Magnetic and electrical properties of oxygen stabilized nickel nanofibers prepared by the borohydride reduction method

Article abstract of DOI:10.1016/j.jmmm.2007.08.034

Fine nickel fibers have been synthesized by chemical reduction of nickel ions in aqueous medium with sodium borohydride. The thermal stability and relevant properties of these fibers, as-prepared as well as air-annealed, have been investigated by structural, magnetic and electrical measurements. As-prepared samples appear to have a novel crystal structure due to the presence of interstitial oxygen. Upon annealing in air, the fcc-Ni phase emerges out initially and develops into a nanocomposite subsequently by retaining its fiber-like structure in nano phase. The as-prepared sample is observed to be weakly magnetic at room temperature, but attains surprisingly high magnetization values at low temperatures. This is attributed to the modified spin structure, presumably due to the presence of interstitial oxygen in the lattice. Development of a weakly ferromagnetic and electrically conducting phase upon annealing in air is attributed to the formation of the fcc-Ni phase. The structural phase transformations corroborate well with magnetic and electrical measurements.

Full text of DOI:10.1016/j.jmmm.2007.08.034

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 320 (2008) 788–795
www.elsevier.com/locate/jmmm
Magnetic and electrical properties of oxygen stabilized nickel nanofibers
prepared by the borohydride reduction method
Ã
V. Srinivasa, , S.K. Barika, Bhaskarjyoti Bodoa,
Debjani Karmakarb, T.V. Chandrasekhar Raob
aDepartment of Physics and Meteorology, Indian Institute of Technology, Kharagpur West Bengal 721 302 India
bTechnical Physics and Prototype Engineering Division, Bhabha Atomic Research Centre, Bombay 400085 India
Received 15 June 2007; received in revised form 14 August 2007
Available online 2 October 2007
Abstract
Fine nickel fibers have been synthesized by chemical reduction of nickel ions in aqueous medium with sodium borohydride. The
thermal stability and relevant properties of these fibers, as-prepared as well as air-annealed, have been investigated by structural,
magnetic and electrical measurements. As-prepared samples appear to have a novel crystal structure due to the presence of interstitial
oxygen. Upon annealing in air, the fcc-Ni phase emerges out initially and develops into a nanocomposite subsequently by retaining its
fiber-like structure in nano phase. The as-prepared sample is observed to be weakly magnetic at room temperature, but attains
surprisingly high magnetization values at low temperatures. This is attributed to the modified spin structure, presumably due to the
presence of interstitial oxygen in the lattice. Development of a weakly ferromagnetic and electrically conducting phase upon annealing in
air is attributed to the formation of the fcc-Ni phase. The structural phase transformations corroborate well with magnetic and electrical
measurements.
r 2007 Elsevier B.V. All rights reserved.
Keywords: Nanostructures; Ni/NiO; Core shell structure; Magnetism; Insulating
1. Introduction
layer on ferromagnetic metal plays a significant role in
modifying the global magnetic properties of nanostruc-
tures. For example, NiO layer formed around Ni can result
in an antiferromagnetic (AFM)–ferromagnetic (FM) inter-
action, thereby reducing net magnetization. Besides,
coupling of FM particles with AFM matrix can be a
source of a large effective anisotropy, which leads to a
marked improvement in the thermal stability of the
moments of FM NPs [4,5]. Influence of surface and
interface anisotropy has also been invoked as the origin
of anomalous magnetic properties of NPs at low tempera-
tures. In fact, anisotropy also fluctuates at the particle
surface and can be larger than that of the core. For
instance, magnetoelastic coupling developed by surface-
core stresses and enhanced by surface magnetostriction can
also contribute to the surface anisotropy, which, through
exchange coupling, can even modify the magnetization of
the core [6]. Further, these FM–AFM interactions can be a
source of large exchange bias, leading to potential spin
Synthesis and characterization of discrete nanostructures
is of significant importance because of their fundamental
role in basic research and technological applications [1–3].
Among various ferromagnetic metals investigated, Ni
nanoparticles (NPs) have attracted much attention because
of their application as catalysts and galvanomagnetic
materials. Increased surface to volume ratio in NPs
enhances surface and interface effects, resulting in novel
phenomena. Besides, NPs could have a spontaneous
surface oxide (SSO) layer on metal particles, leading to
core (metal)–shell (metal oxide) composites.
Nanoparticles in core–shell morphology are of interest in
their own right and have been the subject of extensive
research in the recent past [1,4,5]. The passivating oxide
Ã
Corresponding author. Tel.: +91 3222 283848; fax: +91 3222 255303.
E-mail address: veeturi@phy.iitkgp.ernet.in (V. Srinivas).
0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2007.08.034

ARTICLE IN PRESS
V. Srinivas et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 788–795
789
valve applications. On the other hand, in several solar
absorbing coatings, presence of a dielectric shell around the
metallic core impedes oxidation and appears to be the main
reason for their high-temperature stability [7].
magnetic and electrical properties of these fibers. It is
observed that as-prepared fibers also stabilize in t-Ni
crystal structure. Structural data indicate the evolution of
metallic fcc-Ni phase on thermal treatment of fibers in air,
with t-Ni phase transforming into a nanocrystalline NiO
phase on annealing at 700 1C for 1 h. These observations
corroborate well with detailed studies of their electrical and
magnetic properties.
Synthesis of nanoscale metallic particles is rather difficult
under ambient atmosphere, as reactivity of NPs is
significantly higher due to reduced particle size. Among
various synthesis techniques deployed, the chemical reduc-
tion method is appropriate for producing NPs with
core–shell structures or in different shapes [8,9]. Although
there have been attempts to synthesize well-aligned
nanorods and fibers of various materials, few reports deal
with synthesis of nanofibers or nanostrands of ferromag-
netic metals [10,11]. These include preparation of magnetic
nanorods by sonication of aqueous iron (II) acetate in the
presence of b-cyclodextrin [8,11] and use of other chemical
and physical processes to synthesize Ni/NiO nanorods
[12,13]. Development of nanostructured Ni in new forms
like nanorods or nanostrands is challenging and important
from technological viewpoint. These materials will be
similar to carbon nanofibers and multiwall carbon
nanotubes, besides providing additional vistas based on
electromagnetic, catalytic and metallurgical properties of
Ni. Simple but proprietary chemistry is required to produce
nickel nanofibers for various electromagnetic engineering
applications.
2. Experimental details
Samples were prepared by reducing the nickel salt
NiCl2 ꢀ 6H2O with NaBH4. The reaction was carried out
in aqueous medium at ambient temperature with constant
magnetic stirring. Since this reaction is exothermic, NaBH4
solution was added drop-wise to 0.1 M NiCl2 ꢀ 6H2O
solution taken in a beaker over a period of 1 h in order
to maintain the reaction temperature close to 25 1C. This
ensued the formation of black slurries of Ni accompanied
by evolution of H2 gas. Subsequently, slurries were filtered
and thoroughly washed with distilled water to remove all
adhering ions, followed by a wash with acetone to remove
water. The sample so obtained is stable in ambient
atmosphere and was pulverized properly to obtain fine
powders for measurements.
The structure of as-prepared and annealed samples was
investigated using Philip’s X-ray diffractometer (Model
1710) with CuKa radiation (l ¼ 0.15418 nm). Microstruc-
ture was studied with a Philips CM-12 transmission
electron microscope (TEM). Thermal analysis of the
sample was carried out using a Perkin Elmer Instruments
thermogravimetric (TGA)/differential thermal analyzer
(DTA), in the temperature range 30–800 1C, with a heating
rate of 10 1C/min in flowing argon (100 ml/min). Ambient
temperature magnetic measurements were performed using
a home-made vibrating sample magnetometer (VSM),
while low-temperature magnetization data were recorded
using a SQUID magnetometer (Quantum Design, MPMS).
Temperature variation of resistance of sample pellets
(dimensions 10 mm diameter and 2 mm thick), compacted
under 5-ton pressure, was measured by a Keithley
electrometer.
Co-reduction of transition metals by reducing agent
sodium borohydride (NaBH4) yields metal or metal borides
[14–16]. However, borohydride reduction chemistry is quite
complex and reaction mechanism differs markedly between
aqueous and non-aqueous media. The end product is thus
very sensitive to reaction conditions [14]. Glavee et al.
reported the formation of both Ni and NiO when the
reaction was carried out in aqueous medium under ambient
conditions. Recently, Roy et al. have discussed the reaction
mechanism in ambient atmosphere and demonstrated the
effect of oxygen on the structure of the end product. As-
prepared powder was identified as nickel in a tetragonal
crystal structure (t-Ni), stabilized by the incorporation of O
atoms into the Ni lattice [16]. However, the weakly
magnetic t-Ni structure transforms into nanocrystalline
NiO upon annealing in air at 700 1C and exhibits super-
paramagnetic behavior at room temperature (RT). These
studies indicate that (i) it is imperative to anneal the
samples at temperatures as low as possible to restrain grain
growth and obtain the NPs of Ni or NiO; (ii) the oxidation
kinetics of nickel NPs in the temperature range 250–350 1C
can be described by a diffusion-controlled mechanism,
while there is a deviation from this mechanism at higher
temperatures due to particle sintering and agglomeration
effects. Therefore, low-temperature annealing studies on
these samples for prolonged periods can be useful to
understand oxygen’s passivation effects and to develop
core-shell structured nanomaterials.
3. Results and discussion
3.1. Structure and stability
The XRD pattern of the as-prepared powder shows a
crystalline structure with some degree of disorder (see
Fig. 1). The pattern corresponds neither to fcc-Ni, NiO,
nor to a or b hydroxides reported in literature [18]. In order
to ascertain whether or not nickel is the major constituent
of the sample, as-prepared powder was annealed under
hydrogen atmosphere at 700 1C in a separate experiment.
The XRD pattern of the hydrogen-annealed sample
(not shown in Fig. 1) shows strong fcc-Ni peaks [17]. The
fact that no NiO peaks were observed in the as-prepared
In this article, we report the formation of Ni nanofibers
by reduction of Ni2+ ions with NaBH4 in aqueous solution
in air. Further, we have investigated the structural,

ARTICLE IN PRESS
V. Srinivas et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 788–795
790
The sample annealed for 6 h shows XRD peaks developing
O
*
at 2y ¼ 44.51, 51.81 and 761, corresponding to the most
intense peaks of fcc-Ni with miller indices (1 1 1), (2 0 0) and
(2 2 0) respectively. Contrary to our expectation, these
peaks grow further in intensity on annealing in ambient
atmosphere up to 24 h, while those below 2yꢁ351,
representative of the t-Ni phase, start to diminish. For
samples annealed beyond 24 h at 300 1C, characteristic fcc-
Ni peaks disappear slowly, giving raise to new broad peaks
of XRD pattern. However, on close examination of XRD
patterns of 36 and 72 h annealed samples and the one
annealed at 700 1C (for 1 h in air), it can be speculated that
the samples annealed beyond 24 h contain NiO as major
component. The crystallite size obtained from Scherrer’s
formula is about 5 nm. However, XRD patterns of 700 1C
(1 h) and 300 1C (72 h) annealed samples have some subtle
differences and those will be discussed in the later part of
this article.
Fig. 2 shows TEM and selected area electron diffraction
(SAED) patterns of as-prepared and 72 h annealed
samples. These images show fiber-like structures with an
average width of about 10 nm and length of several
hundreds of nm. The high-resolution images of as-prepared
and annealed samples exhibit a fine structure with lattice
spacing around 5.4 A1. The SAED patterns correspond
clearly to a polycrystal, with some degree of disorder as
evidenced by rings instead of spots. This is in agreement
with XRD patterns discussed earlier. It is clear that the
samples are in the form of strands or fibers rather than NPs
unlike those reported in the earlier study [16]. We speculate
that anisotropic growth of NPs results in these strands of
nanoparticles, which are stable even after annealing
for 72 h at 300 1C. As reduction reaction was carried out
very slowly to prevent any increase in temperature, the
particles probably could arrange themselves in the form
of fibers.
The stability of as-prepared samples was investigated
through thermal analysis experiments under argon atmo-
sphere. The DTA/TGA data (see Fig. 3) indicate a weight
loss in the region 50–160 1C with broad endothermic peaks
in DTA plots. This loss could be due to the desorption of
surface and intercalated water. A second endothermic peak
appears at 390 1C, followed by a steep loss (ꢁ20%) in mass
around 425 1C. As XRD patterns of 425 1C annealed
samples show the emergence of fcc-Ni peaks, this mass loss
is possibly due to the desorption of dissolved oxygen from
the tetragonal lattice. Oxygen desorption leads to the
collapse of the tetragonal structure of Ni fibers and
subsequent crystallization of the fcc-phase. These experi-
ments suggest that the structural transformation mechan-
ism in Ni nanofibers is no different from that seen in NPs
earlier [16].
O
NiO
Ni
O
O
700C 1hr
O
O
300C 72hr
300C 36hr
300C 24hr
*
*
*
*
*
300C 6 hr
*
As Prepared
t-Ni
20
30
40
50
2Θ (Degree)
60
70
80
Fig. 1. X-ray diffractograms of 0.1 M samples: ‘as-prepared’, annealed in
air at 300 1C for different periods of time and 700 1C air annealed for 1 h.
sample and it transforms into pure Ni on annealing at
higher temperatures under inert atmosphere suggests that
no impurities other than loosely bound oxygen atoms were
present to begin with. Further, any non-stoichiometric and
Ni-defective oxides, if present, are unstable. The XRD
peaks of the as-prepared sample could be indexed to a
tetragonal t-Ni crystal structure with space group 14/mcm
[16]. Roy et al. suggested that the presence of dissolved
oxygen modifies the usual fcc-crystal structure of virgin Ni
particles, straining and making them tetragonal [16].
In order to further investigate the structure and stability
of as-prepared powder, low-temperature isothermal an-
nealing was carried out in air for prolonged periods
of time. Fig. 1 compares the X-ray diffractograms of
as-prepared Ni powder (dried at ambient temperature)
and those of samples annealed in air at 300 1C up to 72 h.
3.2. Magnetic properties
The magnetic state of ‘‘as-prepared’’ and annealed samples
was assessed through ambient temperature magnetization

ARTICLE IN PRESS
V. Srinivas et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 788–795
791
Fig. 2. (a) TEM micrograph of as-prepared fibers; (b) HRTEM image showing lattice spacing; (c) fibers air annealed at 300 1C for 72 h; (d) electron
diffractogram of (a).
100
95
90
85
80
and allows one to investigate the contribution of core and
shell [19–24].
Fig. 4(a) shows the ambient temperature M–H curves of
‘‘as-prepared’’ and air-annealed samples. The magnetic
parameters obtained from Fig. 4(a) are plotted in Fig. 4(b),
bringing out some interesting features: (i) as-prepared
sample exhibits very low magnetization values and a linear
response with applied field: (ii) all air-annealed samples
show a sharp increase in magnetization at low fields and a
hysteretic behavior; (iii) saturation magnetization (Ms) and
remnant magnetization (Mr) values are high for the 24 h
annealed sample, and decrease drastically on further
annealing; (iv) coercivity increases with annealing time.
As regards to as-prepared sample, low magnetization
values and linear response clearly suggest paramagnetic/
weak magnetic behavior, presumably due to altered crystal
structure. Upon annealing in air, the growth in ferromag-
netic component could be attributed to the evolution of
fcc-Ni, as evidenced by XRD patterns. While magnetiza-
tion values of annealed samples are relatively low, their
coercivity values are in agreement with those of Ni NPs
reported in literature [20]. Further, an increase in HC and
the decrease of magnetization values in samples annealed
beyond 24 h could be attributed to the increase in volume
fraction of the NiO phase. In the case of 72 h air-annealed
sample, hysteretic behavior at 300 K suggests the presence
of fcc-Ni core, while low magnetization values could be
attributed to the presence of NiO.
DTA
TGA
0
200
400
600
800
1000
Temperature (°C)
Fig. 3. DTA and TGA profiles of the 0.1 M ‘as-prepared’ sample obtained
with a heating rate of 10 K/min in argon atmosphere.
data. The magnetic properties of nickel are known
to be highly sensitive to oxygen contamination. Chemi-
sorption of oxygen onto nickel particles reduces
magnetization, which can be explained by the paramag-
netic state of nickel atoms in oxygen environment.
Moreover, magnetization is a volume-dependent property
It needs to be ascertained whether the magnetic phases
present in this system are in conventional ferromagnetic
state or single domain blocked state. The M versus T
curves were measured under zero field cooling (ZFC) and

ARTICLE IN PRESS
V. Srinivas et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 788–795
792
0
15
30
45
60
75
6
4
6
4
2
0
200
150
100
50
2
0
-2
-4
-6
0
-4000
-2000
0
2000
4000
0
15
30
45
60
75
H (Oe)
Ann. Time (Hrs)
0.06
0.04
0.02
0.00
0
50 100 150 200 250 300
T (K)
Fig. 4. (a) Magnetization (M) as a function of applied field (H) at ambient temperature for 0.1 M samples: ‘as-prepared’ (r) and annealed at 300 1C for 24
(Ã), 48(o) and 72 (D) h. Inset: An expanded view of the plots, clearly showing M vs. H curves for ‘as-prepared’ and ‘72 h air-annealed’ samples. (b)
Variation of MS, MR and HC (at ambient temperature) as a function of annealing time at 300 1C. (c) FC and ZFC magnetization of the ‘72 h air-annealed’
sample measured as a function of temperature, under 100 Oe applied field. Inset: ‘as-prepared’ sample.
field cooling (FC) conditions, for an applied field of
H ¼ 100 Oe, from 5 K to 300 K (Fig. 4c) for the 72 h air-
annealed sample. The observed irreversibility in FC and
ZFC curves indicates the metastable nature of the sample
magnetization, which can be due to the blocked state of Ni
core. A broad maximum observed in the ZFC curve
around 150 K (not seen in the ‘as-prepared’ sample) may be
attributed to the presence of NiO shell. The sharp rise
observed in the ZFC curve near 18 K is typical of t-Ni
phase, more pronounced in the ‘as-prepared’ sample
(see inset). These measurements indicate that the
72 h-annealed sample has a fraction of t-Ni phase along
with Ni core and NiO shell.
Feeble magnetization values have also been reported in
hcp Nickel films and fine particles at ambient temperature
[24,25]. Such low magnetization values and irreversibility in
ZFC, FC magnetization curves are signatures of a different
magnetic state arising out of a modified crystal structure
(atomic positions) and elongated bond distances in
comparison to fcc-Ni [25]. In order to obtain better insight
into this magnetic state, M vs. H curves of ‘as-prepared’,
‘72 h annealed (at 300 1C)’ and ‘1 h annealed (at 700 1C)’
are measured at ambient (Fig. 5(a)) and at 5 K (Fig. 5(b)).
It is clear from Fig. 5(a) that the 72 h-annealed sample
shows larger magnetization values than the as-prepared
sample, while the 700 1C annealed sample also shows
larger but non-hysteritic magnetization. The ambient
temperature M–H curve of this sample fits well to the
Langevin function
ꢀ
ꢁ
1
MðaÞ ¼ MS cothðaÞ ꢂ þ wH,
(1)
a
in which MS is the saturation magnetization, a ¼ mH/KBT,
m is the magnetic moment of a particle, KB is the
Boltzmann constant and w is the susceptibility, yielding
an average particle moment of ꢁ1322 mB and a saturation
magnetization of 1.6 emu/g. The reasonably good Langevin
fit clearly suggests that the NiO particles are in super-
paramagnetic state at this temperature. At 5 K all three
samples show hysteresis loops suggesting the existence of a
magnetically ordered phase. The 700 1C annealed samples
exhibits a coercivity (HC) of ꢁ4 kOe with non-saturation of
magnetization even at fields upto 60 kOe. A similar
behavior has been reported in NiO NPs and attributed to
broken bonds on the surface of NiO nanofibers. Non-
collinear magnetic structures formed due to the combined
effects of missing bonds and exchange interactions [20,23]
can also give rise to a non-saturated magnetization

ARTICLE IN PRESS
V. Srinivas et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 788–795
793
Fig. 5. (a) Comparison of M-H plots at 300 K for as-prepared, 300 1C and 700 1C annealed samples. (b) M–H plots at 5 K, showing hysteresis for all
samples. The samples are in a ‘blocked’ state at this temperature.
Fig. 6. (a) Electrical resistivity as a function of temperature (warm-up and cooling-down curves) of a virgin sample; (b) variation of resistivity (at ambient
temperature) as a function of annealing time at 300 1C. Inset: activation behavior of resistivity.
behavior with high HC. Surprisingly, as-prepared and
300 1C air-annealed samples show inordinately high
magnetization values (470 emu/g) at 5 K in comparison
to ambient temperature values and that of bulk Ni
(ꢁ55 emu/g). This suggests that the major contribution to
magnetization comes from the t-Ni phase observed in
XRD patterns of the as-prepared sample. Similar magnetic
characteristics observed in air-annealed sample suggest that
the t-Ni phase is stable at 300 1C, even though this phase
could not be identified in XRD patterns. At present we
attribute this anomalous behavior to the non-collinear spin
structure in the t-Ni phase. As this phase completely
transforms to fcc-Ni or NiO, magnetization values drop to
the values close to bulk Ni and NiO, respectively.
3.3,. Electrical transport
Temperature variation of electrical resistivity of as-
prepared compacted powder was studied in order to
corroborate the results obtained from structural and
magnetic measurements. As shown in Fig. 6(a), the
resistivity response was recorded during both heating and
cooling cycles. As the temperature is increased from
ambient to 100 1C, the resistivity decreases initially and
then starts to increase rapidly at 150 1C, going through a
maximum at 250 1C before attaining a lower value at
320 1C. These results are consistent with thermal analysis
data where the initial decrease can be attributed to
departure of surface and intercalated water. The increase

ARTICLE IN PRESS
V. Srinivas et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 788–795
794
in resistivity is due to disorption of oxygen that reacts with
the outer surface and facilitates the growth of surface NiO
while the core becomes progressively richer in metallic Ni,
thus decreasing resistivity. On further heating the expo-
nential decrease in resistivity is due to the formation of
the stable insulation layer (NiO), surrounding the metal
(Ni) layer. Irreversible behavior and an exponential
increase in resistivity during cooling run clearly suggest
that the ‘as-prepared’ sample is in a structurally metastable
state, consistent with the structural and magnetic measure-
ments discussed in previous sections. These experiments
also indicate that on heating in air the as-prepared
metastable material transforms into a stable, weakly
ferromagnetic and insulating state. Also the irreversible
behavior of the as-prepared sample from the cooling graph
agrees with structural analysis. As shown in Fig. 6(b), the
resistivity of air-annealed samples, plotted as a function of
annealing time, shows a sharp decrease in resistance with
annealing time up to 36 h due to the contribution from the
metallic phase (fcc-Ni) and then increases due to the
development of NiO layer on the surface of the samples.
Electrical resistivity as a function of temperature shows a
typical activated behavior (shown in the inset). These
results suggest that oxygen plays a significant role in
modifying the magnetic and electrical properties of the
as-prepared powders.
Structural, magnetic and electrical resistivity measure-
ments clearly indicate that the as-prepared sample does not
correspond to fcc-Ni and appears to be stabilized by
oxygen dissolved in Ni lattice. Subsequent, isothermal
annealing for a certain period of time in air results in the
emergence of a ferromagnetic metal of a like (fcc-Ni) phase
at the cost of t-Ni. But on further annealing a mixed phase
results, exhibiting the features of both t-Ni and fcc-Ni or
fcc-NiO phases. Electrical resistivity and thermal analysis
as a function of temperature support this observation.
The dissolved oxygen atoms play a pivotal role in
drastically reducing the magnetization of the ‘as-prepared’
sample, by inducing some degree of frustration
among Ni moments. On annealing in ambient atmosphere,
interstitial oxygen atoms move out of the t-Ni cell
outwardly due to the thermal agitations and result in
regaining normal fcc structure. It is also possible that
these oxygen atoms may react with the outer surface
of the particle and form a NiO shell. In the second stage,
oxygen from air starts reacting with Ni and shell
becomes thicker and eventually the particle becomes
NiO. Although the XRD pattern of the 72 h annealed
sample shows no indication of t-Ni phase, magnetization
data (ZFC/FC and M-H curves) provide conclusive
evidence of its presence. Further, our study shows that
the t-Ni does not directly transform to fcc-NiO. The
transformation takes place in two steps: first the t-Ni phase
develops into fcc-Ni by disorption of oxygen and then
reacts with available oxygen to transform into NiO.
However, from this study it appears that annealing at
300 1C for 72 h in ambient atmosphere leads to a
nanocomposite comprising of t-Ni and NiO, as reflected
clearly in magnetic and electrical measurements.
4. Conclusion
We have synthesized Ni-nanofibers with a diameter of
about 12 nm through a chemical reduction method. As-
prepared samples are essentially Ni, albeit in a modified
structure, due to the presence of interstitial oxygen. This
structure shows abnormally high magnetization values at
low temperature compared to fcc-Ni. When the samples are
annealed up to 24 h at 300 1C in air, fcc-Ni begins to
emerge. On further annealing beyond 24 h a NiO layer
develops around Ni. Magnetization and electrical resistiv-
ity measurements on as-prepared and annealed samples
suggest that a conducting magnetic phase of Ni evolves on
annealing in air for 24 h, but beyond this the insulating and
weakly magnetic NiO phase is formed. These results
corroborate well the structural data and are explained
with a core–shell model.
Acknowledgements
The authors wish to thank Prof. S. Ram of Materials
Science Center, IIT, Kharagpur, for his assistance in
synthesis of samples. Partial financial support from the
Board of Research in Nuclear Sciences (DAE), India is
acknowledged.
References
´
[1] J. Martin, J. Nogues, K. Kai Liu, J.L. Vicent, Ivan. K. Schuller,
J. Magn.Magn. Mater. 256 (2003) 449.
[2] J. Nogues, J. Sort, V. Langlais, V. Skumryev, S. Surinach,
´
J.S. Munoz, M.D. Baro, Phys. Rep. 422 (2005) 65.
[3] L. Lu, M.L. Sui, K. Lu, Science 294 (2000) 1901.
[4] V. Skumryev, S. Stoyanov, Y. Zhang, G. Hadjipanayis, D. Givord,
J. Nogues, Nature 423 (2003) 850.
[5] S. Doppiu, V. Langlais, J. Sort, S. Surinach, M.D. Baro, Y. Zhang,
G. Hadjipanayis, J. Nogues, Chem. Mater. 16 (2004) 5664.
[6] S.W. Sun, R.C. Ohandley, Phys. Rev. Lett. 66 (1991) 2798.
[7] J.R. Cole, N.J. Halas, Appl. Phys. Lett. 89 (2006) 153120.
[8] X. Lue, U. Meridor, P. Zhao, G. Song, A. Frydman, A. Gedanken,
J. Magn. Magn. Mater. 301 (2006) 13;
Y. Zhang, Z. Huang, F. Tang, J. Ren, Solid State Commun. 138
(2006) 132;
S.Y. Lian, Z.H. Kay, E.B. Wang, M. Jiang, C.W. Hu, L. Xu, Solid
State Commun. 129 (2004) 485.
[9] J. Park, et al., Adv. Mater. 17 (2005) 429;
Y.T. Jeon, et al., J. Phys. Chem. B 110 (2006) 1187;
B.B. Nayak, S. Vitta, A.K. Nigam, D. Bahadur, Thin Solid Films 505
(2006) 109;
D.E. Zhang, X.M. Ni, H.G. Zheng, Y. Li, X.J. Zhang, Z.P. Yang,
Mater. Lett. 59 (2005) 2011.
[10] N. Cordente, C. Amiens, F. Senocq, M. Respaud, M.-J. Casanove,
B. Chaudret, Nano letters 1 (2001) 565.
[11] R. Vijaya Kumar, Yu. Kotypin, X.N. Xu, Y. Yeshurun,
A. Gedanken, I. Felner, J. Appl. Phys. 89 (2001) 6324.
[12] M. Fraune, U. Rudiger, G. Guntherodt, S. Cardoso, P. Freitas, Appl.
Phys. Lett. 77 (2000) 3815.
[13] N. Cordente, C. Amiens, B. Chaudret, M. Respaud, F. Senocq,
M.-J. Casanove, J. Appl. Phys. 94 (2003) 6358.

ARTICLE IN PRESS
V. Srinivas et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 788–795
795
[14] G.N. Glavee, K.J. Klabunde, C.M. Sorensen, G.C. Hadjipanayis,
Langmuir 10 (1994) 4726.
[21] Y.D. Yao, Y.Y. Chen, M.F. Tai, D.H. Wang, H.M. Lin, Mater. Sci.
Eng. A217–218 (1996) 281.
[15] J. Legrand, A. Taleb, S. Gota, M.J. Guittet, C. Petit, Langmuir 18
(2002) 4131.
[22] W. Suwanwatana, S. Yarlagadda, J.W. Gillespie Jr., J. Mater. Sci. 38
(2003) 565.
[16] A. Roy, V. Srinivas, S. Ram, J.A. De Toro, U. Mizutani, Phys. Rev.
B 71 (2005) 184443.
[23] R.H. Kodama, S.A. Makhlouf, A.E. Berkowitz, Phys. Rev. Lett. 79
(1997) 1393;
[17] S. Vitta, J. Appl. Phys. 101 (2007) 6782.
[18] M. Rajamathi, G.N. Subbanna, P. Vishnu Kamath, J. Mater. Chem.
7 (1997) 2293.
S.A. Makhlouf, F.T. Parker, F.E. Spada, A.E. Berkowitz, J. Appl.
Phys. 81 (1997) 5561.
[24] S. Sako, K. Ohshima, M. Sakai, S. Bandow, J. Vac. Sci. Technol. B
15 (1997) 1338.
[19] H. Kisker, T. Gessmann, R. Wurschum, H. Kronmuller, H.-E. Schaefer,
Nanostruct. Mater. 6 (1995) 925.
[25] Y.T. Jeon, J.Y. Moon, G.H. Lee, J. Park, Y. Chang, J. Phys. Chem.
B 110 (2006) 1187;
J. Vergara, V. Madurga, J. Mater. Res. 17 (2002) 2099.
[20] H. Bi, S. Li, Y. Zhang, Y. Du, J. Magn. Magn. Mater. 277 (2004)
363.