Polymer stabilized Ni-Ag and Ni-Fe alloy nanoclusters: Structural and magnetic properties

Article abstract of DOI:10.1016/j.jmmm.2009.11.027

We report here the structural and magnetic behaviors of nickel-silver (Ni-Ag) and nickel-iron (Ni-Fe) nanoclusters stabilized with polymer (polypyrrole). High resolution transmission electron microscopy (HRTEM) indicates Ni-Ag nanoclusters to stabilize in core-shell configuration while that of Ni-Fe nanoclusters in a mixed type of geometry. Structural characterizations by X-ray diffraction (XRD) reveal the possibility of alloying in such bimetallic nanoclusters to some extent even at temperatures much lower than that of bulk alloying. Electron paramagnetic resonance (EPR) spectra clearly reveal two different absorption behaviors: one is ascribed to non-isolated Ni²⁺ clusters surrounded by either silver or iron giving rise to a broad signal, other (very narrow signal) being due to the isolated superparamagnetic Ni²⁺ clusters or bimetallic alloy nanoclusters. Results obtained for Ni-Ag and Ni-Fe nanoclusters have been further compared with the behavior exhibited by pure Ni nanoclusters in polypyrrole host. Temperature dependent studies (at 300 and 77 K) of EPR parameters, e.g. linewidth, g-value, line shape and signal intensity indicating the significant influence of surrounding paramagnetic silver or ferromagnetic iron within polymer host on the EPR spectra have been presented.

Full text of DOI:10.1016/j.jmmm.2009.11.027

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Journal of Magnetism and Magnetic Materials 322 (2010) 934–939
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Polymer stabilized Ni–Ag and Ni–Fe alloy nanoclusters: Structural and
magnetic properties
n
L. Kabir, A.R. Mandal, S.K. Mandal
Department of Physics, Visva-Bharati, Santiniketan-731 235, India
a r t i c l e i n f o
a b s t r a c t
Article history:
We report here the structural and magnetic behaviors of nickel–silver (Ni–Ag) and nickel–iron (Ni–Fe)
nanoclusters stabilized with polymer (polypyrrole). High resolution transmission electron microscopy
Received 21 September 2009
Received in revised form
(
HRTEM) indicates Ni–Ag nanoclusters to stabilize in core-shell configuration while that of Ni–Fe
1
1 November 2009
nanoclusters in a mixed type of geometry. Structural characterizations by X-ray diffraction (XRD) reveal
the possibility of alloying in such bimetallic nanoclusters to some extent even at temperatures much
lower than that of bulk alloying. Electron paramagnetic resonance (EPR) spectra clearly reveal two
Available online 24 November 2009
Keywords:
Bimetallic alloy
Nanocluster
EPR
2+
different absorption behaviors: one is ascribed to non-isolated Ni clusters surrounded by either silver
or iron giving rise to
a
broad signal, other (very narrow signal) being due to the isolated
2+
superparamagnetic Ni
clusters or bimetallic alloy nanoclusters. Results obtained for Ni–Ag and
Magnetic property
Ni–Fe nanoclusters have been further compared with the behavior exhibited by pure Ni nanoclusters in
polypyrrole host. Temperature dependent studies (at 300 and 77 K) of EPR parameters, e.g. linewidth,
g-value, line shape and signal intensity indicating the significant influence of surrounding paramagnetic
silver or ferromagnetic iron within polymer host on the EPR spectra have been presented.
&
2009 Elsevier B.V. All rights reserved.
1
. Introduction
Recent experiments and molecular simulations indicate the
significant decrease in melting temperature and latent heat in
such nanoalloy systems compared to their bulk values [20–22].
Apart from few experimental and molecular simulations based
work giving some generic features, various issues like atomic
ordering, interdiffusion of atoms, structural stability and their
effect on the physical properties in such nanoalloy systems needs
to be explored.
In this paper we focus on such bimetallic systems consisting of
Ni–Ag and Ni–Fe elements synthesized in the form of nanoclus-
ters. Systems of Ni–Ag in nanoscale have scarcely been addressed
because of immiscible nature of the bimetallic constituents [23].
These nanoclusters may find superiority in applications like high
density magnetic recording media, sensors and spintronic devices.
Such nanostructures also offer tremendous potentiality in carry-
ing pharmaceutical particles and hence can be used in drug
delivery applications. Here, we report on the chemical synthesis
of such bimetallic Ni–Ag and Ni–Fe nanoclusters and obtain
their structural and magnetic properties. We show how the
temperature dependent magnetic relaxation behaviors change in
such low-dimensional bimetallic systems investigated by electron
paramagnetic resonance spectroscopy (EPR).
Magnetic nanostructures are found to have diverse applica-
tions today and have become the subject of key area of research in
nanoscience and nanotechnology. The major potential area of
applications not only includes magnetic recording media, sensors
and importantly spintronic devices, but it also extends in the key
area of biotechnological research, e.g. cell sorting, drug delivery,
optical coding, ferrofluids, etc [1–8]. The tiny magnetic material
objects can easily be manipulated by suitably changing their size,
shape and composition giving rise to unique properties compared
to bulk [9–11]. Besides, they can be addressable by externally
applied magnetic field and thereby widen the scope of obtaining
tunable magnetic nanodevices. The literature indicates a great
deal of research carried out so far on various ferromagnetic
materials like Fe, Co, Ni (d-block transition metals),
2 3
a-Fe O or
g
-Fe , etc. in their various nanostructures form, e.g. nanopar-
2 3
O
ticles, nanorods and nanowires. In this context, of late bimetallic
magnetic nanomaterials or nanoalloys have regained considerable
research interests for their unique physical properties [12–15].
These bimetallic nanostructures are composed of either
magnetic–nonmagnetic or magnetic–magnetic elements. Such
bimetallic nanoalloys are found to exhibit several anomalies in
terms of lattice constant, thermal expansion coefficient and
average magnetic moments and chemical reactivity [16–19].
2. Experimental
n
Synthesis of Ni, Ni–Ag and Ni–Fe nanoclsuters is accomplished
Corresponding author. Tel.: +91 3463 261016; fax: +91 3463 262728.
E-mail address: sk_mandal@hotmail.com (S.K. Mandal).
by simple wet chemical technique. All the regents used here are
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2009.11.027

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L. Kabir et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 934–939
935
analytically pure and no further purifications are done. For the
synthesis of Ni nanoclusters, the procedure takes place first by
3. Results and discussion
2
+
exchanging Ni ions into the pyrrole and then the nickel ions are
reduced to nickel metal by sodium borohydride (NaBH ). Two
separate solutions of Ni(NO (Purified Merck Limited, India) and
It is found that bimetallic nanoclusters can form alloy or
segregate with each other in core-shell structures [19]. In case of
Ni–Ag system, Ni and Ag are immiscible in their bulk form in any
proportion as indicated by their phase diagram. Various efforts
have been made to synthesize Ni–Ag alloys, but giving no
signature of the formation of alloy, only in some cases giving rise
to the segregation of Ni atoms onto the core and with Ag atoms
preferentially at the surface forming core-shell clusters [22]. The
structural characterization done by transmission electron micro-
scope (TEM) clearly indicates the possible formation of nanoalloy
clusters for Ni, Ni–Ag and Ni–Fe and is given in Fig. 1. Fig. 1 shows
the typical TEM and that of HRTEM for Ni (a, d), Ni–Ag (b, e) and
Ni–Fe (c, f), respectively. The TEM images indicate that although
there is a distribution of nanoclusters size, the average crystallite
size didn’t change appreciably for all the three samples. The
average nanoclusters size is found to be D (=2R, R being the
radius) ꢀ2072 nm and plausibly caused by the similar
experimental conditions while synthesizing such nanoclusters.
We also present the HRTEM images of individual alloy
nanoclusters in Fig. 2. The image given in Fig. 2(a) shows the
nanocluster of Ni–Ag and clearly the formation of core-shell
structures, while that of a (inter-) mixed type of cluster for Ni–Fe
as indicated by Fig. 2(b). Both these structures are possible
patterns of formation of alloy nanoclusters as indicated by
theoretical calculations [19]. The core-shell configuration of Ni
4
3 2
)
sodium borohydride (Spectrochem, India) are prepared in a
solvent of ethanol (GR, Jiangsu Huaxi, China) and distilled water.
The volume ratio of ethanol and water is maintained 2:3
throughout to prepare the solution. Both the solutions are
continuously stirred for two hours keeping the bath temperature
ꢀ
60 1C. Finally, the sodium borohydride solution is slowly added
2
+
to the nickel (Ni ) solution keeping the bath temperature same
and stirred for one hour. This instantly results in the formation of
Ni within polypyrrole (PPY) matrix. The precipitate is washed in
distilled water for three times, filtered and dried at room
temperature in vacuum. The redox type of reaction mechanism
for the formation of Ni in polypyrrole matrix can be proposed in
the following way:
2
þ
2þ
Ni þðPPYÞcomplex-Ni ðPPYÞcomplex
2
þ
-
3
Ni ðPPYÞcomplex þNO þ2NaBH
4
þ6H O-NiðPPYÞcomplex
2
þ2BðOHÞ þ2NaNO
3
þ4H
2
3
The same procedure is adopted while preparing Ni–Ag and
Ni–Fe nanoclusters in polypyyrole matrix. In the latter cases,
initially separate solutions of AgNO
India) and Fe(NO (Purified Merck Limited, India) are prepared.
These solutions are then separately added to the stock solution of
Ni(NO to obtain mixed precursor solution of Ni–Ag and Ni–Fe,
3
(Purified Merck Limited,
(
core)–Ag (shell) as observed earlier is possibly caused by the
3 2
)
lower surface energy of Ag and large lattice mismatch between
the constituents leading to a loosely bonded silver shell covering
the nickel core [24]. On the other hand, intermixed type
of nanocluster is common to many bimetallic systems like
Ni–Fe [25,26]. Of course, the process of coordination of
atoms segregating each other to form core-shell, mixed or any
other configurations is complex and the surface kinetics is
largely influenced by the chemical nature of individual
3 2
)
respectively. These solutions are then reacted with sodium
borohydride under rapid stirring and are continuously stirred
for two hours keeping the bath temperature ꢀ60 1C. The
precipitates of Ni–Ag and Ni–Fe in polypyyrole obtained in this
way are finally washed in distilled water for three times, filtered
and dried at room temperature in vacuum.
Fig. 1. Typical TEM (a–c) and HRTEM (e–f) images obtained for Ni, Ni–Ag and Ni–Fe samples, respectively.

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36
L. Kabir et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 934–939
Fig. 2. HRTEM images obtained for an isolated (a) Ni–Ag and (b) Ni–Fe nanocluster.
constituents, composition, growth process, thermodynamical
parameters, etc.
The XRD pattern of synthesized Ni, Ni–Ag and Ni–Fe nanorods
is given in Fig. 3(a–c). For the sake of clarity, bulk XRD data of Ni is
also included in Fig. 3(d). The XRD of synthesized Ni nanoclusters
Ni-Fe
*
*
*
shows mainly two distinct peaks at
respectively. The peak centered at 2
difficult to index the characteristic peak within the limit of
2y ꢀ39.71 and 711,
y
ꢀ39.71 is broad, making it
resolution. However, in comparison with the bulk Ni data, the
Ni-Ag
Ni
peak in the range
characteristic Ni peaks (1 1 0) and (0 0 6) superposed with each
other. The other peak at 2
ꢀ711 is well resolved and also broad,
2y=37–421 can be identified as the
*
y
can be indexed to Ni (3 0 0). The broad nature of the peaks is
indicative of the nanocrystalline nature of the sample and also to
some extent due to the inherent strain present in such
synthesized nanoclusters. The addition of silver into nickel
results in significant changes in the XRD pattern as obtained for
synthesized Ni–Ag nanoclusters, in terms of position of the peaks,
relative orientations and intensities. The intense peaks Ni (1 1 0)
and Ni (3 0 0) observed for Ni nanoclusters are drastically
broadened and almost subdued for Ni–Ag nanoclusters. On the
Bulk
other hand, new peaks at 2
be indexed to (1 1 1) and (2 2 0) reflections of cubic silver present
in a mixed state with nickel. The only peak at 2
assigned to (1 1 1) reflections of Ni–Ag nanoalloy phase. Such a
formation of Ni–Ag alloy phase has been recently reported for
laser ablated and solution grown films [27,28]. In case of Ni–Fe, in
addition to the (1 1 0) and (3 0 0) reflections of Ni, some new
y
ꢀ44.51 and 76.51 appears which can
y
ꢀ51.71 can be
2
0
30
40
50
60
70
80
90
2
θ (degree)
Fig. 3. X-ray diffraction (XRD) spectra obtained for Ni, Ni–Ag and Ni–Fe samples.
n
The data has been compared with bulk obtained from JCPDS file. The asterisk ( )
indicates the corresponding alloy phase of bimetallic nanoclusters.
Bragg reflections appear at 2
y ꢀ24.91, 451 and 85.51, which can be
indexed to alloy phase of Ni–Fe. These peaks are broad and not
well resolved compared to Ni peaks. The nickel peaks observed in
both Ni–Ag and Ni–Fe is quite similar to that of Ni nanoclusters
apart from a slight shift in peak positions. Clearly, both the Ni–Ag
and Ni–Fe samples indicate a coexistence of isolated Ni as well as
nanoalloy phase due to interfacial mixing and segregation of the
bimetallic constituents. The composition analysis of the alloy
clusters are performed by energy dispersive X-ray analysis (EDAX)
and indicate the following composition of the elements: Ni–Ag:
Ni-7.2% and Ag-92.8%, Ni–Fe: Ni-45% and Fe-55%.
Figs. 4 and 5 present the first derivative of EPR signals [(dP/dH)
vs H] for Ni, Ni–Ag and Ni–Fe nanoclusters recorded at room
temperature (300 K) and liquid nitrogen temperature (77 K),
respectively. EPR measurements are carried out at 9.833 GHz
using a Bruker BioSpin GmbH spectrometer with a judicious
choice of modulation frequency (50–100 kHz) and modulation
amplitude (10–15 G), and without any appreciable spurious
signals. The EPR parameters calculated are given in Table 1. The
room temperature spectrum for Ni nanoclusters consists of two
lines: a weaker sharp line not properly resolved (indicated by the
arrow mark) superposed on a broader resonance line. We call the
broad signal at the lower resonance field as I-type and that of
the sharper one as II-type. The broad I-type signal of Ni
nanoclusters becomes narrower with reduction in temperature
to 77 K and shows a single, nearly symmetric line. The II-type line
gets vanished with lowering in temperature to 77 K. The g-value
obtained for I-type line is found to significantly increase from
1.9679 to 2.2449 as the temperature changes from 300 to 77 K. In
contrast to pure Ni nanoclusters, both the spectra for Ni–Ag and
Ni–Fe nanoclusters at 300 K consist of two distinct lines: a very
sharp line superimposed on a broader line with a significant
difference in their resonance field. For Ni–Ag nanoclusters, the
integral EPR intensity at 300 K of II-type line (g=1.9690) is very
low compared to that of I-type line (g=2.1902). With lowering in

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L. Kabir et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 934–939
937
temperature to 77 K, the II-type line for Ni–Ag is completely
smeared out and the spectrum shows only a single Lorentzian
inhomogeneously broadened and suffered a significant change
(
increase) in the resonance field compared to room temperature.
The g-value is appreciably reduced to 1.8956 and the integral
intensity becomes higher as the temperature reduces to 77 K. The
spectrum for Ni–Fe at 300 K evolves quite similar to Ni–Ag, but
there is a significant change in g-values and EPR intensities for
both the lines. In this case, the II-type line is quite sharp and
intense than what observed for Ni and Ni–Ag nanoclusters. The g-
value is observed to be ꢀ1.9673 for the broad I-type line and that
of ꢀ1.9431 for II-type line, respectively. Interestingly, as the
temperature decreased to 77 K, EPR spectrum retains both the
lines with marked changes in intensity, g-values and linewidth.
The integral intensity of I-type line is found to increase than the
corresponding room temperature EPR signal. Also, the I-type line
is largely broadened with significant decrease in resonance field
and increase in g-value at 77 K. The g-value is calculated to be
ꢀ
2.0372 for I-type line and ꢀ1.9557 for II-type line, respectively.
The origin of II-type line (Lorentzian in shape) can be ascribed
2
+
to isolated Ni
ion clusters or nanoalloy clusters dispersed in
2
+
polymer [29]. Such isolated region of Ni ions in the polymer
may also be found in our samples. In true sense to say, those
isolated ions are surrounded by largely diluted paramagnetic
silver or iron ions, presence of which making no significant
2
+
interactions with Ni ions to influence the EPR signals. Note that
at room temperature the intensity for the II-type line is very low
for pure Ni nanoclusters sample and subsequently increases with
the modified interface Ni/Ag and Ni/Fe. The sharp narrow II-type
resonance line occurs due to the superparamagnetic Ni clusters
for pure Ni sample or nanoalloy clusters in case of Ni–Ag and Ni–
Fe samples. The observed weak intensity of II-type line for pure Ni
sample indicates a very dilute concentration of isolated Ni ion
clusters. On the other hand, for Ni–Ag and Ni–Fe samples the
intensity of II-type line plausibly enhanced because of the
presence of superparamagnetic nanoalloy clusters. Superpara-
magnetic relaxation is further confirmed by the broadening of
linewidth with reduction in temperature [30]. Now the possibility
of Ni–Fe alloy formation is higher (miscible in bulk) than that of
Ni–Ag and quite reasonably making a significant difference in
their respective II-type line intensity. At room temperature, faster
spin relaxation of magnetization directions of superparamagnetic
clusters results in the narrow resonance line [11]. As the
temperature decreases, linewidth increases (without any appreci-
able change in the resonance field and g-value) due to the
reduction of thermal fluctuation rate of the magnetization
directions of the superparamagnetic clusters, usually called the
blocking effect. Such thermally blocked state results in complete
smearing out or weaker resonance line when cooled to lower
temperature as observed in our case. The blocking temperature of
course depends on the size or size distribution of the nanoclusters
and their interface. In our case, complete blocking of the
superparamagnetic state occurs for Ni and Ni–Ag nanoclusters
at 77 K and results in smearing out of the II-type line or too weak
to be observed, while for Ni–Fe nanoclusters, there are some
fraction of small Ni clusters still undergoing superparamagnetic
oscillations giving rise to weaker resonance line at 77 K.
Fig. 4. Room temperature (300 K) EPR spectra obtained for Ni, Ni–Ag and Ni–Fe
samples.
The broad I-type line observed for all the samples is not
superparamagnetic in origin. The origin of this line can be
attributed due to the interaction of Ni nanoclusters with the
surroundings [31]. For pure Ni sample, the interaction is between
magnetically non-isolated Ni clusters within polymer matrix. The
linewidth of I-type spectrum can arise out of the dipole–dipole
and exchange interaction between the interacting spins. The
dipole–dipole interactions superimposed on the Zeeman’s inter-
actions can give rise to the broadening of the EPR spectrum [32].
Fig. 5. EPR spectra obtained for Ni, Ni–Ag and Ni–Fe samples at liquid nitrogen
temperature (77 K).

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L. Kabir et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 934–939
Table 1
EPR parameters obtained for Ni, Ni–Ag and Ni–Fe samples at 300 and 77 K, respectively.
Sample
Temperature (K)
Resonance field (mT)
Linewidth
I-type line
D
H (mT)
g-value
A/B
I-type line
II-type line
II-type line
I-type line
II-type line
I-type line
Ni
300
337.8
296.1
303.5
350.5
337.9
326.3
338
-
ꢀ300
131
77
2.5
-
1.9679
2.2449
2.1902
1.8965
1.9673
2.0372
1.9667
-
1.05
1.07
1.3
77
Ni–Ag
Ni–Fe
300
337.6
-
1.5
-
1.9690
-
77
342
119
301
1.63
1.06
1.09
300
342.1
339.9
1.4
5.4
1.9431
1.9557
77
The magnitude of broadening of course depends on the concen-
tration of spins and the distance between them. On the other
hand, exchange type of interactions lead to the narrowing of
linewidth. The competing effect of the above two effectively
determines the linewidth. Due to large magnetocrystalline
anisotropy coupled with strong spin–spin interaction, the line-
width of the EPR spectra (I-type) significantly increases in case of
Ni–Ag and Ni–Fe with reduction in temperature. For pure Ni
sample, the exchange takes place between identical Ni spins
giving rise to the narrowing of the resonance signal when cooled
to lower temperature. As observed for pure Ni and Ni–Fe samples,
the effective g-value increases with decrease in temperature
indicating gradual strengthening of magnetic ordering. While for
Ni–Ag, g-value is found to decrease with lowering in temperature
to 77 K, clearly indicating a different resonance behavior. We also
comment on the effect of surroundings on the line shape of the
EPR spectra. This can be viewed by calculating the asymmetry
parameter A/B, where A and B are the low field and high field
amplitudes of the EPR signal, respectively (Table 1) [11]. In case of
Ni and Ni–Fe samples, A/B is found to be E1 within the error
limit and nearly temperature independent. To be noted that slight
error may always be encountered in the calculation of A/B due to
difficulties in obtaining the correct baseline of the broad EPR
signal due to the mismatch in low field and high field baselines.
However, Ni–Ag sample exhibits a characteristic Dysonian line
shape i.e. A/B41 at room temperature and becomes more
asymmetric with the decrease in temperature. This symmetric
Dysonian line shape results from the mixture of absorptive and
dispersive components of susceptibility and hence results in a
non-uniform distribution of the microwave fields for nanoclusters
of size larger than the skin depth [33,34]. Clearly, the presence of
paramagnetic silver in the surroundings of nickel than polymer
(Ni–Ag) or mixed pattern (Ni–Fe) in conformity with theoretical
calculations. Existence of such nanoalloy phase is further confirmed
by the EPR spectra. Influence of paramagnetic silver or ferromag-
netic iron on the spin resonance behavior of Ni is further analyzed
with the temperature dependent variation of EPR parameters.
Indeed, as our discussion is restricted to the data available for
temperatures 300 and 77 K only, a detailed analysis of the EPR
parameters recorded in a wide range of temperature may provide
more insight into the resonance behaviors of such important
magnetic nanoclusters and their interfacial alloying behavior.
2
+
Acknowledgement
The authors wish to acknowledge with thanks CSIR, India, for
financial support to this program.
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