Electrodeposition of Sm-Co nanoparticles from aqueous solutions

Article abstract of DOI:10.1016/j.jmmm.2004.04.131

Supported Sm-Co nanoparticles have been synthesized by short pulse electrodeposition using aqueous solutions containing glycine as complexant and buffering agent. Nanoparticle composition is a function of pulse amplitude and pulse duration. Short pulses in particular minimize oxygen incorporation, down to 3 at%. X-ray photoelectron spectroscopy and X-ray diffraction data support the hypothesis that metallic alloys have indeed been obtained by this technique, along with mixed oxides of the metals. In-plane coercivities of up to 5.3 kOe have been achieved in as-plated nanoparticle assemblies when the relative Sm content was about 20 at% and particle size around 80 nm. These Sm-Co nanoparticles hold the promise to be a practical and inexpensive material for use in the synthesis of permanent magnets by powder processing.

Full text of DOI:10.1016/j.jmmm.2004.04.131

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 283 (2004) 89–94
www.elsevier.com/locate/jmmm
Electrodeposition of Sm–Co nanoparticles from
aqueous solutions
Ã
Jianqi Zhang1, Paul Evans2, Giovanni Zangari
Department of Metallurgical and Materials Engineering, Center for Materials for Information Technology, University of Alabama,
P.O. Box 870209, Tuscaloosa AL 35487-0209, USA
Received 21 November 2003; received in revised form 5 April 2004
Available online 8 June 2004
Abstract
Supported Sm–Co nanoparticles have been synthesized by short pulse electrodeposition using aqueous solutions
containing glycine as complexant and buffering agent. Nanoparticle composition is a function of pulse amplitude and
pulse duration. Short pulses in particular minimize oxygen incorporation, down to 3 at%. X-ray photoelectron
spectroscopy and X-ray diffraction data support the hypothesis that metallic alloys have indeed been obtained by this
technique, along with mixed oxides of the metals. In-plane coercivities of up to 5.3 kOe have been achieved in as-plated
nanoparticle assemblies when the relative Sm content was about 20 at% and particle size around 80 nm. These Sm–Co
nanoparticles hold the promise to be a practical and inexpensive material for use in the synthesis of permanent magnets
by powder processing.
r 2004 Elsevier B.V. All rights reserved.
Keywords: Sm–Co; Electrodeposition; Nanoparticles; High-coercivity materials
PACS: 75.50.Tt; 75.50.Vv
The capability to synthesize ferromagnetic
nanometer size particles with large magnetic
anisotropy and small size dispersion is of practical
interest in the development of low noise, thermally
stable magnetic recording media [1] and for the
fabrication of nanocomposite exchange coupled
permanent magnets [2]. Previous work has mainly
focused on the synthesis of FePt nanoparticles
with the L10 structure by colloidal chemistry and
successive annealing [1,3]. Few attempts on the
Ã
Corresponding author. Present address: CESE and Depart-
ment of Materials Science and Engineering, University of
Virginia, Charlottesville VA 22904-4745, USA. Tel: +1-205-
348-2507; fax: +1-205-348-2346.
E-mail address: gz3e@virginia.edu (G. Zangari).
1Present address: Department of Chemical Engineering,
Louisiana State University, Baton Rouge, LA 70803, USA.
2Present address: Department of Pure and Applied Physics,
Queen’s University Belfast, Belfast BT7 1NN, Northern
Ireland, UK.
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2004.04.131

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J. Zhang et al. / Journal of Magnetism and Magnetic Materials 283 (2004) 89–94
other hand have been undertaken to synthesize
nanoparticles of rare earth-transition metal (RE-
TM) alloys by such methods [4], although these
alloys in the bulk exhibit the highest magnetic
anisotropy (SmCo5) and the highest energy pro-
duct (NdFeB). The difficulties involved in synthe-
sizing the complicated unit lattices of the hard
magnetic phases in these alloys [5] and the
susceptibility of the RE element to oxidation are
two possible reasons for the scarcity of efforts in
this respect.
Table 1
Electrolyte composition
Chemical compound
Concentration (mol/l)
Co(NH2SO3)2
SmCl3
0.12 M
0.9 M
0.36 M
1 M
NH2CH2COOH
NH4(NH2SO3)
pH 4.65
Electrodeposition (ECD) of metal nanoparticles
on solid surfaces with low surface energy, such as
highly oriented pyrolitic graphite or hydrogen-
terminated Si is similar to precipitation from
colloidal solutions, because the metal interacts
with the substrate only through weak van der
Waals forces [6]. Electrodeposition presents the
additional advantage that the driving force for
metal formation (the substrate potential) can be
controlled much more precisely and on a much
shorter time scale than during chemical reduction.
These features have enabled the synthesis of
metal nanoparticles with very small size dispersion
(6%) [6].
Electrodeposition of RE-TM alloys from aqu-
eous solutions is very difficult to implement, due to
the low equilibrium potential of the RE element
(E0oꢀ2.3VSHE), which is much lower than the
decomposition potential of water (2H2O+4eꢀ-
2OHꢀ+H2; E0=ꢀ0.828VSHE). At the potentials
necessary to reduce the RE element, strong
hydrogen evolution and a consequent increase in
pH would take place at the interface. Both the RE
and TM would hydrolyze at pH>6 [7]; in
addition, the RE may interact strongly with
dissolved oxygen or hydroxyl ions. Both these
phenomena would favor the formation and
incorporation of oxides or hydroxides of the
metals (mainly of the RE) instead of the metallic
alloy. For this reason, electrodeposition from
organic solutions or molten salts has been almost
exclusively pursued in the attempt to grow RE-TM
alloys [8]. However, more recently it has been
shown that it is possible to grow RE-TM alloys
from aqueous solutions [9,10]. In particular, the
patent by Nobe et al. [10] claims that high-quality
electrodeposits of RE-TM alloys could be
obtained by addition in the solution of amino-
acids with a single carboxyl group, such as glycine
(NH2CH2COOH). Trivalent lanthanide ions, and
Sm in particular, are in fact known to form stable
complexes with oxygen donor ligands [11].
In this work, we report on the synthesis and
magnetic properties of Sm–Co nanoparticles with
high coercivity by short pulse electrodeposition
from aqueous electrolytes, using glycine as a
complexing and buffering agent.
Sm–Co alloy nanoparticles were electrodepos-
ited from the electrolyte reported in Table 1 [12],
using short (5–100 ms) galvanostatic pulses. In
order to avoid nanoparticle coalescence and the
formation of continuous films, the electrodeposi-
tion process was limited to the application of one
single pulse. All solutions were freshly prepared
with analytical-grade reagents and 18 MO water.
The substrates were single crystal n-Si(1 0 0) (Sb-
doped) with a high dopant concentration of
1017 cmꢀ3. An ohmic back contact was made to
the Si using a Ga75In25 eutectic, which is liquid at
room temperature. To hydrogen terminate the Si
surface, immediately prior to deposition the
substrates were etched in a 10 wt% solution of
HF for 20 s, then rinsed in deionized water and
dried in a stream of nitrogen. A Pt net was used as
a counter-electrode and the solution was not
stirred during deposition. The electrolyte was
sparged with N2 gas for 20 min before deposition
and also during the deposition to minimize the
amount of oxygen dissolved in solution. Composi-
tion of the supported nanoparticles was deter-
mined by energy dispersive spectroscopy attached
to a Philips XL 30 SEM, while the chemical states
of the elements in the alloy were determined by
X-ray photoelectron spectroscopy (XPS; Kratos

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J. Zhang et al. / Journal of Magnetism and Magnetic Materials 283 (2004) 89–94
91
100
80
Axis 165). Atomic force microscopy (AFM; DI
Dimension 3000) was used to examine the surface
morphology of the supported nanoparticles and to
determine particle size and their size distribution.
X-ray diffraction (XRD, Philips X’Pert) was
utilized to determine crystalline structure. Mag-
netic properties were measured at room tempera-
ture and up to 14 kOe, using an alternate gradient
force magnetometer (Princeton). Hydrogen evolu-
tion is an important side reaction during the
electrodeposition process, and its extent is strongly
dependent on pulse length. As a consequence, it is
very difficult to determine the efficiency of the
deposition process—and thus the amount of
deposited material—for short deposition times.
This forbids us to calculate or quote absolute
magnetization values in the following.
Figs. 1(a) and (b) show the relative atomic
percentage of Sm ðSm=ðSm þ CoÞÞ in the nano-
particles as a function of pulse amplitude (current
density cd 0.1–1.5 A/cm2) and pulse duration
(5–100 ms). Co relative atomic percentage is the
balance. A strong dependence of the composition
of the nanoparticles on both variables was
observed. The nanoparticles contain also a rele-
vant amount of oxygen, which also varies widely
with deposition parameters. As an example, Fig. 2
displays the oxygen atomic ratio (O/(O+Sm+Co))
as a function of pulse time for a pulse amplitude of
0.2 A/cm2. The observed trend of the metal ratio
(Sm/(Sm+Co)) with pulse amplitude follows the
dependence of alloy composition vs. current
density normally observed in alloy ECD, with
the amount of the less noble metal increasing with
cd. With increasing pulse duration instead Sm
content decreases, while oxygen content increases
up to a maximum of about 50 at%. This indicates
that direct current electrodeposition—which
would occur for longer times and thus lead to
high oxygen incorporation—would not be prac-
tical for the synthesis of RE-TM metallic alloys.
The decrease in Sm content with longer pulse
durations can be understood in terms of an
initially fast reduction kinetics of Sm3+, which
slows down once Sm slowly hydrolyses [13] as a
result of the water splitting reaction and related
pH increase. The increase in oxygen content in the
films with increasing pulse duration is probably a
t
pulse = 20 ms
60
40
20
0
0
0.4
0.8
1.2
1.6
(a)
Pulse Amplitude (A/cm2)
100
80
60
40
20
0
0.1 A/cm2
0.5 A/cm2
1 A/cm2
0
20
40
60
80
100
(b)
Pulse time (ms)
Fig. 1. Nanoparticle composition expressed as atomic ratio of
the metallic elements (Sm/(Sm+Co)) as a function of (a) pulse
amplitude, for a pulse duration of 20 ms, (b) pulse duration, for
three representative pulse amplitudes.
consequence of the hydrolysis of Sm and Co
following the pH increase, which leads to the
successive formation, surface adsorption and
incorporation of Sm(OH)x as well as Co(OH)x
compounds in the growing nanoparticles. It is
important to note that for short pulse duration
(few ms) the oxygen content can be as low as 3 at%
(Fig. 2). This is essential to achieve the synthesis of
metallic alloys, which would also maximize the
magnetic moment of the nanoparticles.
XPS analysis (Fig. 3 shows a typical survey
scan) demonstrates that oxygen and carbon are the

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mostly in the metallic form in the samples with low
50
40
30
20
10
0
oxygen content, as expected. XRD shows the
formation of a HCP phase with lattice constants
slightly larger than that of Co, with variable
crystallinity. In summary, the analytical and
structural evidence point towards the synthesis of
nanoparticles consisting of a mixture of a HCP
Sm–Co metallic alloy phase and a variable fraction
of mixed oxides of the metals. The oxide fraction
can be minimized by keeping the pulse time as
short as possible. More quantitative conclusions in
this respect cannot be drawn at this point.
0.1 A/cm2
0.3 A/cm2
1 A/cm2
1.5 A/cm2
Typical morphologies of the supported particles
grown at 0.2 A/cm2 for increasing time
(20–100 ms) are shown in the AFM images of
Fig. 4. As expected, Volmer–Weber growth of
Sm–Co with the formation of seemingly hemi-
spherical clusters is observed. Isolated nanoparti-
cles are clearly seen on the background of the Si
substrate at short pulse duration, with a very
limited size distribution. The average particle size
increases with pulse time and decreases with pulse
amplitude, as expected from nucleation and
growth theory of electrodeposition in non-inhib-
ited conditions [14]. With increasing pulse time,
particle growth eventually leads to particle coales-
cence and to the formation of continuous films.
As shown above, pulse duration has an influence
both on alloy composition and on nanoparticle
size; it follows that in order to control both
quantities it is necessary to fine tune pulse
amplitude and duration accordingly. In particular,
this is essential in order to achieve optimum
magnetic properties, as shown in the following.
All the particles in the as-plated state are ferro-
or superparamagnetic. In most cases particle
coercivity Hc is low (o100 Oe); however, maxima
in Hc of up to 5.3 kOe in plane were observed at
intermediate pulse amplitude and duration. It was
found that deposition conditions leading to high
coercivities were those which yielded alloys with
Sm relative atomic ratio (Sm/(Sm+Co))ꢁ20 at%
and apparent average grain size—as measured by
AFM—of about 80 nm (Figs. 5 a and b). A steep
decrease in coercivity was observed as soon as
grain size or composition departed from these
optimum conditions. The observed maxima in
Hc are reasonable; in fact, HCP Sm–Co solid
0
20
40
60
80
100
Pulse time (ms)
Fig. 2. Oxygen relative content (O/(O+Sm+Co)) in the
nanoparticles as function of pulse duration, for some repre-
sentative pulse amplitudes.
Sm 3d
8000
Sm Auger
Co 2p
Sm Auger
O 1s
Sm 4p
Sm 4d
C 1s
4000
After Etching
-200
Before Etching
0
-1200 -1000 -800
-600
-400
0
Binding Energy (eV)
Fig. 3. Typical XPS survey scan of supported CoSm nanopar-
ticles (0.7 A/cm2, 10 ms), showing that C and O are the only
impurities in the material.
only impurities in the film, C content being of the
order of 1 at%. A detailed analysis of the chemical
states of the metals was also performed by
analyzing the 3d core levels of Sm and the 2p core
levels of Co. Prior to sputtering, both Sm and Co
are in their oxidized states, most probably as a
consequence of the exposure to air. After sputter-
ing away the surface layer, however, Sm is in a
mixed metallic/oxidized form, and Co shows a
2 eV chemical shift in its 2p peaks, 2.5 eV being the
typical shift for TM hydroxides. Co and Sm are

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93
104
1000
100
i t y
10
0
20
40
60
80
100
(a)
Sm/(Sm-Co) at%
104
1000
100
10
0
100
200
300
400
(b)
Average grain size, nm
Fig. 5. Nanoparticle coercivity as a function of (a) Sm relative
atomic ratio (Sm/(Sm+Co)) (b) average grain size as measured
by AFM. Note that the data seem to follow a unique trend
although nanoparticles grown under different conditions are
included in the graph.
solutions containing 20 at% Sm would exhibit a
magnetocrystalline anisotropy higher than Co due
to the strong deformation of the HCP Co
structure. In addition, the observed apparent grain
size of high Hc nanoparticles is well in the single
domain range [15], which favors magnetization
rotation and high coercivity. The observed mag-
netic properties indirectly support the hypothesis
that Sm–Co alloys have indeed been synthesized
by this technique, along with mixed oxides of the
metals.
Fig. 4. Morphology of the supported nanoparticles as deter-
mined by AFM imaging. Pulse amplitude 0.2 A/cm2, pulse
duration as indicated.

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J. Zhang et al. / Journal of Magnetism and Magnetic Materials 283 (2004) 89–94
outstanding magnetic properties, which could be
used, for example, in the powder processing of
permanent magnets.
1
0.5
0
This work was supported by US DoD/DARPA
through ARO under grant DAAD19-03-1-0038
and made use of the NSF-MRSEC shared facil-
ities, grant no. DMR 98-09423.
20 ms
60 ms
100 ms
-0.5
-1
Note added in proof
-8000
-4000
0
4000
8000
Recently, a paper has been published which
discusses a possible mechanism for RE–TM
codeposition when glycine is used as a complexing
agent [16].
Magnetic Field (Oe)
Fig. 6. In-plane hysteresis loops of nanoparticle assemblies
grown at 0.5 A/cm2 with varying pulse duration.
A continuous variation in the magnetic proper-
ties with increasing pulse time was in general
observed (Fig. 6). Nanoparticles grown for short
times (o20 ms) exhibit very low coercivity and
high saturation fields, probably as a consequence
of their small size. Nanoparticles grown for longer
times instead show open hysteresis loops, with
high Hc for intermediate pulse times and decreas-
ing coercivity at longer pulse times. The decrease
in coercivity occurs when the gradual increase in
grain size leads to grain coalescence and to the
formation of a continuous film. This likely
corresponds to the transition from single domain
to exchange coupled behavior.
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Summarizing, we have successfully synthesized
supported Sm–Co nanoparticles of various com-
positions and with oxygen content down to 3 at%.
Although we have no definitive evidence for alloy
formation, oxidation states of the elements and
crystalline structure seem to imply that at least a
relevant fraction of the particle volume is com-
prised of a metallic alloy, the remaining fraction
consisting of mixed oxides of the metals. Further
evidence for the synthesis of a metallic alloy is the
observation of high coercivities (up to 5.3 kOe)
in as-plated nanoparticles when Sm relative ato-
mic ratio was ꢁ20 at% and particle size about
80 nm. This result opens novel opportunities for
the synthesis of magnetic nanopowders with
[10] K. Nobe, M. Schwartz, L. Chen, N.S. Myung, US Patent #
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Fall 2001 Meeting of the Electrochemical Society, ECS
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