Morphologies and magnetic properties of FeCo nanoparticles modulated by changing the types of ligands of Co

Article abstract of DOI:10.1016/j.jmmm.2012.05.014

A new way of preparing FeCo nanoparticles has been developed using the co-precipitation technique based on the appropriate Co precursors of different types of ligands. These new precursors have been prepared by the reaction of cobalt acetate with three various coordination compositions for forming cobalt complex types. This technique provides proper control on nanoparticle size distribution after annealing to 500 °C. The variation of the magnetic properties with the type of ligands of Co can arise from the changes of the microstructures and crystalline anisotropies. Maximum coercivity values of 480 Oe were obtained. It is found that the precursors with special structures can prevent from agglomeration without presence of any surfactants.

Full text of DOI:10.1016/j.jmmm.2012.05.014

Journal of Magnetism and Magnetic Materials 324 (2012) 3652–3657
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Morphologies and magnetic properties of FeCo nanoparticles modulated by
changing the types of ligands of Co
Sima Alikhanzadeh-Arani a, Masoud Salavati-Niasari b,n, Mohammad Almasi-Kashi c
a Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Islamic Republic of Iran
b Institute of Nano Science and Nano Technology, University of Kashan, Kashan, PO Box 87317–51167, Islamic Republic of Iran
c Department of Physics, University of Kashan, Kashan, PO Box 87317–51167, Islamic Republic of Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
A new way of preparing FeCo nanoparticles has been developed using the co-precipitation technique
based on the appropriate Co precursors of different types of ligands. These new precursors have been
prepared by the reaction of cobalt acetate with three various coordination compositions for forming
cobalt complex types. This technique provides proper control on nanoparticle size distribution after
annealing to 500 1C. The variation of the magnetic properties with the type of ligands of Co can arise
from the changes of the microstructures and crystalline anisotropies. Maximum coercivity values of
480 Oe were obtained. It is found that the precursors with special structures can prevent from
agglomeration without presence of any surfactants.
Received 4 March 2012
Received in revised form
24 April 2012
Available online 7 June 2012
Keywords:
FeCo
Nanoparticle
Organic ligands
Magnetic material
& 2012 Elsevier B.V. All rights reserved.
1. Introduction
modified mechanochemical procedure [22] and high-energy ball-
milling [23]. Precipitation from solution is a simple method to
FeCo alloys are used in various engineering applications
because of their superior properties such as high saturation
magnetization, low coercivity, high Curie temperature, high
permeability and anisotropy, parameters that cannot be matched
by any other alloy system [1,2]. Magnetic properties of nanos-
tructured materials are affected by their morphology and particle
size. Researches on FeCo nanoparticles [3], nanoneedles [4],
nanowires [5] and thin films [6,7] have proven various outcomes.
Besides the interesting magnetic phenomena at the nanoscale
materials, supported magnetic nanoparticles are utilized in appli-
cations such as ultra-high density storage devices [8] and nano-
medicine applications [9] as the magnetic carriers [10]. These
demand nanostructures with both sufficiently high magnetic
anisotropy energy and high saturation magnetization for fast
reading and writing of magnetic bits [11]. Materials with high
magneto-crystalline anisotropy energy such as CoPt or FePt alloys
have attracted much attention for this purpose in the past
[12–16]. Also, the obtained results point to the potential of using
FeCo/graphitic shell nanocrystals for integrated diagnosis and
therapeutic applications [17].
prepare magnetic nanoparticles with a rigorous control over their
size and shape [24–29]. At the moment, a major interest is in the
development of organometallic or inorganic complexes for the
synthesis of nanostructures [30–33]. In this work, we synthesized
FeCo nanoparticles by ultrasound-assisted co-precipitation
method due to the enhanced dispersion of active components
[34]. Ultrasound is well known to have a significant effect on
reduced thermal and concentration gradients, reduced tempera-
ture and reduced particle size. We offer a new method of
preparing FeCo alloy materials using the co-precipitation proces-
sing technique based on appropriate Fe and Co precursors with-
out using any nucleating agent or surfactant. We studied effects of
novel coordination compositions of cobalt as the precursors on
the morphology and magnetic properties of products.
2. Experimental
2.1. Materials and characterization
All the chemical reagents used in our experiments were of
analytical grade and were used as received without further
purification. Three precursors of Co were prepared by the reaction
of high-purity acetate salt of Co with an organic ligand. Appro-
priate amount of acetylacetone (C5H8O2), 2-hydroxyacetophenon
(C8H8O2) or salicylaldehyde (C7H6O2) diluted in methanol (50 mL)
was added dropwise to a solution of Co(AC)2 (3 g, 0.012 mol)
Several synthetic methods have been applied to synthesize
magnetic FeCo alloy nanoparticles [18], such as self-assembly
techniques [19], sonoelectrochemical [20], co-precipitation [21],
n Corresponding author. Tel.: þ98 361 591 2383; fax: þ98 361 5552935.
E-mail address: salavati@kashanu.ac.ir (M. Salavati-Niasari).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jmmm.2012.05.014

S. Alikhanzadeh-Arani et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3652–3657
3653
dissolved in 50 ml methanol under stirring. Each of the reaction
3. Results and discussion
mixtures was heated under reflux at 80 1C for 12 h and the
solvent was removed under reduced pressure. The resulting solid
residue was filtered and dried to obtain the precursors,
[Co(acac)2]; (Co(II) acetylacetonate), [Co(hap)2]; bis(2-hydroxya-
cetophenonato)cobalt(II) and [Co(sal)2]; bis(salicylaldehyde)co-
balt(II). Fig. 1 shows the schematic structures of these final
products. Instrumental details for FT-IR, SEM, XRD, Elemental
Analysis, TEM, EDX, AGFM spectroscopy were the same as
described earlier [30–33].
FT-IR spectroscopy is a useful tool to understand the functional
group of any organic molecule. The infrared spectra of Sample no.
1 at 80, 350 and 500 1C are shown in Fig. 2. We can see the OH
stretch at ca. 3374–3440 cmꢁ1 in all three temperatures that is
the representative of H2O indicating that the surface was hydro-
xylated because the sample was exposed to air and some water
was adsorbed probably on the external surface of the samples
during handling to record the spectra [35]. The only difference
among these peaks is where the peaks’ intensity is varied or a
slight shift in the peaks’ position is detected. A strong band
appears at about 1630 cmꢁ1 at 80 and 350 1C which is attributed
to stretching vibrations of the carbonyl group indicating the
existence of the [Fe(C2O4) ꢀ 2H2O] compound as reported in the
literature [36]. Also, double absorption peaks appeared at 1314
and 1356 cmꢁ1 correspond to the C–O (uC–O) bending model of
the oxalate which clearly disappeared at 500 1C without any
2.2. Physical measurements
Phase identification was carried out for the as-precipitated and
heat treated samples by an X-ray diffraction (XRD) method with a
Rigaku D-max C III, X-ray diffractometer using Ni-filtered Cu K
a
radiation. Scanning electron microscopy (SEM) images were
obtained on Philips XL–30ESEM equipped with an energy
dispersive X-ray spectroscopy. The compositional analysis was
done by energy dispersive X-ray (EDX, Kevex, Delta Class I). The
morphology of the as-precipitated and heat treated samples was
observed with a transmission electron microscope (TEM) of
Philips EM208 transmission electron microscope with an accel-
erating voltage of 200 kV. Fourier transform infrared (FT-IR)
spectra were recorded on Shimadzu Varian 4300 spectrophot-
ometer in KBr pellets. The magnetic properties of the samples
were measured with a vibrating sample magnetometer (VSM).
2.3. Synthesis of FeCo alloys
To synthesize FeCo-alloyed nanoparticles, Fe(C2O4) ꢀ 2 H2O and
three different precursors of cobalt Co(acac)2, Co(hap)2 or
Co(sal)2have been used with a Fe–Co ratio of 50:50 for preparing
Sample nos. 1, 2 or 3, respectively. After being under ultrasonic
radiation for 30 min at room temperature, the mixtures were
heated to 60 1C under stirring. To prevent the oxidation of
particles, a solution of Na2SO3 dissolved in the distilled water
was added. FeCo nanoparticles were synthesized by reduction of
aqueous Fe2þ and Co2þ with potassium borohydride (KBH4)
under Ar/N2 gas. After a reaction for 30 min and again ultrasoni-
cation for 45 min, the final products were separated by centrifu-
gation, washed several times with distilled water and dried in the
vacuum at 80 1C. Finally, the produced powders were annealed at
500 1C for 1 h under Ar atmosphere. The furnace was turned off
and the samples were furnace-cooled to room temperature.
Fig. 2. FT-IR spectra of Sample no. 1 annealed at 80 1C, 350 1C and 500 1C for 1 h.
Fig. 1. Schematic molecular structures of Co complex precursors.

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evidence of oxalate. The peaks shown at ca. 489–577 cmꢁ1 are
attributed to the stretching vibration mode of Fe or Co–oxid
[36,33]. Similarly, the other samples were annealed at 500 1C for
one hour as well.
Results of powder X-ray diffraction show that the as-synthe-
sized Sample no. 1 at 80 1C, has body-centered cubic structure
with preferred (1 1 0) orientation at about 2
y¼451 (Fig. 3a).
Increasing the temperature to 500 1C causes growth of (2 0 0)
orientation at the sample (Fig. 3b), but weak traces of Fe3O4
phase, as indicated by peaks (3 1 1), (4 0 0) and (4 4 0), can be
seen confirming results of the FT-IR spectra. Similarly, an increase
in the temperature by 500 1C in Sample no. 2 resulted in slight
oxidation of FeCo (Fig. 3c), whereas Sample no. 3 included only
Fe–Co alloy without any segregation of Fe3O4 or a secondary
phase at the same temperature (Fig. 3d).
Fig. 3. XRD diffraction patterns for Sample no. (a) 1 annealed at 80 1C, (b) 1
annealed at 500 1C, (c) and (d) 2 and 3 annealed at 500 1C respectively. The peaks
are indexed with (*) denote Fe3O4 phase.
The XRD investigation revealed (1 1 0) and (2 0 0) peaks in
Sample no. 3 corresponding to those in the pure FeCo (by
comparison with the JCPDS Card no. 049–1567). The average size
of the FeCo alloy nanoparticles, calculated using the Debye–
Scherrer formula based on the full width at half-maximum
(FWHM) of the (1 1 0) diffraction peak in Fig. 3 was 18.2 nm,
24.6 nm and 17.5 nm for Sample nos. 1, 2 and 3 after annealing,
respectively. The sharp and distinct peaks reveal the good crystal-
linity of products despite reported in co-precipitation rout else-
where [21], probably owing to the effect of ultrasonic process
causing the more effective mixing and faster energy transfer. The
EDX spectra of the annealed Sample no. 3 at the peaks of Fe K and
Co K confirmed the presence of both metals and no oxygen peak is
observed as shown in Fig. 4. EDX quantitative microanalysis
indicates an average composition of Fe (48.8 at%) and Co
(51.2 at%).
The nanoparticle surfaces get oxidized very quickly on expo-
sure to the air due to their high surface energy [29]. We think that
the formation of Fe3O4 in Sample nos. 1 and 2 can be due to
existence of methyl groups at Co precursors. The methyl groups
play an important role as an electron donor for Co atoms in
Sample nos. 1 and 2. Also, the generated steric hindrance of
methyl groups in the structures of [Co(acac)2] and [Co(hap)2]
around the Co atoms (see Fig. 1) prevents Fe atoms from getting
Fig. 4. EDX spectrum of Sample no. 3 after annealing at 500 1C. The Si line
originates from the grid holding the sample.
Fig. 5. Representative SEM and TEM images of Sample no. 1, respectively: (a) and (c) before, (b) and (d) after annealing at 500 1C.

S. Alikhanzadeh-Arani et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3652–3657
3655
too close to Co atoms. So, Fe atoms tend to combine with oxygen
atoms. It can be expected that the yield of the final product
obtained by the three different methods do not vary so differ-
ently, because the coordination groups of the three utilized
ligands are mostly the same and the cavity size of them is nearly
comparable. While the XRD results showed that the yield of the
final FeCo nanoparticle as the main product in Sample no. 3 is
higher and more desirable than that of Sample nos. 1 and 2 due to
the existence of Fe3O4 as a by-product.
The SEM micrographs show a granular morphology for Sample
no. 1. It can be clearly seen, that the particles size of Sample no. 1
after annealing becomes slightly larger and denser than that
before annealing (Fig. 5a and c). The grains size was initially
10–60 nm in diameter (Fig. 5b), whereas it increased to 90 nm
after annealing (Fig. 5d). The strong magnetic interaction between
the Co–Fe alloy nanoparticles causes agglomeration.
Fig. 6 shows the SEM and TEM images of Sample no. 2 after
annealing. The size of particles became smaller than that of
Sample no. 1 due to the existence of benzene rings around the
central Co that are the better protecting agent than methyl groups
in Sample no. 1. The TG analysis has been shown that the
decomposition of 2-hydroxyacetophenato ligand completes at
two stages up to 460 1C [30] owing to the breaking the bonds of
CH3 groups and C6H6 rings. It is expected that the morphology of
Sample no. 3 would be more homogeneous with a more uniform
size distribution because its ligand consists only of the benzene
rings. The SEM and TEM images of Sample no. 3 shown in Fig. 7
confirm this topic. The degree of agglomeration is reduced and
the shape of the grains is close to tiny sphere (see Fig. 7a) with an
average diameter of less than 40 nm shown in Fig. 7b. So, we
found that by using this new precursor and via a simple route
without presence of any surfactants, it is possible to synthesize
the FeCo nanoparticles after annealing. Indeed, precursors with
special structures and shapes can prevent the agglomeration.
The as-synthesized nanoparticles of Sample no. 1 are ferro-
magnetic at room temperature with saturation magnetization
around 35 emu gꢁ1 (Fig. 8a). It is clear that heat treatment after
annealing causes an increase in Ms to 54.5 emu gꢁ1. Also, a clear
increase in the coercivity (from 102 to 262 Oe) and Mr (from
1.8 to 10.5 emu gꢁ1) is observed after annealing (Fig. 8b) because
the size of particles and the magnetic exchange between the
domains increase with increasing temperature. So, the nanopar-
ticles of larger size require a large magnetic field to change the
direction of magnetization resulting in large coercivity [27].
After annealing, the particles have enough time and tempera-
ture for a well-arranged crystallization, leading to a crystal
order and enhanced magnetic saturation, as a result [29]. Also,
after annealing, the particles became air-stable with enhanced
magnetization, which may be attributed to the fact that the
Fig. 7. Representative (a) SEM and (b) TEM images of Sample no. 3 after annealing
at 500 1C.
Fig. 6. Representative (a) SEM and (b) TEM images of Sample no. 2 after annealing
at 500 1C.

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60
30
0
60
30
0
-30
-30
-60
Sample no.2
after annealing
before anealing
-60
-8000
-4000
0
4000
8000
-8000
-4000
0
4000
8000
150
100
50
20
10
0
0
-50
-100
-150
Sample no.3
-10
-20
after annealing
before anealing
-8000
-4000
0
4000
8000
-400 -300 -200 -100
0
100 200 300 400
Apllied field (Oe)
40
20
0
Fig. 8. Hysteresis loops of (a) Sample no. 1 before and after annealing at 500 1C;
(b) the magnified loops of (a) from ꢁ400 to 400 Oe.
Sample no. 1
Sample no. 2
Sample no. 3
annealing process may lead to a homogeneous atomic distribu-
tion of iron and cobalt within individual nanoparticles as reported
earlier [28].
-20
-40
The magnetic properties of Sample nos. 2 and 3 after annealing
are shown in Fig. 9. Ms of Sample no. 2 was 50 emu gꢁ1 that is
close to that of Sample no. 1 (see Fig. 8). But an increase in Ms
(E148 emu gꢁ1) was observed at Sample no. 3 (Fig. 9b). The high
Ms can be due to lack of Fe3O4 trace at Sample no. 3 because the
oxide shell content in each metallic particle causes a decrease in
Ms as discussed earlier [4,28]. In comparison, Hc in Sample no. 2
(ꢂ480 Oe) and Mr (ꢂ21 emu g–1) in Sample no. 3 are higher than
the others (see Fig. 9c). The small difference in Hc among the
samples may be mainly related to the different morphologies and
thus different shape anisotropies [24]. This study illustrates the
pertinence of the size and magnetic properties of FeCo nanopar-
ticles to precursor that can be seen in Table 1.
-600
-400
-200
Applied field (Oe)
0
200
400
600
Fig. 9. Hysteresis loops after annealing at 500 1C of (a) Sample no. 2; (b) Sample
no. 3 and (c) the magnified loops of three samples for comparison.
Table 1
Morphological characteristics (TEM) and magnetic properties at room tempera-
ture of FeCo nanoparticles (Ms¼saturation magnetization, Hc¼coercivity field,
Mr¼residue magnetization, the annealing temperature is 500 1C).
Sample no.
Particle size (TEM)
(nm)
Ms
Hc
Mr
(emu gꢁ1
)
(Oe)
(emu gꢁ1
)
1 before annealing
1 after annealing
2 after annealing
3 after annealing
10–60
o100
o60
35
54.5
50
102
262
480
331
1.8
10.5
8.8
4. Conclusion
10–40
148
21
In summary, a simple methodology to achieve FeCo nanopar-
ticles in one reduction step with appropriate precursors of Co via
ultrasonic process has been demonstrated. We found that the
magnetic properties are structure and size-dependent. More
importantly, values of Ms, Mr, HC and morphology of products
clearly depend on the type of the utilized precursor, which are the
basic factors for practical applications. So, we think that by
focusing on the new precursors, it can be possible to achieve an
arbitrary magnetic alloy with desired effects. In this study, three
coordination composition precursors for forming cobalt complex
types were used. The nanoparticle of FeCo alloy were then
obtained after annealing to 500 1C without any need for nucleat-
ing or protective agents for controlling the process.
Acknowledgments
Authors are grateful to the council of University of Kashan and
Islamic Azad University (Science and Research Branch of Tehran)

S. Alikhanzadeh-Arani et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3652–3657
3657
for their unending effort to provide financial support to undertake
this work.
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