Synthesis and magnetic characterization of Ni nanoparticles and Ni nanoparticles in multiwalled carbon nanotubes

Article abstract of DOI:10.1016/j.jallcom.2005.10.012

Nickel nanoparticles were synthesized by reduction of nickel acetylacetonate in a monosurfactant system. These nanoparticles, mostly amorphous, were used as catalyst for the growth of multiwalled carbon nanotubes by the catalytic decomposition of methane at 500 °C. TEM analysis reveals a wide size distribution of the diameter of the particles centred around two main values. A detailed characterization of the magnetic properties of the Ni nanoparticles and Ni nanoparticles carbon nanotubes embedded is hereby presented. Both the systems show superparamagnetic behaviour above the blocking temperature T_B. Magnetization data are well fitted by an equation formed by two weighted Langevin functions and display the correct scaling of M/M_S versus H/T for superparamagnetic nanoparticles. The hysteresis loops obtained below T_B agree with the ferromagnetism of single-domain particles, confirmed also by the expected temperature dependence of the coercive field H_C. Differences in the coercive fields at increasing and decreasing applied magnetic fields are to be ascribed to a NiO layer originating an exchange bias with inner Ni. The smallness of the coercive fields difference (around 5 Oe) confirms that the used preparation method produces a very thin NiO layer around Ni nanoparticles.

Full text of DOI:10.1016/j.jallcom.2005.10.012

Journal of Alloys and Compounds 419 (2006) 32–39
Synthesis and magnetic characterization of Ni nanoparticles and Ni
nanoparticles in multiwalled carbon nanotubes
a,∗
a
b
c
Daniele Gozzi , Alessandro Latini , Gustavo Capannelli , Fabio Canepa ,
c
d
e
Myrta Napoletano , Maria Roberta Cimberle , Matteo Tropeano
a
Dipartimento di Chimica, Universit a` di Roma La Sapienza, Piazzale Aldo Moro 5, 00185 Roma, Italy
INSTM and Dipartimento di Chimica e Chimica Industriale, Via Dodecaneso 31, 16146 Genova, Italy
b
c
INFM-LAMIA and Dipartimento di Chimica e Chimica Industriale, Via Dodecaneso 31, 16146 Genova, Italy
d
CNR-IMEM Dipartimento di Fisica, Via Dodecaneso 33, 16146 Genova, Italy
e
INFM-LAMIA, C. so Perrone, Genova, Italy
Received 15 September 2005; received in revised form 10 October 2005; accepted 14 October 2005
Available online 21 November 2005
Abstract
Nickelnanoparticlesweresynthesizedbyreductionofnickelacetylacetonateinamonosurfactantsystem. Thesenanoparticles, mostlyamorphous,
◦
were used as catalyst for the growth of multiwalled carbon nanotubes by the catalytic decomposition of methane at 500 C. TEM analysis reveals
a wide size distribution of the diameter of the particles centred around two main values. A detailed characterization of the magnetic properties of
the Ni nanoparticles and Ni nanoparticles carbon nanotubes embedded is hereby presented. Both the systems show superparamagnetic behaviour
above the blocking temperature T
correct scaling of M/M versus H/T for superparamagnetic nanoparticles. The hysteresis loops obtained below T
of single-domain particles, confirmed also by the expected temperature dependence of the coercive field H . Differences in the coercive fields at
B
. Magnetization data are well fitted by an equation formed by two weighted Langevin functions and display the
S
B
agree with the ferromagnetism
C
increasing and decreasing applied magnetic fields are to be ascribed to a NiO layer originating an exchange bias with inner Ni. The smallness of
the coercive fields difference (around 5 Oe) confirms that the used preparation method produces a very thin NiO layer around Ni nanoparticles.
©
2005 Elsevier B.V. All rights reserved.
PACS: 61.46.+w; 75.20.−g; 75.50.Tt
Keywords: Carbon nanotubes; Magnetic properties; Catalytically grown carbon
1
. Introduction
systems and silent rooms to wireless communications has been
increased.
The synthesis and chemico–physical characterization of
Different techniques have been adopted to obtain ferromag-
netic elements with nanometric dimensions: arc discharge [5],
evaporation [6], sputtering [7] and sonochemical method [8].
However, the high surface to mass ratio has the consequence
to increase enormously the reactivity of the nanoparticles that
could prevent their technological use. Therefore, the coating of
these magnetic nanoparticles by protective layers or the syn-
thesis of magnetic particles in carbon capsules has a particular
interest since it can prevent any kind of reaction, particularly the
oxidation, provided the temperature is kept close to the room
temperature.
elements as Fe, Co or Ni in nanometric scale is nowadays
an important topic in materials science not only for their
peculiar properties related to the reduced size but also for their
relevance from a technological point of view. Possible potential
applications of the nanostructured materials as catalysts (high
value of the surface area), high-density magnetic recording
media and medical diagnostics have been recently emphasized
1–3]. More recently [4], the use of composites with nano-
sized ferromagnetic particles as absorbers of electromagnetic
waves for technological applications starting from radar
[
Finally, magnetic nanoparticles can be used as origin for the
growth of carbon nanotubes. This is a new form of carbon config-
urationally equivalent to two-dimensional graphene sheet rolled
into a tube and can offer amazing possibilities to create future
∗
Corresponding author. Tel.: +39 06 4991 3849; fax: +39 06 4991 3849.
E-mail address: Daniele.Gozzi@uniroma1.it (D. Gozzi).
0
925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2005.10.012

D. Gozzi et al. / Journal of Alloys and Compounds 419 (2006) 32–39
33
nanoelectronics devices, circuits, and computers through further
circuitry integration.
of the complete substitution of Ar with CH4 also amu = 40 was added to the
previous set.
After about 16.9 h from the Ar/CH4 substitution, the reactor was cooled
We prepared either Ni nanoparticles, mostly in the amor-
phous state, and carbon nanotubes embedding Ni nanoparticles
using the Ni nanoparticles as “seeds” for the growth of carbon
nanotubes (randomly oriented). We present, here, the informa-
tion obtained on these two systems by transmission electron
microscopy (TEM) and magnetic measurements as well as some
experimental evidences on the multiwalled carbon nanotubes,
MWCNT, growth.
◦
down to room temperature at 300 C/h keeping still the same CH4 flow-rate. The
final mass gain, ꢀm, entirely attributable to the formation of MWCNTs, was
0.06347 g. The solid reaction product was recovered from the reactor, washed
thoroughly with NaOH ∼2 M to remove the silica support, then with deionised
water and acetone and finally dried under vacuum. The combustion analysis of
MWCNT recovered gave: C = 95.4 ± 0.3%, H = 0.2 ± 0.1% and Ni up to 100,
i.e., cNi = 4.4 ± 0.3%.
The structural investigation was carried out by means of transmission elec-
tron microscope (TEM) model JEOL JEM 2010 FEEM 200 KV from JEOL Ltd.
The samples for TEM analysis were firstly finely dispersed in isopropanol and,
then, exposed to ultrasonic vibrations to decrease the aggregation. The diam-
2
. Experimental
ꢀ
eter distribution has been evaluated over more than 1000 particles. X Pert Pro
diffractometer by Philips, working at Cu K␣1 wavelength, was utilized for XRD
analyses.
Ni nanoparticles were synthesized according to the method developed by
Hou and Gao [9], i.e., by reducing Ni acetylacetonate by NaBH4 in a mono-
surfactant system. The combustion and ICP (Ion Coupled Plasma) analyses
of the product after centrifugation and vacuum drying gave the composition:
C = 5.8 ± 0.3; H = 1.0 ± 0.1; N = 0.4 ± 0.1; Ni = 4.6 ± 0.1 and inorganic inert
residuals up to 100. All the figures are given by wt.%. The catalyst for the
growth of MWCNTs was prepared through a dispersion of the Ni nanoparticles
Magnetic measurements were performed using a Quantum Design SQUID
MPMS magnetometer in the 5–350 K temperature range and in applied µ0H
magnetic fields between ± 5.5 Tesla (T). Additional magnetization measure-
2
000
ments up to 9 T were obtained by a Maglab
from Oxford Instruments Inc.
equipment (extraction method)
2
−1
over high surface area (960 ± 1 m g ) MCM-41 mesoporous silica. To do this
0
.18371 g of the synthesized material, i.e., 0.0084 ± 0.0002 g of Ni nanopar-
3. Results
ticles were suspended in 30 ml of n-hexane and 0.55098 g of MCM-41 silica
were soaked in the suspension under stirring at room temperature. The stir-
ring was continued until the surfactant liquid became transparent. Then, the
3.1. Kinetics of MWCNT growth
◦
temperature was gently raised up to about 70 C and kept constant until to the
solvent full evaporation. The solid catalyst was recovered and dried under vac-
uum. Thus, the Ni nanoparticle concentration, ccat, in the catalyst dispersion is
The ionic intensities of the amu set acquired by the QMS is
reported in Fig. 2. On the left scale are reported the amu of CH4
1
.5 ± 0.4 wt.%.
(amu = 16) and its fragments as well as amu = 40, i.e., Ar. The
The quantity, q, of 0.15548 g of Ni nanoparticle dispersion in MCM-41 silica
was placed in a fixed bed catalytic micro reactor (Fig. 1). The catalyst was heated
ionic intensity of H2 is reported on the right scale. The origin of
the time scale is fixed at the time when the valve of Ar becomes
off and the valve of CH4 is opened (see Fig. 1). At the time
◦
◦
◦
up to 500 C at 300 C/h in an Ar stream. When at 500 C, Ar was substituted
3
−1
by CH4 at 5 cm min at STP. The kinetics of reaction:
◦
origin the micro reactor is already at 500 C. The time at which
CH4(g) → C(nano) + 2H2(g)
(1)
the catalyst sees CH4 without Ar is the time corresponding to
was monitored by a Balzers Quadstar© quadrupole mass spectrometer, QMS,
operating in multiple ion detection, MID, mode. The admission of the reacted gas
mixture into the QMS chamber was realized by a system (see Fig. 1) constituted
by a stainless steel capillary (1 m long × 150 ␮m i.d.), a rough vacuum pumping
*
the disappearing of Ar signal. This occurs at time t . When the
system is cooled down at room temperature, the rate of reaction
(1) goes rapidly to zero and, accordingly, the ionic intensities
and a dosimeter valve (Balzers UDV 040). The capillary was maintained at
related to methane and hydrogen change with opposite trends,
◦
1
50 C and at fixed pressure gradient (1 bar rel. at the inlet and 1 mbar at the
as expected. The final ionic intensity of H cannot be zero it
2
outlet). Methane utilized was 97.5% with 2.5% of H2.
being the main impurity of CH4 used.
Because from preliminary experiments it was confirmed that only reaction
(
1) takes place, only amu corresponding to CH4 (and its fragments CH3, CH2,
CH, C) and H2 were set for the MID detection. In order to establish, the time
*
Fig. 2. Ionic intensities from QMS during the MWCNT growth. Time t is the
Fig. 1. Schematic view of the experimental assembly for MWCNT growth.
time at which Ar is completely replaced by CH4.

3
4
D. Gozzi et al. / Journal of Alloys and Compounds 419 (2006) 32–39
Fig. 3. TEM of Ni nanoparticle after preparation.
3
.2. XRD and TEM
In Fig. 3, a typical HREM image of Ni nanoparticles is
Fig. 5. (A) TEM micrograph of MWCNTs with Ni nanoparticle embedded. A
tip shape nanoparticle attached to a MWCNT is shown in the inset. (B) XRD
spectrum of Ni-MWCNTs at Cu K␣1 radiation. Lines of a reference graphite
syn and fcc Ni are from PCPDFWIN version 2.2, June 2001 data base (ICPDS-
ICDD), respectively, #87-0712 and #75-1621.
reported, while in Fig. 4 the size distribution histogram of the
Ni nanoparticles is presented. This figure displays a wide distri-
bution of particle diameters centered around two main values:
about 70% of the particles has a diameter of about 5 nm, while
only about 25–30% of the particles present a diameter between
a double Gaussian function, reported as dot line in the same
Fig. 4: we obtain two mean diameters of the particles, namely
d1 = 4.6 nm and d2 = 15.4 nm.
1
0 and 20 nm. This bimodal distribution has been fitted with
The XRD spectrum of the Ni nanoparticles displays no peaks
originating from the face-centred cubic (fcc) Ni structures show-
ing that a great part of Ni is in the amorphous state. Only peaks
due to the inert inorganic residuals coming from the synthesis
appear.
Ni nanoparticles embedded in MWCNTs are shown in
Fig. 5A. Each MWCNT carries at one of its ends a Ni nanopar-
ticle. The analysis of the distribution of Ni nanoparticles in
MWCNTs, not reported here, gives the same bimodal result.
Differently from the Ni nanoparticles, the XRD spectrum of Ni-
MWCNTs, reported in Fig. 5B, shows clearly the fcc features of
the Ni nanoparticles linked to MWCNTs, as well as the typical
finger print of a hexagonal graphite structure. The comparison
with one of the closest spectra reported in the ICPDS data base
gives the indication that the graphene planes in MWCNTs, d0 0 2
spacing, are separated 75 pm more than graphite assumed as ref-
erence. The presence of Ni features that appear as weak signals,
because Ni concentration is low, but in the expected intensity
Fig. 4. Size distribution histogram of Ni nanoparticles as obtained from the
TEM analysis.

D. Gozzi et al. / Journal of Alloys and Compounds 419 (2006) 32–39
35
Fig. 6. Temperature dependence of the magnetization of: (a) Ni nanoparticles and (b) Ni nanoparticles in MWCNT obtained at 5 and 50 mT.
pattern, confirms that the transition amorphous/crystalline at
the temperature of the CH4 decomposition has been promoted
during the heating of the Ni nanoparticles. The absence of no
preferential orientation suggests that transition leads only to
polycrystalline nanoparticles.
3
.3. Magnetic measurements
Fig. 6 shows the temperature dependence of the magnetiza-
tion between 5 and 300 K of the Ni nanoparticles (a) and Ni
nanoparticles in carbon nanotubes (b), respectively. The data
were taken in zero-field-cooling (ZFC) and field-cooling (FC)
conditions at two different applied magnetic fields, i.e., 5 and
5
0 mT. ZFC and FC curves show a bifurcation that decreases at
increasing magnetic fields. Furthermore, the ZFC curves show
a large decrease of the magnetization: this decrease, quite evi-
dent in Ni nanoparticles, is less strong in nano Ni in MWCNT
(
note in Fig. 6, the behaviour of ZFC curve at 50 mT where the
magnetization is nearly constant below the bifurcation point).
This last behaviour may be due to weak magnetic interactions
in the system as we will discuss in the following. We observe
also that the magnetization values for Ni nanoparticles are lower
with respect to the values of nano Ni in MWCNT in the same
field and temperature conditions.
In the low temperature regime, the Ni nanoparticles and Ni
nanoparticles in MWCNT show hysteretic behaviour like ferro-
magnetic systems: the hysteresis loops at different temperatures
have been performed after a ZFC procedure and with a thin field
span (≤1 mT) up to 0.011 T. They are presented in Fig. 7a and b,
respectively. For Ni nanoparticles, we observe hysteresis loops
decreasing at increasing temperature with relatively low values
of the magnetization (4 emu/gNi at 0.2 T and 5 K). Once closed,
the magnetization doesn’t saturate, but increases continuously
with a slope, which depends on the applied temperature. On the
contrary, Ni nanoparticles in MWCNT display larger hysteresis
loops saturating at relatively high values (12 emu/gNi at 0.4 T
Fig. 7. Magnetization loops of: (a) Ni nanoparticles and (b) Ni nanoparticles in
MWCNT at selected temperatures.

3
6
D. Gozzi et al. / Journal of Alloys and Compounds 419 (2006) 32–39
amount of CH4 converted in MWCNT. Therefore, some facts
should be taken into account: (i) constant response uncertainty
of QMS on long times especially when considering low ionic
intensity values like that in the case of H2; (ii) H2 is already
*
present in CH4; (iii) variable composition occurring before t .
In the previous reasoning, we assumed that all carbon produced
grows as MWCNTs. This agrees well with TEM analysis. There-
fore, to evaluate quantitatively the conversion, η, of methane in
MWCNT, equation below has been used:
ꢀ
ꢂ
iH /2fH
2
2
η =
ꢁ
ꢀ
× 100
4
0
iCH /fCH + iH /2fH
i
i
2
2
T
ꢂ
iH /2fH
2
2
−
ꢁ
× 100
(2)
4
0
iCH /fCH + iH /2fH
i
i
2
2
T
room
where i and f stand, respectively, for ionic intensity and calibra-
tion factor. The second term on the right side of Eq. (2) represents
the apparent conversion at room temperature due to the H2 con-
tent in the feeding methane. The summation is extended to all
the ionic species of CH4 measured by QMS. Data calculated
through Eq. (2) are plotted in Fig. 9. By comparing Fig. 9 with
*
Fig. 2, it appears clear that there is a time interval, after t , and
lasting until t0, during which the slope of iH decreases. This
2
could be attributed to some consumption of H2 by the catalyst.
At this stage, it is difficult to establish the real process or pro-
cesses occurring. It is reasonable to associate this phenomenon
to both the reduction of some NiO, coating the Ni nanoparticles,
and chemisorption and/or absorption of H2 by the nanoparticles
themselves.
The ratio, γ, between the weight, w, of Ni consumed by the
embedding in MWCNTs and quantity, w0, of Ni in the catalyst
dispersion q is given by:
w
c ꢀm
Ni
γ =
=
≤ 1
(3)
Fig. 8. Normalized magnetization (M/MS) as a function of H/T for Ni nanopar-
ticles (a) at T = 270, 300 K and for Ni nanoparticles in MWCNT (b) at T = 250,
w0
ccatq
In the present experiment, γ = 1.2 ± 0.5. The large value of the
error comes from the calculation of the absolute maximum error
using the respective errors on cNi and ccat. In practice, γ can be
considered close to 1. This leads at least two considerations: (a)
3
00 K. For both plots, the solid line is the fit with a weighted sum of two Langevin
functions, as described in the text.
and 5 K) and showing no temperature dependence. Also, the
remanent magnetization (MR) and the coercive field (HC) are
larger for nano Ni in MWCNT with respect to Ni nanoparticles
in the same temperature conditions.
High temperature magnetization data normalized to the sat-
uration magnetization MS, are reported as a function of the H/T
ratio together with the Langevin fit of the data in Fig. 8a and b,
respectively.
4
. Discussion
4
.1. MWCNT growth
Assuming that the observed experimental curves reported in
Fig. 2 are only due to reaction (1), the growth rate of MWCNT
would be simple proportional to the ionic intensity of H2. This is
true even if being approximate in order to evaluate correctly the
Fig. 9. Conversion of methane in MWCNT as function of time.

D. Gozzi et al. / Journal of Alloys and Compounds 419 (2006) 32–39
37
practically, all the Ni nanoparticles into the MCM-41 dispersion
are suitable to allow the growth of MWCNT. In fact, looking
at Fig. 9, it appears clear that before stopping the reaction the
rate of CH4 conversion, i.e., MWCNT formation rate, is already
decreasing; (b) in contrast, the low value of η indicates that a
large number of preferential paths are expected to be formed
in the catalytic bed of the microreactor due to the high specific
volume of MWCNT produced during the reaction progress.
It has been shown [10] that using a catalyst of different chem-
ical nature and micrometer sized, MWCNT growth occurs on
self-generated tip-shaped single crystal nanoparticles. In the
present case, looking at Fig. 5, it appears clear that most of
the Ni nanoparticles are round shaped except for a few excep-
tions as reported in the inset of same Fig. 5. As stated before
(
see Section 3.2), no evidence here exists that a MWCNT grows
on a single crystal catalyst nanoparticle. It could be inferred
that a different growth mechanism is at working here though
the main chemical process (CH4 catalytic decomposition) and
experimental conditions are exactly the same.
4
.2. The low T magnetic properties of Ni nanoparticles and
nano Ni in MWCNT
Fig. 10. High field hysteresis loop of Ni nanoparticles at 5 K.
The starting point for the analysis of the magnetic proper-
ties of the two systems, i.e., Ni nanoparticles and nano Ni in
carbon nanotubes, is the observation of their common super-
paramagnetic behaviour in addition to the analysis of the mag-
netic differences between crystalline and amorphous magnetic
nanoparticles.
Magnetic nanoparticles behave like single domain particles
depending on the size of the particle: on the basis of the magnetic
domain theory [11,12], the critical radius is strongly related to
the saturation magnetization of the material, its exchange con-
stant and anisotropy constant. So, it is possible to calculate it
and for Ni the value is RC = 21.3 nm. In the amorphous state, a
magnetic nanoparticle behaves like a perfect superparamagnet at
all temperatures: no hysteresis cycle is observed nor a saturation
value at high-applied magnetic fields [13].
The hypothesis that our Ni nanoparticles are mainly formed
by amorphous particles is suggested by the absence of Ni peaks
in the XRD measurements and it was also confirmed by other
experimental evidences. The hysteresis cycle at 5 K up to 9 T is
displayed in Fig. 10. At very low fields, we have a sharp increase
due to the crystalline fraction of the Ni nanoparticles, which
behaves like a ferromagnet. The continuous increase of the
magnetization above 0.2 T is the superparamagnetic behaviour
of the amorphous Ni. It should be pointed out that also at
Fig. 7b), onthecontrary, belowtheblockingtemperature, behave
like a ferromagnetic system: a saturation is reached and the tem-
perature has low or null effect.
A small difference between H and HC+, i.e., the coercive
C−
fields obtained with decreasing and increasing applied magnetic
fields, is observed in both systems at all the measured tempera-
tures. This difference may be attributed to the existence of a very
narrow NiO layer around the Ni particle, which can create an
exchange bias between ferromagnetic Ni and antiferromagnetic
NiO [14]. In our case, the smallness of ꢀH = (H + H )/2
C
C+
C−
(around 5 Oe) suggests that the synthesis here adopted has
the result to prepare metallic nanoparticles less oxidized with
respect to other preparation routes also in comparison with other
results [15].
From the theory of superparamagnetism, it is possible to
obtain the blocking temperature from the temperature depen-
dence of the coercive field using the equation [14,16]:
ꢃ
ꢆ
ꢄ
ꢅ
1/2
T
HC = HC0 1 −
(4)
T
B
where HC0 is the coercivity at 0 K.
9
T the saturation limit is not reached, as expected for a pure
The expected low temperature linear dependence of the coer-
cive fields with respect to the root of the temperature is presented
for Ni nanoparticles in nanotubes in Fig. 11 while in the inset
the data for Ni nanoparticles are reported. From these data, one
could obtain the blocking temperatures TB = 74 and 68 K, for Ni
nanoparticles and nano Ni in MWCNT, respectively. We remark
anyway that these temperature values are meaningless since in
systems where a size distribution of the nanoparticles is present,
a distribution of the blocking temperatures is expected, being
TB proportional to the volume of the nanoparticle through the
superparamagnet.
This hypothesis explains also the hysteresis cycle behaviours
of Ni nanoparticles presented in Fig. 7a. The hysteresis cycle
is due to the small fraction of crystalline Ni nanoparticles and
the continuous increase of the magnetization after the cycle is
due to the rotation of the magnetic moments of the amorphous
Ni towards the applied field: this rotation is more effective at
lower temperatures due to the lower thermal energy opposing
to the magnetic field effect. Ni nanoparticles in MWCNTs (see

3
8
D. Gozzi et al. / Journal of Alloys and Compounds 419 (2006) 32–39
by the wide size and blocking temperature distributions hinder-
ing any further analysis.
4.3. The high T magnetic properties of Ni nanoparticles
and nano Ni in MWCNT
In the theory of superparamagnetism, the magnetization fol-
lows the Langevin function:
ꢄ
ꢅ
M
µH
kBT
=
coth
− µH
(8)
MS
kBT
where MS is the saturation magnetization, H the magnetic field,
kB the Boltzmann constant, T the temperature and µ is the aver-
age magnetic moment of the nanoparticles.
Fig. 11. Linear dependence of the coercivity HC for Ni nanoparticles in
MWCNT with respect to T . In the inset, the same linear dependence is reported
for Ni nanoparticles.
1
/2
The fit of both our magnetic data with a single Langevin
function was, anyway, disappointing. In fact, the wide distribu-
tion of particle sizes in our samples resulting in a distribution
of magnetic moments requires the use of a weighted sum of
Langevinfunctions[18]oratleastthesumoftwoLangevinfunc-
tion related to the bimodal size distribution function observed in
our samples [19]. In such way, a good fit of the magnetization in
the whole field range was obtained and the results of the fit are
shown, as solid line, in Fig. 8a and b for Ni nanoparticles and
nano Ni in MWCNT, respectively.
relation:
KꢁVꢂ
TB =
(5)
2
5kB
where K is the magnetic anisotropy energy per unit volume.
So, by using the bimodal size distribution obtained by TEM
analysis, we have calculated the TB values corresponding to the
two mean size values of the Ni nanoparticles in MWCNT: in
For Ni nanoparticles, we find from the fit that 71% of our
particles carry a mean magnetic moment µ = 17 000µ while
1
B
◦
this case, the crystallization of the nickel (Tcr = 333 C) [17] is
2 B
for 29% the mean magnetic moment µ is 150 000µ . Since the
complete due to the higher working temperature applied for the
synthesis of the nanotubes (500 C). By the relationships:
experimental magnetic moment of the single particle is related
to the volume of the particle by the following equation:
◦
TB(HC) = 0.72ꢁTB1ꢂ + 0.28ꢁTB2ꢂ,
µ = MSꢁVꢂ
(9)
ꢄ
ꢅ
3
ꢁ
TB1ꢂ
ꢁV1ꢂ
ꢁr1ꢂ
=
=
= 0.0266
(6)
where MS is the saturation magnetization of bulk nickel, our
values are consistent, under the hypothesis of spherical shape
of the particles, with diameters of r1 = 4.2 nm and r2 = 8.8 nm,
respectively.
On the other hand, for Ni nanoparticles in nanotubes, we
obtain a similar distribution of the two magnetic moments and
hence of the dimensions: around 76% of Ni nanoparticles in
nanotubes have µ1 = 16 000µB and 24% have µ2 = 150 000µB.
These values lead to r1 = 4.1 nm and r2 = 8.8 nm.
In both the systems, a good agreement with the TEM results,
i.e., the mean radius value and the percentage, is obtained from
the Langevin fit.
ꢁ
TB2ꢂ
ꢁV2ꢂ
ꢁr2ꢂ
where TB(HC) is the blocking temperature obtained from the fit
of Fig. 11, r1 and r2 are the mean radii of the bimodal distribu-
tion obtained from TEM and coefficients 0.72 and 0.28 are the
weights of the two types of nanoparticles obtained from the inte-
gration of the areas of the two peaks reported in the histogram
of Fig. 4. The calculated two different mean blocking tempera-
tures are: ꢁTB1ꢂ = 6 K and ꢁTB2ꢂ = 230 K. This analysis, showing
the existence of a high blocking temperature, explains why the
merging of the ZFC and FC curves, that must happens at TB, is
observed above T = 200 K.
Finally, from Eq. (4), we obtain HC0(1) = 29 mT for Ni
nanoparticles and HC0(2) = 48 mT for Ni in MWCNT. It should
be noted that both values are well below the value of 95 mT
obtained from the equation:
5
. Conclusions
The present work belongs to a research program oriented to
functionalise CNTs by single or polycrystalline metallic or inter-
metallic nanoparticles that have by themselves relevant physical
and/or chemical properties such as electrical, electronic, mag-
netic, catalytic, piezoelectric and so on. The first objective is
to check if and how the CNTs change their properties or the
changes are only at expenses of the embedded nanoparticles.
Next step is to envisage and test some specific application in the
field of the nanotechnologies.
K1
HC0 = 0.64
(7)
MS
for randomly oriented and non interacting particles, using
the saturation magnetization MS = 541 emu/cm3 and taking
for the anisotropy constant K1 the low temperature value
5
3
(
K1 = −8 × 10 erg/cm ). Weak magnetic interactions could
decrease the HC0. In our case, the HC0 calculation is affected

D. Gozzi et al. / Journal of Alloys and Compounds 419 (2006) 32–39
39
The main difficulty encountered in the present work is the
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appropriate working temperature.
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Two of the authors (DG, FC) wish to thank the Ministero
Istruzione, Universit a` e Ricerca (MIUR) for the supported
research PRIN2004 “Caratterizzazione Chimico fisica di
composti intermetallici in forma di bulk e in forma di
film”.
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