Preparation and magnetic properties of anisotropic (Sm,Pr)Co5/Co composite particles

Article abstract of DOI:10.1016/j.jmmm.2011.10.014

Anisotropic (Sm,Pr)Co₅/Co nanocomposite particles have been fabricated by chemical coating the 2 h ball milled (Sm,Pr)Co₅ flakes with Co nanoparticles. The Co nanoparticles were synthesized with mean particle sizes in the range of 2050 nm. The nanocomposite particles present [0 0 1] out-of-plane texture and improved magnetic properties, e.g., an enhanced remanent magnetization of 72 emu/g for (Sm,Pr)Co₅/Co and 66 emu/g for (Sm,Pr)Co₅. In addition, by using the 8 h ball milled powders (much smaller than the 2 h ball milled powders) as the starting materials, Co nanoparticles can also be successfully coated on the surface of the flakes. A plausible mechanism for the formation of Co nanoparticles on the surface of (Sm,Pr)Co₅ flakes is discussed.

Full text of DOI:10.1016/j.jmmm.2011.10.014

Journal of Magnetism and Magnetic Materials 324 (2012) 889–892
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Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Preparation and magnetic properties of anisotropic
(Sm,Pr)Co5/Co composite particles
n
Xiaoliang Wang a, Huanlong He a, Fengqing Wang a, Yan Chen a, Lei Xu a, Xiaohong Li b, Xiangyi Zhang a,
a State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, 066004 Qinhuangdao, PR China
b College of Physics Science and Technology, Hebei University, 071002 Baoding, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Anisotropic (Sm,Pr)Co5/Co nanocomposite particles have been fabricated by chemical coating the 2 h
ball milled (Sm,Pr)Co5 flakes with Co nanoparticles. The Co nanoparticles were synthesized with mean
particle sizes in the range of 20–50 nm. The nanocomposite particles present [0 0 1] out-of-plane
texture and improved magnetic properties, e.g., an enhanced remanent magnetization of 72 emu/g for
(Sm,Pr)Co5/Co and 66 emu/g for (Sm,Pr)Co5. In addition, by using the 8 h ball milled powders (much
smaller than the 2 h ball milled powders) as the starting materials, Co nanoparticles can also be
successfully coated on the surface of the flakes. A plausible mechanism for the formation of Co
nanoparticles on the surface of (Sm,Pr)Co5 flakes is discussed.
Received 21 July 2011
Received in revised form
8 October 2011
Available online 17 October 2011
Keywords:
Sm–Co magnet
Anisotropy
Powder coating
Nanocomposite magnet
& 2011 Elsevier B.V. All rights reserved.
1. Introduction
explore simple mechanical blending of soft magnetic particles
with more coarse anisotropic hard magnetic powders [14,15].
Nanocomposite exchange coupled magnets consisting of a fine
mixture of hard- and soft-magnetic phases, e.g., R2Fe14B/ -Fe
However, the hard magnetic particles were relatively large and
it is extremely difficult to fabricate the soft magnetic phase with
controlled morphology. By contrast, coating of hard magnetic
particles with a layer of soft magnetic material may enhance the
interaction between the soft and hard phases and thus the
magnetic performance. One challenge for the powder coating
technologies is to disperse the soft phase throughout the magnet,
especially for coating the hard magnetic particles produced by a
ball milling treatment. Several studies have been performed to
explore the powder coating technologies [13,16,17]. Nevertheless,
only a few attempts have been made toward coating the ball
milled hard magnetic particles with the soft magnetic nanopar-
ticles by chemical reduction [16,17,21].
In this paper, we report a simple chemical coating approach to
make anisotropic (Sm,Pr)Co5/Co nanocomposites. Using the 2 and
8 h ball milled (Sm,Pr)Co5 flakes as the starting materials, the
cobalt soft magnetic nanoparticles can be uniformly deposited on
the surface of the flakes using the current method. The nanocom-
posite particles present [0 0 1] out-of-plane texture and improved
magnetic properties.
a
(R¼rare earth), have attracted much scientific and technological
interests for the development of next generation permanent
magnets with a high energy density [1–3]. Theoretical calcula-
tions predict that anisotropic nanocomposite magnets have the
potential to achieve a maximum energy product (BH)max above
100 MGOe [4]. Substantial efforts have been made to manipulate
the microstructures of nanocomposite magnets to satisfy the
requirements of theoretical models. Unfortunately, at present,
most of the nanocomposite magnets fabricated by the conven-
tional top–down methods, e.g., melt spinning and mechanical
alloying, are isotropic [5–10]. More recently, studies show that the
(0 0 l) texture for Nd2Fe14B nanocrystals can be achieved by
a thermo-mechanical processing of amorphous alloys [11,12].
An alternative bottom–up approach is a potential method to
fabricate anisotropic nanocomposite magnets, where magnetically
hard and soft nanoparticles are first prepared, assembled together
and then compacted in order to form the nanocomposite magnet
[13–17]. Although substantial progress has been made in the
synthesis of hard-magnetic particles with crystal texture and
an ultrafine and uniform size, the fabrication of anisotropic
nanocomposite magnets with tailored morphology is still a great
challenge [12,18–21]. A number of attempts were made to
2. Experimental procedure
The (Sm0.58Pr0.42)Co5 flakes were prepared by high-energy ball
milling (HEBM) for 2 and 8 h using a SPEX 8000 mill. A ball-to-
powder weight ratio of 10:1 was used. Heptane (99.8%) was
used as the milling medium and oleic acid (99%) as the surfactant.
n Corresponding author. Tel.: þ86 335 8057018.
E-mail address: xyzh66@ysu.edu.cn (X. Zhang).
0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2011.10.014

890
X. Wang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 889–892
The synthesis of (Sm,Pr)Co5/Co particles was carried out by
employing mixtures of cobalt chloride (CoCl2 ꢀ 6H2O) and the ball
milled (Sm,Pr)Co5 flakes in the desired Co:(Sm,Pr)Co5 weight ratio
in ethanol solution. The mixture was dispersed into the solution
under continuous ultrasonic mixing at 50–70 1C for ꢁ5 min.
Poly(vinylpyrrolidone) (K30) (1 g) was added as capping agent
and dispersant to protect the particles from oxidation and avoid
their aggregation. Then another solution consisting of 1.5 g NaOH
and 6 ml hydrazine monohydrate liquid (80%) was added. The
process of the reaction can be described as below:
by ball milling for 2 h in heptane and oleic acid, which is in
agreement with the report of Cui et al. [22]. Fig. 1(b) and
(c) shows the XRD patterns of the magnetically non-aligned and
aligned nanocomposite particles. The diffraction peaks can be
indexed as the standard SmCo5 (JCPDS NO. 35-1400) and hexa-
gonal Co (JCPDS NO. 05-0727). In Fig. 1(c), the (0 0 2) peak of
(Sm,Pr)Co5 hard phase becomes the dominant reflection, indicat-
ing a strong c-axis crystallographic alignment of (Sm,Pr)Co5 phase
in the nanocomposite particles.
Typical SEM images of the (Sm,Pr)Co5 flakes milled for 2 h
and the composite powders are presented in Fig. 2. In this figure,
(a) and (b) show the magnetic powders with flaky-like morphol-
ogy at different magnifications. It can be seen that the surface of
the as-prepared (Sm,Pr)Co5 flakes are smooth, and their thick-
2Co2þ þN2H4 þ4OHꢂ-2CokþN2mþ4H2O
ð1Þ
After strong stirring under ultrasonic for 30 min, the black
magnetic powders were recovered by centrifugation and rinsed in
ethanol.
nesses is in the range of 20–100 nm and length is 0.5–5 mm.
Fig. 2(d) and the corresponding high-magnification SEM image
(see Fig. 2(e)) exhibit the feature of nanocomposite flakes when
0.03 g (Sm,Pr)Co5 powers reacted with 0.024 g cobalt chloride.
From Fig. 2(d) and (e), it can be clearly seen that as-prepared
(Sm,Pr)Co5 flakes are decorated by uniform nanoparticles in the
range of 20–50 nm in diameter. The energy dispersive X-ray
spectroscopy performed on the surface of particles showed the
atomic ratio of the samples at different stages of reaction
(Fig. 2(c) and (f)). The loss of a small amount of Sm and Pr from
the as-prepared (Sm,Pr)Co5 may be mainly due to the minor
oxidation of Sm and Pr during the HEBM processes. The composi-
tion of this heterostructure is confirmed by the EDS studies,
which reveals the presence of Sm, Pr, Co and O element. In
Fig. 2(f), a relatively high content of cobalt element was detected.
It can also be seen from Fig. 2(c) and (f) that the concentration of
oxygen is relatively high. The oxygen likely came from two
sources, one from the oxidation of Sm and Pr during the HEBM
The surface morphology and composition of the products were
investigated by field emission scanning microscopy (FESEM)
equipped with energy dispersion X-ray spectrometer (EDS). The
microstructures of the products were investigated by employing
an x-ray diffraction (XRD) with Cu
Ka radiation. Magnetic
measurements were performed using a vibrating sample magnet-
ometer (VSM) with an applied field of up to 9 T at room
temperature. The alignment of the flakes was done by applying
a 4 kOe field and immobilizing the flakes with epoxy resin or wax
for XRD and VSM measurements, respectively.
3. Results and discussion
XRD patterns of the as-prepared magnetically non-aligned and
aligned flakes are presented in Fig. 1(a). It can be seen that after
magnetic alignment (see the inset in Fig. 1(a)) crystallographically
anisotropic (Sm,Pr)Co5 flakes with [0 0 1] texture were obtained
Fig. 2. SEM images and EDS spectrums of the (Sm,Pr)Co5 flakes and (Sm,Pr)Co5/Co
composites. (a, b) typical SEM images of the ball milled (Sm,Pr)Co5 flakes for 2 h.
(c) Typical EDS spectrum of the (Sm,Pr)Co5 flakes and the atomic ratio (inset
in (c)). (d, e) typical SEM images of (Sm,Pr)Co5/Co composites. (f) Typical EDS
spectrum of the (Sm,Pr)Co5/Co composites and the atomic ratio (inset in (f)).
Fig. 1. XRD patterns of nonaligned (a) and magnetically aligned (inset in (a))
(Sm,Pr)Co5 flakes, nonaligned (b) and magnetically aligned (c) (Sm,Pr)Co5/Co
composites fixed in epoxy resin.

X. Wang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 889–892
891
process and another from the residual oleic acid of powders.
Obviously, coating processing should be improved to minimize
oxygen in future experiments.
similar results were reported by Gibson and Putzer [23]. The
findings, that the Co nanoparticles are distributed uniformly on
the surface of the (Sm,Pr)Co5 flakes, were a bit surprising because
ferromagnetic particles tend to form tight aggregates as a result of
magnetic attractions between the particles [25]. However, the
cobalt nanoclusters are small enough to be strongly influenced by
Brownian forces. Because of the shape anisotropy of the Co
nanoparticles, these forces result in the rotation of the particles
(Brownian rotation). This effect probably inhibits the intimate
contact between particles that is necessary for agglomeration
[23]. Besides that, PVP has been widely used as a steric stabilizer
or capping agent to protect nanoparticles from agglomeration
[26]. According to the classical nucleation theory, heterogeneous
nucleation is energetically easier than homogeneous nucleation
[27]. The liquid/solid interfaces act as anchor sites and conse-
quently make the in situ formed nanoparticles attach on the
surface and edges of flakes. Therefore, a high density of uniform
Co nanoparticles decorating (Sm,Pr)Co5 flakes could be obtained.
The hysteresis loops of the aligned (Sm,Pr)Co5/Co nanocompo-
site powders and the aligned uncoated powders are shown in
Fig. 3, which are measured along the direction parallel to the
c-axis. As can be seen for the case of Co coating, the saturation
magnetization Ms increases distinctly. The (Sm,Pr)Co5/Co compo-
sites show an enhanced remanent magnetization Mr of 72 emu/g
with a coercivity Hc of 8.9 kOe as compared with those of
(Sm,Pr)Co5 flakes, Mr¼66 emu/g and Hc¼10.8 kOe.
Based on the XRD, FESEM and EDS studies, the nanoparticles
coated on the surface of the flakes are determined to be Co
nanoparticles (Fig. 2). In theory, Eq. (1) is thermodynamically
feasible and very exothermic [23]. However, it is still kinetically
slow [23,24]. It can be observed that the equilibrium molar ratio
of N2H4 to Co2þ is equal to 0.5. However, the ratio experimented
was purposely much larger than 0.5 to accelerate the reaction.
We propose that the monodispersed colloidal Co nanoparticles
can be the effect of the alkalinity of the reaction solution since it
was found that the formation of Co nanoparticles is easier at this
condition. In an alkaline medium, a large concentration of hydro-
xyl ions keeps the Co ions in its active state of Co(OH)24ꢂ, which
then promotes the reaction. Thus, to narrow down the size
distribution of the particles in the solution, an alkaline environ-
ment during the reduction is required. We suggest that the Co
nanoparticles are in a disk-like shape using the present method,
Fig. 4 shows the SEM images of the 8 h ball milled (Sm,Pr)Co5
flakes and the (Sm,Pr)Co5/Co composite powders after chemical
coating for 30 min. It can be seen that most of the powders appear
as flakes with thickness about 20 nm and a length of about 1.5 mm
(Fig. 4a and b), much smaller than those of the 2 h ball milled
products. Typical SEM images of the (Sm,Pr)Co5/Co composites
are given in Fig. 4c and d. It can be seen that the (Sm,Pr)Co5 flakes
are coated with uniformly distributed nanopartices. Fig. 5 shows
XRD patterns of the ball milled (Sm,Pr)Co5 flakes and (Sm,Pr)Co5/
Co composites. The diffraction peaks can be indexed as the
standard SmCo5 and hexagonal Co, indicating that the nanopar-
ticles attaching on the surface of (Sm,Pr)Co5 flakes are soft
Fig. 3. Hysteresis loops of the aligned (Sm,Pr)Co5 flakes ball milled for 2 h and the
aligned (Sm,Pr)Co5/Co composites.
Fig. 4. SEM images of the (Sm,Pr)Co5 flakes. (a, b) typical SEM images of the ball milled (Sm,Pr)Co5 flakes for 8 h. (c, d) typical SEM images of (Sm,Pr)Co5/Co composites.

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X. Wang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 889–892
51031004 and 50871095) and the National Basic Research Program
of China (Grant no. 2011CB808204).
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The authors gratefully acknowledge the financial support of the
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