Synthesis of Sm-Co and Sm-Co/Fe nanocrystals by reductive annealing of nanoparticles

Article abstract of DOI:10.1016/j.jallcom.2010.10.164

We report here a simple route for the chemical synthesis of SmCo ₅ and Sm₂Co₁₇ single-phase and SmCo ₅/Fe composite nanocrystals by polyol reduction of metal precursors and subsequent calcium reductive annealing. Sm₂O₃ and Co nanoparticles were synthesized first, and nanocrystalline SmCo₅ or Sm₂Co₁₇, compound with tunable composition was then obtained after the annealing. The composition of the compound was controlled by varying the initial molar ratio of the metal precursors. At room temperature, the obtained SmCo₅ and Sm₂Co₁₇ show coercivity of 12 kOe and 4 kOe, and saturation magnetization of 70 emu/g and 85 emu/g, respectively. To prepare SmCo₅/Fe nanocomposites, Fe, Co and Sm ₂O₃ nanoparticles were synthesized in one-pot by polyol reduction of the metal precursors.

Full text of DOI:10.1016/j.jallcom.2010.10.164

Journal of Alloys and Compounds 509 (2011) 2132–2136
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Synthesis of Sm–Co and Sm–Co/Fe nanocrystals by reductive annealing of
nanoparticles
∗
Girija S. Chaubey, Narayan Poudyal, Yuzi Liu, Chuanbing Rong, J. Ping Liu
Department of Physics, University of Texas at Arlington, Arlington, TX 76019, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 July 2010
Received in revised form 22 October 2010
Accepted 27 October 2010
Available online 4 November 2010
We report here a simple route for the chemical synthesis of SmCo5 and Sm Co single-phase and
2
17
SmCo5/Fe composite nanocrystals by polyol reduction of metal precursors and subsequent calcium
reductive annealing. Sm2O3 and Co nanoparticles were synthesized first, and nanocrystalline SmCo5
or Sm2Co17, compound with tunable composition was then obtained after the annealing. The composi-
tion of the compound was controlled by varying the initial molar ratio of the metal precursors. At room
temperature, the obtained SmCo5 and Sm2Co17 show coercivity of 12 kOe and 4 kOe, and saturation mag-
netization of 70 emu/g and 85 emu/g, respectively. To prepare SmCo5/Fe nanocomposites, Fe, Co and
Sm2O3 nanoparticles were synthesized in one-pot by polyol reduction of the metal precursors.
Keywords:
Nanoparticle
Annealing
Nanocomposite
Magnetic properties
©
2010 Published by Elsevier B.V.
1
. Introduction
nanocrystals by reductive annealing of the Co@Sm O core/shell
2 3
nanoparticles [20]. It was reported that, by properly tuning the
Rare-earth-transitional-metal compounds exhibit magnificent
dimension of the Sm O3 shell, different phases of Sm–Co system
2
magnetic and optical properties. In particular, Sm–Co compounds
have superior intrinsic magnetic properties such as very high Curie
temperatures and magnetocrystalline anisotropy, and therefore are
applied as high performance permanent magnets and are expected
to be applied in high-density magnetic recording media [1,2]. For
permanent magnet applications, exchange-coupled nanocompos-
ite magnets containing magnetically hard/soft phases with high
energy product (BH)max are desirable [3–6]. The intimate con-
tact between the hard phase with high coercivity and the soft
phase with high magnetization results in a high energy product
due to the inter-phase exchange-coupling. However, synthesis of
Sm–Co based nanocomposite remains a challenging task due to the
poor chemical stability of the Sm-containing nanoparticles. Sev-
eral techniques such as ball milling [7–10], melt spinning [11–13],
electrodeposition [14,15] and spark plasma sintering [16] were
developed for fabrication of Sm–Co based nanostructured mag-
nets. However, preparation of Sm–Co nanostructured magnets with
controllable nanoscale grain size and narrow size distribution still
remains a challenge. Numerous attempts to synthesize SmCo5
nanoparticles with high-temperature reductive decomposition of
organometallic precursors have been reported [17–19], though no
room-temperature magnetic hysteresis was reported until a two-
step synthetic strategy was developed by us to prepare SmCo5
can be obtained after the annealing. By adding Fe O nanoparticles
3
4
into the Sm–Co–O matrix before the annealing, SmCo/Fe nanocom-
posite nanocrystals were produced [21]. Recently, Matsushita et al.
have synthesized SmCo5 nanoparticles using polyol method with
room temperature coercivity of 1500 Oe [22]. One of the major hur-
dles for the synthesis of SmCo alloy is its high reactivity towards
oxygen when exposed to air. The oxidation kinetics and oxidation
morphology of SmCo alloy were recently reported [23,24]. A high
density gas aggregation nanoparticle gun [25] and several other
synthetic efforts to make SmCo alloy and SmCo-based nanocom-
posites [26] have also been reported recently.
In this paper, we report a simple method to synthesize SmCo5
(or Sm Co17) and SmCo5/Fe composite nanocrystals with well con-
2
trolled composition and grain size by high temperature reduction
of metal and metal oxide nanoparticles.
2. Experimental procedures
All the reagents used in this synthesis are commercially available. Iron (III) acety-
lacetonate (Fe(acac)3), cobalt (II) acetylacetonate (Co(acac)2), 1,2-hexadecanediol
(
HDD), trioctylphosphine (TOP) and oleic acid were purchased from Sigma–Aldrich.
Samarium (III) acetylacetonate (Sm(acac) ) was obtained from Strem Chemical. Cal-
3
cium powder was purchased from American Element Company. All the reactions
were carried out under inert atmosphere.
2
.1. Synthesis of Sm2O3 and Co nanoparticles
∗ Corresponding author. Tel.: +1 817 272 2815; fax: +1 817 272 3637.
Co(acac)2 (1 mmol, 0.257 g), Sm(acac)3 (0.2 mmol, 0.09 g), 1,2-hexadecanediol
E-mail address: pliu@uta.edu (J.P. Liu).
(3 mmol, 0.77 g) and oleic acid (3 ml) were charged to 125 ml round bottom flask at
0
925-8388/$ – see front matter © 2010 Published by Elsevier B.V.
doi:10.1016/j.jallcom.2010.10.164

G.S. Chaubey et al. / Journal of Alloys and Compounds 509 (2011) 2132–2136
2133
Our extended experiments showed that this method can be potentially scalable
for large scale production. This synthetic strategies involved one-pot synthesis of
mixture of nanoparticles. By simply multiplying the amount of the precursors in
designated molar ratio, large scale nanoparticles can be obtained.
The morphology of the nanoparticles was observed using transmission electron
microscopy (TEM). Samples for TEM analysis were prepared by drying a hexane dis-
persion of the particles on amorphous carbon coated copper grids. X-ray diffraction
(
XRD) patterns of the particle assemblies were collected on a diffractometer with
Cu K-␣ radiation (ꢀ = 1.5406 A˚ ). Magnetic measurements were carried out using a
superconducting quantum interference device (SQUID) magnetometer. The compo-
sition analysis was done by energy dispersive X-ray spectroscopy (EDX) and coupled
plasma-atomic emission spectroscopy (ICP-AES).
3. Results and discussion
The reductive thermal decomposition of Co(acac) and Fe(acac)3
2
in the presence of surfactants resulted in formation of Co and Fe
nanoparticles where as Sm O3 nanoparticles was formed by the
Scheme 1. Schematic illustration of synthesis of nanocrystalline SmCo5 or Sm2Co17
2
from the mixture of Co and Sm2O3 nanoparticles.
thermal decomposition of Sm(acac)3. Oleic acid and trioctylphos-
pine were capping ligands to stabilize the nanoparticles and
1,2-hexadecanediol was reducing agent. Fig. 1A shows the TEM
room temperature. After purging with argon for 30 min, trioctylphosphine (2.7 ml)
was injected and the reaction mixture was heated to 100 C for 10 min. Subsequently,
the reaction mixture was heated to a temperature in the range 330–340 C and
refluxed for 3 h. Argon gas was flowed through out the reaction. Most of the surfac-
tants were evaporated and a black dense product was obtained by adding 20 ml of
ethanol and centrifugation. The product was washed 2–3 times using mixture of hex-
ane (10 ml) and ethanol (40 ml) and separated via centrifugation. Finally, Sm2O3 and
Co nanoparticles having an average diameter of 25 nm were redispersed in hexane.
◦
image of as-synthesized mixture of Sm O3 and Co nanoparticle
2
◦
assemblies and Fig. 1B shows a TEM image of Co, Fe and Sm O3
2
nanoparticles in flower-like assemblies.
Fig. 2A shows XRD patterns of Sm2O3 and Co samples in different
stages. The pattern (a) is for as-synthesized sample, one can only
find peaks corresponding to Co phase, indicating that Sm O may
2
3
be in an amorphous state. To confirm the presence of Sm O , the
as-synthesized sample was annealed at 900 C in nitrogen atmo-
sphere. XRD measurement of the annealed sample confirmed the
2
3
2
.2. Synthesis of Sm2O3, Co and Fe nanoparticles
◦
Co(acac)2 (2 mmol, 0.514 g), Sm(acac)3 (0.4 mmol, 0.18 g), 1,2-hexadecanediol
(
8 mmol, 2.06 g) and oleic acid (5 ml) and an appropriate amount of Fe(acac)3 were
presence of both Sm O and Co in the sample (Fig. 2A(b)). To obtain
2
3
charged to a 125 ml round bottom flask at room temperature. After purging with
argon for 30 min, trioctylphosphine (2.7 ml) was injected and the reaction mixture
hard magnetic Sm–Co phase, the nanoparticles were annealed at
◦
◦
◦
900 C in presence of calcium. It is obvious that the Sm2O3 par-
was heated to 100 C for 30 min. The reaction was then heated to 330–340 C and
refluxed for 3 h before cooling down to room temperature. 20 ml of ethanol was
added to the product and centrifuged to separate the black product. The product was
then washed 2–3 times with hexane and ethanol and separated via centrifugation.
Finally, a nanoparticle mixture comprising of Sm2O3, Co and Fe were obtained.
ticles were reduced and diffused into cobalt nanoparticles to form
the hard magnetic Sm–Co phases. Similar result was obtained when
◦
Co/Sm O3 core/shell nanoparticles were annealed at 900 C with
2
calcium [20].
The final composition of the Sm/Co was controlled by tuning the
initial molar ratio of the Sm and Co metal precursors. For example,
the atomic ratio of Co to Sm with values of 86 to 14 or 90 to 9
was obtained by feeding an initial molar ratio of 5:1 and 9:1 for Co
and Sm metal precursors, respectively. Slight deviation in the final
composition of Sm/Co ratio from their initial precursor’s molar ratio
is due the minimal evaporation of Sm during annealing process.
The final composition of the particle was determined by EDX and
ICP-AES.
2
.3. Synthesis of SmCo5, Sm2Co17 nanomagnets and SmCo5/Fe nanocomposite
magnets
Dried powder of the as-synthesized Sm2O3 and Co nanoparticles or Sm2O3,
Co and Fe nanoparticles (300 mg) were ground together with potassium chloride
150 mg) and calcium powder (450 mg) and mixed thoroughly inside an inert-gas
(
glove box. The mixture was then loaded in an iron boat and transferred to a tube fur-
nace. The tube was purged with the forming gas (Ar + 7% H2) for 30 min. The furnace
◦
◦
was first heated to 120 C for 30 min and then to 900 C for 1–1.5 h under continuous
flow of the forming gas. The ash colored product was then washed several times with
deionized water to remove Ca, CaO and KCl powders. Finally, the products SmCo5,
or Sm2Co17, or SmCo5/Fe were obtained. Schemes 1 and 2 show the synthesis routes
for the formation of SmCo5 or Sm2Co17 nanocrystals and SmCo5/Fe nanocomposites,
respectively.
However, the grain size of the resulted Sm–Co particles is quite
large because of the high annealing temperature. In order to control
the grain growth, we added potassium chloride (KCl) salt, similar
as our previously reported salt-matrix annealing method [27,28],
into the mixture of Sm O , Co nanoparticles and calcium powder
2
3
◦
and then heat treated the mixture at 900 C in forming gas. It was
observed that presence of KCl salt reduces grain growth effectively,
due to the fact that the salt impedes interdiffusion between the
particles. In the mean time, large amount of KCl salt also slowered
down the reduction of the Sm O nanoparticles. We varied weight
2
3
ratio of Sm O , Co, Ca and KCl to optimize the composition and
2
3
annealing conditions and found that the ratio in weight of 2:3:1
between Sm O and Co nanoparticles, Ca and KCl resulted in a best
2
3
control of grain growth while having a complete Sm oxide reduc-
tion. The ash colored product was then washed several times with
deionized water to remove Ca, CaO and KCl powders. Fig. 2A(c, d)
shows the XRD patterns of the samples after the reductive anneal-
ing. The diffraction peaks matched well with the standard SmCo5
and Sm Co patterns (JCPDS No. 35-1400 and JCPDS No. 35-1368),
2
17
indicating that high temperature reduction and interface diffusion
lead to the formation of hexagonal structured SmCo5 and Sm Co
phases, respectively. The average grain size calculated from the
Scheme 2. Schematic illustration of synthesis of nanocrystalline SmCo5/Fe
nanocomposite from the mixture of Co, Fe and Sm2O3 nanoparticles. Black spot
represent Fe nanograins in SmCo5 matrix.
2
17

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G.S. Chaubey et al. / Journal of Alloys and Compounds 509 (2011) 2132–2136
Fig. 1. Bright-field TEM images of (A) as-synthesized Co and Sm2O3 nanoparticles and (B) as-synthesized Co, Fe and Sm2O3 nanoparticles.
XRD patterns using the Scherrer equation [29] is 25 nm for SmCo5
TEM image of the as-synthesized nanoparticles of Co, Fe and
Sm O is shown in Fig. 1B. It can be seen that nanoparticles of
phase and 28 nm for Sm Co17 phase. Few very low intensity peaks
2
2
3
are present in Fig. 2A(d), this could be due to the trace amount
of Sm O in the sample. Fig. 2B shows the typical HRTEM image
size in the range 5–10 nm were self-assembled together to form
flower-like assemblies. X-ray diffraction measurement on the as-
synthesized sample confirmed the presence of Fe and Co (Fig. 3A).
Sm O peaks were not observed in the as-synthesized sample due
2
3
of SmCo5 nanocrystals obtained after the high-temperature reduc-
tion annealing. The interfringes spacing of 0.29 nm was observed
in each nanocrystal grain which corresponds to the (1 0 1) planes
of the SmCo5 phase. SAED patterns further confirmed the SmCo5
phase (not shown here).
Exchange-coupled magnets are nanocomposites that are com-
posed of magnetically hard and soft phases that interact by
magnetic exchange coupling. The synthesis of single phase
Sm–Co nanocrystals was further extended to synthesize SmCo5/Fe
nanocomposites by modifying the synthesis procedure. The syn-
thesis procedure of SmCo5/Fe nanocomposite was similar to SmCo5
2
3
to its amorphous nature as discussed earlier.
The proportion of Fe nanoparticles in the final product can be
controlled by varying the initial molar ratio of the iron-precursor
while keeping the Sm and Co precursor molar ratio constant.
◦
Upon reductive annealing at 900 C in presence of Ca and KCl salt-
matrix, nanocomposite SmCo5/Fe was obtained [21]. The sample
was in powder form having micron size SmCo5 particles contain-
ing nanocrystals of SmCo5 and Fe. High resolution TEM analysis
of the annealed powder sample shows the presence of both typi-
cal Sm–Co and Fe nanocrystals grains. It can be seen from Fig. 3B
that the sample consists of nanoscale grains of two phases as indi-
cated by dash lines, one with lattice fringe distance at 0.29 nm and
another at 0.20 nm, corresponding to the (1 0 1) planes in the SmCo5
and the (1 1 0) planes in bcc-Fe, respectively. This was further con-
firmed by the XRD analysis (Fig. 3A). XRD patterns revealed that
the slight diffusion of Fe into SmCo5 during the reductive anneal-
particles synthesis except that Fe (acac) was added to the reaction
3
mixture as a Fe-source in addition to Sm and Co metal precursors,
as shown in Scheme 2. Under present reaction condition, the for-
mation of FeCo alloy [30] is unlikely due to the presence of Sm O3
2
nanoparticles which hinders atomic diffusion of Fe and Co. It was
reported earlier that the exchange-coupled SmCo5/Fe nanocom-
posite can be obtained [21] by reductive annealing of the mixture
of Sm–Co-O and Fe O nanoparticle.
ing resulted in the formation of Sm Co phase [21]. Fe peak was
3
4
2 17
◦
Fig. 2. (A) X-ray diffraction patterns of (a) as-synthesized mixture of Co and Sm2O3 nanoparticles (b) mixture Co and Sm2O3 nanoparticles annealed at 900 C in nitrogen
atmosphere and (c) nanocrystalline SmCo5 (d) nanocrystalline Sm2Co17, prepared by reductive annealing of mixture of Co and Sm2O3 nanoparticles (B) HRTEM image of the
nanocrystalline SmCo5 product.

G.S. Chaubey et al. / Journal of Alloys and Compounds 509 (2011) 2132–2136
2135
Fig. 3. (A) X-ray diffraction patterns of (a) as-synthesized Co, Fe and Sm2O3 nanoparticles mixture (b) SmCo5Fe0.6 (c) SmCo5Fe1.2 nanocomposite (B) HRTEM image of an
assembly of nanocrystalline SmCo5/Fe structure, with dashed lines showing the nanocrystalline grain boundaries of either Sm–Co or Fe.
difficult to characterize in these X-ray diffraction patterns as its
(
1 1 0) peak is overlapped with Sm Co17 (0 0 4) peak. However, it
2
can be clearly seen in Fig. 3A that the intensity corresponding to
1 1 0) peak of bcc-Fe increases with increasing Fe fraction in the
(
nanocomposite providing additional evidence of presence of Fe in
the nanocomposite.
Fig. 4 shows room temperature hysteresis loops of the SmCo5
nanocrystals and SmCo5/Fe nanocomposite. It can be seen that the
coercivity up to 12 kOe and saturation magnetization of 70 emu/g
was obtained for SmCo5. The room-temperature magnetic hystere-
sis loops for SmCo5/Fe nanocomposite are also shown in Fig. 4.
It can be seen that with increasing Fe fraction in the nanocom-
posite, the coercivity decreases whereas magnetization increases.
The energy products (BH)max are about 6.0, 7.1 and 7.8 MGOe for
SmCo5, SmCo5.5Fe0.6 and SmCo6.5Fe1.2, respectively. A strong pos-
itive peak in the ␦M plot suggested the strong exchange coupling
interaction between the nanograins of the hard and soft phases (as
shown in Fig. 5) [31]. However, the intercept of x-axis in ␦M plot is
higher than the coercivity of SmCo5/Fe nanocomposite indicating
not homogeneous distribution of soft fraction in nanocomposite
sample. By optimizing the soft fraction in SmCo5/Fe nanocompos-
Fig. 5. The ␦M plot for SmCo5/Fe nanocomposite.
ite, the exchange coupling between two magnetic phases can be
further increase and hence the squareness of the magnetic loops
and the energy products can be improved. The consolidation exper-
iments on these nanocomposite materials are under investigation.
4. Conclusions
In summary, a simple procedure for synthesis of SmCo5 and
Sm Co nanocrystals and SmCo5/Fe nanocomposites has been
2
17
reported. The room-temperature magnetic coercivity of 12 kOe
was obtained for SmCo5 nanostructure. Addition of Fe nanoparti-
cles resulted in exchange-coupled SmCo5/Fe nanocomposites. The
increase of Fe fraction in SmCo5/Fe composite has resulted in a
decrease in coercivity and an increase in saturation magnetization.
The reported synthetic procedures are easily scalable for large scale
production.
Acknowledgements
This work was supported by US DoD/MURI grant N00014-
05-1-0497. This work was also supported by the Center of
Fig. 4. Room temperature magnetic hysteresis loops of nanocrystalline SmCo5 and
SmCo5/Fe nanocomposite.

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G.S. Chaubey et al. / Journal of Alloys and Compounds 509 (2011) 2132–2136
Nanostructured Materials and Characterization Center for Mate-
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