Magnetic and microwave absorbing properties of Co-Fe thin films plated on hollow ceramic microspheres of low density

Article abstract of DOI:10.1016/j.jmmm.2003.09.012

Conductive and magnetic microspheres are fabricated by plating of Co-Fe alloy thin films on hollow ceramic microspheres of low density for the application to lightweight microwave absorbers. Metal plating was carried out in a two-step electroless plating process (pre-treatment of sensitizing and subsequent plating). Uniform coating of the film with about 2μm thickness was identified by SEM. High-frequency magnetic and microwave absorbing properties were determined in the rubber composites containing the metal-coated microspheres. Due to the conductive and ferromagnetic behavior of the Co-Fe thin films, high dielectric constant and magnetic loss can be obtained in the microwave frequencies. In particular, the magnetic loss increases with Fe content in the alloy films and its frequency dispersion can be explained by ferromagnetic resonance theory. Due to the electromagnetic properties, high absorption rate and thin matching thickness are predicted in the composite layers containing the metal-coated microspheres of low density (about 0.8g/cc) for the electromagnetic radiation in microwave frequencies.

Full text of DOI:10.1016/j.jmmm.2003.09.012

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 271 (2004) 39–45
Magnetic and microwave absorbing properties of Co–Fe thin
films plated on hollow ceramic microspheres of low density
Sung-Soo Kima,*, Seon-Tae Kima, Joon-Mo Ahnb, Keun-Hong Kimb
a Department of Materials Engineering, Chungbuk National University, Gaesindong San 41, Cheongju 361-763, South Korea
b Agency for Defense Development, Yoosung P.O. Box 35, Taejon, South Korea
Received 6 June 2003; received in revised form 27 August 2003
Abstract
Conductive and magnetic microspheres are fabricated by plating of Co–Fe alloy thin films on hollow ceramic
microspheres of low density for the application to lightweight microwave absorbers. Metal plating was carried out in a
two-step electroless plating process (pre-treatment of sensitizing and subsequent plating). Uniform coating of the film
with about 2 mm thickness was identified by SEM. High-frequency magnetic and microwave absorbing properties were
determined in the rubber composites containing the metal-coated microspheres. Due to the conductive and
ferromagnetic behavior of the Co–Fe thin films, high dielectric constant and magnetic loss can be obtained in the
microwave frequencies. In particular, the magnetic loss increases with Fe content in the alloy films and its frequency
dispersion can be explained by ferromagnetic resonance theory. Due to the electromagnetic properties, high absorption
rate and thin matching thickness are predicted in the composite layers containing the metal-coated microspheres of low
density (about 0.8 g/cc) for the electromagnetic radiation in microwave frequencies.
r 2003 Elsevier B.V. All rights reserved.
Keywords: Microwave absorber; Cobalt-iron thin films; Hollow microspheres; Electroless plating; Complex permeability
1. Introduction
which restricts their usefulness in applications
requiring lightweight mass [1,2]. Moreover, those
materials have difficulties in increasing the perme-
ability in GHz region because of Snoek limit
for ferrites [3] or eddy current loss for magnetic
metals [4].
As one of the ways to overcome these problems,
the use of metal thin films coated on microspheres
(or microballoons) of low density has been
suggested. Gindrup [5] proposed an electromag-
netic radiation absorptive composition, which
comprises microballoons having a thin coating of
metal (Ag) on the microsphere surface. The result
has significance in providing a new technique to
In Recent years, microwave absorptive materials
draw more attention because of its widespread
applications for many electromagnetic compat-
ibility (EMC) purposes. A number of materials
have been described in the prior arts, which are
capable of absorbing electromagnetic radiation.
However, the conventional absorptive materials
such as metal powders and ferrites are quite heavy,
*Corresponding author. Tel.: +82432612418; fax:
+82432713222.
E-mail address: sskim@chungbuk.ac.kr (S.-S. Kim).
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2003.09.012

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S.-S. Kim et al. / Journal of Magnetism and Magnetic Materials 271 (2004) 39–45
produce lightweight absorbers in microwave
frequencies. However, since the coating on micro-
spheres is confined to only single layer of Ag
(which has only conduction loss), the micro-
wave absorption was inferior to conventional
absorptive materials such as ferrite or carbonyl
iron.
Seven grams of microspheres were placed in
a funnel with a catalyst solution of PdCl2
(0.01 mol/l), SnCl2 ꢀ 2H2O (0.01 mol/l) and HCl
(100 mol/l). This mixture was shaken at 50ꢁC for
30 min. The microspheres were then washed with
distilled water, gravity filtered, and air dried at
60ꢁC.
In resonant absorbers of quarter wavelength,
zero-reflection can be obtained by access to wave
impedance matching at the surface of the absorb-
ing layer, which requires a proper combination of
magnetic permeability and dielectric permittivity
at a given thickness and frequency [6]. Those
material parameters in high frequencies can be
controlled by use of magnetic metals (permittivity
control by electrical property and permeability
control by magnetic property) in a single-layered
microwave absorber.
The present study proposes Co–Fe alloy thin
films as the coating layer on the hollow micro-
spheres. According to the bulk magnetic properties
of Co–Fe alloy [7], the crystal anisotropy field is
sensitively reduced by Fe addition in Co-rich
compositions by change of easy-axis of magnetiza-
tion. It was expected that frequency dispersion
of magnetic permeability could also be controlled
by Fe content in the alloy films. The Co–Fe thin
films were coated on the microspheres by electro-
less plating and their high-frequency properties
(complex permeability and permittivity) and mi-
crowave absorbance were investigated in GHz
frequencies.
1 g of the pre-treated microspheres was placed in
500 ml of plating solution containing 8.0 g of salt
of cobalt (CoSO4 ꢀ 7H2O) and salt of iron (FeSO4 ꢀ
7H2O), 8.5 g of reducing agent (NaPH2O2 ꢀ H2O),
57.5 g of complexing agent (Na2C2H4O6 ꢀ H2O)
and 10 ml of pH modifier (NH4OH), and the
mixture was tumbled at 80ꢁC for 1 h. The mol
ratio of FeSO4 ꢀ 7H2O and CoSO4 ꢀ 7H2O was
controlled in the range that the Fe content in the
film is 5–10 at%. During plating, pH was con-
trolled to be 8.5. The metal-coated microspheres
were gravity filtered, washed with distilled water
and air dried at 60ꢁC. The batch was repeated
three times for the complete coating of the metal
film.
Uniformity of coating layer was identified by
two methods. One is by measurement of apparent
electrical resistance of powder compacts and
the other is by direct observation using SEM.
Apparent resistance was measured in a compact
of 0.05 g metal-coated microspheres pressed
slightly in an insulated cylinder mold of 10 mm
inner diameter.
The complex permeability and permittivity
was measured in the composite specimens of the
metal-coated microspheres dispersed silicone rub-
ber matrix. The mixing ratio of particles to
rubber was 1:1 by weight (5.5:4.5 in volume ratio).
Toroid sample (inner diameter of 3 mm, outer
diameter of 7 mm, and thickness of about 2 mm)
was inserted in a standard coaxial sample holder
(APC-7 beadless air line), and reflection coefficient
(S11 parameter) and transmission coefficient (S21
parameter) were measured by using HP8722D
network analyzer. Frequency range of the mea-
surement was 1–18 GHz. The complex per-
mittivity and permeability was calculated from
the S11 and S21 parameters. Reflection loss
was determined by measuring the S11 parameter
after rear face of sample was terminated by
metal.
2. Experimental procedure
Commercially available hollow microspheres
were obtained from PQ Cooperation (Valley
Forge, PA, USA). The microspheres have a hollow
ceramic (silicate) shell of a few mm thickness and
thus having a low density of 0.2 g/cc. Metals (Co
and Co–Fe alloy) plating was carried out in a two-
stepoepration in which the surface of micro-
spheres is first sensitized by treatment with salts of
metal (PdCl2–SnCl2), followed by chemically
reducing the salts of metals (CoSO4 ꢀ 7H2O and
FeSO4 ꢀ 7H2O) using a mild reducing agent of
NaPH2O2 ꢀ H2O.

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S.-S. Kim et al. / Journal of Magnetism and Magnetic Materials 271 (2004) 39–45
41
3. Results and discussion
small. Low value of apparent electrical resistance
(about 0.1 O) measured in the powder compacts of
the Co-coated microspheres also provides an
indirect evidence of complete coating.
Starting with 1.0 g non-coated microspheres, the
weight gain was measured to be 3.5 g after plating.
Due to this weight gain, the density was also
increased from 0.2 g/cc (non-coated microspheres)
to 0.8 g/cc (after plating). The thickness of film
roughly estimated from the weight gain and
theoretical density was about 2 mm, which was
also identified by SEM observation.
Fig. 3 shows the complex permeability (mr ¼
m0r À jm0r0) and permittivity (er ¼ er0 À je0r0) determined
in the composite of non-coated microspheres and
silicone rubber matrix of which weight ratio was
1:1. Because of non-magnetic property of the
constituent materials, the complex permeability
was determined to be m0r ¼ 1 and mr00 ¼ 0: Small
value of dielectric constant (e0r ¼ 2:6) and negligi-
bly small dielectric loss (e0r0D0) was estimated.
The Co film plated on the microspheres
increases both magnetic permeability and dielec-
tric permittivity as shown in Fig. 4. In particular,
the increase of magnetic loss (m0r0) and dielectric
constant (e0r) is evident. Slightly increasing function
of m0r0 (from zero at 1 GHz to 0.26 at 18 GHz) and
decreasing function of m0r (from 1.2 at 2 GHz to 0.8
at 18 GHz) with frequency is observed. Due to
high anisotropy field of Co thin film, the magnetic
loss appears in the high frequencies.
Fig. 1 shows the SEM observation of the non-
coated silica microspheres and their surface
structure. Spherical particles with an average
diameter of about 70 mm were observed. No grain
boundary observed in the surface structure in-
dicates that the silica shell is composed of glassy
phase.
Fig. 2 shows the microstructure of the micro-
spheres coated with Co films by electroless plating.
Uniform coating of Co film and its smooth surface
morphology can be observed. Some small irregular
particles are considered to be free Co precipitates
formed during plating. But their amount is quite
Fig. 1. SEM morphology of (a) hollow microspheres and (b) its
surface structure.
Fig. 2. SEM morphology of Co-coated microspheres.

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S.-S. Kim et al. / Journal of Magnetism and Magnetic Materials 271 (2004) 39–45
2
1
0
3
'
εr
2
1
µr
'
µr''
εr''
0
3
6
9
12
15
18
FREQUENCY [GHz]
Fig. 3. Material parameters of non-coated microspheres
dispersed in rubber matrix.
Fig. 5. Complex permeability of composites containing micro-
spheres coated with Co–Fe thin films: (a) m0r and (b) mr00:
Fig. 4. Material parameters of Co-coated microspheres dis-
persed in rubber matrix.
High value of dielectric constant (e0rD17) is
attributed to metallic behavior of Co thin film. The
space charge polarization between adjacent con-
ductive particles (separated by insulating rubber
matrix) gives rise to a high value of dielectric
constant. However, the conduction loss (repre-
sented by e0r0) was observed to be small (loss
tangent less than 0.1).
Fig. 5 shows the frequency dispersion of
complex permeability of Co–Fe films with varia-
tion of Fe content (5–10 at%). The coating
composition was controlled by the mole ratio of
metal sources in the plating solution under the
assumption that the film has the same composition
as that of plating solution. The value of mr0
increases with Fe content in the lower frequency
region below 9 GHz (Fig. 5(a)). For instance, at
1 GHz m0r increases from 1.2 (5 at% Fe) to 2.6
(9 at% Fe). Magnetic loss (m0r0) also increases with
the Fe content and dispersion peak of m0r0 moves to
lower frequency (from 18 GHz for 5 at% Fe film to
1.2 GHz for 9 at% Fe film) as shown in Fig. 5(b).
For 10 at% Fe film, the m0r0 peak frequency is
considered to be lower than 1 GHz. If the
resonance frequency is below 1 GHz for the 10%
Fe coating, both real and imaginary parts of

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S.-S. Kim et al. / Journal of Magnetism and Magnetic Materials 271 (2004) 39–45
43
permeability can be smaller than that of 8 and
9 at% Fe films in the frequencies above 1 GHz as
shown in Fig. 5. The similar frequency spectrum of
permeability can be found in CodZn2ÀdZ hexago-
nal ferrites where the higher the initial perme-
ability (the lower the resonance frequency), the
smaller the value of permeability (both real and
imaginary) in the high frequencies [3].
Fig. 6 shows the hysteresis loops determined in
the microspheres coated with Co and Co-10 at%
Fe films. The saturation magnetization Ms in-
creases from 54 emu/g (Co film) to 134 emu/g (Co-
10 at% Fe film), which is due to higher magnetic
moment of Fe atom. On the while, the coercive
force Hc decreases from 450 Oe (Co) to 110 Oe
(Co-10 at% Fe). According to the bulk magnetic
properties of Co–Fe alloy [7], the magnetocrystal-
line anisotropy is changed from uniaxial c-axis to
in-plane of HCP at 1.0 at% Fe, and then to cubic
anisotropy of FCC at 5.2 at% Fe. Fe substitution
in Co lattices in the composition range reduces the
crystal anisotropy field and, therefore, lower Hc
was predicted in the microspheres coated with Co–
Fe films. The similar behavior of crystalline
anisotropy was observed in M-type hexaferrites,
where easy axis is changed from c-axis to in-plane
by substitution of Ti–Co ions for Fe sites [8].
According to the ferromagnetic resonance the-
ory [9], the resonance frequency fr is proportional
to the anisotropy field HA as expressed by the
equation,
g
2p
fr ¼
HA;
ð1Þ
where g is gyromagnetic ratio. For the films coated
on the spherical particles, shape anisotropy field
can be negligible. HA is then crystal anisotropy
field, which is given by
2K
60
40
HA
¼
;
ð2Þ
MS
where K is crystalline anisotropy constant. Since
the coercive force is, in general, proportional to
the anisotropy field, the resonance frequency
increases with the coercive force. The result of
permeability dispersion is quite well consistent
with the ferromagnetic resonance theory.
M
s = 53.66
20
0
Hc = 450 [Oe]
-10000
-5000
5000
10000
[Oe]
0
-20
-40
The complex permittivity is insensitive to Fe
content as shown in Fig. 7. Slightly different
(a)
-60
150
Co-6%Fe
Co-8%Fe
'
εr
25
20
15
10
5
100
50
Ms = 133.75
Hc = 110 [Oe]
Co-7%Fe
Co-9%Fe
0
-10000
-5000
5000
10000
0
''
εr
[Oe]
-50
-100
-150
0
1
3
6
9
12
15
18
(b)
FREQUENCY [GHz]
Fig. 6. Hysteresis loops determined in the microspheres coated
with (a) Co and (b) Co-10% Fe thin films.
Fig. 7. Complex permittivity of rubber composites containing
microspheres coated with Co–Fe thin films.

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S.-S. Kim et al. / Journal of Magnetism and Magnetic Materials 271 (2004) 39–45
values of e0r ¼ 20224 and small value of e0r0
(loss tangent less than 0.1) was determined.
The electrical conductivity does not change con-
siderably with Fe content in the films, which
results in not so different values of dielectric
constant.
For the composite layer (of metal-coated micro-
spheres and rubber matrix) terminated by metal
plate, the input impedance Zin at the surface is
given by
0
-5
-10
-15
-20
-25
Co-8%Fe film
Co-6%Fe film
Co-5%Fe film
Co-6%Fe film (measured)
ꢀ
ꢁ
rffiffiffiffi
pffiffiffiffiffiffiffiffi
m
er
j2pd
l
Zin ¼ Z0
rtanh
ermr ;
ð3Þ
1
3
6
9
12
15
18
(a)
FREQUENCY [GHz]
where Z0 is wave impedance of free space
(376.7 O), l is wavelength in free space, and d is
thickness of composite layer. Since the reflection
coefficient G is proportional to the difference
between Zin and Z0 as expressed in Eq. (4), the
reflection loss can be calculated from the measured
material parameters (mr and er) as a function of
frequency and thickness.
0
-5
-10
-15
-20
-25
-30
Co-5%Fe film
Co-6%Fe film
Co-8%Fe film
Zin À Z0
Zin þ Z0
G ¼
:
ð4Þ
Fig. 8 shows the reflection loss determined in the
rubber composites containing the microspheres
coated with Co–Fe films. At the thickness of
1.5 mm, the microwave absorption (more than
10 dB) occurs in the X-band frequencies (8–
12 GHz) as shown in Fig. 8(a). The measured
value of reflection loss is also given for the sample
of 6 at% Fe coating. There is a small difference in
maximum attenuation between theoretical
(À18 dB: 98% power absorption) and experimen-
tal result (À16 dB: 97% power absorption). How-
ever, the difference is quite small in power
absorption. By increasing the thickness to 2.0 mm,
the absorption band moves to the lower C-band
frequencies (4–8 GHz) as shown in Fig. 8(b).
The maximum absorption (more than 20 dB) was
predicted at the composition of 6 at% Fe. A slight
difference in microwave absorbance is observed by
the variation of film composition. The absorption
band moves slightly to lower frequency with
increase of Fe content in the films, which might
be due to higher magnetic loss as shown in
Fig. 5(b).
1
3
6
9
12
15
18
(b)
FREQUENCY [GHz]
Fig. 8. Reflection loss determined in the rubber composites
containing the microspheres coated with Co–Fe thin films: (a)
d ¼ 1:5 mm and (b) d ¼ 2 mm.
It has been reported that for the previously
developed X-band absorbers composed of ferrite
particles and rubber matrix, the typical value of
thickness is about 3 mm [10]. The predicted
matching thickness of this study is quite small
compared with the conventional ferrite composite
absorber, and the result is attributed to the higher
magnetic loss and dielectric constant of the Co–Fe
films. Moreover, the density of the metal-coated
microspheres is very low (0.8 g/cc) as compared
with ferrites (5.7 g/cc). The proposed conductive
and magnetic microspheres coated with Co–Fe
films has advantages in reducing both mass and
thickness when used as the absorbent fillers of
microwave absorbers.

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45
4. Conclusions
Acknowledgements
Lightweight and thin microwave absorbers
could be demonstrated by using the hollow
ceramic microspheres coated with Co–Fe thin
films as the absorbent fillers in rubber matrix.
Highly conductive and magnetic microspheres of
low density (about 0.8 g/cc) could be prepared by
plating the Co–Fe thin films on the microspheres.
Due to metallic and ferromagnetic behavior of the
Co–Fe films, high dielectric constant and magnetic
loss could be obtained in microwave frequencies. It
was found that the higher the Fe content in the
alloy films, the higher the magnetic loss (m0r0) and
the lower the dispersion peak frequency of m0r0: The
result could be well explained by ferromagnetic
resonance theory. Due to these electromagnetic
properties, high absorption rates for electromag-
netic radiation and thin matching thickness were
predicted in the rubber composites containing the
metal-coated microspheres. Reflection loss less
than À20 dB were predicted in X-band frequencies
with a thickness of 1.5 mm. The proposed absorber
is well advanced in both mass and thickness in
comparison with the conventional ferrite absorber.
Helpful discussion with Mr. E.J. Kim and Mr.
J.H. Kim is greatly acknowledged. This work was
supported by Korea Research Foundation.
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