Ferromagnetic-like behavior of ultrafine NiO nanocrystallites

Article abstract of DOI:10.1016/j.jmmm.2003.11.017

Ultrafine NiO nanocrystallites (NCs) with grain size of 5nm have been synthesized by vacuum thermolysis method. These NCs exhibit a ferromagnetic-like behavior, and a large coercivity of 327.14 kA/m exists together with a large exchange bias field of 148.48 kA/m at 4.2 K. A large net magnetization of 205.72 μ_B per particle is evaluated for the 5nm NCs. The ferromagnetic-like magnetic properties are attributed to the combined effects of missing bonds and the lattice distortion.

Full text of DOI:10.1016/j.jmmm.2003.11.017

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 277 (2004) 363–367
Ferromagnetic-like behavior of ultrafine NiO nanocrystallites
a,
Hong Bi *, Shandong Li , Yuchuan Zhang , Youwei Du
b
a
b
a
School of Chemistry and Chemical Engineering, Anhui University, Feixi Road 3, Hefei 230039, China
b
Department of Physics, National Laboratory of Solid State Microstructure, Nanjing University, Nanjing 210093, China
Received 4 June 2003; received in revised form 11 October 2003
Abstract
Ultrafine NiO nanocrystallites (NCs) with grain size of 5 nm have been synthesized by vacuum thermolysis method.
These NCs exhibit a ferromagnetic-like behavior, and a large coercivity of 327.14 kA/m exists together with a large
exchange bias field of 148.48 kA/m at 4.2 K. A large net magnetization of 205.72 m per particle is evaluated for the 5 nm
B
NCs. The ferromagnetic-like magnetic properties are attributed to the combined effects of missing bonds and the lattice
distortion.
r 2003 Elsevier B.V. All rights reserved.
PACS: 61.46.+w; 61.72.ꢀy; 75.50.Ee
Keywords: Nanoscale materials; Lattice distortion; Antiferromagnetics
1
. Introduction
ments [8]. The net magnetic moments are attrib-
uted to an uncompensated number of spins on two
sublattices. Indeed, large magnetic moments in
AFN have been observed [9–11]; however, their
origin is not clear up to now. Kodama et al.
proposed a multi-sublattice model to explain the
large magnetic moments that are far larger than
the values predicted by two-sublattice model [12].
The break of symmetry and reduced coordination
at surface in AFN have been paid much attention
to describe the deviation of magnetic properties
from those of bulk materials [13,14]. However, the
effect of atomic configuration, such as lattice
distortion or defects, on the magnetic properties
of AFN has often been ignored. In this paper, the
origin of unexpected large magnetic moments in
NiO nanocrystallites (NCs) is discussed in the light
of lattice distortion and missing bonds.
Magnetism of small particles has generated
increasing interest due to their unique magnetic
properties as well as their technological applica-
tions [1–3]. Antiferromagnetic nanoparticles
(
AFN) have recently gained increasing attention
by virtue of their potential for exhibiting magne-
tization reversal by quantum tunneling [4–6]. N e! el
first noticed the magnetism of the fine antiferro-
magnetic particles [7]. He suggested that fine
antiferromagnetic particles possibly exhibit super-
paramagnetic or spin glass behavior, even weak
ferromagnetism with a permanent magnetic mo-
*Corresponding author. Tel.: +86-0551-5106881; fax: +86-
0
0
551-2824516.
E-mail address: bihong@vip.sina.com (H. Bi).
304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2003.11.017

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2
. Experimental
0.8
0
0
0
.7
.6
.5
0.03
0.02
0.01
0.00
-0.01
A typical synthesis procedure of Ni(OH) is as
2
follows: 10 g NiCl ꢁ 6H O was dissolved in 200 ml
2
2
H O at room temperature, then 3.4 g NaOH was
2
TB
added into the solution under stirring, green
precipitates were rapidly produced, then they were
filtered, washed by distilled water twice, and then
dried in air. NiO nanoclusters were synthesized by
thermally decomposing Ni(OH)2 at 573 K for
0.4
0
50 100 150 200 250 300
Temperature (k)
0
0
0
.3
.2
.1
ZFC
FC
2
10 min in a vacuum furnace. The magnetic
0.0
properties and magnetic relaxation behavior of
NiO NCs were measured by use of vibrating
sample magnetization (VSM) from 300 to 4.2 K.
The microstructure of NiO NCs is characterized
by X-ray diffraction (XRD), using Cu Ka radia-
tion, transmission electron microscopy (TEM) and
high-resolution TEM (HRTEM).
0
50
100
150
200
250
300
Temperature (K)
Fig. 2. ZFC–FC curves of NiO NCs. The cooling field and
measurement field are 1 and 0.01 T, respectively. The inset
shows magnification of ZFC–FC curves at lower magnetiza-
tion.
vior in order to understand the magnetic proper-
ties of NiO NCs in detail. Fig. 2 shows the zero
field cooling (ZFC) and field cooling (FC) curves
of NiO NCs. The cooling field is 1 T, and the
measurement field is 0.01 T. As illustrated in
Fig. 2, the ZFC and FC curves strongly bifurcate
at lower temperature. The FC magnetization
rapidly increases with decreasing temperature
and becomes far larger than the ZFC magnetiza-
tion at 4.2 K. The temperature corresponding to
the maximum in the ZFC curve is usually assigned
to the average blocking temperature (TB). The TB
is about 180 K for NiO NCs (see Fig. 2 inset). This
phenomenon indicates that NiO NCs exhibit weak
ferromagnetic character in spite of the antiferro-
magnetics for bulk material.
The magnetic properties at low temperature
were investigated by means of FC hysteresis loop.
Fig. 3 shows the FC hysteresis loop at 4.2 K in
presence of a cooling field of 2.5 T. It is evident
that NiO NCs exhibit a ferromagnetic-like loop. A
large coercivity of 327.14 kA/m is present together
with a large exchange bias field of 148.48 kA/m at
3
. Results and discussion
Fig. 1 shows the hysteresis loops of Ni(OH)2
and its decomposed product NiO NCs at 300 K.
It can be seen that Ni(OH)2 exhibits a typical
antiferromagnetism, while the NiO NCs present
a superparamagnetic behavior although NiO bulk
material is antiferromagnetic [15,16]. So it is
necessary to study the magnetic relaxation beha-
0.6
0.4
0.2
0.0
NiO
Ni(OH)2
-0.2
-0.4
-0.6
4.2 K (the sample does not reach saturation under
an applied field of 2.5 T).
-
2
-1
0
1
2
The average intrinsic magnetic moments of NiO
NCs were simply estimated by using the saturation
2
magnetization of 2.14 A m /kg at 4.2 K. It is
assumed that NiO is a cubic structure with lattice
Applied field (T)
Fig. 1. The hysteresis loops of Ni(OH)
00 K.
2
and the NiO NCs at
3

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H. Bi et al. / Journal of Magnetism and Magnetic Materials 277 (2004) 363–367
365
4
3
000
500
2.5
2.0
1.5
1.0
0.5
0.0
3000
2
2
500
000
1500
000
1
5
00
0
2
0
30
40
50
60
70
80
90
-0.5
-1.0
-1.5
-2.0
-2.5
(
a)
2θ (degree)
H =327.14 kA/m
C
0
.045
.040
.035
H =148.48 kA/m
E
0
0
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5
0.030
Applied field (T)
D =5.03 nm
d/d=3.69E-3
0
0
.025
.020
∆
Fig. 3. The FC hysteresis loop of NiO NCs at 4.2 K with a
cooling field of 2.5 T.
1.2
1.4
1.6
1.8
4
2.0
sinθ
2.2
2.4
2.6
(
b)
Fig. 5. (a) XRD pattern of NiO NCs and the linear fitting using
Williamson–Hall relationship.
1
8
gram is estimated as NE1:12 Â 10 : Thus the
magnetic moments can be calculated as about
2
05:72 m per particle.
B
This ferromagnetic-like behavior is generally
suspected arising from the nonstoichiometry of the
sample, a presence of superparamagnetic metallic
3
+
Ni clusters, or Ni
17,18]. However, the XRD, TEM, and HRTEM
results shown below reveal that there is no trace of
ions within the NiO lattice
[
3
+
presence of metallic Ni, Ni
ions, and other
ferromagnetic impurities. Therefore, the ferromag-
netic behavior in NiO nanoparticles is originating
from NiO nanoparticle system.
In order to study the origin of the magnetic
properties, TEM, HRTEM observations and
XRD stains analysis have been carried out.
Shown in Fig. 4 are (a) the TEM micrograph
and selected-area electron diffraction (SAED)
pattern ((a) inset), and (b) HRTEM micrograph
for the sample. The diffraction rings of SAED well
agree with that of NiO crystalline. The TEM
micrograph and SAED pattern suggest that
homogeneous NiO grains with about 5 nm in size
were obtained. It should be mentioned here that
Fig. 4. (a) TEM micrograph and SAED pattern ((a) inset), and
(b) HRTEM micrograph for the NiO NCs. The diffraction
rings in the inset of (a) well agree with NiO crystalline
suggesting that only NiO phase exists in the sample.
(
constant of a ¼ 4:1771 A. In the sample, the
average particle size is 5 nm (from TEM observa-
3
+
ferromagnetic phase such as metallic Ni, Ni
ions, or other ferromagnetic impurities do not exist
in the sample from its SAED pattern. The
tion (Fig. 4(a)) and XRD pattern calculation
(Fig. 5(a)) and so the number of particles per

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2
+
HRTEM micrograph shows a large amount of
surface defects (crystallite A) and lattice distortion
magnetization per Ni calculated by Kodama for
the ultrafine NiO particles system (about 5 nm in
size) has a close value with the studied sample
(
crystallite B) for the NiO NCs.
In order to further confirm this result, the XRD
2
+
(about 0:02 m per Ni ) [12]. Maybe the present
B
pattern and the strain analysis were carried out.
Fig. 5(a) shows the XRD spectrum of NiO NCs.
XRD pattern also demonstrates that only NiO
phase is found out in the sample. The broad
diffraction peaks for the vacuum annealed sample
suggest that ultrafine NiO particles were obtained.
Williamson–Hall relationship was used to calcu-
late the effect of the grain size and lattice
distortion ratio (strain) on the lattice expansion
result confirms those in Kodama et al. by
extending them to a smaller size.
Based on the result of XRD strains analysis, it
should be mentioned that a lager deviation from
the antiferromagentic characteristics for the stu-
died ultrafine particle system than that for 15 nm
NiO particle system even in our sample exhibits a
weak ferromagnetic-like behavior at low tempera-
ture and this suggests that the large magnetization
found in our sample may be attributed not only to
the broken bonds and the reduced coordination,
but also to the lattice distortion. It is well known
that the lattice distortion usually occurs for the
nanoscaled particles. XRD and HRTEM results
have also verified this phenomenon in the studied
sample. For antiferromagnetic oxides, such as
NiO, MnO, Co O , the magnetic coupling through
[
19]. It is expressed as:
0
:9l
4ðDdÞ sin y
d cos y
bTotal ¼ bSize þ bStrain
¼
þ
;
ð1Þ
t cos y
where bTotal is the full-width at half-maximum of
the XRD peak, l is the incident X-ray wave length,
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 against
3
4
the super-exchange interaction is sensitive to the
local environment, such as the length and angle of
bonds. The variation of lattice parameters should
influence the strength of super-exchange interac-
tion, even change the sign of magnetic coupling
resulting in the appearance of weak ferromagnet-
ism. Considering the XRD fitting result, a large
lattice expansion takes place for 5 nm NiO NCs. It
is believed that the weak ferromagnetism is
attributed to the combined effects of the lattice
distortion and the broken bonds.
4
sin y yields the crystal size from the intercept
value, and the strain (Dd=d) from the slope.
Diffraction angles were measured from 25 to
ꢂ
ꢂ
8
5 , which cover five peaks for NiO NCs (Fig. 5(a)
and (b) shows the linear fitting result by use of
Williamson–Hall relationship. From this plot, an
average grain size of 5.0370.71 nm (which agrees
with the TEM and HRTEM results) and a lattice
ꢀ
3
distortion ratio (strain) of (3.6970.93) Â 10 are
obtained, indicating a large lattice distortion in the
NiO NCs.
The large coercivity and exchange bias in 5 nm
NiO NCs are described as the exchange coupling
between surface and inner spins. Much recently, in
some nanoscaled ferromagnetic or anntiferromag-
neitc oxides system, such as NiFe O , CoFe O ,
Generally, the large permanent magnetic mo-
ments and superparamagnetic characteristics of
nanoscaled antiferromagnetic particles are attrib-
uted to the broken symmetry at the surface and
broken bonds [20–22]. The origin of the magnetic
properties in AFN was explained by Kodama
in terms of broken bonds and exchange restriction
2
4
2
4
a-Fe O , and MnO, and so on, [20,22–27] a
2
3
surface layer with non-collinear spins has been
demonstrated by means of M o. ssbauer spectra and
polarized neutron diffraction, etc. [20,23,26,27]. A
clear separation of core and shell magnetic proper-
ties has been reported. In this picture, the
nanocrystallite is described as a magnetic bilayer
with a spherical symmetry. One should then
consider two magnetic media with different
anisotropies, interactions and magnetic structure.
The influence of the surface anisotropy has also
[
12]. Non-collinear magnetic structures are formed
due to the combined effects of missing bonds
and exchange restriction. In comparison with
Kodama’s result of 700 m per particle with grain
B
size of 15 nm [12], the net magnetic moments per
unit volume of our NiO NCs are 7.9 times larger
than that of his value. Considering the non-linear
effect of non-collinear magnetic structures, the net

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H. Bi et al. / Journal of Magnetism and Magnetic Materials 277 (2004) 363–367
367
been invoked as the origination of the anomalous
magnetic properties of NCs at low temperature.
[7] L. N e! el, in: C. Dewitt, B. Dreyfus, P.D. de Gennes (Eds.),
Low Temperature Physics, Gordon and Breach,
New York, 1962, p. 413.
[8] C.A. Mulder, A.J. van Duyneveldt, J.A. Mydosh, Phys.
Rev. B 23 (1981) 1384.
4
. Conclusion
[
9] J.T. Richardson, W.O. Milligan, Phys. Rev. 102 (1956)
1
289.
[
[
[
[
10] J. Cohen, K.M. Creer, R. Pauthenet, K. Srivastava,
J. Phys. Soc. Japan 17 (Suppl. B-I) (1962) 685.
11] W.J. Schuele, V.D. Deetscreek, J. Appl. Phys. 33 (1962)
A ferromagnetic-like behavior of ultrafine NiO
NCs with grain size of 5 nm was reported. The
large net magnetization is attributed to the
reduced coordination and broken bonds combined
with the lattice distortion. The coexistence of large
coercivity and exchange bias is explained in terms
of the exchange coupling between surface and
inner spins. Maybe the present results confirm
those of Kodama et al. by extending them to a
smaller particle size.
1
136.
12] R.H. Kodama, S.A. Makhlouf, A.E. Berkowitz, Phys.
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H.C. Ri, J. Am. Chem. Soc. 124 (2002) 12094.
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Acknowledgements
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17] I.S. Jacobs, C.P. Bean, in: G.T. Rado, H. Suhl (Eds.),
Magnetism, Vol. III, Academic Press, New York, 1963,
p. 294.
This work was financially supported by the
National Science Foundation of Anhui Provin-
cial Educational Committee under grant no.
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