Sol-gel synthesis and anomalous magnetic behaviour of NiO nanoparticles

Article abstract of DOI:10.1016/j.jpcs.2007.06.010

Nickel oxide nanoparticles (size ～4-22 nm) have been synthesized by sol-gel process using nickel acetate tetrahydrate and oxalic acid as precursors and ethanol as solvent. The process essentially involves formation of gel, drying at 110 °C for 24 h to produce nickel oxalate dihydrate and subsequent calcination at 300 °C or above in air to decompose and yield NiO powder. Their anomalous magnetic behaviour includes: (i) irreversibility in magnetization (M) vs temperature (T) curves under zero field cooled (ZFC) and field cooled (FC) conditions below the bifurcation temperature T₁; (ii) presence of two maxima (broad and sharp at T₂ and T₃, respectively) in the M_ZFC(T) and χ_ZFC(T) curves; (iii) decrease of both T₁ (325-115 K) and T₂ (265-95 K) with increasing crystallite size; (iv) shift of T₂ and T₃ towards lower and higher temperatures, respectively, in M_ZFC(T) curves with increase of applied magnetic field (typical value of T₂ and T₃ at 20 kOe being 90 and 30 K, respectively, for average crystallite size ～6.8 nm); (v) bifurcation of M_ZFC(T) and M_FC(T) curves even at high measuring fields; and (vi) stretched S - or eye-like shapes of M-H plots at 5 K with a shift of hysteresis loops M_FC(H) by 590 Oe. Further, the present results provide evidence for the non-existence of spin-glass behaviour in NiO. Instead, the observations suggest core-shell description for NiO nanoparticles. While the core behaves like a ferrimagnet because of prevailing lack of spin compensation due to reduced size (peak at T₂ is associated with the blocking process), shell contains randomly oriented spins with low co-ordination and are possibly responsible for sharp increase in M_ZFC below 30 K as a result of collective freezing effect. Finally, the increase of blocking temperature (T_B) as well as energy band gap (E_g) observed with decreasing particle size seems to have common origin and caused by size effects, viz. confinement of charge carriers and increase in the uncompensated core spins, respectively.

Full text of DOI:10.1016/j.jpcs.2007.06.010

ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 68 (2007) 1951–1964
www.elsevier.com/locate/jpcs
Sol–gel synthesis and anomalous magnetic behaviour
of NiO nanoparticles
Ã
Subhash Thota , Jitendra Kumar
Materials Science Programme, Indian Institute of Technology, Kanpur 208016, India
Received 27 October 2006; received in revised form 11 May 2007; accepted 2 June 2007
Abstract
Nickel oxide nanoparticles (size ꢀ4–22 nm) have been synthesized by sol–gel process using nickel acetate tetrahydrate and oxalic acid
as precursors and ethanol as solvent. The process essentially involves formation of gel, drying at 110 1C for 24 h to produce nickel oxalate
dihydrate and subsequent calcination at 300 1C or above in air to decompose and yield NiO powder. Their anomalous magnetic
behaviour includes: (i) irreversibility in magnetization (M) vs temperature (T) curves under zero field cooled (ZFC) and field cooled (FC)
conditions below the bifurcation temperature T ; (ii) presence of two maxima (broad and sharp at T and T , respectively) in the
1
2
3
1 2 2
MZFC(T) and wZFC(T) curves; (iii) decrease of both T (325–115 K) and T (265–95 K) with increasing crystallite size; (iv) shift of T and
3 2
T towards lower and higher temperatures, respectively, in MZFC(T) curves with increase of applied magnetic field (typical value of T
and T at 20 kOe being 90 and 30 K, respectively, for average crystallite size ꢀ6.8 nm); (v) bifurcation of MZFC(T) and MFC(T) curves
3
even at high measuring fields; and (vi) stretched ‘‘S’’- or eye-like shapes of M–H plots at 5 K with a shift of hysteresis loops MFC(H) by
5
90 Oe. Further, the present results provide evidence for the non-existence of spin-glass behaviour in NiO. Instead, the observations
suggest core–shell description for NiO nanoparticles. While the core behaves like a ferrimagnet because of prevailing lack of spin
compensation due to reduced size (peak at T is associated with the blocking process), shell contains randomly oriented spins with low
co-ordination and are possibly responsible for sharp increase in MZFC below 30 K as a result of collective freezing effect. Finally, the
increase of blocking temperature (T ) as well as energy band gap (E ) observed with decreasing particle size seems to have common
2
B
g
origin and caused by size effects, viz. confinement of charge carriers and increase in the uncompensated core spins, respectively.
r 2007 Elsevier Ltd. All rights reserved.
PACS: 75.50.ꢁy; 75.50.Tt; 75.75.+a
Keywords: B. Sol–gel growth; D. Magnetic properties
2
1
. Introduction
permanent magnets, ultra-high density (ꢀtera bit/in. )
information data storage devices, magnetic sensors, refrig-
eration systems, catalysis, targeted drug delivery, quantum
computing, magnetic resonance imaging, etc. [15,16].
Surface and finite size effects induce an anomalous
magnetic behaviour which differs markedly from the
corresponding bulk material. For example, with the
decrease of crystallite size, surface area to volume ratio
increases. This leads to an increase in the (i) fraction of
atoms present on or near the surface and (ii) number of
defects and missing bonds. Consequently, the co-ordina-
tion number gets reduced which, in turn, causes modifica-
tion in the exchange interaction between the surface atoms.
Furthermore, the symmetry discontinuity at the surface
Magnetic ultra-fine particles have attracted attention in
recent years because of their novel behaviour, enormous
scientific potential and technological applications [1–4].
Some of their novel properties are: enhanced magnetic
moments [5,6]; exchange-coupled dynamics [7]; spin-wave
quantization [8,9]; giant magnetoresistance [10,11]; en-
hancement of remanence, coercivity and magnetocaloric
effects [12]; glassy behaviour [13]; quantum tunnelling [14];
etc. These new properties have found applications in
Ã
Corresponding author. Tel.: +91 512 2597082; fax: +91 512 2597664.
E-mail address: subhasht@iitk.ac.in (S. Thota).
0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2007.06.010

ARTICLE IN PRESS
S. Thota, J. Kumar / Journal of Physics and Chemistry of Solids 68 (2007) 1951–1964
1952
results in an additional and dominant contribution to the
total crystalline anisotropy in small particles. Of particular
interest are ultra-fine particles of transition metal oxides
at 5 K under 20 kOe. While the offset of FC hysteresis loop
is believed to arise due to exchange interaction between the
uncompensated spins in the surface region and antiferro-
magnetic core of the particle, other observations need
explanation and open issues pertaining to the exact
magnetic nature of nanoparticle in general and uncompen-
sated spins in particular. Kodama and Berkowitz [25]
invoked a new finite size effect involving multiple (instead
of two) spin configurations to explain anomalous magnetic
behaviour (viz., large magnetic moments and coercivities
and considerable hysteresis loop shifts ) in NiO nanopar-
ticles. In this, the reduced co-ordination of surface spins
induces an overall change in the magnetic order of the
particle. Further, a weak magnetic coupling exists between
the multi-spin configurations. This allows a variety of
reversal paths for the spins upon cycling the applied field,
leading thereby to large coercivities and loop shifts. Tiwari
and Rajeev [32,33] have recently reported signatures of
spin-glass (SG) freezing in NiO nanoparticles (prepared by
sol–gel process taking aqueous solution of nickel nitrate
and sodium hydroxide at pH of 12) using ac susceptibility,
dc magnetization and time decay of thermoremanent
magnetization.
(
e.g., MnO, CoO and NiO) which have been investigated
by several research groups due to their abnormal magne-
tization behaviour, small size and surface effects [19–21].
The revolutionary work of Ne
to renewed focus on the antiferromagnetic nanoparticles
AFN) mainly due to the magnetization reversal phenom-
´
el [17,18] and Brown [22] led
(
enon by quantum tunnelling and inherent enhanced surface
and interface effects observed [23,24]. Bulk antiferromag-
netic materials contain equal number of spins in mutually
opposite directions (e.g., up and down) and thus are fully
compensated. However, as the dimension is reduced, the
two types of spin become progressively unequal in number
(
especially at the surface), remain uncompensated and
produce substantial magnetization at low temperatures.
Origin of such size-dependent magnetization behaviour is
still not very well understood.
Among all the antiferromagnetic oxide systems,
nickel(II) oxide is unique and extensively studied [23–30].
It exhibits a slightly distorted face-centred cubic (fcc)
(
NaCl-type) structure that gains stability by squeezing
inward along the /1 1 1S direction. The resultant structure
is represented by rhombohedral unit cell with
It is obvious that despite of several reports and a lot of
new observations, inconsistency continuous to persist with
regard to the behaviour of NiO nanoparticles. Further, the
literature survey suggests that the characteristics of the
product depend largely on the crystallite size and distribu-
tion besides the synthesis route and prevailing experimental
conditions, e.g. NiO can be pale apple green or jet black
depending up on the stoichiometry. While the green
corresponds to the composition NiO and is an insulator,
a
˚
0
¯
a ¼ 2.9518 A, a ¼ 6014.2 , Z ¼ 1 and space group R3m at
1
8 1C. When it is cooled to 77 K, the distortion increases
˚
0
and parameters become a ¼ 2.9469 A and a ¼ 60112 .
Further, the rhombohedral phase is characterized by
2
+
antiferromagnetic ordering with Ni
1 1 1) layers having their spins in mutually opposite
directions and coupling of d electrons through p electrons
ions in alternate
(
2
ꢁ
2+
of O ions disposed in between (i.e., via a super exchange
process). At 523 K, nickel oxide transforms to a fcc (NaCl-
the black material has deficiency of Ni ions (represented
as Ni0.98O1.00) and behaves like a p-type semiconductor
[38]. An attempt has therefore been made to reinvestigate
the nickel oxide nanoparticles system in detail. For
synthesis, sol–gel method [36] involving non-aqueous
instead of popular aqueous solution route [29] has been
employed mainly because it is simple, cost-effective,
requires low temperature for processing, and offers several
advantages [37]. The powder products have been char-
acterized with regard to their thermal stability (TGA),
phase(s) (XRD), microstructure (SEM), magnetic proper-
ties (VSM, SQUID), and optical absorption.
˚
type) structure with a ¼ 4.1684 A, Z ¼ 4 and space group
Fm3m and a paramagnetic state simultaneously [38]. NiO is
also a potential material for room temperature spin-valve
´
device applications because of its Neel temperature (TN)
being 523 K [34].
Richardson et al. [28] attributed the observed super-
paramagnetism and weak ferromagnetism in nickel oxide
to reduced particle size below 100 nm as the contribution
towards the magnetic moment arising from other sources
3
+
such as the presence of non-stoichiometry and/or Ni
ions are indeed insignificant. Needless to say that the
measured magnetic susceptibility was found to be inversely
proportional to the particle size, quite consistent with the
2. Experimental details
Nee
´
+
l’s description of incomplete spin compensation of
ions at the bounding planes of the particles [17].
For synthesis, nickel acetate tetrahydrate ((CH COO)
3
2-
2
Ni
Ni ꢂ 4H O) is first dissolved in ethanol under constant
2
Some unusual magnetic behaviour reported in NiO
nanoparticles (size ꢀ5.3 nm) by Makhlouf et al. [31]
include: (i) reversible magnetization above the blocking
stirring for 2 h at 40–50 1C to obtain a light greenish sol.
Oxalic acid solution (0.23 M in ethanol) is then added
slowly to the warm sol to yield a greenish thick gel. The gel
product is subsequently dried/digested at 80–110 1C for
24 h and subjected to calcination at 500 1C for 2 h. This
process produces a fine black powder. Thermogravimetric
analysis of the product has been carried out in air by
monitoring its weight while heating at a constant rate of
temperature (T ) not fitting the Langavin function,
B
(
ii) irreversibility in M–H curves at 5 K even after cooling
under high fields ꢀ50 kOe, (iii) large coercivity value below
T , and (iv) broadening and shift in field cooled (FC)
B
hysteresis loop along the applied field direction by ꢀ647 Oe

ARTICLE IN PRESS
S. Thota, J. Kumar / Journal of Physics and Chemistry of Solids 68 (2007) 1951–1964
1953
4
1C/min. For characterization, an X-ray powder diffract-
1
00
ometer (Rich Seifert model ISO Debye flux 2002), a
vibrating sample magnetometer (VSM; ADE Technologies
model EV-7 VSM), superconducting quantum interference
device (SQUID) based magnetometer (Quantum Design,
MPMS-XL5), UV–vis–NIR spectrophotometer (Varian
Cary 5000) and a scanning electron microscope (FEI
Quanta 200 HV) have been employed.
A
B
C
9
0
80
D
E
7
6
5
4
3
0
0
0
0
0
3
. Results and discussion
3
.1. Formation of nickel oxide
F
G
Fig. 1 shows the XRD pattern of the sol–gel product
after drying at 110 1C for 24 h. It corresponds to nickel
oxalate dihydrate (NiC O ꢂ 2H O) having an orthorhom-
0
200
400
600
800
1000
Temperature (°C)
2
4
2
Fig. 2. Weight (%) vs temperature plot of nickel oxalate dihydrate
precursor; the temperature is raised at the rate of 4 1C/min, in air.
˚
bic structure with lattice parameters a ¼ 11.84 A,
˚
˚
b ¼ 5.345 A, c ¼ 15.716 A, Z ¼ 8, and space group Cccm
39]. This result is in contrast to that of Wang et al. [41],
[
the temperature range 320–367 1C, but substantial beyond
who obtained nickel oxalate hydrate similarly, but with
nickel nitrate (instead of nickel acetate) as precursor and
reported a monoclinic phase (known lattice parameters
3
67 1C (i.e., point E). At ꢀ425 1C (point F), total weight
loss corresponds to ꢀ60.5% as against 59.1% expected for
the formation of nickel oxide (NiO). Above 425 1C, there is
no appreciable change in weight. These results suggest the
scheme of NiO formation as under:
˚
˚
˚
being a ¼ 11.77 A, b ¼ 5.333 A, c ¼ 9.833 A, b ¼ 127.21,
Z ¼ 4 and space group C2/c [40]). For thermogravimetric
analysis, weight of the nickel oxalate dihydrate has been
monitored by placing it in a furnace and raising the
temperature at a rate of 4 1C/min up to 900 1C in air. Fig. 2
shows the wt% vs temperature plot. It depicts weight loss
in three stages up to ꢀ320 1C (i.e., point D) with
substantial loss in temperature range of 257–320 1C
ꢃ
320ꢁ425 ꢃ
1
18ꢁ320
C
C
NiC2O4 ꢂ 2H2O
ꢁ!
NiC2O4
ꢁ!
NiO
.
Nickel oxalate dihydrate
Nickel oxalate ðanhydrousÞ
Nickeloxide
Fig. 3 shows a few typical scanning electron micrographs
recorded in secondary electron (SE) mode of the product at
various stages of synthesis. The gel depicts a network of
leaves and islands (Fig. 3a). When the gel is dried at 80 1C
for 24 h, it gives fibrous type morphology (Fig. 3b).
Emergence of spherical particles after calcination at 300
and 400 1C in air for 2 h each (Fig. 3c,d). On examining
very closely, one finds spherical particles to consist of a
large number of tiny particles (Fig. 3e) at high magnifica-
tion the product obtained after calcination at 300 1C in air
for 3 h. The thermogravimetric analysis clearly suggests
that any temperature above 425 1C is quite suitable for
obtaining nickel oxide from the nickel oxalate dihydrate,
produced after drying/digestion of the sol–gel product at
(
C–D). The overall weight loss up to ꢀ320 1C is ꢀ18.6%
as against the weight of crystallizing water of 19.7% in
nickel oxalate dihydrate. Obviously, an anhydrous nickel
oxalate emerges at ꢀ320 1C. The weight loss is meagre in
110 1C for 24 h. In order to follow the formation of NiO
closely, powder of nickel oxalate dihydrate has been
calcined in air at 300, 400, 500, and 600 1C for 2 h each.
XRD patterns of the resulting powders are shown in Fig. 4.
They all reveal formation of nickel oxide corresponding to
a fcc structure with lattice parameter ‘a’ decreasing
with increase in the calcination temperature, i.e., the
˚
value lying in the range 4.18070.005 to 4.16870.005 A
for the calcination temperature of 300–600 1C. Further,
it may be noticed that the height of diffraction peaks
increases, while the width decreases progressively with
rise in the calcination temperature. The average particle
size determined from the Scherrer formula using the full
width at half maximum (FWHM) values of various
2
0
30
40
50
60
70
2
θ (degree)
Fig. 1. X-ray diffraction pattern of sol–gel product after drying at 110 1C
for 24 h. It corresponds to nickel oxalate dihydrate (NiC O).
ꢂ 2H
2
O
4
2

ARTICLE IN PRESS
S. Thota, J. Kumar / Journal of Physics and Chemistry of Solids 68 (2007) 1951–1964
1954
Fig. 3. Scanning electron micrographs (SE mode) of sol–gel product: (a) as prepared, (b) after drying at 80 1C for 24 h, (c, d) after calcination of dried gel
at 300 and 400 1C, respectively, in air for 2 h each, (e) same as (c) but at high magnification showing tiny particles.
diffraction peaks as 4.1, 6.8, 13.4 and 22 nm for samples
calcined for 2 h each at 300, 400, 500, and 600 1C,
respectively.
cooled (FC) conditions. The value of magnetic field used is
500 Oe and the samples are calcined at different tempera-
tures in the range 300–600 1C for 2 h each. The striking
features of these curves are (Table 1):
3
3
.2. Dc magnetization
(
a) Splitting or bifurcation of the MZFC and MFC curves
below a particular temperature (T ), determined by the
.2.1. Temperature dependence
The static as well as dynamic magnetic behaviour of
1
heat treatment conditions, i.e., T shifts progressively
1
nickel oxide powders have been investigated as a function
of temperature and field. Fig. 5(a–e) shows the temperature
dependence of dc magnetization of various nickel oxide
samples under both zero field cooled (ZFC) and field
towards lower temperatures with increase of the
calcination temperature. As mentioned in Section 3.1,
higher calcination temperature amounts to increase of
average crystallite size.

ARTICLE IN PRESS
S. Thota, J. Kumar / Journal of Physics and Chemistry of Solids 68 (2007) 1951–1964
1955
6
00°C for 2h
e
5
4
00°C for 2h
00°C for 2h
d
c
3
00°C for 2h
b
8
0°C for 2h
a
30
40
50
60
70
80
90
100
110
120
2
θ (degree)
Fig. 4. X-ray diffraction patterns of sol–gel product after: (a) drying at 80 1C for 24 h and (b–e) subsequent calcination at 300, 400, 500 and 600 1C,
respectively, for 2 h each. (b–e) Correspond to fcc (NaCl-type) phase of NiO; notice sharpening of peaks with increase of calcination temperature.
(
b) The MZFC curves exhibit two peaks, i.e., a broad
centred at a temperature designated as T , below the
e
0
0
0
.06
.04
.02
ZFC
FC
2
bifurcation temperature (T ) and another sharp in
1
the low-temperature regime for samples calcined at
temperature above 350 1C with maxima being at T3
0
0
.00
.09
(
ꢀ15 K); the value of T decreases with increase of
2
d
the calcination temperature (Table 1). Also, the broad
peak at T becomes sharp gradually with increase of
0
0
.06
.03
2
calcination temperature.
c) MFC curves (Fig. 5) invariably depict progressive
(
0
0
.00
.15
increase in the magnetization value with decrease
in temperature below T (though with varying rates
c
1
in different temperature regimes) but a sharp rise in
magnetization below a critical low temperature of 20 K.
d) The value of magnetization at any temperature
decreases with increase of calcination temperature or
average particle size.
0
0
.10
.05
(
0
0
.00
.20
b
a
0
0
.15
.10
Above the bifurcation temperature (T ) magnetization
1
MZFC and MFC curves of the samples exhibit super-
paramagnetic (SPM) characteristics. Each particle behaves
like a single domain with spins of all the comprising
magnetic species lying in one direction. However, spin
orientation of different particles vary randomly in space
0
0
.05
.30
0
0
0
0
.24
.18
.12
.06
(like that of atoms in paramagnetic substance) and
0
50
100
150
200
250
300
350
magnetization decreases with increase of temperature
following a Langevin-type function. Each particle can be
considered to have a core of an antiferromagnetic NiO and
a shell containing random distribution of spins, causing
increase in the surface anisotropy. If the core part exhibits
uncompensated spins at the core–shell interface, the system
T (K)
Fig. 5. Zero field cooled (ZFC) and field cooled (FC) magnetization vs
temperature plots at a field of 500 Oe of NiO powder formed after
calcination of dried gel product at: (a) 300, (b) 350, (c) 400, (d) 500 and
(
e) 600 1C for 2 h each.

ARTICLE IN PRESS
S. Thota, J. Kumar / Journal of Physics and Chemistry of Solids 68 (2007) 1951–1964
1956
Table 1
Calcination temperature T (1C), average particle size p (nm), peak temperatures in MZFC vs T plots at 500 Oe
20
( ꢄ 10ꢁ ) (J)
5
3
( ꢄ 10ꢁ4) (J/m2)
T (1C) P (nm)
T
1
(K)
T
2
(K) M (emu/g)
H
C
(Oe) at
200 K 5 K
3000 1160 0.40
M
r
(emu/g) at
E
a
K
eff ( ꢄ 10 ) (J/m )
K
S
5
K
200 K
3
3
4
5
6
00
50
00
00
00
4.1
4.7
325
291
202
145
115
265
235
162
115
95
0.139
0.099
0.069
0.042
0.034
0.116
9.14
8.10
5.62
3.96
3.28
25.34
13.99
3.512
0.300
0.055
17.31
11.19
4.00
6.8
–
–
–
–
13.4
22.0
2000
3100
214
113
0.275
0.217
0.114
0.106
0.34
0.67
Bifurcation (T
1
), blocking (T
surface anisotropy constant (K
2
), magnetization at T
).
2
, coercivity (H
C
r a
), remenence (M ), anisotropy energy (E ), effective anisotropy constant (Keff), and
S
essentially behaves like a ferrimagnet with appreciable
magnetic moment. The net magnetization then increases
progressively with decrease in particle size due to corre-
sponding increase in the surface-to-volume ratio (Table 1).
1
1
0
.1
.0
.9
6
00 Oe
1
000 Oe
000 Oe
2
0000 Oe
1
20000 Oe
The peak at a lower temperature T in M
2
curve is
ZFC
0.8
associated with progressive blocking process of uncompen-
sated magnetic spins of the antiferromagnetic core.
Moreover, this transition is broad because of the prevailing
size distribution of the crystallites and, in turn, the energy
barriers, i.e., the spread in relaxation time occurring due to
different sizes and random orientation of anisotropy axis
0
0
.7
.6
0.5
0
0
.4
.3
[
45]. With increase in the measuring temperature, crystal-
lites of progressively larger barrier align with the field,
contributing to the enhancement of the magnetization
value. The temperature T2 corresponds to the highest
blocking temperature associated with the large population
of crystallite whose moments get aligned with the
0.2
0
0
.1
.0
0
50
100
150
200
250
300
T (K)
particular applied field. Beyond T , thermal energy over-
2
comes the applied field effect leading to an overall decrease
of magnetization value gradually.
Fig. 6. Zero field cooled magnetization (MZFC) vs temperature curves of
NiO (obtained after calcination of dried gel product at 400 1C for 2 h at
various applied fields (Haf).
A sharp increase in the MZFC noticed at low-temperature
regime (10–30 K) in all the samples is possibly due to a new
contribution and can be related to the collective freezing of
disordered spins in the shell region, magnetic phase
transition of the core, and/or presence of a secondary
phase of low Curie temperature. At low temperatures,
surface spin fluctuations are subsided and short-range
magnetic correlations assume significance causing nuclea-
tion and growth of exchange coupled clusters, which
eventually develop to a cluster-glass-like state on collective
freezing. This noteworthy feature in MZFC vs T plots has
also been found in amorphous nanoparticles of Fe–Ni–B
and Co–Ni–B alloy system of average size of 3 nm [42].
Moreover, Monte-Carlo simulations undertaken by assum-
ing core–shell model further confirm the strong influence of
the disordered shell spin in determining the magnetic
behaviour of non-interacting particles [43].
respectively. The gradual shift in both the bifurcation
temperature (T ) and peak temperature (T ) in M
ZFC
1
2
curves towards a lower temperature with increase in the
average crystallite size is also observed in NiO, MnO and
CoO, recently [25,57,58]. On the other hand, Richardson
and Milligan [29] observed a shift in peak temperature (T2)
towards higher (instead of lower) temperature with
increase in particle size from their dc susceptibility
measurements at 3500 Oe in NiO. All the MFC curves
show continuous increase of the magnetization with
decrease in temperature (Fig. 5), similar to the behaviour
observed in non-interacting system [42,43,46]. Needless to
say that MFC assumes a constant value at very low
temperatures in system containing interacting magnetic
particles [44].
Winkler et al. [35] have prepared NiO nanoparticles of
average size ꢀ3 nm through hydroxide route and claimed
them to be non-interacting. They observed two maxima
in the MZFC vs T plot centred around 60 and 15 K
corresponding to lack of spin moments compensation of
particle core and freezing of surface cluster spins,
3.2.2. Field dependence
Fig. 6 shows typical dc magnetization MZFC vs
temperature curves of a nickel oxide sample obtained by
calcination of the dried gel product at 400 1C for 2 h at

ARTICLE IN PRESS
S. Thota, J. Kumar / Journal of Physics and Chemistry of Solids 68 (2007) 1951–1964
1957
various magnetic fields (the range being 0.6–20 kOe). These
curves clearly reveal that with increase in field the peak
develops and the associated temperature T2 decreases
eye-like shapes matching well with the typical behaviour of
feebly interacting nanoparticles [43]. Clearly, the hysteresis
loops are broaden with increase enclosed area. Also, loops
show necking (narrowing) and swelling to concave shape
(eye-like) at small magnetic fields for samples of average
particle size of 6.8 and 13.4 nm, respectively (Fig. 7c,e). The
notable feature is first decrease and then increases of
coercivity value from 3000 to 2000 Oe and, subsequently, to
3100 Oe for powder of average particle size of 4.1 nm. The
eye-like hysteresis loop is similar to that observed by
progressively whereas peak corresponding to T broadens
3
and becomes visible due to a gradual shift towards higher
temperature. Also, the magnetization value improves with
the increase of field at any given temperature. The trends
further suggest that the strong irreversibility associated
with the blocking process (i.e., peak T ) of the magnetic
2
particles may disappear at sufficiently high applied field
(
˚
H ) when the anisotropy effect is overcome and the
af
Kodama et al. [26] at 5 K for nickel oxide particles of 315 A
moments get fully saturated. Theoretically, applied field
H ) is expected to lower the barrier between the two easy
diameter prepared by calcining the dried gel of Ni(OH)2
precipitate at and above 523 K for 3 h. Fig. 7(a) matches
well with the hysteresis loops reported by others [31,35] for
nickel oxide particles of smaller average size (3 or 5.3 nm).
At very low temperatures (o30 K), a second maxima
(
af
axes of moment orientation [45]. Obviously, a crystallite
then needs less energy to overcome the crystal-field
anisotropy and a small amount of thermal energy would
reverse the magnetization. It is because of this reason
perhaps the blocking temperature decreases with increase
in the applied field. The lowest blocking temperature
observed is ꢀ90 K at the external field of 20 kOe in NiO
powder obtained by calcination of dried gel product at
(at T ) is observed in M
vs T. Such a behaviour is
ZFC
3
believed to arise from disordered surface spins of the
particles. These spins do not saturate even at high external
field (e.g., ꢀ20 kOe in the present case) and are responsible
for narrowing of hysteresis loops with rapid variation in
the magnetization at low fields in M–H curves (Fig. 7c).
4
00 1C for 2 h (Fig. 6 and Table 1). This behaviour is
expected as at very high external magnetic fields irrever-
sibility phenomena can disappear altogether with the
blocking process or SG freezing either occurring at very
low temperatures or not taking place at all.
Fig. 7 shows a set of hysteresis loops of nickel oxide by
calcination of dried gel product at 300, 400 and 500 1C for
This corresponds to a decrease of coercivity (H ) value and
C
manifests as a minimum in the HC vs T plot [46]. The
nature of M–T plot (Fig. 6) observed below 30 K appears
to be the result of yet another blocking process of the
moments asociated with the disordered surface spins.
However, the blocking temperature is found to increase
(rather decrease observed in the first blocking process,
2
h each and recorded with a VSM at ꢀ290 K and a
SQUID under ZFC conditions at 5 and 200 K. Interest-
ingly, magnetization value is invariably decreasing with rise
in the calcination temperature in both the cases. The nature
of M–H plots at 290 K obtained with VSM (Fig. 7b,d,f)
indicates gradual disappearance of the hysteresis loops
with increase in the calcination temperature (and, in turn,
particle size) and becomes reversible. It may be mentioned
that the high-temperature dc magnetization measurements
described in Section 3.4 reveal NiO products to be
essentially antiferromagnetic below ꢀ523 K. Nevertheless,
despite of being antiferromagnetic, the sample obtained
after calcination of dried sol–gel product at 300 1C for 2 h
shows hysteresis loop at 290 K. It is due to small average
particle size, increasing tendency of uncompensated spins,
and high bifurcation temperature (320 K) observed in M–T
curves (Fig. 5(a)). That means, favourable conditions for
antiferromagnetic ordering (or spin compensation) develop
gradually with increase in particle size of the polycrystalline
NiO. The extent of spin compensation is determined by the
particle size itself. While the powder of average particle size
of 4.1 nm exhibits hysteresis, that of 6.8 and 13.4 nm
show to reversible magnetization characteristics at 2901 C
associated with T ) with increase of applied magnetic
2
field (Haf).
Fig. 8 shows magnetization vs field (M–H) curves at 5 K
of a nickel oxide obtained by calcination of dried gel
product at 400 1C for 2 h after cooling (i) under zero field
and (ii) at 20 kOe. They display stretched S—shapes
without saturation even at high magnetic field of 40 kOe.
The values of coercivity (H ) and remanence magnetiza-
C
tion (M ) are 1050 Oe and 0.237 emu/g, respectively. In FC
r
case, the sample shows a magnetization value of 0.64 emu/g
even at zero applied field. The magnetization curves with
increase and decrease of external magnetic field almost
overlap and the hysteresis loop remains open in the first
quadrant. The hysteresis loop corresponding to MFC shows
an overall broadening and shifts in comparison to that
observed under ZFC condition by about 590 Oe towards
the direction opposite to the cooling field at 5 K (i.e., below
T ). This behaviour has been explained differently in the
3
literature, e.g., Kodama et al. [26] involved multiple
sublattice spin configurations in nanoparticles with a new
finite size effect in which surface spins due to their reduced
co-ordination cause magnetic ordering throughout the
particle. Nanoparticles of antiferromagnetic material con-
tain multiple (instead of two) sub-lattice configurations
with week coupling. So, a variety of reversal paths may
exist for spins upon cycling the field. Also, nanoparticles
of antiferromagnetic material comprise of a core with
ferrimagnetically aligned spins and a surface layer with
(
2
Fig. 7b,d,f). SQUID M–H plots (Fig. 7a,c,e) obtained at
00 K continue to display similar trends, but with higher
magnetization values and signatures of soft magnetic
material with increasing particle size. The rapid increase
in magnetization at small fields can be noticed in Fig. 7c.
However, at 5 K, the M–H plots contain stretched S- or

ARTICLE IN PRESS
S. Thota, J. Kumar / Journal of Physics and Chemistry of Solids 68 (2007) 1951–1964
1958
SQUID
VSM
0.12
NiO 300°C-2h
NiO 300°C-2h
3
2
1
0
5
K
0.10 290K
200 K
0
0
0
0
.08
.06
.04
.02
0.00
0.02
0.04
0.06
0.08
0.10
0.12
-
-
-
-
-
-
-1
-2
-3
-
40000
-20000
0
20000
40000
-20000-15000-10000 -5000
0
5000 10000 15000 20000
2
1
1
.0
.5
.0
0.08
0.06
NiO 400°C-2h
NiO 400°C-2h
290 K
5
2
K
00 K
0.04
0.02
0.5
0
.0
0.00
-
-
-
-
0.5
1.0
1.5
2.0
-0.02
-0.04
-0.06
-0.08
-
20000-15000-10000 -5000
0
5000 10000 15000 20000
-
40000
-20000
0
20000
40000
0
0
.06
.04
0
.6
.4
.2
.0
NiO 500°C-2h
NiO 500°C-2h
290K
5
K
200K
0
0
0
0.02
0.00
0.02
0.04
0.06
-
-
-
0.2
0.4
0.6
-
-
-
-
40000
-20000
0
20000
40000
-20000-15000-10000 -5000
0
5000 10000 15000 20000
H (Oe)
H (Oe)
Fig. 7. Magnetization (MZFC) vs field (H) curves of NiO formed after calcination of dried gel product at 300, 400, and 500 1C for 2 h each: (a, c and e)
using SQUID at 5 K and (b, d, and f) using VSM at 290 K.

ARTICLE IN PRESS
S. Thota, J. Kumar / Journal of Physics and Chemistry of Solids 68 (2007) 1951–1964
1959
2
1
1
0
0
.0
.5
.0
.5
.0
FC at 5 K
-0.5
-1.0
-1.5
2
1
1
0
0
.0
.5 ZFC at 5 K
.0
.5
.0
-
-
-
-
0.5
1.0
1.5
2.0
-
40000
-20000
0
20000
40000
H (Oe)
Fig. 8. Hysteresis loops at 5 K of NiO formed after calcination of dried gel product at 400 1C for 2 h following cooling at: (a) zero field and (b) field of
0 kOe.
2
a SG configuration. The surface spins have multiple
configurations, but get frozen at very low temperature
be understood by assuming a core–shell model. While the
core part corresponds to antiferromagnetic character, spin
disorder prevails at the surface (shell region) and plays an
important role in determining the overall behaviour. As the
average particle size decreases, core gets automatically
reduced and spin density at the core–shell interface
(
possible sources such as non-stoichiometry and/or presence
say o50 K). The magnetization contribution due to other
3
+
of Ni
ions are found to be insignificant and therefore
cannot account for the abnormal magnetic behaviour of
NiO [28].
Although there occurs: (i) splitting/bifurcation of MFC
and MZFC curves below the bifurcation temperature (T ) at
1
high applied fields and (ii) shift of hysteresis loop of MFC
from MZFC in the direction opposite to the cooling field
increases vis-a-vis total core, dominating thereby the
`
magnetic behaviour. The shell portion separates the
neighbouring crystallites such that the interaction between
cores becomes difficult, rendering the system to be weakly
interacting. The shell region however contains reasonable
number of spins in disordered state with reduced co-
ordination. These spins interact with each other and give
rise to different canted spin structures. The possible
interactions can then be shell–core and shell–shell, besides
between the spins within the shell.
(
Fig. 8) yet, the most striking feature associated with the
glassy state of nanoparticles, viz. the flattening of MFC vs T
plot at low temperatures is totally absent. Instead, MFC
value increases progressively with decrease in temperature
(
Fig. 5). Usually, non-flattening of FC susceptibility below
TB suggests the non-interacting nature [42,43]. If the
particles are perfectly non-interacting, MFC should vary
as 1/T which is certainly not observed here. For non-
Let us return to the MZFC measurements, where the
sample is first cooled to 5 K in zero magnetic field and then
the magnetization measured with heating at a constant rate
under a small uniform external field of 500 Oe. At a low
temperature (say 5 K), an ensemble of small individual
crystallites gets frozen and each particle tends to magnetize
along its preferred crystal direction so as to minimize the
interacting particles, T should increase linearly with the
B
size of the particles which is not observed. In fact, an
opposite trend is observed (Table 1) [because the energy
barrier separating the low-energy states is proportional to
2
the volume of the particle, TB / VðHK ꢁ HÞ , where V is
magneto-crystalline energy. The energy density expression
2
the particle volume, H is a constant, and H is the field of
K
of the particle contains a term Keff sin y, where K is a
eff
measurement (500 Oe in the present case)]. The magnitude
of O ¼ 0.056–0.083 determined from the ac susceptibility
measurements (we shall see later in Section 3.3) exactly
belongs to interacting particles. For completely non-
interacting particles, O should be greater than 0.10 whereas
for spin glasses, O is less than 0.03 [56]. Further, the
anomalous magnetic behaviour of NiO nanocrystallites can
constant representing the energy density associated with
the magneto-crystalline anisotropy and y is the angle
between the easy axis and magnetization vector. Thus, the
energy is minimized when y ¼ 0 or 1801, i.e., magnetization
vector lying parallel or anti-parallel to the easy axis.
Moreover, the magnetic moments of different particles are
randomly oriented leading to the resulting magnetization

ARTICLE IN PRESS
S. Thota, J. Kumar / Journal of Physics and Chemistry of Solids 68 (2007) 1951–1964
1960
as zero or negligibly small value. With increasing tempera-
ture, the magnetic moments of the particles can fluctuate
rapidly and a crystallite of volume V needs an activation
energy of E ( ¼ K V) to flip its magnetization from y ¼ 01
(E ), anisotropy constants K and K , etc. are summarized
a eff S
in Table 1. Here, the crystallite is assumed to be spherical
with the average diameter given in Table 1 for evaluation of
various parameters. Notice that the values of both the
anisotropy constants (K ) and (K ) decrease with increase
a
eff
to 1801 or vice versa. For small particles, E can be
a
eff
S
5
3
comparable (or smaller) to thermal energy (k T). In such
cases, thermal fluctuations can easily flip the magnetiza-
of the particle size; typical values being K ¼ 3.5 ꢄ 10 J/m
eff
B
ꢁ
4
2
and K ¼ 4 ꢄ 10 J/m for crystallite of the average
S
5
tion. The relaxation time (t) associated with this process is
well described by the Neel–Arrhenius law [47]:
´
size of ꢀ6.8 nm. The reported values of K are 2.5 ꢄ 10
eff
4
3
and 1.0 ꢄ 10 J/m for NiO of average particle size of 2
and 7 nm, respectively [48]. Yet, Kondoh [49] found
Ea=kBT
t ¼ to e
,
5
3
a value of Keff as 5 ꢄ 10 J/m for NiO. Lai et al. [50]
calculated the effective crystalline anisotropy constant of
Keff epitaxial (1 1 1) grown NiO film from micromagnetic
ꢁ
9
where to is typical of the order of 10 s, kB is the
Boltzmann constant, and T is the temperature. Accord-
ingly, the thermal fluctuations become rapid on heating (as
t decreases) and get slow-down on cooling (t increases).
When ‘t’ is much more than the measuring time (t), the
system appears to be static. The temperature at which the
measuring time (t) becomes equal to the relaxation time (t)
is also called the blocking temperature (T ). Below T ,
3
3
modelling as 2.4 ꢄ 10 J/m . Large values of K are usually
eff
attributed to dipolar interactions and/or surface effects [51]
(Table 2).
3.3. Ac susceptibility (temperature, frequency and field
dependence)
B
B
each magnetic particle gets locked into one of the two
states of minimum energy described here. Typical measure-
ment time for static magnetic measurements is ꢀ100 s [45].
In order to understand the magnetodynamic properties
of the NiO ultra-fine crystallites, their temperature, field
and frequency dependence of ac magnetic susceptibility has
also been studied. Since the experimental time window
of the undertaken measurement is determined by the
frequency (f) of the ac field, a broad range of time span can
be easily set by changing the frequency itself. The ac
magnetic susceptibility measurements yield two quantities,
namely the magnitude w, and the phase shift j which, in
turn, yield in-phase (real) and out-of-phase (imaginary)
Under this condition, the activation energy (E ) turns out
a
to be 25k T and hence allows experimental estimation of
B
the anisotropy energy, if blocking temperature is known.
Once the activation energy (E ) is determined, the value of
a
the constant Keff can be found out for a crystallite of
volume (V) since E ¼ K V. Similarly, E is also related to
a
eff
a
crystallite surface area (S) as E ¼ K S, where K is the
a
S
S
surface anisotropy constant. In ZFC case, after the sample
is cooled to 5 K and a low magnetic field of 500 Oe is
applied for magnetization measurement, moments con-
tinue to remain locked into the preferred crystal directions
as before. As the heating is resumed, thermal energy begins
to disturb the system, and the moments now try to align in
the direction of applied field so that net magnetization
gradually increases and reaches a maximum where a large
population of moments gets aligned with the external field.
The temperature at which the magnetic moment reaches a
0
00
parts of susceptibility as w and w . These parameters are
very sensitive to thermodynamic phase changes and have
often been employed to determine the transition tempera-
0
0
tures. Fig. 9 shows temperature dependence of the w . of
nickel oxide (obtained by calcination of the dried sol–gel
product at 400 1C for 2 h) at frequencies of 1, 10, 100 and
1000 Hz of ac field (amplitude 1 Oe) superimposed on dc
field of 10 Oe under both ZFC (a) and FC (b) conditions.
The insets depict the curves in compressed form covering
the temperature interval 50–250 K to reveal the peak in a
better way. All these susceptibility curves depict two
maximum is called the blocking temperature (T ). As the
B
temperature rises above T , thermal vibrations overcome
B
the applied field effect and cause randomization of the
magnetic moment again leading to decrease in the
magnetization level progressively. The values of various
parameters like blocking temperature, activation energy
maxima: a broad centred at T ꢀ170–180 K and a sharp
2
one at T below 30 K. Fig. 9 clearly depicts increase of
3
blocking temperature (T ) with increasing frequency in
2
both the ZFC and FC situations. Also, the maximum at T2
Table 2
The values of external applied field (H), blocking temperature (T
B a
), anisotropy energy (E ), and effective anisotropy constant (Keff) of nickel oxide sample
calcined at 400 1C for 2 h
External field, H
(Oe)
Blocking
temperature, T
Magnetic moment
at T , M (emu/g)
Anisotropy energy,
20
( ꢄ 10ꢁ ) (J)
Effective anisotropy
5
3
B
(K)
B
B
E
a
constant, Keff ( ꢄ 10 ) (J/m )
6
00
160
160
150
115
90
0.065
0.105
0.190
0.638
1.011
5.52
5.52
5.17
3.96
3.10
3.35
3.35
3.14
2.41
1.88
1
2
000
000
1
2
0000
0000

ARTICLE IN PRESS
S. Thota, J. Kumar / Journal of Physics and Chemistry of Solids 68 (2007) 1951–1964
1961
7
7
.5
.0
11
3
.9
2
2
1
1
1
1
1
2
0
8
6
4
2
0
8
6
4
2
ZFC at 10 Oe
ZFC at 10 Oe
Hz
ZFC
1
Hz
10 Hz
00 Hz
1
Hz
0 Hz
00 Hz
000 Hz
3.6
3.3
3.0
1
0
9
8
7
6
5
4
3
2
1
1
1
1
6.5
6.0
5.5
1000 Hz
10
Hz
1
100 Hz
1
1000 Hz
2
2
2
.7
.4
.1
5
4
4
3
.0
.5
.0
.5
1.8
60
80 100 120 140 160 180 200 220 240
60
80 100 120 140 160 180 200 220 240
T (K)
T (K)
0
50
100
150
200
250
300
0
50
100
150
200
250
300
11
7
7
.5
.0
3
.8 FC
2
2
1
1
1
1
1
2
0
8
6
4
2
0
8
6
4
2
FC at 10 Oe
Hz
FC at 10 Oe
1 Hz
1
Hz
10 Hz
00 Hz
1000 Hz
3.6
1
10
9
8
7
6
5
4
3
2
1
1
3
3
.4
.2
10 Hz
00 Hz
6.5
10 Hz
100 Hz
1000 Hz
1
6
5
5
4
4
3
.0
.5
.0
.5
.0
.5
3.0
.8
2
1000 Hz
2.6
2.4
2.2
2.0
1.8
60 80 100 120 140 160 180 200 220 240
T (K)
60
80 100 120 140 160 180 200 220 240
T (K)
0
50
100
150
200
250
300
0
50
100
150
200
250
300
0
00
Fig. 9. Ac magnetic susceptibility (real w (T) and imaginary w (T) components) vs temperature plots of NiO, obtained by calcination of dried gel at 400 1C
for 2 h, at 10 Oe with ac excitation field of amplitude 1 Oe and various frequencies superimposed under (a, b) zero field cooled and (c, d) field cooled
conditions. The insets depict the same curves in the temperature range 50–250 1C after modifying the scales to show peak better.
0
00
prevails in both w vs T and w vs T plots. Notice the peak
shift towards a higher temperature with increasing
case) per decade frequency from the expression [52]
DTm
0
frequency. Moreover, the value of w decreases while as
O ¼
.
Tm Dlog ðf Þ
0
0
10
that of w increases with increasing frequency. The broad
nature of maxima and shift of T2 towards higher
temperature with increase of frequency of the driving ac
field observed in both the curves indicate usual behaviour
of SPM system. Also, there appears to be a close
The value of O is found to be in the range 0.0045–0.06 and
0.1–0.13 for many canonical SG and SPM systems,
respectively [32,51]. In the present case, the value of O turns
out to be in the range of 0.056–0.083 which begins towards
the higher end of SG but falls somewhat below the mark
designated for SPM system.
0
resemblance between w and dc magnetization and support-
ing the belief that the SPM behaviour is originated from
the uncompensated antiferromagnetic core of the particle
itself. On the other hand, the presence of a sharp peak at T3
below 30 K corresponds to SG characteristics or another
phenomena believed to be associated with the shell of the
particle only.
The useful criterion for distinguishing between the freezing
and the blocking processes lies in determining the relative
shift (O) of the peak temperature (T being T in the present
To examine the issue further magnetic susceptibility has
been measured at different dc field with a fixed ac driving
field (of amplitude 1 G and frequency 10 Hz) superimposed
0
under ZFC condition. Fig. 10 shows w as a function of
temperature with external dc field of 10, 100, 200, 400 and
0
600 Oe. Clearly, as the dc field increases the value of w
0
0
decreases whereas that of w increases. Also, the peak
0
temperature in w vs T plot decreases marginally with
m
2

ARTICLE IN PRESS
S. Thota, J. Kumar / Journal of Physics and Chemistry of Solids 68 (2007) 1951–1964
1962
2
2
1
1
1
1
1
2
0
8
6
4
2
0
8
6
4
2
7
.0
ZFC at 10 Hz
10 Oe
6.5
1
2
4
6
00 Oe
00 Oe
00 Oe
00 Oe
6
5
.0
.5
5.0
4
4
3
.5
.0
.5
60
80 100 120 140 160 180 200 220 240
T (K)
0
50
100
150
200
250
300
T (K)
3.50
ZFC at 10 Hz
1
0
9
8
7
6
5
4
3
2
1
1
0 Oe
3.25
1
2
4
6
00 Oe
00 Oe
00 Oe
00 Oe
3
2
.00
.75
2.50
2.25
2.00
1.75
60
80 100 120 140 160 180 200 220 240
T (K)
0
50
100
150
T (K)
200
250
300
0
00
Fig. 10. Temperature dependence of ac magnetic susceptibility (real w (T) and imaginary w (T) components) of NiO, obtained by calcination of dried gel at
00 1C for 2 h, at various fields. With ac excitation field of amplitude 1 Oe and frequency 10 Hz superimposed under zero field condition. The insets depict
that the same curves in the temperature range of 50–250 K after modifying the scales to show the peaks better.
4
2
/3
increase of dc field and is consistent with the trends
0
dependence of temperature (T ) with either H or H ,
2
observed in dc magnetization curves. But, in the case of w
vs T plots, the peak temperature increases slightly and then
which is generally attributed to the existence of SG
phase [53]. Tiwari and Rajeev [32] have recently drawn
the inference about the SG behaviour in NiO from
attains a constant value of 180 K with increase of dc field
range 10–600 Oe). These results do not show any linear
(
(i) unreasonable values found for energy barrier, E and
a

ARTICLE IN PRESS
S. Thota, J. Kumar / Journal of Physics and Chemistry of Solids 68 (2007) 1951–1964
1963
attempt frequency (n ) for magnetization flips; (ii) linear
system. Further, the frequency-dependent (1–1000 Hz) and
field-dependent (10–600 Oe) ac susceptibility (w) measure-
ment data in conjunction with the above results provide
0
0
0
variation of temperature (corresponding to peak in the w
vs T plot) with H following de Almeida–Thouless (AT)
2
/3
line [53,54], and showing SG transition temperature (T ) as
f
evidence and rule out the SG behaviour (that means T is
2
ꢀ
167 K (usually referred as the blocking temperature); and
iii) disappearance of peak noticed in MZFC vs T at high
fields (ꢀ20 kOe). On the contrary, the present results show:
i) reasonable values of E and n , (ii) absence of linear
the blocking T and not the SG transition temperature T )
B
g
(
for NiO nanoparticles. Instead, the observations suggest
core–shell description for NiO nanoparticles. While the
core behaves like a ferrimagnet because of prevailing lack
of spin compensation due to reduced size, shell contains
randomly oriented spins with low co-ordination and is
possibly responsible for sharp increase in MZFC below 30 K
as a result of collective freezing effect and or presence of
secondary phase of low Curie temperature. Finally, the
increase of blocking temperature from 115 to 265 K as well
as energy band gap from 3.57 to 3.68 eV observed with
decreasing average particle size (13.4–4.1 nm) seems to
have a common origin and caused by size effects, viz.
confinement of charge carriers and increase in the
uncompensated core spins, respectively.
(
a
0
2
/3
variation of T with H , (iii) T as blocking temperature
and not the SG transition temperature, (iv) existence of
peak corresponding to temperature T at high fields of
2
2
2
2
0 kOe and (v) hysteresis loops even at 5 K. The room
temperature optical absorption studies further reveal
increase of energy band gap with decreasing crystallite
size, the values being 3.57, 3.63 and 3.68 eV for NiO
powders of average particle size of 13.4, 6.8 and 4.1 nm,
respectively. Such a result in fact, corresponds to a blue
shift of the optical absorption edge usually observed in
semiconductor nanoparticles due to quantum size effects
[
55]. The extent of quantum confinement increases with
decrease in particle size and leads to widening of the energy
band gap. The increase blocking temperature as well
as energy band gap observed with decreasing particle
size in the present work appears to have common origin
and caused mainly by size effect (i.e., confinement of
charge carriers and increase in the surface spin density,
respectively).
References
[1] R.W. Chantrell, K. O’Grady, in: R. Gerber, C.D. Wright, G. Asti
(Eds.), Applied Magnetism, Kluwer Academic Publishers, The
Netherlands, 1994, p. 113.
[
[
2] D. Weller, A. Moser, IEEE Trans. Magn. 35 (1999) 4423.
3] G.C. Hadjipanayis, G.A. Prinz, Science and Technology of Nanos-
tructurated Materials, Plenum Press, New York, 1991.
4] Q.A. Pankhust, J. Connolly, S.K. Jones, J. Dobsan, J. Phys. D: Appl.
Phys. 36 (2003) R167.
[
[
4
. Conclusions
5] J.P. Bucher, D.C. Douglass, L.A. Bloomfield, Phys. Rev. Lett. 66
(
1991) 3052.
Nickel oxalate dihydrate (NiC O ꢂ 2H O) synthesized by
2
4
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