Post on 15-Jan-2023
Microstructure and Magnetic Propertiesof Carbon-Coated Nanoparticles
Xiangcheng Sun*
Center for Materials for Information Technology (MINT),
The University of Alabama, Tuscaloosa, Alabama, USA
ABSTRACT
In the present work, microstructure and superparamagnetic properties of two types of
carbon-coated magnetic Ni and Fe nanoparticles [Ni(C) and Fe(C)] are reviewed. High-
resolution transmission electron microscopy (HRTEM), electron diffraction (SAED), and
x-ray diffraction (XRD) analyses have been used to reveal the distinct structural morphol-
ogies of Ni and Fe nanoparticles. Moreover, novel carbon-coated Ni nanoparticle assemblies
offer us great opportunities for studying the mechanism of superparamagnetism in particle
assemblies. Magnetization measurements [M(T) and M(H) curves] for assemblies of Ni
nanoparticles indicate that modified superparamagnetic properties at T> TB, have been
found in the assemblies of Ni(C) particles. The blocking temperature, TB, is determined to be
near 115K under a certain applied field. Above TB, the magnetization M(H, T) can be
described by the classical Langevin function L using the relation, M=Ms (T¼ 0)¼ coth
(mH=kT)� kT=mH. It is suggested that these assemblies of carbon-coated Ni nanoparticles
have typical single-domain, field-dependent superparamagnetic relaxation properties. Finally,
Mossbauer spectra and hyperfine magnetic fields at room temperature for the assemblies of
Fe(C) nanoparticles confirm their distinct nanophases that were detected by structural
analysis. Modified superparamagnetic relaxation is observed in the assemblies of Fe(C)
nanoparticles, which is attributed to the nanocrystalline nature of the carbon-coated
nanoparticles.
Key Words: Ni nanoparticles assemblies; Fe nanoparticles assemblies; Superparamagnetic
property; HRTEM lattice imaging; Nanodiffraction; X-ray diffraction; Mossbauer spectra;
Hyperfine magnetic fields; Interparticle interactions; Magnetization measurements; Blocking
temperature.
*Correspondence: Xiangcheng Sun, Center for Materials for Information Technology (MINT), The University of Alabama,
Tuscaloosa, AL 35487-0209, USA; Fax: 205-348-2346; E-mail: xcsun@bama.ua.edu.
DOI: 10.1081=DIS-120021812 0193-2691 (Print); 1532-2351 (Online)
Copyright # 2003 by Marcel Dekker, Inc. www.dekker.com
557
JOURNAL OF DISPERSION SCIENCE AND TECHNOLOGY
Vol. 24, Nos. 3 & 4, pp. 557–567, 2003
INTRODUCTION
Studies on system of magnetic nanoparticles have
been the object of intensive research with respect to their
many technological applications and unique magnetic
properties.[1–3] Nanometer magnetic particles exhibit
specific properties such as superparamagnetism[4] due
to their very small sizes and fundamental change in the
coordination, symmetry, and confinement.[1–3] Superpara-
magnetism is regarded as a unique feature of magnetic
nanoparticles, and has great relevance to modern poten-
tial technologies including magnetic resonance imaging
contrast agents, data lifetime in high density information
storage, ferrofluid technology.[5,6] However, due to the
various contributions (i.e., magnetoccrystalline, surface,
shape, dipolar anisotropy), the transition from superpara-
magnetic state to collective magnetic excitations (i.e.,
spin-glass) could be occurring in the interacting magnetic
nanoparticles systems.[7,8] The study of the mechanism
of superparamagnetic properties in systems of fine mag-
netic particles will facilitate the control of superparamag-
netic behavior, and be favorable for their technological
applications.[9]
Following the discovery of the method for the
preparation of fullerenes in macroscopic quantities by
the so-called Kratschmer–Huffman carbon arc
method,[10] some novel magnetic systems of carbon
coated ferromagnetic materials, e.g., Fe, Co, and Ni,
had successfully been obtained.[1,4] Interest was paid to
the properties of the encapsulated particles, and in
particular to those involving novel magnetic properties
of ferromagnetic materials.
As an alternative magnetic nanoparticle system,
novel carbon encapsulated metal nanoparticles offer us
a unique opportunity for developing superparamagnetic
relaxation with desirable properties, because the encap-
sulated species are likely to be immune to environmental
effects owing to the protective carbon sheets around
them.[1,11–13] Most important, interparticle interaction
effects are limited by carbon encapsulated layers in
those magnetic nanoparticles systems.
We herein use the modified arc-discharge method to
generate two types of carbon-coated magnetic nanopar-
ticles, Ni(C) and Fe(C) nanoparticles, in a methane
atmosphere. The remarkable microstructures were
revealed by high-resolution transmission electron micro-
scopy (HRTEM) and x-ray diffraction (XRD). Magnetic
measurements were performed by superconducting quan-
tum interference device magnetometer (SQUID) and
Mossbauer magnetometer at different temperature and
magnetic fields. The relationships between the micro-
structure and novel magnetic behavior have also been
established.
EXPERIMENTAL
The detailed experimental apparatus (modified arc-
discharge) was fully illustrated elsewhere.[14] The pure
material (bulk Ni and Fe) to be evaporated was laid on a
water-cooled copper stage, which served as the anode.
A copper arm that was also water-cooled supports the
upper carbon rod, which served as the cathode. After the
chamber was evacuated, the desired gas of methane was
backfilled as a reactant gas to reach the desired pressure.
The distance between the two electrodes can be adjusted
from outside the chamber, so that the arc can be started
and controlled during a continuous operation.
A JEOL-2010EX HRTEM operated at 200 keV, was
used to reveal the internal structure and morphology of
the particle. It also allows one to record selected area
diffraction (SAED) analysis information. X-ray diffrac-
tion was conducted using CuKa radiation in a Siemens
x-ray difractometer to identify the different nanophases
and the crystal structure of nanoparticles.
Mossbauer spectra were recorded at room tempera-
ture using an Austin Science Associates Model S-600
spectrometer with a 57Co=Rh source. The absorption
spectra were computer fitted by using the NORMOS
program that used the input parameters as a first approxi-
mation to fit the experimental curve.
The magnetization (D.C. susceptibility) measure-
ments were carried out using a quantum design SQUID
magnetometer in the temperature range from 2 to 300K at
different applied magnetic fields. For the zero-field-cooled
(ZFC) magnetization measurements, the sample was first
cooled down to 2K without the applied field, after which
the magnetic field was applied. The sample was then
slowly warmed up to 300K for subsequent measurements
of the magnetic moments (ZFC). Thereafter, for the field-
cooled (FC) magnetization measurement, the sample was
cooled down to 2K without turning the magnetic field off;
measurements of magnetic moment could be obtained at
each intermediate temperature (FC). The magnetization as
a function of temperature [M(T)] reveals some of the main
features of a superparamagnetic system. The average
blocking temperature (TB), below which the particle
moments are blocked, is usually considered an important
parameter when characterizing the magnetic behavior of a
magnetic nanoparticle system. In general, TB can be
obtained by analyzing the ZFC=FC magnetization vs.
temperatures curves. Above TB, thermal fluctuations can
flip the direction of the magnetic moment; and no hyster-
esis is observed in the M(H) measurement. When the
particles interact, magnetic dipole–dipole interactions
become significant (stronger). This dipole interaction
may be either ferromagnetic or anti-ferromagnetic
depending upon the dipole orientations. In those cases,
558 Sun
the superparamagnetic relaxation process becomes highly
blocked and the magnetic dipole–dipole interactions may
become important in determining the magnetic state of the
nanoparticle system. Thus, a system of nanometer particles
with disordered magnetic moments may exhibit spin-glass
behavior (collective behavior), such as that observed in the
g-Fe2O3 magnetic particle systems.[15]
RESULTS AND DISCUSSION
It is well known that, carbon-coated nanoparticles
often have mutilayered carbon layers that are thought to be
sometimes closed like fullerenes. It has been shown that
such cages are airtight and protect the entrapped materials
from hydrolyzing and oxidation.[11–13]
The Assemblies of Ni(C) Particles
Systems
Powder XRD profiles in Fig. 1 clearly show the
presence of a majority of fcc Ni, with a small amount of
hexagonal Ni3C present from carbon encapsulated Ni
nanoparticles. Additionally, that the hexagonal Ni3C
phase is detectable using XRD suggests that the hexago-
nal Ni3C most likely is one single phase along with
majority fcc Ni phase in this assembly of Ni(C)
nanoparticles. The average grain size estimated from
the XRD patterns using the Scherrer formula shows
that those Ni nanoparticles have an average grain size
of 10–15 nm, which is consistent with the corresponding
HRTEM images (Fig. 2).
High-resolution transmission electron microscope
images and SAED patterns (Fig. 2) also indicate that
most of particles are spherical in shape and do not show
any well-developed facets. It is worth noting from Fig. 2
that several carbon layers encapsulating the Ni lattice
planes are quite visible. The contiguous carbon fringes
around the Ni nanocrystal are good evidence for com-
pleted encapsulation by carbon layers. Both fcc-Ni (1 1 1)
and hexagonal-Ni3C (1 0 1) phases are clearly observed
from SAED patterns of Ni(C) particles. In addition, the
(0 0 2) reflection is also present from those encapsulated
carbon layers. Those results are in good agreement with
the XRD analysis and HRTEM observations.
One significant morphological characteristic to note,
from the HRTEM images (Fig. 2), is that neither gaps nor
intermediate phases are observed between the outer
carbon layer and the core materials in the Ni(C) carbon
cages. These characteristics typically reflect a growth
history of these nanoencapsulated Ni particles, in which
carbon atoms dissolve into a molten or solid carbon–
metal alloy and then graphite precipitates at the surface
during carbon-arc discharge plasma process.[16]
Figure 3 (a,b) illustrates the magnetization vs. field
plots (M vs. H hysteresis loops) at 300K (a) and 2K (b)
for assemblies of Ni(C) nanoparticles. Figure 3(a) indi-
cates a rapid increase with increasing applied magnetic
field without saturation due to the superparamagnetic
relaxation and the non-collinear moment of the surface
Figure 1. Powder XRD patterns of Ni(C) nanoparticles at room temperature. Source: Ref.[21].
Microstructure and Magnetic Properties of Carbon-Coated Nanoparticles 559
spins in the smaller particle assembly.[17] Hysteresis is
absent with a little remanence and coercivity (HC), which
suggests the presence of a long-range magnetic dipole–
dipole interaction among the assemblies of superpara-
magnetic Ni(C) particles. With decreasing temperature,
the magnetization of the samples increases and exhibits a
symmetric hysteresis loop under both ZFC and FC at 2 K,
indicating a transition from superparamagnetic to
ferromagnetic behavior. Specifically, the temperature
dependence of the magnetization under the ZFC
Figure 2. High-resolution transmission electron microscopy image and corresponding SAED patterns of Ni(C) nanoparticles. Note,
arrows indicate the carbon layers; G symbol is carbon phase; C symbol is Ni3C phase. Source: Ref.[21].
Figure 3. (a, b) Field dependence of magnetization for the assemblies of Ni(C) nanoparticles at 2 K and 300 K, respectively.
Source: Ref.[21].
560 Sun
(MZFC) and FC (MFC) conditions gives further confirma-
tion of superparamagnetism in the assemblies of Ni(C)
nanoparticles, as shown in Fig. 4.
An important feature to note from Fig. 4 is the
temperature at which the MZFC curve exhibits a cusp
that defines the blocking temperature (TB). Below TB, the
superparamagnetic transition is blocked. In other words,
the magnetization cannot relax during the time of the
measurements. In particular, the well-defined TB of the
MZFC at 115K and broad blocking temperature ranges
are consistent with the particle size distribution as
observed from HRTEM images. Furthermore, the
continued increase of the MZFC and the irreversibility
occurring below 225K are also consistent with the
presence of superparamagnetic particles.[17]
Concerning the assemblies of Ni(C) nanoparticles,
above TB, the magnetization could be free to align with
the fields during the measurement time, and the curves of
magnetization vs. applied field should exhibit no hysteri-
sis loop [see Fig. 3(a)]. In such a superparamagnetic
state, the relative magnetization M=Ms can be described
by the standard Langevin function,[18–20] L, using the
relation:
M
Ms
(T ¼ 0) ¼ cothmH
kT
� ��
kT
mH
where Ms (T¼ 0) is the 0 K saturation magnetization of
the sample, and m is the particle moment given by the
MshV i where Ms is the saturation magnetization of the
bulk phase and hVi is the average particle volume.
Figure 5 shows the magnetization M(H) curves against
H=T for those Ni(C) nanoparticles assemblies. This scaling
is consistent with a superparamagnetic response. The
saturation magnetization moment can be estimated to be
near 22.5 emu=g. By using the data of Fig. 5 in the super-
paramagnetic regime, a least-squares fit to the magnetiza-
tion data reveals a moment m� 4.6� 10�15 emu or
�5.06� 105mB, which is consistent with 8� 105 Ni
atoms per particle. Further using the value of the saturation
magnetization s0¼ 510 emu=cm3, we can estimate an aver-
age particle volume hVi ¼ 8.93� 10�18 cm3, which corre-
sponds to spherical particles with radius r� 12.87 nm, is
good agreement with the above HRTEM observations.
Consistent with the theory of superparamagnet-
ism,[13,21,22] the coercivity, HC, has the temperature depen-
dence of HC¼HC0[1� (T=TB)1=2] at lower temperatures.
Figure 6 shows the variation of the coercivity with tem-
perature for the assemblies of Ni(C) particles. It is evident
from this figure that the coercivity (HC) vs. temperature (T)
data can be fit to the above expression. It is interesting to
note, from Fig. 6, that the zero temperature coercivity (HC0)
is determined to be �425 Oe and the blocking temperature
TB, to be�120K. Note that this TB value is very close to the
one that inferred from the sharp maximum of MZFC curves.
It is known that a small magnetic particle becomes
single-domain below some critical size due to the inter-
play between the energy of dipole fields and domain wall
Figure 4. Temperature dependence of magnetization for the assemblies of Ni(C) nanoparticles at applied field of 1000 Oe, the
blocking temperature is about 115 K. Source: Ref.[21].
Microstructure and Magnetic Properties of Carbon-Coated Nanoparticles 561
creation.[23] Consequently, Ni(C) nanoparticles, with an
average size of about 10.5 nm, may be considered as
having a single magnetic domain, the origin of its
magnetic hysteresis being spin rotation. Thus, the
magnetic behavior is size-dependent and should be
considered in conjunction with thermal energy and sur-
face anisotropy aspects.[24] In particular, single-domain
particles below a certain critical diameter are greatly
affected by temperature fluctuations. From Table 1, at
300 K, it is clear that the Mr and HC values of the
Figure 5. The plot of magnetization [M(H)] as a function of H=T with Langevin fitting for the assemblies of Ni(C) nanoparticles.
Source: Ref.[21].
Figure 6. Plot of HC (T) vs. T1=2 for the assemblies of Ni(C) nanoparticles. The TB is near 120 K. Source: Ref.[21].
562 Sun
assemblies of Ni(C) particles are significantly lower than
those for the microcrystalline Ni. Compared with 300 K,
the assemblies of Ni(C) particles show much larger
remanent magnetization (Mr) and coercivity (HC) at 2 K.
This implies that, the thermal energy for demagnetizing
become dominant over spontaneous magnetization at
300 K for the assemblies of Ni(C) nanoparticles. At 2 K,
however, the thermal energy effects significantly
decrease, so that the interparticle dipole–dipole interac-
tion and surface anisotropy energy may play a significant
role in determining the magnetic behavior.[24] It can be
supposed that like Ni(C) superparamagnetic particle
assemblies exhibit modified superparamagnetic behavior.
The larger decrease of remanent magnetization (Mr) and
coercivity (HC) at T> TB for the assemblies of Ni(C)
particles is a unique characteristic associated with mod-
ified superparamagnetism.
In addition, from Table 1, the saturation magnetiza-
tion (Ms) of the assemblies of Ni(C) nanoparticles is
suppressed in magnitude compared to that of the bulk Ni,
from microstructural analysis (HRTEM) results. We
attribute this result to a small amount of carbon solubility
in Ni particles. Hwang et al.[24] suggest that the decrease
in Ms is due to the nonmagnetic or weakly magnetic
interface that leads to a decrease in the effective magnetic
moment per unit mass. So the possibility is that some
carbon (which is diamagnetic), may have remained inside
the Ni nanocrystals as a metastable solid solution,
thereby reducing the contribution of the effective mag-
netic moment during magnetization process.[24]
Table 1. Comparison of saturation magnetization (Ms), remanent magnetization (Mr),and coercivity (HC) values for carbon encapsulated Ni nanocrystals and Ni in bulk.
T (K) Ms (emu=g) Mr (emu=g) HC (Oe) d (nm)
Ni(C) 300 22.5 1.37 7 10� 0.2
Ni(C) 200 23.9 2.11 46 10� 0.2
Ni(C) 100 24.9 2.70 117 10� 0.2
Ni(C) 50 25.2 7.08 198 10� 0.2
Ni(C) 10 25.2 7.37 316 10� 0.2
Ni(C) 2 25.2 10.55 367 10� 0.2
Bulk Ni 300 55.0 2.70 100 2–3 mm
Source: Ref.[21].
Figure 7. X-ray diffraction patterns for Fe(C) nanoparticles. Source: Ref.[11].
Microstructure and Magnetic Properties of Carbon-Coated Nanoparticles 563
On the other hand, one may suggest that the exis-
tence of the small amount of metastable hexagonal Ni3C
phase in the assemblies of Ni(C) nanoparticles has also
affected the magnetization process. The metastability
Ni3C is generally attributed to the low maximum solubi-
lity of carbon in nickel (2.7 at.% at 1600 K). The meta-
stable hexagonal Ni3C phase, in particular is expected to
be non-ferromagnetic due to strong hybridization
between Ni and C orbitals except that it remains in a
disordered state.[25] Therefore, the saturation magnetiza-
tion (Ms) from the minority of metastable Ni3C phase
effect should be ruled out of this study. Thus, the gradual
decrease in Ms could be attributed to the nanocrystalline
nature of the encapsulated particles, coupled with possi-
ble carbon solution in Ni nanoparticle.[24]
Assemblies of Fe(C) Particles
Powder XRD patterns of Fe(C) nanoparticles are
showed in Fig. 7. It is very apparent that a-Fe (bcc), g-Fe
(fcc), and Fe3C (orthorhombic) nanophases are detected
in the assemblies of Fe(C) nanoparticles; these three
phases are usually found in similarly prepared Fe��C
nanocomposites.[11,16] As revealed by high-resolution
images (HRTEM) of Fig. 8(a, b) and Fig. 9(a, b), these
Fe(C) nanoparticles (a-Fe, g-Fe, and carbide Fe3C) are
Figure 8. (a, b) High-resolution transmission electron micro-
scopy image (a) and SAED patterns (b) for a-Fe(C) and g-Fe(C)
nanoparticles. Note that, arrows indicate the carbon layers.
Source: Ref.[11].
Figure 9. (a, b) High-resolution transmission electron micro-
scopy image (a) and SAED patterns (b) for Fe3C phases in
Fe(C) nanoparticles. Note that, arrows indicate the carbon layers
and structure defects. Source: Ref.[11].
564 Sun
typically spherical in core-shell shape, and completely
encapsulated by the (wrapping shell) carbon layers,
which thickness is uniform over the surface of each
particle. The size of three core nanocrystals is generally
10–15 nm, and note that each core nanocrystal is tightly
covered by 10 to 40 carbon layers rather than a single
carbon layer only.[11,16,26] Meanwhile, SAED patterns of
Fig. 8 and Fig. 9 further confirm the presence of a-Fe,
g-Fe, and carbide Fe3C nanophases covered with carbon
layers. Especially, XRD and SAED patterns indicate that
no intermediate phases are observed between the wrap-
ping carbon layers and three core nanocrystals.[12] These
striking structural properties are in good agreement with
our previous work of other carbon coated transition metal
particles.[12–14,21,22]
Typical hyperfine fields and Mossbauer spectra of
the assemblies of Fe(C) nanoparticles at room tempera-
ture are shown in Fig. 10(a, b). The spectra of Fig. 10(a)
Figure 10. (a, b) Mossbauer spectra and hyperfine field distributions for Fe(C) nanoparticles. Source: Ref.[11].
Microstructure and Magnetic Properties of Carbon-Coated Nanoparticles 565
are fitted into the following components: the three sextets
whose magnetic hyperfine field (Hhyp) values of 335 kOe,
271 kOe, and 200 kOe are assigned to a-Fe, and two
carbide Fe3C nanophases, respectively;[27–29] also, the
singlet near zero velocity with an isomer shift
(IS)¼�0.126 mm=s is assigned to g-Fe. Generally
speaking, Mossbauer spectra results at room temperature
are in good agreement with those of earlier XRD analysis
and HRTEM observations, which confirm these distinct
nanophases in Fe(C) nanoparticles. In addition, the
evidence of modified superparamagnetic absorption
peaks could be observed at room temperature from
Fig. 10(b). However, such superparamagnetism can be
blocked by interparticle dipole interactions due to the fact
of the encapsulated particles are in contact with each
other, which suggests that the assemblies of Fe(C)
nanoparticles display modified superparamagnetic
relaxation at room temperature.[28] As a matter of fact,
it is necessary to perform further studies on magnetiza-
tion measurements, and confirm these specific magnetic
properties.
CONCLUSIONS
Two types of carbon-coated Ni and Fe nanoparticles
[Ni(C) and Fe(C)] have been successfully synthesized
using a modified graphite arc-discharge method. High-
resolution transmission electron microscopy, SAED, and
XRD analyses have been used to characterize these
distinct structural morphologies. The presence of the
majority of carbon encapsulated fcc-Ni and the minority
of Ni3C phases is confirmed in those Ni(C) particles;
meanwhile the presence of carbon encapsulated a-Fe,
g-Fe, and Fe3C phases have also been identified in those
Fe(C) particles.
Magnetization measurements [M(T) and M(H)
curves] for the assemblies of Ni nanoparticles indicated
that modified superparamagnetic properties at T> TB,
were exhibited in the assemblies of Ni(C) particles. The
blocking temperature, TB, was determined to be near
115K under a certain applied field. Moreover, a gradual
decrease in saturation magnetization (Ms) was observed,
which was attributed to the nanocrystalline nature of the
encapsulated particles, coupled with possible carbon
solution in Ni nanocrystals. Mossbauer spectra and
hyperfine magnetic fields at room temperature for the
assemblies of Fe(C) nanoparticles confirmed the distinct
nanophases revealed by XRD analysis and HRTEM
observation. Modified superparamagnetic relaxation
was also observed in the assemblies of Fe(C) nanoparti-
cles, which was attributed to the nanocrystalline nature of
the carbon-coated nanoparticles.
ACKNOWLEDGMENTS
Thanks to Dr. M. J. Yacaman, Dr. R. Escudero,
Dr. F. Morales, Dr. J. R. Gasga, Dr. N. Nava,
Mrs. A. Gutierrez, Mr. R. H. Reyes, Mr. L. Rendon,
and Mr. S. Tehuacanero for their technical help and
fruitful suggestions.
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Received December 17, 2002
Accepted February 10, 2003
Microstructure and Magnetic Properties of Carbon-Coated Nanoparticles 567