Open Journal of Composite Materials, 2012, 2, 119-124
http://dx.doi.org/10.4236/ojcm.2012.24014 Published Online October 2012 (http://www.SciRP.org/journal/ojcm)
119
Magnetic Properties Hard-Soft SmCo5-FeNi and
SmCo5-FeCo Composites Prepared by Electroless Coating
Technique
M. Lamichanne1, B. K. Rai1, S. R. Mishra1*, V. V. Nguyen2, J. P. Liu2
1Department of Physics, The University of Memphis, Memphis, USA; 2Department of Physics, The University of Texas, Arlington,
USA.
Received May 25th, 2012; revised June 27th, 2012; accepted July 8th, 2012
ABSTRACT
Composites of SmCo5-FeNi and SmCo5-FeCo, hard-soft magnetic materials, have been synthesized via electroless
plating of magnetically hard SmCo5 powder particles with magnetically soft FeNi and FeCo, respectively. The influence
of coating thickness of soft magnetic layers on the structure and magnetic properties of the composite has been studied.
Overall FeNi coating was found to be less dense compared to FeCo for the same plating duration. Structurally the coat
ing was found to be nodular in morphology. These coating have dramatic effect on the overall magnetic property of the
composite. As compared to FeNi coated SmCo5 composite, two-fold increase in the saturation magnetization has been
observed upon coating SmCo5 (Ms ~ 28 emu/g) with FeCo to a value 56 emu/g. The coercivity of composite powder
was found to decrease with increasing the coating layer thickness. The absence of exchange spring behavior in the
hard-soft composite is attributed to magnetically soft layer thickness exceeding the theoretical length limit for ex-
change-spring coupling.
Keywords: Electroless Plating; Hard-Soft Composites; Exchange Spring
1. Introduction
Permanent magnetic materials are characterized by their
figure of merit, (BH)max energy product, which is the
maximum rectangular area under the second quadrant of
the hysteresis loop. The increase in (BH)max is possible
by increasing the coercivity (HC) and the saturation
magnetization (Ms). For materials with high HC values
(HC > 2Ms), the theoretical limit for the energy product
is limited only by Ms and is given by (BH)max  (2Ms)2.
Methods to overcome this limitation have focused on
developing high anisotropy materials with high Ms and
Curie temperatures (TC), materials which are based on
rare-earth (RE)-transition metal (TM) intermetallics such
as SmCo5, Sm2Fe17 and their boride or nitride com-
pounds such as Nd2Fe14B and Sm2Fe17Nx [1-3]. In spite
of the considerable improvement in the magnetocrystal
line anisotropy, these materials still exhibit lower Ms
than Co, Fe or Fe65Co35, which have 4Ms values of 18,
21, and 24 kG, respectively [1].
The concept of an “Exchange-Spring” or “Exchange-
hardened” magnet was proposed by Kneller and Hawig
[4] to overcome the limitations outlined above. The
spring magnet is essentially a composite of two ferro-
magnetic (FM) phases; hard and soft magnetic phases.
The soft phase, rich in transition metal, enhances satura
tion magnetization, while the hard phase provides the
required magnetic anisotropy and stabilizes the exchange
coupled soft phase from demagnetization. Most of the re-
ported literature on spring magnets is based on Nd2Fe14B
or Sm2Fe17 (hard)-α-Fe or Fe3B (soft) [5-7]. Insufficient
temperature stability and poor corrosion re sistance are
main factors limiting applications of Nd2Fe14B-based
magnets. However, the family of sa marium based alloys
such as SmCo5 and Sm2Fe17Nx could be an attractive
candidate material since it can be used as the hard phase
due to its high magnetization (SmCo5, Ms ~ 0.84 MAm−1,
Sm2Fe17Nx, Ms ~ 1.22 MAm−1) and high Curie tempera-
ture, TC, (SmCo5, TC ~ 720˚C, Sm2Fe17Nx, TC ~ 476˚C)
which are comparable to those of Nd2Fe14B (Ms ~ 1.28
MAm−1, TC ~ 312˚C, HC ~ 6.0 MAm−1) [8].
The exchange-spring magnets have been fabricated by
melt-spinning, hot-compaction [9], mechanical alloying
[10,11], however these techniques yields nanocomposites
with random dispersion and orientation of hard magnetic
phase and often having no control on the growth of soft-
phase [6,12]. Thus, if the soft phase size could be re-
*Corresponding author.

Magnetic Properties Hard-Soft SmCo5-FeNi and SmCo5-FeCo Composites Prepared by Electroless Coating Technique
120
duced and highly dispersed throughout the magnet, then
the interaction between hard-soft phases could be sig-
nificantly enhanced, leading to significant improvement
in (BH)max energy product. Various techniques such as
sputtering, pulse laser deposition (PLD), electrolytic
coating, and electroless coating [13] have been explored
to increase the interaction between hard-soft phases of
the composite.
Majority of electroless coating studies are limited to
Nd-Fe-B powder [14-16] and were carried out mainly
with a purpose of improving the corrosion resistance of
the magnet. However, the use of electroless plating
method has not been yet fully explored for the purpose of
developing hard-soft exchange-spring magnets [17].
Considering compatible magnetic properties of SmCo5 to
Nd-Fe-B, and high magnetization of FeNi and FeCo as
compared to
 -Fe, the present study explores the synthe-
sis of hard-soft phase SmCo5-FeNi/FeCo composites via
electroless plating technique. Electroless plating method
is known to produce uniform adhesive coating on a wide
variety of substrate. In the electroless plating method, the
autocatalytic plating baths contain reducing agents ready
to react with the substrate in the presence of catalyst, a
metal which is to be deposited on the substrate. The
property of the coated film depends on time, temperature,
and catalyst concentration. In this article, magnetic prop-
erties of hard-soft phase composite are studied as a func-
tion of thickness of the soft magnetic layer.
2. Experiment
The hard-soft composites, with a choice of SmCo5 as a
hard phase and FeNi and FeCo as soft phase, were pre-
pared using modified electroless plating method [17].
SmCo5 powder of average grain size ~50 micron and
99.5% purity was obtained from Sigma Aldrich. SmCo5
was degreased in acetone solution and activated in 0.5 M
sodium hypoposhite at 90˚C for 10 min. An electroless
plating bath was prepared by using 0.09 mol of nickel
sulfate (NiSO4·6H2O) or cobalt sulfate (CoSO4·7H2O),
0.07 mol of iron sulfate (FeSO4·7H2O), 0.5 mol of so-
dium hypophosphite (NaH2PO2·H2O), 0.3 mol of sodium
citrate (Na3C6H5·2H2O), and 0.1 mol of ammonium sul-
fate ((NH4)2SO4). Activated SmCo5 powder was intro-
duced into the electroplating bath. The pH value of the
plating bath was maintained at 8 - 10 by using sodium
hydroxide. The plating was done at temperature 80˚C.
The electroless plating of FeNi was carried out for the
duration of 15, 30, and 60 minutes and for FeCo was
carried out for 30, 60, 90, and 120 min. Afterward elec-
troplated SmCo5 composite powder was rinsed with the
de-ionized water and dried in the presence of argon gas
at 90˚C for three hours. The X-ray diffraction (XRD)
patterns of the composite were obtained using Bruker D8
Advance diffractometer using Cu K

radiation. The sur-
face morphology and composition of the composites
were studied using scanning electron microscope (SEM)
and energy dispersive X-ray (EDX) spectroscopy, re-
spectively. The room temperature magnetic measure-
ments were made by the Alternating Gradient Magne-
tometer (AGM, Princeton Measurement Corp.) in the
maximum field of 14 kOe.
3. Results and Discussion
X-ray diffraction pattern of SmCo5 powder electoplatted
with FeNi and FeCo is shown in Figures 1(a) and (b),
respectively. Presence of strong FeNi and FeCo peaks is
recorded. With the electroless plating time, the peak in-
tensity of FeNi and FeCo increases and correspondingly
the peak intensity of SmCo5 decreases. Difference in the
growth of FeNi and FeCo is also evident. The presence
of broad FeNi peaks shows that initially FeNi layers are
amorphous. Furthermore, the presence of SmCo5 peaks
even after 60 minute of plating time shows the formation
of thin layer of FeNi. However, the full-width-at-half-
maximum (FWHM) of FeNi peak de creases with the
plating time. This decrease of FWHM of FeNi peaks
indicate a crystalline growth of FeNi, a growth which
results upon thickening of FeNi layers. On the other hand,
even for short plating duration ~15 min, thick and crystal-
line FeCo layers are formed as evident from the presence
of sharp FeCo diffraction peaks. Furthermore, gradual
reduction in SmCo5 peaks is noticed with the plating time;
which clearly indicates the thickening of FeCo layers
over SmCo5 particles.
Figures 2 and 3 show SEM images of FeNi and FeCo
coated SmCo5 composite powders, respectively. These
SEM images show that in all cases the coated layer is
neither smooth nor uniform. A nodule like coating struc-
ture appears. The EDX elemental analysis confirms that
both FeNi and FeCo are in 30:70 weight ratios. This is
further confirmed by the X-ray diffraction peaks indexed
by ICDD data base. The ICDD index number 71-8326
and 50-0795, for FeNi and FeCo, respectively show that
soft layers are iron deficient. Pres ence of trace amount
of phosphorous is also detected in EDX.
Figures 4(a) and (b) show the room temperature hys-
teresis loops of SmCo5-FeNi and SmCo5-FeCo compos-
ites, respectively. The hysteresis loops of electroless
plated SmCo5 particles are also compared with SmCo5
particles. As it can be seen from these plots, that FeNi
and FeCo both have different influence on the overall
magnetic properties of these composites. Table 1 lists the
room temperature magnetic parameters extracted from
the hysteresis loop of these composites.
The observed decrease in magnetization of FeNi
coated SmCo5 composite (15 minute sample) may result
partly from the increased weight of the sample and
amorphisity of the FeNi layer. With the increase in the

Magnetic Properties Hard-Soft SmCo5-FeNi and SmCo5-FeCo Composites Prepared by Electroless Coating Technique 121
Intensity ( a.u)
7060504030
2 (degree)
SmCo5
FeNi
SmCo
5
t = 15 min.
t = 30 min.
t = 60 min.
(a)
Intensity (a.u)
7060504030 2 (degree)
SmCo
5
FeCo
SmCo
5
t = 30 min.
t = 60 min.
t = 90 min.
t = 120 min.
(b)
Figure 1. (a) X-ray diffraction pattern of SmCo5 powder
coated with FeNi as a functi on of electroless plating time; (b)
X-ray diffraction pattern of SmCo5 powder coated with
FeCo as a function of electroless plating time.
plating time, the magnetization seems to increase be-
cause of the growth of FeNi crystals. This is also evident
from monitoring the FeNi peaks in the XRD results. On
the contrary, in case of SmCo5-FeCo composites, highly
crystalline FeCo coating enhances the saturation mag-
netization of the composite by almost a factor of two.
This is because FeCo has higher magnetization than
FeNi.
The coercivity of composites was observed to decrease
upon coating as depicted in inset Figure 4 and Table 1.
The observed decrease in coercivity in SmCo5-FeCo is
more than that of SmCo5-FeNi. The decrease in coercivi-
ity is schematically explained in Figure 5. Assuming
SmCo5 as a spherical particle coated with thick and
magnetically soft phase layer of FeCo or FeNi. Under the
(a)
(b)
(c)
Figure 2. SEM images of SmCo5-FeNi composites electroless
plated for (a) 15; (b) 30; and (c) 60 minutes.
Figure 3. SEM images of SmCo5-FeCo composites elec-
troless plated for (a) 30; (b) 60; (c) 90; and (d) 120 minutes.
external field the soft layer is magnetized quickly. If this
soft phase layer (shell) is thick and coats entirely the hard
phase sphere (core) then the core cannot see the external
field but only see the field created by the magnetization

Magnetic Properties Hard-Soft SmCo5-FeNi and SmCo5-FeCo Composites Prepared by Electroless Coating Technique
122
1600
1400
1200
1000
Coercivity (Oe)
6040200
Plating Time (minutes)
20
10
0
-10
-20
Magnetization (emu/g)
10x10
3
50-5-10
Field
(
Oe
)
SmCo
5
-FeNi
15 min.
30 min.
60 min.
(a)
1600
1200
800
400
Coercivity (Oe)
12080400
Plating Time ( minutes)
40
20
0
-20
-40
Magnetization (emu/g)
10x10
3
50-5-10
Field (Oe)
SmCo
5
-FeCo
30 min.
60 min.
90 min.
120 min.
(b)
Figure 4. (a) Room temperature hysteresis loops of SmCo5-
FeNi composites. Coercivity of composites as a function of
plating time is shown in the inset; (b) Room temperature
hysteresis loops of SmCo5-FeCo composites. Coercivity of
composites as a function of plating time is shown in the in-
set.
Table 1. Magnetic parameters obtained from hysteresis
loop measured at room temperature.
Coating Time
(min)
Ms
(emu/g) HC (Oe) Mr (emu/g) Mr/Ms
SmCo5 -- 27.99 1770 10.480.37
SmCo5-FeNi15 15.83 1725 6.93 0.43
30 18.49 1325 7.60 0.41
60 20.96 825 6.70 0.31
SmCo5-FeCo30 49.98 550 10.680.21
60 55.38 475 13.660.24
90 56.71 301 8.98 0.15
120 55.97 275 8.82 0.15
H
+
H=
i
H
c
H=0
Figure 5. A model of the magnetic behavior of the hard
phase sphere coated by the thick soft phase layer.
vector of the soft phase layer. When the Hext is applied,
the magnetization in the soft phase shell is saturated first;
the magnetization in the hard phase core has some mod-
erate value dependent on the Ms of the shell. When the
Hext is being reduced to zero, the M inside the shell is
rapidly approaches zero but the M inside the core de-
creases to some value. Next assume Hext goes on to the
value iHC which corresponds to the zero averaged value
of magnetization. Since in this case the M inside the core
is not zero (arrow in the core, right picture of Figure 5,
that means that this value iHC must be smaller than the
value of the coercivity of the core hard phase when no
shell covers it. Thus, thicker shell the smaller the iHC.
Overall exchange-spring hardening is not observed in
these composites. The absence of exchange spring phe-
nomenon in SmCo5-FeNi/FeCo composites may be at-
tributed to large thickness/grain size of soft FeNi and
FeCo layers, which are much larger than the domain wall
width of SmCo5 (

B ~ 3 - 6 nm) [18-20]. This cannot guar-
antee that the entire volume of FeNi or FeCo is exchange
hardened by the SmCo5 phase because the exchange
coupling requires grain sizes of the order of

B ~ 8 nm.
4. Conclusion
In summary we have successfully coated hard SmCo5
particles with soft-magnetic FeNi and FeCo via elec-
troless plating technique. The electroless coating resulted
in a thick uniform coating more so for FeCo than FeNi
for similar plating duration. Adding magnetically soft
phase into the SmCo5-FeNi/FeCo composite enhances its
magnetization but decreases its coercivity. This coerciv-

Magnetic Properties Hard-Soft SmCo5-FeNi and SmCo5-FeCo Composites Prepared by Electroless Coating Technique 123
ity reduction is explained on the bases of increased
amount of magnetic softness of the coated layer. An in-
dependently tuning the size and composition of the indi-
vidual building blocks of the composite is necessary in
order to enhance exchange coupling between hard-soft
phases of the composite. It is expected that composite
magnets of SmCo5-FeNi or FeCo could be obtained with
core-particles’ coercivity exceeding 10 kOe, which can
be attained either via ball milling technique [21-23] or
wet-chemical method [24]. It is also evident from the
present study that the growth rate of soft magnetic layer
needs to be better controlled in order to form a fine uni-
form coating of a few nanometer thicknesses on core
particles to meet the theoretical exchanges spring inter-
action length limit.
5. Acknowledgements
This work was supported by DMR NSF-EAGER, Grant
#0965801.
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