Surface spin pinning effect of polymer decomposition residues in CoCr 2O4 nanocrystallites system

Article abstract of DOI:10.1016/j.jmmm.2004.03.040

CoCr₂O₄ nanocrystallites with and without covering of residues from the decomposed polymer at the surface have been fabricated, respectively. It has been revealed that the magnetic properties of the nanocrystallites covered with the

Full text of DOI:10.1016/j.jmmm.2004.03.040

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 281 (2004) 11–16
Surface spin pinning effect of polymer decomposition residues
in CoCr2O4 nanocrystallites system
Shandong Lia,*, Hong Bib, Zongjun Tianc, Feng Xua, Benxi Gua,
Mu Lua, Youwei Dua
a National Laboratory of Solid State Microstructure, Department of Physics, Nanjing University, Hankou Road 22,
Nanjing 210093, China
b College of Chemistry and Chemical Engineering, Anhui University, Hefei 230039, China
c College of Mechanical and Electronical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
Received 31 January 2004; received in revised form 7 March2004
Available online 20 April 2004
Abstract
CoCr2O4 nanocrystallites with and without covering of residues from the decomposed polymer at the surface have
been fabricated, respectively. It has been revealed that the magnetic properties of the nanocrystallites covered with the
residues deviate greatly from those of the ones without residues. In comparison with the ‘‘naked’’ CoCr2O4
nanocrystallites, for the sample with the covering at the surface, the saturation field, coercivity and loop shift are
enhanced remarkably. It is believed that the significant differences in magnetic properties are caused by the presence of
residues at the surface.
r 2004 Elsevier B.V. All rights reserved.
PACS: 75.75+a; 75.30.Et; 75.30.Gw
Keywords: Surface pinning effect; CoCr2O4 magnetic properties; Nanometer materials
1. Introduction
layer, surface spin canting and a large surface
magnetic anisotropy, etc. have been found in some
ferro- and ferrimagnetic nanocrystallites, suchas
Fe, Ni, NiFe2O4, CoFe2O4, etc. [1–5]. However,
the origin of these anomalous magnetic properties
is not completely clear up to now. So it is also
necessary to broaden the research range for the
further understanding of the mechanism. Despite
the large amount of work that has been focused on
the magnetic properties of the pure nanocrystal-
lites materials, few reports have been on the
organic group covered nanocrystallites.
Magnetic properties of nanocrystallites (NCs),
especially the surface magnetic structure and
properties, are of great current interest, stemming
in part from the development of high-density
magnetic storage media withnanosized constituent
particles or crystallites. Spin-glass-like surface
*Corresponding author. Tel.: +86-25-83593817; fax: +86-
25-83595535.
E-mail address: sdli@ufp.nju.edu.cn (S. Li).
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2004.03.040

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It is well known that cobalt chromite (CoCr2O4)
were characterized by a transmission electron
microscopy (TEM), high resolution transmission
electron microscopy (HRTEM) and an X-ray
diffractometer (XRD). The surface characteristic
of the NCs was analyzed by Fourier transition
infrared spectroscopy (FTIR).
has been widely used in dyes, catalyst and the
substrate for preparation of film [6]. However, few
articles have been reported on the magnetic
properties of nanoscaled CoCr2O4 crystallites,
especially those of ones covered by the polymer
decomposition residues (PDR) at surface. There-
fore, in this paper, the magnetic properties and the
surface spin pinning effect were studied by
comparing two CoCr2O4 nanocrystallites systems
with and without the covering at the surface.
3. Results and discussion
Fig. 1 shows the XRD curves for samples A and
B. The indexes of CoCr2O4 facets signed in Fig. 1
reveal that these two samples are all composed of
CoCr2O4 NCs. The grain sizes calculated by
Scherrer equation for samples A and B are 10.5
and 11.2 nm, respectively. This result has also
confirmed by TEM observation (not shown here).
In order to have a detailed insight into the surface
structure for these two samples, HRTEM has been
performed for samples A and B, shown in Fig. 2.
As illustrated in Fig. 2, the grain features for
samples A and B are very similar, and the grain
sizes are well consistent withthe values of XRD
and TEM. However, the grain surface for sample
A is covered by a thin PDR layer (pointed out by
arrows for some places), while that for sample B is
shown a clean surface. It should be mentioned here
that the PDR only situates at surface of CoCr2O4
NCs for sample A, and does not exist in grain in
form of impurities. Considering XRD, TEM, and
HRTEM results as a whole, it is evident that the
2. Experimental methods
In order to research the surface effect on the
magnetic properties, two kinds of CoCr2O4
nanocrystallites systems have been prepared: one
surface is covered by PDR and the other is naked.
For convenience, they were referred as A and B,
respectively. The preparation procedures for these
two samples are as follows: (1) sample A, 2.0 g
copolymer of styrene-co-maleic acid anhydride
(SMA) was dissolved in 50 ml N,N-dimethylamide
(DMF) at 353 K in a three-neck bottle, then 1.20 g
CoCl2 ꢀ 6H2O and 2.40 g CrCl3 ꢀ 6H2O was put into
it, the solution was kept stirring under 353 K for
2 h, then it was dropped into excessive distilled
water, the precipitate was filtered, washed by
distilled water for several times. The dry, solid
polymer-metal complex was annealed at 593 K for
8 hat vacuum atmosphere, followed by 833 K for
6 h. Finally, the resulted black powder was
oxidized in an air oven at 873 K for 4 h. (2)
Sample B, 4.8 g CoCl2 ꢀ 6H2O and 16 g
Cr(NO3)3 ꢀ 9H2O were dissolved in 200 ml H2O
and mixed evenly under stirring at room tempera-
ture, then 6.4 g NaOH was added, and the mixture
was kept stirring at 353 K for 6 h, a kind of grey-
green precipitate was filtered and washed by
distilled water for several times. Finally the
resulted grey-green powder product was annealed
in an air oven at 573 K for 2 hand subsequently at
773 K for 3 h.
(a)
(b)
The magnetic properties of the CoCr2O4 NCs
were measured by using vibrating sample magnet-
ometer (VSM) witha maximum field of 8 T from
300 to 4.2 K. The microstructures of the materials
30
40
50
60
70
80
2θ (degree)
Fig. 1. The XRD traces for samples (a) A and (b) B.

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S. Li et al. / Journal of Magnetism and Magnetic Materials 281 (2004) 11–16
13
40
35
30
25
20
15
10
5
Sample A
Sample B
at 4.2 K
0
0
1
2
3
4
5
6
7
8
Applied field (T)
Fig. 3. The initial magnetization curves for samples A and B at
4.2 K.
40
sample A
sample B
30
20
10
0
-10
-20
-30
-40
-8
-6
-4
-2
0
2
4
6
8
Fig. 2. HRTEM images for samples (a) A and (b) B. The
arrows in Fig. 2(a) point out some places covered by PDR
layer.
Applied field (T)
Fig. 4. The hysteresis loops of CoCr2O4 nanocrystallites for
samples A and B at 4.2 K witha cooling field of 8 T.
different magnetic properties described below are
attributed to the different surface feature between
samples A and B rather than the difference of
grain sizes or impurities in sample A.
increases. This fact suggests that there is a pinning
field for sample A which hampers the rotation of
magnetic moments to the applied field.
Fig. 4 shows the hysteresis loops of the samples
at 4.2 K in the presence of a cooling field of 8 T. As
illustrated in Fig. 4a shifted loop of 30.6 kA/m and
a very high coercivity of 632.9 kA/m were observed
in sample A. While the coercivity for sample B is
416.0 kA/m, which is evidently lower than that of
sample A. Moreover, the loop shift was not
observed for sample B.
Although samples A and B are all composed of
CoCr2O4 NCs, and have approximately equal
grain size, they exhibit significant difference in
magnetic properties. In order to study the origin
The magnetic properties at low temperature for
these two samples were recorded by a VSM.
Shown in Fig. 3 are the initial magnetization
curves for samples A and B. As illustrated in Fig.
3, the initial magnetization curve of sample B
exhibits a typical ferrimagnetic characteristic. The
magnetization of sample B will approachto
saturation at high field. However, it is difficult
for sample A to reachthe saturation state. Even
the field rises up to 8 T (the maximum field in the
measurement), the magnetization of sample A still

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S. Li et al. / Journal of Magnetism and Magnetic Materials 281 (2004) 11–16
of these differences, besides the observation of
HRTEM, the surface characteristic of these two
systems was also studied by FTIR and the results
are shown in Fig. 5 and Table 1. It can be seen that
there is a great difference in the surface character-
istic for these two samples. As illustrated in Fig.
5(a), three peaks at 955, 854 and 798 cmꢁ1 are
assigned to the vibrating of Co(II)–O bond. The
peaks at 632 and 523 cmꢁ1 originate from the
vibrating of Cr(III)–O bond. This result demon-
strates the CoCr2O4 compound in sample A. In
addition, there is a large amount of the polymer
decomposition residues covering at the surface of
CoCr2O4 NCs. For example, two very strong
peaks at 1346 and 1014 cmꢁ1 are attributed to an
asymmetric and a symmetric vibrating peak of (C–
OH), respectively. Two weak shoulder peaks at
1651 and 1429 cmꢁ1 locating on the left side of the
strong peak at 1346 cmꢁ1 are assigned to the
vibrating peak of R1CH¼CHR2 or RCH¼CH2.
Two small narrow peaks at 749 and 700 cmꢁ1 are
attributed to the vibrating of C–H of mono-
substituted benzyl group. All these residue groups
resulted from the decomposition of SMA polymer
which containing repeated styrene and maleic acid
anhydride units, and absorbed on the surface of
CoCr2O4 NCs during annealing. However, for
sample B only Co(II)–O (located at 949, 885,
757 cmꢁ1) and Cr(III)–O (at 624 and 519 cmꢁ1
)
bonds are observed except for the water peaks at
3421 and 1626 cmꢁ1. The water arises from the
absorption of water moisture in air.
It is known that bulk CoCr2O4 has a normal
spinel 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.
Variation in coordination of surface cations results
in a distribution of net exchange fields. The
exchange bonds will be broken if an oxygen ion
is missing from the surface. For nanoscaled
crystallites, because of the broken symmetry at
the surface, broken bonds, and the super-exchange
interaction fluctuation in bothstrengthand
sign, thus the magnetic structure of the surface
corresponds to a spin glass-like configuration
or to a spin disordered system [7,8]. For the
studied systems, the grain sizes are only about
10 nm, which gives rise to a large ratio of
surface area versus volume for a particle. A large
amount of ions are located at the surface of the
100
749
80
60
40
20
(a) sample A
0
100
80
60
40
20
(b) sample B
0
4000 3500 3000 2500 2000 1500 1000
Wave number (cm-1)
500
Fig. 5. FTIR spectra for samples (a) A and (b) B.
Table 1
The peak positions and their assignments of FTIR traces for CoCr2O4 nanocrystallites
Sample
A
Wavenumber (cmꢁ1
)
Chemical bond
Origination
3424
H–OH
R1CH¼CHR2, RCH¼CH2
–C–OH
H2O
1651, 1429
1346, 1014
749, 700
SMA decomposition
SMA decomposition
SMA decomposition
Oxidation
Mono-substituted benzyl
Co(II)–O
Cr(III)–O
955, 854, 798
632, 523
Oxidation
B
3421, 1626
949, 885, 757
624, 519
H–OH
Co(II)–O
Cr(III)–O
Water moisture
Decomposition
Decomposition

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nanocrystallites. So the surface layer, to great
extent, dominates the magnetic properties.
magnetic spin state between the surface layer and
the inner core has been enlarged.
However, in addition to the size effect and
surface effect on the magnetic properties, in the
case of these two systems with approximately
equal grain size, the significant difference in
magnetic properties should be attributed to the
different surface magnetic structures caused due to
the organic residues at the surface. In fact, the
effect of the organic residues at the surfaces on the
magnetic properties can be interpreted as the
surface pinning effect [9]. The organic residues
bonded to the surface lead to the fact that the
electrons involved can no longer participate in the
superexchange. Accordingly, they will further
reduce the effective coordination of the surface
cations.
The surface spins have multiple configurations
for any orientation of the core magnetization [1].
When the sample is cooled in a high magnetic field
to a low temperature, the spin canted layer is
frozen into a spin-glass-like phase with a minimum
total free energy at low temperature for CoCr2O4
NCs. Furthermore, the spin pinning effect by PDR
results in the difficulty to change surface spin
configuration from one to another. Therefore, an
enhanced coercivity and bias field present are in
the PDR-covered sample. In the same way, the
difficulty to approachteh saturation state for
sample A is attributed to this surface spin pinning
effect.
It is well known that the electronic and magnetic
configurations of nanocrystallites are dramatically
influenced by the chemisorption of polymeric
ligand on the surface of nanocrystallites. For
example, the ferrite ultrafine crystallites coated
by organic molecules, exhibit surface spin canting
and a large surface magnetic anisotropy [1,5,9].
Another example is that in nickel carbonyl clusters
4. Conclusion
The enhancement effects of magnetic properties,
including a loop shift, coercivity and saturation
field, were observed in CoCr2O4 nanocrystallites
with organic residues at the surfaces. These
enhancement effects are believed to originate from
spin surface pinning effect by the organic residues
absorbed at the surfaces of the nanocrystallites.
withCO chemisorption on Ni surface
[10], th e
coordination by CO favors intra-atomic redistri-
bution of the Ni electronic configuration from
3d9ꢁx4sx to 3d10 (similar to Cu). As a result, the
effect of CO ligation is to quenchcompletely the
magnetic moments of the surface Ni atoms. The
bonding of surfactant molecules is equivalent to a
strong ligand field acting on the surface atoms.
Sucha ligand field causes a very strong pinning of
the magnetic moments on metallic ions in the
surface layer, which is transferred well into the
interior of the crystallite by the magnetic exchange
interactions [11,12]. A surface layer withnon-
collinear spins and a clear separation of core and
shell magnetic properties in oxides NCs has been
demonstrated by means of Mo.ssbauer spectra and
polarized neutron diffraction, etc. [5,12–14].
Therefore, it is reasonable that the surface
absorption of the polymer decomposition residues
on CoCr2O4 NCs will act as a kind of ligand
pinning, and significantly influence the surface
magnetic structure of the CoCr2O4 NCs. Due to
the surface spin pinning effect, the difference of
Acknowledgements
This work is financially supported by National
Key Project for Basic Research(G1999064508)
and the Nano-technology Laboratory of Jiangsu
Province.
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