Synthesis and anomalous magnetic properties of CoCr2O4 nanocrystallites with lattice distortion

Article abstract of DOI:10.1016/j.jmmm.2006.02.081

Nanocrystalline Cobalt chromite (CoCr₂O₄) ceramic has been synthesized under a mild condition, rather than by a high-temperature sintering (e.g. >1673 K, in general). A shifted hysteresis loop with an exchange-bias field of 35.7 kA/m and a high coercivity of 627.9 kA/m at 4.2 K was achieved under the cooling field of 2.39×10⁶ A/m. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) results reveal that a strong lattice distortion and a large amount of surface defects exist in CoCr₂O₄ nanocrystallites (NCs). The anomalous magnetic properties, such as bias field and large coercivity, are attributed not only to the nanosize effect but also to the lattice distortion and crystal defects.

Full text of DOI:10.1016/j.jmmm.2006.02.081

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 305 (2006) 448–451
www.elsevier.com/locate/jmmm
Synthesis and anomalous magnetic properties of CoCr O
2
4
nanocrystallites with lattice distortion
a,b,
Ã
a
c
a
a
, Guoxia Zhao , Hong Bi , Zhigao Huang , Heng Lai ,
Shandong Li
a
Rongquan Gai , Youwei Du
b
a
Department of Physics, Fujian Normal University, Fuzhou 350007, ROC
b
National laboratory of Solid State Microstructure, Department of Physics, Nanjing University, Nanjing 210093, ROC
c
School of Chemistry and Chemical Engineering, Anhui University, Hefei 230039, ROC
Received 27 July 2005; received in revised form 14 December 2005
Available online 6 March 2006
Abstract
Nanocrystalline Cobalt chromite (CoCr O ) ceramic has been synthesized under a mild condition, rather than by a high-temperature
2
4
sintering (e.g. 41673 K, in general). A shifted hysteresis loop with an exchange-bias field of 35.7 kA/m and a high coercivity of 627.9 kA/
6
m at 4.2 K was achieved under the cooling field of 2.39 ꢀ 10 A/m. X-ray diffraction (XRD) and high-resolution transmission electron
2 4
microscopy (HRTEM) results reveal that a strong lattice distortion and a large amount of surface defects exist in CoCr O
nanocrystallites (NCs). The anomalous magnetic properties, such as bias field and large coercivity, are attributed not only to the nanosize
effect but also to the lattice distortion and crystal defects.
r 2006 Elsevier B.V. All rights reserved.
PACS: 61.72.Lk; 75.75.+a; 75.50.Tt; 75.30.Et
2 4
Keywords: CoCr O ; Lattice distortion; Nanocrystallites; Exchange bias
1
. Introduction
Magnetism of small particles has generated increasing
[9], the effect of surface-spin-pinning matters on the
magnetic properties has been discussed by comparing the
naked and coated NCs, since a relatively perfect crystal
microstructure and few crystal defects are present in the
CoCr O NCs fabricated by thermolysis of the polymer–
interest due to their unique magnetic properties as well as
their technological applications [1–4]. Of them, the
magnetic properties of nanoscaled transition-metal oxides
have drawn much attention in practical applications, such
as high-density magnetic memory media [5–7]. Cobalt
chromite (CoCr O ) has been widely used as dye, catalyst
2
4
metal complex precursor at a high temperature for a long
time, or by co-precipitation method. In the current paper,
we focus on the effect of lattice distortion and surface
defects on the magnetic properties of the CoCr O NCs. It
2
4
2
4
and substrate for film growth [8]. However, few articles
have reported the synthesis and magnetic properties of
CoCr O nanocrystallites (NCs). By various experimental
has been found that the CoCr O NCs present a strong
2 4
lattice distortion and a large amount of defects when they
are prepared by thermolysis of the polymer–metal complex
precursor at a low temperature for a relatively short time.
Bulk CoCr O is known to have a cubic normal spinel
2
4
researches, it has been found that the main factors that
influence the exchange bias and large, coercivity of
CoCr O NCs are nanosize effect, surface-pinning matters,
2
4
structure, and cannot be found in natural mine, but it is
generally manufactured by sintering the mixture of Co O
and Cr O at high temperature (e.g. 1673 K). In this paper,
2
4
lattice distortion and surface defects. In a previous paper
3
4
2
3
Ã
CoCr O NCs with an average crystallite size about 12 nm
2 4
have been synthesized under a mild condition by thermo-
lysis of polymer–metal complex at a low temperature.
Corresponding author. Tel.: +86 591 28198956;
fax: +86 591 8316 5313.
E-mail address: dylsd007@yahoo.com.cn (S. Li).
0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2006.02.081

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S. Li et al. / Journal of Magnetism and Magnetic Materials 305 (2006) 448–451
449
Some anomalous magnetic properties of CoCr O NCs and
2
0.1
.0
105
100
4
their origin are reported.
. Experimental methods
A typical synthesis procedure of CoCr O NCs is as
DTA
TG
0
95
-
-
-
0.1
0.2
0.3
2
6
13 K
A
90
85
2
4
follows: 2.0 g co-polymer of styrene-co-maleic acid anhy-
0
8
7
0
5
-0.4
-0.5
dride (SMA) were dissolved in 50 ml N,N -dimethylamide
(
DMF) at 353 K in a three-neck bottle, and 1.20 g
B
CoCl ꢁ 6H O and 2.40 g CrCl ꢁ 6H O were then put into
70
2
2
3
2
514 K
-
-
0.6
0.7
C
it. This solution was kept stirred at 353 K for 2 h, and then
it was dropped into excessive distilled water. NaOH
aqueous solution was dropped into it for keeping the
reaction under an alkalescent environment. The precipitate
was filtered, washed by distilled water twice, and then dried
in vacuum at 323 K for 72 h. The dry, blue, solid
polymer–metal complex was annealed at various condi-
tions as described below. It is well known that under these
conditions, some polymer decomposition residues (PDR)
are generally present at the surface of the NCs [9]. In order
to rule out the effect of PDR on the magnetic properties, a
clean surface is required. However, it is difficult to wipe off
the PDR at the surface of the NCs. The sample was
repeatedly washed by aqueous solutions of acid and alkali
for more than 6 times until the vibrating peaks of PDR
were not detected by the Fourier transition infrared
spectroscopy (FTIR). Thus, it could be thought that a
clean surface for the NCs is obtained.
6
5
0
375 K
6
3
00
400
500
600
700
800
900 1000
Temperatue (K)
Fig. 1. DTA/TGA curves of as-prepared sample under air atmosphere.
CoCr O4
*
*
+
2
NaCl
*
*
*
*
+
*
7
73K*3h
+
+
+
*
+
*
*
*
The microstructure was characterized by using XRD
0
(
(
Thermo ARL, X trA), Cu Ka radiation, and HRTEM
JEOL, JEM-2010). The thermal behavior was detected by
623K*3h
as-prepared
differential thermal analyzer/thermogravity analyzer
DTA/TGA) (Netzsch STA 449C). A superconductor
quantum interference device (SQUID) (Quantum Design
Co.) was used to measure the magnetic properties from 300
to 4.2 K.
(
20
30
40
50
θ (degree)
60
70
80
2
Fig. 2. XRD patterns for the samples annealed at various conditions.
3
. Results and discussion
Fig. 1 shows the DTA/TGA curves of the as-prepared
sample under air atmosphere. The DTA curve shows two
endothermal and one exothermal peaks at 375, 514 and
CoCr O NCs with crystallite size of about 4 nm appear.
2
4
Relatively well-crystallized CoCr O NCs can be obtained
2 4
by annealing at 773 K. It should be mentioned here
that the weight loss in stage C of TGA curve corres-
ponds to the decomposition of small amount of organic
residues at the surface of CoCr O NCs and/or to the
6
13 K, respectively, and TGA curve exhibits three weight-
loss stages (referred to as A, B and C, respectively). It is
easy to understand that the weight loss and endothermal
peak at 375 K are attributed to the volatilization of water
since the boiling point of water is 373 K, while that at
2
4
weight loss of residual carbon at the surface of CoCr O
2
4
NCs due to oxidization, since no detectable thermal
effect was observed in DTA curve in this temperature
range.
5
17 K to the decomposition of polymer. In order to
investigate the variation at the exothermal peak at 613 K,
the sample was annealed at 623 K for 3 h under air
atmosphere. In addition, for comparison, the sample was
also annealed at a higher temperature of 773 K for 3 h.
Shown in Fig. 2 are the XRD patterns of the as-prepared
sample and the annealed ones under above conditions. It
can be seen that the as-prepared sample shows an
amorphous-like structure. When it is annealed at 623 K,
besides a small amount of unexpected NaCl, ultra-fine
Before the magnetic measurements, the residual NaCl in
the sample was eliminated. In SQUID, the sample, which
was annealed at 773 K for 3 h, was cooled from 300 K to
6
4.2 K in the presence of magnetic field of 2.39 ꢀ 10 A/m.
Then the field-cooled (FC) hysteresis loop, shown in Fig. 3,
was measured at 4.2 K. A shifted loop with an exchange-
bias field of 35.7 kA/m and a very high coercivity of
627.9 kA/m were observed at 4.2 K.

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450
ꢂ3
(
3.570.1) ꢀ 10 were obtained, indicating a large lattice
1
2
8
4
0
4
8
distortion in CoCr O NCs.
2
4
The lattice distortion of CoCr O NCs is further
2
4
confirmed by HRTEM observation. Shown in Fig. 5 is
¯
¯
the HRTEM image of CoCr O NCs with a ½1 1 2ꢃ axis
2
4
zone parallel to the electron beam. It can be seen that the
strong lattice distortion of the NCs occurs especially at the
-
-
H
= 627.9 kA/m
(311)
C
HE = 35.7 kA/m
-
12
-
6
-4
-2
0
2
4
6
6
Applied field (10 A/m)
Fig. 3. The hysteresis loop of CoCr
6
2
O
4
nanocrystallites at 4.2 K with a
30
40
50
60
70
80
cooling field of 2.39 ꢀ 10 A/m.
(
a)
2θ (degree)
0
0
0
0
.0135
.0130
.0125
.0120
It is interesting to note the co-existence of exchange
bias and a large coercivity in ferrimagnetic CoCr O NCs.
2
4
It is well known that ferromagnetic–antiferromagnetic
FM–AFM) coupled materials exhibit an exchange bias
(
0.0115
0.0110
under the condition of magnetic field cooling [10–12]. The
origin of the exchange bias is attributed to the unidirec-
tional anisotropy due to the exchange coupling between the
interfaces of FM and AFM materials [13,14]. However, no
FM–AFM coupled interfaces exist in our single-phase
CoCr O NCs system. Therefore, it is significant to
0
0
.0105
.0100
1
.2
1.4
1.6
1.8
2.0
2.2
(
b)
4sinθ
2
4
investigate the origin of exchange bias in the NCs.
Fig. 4. (a) XRD trace, and (b) the linear fitting by use of Williamson–Hall
relationship for the sample annealed at 773 K for 3 h.
In general, the atomic spacing is expanded with the
decrease of grain size for NCs [15]. In order to investigate
the origin of the anomalous magnetic properties in
CoCr O NCs, the microstructure and surface morphology
2
4
of the CoCr O NCs were detected by XRD and HRTEM.
2
4
Shown in Fig. 4(a) is the XRD pattern for the sample
annealed at 773 K for 3 h. The crystallite size and lattice
distortion ratio (microstrain) of the NCs are estimated by
Williamson–Hall plots [15] using the equation
0
:9l
4ðDdÞ sin y ,
bTotal ¼ bSize þ bStrain
¼
þ
(1)
t cos y
d cos y
where bTotal is the full-width half-maximum of the XRD
peak, l is the incident X-ray wavelength, y is the diffraction
angle, t is the crystal size, and Dd is the difference of the d
spacing corresponding to a typical peak. A plot of
bTotal cos y vs. 4 sin y yields the crystal size from the
intercept value, and the strain (Dd=d) from the slope.
Here, the effect of the instrumental background width has
been subtracted. Diffraction angles were measured from
2
0 to 801, which covered nine peaks for cubic CoCr O (see
2
4
Fig. 4(a)). Fig. 4(b) shows the linear fitting result by use of
Williamson–Hall relationship for five selected peaks with
high diffraction angle. From this plot, an average grain size
of 1272 nm and a lattice distortion ratio (microstrain) of
¯
¯
Fig. 5. HRTEM image of CoCr
2
O
4
nanocrystallites with ½1 1 2ꢃ axis zone.

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S. Li et al. / Journal of Magnetism and Magnetic Materials 305 (2006) 448–451
451
surface (highlighted by five small arrows). The lines A and
˚
in CoCr O NCs can be attributed not only to the nanosize
2 4
effect mentioned above, but also to the crystal defects and
lattice distortion.
¯
B are along ½2 2 0ꢃ direction (atomic spacing of 2.96 A) and
˚
1 1 1] direction (atomic spacing of 4.84 A), respectively. It
[
is evident that the atoms of the NCs deviate from their
normal sites, especially for the atoms near the surface. The
broadening of the lattice spacing can also be verified by
direct measurement from the image. In addition, some
atomic steps exist at surface partially pointed by letter C
and two big arrows.
4
. Conclusion
CoCr O nanoceramic was synthesized under a mild
2
4
condition. Large lattice distortion and large numbers of
surface defects are present in the CoCr O nanocrystallites.
Besides the nano-size effect on the magnetic properties, the
co-existence of a large coercivity and an exchange bias in
CoCr O nanocrystallites mainly results from the lattice
2
4
The XRD and HRTEM results reveal that strong lattice
distortion and a large amount of surface defects are
presented in the CoCr O NCs. The anomalous magnetic
2
4
2
4
properties deviated from the bulk materials should be
attributed to these special microstructures in CoCr O
distortion and surface defects.
2
4
NCs.
Ferrimagnetic bulk CoCr O has a normal spinel
Acknowledgments
2
4
structure, and the magnetic coupling is carried out through
the super-exchange interaction between the metal ions via
oxygen ions, which is sensitive to the local atomic
environment, such as the length and angle of chemical
bonds etc. For NCs, the broken symmetry at the surface,
broken bonds, some degree of structural disorder result in
the super-exchange interaction fluctuates in both strength
and sign, and thus the magnetic structure, especially at
surface, deviates from the perfect crystal [16,17]. Therefore,
the nanosize effect dramatically influences the magnetic
properties in nanoscaled oxide systems. Much recently, a
special surface spin structure has been reported in some
nanoscaled ferrimagnetic oxide systems, such as NiFe O ,
This work was financially supported by National Key
Project for Basic Research (G1999064508), National
Science Foundation of China (10504010, 2005J023) and
the Nanotechnology Laboratory of Jiangsu Province.
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4
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