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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Microstructure and Magnetic Properties of Carbon-Coated Nanoparticles 567