19452
J. Phys. Chem. C 2009, 113, 19452–19457
Influence of Cobalt Nanoparticles’ Incorporation on the Magnetic Properties of the Nickel
Nanofibers: Cobalt-Doped Nickel Nanofibers Prepared by Electrospinning
Nasser A. M. Barakat,*,†,‡ Bongsoo Kim,§ Chuan Yi,‡ Younghun Jo,| Myung-Hwa Jung,⊥
Kong Hee Chu,# and Hak Yong Kim*,∇
Chemical Engineering Department, Faculty of Engineering, El-Minia UniVersity, El-Minia, Egypt, Center for
Healthcare Technology DeVelopment, Chonbuk National UniVersity, Jeonju 561-756, Republic of Korea,
Department of Chemistry, KAIST, Daejeon 305-701, Republic of Korea, Quantum Material Research Team,
KBSI, Daejeon 305-333, Republic of Korea, Department of Physics, Sogang UniVersity, Seoul, Republic of
Korea, Clean & science Co. Ltd., Samsung Domg, Kangnam Ku, Seoul, Republic of Korea, and Department of
Textile Engineering, Chonbuk National UniVersity, Jeonju 561-756, Republic of Korea
ReceiVed: June 16, 2009; ReVised Manuscript ReceiVed: September 14, 2009
Among the common ferromagnetic metals, cobalt has distinct magnetic properties, so incorporation of cobalt
nanoparticles in the nickel nanofibers, which reveals better magnetic properties compared with the bulk, might
have considerable impact. In this study, we are introducing cobalt-doped nickel nanofibers prepared by
electrospinning. Electrospinning of a colloidal solution rather than a sol-gel (which is widely utilized in the
conventional electrospinning technique) has been invoked as a novel strategy to prepare cobalt nanoparticles/
nickel acetate/poly(vinyl alcohol) nanofiber mats. Physiochemical characterizations indicated that calcination
of the dried electrospun mats in argon atmosphere at 700 °C for 5 h leads to produce cobalt-doped nickel
nanofibers. Overall, magnetic properties studied pointed to improvement of the magnetic parameters of the
synthesized cobalt-doped nickel nanofibers compared with the pristine ones.
Introduction
Materials science has created magnetic materials far more
powerful than those available only a few decades ago, resulting
in a tremendous impact on modern technology. For instance,
ferromagnetic metal nanostructures reveal physical and chemical
properties that are characteristic of neither the atom nor the bulk
counterparts.1 Quantum size effects and the large surface area
of magnetic nanoparticles dramatically change some of the
magnetic properties and exhibit superparamagnetic phenomena
and quantum tunnelling of magnetization because each particle
might be considered as a single magnetic domain.2 Consequently, some metal nanoparticles such as Fe, Co, and Ni have
been given much attention to be utilized in various applications
such as electronic, optical, and mechanic devices, magnetic
recoding media, catalysis, superconductors, ferrofluids, magnetic
refrigeration systems, and contrast enhancement in magnetic
resonance imaging carriers for drugs and targeting.3-9 Many
literatures have been reported, confirming that the magnetic
properties of those materials are highly dependent on the particle
shape.10-14 1D magnetic nanomaterials are expected to have
interesting properties, as the geometrical dimensions of the
material become comparable to key magnetic length scales, such
as the exchange length or the domain wall width.15,16 Practically,
attempts have been done in this regard; for instance nanoscale
* To whom correspondence should be addressed. Tel: +82 63 270 2351.
Fax: +82 63 270 2348. E-mail: [email protected] (H.Y.K.), nasbarakat@
yahoo.com (N.A.M.B).
†
Faculty of Engineering, El-Minia University.
‡
Center for Healthcare Technology Development, Chonbuk National
University.
§
Department of Chemistry, KAIST.
|
Quantum Material Research Team, KBSI.
⊥
Department of Physics, Sogang University.
#
Clean & science Co. Ltd., Samsung Domg, Kangnam Ku.
∇
Department of Textile Engineering, Chonbuk National University.
magnetic logic junctions have recently been fabricated with
ferromagnetic nanowires as building blocks;17 magneto-optical
switches have been prepared using suspensions of ferromagnetic
nanowires.18 Among the 1D nano shapes, nanofibers have
considerable importance because of the longest axial ratio
characteristic. Therefore, nanofibers are the best candidate for
nanodevices and nanomembranes. Electrospinning is the most
popular technique utilized in production of functional nanofibers
because of its simplicity, low cost, and high yield.19 Metal base
nanofibers are produced by electrospinning of a sol-gel
composed of a metal precursor and an accordant polymer. In
the field of pure metal nanofibers, the electrospinning process
has been exploited to synthesize Co, Cu, Fe, and Ni in a
nanofibrous shape by calcination of the electrospun metal
precursor/polymer nanofiber mats in a hydrogen atmosphere.20-22
However, we have recently introduced calcination in an argon
atmosphere as a safe, economically preferable, and more
effective alternative strategy to produce silver, nickel, and cobalt
nanofibers.23-25 It is noteworthy mentioning that, in the case of
nickel metal, the nanofibrous shape strongly affects the magnetic
properties. For instance, the coercivity at room temperature for
the nickel nanofibers was about 100 times the magnitude of
the bulk material.21,24
Soft magnetic materials basically consist of nickel and another
ferromagnetic metal (Fe or Co). High magnetic properties soft
magnetic materials are strongly required, as these materials are
extensively used in power electronic circuits, as voltage and
current transformers, saturable reactors, magnetic amplifiers,
inductors, and chokes. Therefore, producing Ni/Co nanofibers
was the main goal of this study to exploit the distinct advantage
of this marvelous nanoshape for the improvement of the
magnetic properties.
Generally, the electrospun solution is either polymer(s)
dissolved in a proper solvent or metallic precursor/polymer
10.1021/jp905667s CCC: $40.75  2009 American Chemical Society
Published on Web 10/19/2009
Cobalt-Doped Nickel Nanofibers
J. Phys. Chem. C, Vol. 113, No. 45, 2009 19453
Figure 1. SEM images for the Co/Ni(AOc)/PVA electrospun nanofiber mats in low (A) and high (B) magnifications.
solution. The distinct feature of these solutions is that they have
to be completely miscible. In other words, in a case of exploiting
a metallic precursor, it should be soluble in a suitable solvent
because it has to hydrolyze and polycondensate in the final
precursor/polymer mixture to form the gel network. In this study,
we have utilized electrospinning of a colloidal solution as a
novel strategy to produce cobalt-doped nickel nanofibers as a
proposal to further improve the magnetic properties of the nickel
metal. Briefly, cobalt nanopowder/nickel acetate/poly(vinyl
alcohol) colloid has been electrospun, and calcination of the
dried nanofiber mats in an argon atmosphere resulted in the
production of nickel nanofibers embedding cobalt nanoparticles.
The synthesized cobalt-doped nickel nanofibers revealed better
magnetic properties compared with the pristine ones.
2. Experimental Details
2.1. Materials. Nickel(II) acetate tetrahydrate (Ni(AOc),
98%) and poly(vinyl alcohol) (PVA, molecular weight (MW)
) 65 000 g/mol) were obtained from Showa Co., Japan and
Aldrich Co., USA, respectively. Cobalt nanoparticles (99.9%
purity, average particle size ∼28 nm) have been obtained from
NaBond Tech. Co. Ltd., Shenzhen P.R. China. These materials
were utilized without any further treatments. Distilled water was
used as solvent.
2.2. Experimental Work. In this study, three mixtures have
been individually electrospun; Ni(AOc)/PVA, Co nanoparticles/
PVA, and Co nanoparticles/Ni(AOc)/PVA. A PVA aqueous
solution (10 wt %) was utilized in all mixtures. Briefly, Ni(AOc)/
PVA sol-gel was prepared by mixing of aqueous Ni(AOc)
solution (20 wt %) and the prepared PVA solution in a weight
ratio of 1:3. Cobalt nanoparticles have been added to the
prepared Ni(AOc)/PVA solution to produce Co/Ni(AOc)/PVA
colloid containing 0.5 wt % cobalt. For the Co/PVA, the
nanoparticles have been added to the polymer solution to get a
final colloid containing 0.5 wt % solid material. These mixtures
were vigorously stirred at 50 °C for 5 h. Later on, every mixture
was placed in a plastic capillary. A copper pin connected to a
high-voltage generator was inserted in the solution, and the
solution was kept in the capillary by adjusting the inclination
angle. A ground iron drum covered by a polyethylene sheet
was serving as a counter-electrode. A voltage of 20 kV was
applied to this solution. The formed nanofiber mats were initially
dried for 24 h at 80 °C under vacuum. Ni(AOc)/PVA and Co/
Ni(AOc)/PVA dried electrospun nanofiber mats were sintered
at 700 °C for 5 h in an argon atmosphere with a heating rate of
2.3 °C/min.
2.3. Characterization. Surface morphology was studied by
scanning electron microscope (SEM, JEOL JSM-5900, Japan)
and field-emission scanning electron microscope (FESEM,
Hitachi S-7400, Japan) equipped with energy dispersive X-ray
(EDX). Thermal properties have been studied by thermal
gravimetric analyzer (TGA, Pyris1, PerkinElmer Inc., USA).
Information about the phase and crystallinity was obtained by
using Rigaku X-ray diffractometer (XRD, Rigaku, Japan) with
Cu KR (λ ) 1.540 Å) radiation over the Bragg angle ranging
from 30 to 100°. High-resolution images and selected area
electron diffraction patterns were obtained with transmission
electron microscope (TEM, JEOL JEM-2010, Japan) operated
at 200 kV. Magnetic properties of the nanofibers were evaluated
using commercial superconducting quantum interface device
(SQUID) magnetometery. The nanofibers were weighed and
then filled into capsules in an inert gas environment. After this,
the capsules were sealed with paraffin wax to prevent the
nanofibers from air oxidation. The weight of the pristine and
cobalt-doped nickel nanofibers were 11.48 and 5.26 mg,
respectively.
3. Results and Discussion
Incorporation of metal nanoparticles in either polymeric or
metallic nanofibers is a desirable demand because the metal
nanoparticles enhance the physical and chemical properties and/
or provide the nanofibers with new characteristics. As the metals
powders are physically insoluble in any solvent, the reported
metallically doped electrospun nanofiber mats contain metal
compounds rather than zero-oxidation state metal nanoparticles
because soluble metallic compounds’ precursors have to be
utilized in the conventional electrospinning technique. Obtaining
metal nanoparticles from the utilized meal precursors needs
strong reducing agents, which affect the final product morphology. According to our best knowledge, only some noble metals’
(e.g., Ag and Pt) nanoparticles could be successfully incorporated in polymeric or metallic nanofibers.26-29 Electrospinning
of colloids might be an interesting solution for this dilemma as
the metal nanopowder can be used. In this regard, Figure 1
represents the SEM for the electrospun nanofiber mats obtained
from the Co nanoparticles/Ni(AOc)/PVA colloid. As shown in
this figure, the obtained nanofibers are quite smooth and beadsfree. As the main peaks corresponding to nickel and cobalt in
the energy dispersive X-ray (EDX) spectra are obtained at
relatively close binding energy values, we have electrospun Co
nanoparticles/PVA colloid to ensure that the cobalt nanoparticles
were incorporated in the polymeric nanofibers. Figure 2
demonstrates the SEM images and EDX results for the obtained
Co nanoparticles/PVA nanofibers. As shown in this figure,
smooth nanofibers are obtained. Moreover, the EDX results
affirmed incorporation of the cobalt nanoparticles in the
polymeric nanofibers, as the peaks indicating the presence of
cobalt are clearly apparent in the obtained spectra. Considering
the small particle size of the utilized metal nanoparticles (∼28
nm) compared with the electrospun nanofibers (∼450 nm) of
both of PVA and Ni(AOc)/PVA, we can say that the cobalt
nanoparticles are imprisoned inside the polymeric nanofibers
19454
J. Phys. Chem. C, Vol. 113, No. 45, 2009
Barakat et al.
Figure 2. SEM images (up) and EDX results (down) for Co nanopowder/PVA nanofiber mats.
Figure 3. SEM images for the Ni(AOc)-PVA-Co electrospun
nanofiber mats after calcination in Ar atmosphere at 700 °C, (A) and
(B); FE SEM image for a single calcined nanofibers, (C); and (D)
represents the FE SEM image of pure nickel nanofibers.
in both formulations. Therefore, the metallic nanoparticles could
not be observed in either of Figure 1 or Figure 2.
Calcination of the Co/Ni(AOc)/PVA electrospun nanofiber
mats in an argon atmosphere resulted in keeping the general
nanofibrous morphology with an observable decrease in the
average diameter compared with the original ones. As shown
in parts A and B of Figure 3, the sintered powder consists of
nanofibers with an average diameter of ∼230 nm, which is a
half of the average diameter of the electrospun nanofibers (∼450
nm). Part C of Figure 3 demonstrates the FESEM image of a
single nanofiber, as shown in this figure, the surface is rough.
However, the surface of the cobalt-free nanofibers that were
obtained form calcination of Ni(AOc)/PVA is comparatively
smooth, as shown in part D of Figure 3. According to our
previous study,24 the fibers in part D of Figure 3 are composed
of pure nickel. Elimination of the utilized polymer is the main
reason for the decrease of the average diameter.24,25 Actually,
the weight of the electrospun mat decreases greatly during the
calcination process due to decomposition of the utilized Ni(AOc)
Figure 4. TGA in argon atmosphere for Co nanoparticles/Ni(AOc)/
PVA nanofiber mats.
and elimination of the polymer, which is the main content of
the electrospun fibers. Figure 4 shows the thermal gravimetric
analysis (TGA) results in an argon atmosphere. As shown in
this figure, calcination of the electrospun nanofibers leads to
the loss of more than 80% of the original weight.
According to the JCPDS XRD database, cobalt and nickel
do have the same crystal structure. They have FCC crystal lattice
with space group class (S.G) of Fm3m (225). Moreover, the
cell parameters are very close; 3.544 and 3.523 Å for the cobalt
and nickel, respectively (JCDPS 15-0806, Co; and 04-0850,
Ni). Therefore, XRD analysis cannot be utilized to distinguish
between these two metals because their peaks are obtained at
nearly identical diffraction angles. Figure 5 shows the XRD
analysis results for the sintered nanofibers. The strong diffraction
peaks at 2θ values of 44.30, 51.55, 76.05, 92.55, and 98.15°
corresponding to (111), (200), (220), (311), and (222) crystal
planes respectively indicate the formation of pure nickel or pure
cobalt or both. As we have explained in details in our previous
study24 and also by other authors,30 heating of Ni(AOc) in an
inert atmosphere results in the formation of pure nickel.
However, the original electrospun nanofiber mats contain cobalt
Cobalt-Doped Nickel Nanofibers
J. Phys. Chem. C, Vol. 113, No. 45, 2009 19455
SCHEME 1: Conceptual Illustration for the Utilized
Procedure to Produce Co-Doped Nickel Nanofibers
Figure 5. XRD results for the obtained calcined nanofibers.
nanoparticles, which have a high melting point (1495 °C) and
cannot be vaporized at the utilized calcination temperature (700
°C), so one can say that the XRD results explain that the
obtained nanofibers consist of a Co and Ni mixture. The most
important finding is that no oxide formulation for both metals
was detected. Also, obtaining sharp and high peaks reveals good
crystallinity for the produced nanofibers. EDX analysis can be
relatively utilized to distinguish between the two metals, as is
shown in figure 6, which represents the EDX results for the
calcined nanofibers, and both metals could be detected.
Figure 7 shows the transmission electron microscope (TEM)
image for the obtained nanofibers. As shown in the upper insert,
which represents the HRTEM image of the marked area, a
nanoparticle can be observed incorporated inside the nanofiber.
Because nickel acetate was used as solution and was well mixed
with the PVA polymer, we can say that the crystalline matrix
shown in the HRTEM represents nickel; however, the nanoparticle can be assigned as cobalt because the utilized cobalt
nanoparticles have not melted or reacted during the treatment
process. Therefore, one can say that the final obtained product
is cobalt-doped nickel nanofibers. A ring pattern SAED image
has been investigated and is presented in the lower insert in
Figure 7; the ring pattern indicates the marked area is relatively
thick because it contains cobalt nanoparticle. The rings are clear
because the two metals have the same crystal planes, and they
have no dislocations or imperfections observed in the lattice
planes, which indicate good crystallinity of the synthesized
nanofibers. To make the proposed strategy more understandable,
we have built Scheme 1 as a conceptual illustration to show
the utilized procedure for the production of Co-doped Ni
nanofibers.
Figure 8 shows the hysteresis loops for both the cobalt-doped
and the cobalt-free nickel nanofibers at 5 and 300 K. It can be
Figure 6. EDX results for the produced cobalt-doped nickel nanofibers.
observed that both formulations reveal typical ferromagnetic
behavior. The ferromagnetism of the prepared nanofibers is
clearly shown by coercivity (Hc), saturation magnetizations (Ms),
remanent magnetization (Mr), and saturation field (Hs) listed in
Table 1. The saturation magnetization is the maximum induced
magnetic moment that can be obtained in a magnetic field;
beyond this field no further increase in magnetization occurs.
High saturation magnetization magnetic materials are required
for future high-density recording heads as well as high-frequency
inductors. As shown in Figure 8 and Table 1, incorporation of
cobalt resulted in an increase of the saturation magnetization
into almost 40%. Theoretically, cobalt has much more saturation
magnetization compared with nickel; 162.55 and 58.57 emu/g
for cobalt and nickel, respectively.31 Nanostructures usually have
saturation magnetization lower than that for bulk materials.32-34
A logic explanation of that can be drawn as: the high surface
area enhances oxidation of the surface of magnetic nanofibers,
which may create a magnetically dead layer. Moreover, the large
specific area and the imperfection of the crystalline structure at
the surface may also lead to a significant decrease in the
nanofiber saturation magnetization.21 For instance, the saturation
magnetization for cobalt and nickel nanofibers have been
reported as 81.97 and 27.23 emu/g, respectively.21 Therefore,
it was expected that incorporation of cobalt nanoparticles inside
nickel nanofibers will lead to an increase in the magnetic
properties.
Coercivity is the reverse magnetic field required to reduce
the net magnetization to zero. For magnetic materials, it is
necessary to reduce coercivity as a way to control the energy
losses. As shown in the second row in Table 1, the cobalt-doped
nickel nanofibers have relatively lower coercivity compared that
of the pristine at low and high temperatures, which can be
considered as a further improvement of the magnetic properties.
The decrease of the coercivity can probably be explained in
terms of the effect of CoNi bilayers.
Simply, remanent magnetization (Mr) can be defined as the
remaining magnetic momentum after realizing the magnetic
19456
J. Phys. Chem. C, Vol. 113, No. 45, 2009
Barakat et al.
Figure 7. TEM image for the obtained cobalt-doped nickel nanofibers.
The inserts represent the HR TEM (up) and selected area electron
diffraction pattern (SAED) (down).
Figure 9. FC and ZFC of pristine and cobalt-doped nickel nanofibers
at 100 Oe. The insert represents high-magnification plot at the blocking
temperature range for the ZFC curve for both formulations.
Figure 8. Magnetic properties of pristine and cobalt-doped nickel
nanofibers at 5 and 300 K.
TABLE 1: Magnetic Parameters of the Synthesized
Cobalt-Doped Nickel Nanofibers Compared with Pristine
Nickel Nanofibers at 5 K and Room Temperature
At 5 K
Parameter
saturation magnetization,
Ms (emu/g)
coercivity, Hc (Oe)
remanent magnetization,
Mr (emu/g)
saturation field,
Hs (emu/g)
Co/Ni
Room temperature
Ni
Co/Ni
Ni
32.7
25.3
32.17
23.12
323.5
10.4
382.52
10.4
41.57
2.34
67.66
3.06
3400
3700
900
800
field. Low remanent magnetization materials are classified as
magnetically clean materials. In some distinct fields, low
remanent magnetization is highly desirable for instance in data
storage applications. As shown in the third row in Table 1, both
cobalt-doped and pristine nickel nanofibers have acceptable
remanent magnetization. At low temperature (5 K), incorporation
of cobalt nanoparticles has no effect; however, an observable
decrease in the remanent magnetization was detected at room
temperature due to cobalt incorporation. In general, the magnetization moment depends on the magnetic field direction until
a certain threshold called the saturation field; so the popular
hysteresis loop is obtained. In other words, beyond the saturation
field, the magnetization moment is independent of the field sign.
Just beyond this threshold, small increase in magnetization can
be achieved; later on also the field magnitude would have no
impact on the magnetization (i.e., reachcing to the saturation
magnetization status). Therefore, some researchers give a rough
definition of the saturation field as the value at which all of the
atomic magnetic dipole moments are aligned with the field, and
the magnetic material in such a case is said to be saturated. As
shown in Figure 8 and Table 1, doped and pristine nickel
nanofibers have very small saturation fields at room temperature
compared with the values obtained at low temperature (i.e., 5
K). Moreover, incorporation of cobalt in the nickel nanofibers
has almost no influence on the saturation field at low and high
temperatures.
The zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves of the doped and primeval nickel nanofibers
recorded under an applied magnetic field of 100 Oe (Figure 9)
show irreversibility, which is typical of the blocking process
for an assembly of superparamagnetic nanostructures.35 The
temperature below which irreversibility in ZFC and FC magnetization occurs (Tirr) is found to be ∼365 K for both
formulations. Moreover, the blocking temperature (TB) at which
the ZFC magnetization peaks decreases as H is increased, as
expected for a superparamagnet,36,37 was detected at almost the
same temperature value (∼252 K) as shown in the lower insert
in Figure 9. However, cobalt incorporation enhances the
magnetization, as shown in Figure 9.
In summary, considering the advantage of nickel metal, it is
mainly utilized as a basic metal in the soft magnetic materials
that do have wide applications, and the synthesized cobalt-doped
nickel nanofibers might be of special interest. Although cobalt
nanofibers have better magnetic properties when compared with
the cobalt-doped and pristine nickel nanofibers, we believe that
the cobalt-doped nickel might be more beneficial than these
metal nanofibers because of high oxidation resistance and good
Cobalt-Doped Nickel Nanofibers
magnetic properties when compared with cobalt and nickel
nanofibers, respectively.
Conclusions
Electrospinning of a colloidal solution can be utilized to
produce metal-doped electrospun nanofibers as a novel strategy
to produce a new class of nanofibers that could not be obtained
by the conventional electrospinning technique. The proposed
strategy was successfully utilized to produce cobalt-doped nickel
nanofibers. Incorporation of cobalt nanoparticles distinctly
enhanced the magnetic properties of the nickel nanofibers. For
instance, the saturation magnetization has been improved to be
40% greater than the pristine nickel nanofibers. The other
magnetic parameters have been also relatively modified. Overall,
as many metals nanofibers have been reported in the literature,
this study can be considered as a new avenue for the researchers
to produce metal-doped metallic nanofibers that will have wide
applications according to the chosen metals.
Acknowledgment. This work was supported by a grant of
the Korean Ministry of Education, Science and Technology (The
Regional Core Research Program/Center for Healthcare Technology & Development, Chonbuk National University, Jeonju
561-756, Republic of Korea). We thank Mr. T. S. Bae and Mr.
J. C. Lim, KBSI, Jeonju branch, and Mr. Jong-Gyun Kang,
Centre for University Research Facility, for taking high-quality
FESEM and TEM images, respectively.
Supporting Information Available: Standard experimental
setup and a photograph for the utilized electrospinning experiment. This material is available free of charge via the Internet
at http://pubs.acs.org.
References and Notes
(1) Babes, L.; Denizot, B.; Tanguy, G.; Le Jeune, J. J.; Jallet, P. J.
Colloid Interface Sci. 1999, 212, 474.
(2) Goya, G. F.; Berquo, T. S.; Fonseca, F. C. J. Appl. Phys. 2003, 94,
3520.
(3) Gangopadhyay, S.; Hadjipanayis, G. C.; Dale, B.; Sorensen, C. M.;
Klabunde, K. J.; Papaefthymiou, V.; Kostikas, A. Phys. ReV. B 1992, 45,
9778.
(4) Schmid, G.; Chi, L; F. AdV. Mater. 1998, 10, 515.
(5) Zhang, D. E.; Ni, X. M.; Zheng, H. G.; Li, Y.; Zhang, X. J.; Yang,
Z. P. Mater. Lett. 2005, 59, 2011.
(6) Hyeon, T. Chem. Commun. 2003, 8, 927.
(7) Shouheng, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A.
Science 2000, 287, 1989.
(8) Mikulec, F. V.; Kuno, M.; Bennati, M.; Hall, D. A.; Griffin, R. G.;
Bawendi, M. G. J. Am. Chem. Soc. 2000, 122, 2532.
J. Phys. Chem. C, Vol. 113, No. 45, 2009 19457
(9) Kim, S. W.; Son, S. U.; Lee, S. S.; Hyeon, T.; Chung, Y. K. Chem.
Commun. 2001, 21, 2212.
(10) Fraune, M.; Rüdiger, U.; Güntherodt, G. App. Phys. Lett. 2000,
77, 3815.
(11) Cordente, N.; Amiens, C.; Chaudret, B.; Respaud, M.; Senocq, F.;
Casanove, M. J. J. App. Phys. 2003, 94, 6358.
(12) Park, J.; et al. AdV. Mater. 2005, 17, 429, and references therein.
(13) Jeon, Y. T.; Lee, G. H.; Park, J.; Chang, Y. J. Phys. Chem. B 2006,
110, 1187.
(14) Nayak, B. B.; Vitta, S.; Nigam, A. K.; Bahadur, D. Thin Solid Films
2006, 505, 109.
(15) Dennis, C. L.; Borges, R. B.; Buda, L. D.; Ebels, U.; Gregg, J. F.;
Hehn, M.; Jouguelet, E.; Ounadjela, K.; Petej, I.; Prejbeanu, I. L.; Thorntorn,
M. J. J. Phys.: Condens. Matter 2002, 14, R1175.
(16) Allwood, D. A.; Xiong, G.; Cooke, M. D.; Faulkner, C. C.;
Atkinson, D.; Vernier, N.; Cowburn, R. P. Science 2002, 296, 2003.
(17) Bentley, A. K.; Ellis, A. B.; Lisensky, G. C.; Crone, W. C.
Nanotechnology 2005, 16, 2193.
(18) Henry, Y.; Ounadjela, K.; Piraux, L.; Dubois, S.; George, J. M.;
Duvail, J. L. Eur. Phys. J. B 2001, 20, 35.
(19) Sigmund, W.; Yuh, J.; Park, H.; Maneeratana, V.; Pyrgiotakis, G.;
Daga, A.; Taylor, J.; Nino, J. C. J. Am. Ceram. Soc. 2006, 89, 395.
(20) Graeser, M.; Bognitzki, M.; Massa, W.; Pietzonka, C.; Greiner,
A.; Wendorff, J. H. AdV. Mater. 2007, 19, 4244.
(21) Wu, H.; Zhang, R.; Liu, X.; Lin, D.; Pan, W. Chem. Mater. 2007,
19, 3506.
(22) Bognitzki, M.; Becker, M.; Graeser, M.; Massa, W.; Wendorff,
J. H.; Schaper, A.; Weber, D.; Beyer, A.; Gölzhäuser, A.; Greiner, A. AdV.
Mater. 2006, 18, 2384.
(23) Barakat, N. A. M.; Woo, K. D.; Kanjwal, M. A.; Kyung, E. C.;
Khil, M. S.; Kim, H. Y. Langmuir 2008, 24, 11982.
(24) Barakat, N. A. M.; Kim, B.; Kim, H. Y. J. Phys. Chem. C 2009,
113, 531.
(25) Barakat, N. A. M.; Kim, B.; Park, S. J.; Jo, Y.; Jung, M. H.; Kim,
H. Y. J. Mater. Chem. 2009, DOI: 10.1039/B904669K.
(26) Jin, W. J.; Lee, H. K.; Jeong, E. H.; Park, W. H.; Youk, J. H.
Macromol. Rapid Commun. 2009, 26, 1903.
(27) Mallick, K.; Witcomb, M. J.; Dinsmore, A.; Scurrell, M. S.
Langmuir 2005, 21, 7964.
(28) Kanjwal, M. A.; Barakat, N. A. M.; Sheikh, F. A.; Khil, M. S.;
Kim, H. Y. Int. J. Appl. Ceram. Technol. 2009, in press.
(29) Formo, E.; Lee, E.; Campbell, D.; Xia, Y. Nano Lett. 2008, 8, 668.
(30) Juan, C. D. J.; Ismael, G.; Angel, Q.; Tito, P. J. Mol. Catal. A:
Chem. 2005, 228, 283.
(31) Wohlfarth, E. P. Ferromagnetic Materials North Holland: Amsterdam, 1980; Vol. 1, p 20.
(32) Yan, X. H.; Liu, G. J.; Haeussler, M.; Tang, B. Z. Chem. Mater.
2005, 17, 6053.
(33) Yan, X. H.; Liu, G. J.; Liu, F. T.; Tang, B. Z.; Peng, H.; Pakhomov,
A. B.; Wong, C. Y. Angew. Chem., Int. Ed. 2001, 40, 3593.
(34) Burker, N. A.; Stover, H. D.; Dawson, F. P. Chem. Mater. 2002,
14, 4752.
(35) Denardin, J. L.; Brandl, A. L.; Knobel, M.; Panissod, P.; Pakhomov,
A. B.; Liu, H.; Zhang, X. X. Phys. ReV. B 2002, 65, 064422.
(36) Dormann, J. L.; Fiorani, D.; Yamani, M. E. Phys. Lett. A 1987,
120, 95.
(37) El-Hilo, M.; Grady, K. O.; Chantrell, R. W. J. Magn. Magn. Mater.
1992, 114, 307.
JP905667S