Antiferro to superparamagnetic transition on Mn doping in NiO

Article abstract of DOI:10.1016/j.ssc.2010.05.003

We report the structural and magnetic properties of Ni_1-xMn _xO (x=0 to 0.05) prepared by chemical method. Within the XRD resolution, no impurity phases could be detected up to 5 at.% Mn doping in NiO. The fcc structure and the lattice parameter of host NiO matrix is not altered on Mn doping. The average crystallite size was found to remain almost constant (28 nm) up to 3 at.% Mn doping, beyond which it decreases to 21 nm for 5 at.% Mn doping in NiO. The magnetic properties on the other hand showed a drastic change with Mn doping. While 0 and 1 at.% Mn doped NiO showed antiferromagnetic behaviour down to 10 K, 3 and 5 at.% Mn doped NiO were superparamagnetic at 300 K with a blocking temperature of 186 and 171 K respectively. Clear hysteresis loops were thus observed for these samples at 10 K. The distribution of blocking temperature of the Mn doped NiO particles matches well with the distribution of particle size as obtained from TEM. The observed antiferro to superparamagnetic transition on Mn doping in NiO is understood on the basis of Mn occupying Ni site and breaking the translational symmetry of the parent antiferromagnetic correlation.

Full text of DOI:10.1016/j.ssc.2010.05.003

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Solid State Communications 150 (2010) 1342–1345
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Solid State Communications
journal homepage: www.elsevier.com/locate/ssc
Antiferro to superparamagnetic transition on Mn doping in NiO
P. Mallicka,e, Chandana Rathb, A. Rathc, A. Banerjeed, N.C. Mishrae,∗
a Department of Physics, North Orissa University, Baripada 757 003, India
b School of Material Science & Technology, Institute of Technology, BHU, Varanasi 221 005, India
c Institute of Physics, Sachivalaya Marg, Bhubaneswar-751005, India
d UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore 452017, India
e Department of Physics, Utkal University, Bhubaneswar, Orissa 751 004, India
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the structural and magnetic properties of Ni1−xMnxO (x = 0 to 0.05) prepared by chemical
method. Within the XRD resolution, no impurity phases could be detected up to 5 at.% Mn doping in NiO.
The fcc structure and the lattice parameter of host NiO matrix is not altered on Mn doping. The average
crystallite size was found to remain almost constant (28 nm) up to 3 at.% Mn doping, beyond which it
decreases to 21 nm for 5 at.% Mn doping in NiO. The magnetic properties on the other hand showed a
drastic change with Mn doping. While 0 and 1 at.% Mn doped NiO showed antiferromagnetic behaviour
down to 10 K, 3 and 5 at.% Mn doped NiO were superparamagnetic at 300 K with a blocking temperature
of 186 and 171 K respectively. Clear hysteresis loops were thus observed for these samples at 10 K. The
distribution of blocking temperature of the Mn doped NiO particles matches well with the distribution of
particle size as obtained from TEM. The observed antiferro to superparamagnetic transition on Mn doping
in NiO is understood on the basis of Mn occupying Ni site and breaking the translational symmetry of the
parent antiferromagnetic correlation.
Received 8 February 2010
Received in revised form
5 April 2010
Accepted 4 May 2010
by P. Chaddah
Available online 10 May 2010
Keywords:
A. Magnetic materials
B. Chemical synthesis
D. Superparamagnetic
D. Doping
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
will facilitate the integration of spintronic devices with advanced
silicon based microelectronic devices [17].
NiO with cubic structure is a prototype p-type transparent
conducting oxide [18,19] due to vacancy at Ni2+ sites [20,21].
Bulk NiO reveals antiferromagnetic behaviour below its Neel
temperature, TN = 523 K. The magnetic structure of NiO consists
of ferromagnetic sheets of Ni2+ parallel to the (111) plane with
opposite spin directions in neighbouring sheets. NiO is readily
dopable by holes but not by electrons [22,23]. TM doping [24–27]
or reduction of particle size [28,29] has been shown to disturb
the magnetic structure of NiO. In the present study, we examine
the evolution of structure and magnetic properties of NiO
nanoparticles on Mn doping (0 to 5 at.%). We show that Mn doping
does not modify the cubic structure and lattice parameters of
NiO. The magnetic order of NiO however undergoes a transition
from antiferromagnetic to superparamagnetic state on Mn doping
beyond 1 at.%.
In the recent years, ferromagnetic semiconductors have been
actively pursued because of their potential as spin polarized carrier
sources and easy integration into semiconductor technology [1].
Theoretical developments [2,3] have shown that transitional metal
(TM) doped wide band gap semiconductors are the most promising
candidates for achieving room temperature ferromagnetism.
Though the absence of any ferromagnetic ordering has been
reported in a few cases [4–6], the discovery of room temperature
ferromagnetism in TM doped oxides [7] has attracted a great deal
of research attention due to their potential for use in spintronic
devices. The origin of the ferromagnetism in these systems
however is still not clear. Some groups attributed it to an intrinsic
origination, while some other groups attributed it to TM metal
cluster or secondary phases [8–16]. For practical applications,
these materials need to exhibit high Curie temperature (>300 K)
with intrinsic ferromagnetism rather than one due to the presence
of secondary phases. Also it is anticipated that if one can
introduce room temperature ferromagnetism in cubic systems, it
2. Experimental details
Pure and Mn doped NiO samples were synthesized by chemical
method with Ni(NO3)2.6H2O and Mn(CH3COO)2.4H2O as starting
materials. Nominal amount of the starting materials were weighed
and dissolved separately into the solution of distilled water and
ethyl alcohol taken in the ratio 2:1. The pH of the solutions were
∗
Corresponding author. Tel.: +91 674 2581079; fax: +91 674 2581079.
E-mail address: nareshcmishra@gmail.com (N.C. Mishra).
0038-1098/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2010.05.003

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1343
maintained in the acidic range by adding nitric acid and acetic
acid to the Ni(NO3)2.6H2O and Mn(CH3COO)2.4H2O solutions
respectively. Individual solutions were stirred for 1 h. These
solutions were then mixed and the mixture was stirred again for
2 h. The resulting mixture was subjected to gelation for 1 day and
then evaporated to dryness to obtain the samples in powder form.
The resultant Mn doped NiO nano-powders were prepared by the
heat treatment of the obtained powder samples, in air at 773 K
for 1 h.
a
10000
25000
20000
15000
10000
5000
0
(200)
YOBS
YCALCZ
Difference
8000
6000
4000
2000
0
(111)
(220)30
40
50
60
70
80
90 100
X-ray diffraction (XRD) (Rigaku Powder Diffractometer) was
used to reveal the structure and microstructure of the samples.
2θ(Degree)
(311) (222)
The XRD data were collected at room temperature using CuK
(331)
α
x=0.05
radiation. Transmission electron microscopy (TEM) measurements
of Mn doped NiO samples were performed (using a JEOL JEM-
2010 (UHR) electron microscope operating at 200 kV) to study
the size distribution and lattice spacing. Magnetic properties
of different Mn doped NiO samples were studied using a 14
T Physical Properties Measurement System (PPMS)—Vibrating
Sample Magnetometer of Quantum design.
x=0.03
x=0.01
x=0
40
60
80
100
2θ (Degree)
20000
15000
b
3. Results and discussion
(200)
The XRD pattern of Ni1−xMnxO samples with x = 0 to 0.05
(Fig. 1(a)) did not reveal the presence of any other phase except
the cubic phase of pure NiO. To delineate the peaks due to minor
secondary phases if suppressed due to high intensity of the peaks
of host fcc NiO phase, we present the XRD spectra of 5 at.% Mn
doped NiO sample with intensity axis given in logarithmic scale
(Fig. 1(b)). Even at this high Mn concentration, no impurity phase
could be observed in the XRD spectra. The Rietveld refinement also
revealed phase purity and fcc structure of 5 at.% Mn doped NiO
(inset of Fig. 1(a)). The lattice parameter obtained from the Rietveld
refinement with Fm3m space group is found to be almost constant
(0.4178 ± 0.0001 nm) for all the Ni1−xMnxO samples.
(111)
(220)
10000
(311)
(222)
(331)
x=0.05
Fig. 2 shows a planar TEM micrograph of 3 and 5 at.% Mn doped
NiO samples along with their histogram showing particle size
distribution. The distribution obtained form several micrographs
of the corresponding samples fit well with Gaussian function
(Fig. 2(c) and (d)). In the cases where non-spherical grains were
seen in TEM micrograph (Fig. 2(a) and (b)), size averaged over
the value measured along different axes was taken. TEM analysis
indicated that the average particle size of 3 and 5 at.% Mn doped
NiO samples are ∼26 and 22 nm respectively. These values
compare well with the crystallite size (28 nm for 0 to 3 at.%
Mn doped and 21 nm for 5 at.% Mn doped NiO) obtained from
the XRD line width using Scherrer formula. The small decrease
of the particle size with Mn doping in NiO is in contrast to
the case of Mn doped (ZnAs)O, where Mn acts as a potential
catalyst for nano-dot formation decreasing the particle size from
40 nm to less than 10 nm upon 5 at.% Mn [30]. The HRTEM
images obtained from the nanoparticles for 3 and 5 at.% Mn
doped NiO are shown in the inset of Fig. 2(a) and (b) respectively.
The d-spacing (0.245 ± 0.002 nm) corresponding to (111) planes
and that (0.21 ± 0.002 nm) corresponding to (200) planes obtained
from TEM, match well with the values obtained from the XRD data
for both the samples.
Magnetization as function of field (M–H) at 10 K is shown
in Fig. 3. A linear dependence of magnetization on the applied
magnetic field was observed in pure NiO sample as is expected for
an antiferromagnetic material. The 1 at.% Mn doped NiO sample
also showed linear dependence of magnetization with applied
magnetic field. Further increasing the Mn concentration in NiO to
3 and 5 at.%, the M–H curve showed hysteresis loop. At 300 K,
the hysteresis loop was not observed for any of the Mn doped
NiO samples (inset of Fig. 3). Instead, the 3 and 5 at.% Mn doped
samples showed rapid increase of magnetization up to a field of
40
60
80
100
2θ (Degree)
Fig. 1. (a) XRD pattern of Ni1−xMnxO (x = 0 to 0.05) samples. The inset shows the
typical XRD pattern of Ni0.95Mn0.05O, simulated by the Rietveld refinement. (b) XRD
pattern of Ni0.95Mn0.05O with intensity in logarithmic scale.
∼1 T, beyond which it showed a linear increase. The rapid increase
of magnetization at low fields indicates superparamagnetic nature
of these particles. The undoped and 1 at.% Mn doped NiO samples
on the other hand did not show such non-linear increase of
magnetization with magnetic fields. All the samples exhibited
unsaturation of magnetization with magnetic field in the high field
regime up to a field of 5 T at both 10 K and 300 K.
In order to understand the complex magnetic behaviour of
Ni1−xMnxO samples at 10 K and 300 K, we measured the
zero field cooled (ZFC) and field cooled (FC) magnetization in
the temperature range from 10 to 350 K at 0.05 T (Fig. 4).
The magnitude of magnetization in ZFC and FC curves at all
temperatures increased with the increase of Mn content in NiO.
Fig. 3 also reveals higher magnetization for samples with higher
Mn content in NiO at all fields. The antiferromagnetic nature of
1 at.% Mn doped NiO seen in its M–H curve at 10 and 300 K (Fig. 3) is
also reflected in its much suppressed FC and ZFC curves (Fig. 4). The
ZFC curve of 3 at.% Mn doped NiO shows a broad peak at 186 K and
that of 5 at.% Mn doped NiO shows the peak at a lower temperature
(171 K).
The peak in ZFC curve can be seen in case of superparamag-
netic and spin glass systems [31]. Generally the temperature de-
pendence of FC magnetization shows saturation below the peak
temperature (freezing temperature) for spin glasses and continues
to increase below that temperature (blocking temperature) for su-
perparamagnets [32]. In our case, the FC magnetization continues

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P. Mallick et al. / Solid State Communications 150 (2010) 1342–1345
a
b
c
d
Fig. 2. Planar TEM micrograph of (a) Ni0.97Mn0.03O and (b) Ni0.95Mn0.05O samples. The inset shows the typical HRTEM images of corresponding samples. The histogram
showing particle size distribution of (c) Ni0.97Mn0.03O and (d) Ni0.95Mn0.05O samples obtained from TEM micrograph.
0.4
0.3
0.2
0.1
0.0
0.4
0.3
0.2
0.1
0.0
ZFC (x=0.01)
FC (x=0.01)
ZFC (x=0.03)
FC (x=0.03)
ZFC (x=0.05)
FC (x=0.05)
x=0.05
x=0.03
0
50
100
150
200
250
300
350
T (K)
Fig. 3. Field dependence magnetization (M–H) curve for Ni1−xMnxO (x
=
Fig. 4. The temperature dependence of ZFC (solid symbol) and FC (open symbol)
magnetization for x = 0.01, 0.03 and 0.05 at a field of 0.05 T. The curves (line with
triangle) represent the number of particles with different blocking temperature, TB
for x = 0.03 and 0.05. The value TB for this curve was calculated considering the
TEM size distribution.
0.00, 0.01, 0.03 and 0.05) at 10 K. The inset shows the M–H curve for x = 0.01,
0.03 and 0.05 at 300 K.
to increase below the peak temperature indicating the superpara-
magnetic behaviour of 3 and 5 at.% Mn doped NiO above the block-
ing temperature as was suggested from their M–H loops (Fig. 3).
The presence of broad peak in ZFC curve is an indication of the dis-
tribution of blocking temperature (TB) for superparamagnetic par-
ticles [33]. The maximum in the ZFC curve indicates the average
TB of the system, while the bifurcation temperature points to the
maximum TB corresponding to the largest nanoparticles in the size
distribution [34].
correlation of the observed TB distribution in the ZFC data with the
distribution of the particle size obtained from the TEM data, we
find the expected TB values for the latter using the relation [35]
TB1
TB2
V1
V2
=
.
(1)
In the above relation, we have taken TB1 ≈ 300 K for NiO particles
having a diameter of 31.5 nm with volume V1 obtained by Kodama
et al. [28] as reference to calculate the TB (TB2) for particles of
The blocking temperature, TB of a superparamagnetic particle
is proportional to the volume of that particle [33]. To find a

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P. Mallick et al. / Solid State Communications 150 (2010) 1342–1345
1345
different sizes (with volume V2) as seen in our TEM data. Fig. 4
shows a good correlation of the distribution of TB obtained from the
ZFC data and that calculated from the TEM data. Our observation of
larger TB for 3 at.% than that of 5 at.% Mn doped NiO is in accordance
with the characteristic behaviour of superparamagnetic system,
since the crystallite size for 5 at.% Mn doped NiO samples is less
than that of 3 at.% Mn doped NiO.
21 nm for 5 at.% Mn doping in NiO. The antiferromagnetic proper-
ties of NiO is retained up to 1 at.% Mn doping. Increasing Mn doping
concentration to 3 and 5 at.% in NiO led to superparamagnetic be-
haviour. The distribution of blocking temperature of the Mn doped
NiO particles matches well with the distribution of particle size as
obtained from TEM. The observed antiferro to superparamagnetic
transition on Mn doping in NiO is understood on the basis of Mn
occupying Ni site and breaking the translational symmetry of the
parent antiferromagnetic correlation.
The observed magnetic behaviour of Mn doped NiO nanoparti-
cles can have contribution from the magnetization resulting from
(i) the finite size effect arising due to the reduced coordination of
the surface spins of NiO nanoparticles, (ii) the secondary Mn re-
lated impurity phases and (iii) the Mn2+ ions occupying the Ni. We
examine the occurrence of these possibilities in our samples.
NiO nanoparticles are shown to exhibit finite size effect, where
8-, 6-, or 4-sublattice spin configurations arise due to the reduced
coordination of surface spins, leading to anomalous magnetic
properties like large moments and coercivity, and loop shifts [28].
In some cases, finite size effect leads to ferromagnetism [29]
(for particles of size, D ≤ 24 nm), superparamagnetism (D ≤
31.5 nm) [28], spin glass behaviour (D ≤ 10 nm) [31] and even
core–shell like structure (4 ≤ D ≤ 22 nm) [36], where the core of
NiO nanoparticle behaves like a ferrimagnet and the shell contains
randomly oriented spins with low coordination. Thus depending
upon the particle size, different anomalous magnetic properties
emerge at the expense of antiferromagnetic property of the host
NiO. In our case, the crystallite size remained almost invariant with
Mn doping. The magnetic behaviour however showed a dramatic
modification with Mn concentration. These results indicate that
more than the finite size effect, the Mn content dictates the
magnetic property of NiO.
Acknowledgements
We thank Prof. P.V. Satyam, IOP, Bhubaneswar for providing
TEM facility. We acknowledge the UGC-DAE CSR, Indore for
providing magnetic measurement facility. DST Govt. of India is
acknowledged for providing PPMS-VSM at UGC-DAE, CSR, Indore
by which magnetic measurements were carried out.
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4. Conclusion
The structural and magnetic properties of chemically synthe-
sized Mn doped NiO samples are reported. We show that the struc-
ture and microstructure of NiO is not influenced considerably, but
the magnetic properties undergo a drastic change with Mn dop-
ing. The average crystallite size was found to remain almost con-
stant (28 nm) up to 3 at.% Mn doping beyond which it decreases to
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