Electromagnetic and microwave absorption properties of FeSr 0.8La0.2Fe11.8Co0.2O19 shell-core composites

Article abstract of DOI:10.1016/j.jmmm.2012.02.107

Shell-core FeSr_0.8La_0.2Fe_11.8Co _0.2O₁₉ composites are prepared by chemical vapor deposition (CVD) for use as microwave absorbing materials. Scanning electron microscopy and X-ray diffraction analyses show that the CVD method yields Sr_0.8La_0.2Fe_11.8Co_0.2O₁₉ powders with a uniform coating of Fe. Compared with Sr_0.8La _0.2Fe_11.8Co_0.2O₁₉, FeSr _0.8La_0.2Fe_11.8Co_0.2O₁₉ composites have higher electrical conductivity, permittivity, and dielectric loss, which gradually increase with increasing Fe content. When Sr _0.8La_0.2Fe_11.8Co_0.2O ₁₉/Fe=7:3, a reflection loss (RL) exceeding -10 dB is obtained in the frequency range of 1014 GHz at a coating thickness of 2.0 mm. A minimum RL of -30 dB was found at 8.0 GHz, corresponding to a matching thickness of 2.8 mm.

Full text of DOI:10.1016/j.jmmm.2012.02.107

Journal of Magnetism and Magnetic Materials 324 (2012) 2177–2182
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Electromagnetic and microwave absorption properties
of Fe–Sr0.8La0.2Fe11.8Co0.2O19 shell-core composites
Zeyang Zhang a, Xiangxuan Liu a,n, Xuanjun Wang a, Youpeng Wu b, Yuan Liu a
a No. 503 Faculty, Xi’an Research Institute of High Technology, Xi’an 710025, China
b Beijing Research Institute of High Technology, Beijing 100085, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Shell-core Fe–Sr0.8La0.2Fe11.8Co0.2O19 composites are prepared by chemical vapor deposition (CVD) for
use as microwave absorbing materials. Scanning electron microscopy and X-ray diffraction analyses
show that the CVD method yields Sr0.8La0.2Fe11.8Co0.2O19 powders with a uniform coating of Fe.
Compared with Sr0.8La0.2Fe11.8Co0.2O19, Fe–Sr0.8La0.2Fe11.8Co0.2O19 composites have higher electrical
conductivity, permittivity, and dielectric loss, which gradually increase with increasing Fe content.
When Sr0.8La0.2Fe11.8Co0.2O19/Fe¼7:3, a reflection loss (RL) exceeding ꢀ10 dB is obtained in the
frequency range of 10–14 GHz at a coating thickness of 2.0 mm. A minimum RL of ꢀ30 dB was found
at 8.0 GHz, corresponding to a matching thickness of 2.8 mm.
Received 16 December 2011
Received in revised form
19 February 2012
Available online 2 March 2012
Keywords:
Strontium ferrite
Carbonyl iron
Shell-core structure
Electromagnetic property
Microwave absorption
& 2012 Elsevier B.V. All rights reserved.
1. Introduction
powders have been found to be tunable by adjusting the iron
content in the samples [15].
The increase in electromagnetic pollution due to the rapid
development of gigahertz-scale electronic systems and telecom-
munications has resulted in intense and growing interest in
electromagnetic absorber technology [1–5]. An absorbent is the
core of an absorbing material. Many studies have been carried out
to investigate various aspects of microwave absorption materials
with high magnetic and dielectric loss [6–8]. M-type strontium
ferrite has a hexagonal molecular structure (SrFe12O19) and is a
promising material for permanent magnets, advanced recorders,
and microwave absorbers because of its fairly large magneto-
crystalline anisotropy, high Curie temperature, relatively large
magnetization, excellent chemical stability, and corrosion resis-
tivity. However, its magnetic and electric properties must be
adjusted to satisfy the requirements of different applications. In
an attempt to improve the microwave absorption properties of
SrFe12O19, La–Co substituted M-type strontium hexaferrites were
prepared in our previous study. Sr0.8La0.2Fe11.8Co0.2O19 was found
In this article, shell-core-structured Fe–Sr0.8La0.2Fe11.8Co0.2O19
composites are prepared by chemical vapor deposition (CVD), and
the influence of the iron shell on the crystal structure, morphol-
ogy, complex permittivity and complex permeability, and micro-
wave absorption properties of the Sr0.8La0.2Fe11.8Co0.2O19 particles
is investigated.
2. Experimental
2.1. Synthesis of Sr0.8La0.2Fe11.8Co0.2O19
Sr0.8La0.2Fe11.8Co0.2O19 hexagonal ferrites were prepared by a
sol–gel-auto combustion method. Based on Sr0.8La0.2Fe11.8-
Co0.2O19, stoichiometric amounts of strontium nitrate, cobalt
nitrate, lanthanum nitrate, and iron nitrate were dissolved in
de-ionized water, and an appropriate amount (1:1 mol ratio of
citric acid to NO3) of citric acid was added to complex the metal
ions completely. The pH value of the sol was adjusted to 6.0 using
ammonia solution (25 wt%). The solution was evaporated at 80 1C
until a viscous gel formed. The gel was air-dried at 120 1C, ignited
in air, and burned into dendritic powders. Finally, the precursor
powder was sintered at 1200 1C for 3 h.
to exhibit better microwave absorption compared with SrFe12O19
.
However, poor dielectric loss was also found to be an intrinsic
disadvantage of Sr0.8La0.2Fe11.8Co0.2O19 for microwave absorption.
Carbonyl iron (CI) powders, which features high saturation
magnetization and relative permeability at radar wave frequen-
cies is a superior material that is widely accepted as microwave
absorbers [9–14]. The high-frequency dielectric properties of CI
2.2. Synthesis of Fe–Sr0.8La0.2Fe11.8Co0.2O19 shell-core composite
n Corresponding author. Tel.: þ86 15902957276; fax: þ86 029 83348220.
E-mail address: xiangxuan_study@yahoo.com (X. Liu).
Synthesis was performed in a CVD setup designed by our group.
typical preparation process for Fe–Sr0.8La0.2Fe11.8Co0.2O19
A
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2012.02.107

2178
Z. Zhang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2177–2182
composites is as follows: First, 1 g of Sr0.8La0.2Fe11.8Co0.2O19 powders
and 5 mL of iron pentacarbonyl [Fe(CO)5] were added into an airproof
reactor and evaporator, respectively. Fe(CO)5 was evaporated at 65 1C,
while Sr0.8La0.2Fe11.8Co0.2O19 powders were heated and maintained at
250 1C. Fe(CO)5 vapor was bowed into the reactor by argon gas at a
rate of 150 mL/min for 3, 9 and 18 min. To obtain a uniform coating,
the reactor was vigorously and continuously stirring mechanically.
Shell-core Fe–Sr0.8La0.2Fe11.8Co0.2O19 composites were obtained after
slow-cooling to room temperature, and the whole experiment was
protected by argon. The weight content of iron in the composites was
determined by comparing the weights of specimens before and after
the experiment. The mass ratios of Sr0.8La0.2Fe11.8Co0.2O19 to Fe were
approximately 9:1, 7:3 and 4:6.
3.2. Crystal structures
Fig. 3 shows the X-ray diffraction (XRD) patterns of Sr0.8La0.2-
Fe11.8Co0.2O19
,
CI, and Fe–Sr0.8La0.2Fe11.8Co0.2O19 composites.
Fig. 3(b) shows peaks at 19.341, 23.221, 30.931, 31.131, 32.611, 34.21,
36.051, 37.801, 41.061, 57.461, and 64.31, which are respectively
attributed to the (102), (006), (110), (008), (107), (201), (203), (205),
(206), (2011), and (220) phases of the Sr0.8La0.2Fe11.8Co0.2O19 particles.
These peaks can be indexed to the pure M-type hexa ferrite structure,
in agreement with JCPDS Card no. 720739. No impurity peaks were
detected, indicating the high purity of the powders. The shell-core-
structured Fe–Sr0.8La0.2Fe11.8Co0.2O19 composites [Fig. 3(c)] had simi-
lar diffraction peaks to Sr0.8La0.2Fe11.8Co0.2O19 and CI, but the intensity
of the peaks decreased. In addition, a peak around 2
observed, indicating that the shell layer was composed of iron.
y¼44.61 was
2.3. The preparation of composites coatings
3.3. Complex permittivity and permeability
The Fe–Sr0.8La0.2Fe11.8Co0.2O19 powders were dispersed into
the epoxide resin via adding solvent and a high energy ultrasonic
treatment for 30 min. Afterwards, hardener was added into the
mixtures, followed by stirring at 1000 rpm for 10 min. Finally, the
composite materials are fabricated on an aluminum substrate
with a standard size (180 mm ꢁ 180 mm ꢁ 3 mm).
Fig. 4 shows the complex permittivity and permeability of pure
Sr0.8La0.2Fe11.8Co0.2O19 and Fe–Sr0.8La0.2Fe11.8Co0.2O19 composites.
Fig. 4(a) and (b) respectively show the real (e0) and imaginary (e00
)
parts of the permittivity of the specimens plotted as a function of
frequency. e0 and e00 represent the energy storage ability and loss
ability, respectively. The e0 and e00 values of the specimens remained
constant with slight fluctuations over the entire frequency range and
increased with increasing weight ratio of Fe. The e0 and e00 of
Sr0.8La0.2Fe11.8Co0.2O19 were almost constant with nearly no differ-
ence throughout the frequency range studied (e0 ¼6, e00 ¼0.5), indicat-
ing poor dielectric loss. When the mass fraction of Fe was increased
by 60%, the e0 values increased from 6 to 12 while the e00 values
increased from 0.5 to 2.3. Dielectric loss commonly results from
electron polarization, iron polarization, and electric dipolar polariza-
tion [16]. The experiments showed that the Fe shell, which forms a
surface layer, greatly increases the conductivity of the samples and
contributes to the enhancement of permittivity. Because of the
metallic properties of the shell, electrons can travel freely and
accumulate on the Fe–Sr0.8La0.2Fe11.8Co0.2O19 interface, forming a
structure similar to a boundary-layer capacitor and generating
interfacial electric dipolar polarization. According to previous
research [17], electron and iron polarization only occur at frequencies
higher than the infrared range. Thus, the enhanced dielectric loss
found in our samples could be attributed to electric dipolar polariza-
tion. Optimizing the weight ratio of Fe would likely cause the
composites to attain the permittivity necessary in microwave absorb-
ing materials.
2.4. Measurement of properties
The phase structure of the powder was identified by X-ray
diffraction (XRD; D/max-IIB, Japan). A VEGA II XMU INCA scan-
ning electron microscope (SEM) was employed for morphological
analysis. EM parameters (i.e., complex permeability and permit-
tivity) were measured using
a vector network analyzer
(HP-8720ES) in the frequency range of 2–18 GHz. The samples
used for EM parameter measurements were prepared by disper-
sing powders into paraffin wax at a mass fraction of 60%, and then
pressing the mixtures into a compact toroidal shape with outer
and inner diameters of 7.0 and 3.0 mm, respectively. The reflec-
tion loss of the prepared absorbers versus the frequency is studied
using an HP 8510B vector network analyzer and standard horn
antennas in an anechoic chamber.
3. Results and discussion
The real (m0) and imaginary (m00) parts of the permeability are
shown in Fig. 5(a) and (b), respectively. An intersecting point at
8.5 GHz was observed on the curve of m0. Higher m0 values were found
for Fe–Sr0.8La0.2Fe11.8Co0.2O19 composites with higher Fe contents
prior to this intersecting point. Inverse changes were detected after
the intersecting point. Meanwhile, the m00 of Fe–Sr0.8La0.2Fe11.8
Co0.2O19 was uniformly smaller than that of pure Sr0.8La0.2Fe11.8-
Co0.2O19 over the range of 2–18 GHz, and decreased with increasing
Fe content. Broad peaks at 3–7 GHz were present in the
Fe–Sr0.8La0.2Fe11.8Co0.2O19 composites. Generally, for ferrite magnetic
materials, the microwave magnetic loss originates mainly from
domain wall resonance and natural ferromagnetic resonance [18].
Domain wall resonance is usually exhibited in the low-frequency
region (o2 GHz). However, resonance due to the spin rotational
component occurs at high-frequency regions. Thus, the resonance
peak observed may be attributed to natural resonance.
3.1. SEM analysis
An SEM image of the Sr0.8La0.2Fe11.8Co0.2O19 particles shows
irregularly shaped platelet-like crystals with smooth surfaces,
as shown in Fig. 1(a). The particles stack on top of each other
due to magnetic attraction between particles and are uni-
formly coated with iron, as illustrated in Fig. 1(b)–(d). The
Fe–Sr0.8La0.2Fe11.8Co0.2O19 micro-structured composites have a
full shell-core structure. When a small amount of Fe(CO)5 vapor
was added, CI particles with average diameters of approximately
0.02–0.1 mm were found in some of the Sr0.8La0.2Fe11.8Co0.2O19
particles [Fig. 1(b)]. Sr0.8La0.2Fe11.8Co0.2O19 particles coated with
CI showed coarse surfaces, as shown in Fig. 1(c). An increase in
the CI content of the Fe–Sr0.8La0.2Fe11.8Co0.2O19 composites
changed the shape of the particles to coralloid, and their arris
disappeared due to increasing thickness of the carbonyl shells.
Fig. 2 (a) and (b) are the surface and section SEM observation,
which show the microstructure of the coating filled with 60%
Fe–Sr0.8La0.2Fe11.8Co0.2O19 composites. It was observed that the
fillers are well dispersed in the EP matrix and no significant
porosity was noticed.
3.4. Microwave absorption properties
The normalized input impedance, Zin, of a metal-backed
microwave absorption layer is derived from the following

Z. Zhang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2177–2182
2179
Fig. 1. SEM micrographs of (a) Sr0.8La0.2Fe11.8Co0.2O19 particles and Fe–Sr0.8La0.2Fe11.8Co0.2O19 composites with mass ratios of (b) 9:1, (c) 7:3, and (d) 4:6.
Fig. 2. SEM micrograph of microstructure of coating surface and section.
expression [19]:
follows [20]:
ꢀ
ꢀ
ꢀ
ꢀ
pffiffiffiffiffiffiffiffiffi
RL ¼ 20log ðZinꢀZ0Þ=ðZin þZ0Þ ;
ð2Þ
Zin ¼ Z0ðmr
=er
Þ
1=2tanh½j2
p
df ermr=cꢂ;
ð1Þ
where Z0 is the characteristic impedance in a vacuum, Z0¼120
p.
where f is the frequency of the EM wave, d is the thickness of the
absorber, and c is the velocity of light in a vacuum.
According to the transmission line theory, reflection loss (RL)
is a function of the reflection coefficient and can be expressed as
Fig. 6 shows the RLs of pure Sr0.8La0.2Fe11.8Co0.2O19 powder
and Fe–Sr0.8La0.2Fe11.8Co0.2O19 composites 2 mm thick in the
frequency range of 2–18 GHz. The RL is sensitive to the Fe content
of Fe–Sr0.8La0.2Fe11.8Co0.2O19 composites. From the reflectivity

2180
Z. Zhang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2177–2182
0
-SrFe12O19
-Fe
-5
-10
-15
-20
-25
-30
c
b
a
Sr0.8La0.2Fe11.8Co0.2O19
Fe 10%
Fe 30%
Fe 60%
2
4
6
8
10
f/GHz
12
14
16
18
20
30
40
50
2θ(°)
60
70
Fig. 6. Microwave absorbing properties of Sr0.8La0.2Fe11.8Co0.2O19 and
Fig. 3. XRD patterns of (a) carbonyl iron particles, (b) Sr0.8La0.2Fe11.8Co0.2O19
Fe–Sr0.8La0.2Fe11.8Co0.2O19 composites at a sample thickness of 2 mm.
particles, and (c) Fe–Sr0.8La0.2Fe11.8Co0.2O19 composites.
3.0
12
11
10
Sr la Fe Co
O
10% Fe
30% Fe
60% Fe
2.5
2.0
1.5
1.0
0.5
0.0
Sr la Fe Co
O
9
8
7
6
5
10% Fe
30% Fe
60% Fe
2
4
6
8
10
12
14
16
18
2
4
6
8
10
12
14
16
18
Frequency [GHZ]
Frequency [GHZ]
Fig. 4. Real and imaginary parts of the complex permittivity of Sr0.8La0.2Fe11.8Co0.2O19 powders and Fe–Sr0.8La0.2Fe11.8Co0.2O19 composites in the frequency range of 2–18 GHz.
1.5
Sr la Fe Co
O
Sr la Fe Co O
0.6
10% Fe
30% Fe
60% Fe
10% Fe
30% Fe
60% Fe
1.2
0.9
2
4
6
8
10
12
14
16
18
2
4
6
8
10
12
14
16
18
Fig. 5. Real and imaginary parts of the complex permeability of Sr0.8La0.2Fe11.8Co0.2O19 powders and Fe–Sr0.8La0.2Fe11.8Co0.2O19 composites in the frequency range of 2–18 GHz.

Z. Zhang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2177–2182
2181
curves obtained, the position of the reflection peak loss moved to
lower frequency regions with increasing Fe content. The accessi-
bility of Fe can significantly improve the absorption properties of
Sr0.8La0.2Fe11.8Co0.2O19 powders. With their poor dielectric loss,
the RL properties of pure Sr0.8La0.2Fe11.8Co0.2O19 powders are
expected to originate mainly from magnetism, such that a mini-
mum RL of ꢀ15 dB was observed at 14.5 GHz. The minimum RLs
result in weak microwave absorption properties due to the imbalance
of the EM wave match. Pure Sr0.8La0.2Fe11.8Co0.2O19 is a magnetic
material with negligible dielectric loss; its poor EM wave match
results in weak absorption at a given thickness. An Fe coating can
enhance the electrical properties of Sr0.8La0.2Fe11.8Co0.2O19. As shown
in Fig. 7, higher dielectric loss for Sr0.8La0.2Fe11.8Co0.2O19/Fe¼7:3 and
4:6 is beneficial for enhancing microwave absorption. The magnetic
loss of specimens decreases with increasing Fe content, resulting in
microwave absorption deterioration. Thus, enhanced microwave
absorption properties are a consequence of proper electromagnetic
matches and enhanced dielectric loss. As well, the observed magnetic
properties of the Fe–Sr0.8La0.2Fe11.8Co0.2O19 composites are due to
their shell-core structure. Sr0.8La0.2Fe11.8Co0.2O19/Fe¼7:3 showed
better interaction than others, resulting in better microwave absorp-
tion properties.
The thickness-dependence of the RL of the composite was
studied. The theoretical values reflection loss of the Fe–CF
composites with different thickness was calculated based on
formulas (1) and (2). Fig. 8 shows the variation of the reflection
loss versus frequency of Sr0.8La0.2Fe11.8Co0.2O19/Fe¼7:3 with layer
thicknesses ranging from 1 mm to 3.5 mm. It is obvious that the peak
for the RL shifted to lower frequencies with increasing thickness;
under this condition, the peak value and absorption bandwidth first
increased and then decreased. For Sr0.8La0.2Fe11.8Co0.2O19/Fe¼7:3,
of Fe–Sr0.8La0.2Fe11.8Co0.2O19 composites with Sr0.8La0.2Fe11.8Co0.2O19
/
Fe ratios of 9:1, 7:3, and 4:6 are ꢀ20, ꢀ28, and ꢀ19 dB at 14, 12, and
11 GHz, respectively.
The dielectric and magnetic dissipation factor (tandm
¼ = ,
m00 m0
tande
¼
=
e00 e0) were calculated to investigate the reasons for the
improved microwave absorption of Fe–Sr0.8La0.2Fe11.8Co0.2O19 com-
posites. Fig. 7 shows that both magnetic loss and dielectric loss
contributed to microwave absorption. As shown in Fig. 7, the
dielectric loss increased with increasing Fe content; these increases
obviously result from the formation of the Fe shell on the surface of
the Sr0.8La0.2Fe11.8Co0.2O19 core. Meanwhile, an inverse change was
found in the magnetic loss. The improvement in microwave absorp-
tion of the Fe–Sr0.8La0.2Fe11.8Co0.2O19 composites is suspected to
originate from the efficient combination of Sr0.8La0.2Fe11.8Co0.2O19
and Fe. Generally, the microwave absorption of materials is consid-
ered to be related to the balance between their complex permeability
and complex permittivity. Either magnetic loss or dielectric loss may
Fig. 7. Dielectric loss and magnetic loss factor of (a) Sr0.8La0.2Fe11.8Co0.2O19 particles and Fe–Sr0.8La0.2Fe11.8Co0.2O19 composites at mass ratios of (b) 9:1, (c) 7:3, and (d) 4:6.

2182
Z. Zhang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2177–2182
reflection loss ꢀ26.5 dB can be obtained at 9.4 GHz for the
0
-5
calculated result. The difference between calculated and mea-
sured can be attributed to the surface irregularity of absorber
sample, the differences in the actual thickness of the layers, etc
[21]. As a result, we concluded that the calculated RL agree closely
with the experimental results in the whole measured frequency
range.
-10
-15
-20
-25
-30
-35
4. Conclusion
1.0mm
1.5mm
2.0mm
2.5mm
2.8mm
3.0mm
3.5mm
Sr0.8La0.2Fe11.8Co0.2O19 was synthesized by a sol–gel method.
CI-coated shell-core Sr0.8La0.2Fe11.8Co0.2O19 composites were suc-
cessfully prepared by CVD, and characterized by XRD and SEM.
Sr0.8La0.2Fe11.8Co0.2O19/Fe composites exhibited excellent microwave
absorption properties in the frequency range of 2–18 GHz. When the
weight ratio of Fe was 30%, a minimum RL value of ꢀ30 dB at
8.0 GHz on a coating with a matching thickness of 2.8 mm was
found. An RL of o ꢀ10 dB was observed in the 10–14 GHz range
with a corresponding thickness of 2.0 mm. Development of the
microwave absorption properties of Fe–Sr0.8La0.2Fe11.8Co0.2O19 com-
posites was attributed to improvements in their dielectric loss.
2
4
6
8
10
f/GHz
12
14
16
18
Fig. 8. Microwave absorbing properties of Sr0.8La0.2Fe11.8Co0.2O19/Fe¼7:3 with
different thickness.
0
-5
References
[1] T. Giaimakopoulou, A. Oikonomou, G. Kordas, Journal of Magnetism and
Magnetic Materials 271 (2004) 224–229.
[2] A. Hirata, S. Matsuyama, T. Shiozawa, IEEE Transactions on Electromagnetic
Compatibility 42 (2000) 386–392.
[3] A. Hirata, M. Morita, T. Shiozawa, IEEE Transactions on Electromagnetic
Compatibility 45 (2003) 109–116.
[4] Xiangxuan Liu, Zeyang Zhang, Youpeng Wu, Composites Part B: Engineering
42 (2011) 326–329.
[5] Zhi Ma, Jianbo Wang, Qingfang Liu, Jing Yuan, Applied Surface Science 255
(2009) 6629–6633.
[6] Ping Xu, Xijiang Han, Jingjing Jiang, Journal of Physical Chemistry C 111
(2007) 12603–12608.
[7] Hongmei Xiao, Xianming Liu, Shaoyun Fu, Composites Science and Technol-
ogy 66 (2006) 2003–2008.
-10
-15
-20
-25
-30
calculated
measured
[8] B. Lu, X.L. Dong, H. Huang, X.F. Zhang, Journal of Magnetism and Magnetic
Materials 320 (2008) 1106–1111.
-35
[9] X.A. Fan, J.G. Guan, W. Wang, G.X. Tong, Journal of Physics D: Applied Physics
42 (2009) 75006–75012.
[10] X.M. Ni, Z. Zheng, K. Xiao, L. Huang, L. He, Materials Chemistry and Physics
120 (2010) 206–212.
[11] Y.B. Feng, T. Qiu, C.Y. Shen, Journal of Magnetism and Magnetic Materials 318
(2007) 8–13.
-40
2
4
6
8
10
f/GHz
12
14
16
18
Fig. 9. Comparison of measured and calculated reflection loss for Fe–Sr0.8La0.2-
[12] Longgang Yan, Jianbo Wang, Yunzhe Ye, Journal of Alloys and Compounds
487 (2009) 708–711.
Fe11.8Co0.2O19 composites.
[13] Meng Wang, Yuping Duan, Shunhua Liu, Journal of Magnetism and Magnetic
Materials 321 (2009) 3442–3446.
[14] Guoxiu Tong, Wenhua Wu, Qiao Huq, Journal of Alloys and Compounds 509
(2011) 451–456.
[15] Y.L. Cheng, J.M. Dai, Y.P. Wu, Journal of Magnetism and Magnetic Materials
32 (2010) 97–101.
the minimum RL was ꢀ30 dB at 8.0 GHz and the corresponding
thickness was 2.8 mm. For a coating thickness of 2 mm, an RL
exceeding ꢀ10 dB was obtained in the frequency range of
10–14 GHz.
As shown in Fig. 9, the measured reflection loss of Sr0.8La0.2-
Fe11.8Co0.2O19/Fe¼7:3 at thickness d¼2.5 mm has been compared
with calculated results. The shapes of both measured and theore-
tically calculated results are similar to the curve pattern and
absolute values. However, there are still some differences
between the measured and calculated of reflection loss. It can
be seen that the value of minimum reflection loss at 9 GHz is
nearly ꢀ35 dB for the measured results while a minimum
[16] B. Gao, L. Qiao, J.B. Wang, Q.F. Liu, F.S. Li, J. Feng, D.S. Xue, Journal of Physics
D: Applied Physics 41 (2008) 35005.
[17] X.F. Zhang, P.F. Guan, X.L. Dong, Applied Physics Letters 96 (2010) 102505.
[18] M.Z. Wu, Y.D. Zhang, S. Hui, T.D. Xiao, S.H. Ge, W.A. Hines, J.I. Budnick,
G.W. Taylor, Applied Physics Letters 23 (2002) 4404.
[19] Y. Naito, K. Suetake, IEEE Transactions on Microwave Theory and Techniques
19 (1971) 65–72.
[20] B. Wessling, Polymer Engineering and Science 16 (1991) 1200–1206.
[21] Qing Yuchang, Zhou Wancheng, Luo Fa, Zhu Dongmei, Journal of Magnetism
and Magnetic Materials 323 (2011) 600–606.