Effect of annealing on phase composition, structural and magnetic properties of Sm-Co based nanomagnetic material synthesized by sol-gel process

Article abstract of DOI:10.1016/j.jmmm.2012.02.038

Sm-Co based nanomagnetic material was synthesized by means of a Pechini-type sol-gel process. In this method, a suitable gel-precursor was prepared using respective metal salts and complexing agent such as citric acid. The gel-precursor was dried at 300 °C and then subjected to various reductive annealing temperatures: 350, 500 and 600 °C. The nanopowders so obtained were characterized for their structure, phase composition and magnetic properties. FT-IR studies on the gel-precursor showed the binding of metal cations with the citrate molecules in the form of metal-citrate complex. The gel-precursor, which was annealed at 350 °C showed the presence of both meta-stable cobalt carbide (Co₂C, Co₃C) and Co ₃O₄ phases; while the sample annealed at 500 °C indicated the sign of SmCo₅ phase. Upon increasing the reductive annealing temperature to 600 °C, crystalline phase such as fcc-Co and Sm₂C₃ were formed prominently. FE-SEM analysis revealed the change in sample morphology from spherical to oblate spheres upon increasing the annealing temperature. VSM measurements demonstrated ferromagnetic nature at room temperature for all the nanopowders obtained irrespective of their after reductive annealing temperature.

Full text of DOI:10.1016/j.jmmm.2012.02.038

Journal of Magnetism and Magnetic Materials 324 (2012) 2158–2162
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Effect of annealing on phase composition, structural and magnetic properties
of Sm-Co based nanomagnetic material synthesized by sol-gel process
a
b
a,n
G. Suresh , P. Saravanan , D. Rajan Babu
a
Advanced Materials Research Centre, School of Advanced Sciences, VIT University, Vellore 632 014, India
Defence Metallurgical Research Laboratory, Hyderabad 500 058, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Sm-Co based nanomagnetic material was synthesized by means of a Pechini-type sol-gel process. In this
method, a suitable gel-precursor was prepared using respective metal salts and complexing agent such as
citric acid. The gel-precursor was dried at 300 1C and then subjected to various reductive annealing
temperatures: 350, 500 and 600 1C. The nanopowders so obtained were characterized for their structure,
phase composition and magnetic properties. FT-IR studies on the gel-precursor showed the binding of
metal cations with the citrate molecules in the form of metal-citrate complex. The gel-precursor, which was
Received 1 September 2011
Received in revised form
1
February 2012
Available online 28 February 2012
Keywords:
Chemical synthesis
Sol-gel process
annealed at 350 1C showed the presence of both meta-stable cobalt carbide (Co
while the sample annealed at 500 1C indicated the sign of SmCo phase. Upon increasing the reductive
annealing temperature to 600 1C, crystalline phase such as fcc-Co and Sm were formed prominently.
FE-SEM analysis revealed the change in sample morphology from spherical to oblate spheres upon
increasing the annealing temperature. VSM measurements demonstrated ferromagnetic nature at room
temperature for all the nanopowders obtained irrespective of their after reductive annealing temperature.
2 3 3 4
C, Co C) and Co O phases;
5
Samarium cobalt
Magnetic nanoparticle
2 3
C
&
2012 Elsevier B.V. All rights reserved.
1
. Introduction
components was not realistic and led to misconception [12–14].
In the wet-chemical synthesis, the redox potential of the reducing
reagent must be more negative than that of the metal ions which is
an important factor in controlling the formation of Sm-Co phases.
A stronger reducing reagent is supposed to be preferred for the
reduction of metal ions having higher electronegativity [15]. As the
oxidation potential of Sm and Co is (ꢀ2.301 V) and (ꢀ0.28 V)
respectively [16], the co-reduction of both the metal ions should
be attempted during the wet-chemical process, in order to achieve
the required Sm-Co crystalline phase. In this context, Pechini-type
sol-gel process is considered as one of the most successful routes
for synthesizing metal alloy nanoparticles with required phase
homogeneity, as compared to other wet-chemical methods such as
polyol and co-precipitation. This technique caters the ability of
organic acids for the formation of metal organic complexes [17];
upon suitable reductive annealing, these complexes transformed into
respective alloy nanoparticles. Herein, we intend to employ such a
process to synthesis Sm-Co based nanomagnets with a variety of
crystalline phases by suitably controlling the reductive annealing
temperature.
Among the rare-earth permanent magnets, the highly anisotropic
Sm-Co compounds such as SmCo
technologically important on account of their intrinsic magnetic
properties. Of all the Sm-Co intermetallic compounds, SmCo and
Sm Co17 are the two important hard magnetic phases [1] and they
crystallize in hexagonal CaCu and Th Ni17 or rhombohedral-Th Zn17
5 7 2 5
, SmCo , Sm Co17, and Sm Co19 are
5
2
5
2
2
structure, respectively [2,3]. Both these magnetic phases have been
extensively investigated in the past decades for their applications as
5
permanent magnets at elevated temperatures; in particular, SmCo is
widely known for its intrinsic characteristics such as high magneto-
crystalline anisotropy and high Curie temperature [3–5]. In view of
the perceived advantages of Sm-Co compounds in their bulk form; in
recent years, considerable efforts have been made on the synthesis of
nanomagnetic materials of Sm-Co compounds utilizing a variety of
conventional processing routes, viz., ball-milling [6], thermal decom-
position [5], reductive annealing [7], polyol process [8,9], spin coating
[
10] and superhydride reduction [11]. Very recently, Harris et al.
reported that the wet-chemical synthesis of Sm-Co nanoparticles
through polyol process, which resulted in Co C, Co C and Sm-Co
phases [12]. As the X-ray diffraction patterns of Sm-Co, Co C and
C phases are quite similar, the identification of individual phase
2
3
2
Co
3
2. Materials and method
n
In order to prepare a thermally stable sol, 2 mmol of samarium
acetate tetrahydrate, 10 mmol of cobalt acetate tetrahydrate,
16 mmol of citric acid and 16 mmol of ethylene glycol were
Corresponding author. Tel.: þ91 94434 71434; fax: þ91 416 220 2804.
E-mail addresses: drajanbabu@vit.ac.in,
rajanbabud@hotmail.com (D. Rajan Babu).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2012.02.038

G. Suresh et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2158–2162
2159
dissolved in 50 ml of de-ionized water. The contents were heated
to 80 1C with gentle stirring to form a highly viscous gel and
subsequently the gel was allowed to dry at 300 1C in air and
named as SMC-1. The powder samples of SMC-1 were further
at different temperatures: 350, 500 and 600 1C under H
2
atmo-
sphere. Fig. 1 shows XRD patterns obtained for all the samples
indicating the formation of various crystalline phases. Sharp
diffraction peaks could be observed from the XRD pattern of
SMC-1 and the observed peaks indicate the crystallization of
2
annealed at 350, 500 and 600 1C in H atmosphere for 30 minutes
and named as SMC-2, SMC-3 and SMC-4, respectively.
Co
drying of gel in ambient conditions. The gradual decomposition of
Co phase was observed upon annealing the sample under H
2
3 4
O phase (JCPDS card No. 042-1467); which is due to the
3
O
4
2.1. Characterizations
atmosphere at different temperatures. Furthermore, the growth of
other phases can be witnessed. Fig. 2(a) shows the XRD patterns
of SMC-2 and SMC-3 and the XRD peaks appear to be broad,
which may be due to the small crystallite size [18]. The analysis of
XRD patterns revealed the presence of several other crystallo-
X-ray diffraction (XRD) patterns were recorded using Bruker D8
˚
Advance diffractometer with Cu K
a
1
radiation (
l
¼1.5406 A) at a
scanning speed of 0.0051/sec. Fourier transform infrared (FT-IR)
spectra were recorded using a Nicolet Avator 330 spectrometer.
Thermogravimetric analysis (TGA) was carried out at a rate of 10 1C/
graphic phases such as SmCo
presence of SmCo phase cannot be ruled out; though the XRD
patterns of Co C and SmCo are almost similar [12]. The SMC-2
has majority of Co C phase and minority of oxide phases. From
the XRD pattern of sample SMC-3 (Fig. 2(a)), crystallization of
SmCo and partial crystallization of fcc-Co phase can be clearly
noticed. The XRD pattern of Co C, Co C and SmCo phases
are consistent with that of their bulk with JCPDS card numbers:
65–1457, 043-1144 and 065–4157, respectively. From the XRD
pattern of SMC-4 (Fig. 2 (b)), the presence of fcc-Co, Sm , SmO
and Co phases are witnessed with no evidence of SmCo
phase. The SmCo phase, which is crystallized in SMC-3 has
5 2 3 3 4
, Co C, Co C, Co O and CoO. The
5
2
min in N atmosphere using SDT Q600, TA Instrument, USA
2
5
equipped with Thermal Advantage software. Field emission-scan-
ning electron microscope (FE-SEM) imaging was carried out in FEI
Quanta FEG 200 equipped with energy dispersive X-ray (EDX)
spectrometer. The magnetic hysteresis measurements were carried
out in room temperature using Lakeshore vibrating sample magnet-
ometer (VSM) 7410 at a maximum applied field of 20 kOe.
3
5
2
3
5
0
2 3
C
3
O
4
5
3
. Results and discussion
5
decomposed in SMC-4 due to the secondary phase crystallization
of fcc-Co (JCPDS card No. 015-0806). The secondary crystallization
of fcc-Co is generally observed in cobalt-based alloys, when the
sample is subjected to annealing sufficiently higher temperature
The synthesis of Sm-Co based nanomagnetic material using
the Pechini-type sol-gel technique has two steps: (i) formation of
metal-citrate gel precursor and (ii) annealing of the gel precursor
[19]. The decomposition of cobalt carbide phases is due to their
Fig. 1. XRD patterns of gel precursors subjected to different annealing
temperatures.
Fig. 3. TGA trace of SMC-1.
Fig. 2. (a) XRD patterns of SMC-2, SMC-3 and (b) XRD pattern of SMC-4.

2
160
G. Suresh et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2158–2162
meta-stable nature and these phases are generally stable up to
00 1C [20]. The peak observed at 27.51 reveals the presence of
Sm ; while the peak noticed at 31.61 reveals the presence of
SmO and Co phases.
6
2 3
C
3 4
O
The TGA trace of SMC-1 (Fig. 3) shows weight loss in four
steps. The observed weight loss was gradual from room tempera-
ture to 160 1C (step-1), 160 to 320 1C (step-2), 320 to 600 1C (step-
3) and 600 to 773 1C (step-4). The thermal decomposition of the
metal citrate complex is the most important and the most
complex stage [21]. The decomposition involves a few steps,
which includes dehydration and decomposition of the anhydrous
citrate complex and free citric acid, through intermediates to the
respective metal oxides [22]. The detailed analysis of TGA trace
revealed the weight loss in step-1 as 2.61%. This could be due to
the decomposition of trapped water molecules. It is apparent
from Fig. 3, the decomposition of anhydrous citrate precursor
started at 160 1C and hence the weight loss prior to the tempera-
ture 160 1C could be due to the trapped water molecules. Step-2
corresponds to the decomposition of intermediate phases such as
aconitate, itaconic and other phases leading to the respective
metal oxides [22], which is calculated as 2.62%. Step-3 corre-
sponds to the decomposition of oxycarbonate compound, which
was found to be stable up to 440 1C and then it decomposed at
Fig. 4. FT-IR spectra of as-prepared gel precursor and its annealed counterparts at
different temperatures.
2 3
490 1C. The meta-stable Co C and Co C phases also decomposed
at 490 1C [23], accompanied by a weight loss of 6.36% in this
region. Step-4 evidently shows a weight loss of 7.63% that
corresponds to the decomposition of Sm
2 3
C . When these carbides
(
Sm ) are heated, they vaporize incongruently and form Sm C
2
C
3
2
and metallic Sm [24,25]. Above 776 1C, weight losses are negli-
gible. A total weight loss of 19.22% was observed, which is
convincing and confirms the complete decomposition of all the
non-metallic phases.
The FT-IR spectra of samples: SMC-1, SMC-2, SMC-3 and SMC-4
are shown in Fig. 4. The spectrum of the gel-precursor showed a
ꢀ
1
broad band around 3400 cm indicates the O-H stretching due to
the trapped water. The intensity of this band decreased when the
sample was annealed to higher temperatures. A sharp peak around
ꢀ
1
1
600 cm is due to the C¼O stretching indicating the presence of
carboxylic group. The low intensity peaks at 2922, 2855 and
ꢀ
1
1316 cm
are due to the C-H stretching of alkanes [26]. Two
ꢀ
1
sharp peaks that appear at 669 and 573 cm
in the spectrum of
SMC-1 might be due to the result of metal-oxide vibrations [27]
and these two low intensity peaks are also observed in all other
spectra as well.
The FE-SEM image of the sample (SMC-2) obtained after
reductive annealing at 350 1C is shown in Fig. 5. From the figure,
it can be seen that the sample comprised in the form of
Fig. 5. (a) FE-SEM image of spherical granules of SMC-2.
Fig. 6. FE-SEM images of samples: (a) SMC-3 and (b) SMC-4.

G. Suresh et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2158–2162
2161
spherical granules with sizes in the range of sub-100 nm. The
spherical granular nature of the sample is due to the dipolar
interaction of the nanoparticles apart from the enormous
surface energy [28]. Fig. 6 (a) and (b) shows the FE-SEM images
of SMC-3 and SMC-4, respectively. Both the micrographs reveal
agglomerated particles with irregular shapes due to the annealing
of samples at sufficiently higher temperatures (500 and 600 1C).
The carbonaceous coating from the citrate molecules stabilized
the nanoparticles. When the sample coated with carbon subjected
to annealing, the carbon coating decomposes and induces
the particles to grow into irregular shapes and the above
fact is witnessed in the FE-SEM images of the samples
and other Sm-Co phases in the samples. Although such phases are
not seen in the XRD patterns and it is expected that these phases
may be present in the amorphous form [32,33]. The M
SMC-2 are found to be 23.7 emu/g and 905 Oe, respectively. The
observed increase in M could be due to the coexistence of Co
along with the Co of SMC-2 is found to
C (x¼2, 3) phases. The H
decrease when compared to SMC-1, which could be due to the
existence of soft magnetic cobalt carbide phases (i.e. Co C) [12].
The M and H of SMC-3 were 39.3 emu/g and 1193 Oe respectively
and these values were found to increase when compared to SMC-2,
which was due to the maximum decomposition of Co phase
and the crystallization of SmCo alloy. This can be observed from
the XRD pattern of SMC-3 (Fig. 2(a)). The decomposition of Co
phase could be understood from the decrease in oxygen content
(Table 1) of the EDX spectra (spectra not shown). The M and H of
s c
and H of
s
3 4
O
x
c
3
s
c
3 4
O
5
(
Fig. 6(a) and (b)). EDX analysis was carried out to determine
3 4
O
the elemental composition of the samples and the results are
tabulated in Table 1.
s
c
The magnetic measurement of all the samples was carried out
at room temperature and the typical M-H curves are shown in
Fig. 7(a). All the samples demonstrated ferromagnetic behavior
irrespective of their annealing conditions. Table 2 lists the mag-
netic properties of the four samples. From Fig. 7(b) of
SMC-1, it could be observed that the saturation magnetization
SMC-4 reached 100.2 emu/g and 1091 Oe, respectively, implies
that the sample exhibits metallic Co, which is confirmed with the
s
XRD analysis (Fig. 2(b)). The observed M (101 emu/g) coincides
with the reported value [34], which is due to the existence of
fcc-Co, as observed in the XRD pattern. The enhanced value
of H
and the traces of amorphous Sm-Co. The crystallization of fcc-Co
and Sm phases induces decomposition of SmCo phase [25]. The
controlled crystallization of Co and Sm phases such as those
c
(1091 Oe) might be due to the presence of cobalt carbide
(
M
s
) of SMC-1 is very low (2.2 emu/g) due to the existence of oxide
) phase, which is apparent from the XRD pattern (Fig. 1) and
) was found to be 1224 Oe, which is quite high
among all the samples. The XRD pattern of SMC-1 can be indexed
with the Co phase, which is an antiferromagnetic material. The
Neel temperature (T ) of Co is close to room temperature and it
is a size dependent property. Despite the fact that weak ferromag-
netism has been reported in the case of Co nanoparticles [29];
(Co O
3 4
C
2 3
5
the coercivity (H
c
2 3
C
obtained in the present study will eventually drive the facile sol-
gel technique to synthesize Sm-Co nanoparticles in large scale.
3 4
O
N
3 4
O
4. Conclusions
3 4
O
the observed moderate ferromagnetic behavior for the sample
SMC-1 obtained in the present study could be due to the following
facts: (i) finite size effects [30,31] and (ii) the presence of carbide
Although more work is needed to attain the expected Sm-Co
crystallographic phase, our results are convincing for the
Table 2
Table 1
Magnetic properties of samples prepared at various annealing conditions.
Elemental composition of samples calculated using EDX.
Sample Id
M
s
(emu/g)
H
c
(Oe)
M
r
(emu/g)
Squareness
Sample ID
Sm (wt %)
Co (wt %)
O (wt %)
C (wt %)
r s
ratio (M /M )
SMC-1
SMC-2
SMC-3
SMC-4
21.21
19.2
45.53
48.97
62.63
60.04
27.86
22.93
4.43
5.37
8.9
SMC-1
SMC-2
SMC-3
SMC-4
2.2
1224
905
1.0
9.5
0.47
0.40
0.47
0.40
23.7
39.3
27.56
30.99
5.39
4.74
1193
1091
18.4
40.2
4.22
100.2
Fig. 7. (a) M-H loop of samples annealed at different temperatures and (b) M-H loop of SMC-1.

2
162
G. Suresh et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2158–2162
synthesis of Sm-Co alloy nanoparticles obtained through the
facile sol-gel process. The major difficulty faced in the present
work was the control of oxidation of metals and consecutive
removal of oxides to achieve alloy nanoparticles. The existence of
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Sm which deteriorated the SmCo crystallographic phase
5
phase is witnessed only in the case of SMC-3 sample. The
[
2
C
3
5
completely. All the synthesized samples showed room tempera-
ture ferromagnetic behavior. The control of oxidation and growth
of fcc-Co by modifying the existing synthesis protocols may
trigger a new path way for the large-scale synthesis of nanopar-
ticles of Sm-Co alloys towards fabrication of nanostructured hard
magnets for sophisticated applications.
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Authors G.S. and D.R.B. thank VIT University for the financial
support, G.S. acknowledges the VIT University for the Research
Associateship offered to him.
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