Renaud, P. Fejes, Science 303 (2004)
821.
[5] S. Mørup, C. Frandsen, Phys. Rev. Lett. 92 (2004) 217201.
[6] J. Won, M. Kim, Y.W. Yi, Y.H. Kim, N. Jung, T.K. Kim, Science 309 (2005) 121.
[7] M. Jamet, A. Barski, T. Devillers, V. Poydenot, R. Dujardin, P. Bayle-Guillemaud,
J. Rothman, E. Bellet-Amalric, A. Marty, J. Cibert, R. Mattana, S. Tatarenko, Nat.
Mater. 5 (2006) 653.
[8] M.M. Ibrahim, S. Darwish, M. Seehra, Phys. Rev. B 51 (1995) 2955.
[9] S. Gider, D. Awschalom, T. Douglas, S. Mann, M. Chaparla, Science 268 (1995)
77.
[10] T. Ambrose, C.L. Chien, Phys. Rev. Lett. 76 (1996) 1743.
[11] R.H. Kodama, S.A. Makhlouf, A.E. Berkowitz, Phys. Rev. Lett. 79 (1997) 1393.
[12] Y. Wang, C.M. Yang, W. Schmidt, B. Spliethoff, E. Bill, F. Schuth, Adv. Mater. 17
(2005) 53.
[13] E.L. Salabas, A. Rumplecker, F. Kleitz, F. Radu, F. Schu¨th, Nano Lett. 6 (2006)
2977.
Fig. 10. Exchange bias field and coercivity as a function of temperature.
[14] D.A. Resnick, K. Gilmore, Y.U. Idzerda, M.T. Klem, M. Allen, T. Douglas,
E. Arenholz, M. Young, J. Appl. Phys. 99 (2006) 08Q501.
[15] I. Lopes, N.E. Hassan, H. Guerba, G. Wallez, A. Davidson, Chem. Mater.
18 (2006) 5826.
4. Conclusion
´
[16] L. Neel, C. Dewitt, B. Dreyfus, P.D. de Gennes (Eds.), Low Temp. Phys., Gordon
and Beach, New York, 1962, p. 413.
[17] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000)
496.
[18] S.A. Makhlouf, J. Magn. Magn. Mater. 246 (2002) 184.
[19] C. Nethravathi, S. Sen, N. Ravishankar, M. Rajamathi, C. Pietzonka, Bernd
Harbrecht, J. Phys. Chem. B 109 (2005) 11468.
[20] M. Sato, S. Kohiki, Y. Hayakawa, Y. Sonda, T. Babasaki, H. Deguchi, M. Mitome,
J. Appl. Phys. 88 (2000) 2771.
[21] W.L. Roth, J. Phys. Chem. Solids 25 (1964) 1.
[22] R. Xu, H.C. Zeng, J. Phys. Chem. B 107 (2003) 12643.
[23] Y. Ichiyanagi, S. Yamada, Polyhedron 24 (2005) 2813.
[24] Z.X. Tang, C.M. Sorensen, K.J. Klabunde, G.C. Hadjipanayis, Phys. Rev. Lett. 67
(1991) 3602.
[25] K. Chen, A.M. Ferrenberg, D.P. Landau, Phys. Rev. B 48 (1993) 3249.
[26] S. Li, H. Bi, B. Cui, F. Zhang, Y. Du, Z. Jiang, C. Yang, Q. Yu, Y. Zhu, J. Appl. Phys.
95 (2004) 7420.
In summary, the magnetic properties of AFM Co3O4 nanopar-
ticles prepared by hydrothermal method have been investigated.
´
A deviation of its Neel temperature from the bulk has been found,
which can be well described by the theory of finite-size scaling. At
low temperature, the FC hysteresis loop exhibits enhanced
coercivity and loop shift. An exchange bias system with the AFM
core and FM surface spin is employed to explain anomalous
magnetization of Co3O4 nanoparticles. The finite size effect as well
as the uncompensated surface spin is the origin of the weak
ferromagnetism in the AFM Co3O4 nanoparticles.
Acknowledgments
This work was supported by the Chinese Academy of Sciences
and the Innovation Fund of Institute of Physics, Chinese Academy
of Sciences.