Journal of Materials Science and Chemical Engineering, 2013, 1, 8-13
http://dx.doi.org/10.4236/msce.2013.14002 Published Online September 2013 (http://www.scirp.org/journal/msce)
Microwave Absorbing Properties of W-Type Hexaferrite
Ba(MnZn)xCo2(1−x)Fe16O27
Xianming Qin1, Ying Cheng2, Kesheng Zhou2, Shengxiang Huang2, Xia Hui2*
1Xiamen Hongye Project Build Technique Co. Ltd., Xiamen, China
2School of Physics and Electronics, Central South University, Changsha, China
Received June 18, 2013; revised July 18, 2013; accepted August 25, 2013
Copyright © 2013 Xianming Qin et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
MnZn-doped W-type barium cobalt ferrite powder composites of Ba(MnZn)xCo2(1−x)Fe16O27 (x = 0.1, 0.2, 0.3, 0.4, and
0.5) were prepared in a sol-gel process. The microwave absorbing properties of the composites in the range of 2 - 18
GHz and their electromagnetic loss mechanisms were studied. The results demonstrated that the synthesized
Ba(MnZn)xCo2(1−x)Fe16O27 samples possess a W-type phase of the crystal structure with a hexagonal flaky shape in mi-
cro-morphology, and the samples exhibited a soft magnetic trait that enables improving their microwave absorption
properties through suitable MnZn doping. For Ba(MnZn)0.4Co1.2Fe16O27 with a thickness of 2.8 mm, the reflection loss
peak was −40.7 dB at a frequency of 7.3 GHz, with a bandwidth of 6.6 GHz at a loss of less than −10 dB. The micro-
wave absorption primarily resulted from magnetic losses caused by magnetization relaxation, domain wall resonance,
and natural resonance.
Keywords: Sol-Gel Process; W-Type Ferrite; Microwave Absorbing Materials; Electromagnetic Loss
1. Introduction
Ferrite is one type of the important electromagnetic-wave
absorbing materials, of which spinel and magnetoplum-
bite hexagonal ferrites are the most widely used in appli-
cations. However, because of Snoek’s limitation, the
working frequency of those spinel ferrites is generally
confined to the range below 3 GHz [1-5]. Hexagonal
ferrites, which exhibit a planar anisotropy and ferromag-
netic resonance in the GHz range caused by a high mag-
netocrystalline anisotropy field, are applicable for use as
electromagnetic-wave absorption materials working in
the GHz wavelength range [5-9]. Hexagonal ferrites can
be classified into many types, including M-, Y-, W-, X-,
Z-, and U-type phase, according to their chemical for-
mulas and crystal structures. The M-type is the most
studied among those hexagonal ferrites. It has been con-
firmed that different ions can be substituted for Ba2+ and
Fe3+ in M-type phase ferrite, such as La for Ba2+ and
Co-Ti, Mn-Ti, Mg-Ti, Co-Mn-Ti, Mn-Cu-Ti, Mn-Cu-Zr
and Zn-Zr-Sn for Fe3+. Consequently, the electromag-
netic parameters of the ferrites can be adjusted or con-
trolled, and their microwave absorption performance can
be improved [2-7,10-17].
W-type phase ferrite can be viewed as a complex fer-
rite consisting of M-type phase and spinel phase block,
which is M + 2S (M: BaFe12O19, S: Fe2+Fe2O4). In
BaFe12O19, chemical doping is commonly utilized by
replacing Fe2+ with Co2+ to improve its thermal stability
and other properties, which results in a W-type barium-
cobalt ferrite of BaCo2Fe16O27. Because of the existing
Fe2+ and Fe3+ cation sites, Fe cations are easily substi-
tuted by other divalent and trivalent cations; the mag-
netic parameters, including saturation magnetization,
Curie temperature, coercivity, magnetic anisotropy and
ferromagnetic resonance frequency, are thus more ad-
justable. However, there are fewer studies on microwave
electromagnetic characteristics of W-type phase ferrites
than on the characteristics of M-type ferrites. The substi-
tution of Co2+ or Ba2+ by other metal ions was deter-
mined to help improve the microwave absorption proper-
ties of BaCo2Fe16O27. Materials with improved micro-
wave absorption include Ba(ZnxCo1−x)2Fe16O27 [10],
BaZn1.1Co0.9Fe16O27 [13], Ba1−xErx(Zn0.3Co0.7)2Fe16O27
[18] and Ba(Zn0.5Co0.5)2Fe16O27 [19]. The microwave
electromagnetic effects are expected to be richer when
Co2+ in BaCo2Fe16O27 is substituted by two or more
metal ions.
In this paper, MnZn-doped W-type barium cobalt fer-
*Corresponding author.
C
opyright © 2013 SciRes. MSCE

X. M. QIN ET AL. 9
rite composites of Ba(MnZn)xCo2(1−x)Fe16O27 were pre-
pared using a sol-gel process. The microwave absorption
properties of Ba(MnZn)xCo2(1−x)Fe16O27 in the frequency
range of 2 - 18 GHz are investigated, and the electro-
magnetic loss mechanism is discussed.
2. Experiments
2.1. Preparation of Ba(MnZn)xCo2(1−x)Fe16O27
Powders
Ba(MnZn)xCo2(1−x)Fe16O27 (x = 0.1, 0.2, 0.3, 0.4 and 0.5)
were synthesized using a sol-gel process. According to
the stoichiometric ratio of Ba(MnZn)xCo2(1−x)Fe16O27, the
weighed Fe3+, Co2+, Zn2+, and Ba2+ nitrate and manganese
acetate were dissolved in deionized water and stirred to
form a uniform brown transparent solution. Citric acid
was added to the above solution according to the molar
ratio of 1:1.5 between the metal ions and the citric acid,
and the ammonia water was dripped to ensure the pH
value that was in the range of 6 - 7. Next, the solution
was strung for 4 h in a water bath at 80˚C to form a sol.
The solution was then dried in an oven (100˚C) to obtain
the dry gels. The dry gels were slightly grinded after ashing
at 450˚C. Finally, the black Ba(MnZn)xCo2(1−x)Fe16O27
ferrites were obtained by cooling the powder, which was
sintered in an electric furnace at 1235˚C for 4 h under the
atmosphere circumstances.
2.2. Measurements
The crystal structure of Ba(MnZn)xCo2(1−x)Fe16O27 (x =
0.1, 0.2, 0.3, 0.4 and 0.5) was investigated through X-ray
diffraction (XRD) using a system from X’pert PRO of
Holland PANalytical that utilized a Cu target at a volt-
age/current of 35-kV/25-mA. Scanning electron micros-
copy (SEM) examination was performed using QUANTA
200 to obtain the morphology and size of the powder
particles of Ba(MnZn)xCo2(1−x)Fe16O27. The hysteresis
loops of the Ba(MnZn)0.4Co1.2Fe16O27 particles were
measured using a vibrating sample magnetometer (VSM).
The prepared crystalline powders were mixed with
paraffin wax at a ratio of 8:3 in weight by heating and
pressed into annular samples. The complex permittivity
and permeability of the samples were measured using an
AV3629 microwave vector network analyzer in the fre-
quency range of 2 - 18 GHz. The relationship between
microwave reflectivity (R) and frequency (f) can be ob-
tained according to a previous formula [20,21] that used
the measured data.
3. Results and Discussion
3.1. Crystal Structure, Morphology and
Hysteresis Loop of the Powders
Figure 1 shows the XRD pattern of the
˚
Figure 1. XRD pattern of Ba(MnZn)xCo2(1−x)Fe16O27.
Ba(MnZn)xCo2(1−x)Fe16O27 (x = 0.1, 0.2, 0.3, 0.4 and 0.5)
ferrites calcined at 1235˚C for 4 h. Compared with the
characteristic diffraction peaks of the standard XRD card
(PDF#19-0098) for W-type phase ferrite, the crystal
structures of Ba(MnZn)xCo2(1−x)Fe16O27 samples were
measured to be a single W-type phase structure. Nearly
no variations of the crystal structure were observed for
all MnZn-doped Ba(MnZn)xCo2(1−x)Fe16O27 samples
when x was equal to 0.1, 0.2, 0.3 0.4 and 0.5.
Figure 2 show SEM images of
Ba(MnZn)0.4Co1.2Fe16O27 and Ba(MnZn)0.3Co1.4Fe16O27
powders after high-temperature annealing at 1235˚C for
4 h. The SEM images indicate that both the morphology
of the Ba(MnZn)0.4Co1.2Fe16O27 and
Ba(MnZn)0.3Co1.4Fe16O27 are hexagonal flaky with a
clear outline. The average size was 5 μm with a narrow
distribution. As a result, doping with a small amount of
MnZn does not significantly affect the grain size and
morphology.
Figure 3 shows the hysteresis loop of
Ba(MnZn)0.4Co1.2Fe16O27 powders calcined at 1235˚C for
4 h. The coercitity (Hc), magnetization (Ms) and reten-
tivity (Mr) are 28.702 Oe, 67.320 emu/g, 3.7964 emu/g,
respectively. The hysteresis loop indicates that
Ba(MnZn)0.4Co1.2Fe16O27 is a soft magnetic material.
3.2. The Microwave Absorption Properties
Figure 4 shows the variation of reflection loss versus
frequency for Ba(MnZn)xCo2(1−x)Fe16O27 (x = 0.1, 0.2,
0.3, 0.4 and 0.5) at a thickness of 2.8 mm. For x = 0.1,
the reflection loss exhibits a peak at approximately −12
dB at a frequency of 8.1 GHz, with a bandwidth of 5
GHz for losses less than −10 dB. For x = 0.2, the reflec-
tion loss exhibits a peak at approximately −16 dB at 5.0
GHz, with a bandwidth of 6 GHz for losses less than −10
dB. For x = 0.3, the reflection loss exhibits a peak at ap-

X. M. QIN ET AL. 10
(a)
(b)
Figure 2. SEM images of powders (a)
Ba(MnZn)0.4Co1.2Fe16O27 and (b) Ba(MnZn)0.3Co1.4Fe16O27.
Figure 3. Hysteresis loop of Ba(MnZn)0.4Co1.2Fe16O27.
proximately −23 dB at 8.0 GHz, with a bandwidth of 6.5
GHz for losses less than −10 dB. For x = 0.4, the reflec-
tion loss exhibits a peak at approximately −40.7 dB at 7.3
GHz, with a bandwidth of 6.6 GHz for losses less than
−10 dB. For x = 0.5, the reflection loss exhibits a peak at
approximately −12 dB at 5.5 GHz, with a bandwidth of 3
GHz for losses less than −10 dB. It was shown that the
compositions Ba(MnZn)0.3Co1.4Fe16O27 and
Ba(MnZn)0.4Co1.2Fe16O27 have obviously higher micro-
wave absorptions than the other compositions. This
would explained by doped MnZn elements can change
the electric and magnetic structures of the system, result
in the change of the magnetocrystalline anisotropy field,
the increase of the domain rotation resistance, pinning of
domain wall displacement, thereby the magnetic loss is
increased and the resonance frequency is changed.
However, overdope will increase the lattice distortion,
result in the increase of electric resistivity and the de-
crease of Ohmic loss. Thus, the appropriate doping can
enhance the impedance matching and microwave at-
tenuation and change the position of the absorption peak.
As a result, W-type barium-cobalt ferrite doped by cer-
tain amounts of MnZn can act as a wide-band microwave
absorber. As shown in Figure 4, the frequency position
of the absorption peak changes with the amount of doped
MnZn. This absorption peak shifts with doping because
the amount of doped material will affect the magneto-
crystalline anisotropic field and thus cause a change in
the natural resonance frequency.
Figure 5 shows the variation of the reflection loss
versus frequency for Ba(MnZn)0.4Co1.2Fe16O27 at differ-
ent thicknesses. For d = 2.0 mm, the maximum reflection
loss is approximately −17 dB at a frequency of 4.8 GHz,
with a bandwidth of 3 GHz for losses less than −10 dB.
For d = 2.4 mm, the maximum reflection loss is ap-
proximately −22.5 dB at a frequency of 5.5 GHz, with a
bandwidth of 5 GHz for losses less than −10 dB. For d =
2.8 mm, the maximum reflection loss is approximately
369 12151
-50
-40
-30
-20
-10
0
8
R/dB
f /GHz
x=0.1
x=0.2
x=0.3
x=0.4
x=0.5
Figure 4. Variation of the reflection loss with frequency of
Ba(MnZn)xCo2(1−x)Fe16O27 ferrites at a thickness of 2.8 mm.

X. M. QIN ET AL. 11
36912 15 18
-50
-40
-30
-20
-10
0
R/dB
f /GHz
x=0.1
x=0.2
x=0.3
x=0.4
x=0.5
Figure 5. Variation of the reflection loss with frequency of
Ba(MnZn)0.4Co1.2Fe16O27 ferrites at different thicknesses.
−40.7 dB at a frequency of 7.3 GHz, with a bandwidth of
6.6 GHz for losses less than −10 dB. For d = 3.2 mm, the
maximum reflection loss is approximately −32.5 dB at a
frequency of 9.0 GHz, with a bandwidth 6.5 GHz for
losses less than −10 dB. The absorption peak obviously
exhibits a shift towards higher frequencies (blue shift)
with an increase in the thickness of the samples. Thus,
the matching thickness of Ba(MnZn)0.4Co1.2Fe16O27 as a
microwave absorber is 2.8 mm.
3.3. Electromagnetic Loss Mechanisms
Figure 6 shows the spectra of the complex permittivity
and permeability of Ba(MnZn)0.4Co1.2Fe16O27. The
imaginary part of the permittivity is small, and the
real part is in the range of 5.5 - 6.5 over the fre-
quency range of 2 - 18 GHz. The real part of the perme-
ability decreases with an increase of frequency, but it
does not change when the frequency exceeds 10 GHz.
Two peaks of the imaginary part
ε''
ε'
μ
′′ are located at 4.5
GHz and 8 GHz, but
μ
′′ decreases when the frequency
exceeds 4.5 GHz.
Using the dielectric loss tangent tan
δ
e = rr
and
the magnetic loss tangent tan
δ
m =
ε'' ε'
rr
'' '
μμ
, the variation
of tan
δ
e and tan
δ
m over the frequency range of 2 - 18
GHz can be determined. As shown in Figure 7, tan
δ
e is
less than 0.1 and gradually changes with frequency over
the entire frequency range, which indicates the presence
of weak dielectric loss. This low loss may occur because
doping will produce intrinsic electric dipole moments,
thus forming defected dipoles. In the presence of a mi-
crowave electromagnetic field, polarization relaxation in
the dipoles can lead to dielectric losses.
Relative to the dielectric loss tan
δ
e, the magnetic loss
tan
δ
m is much greater over the frequency range of 2 - 18
GHz. The value of tan
δ
m is greater than 0.3 over the fre-
quency range of 2 - 15 GHz. There are two peaks on the
curve of tan δ
m, with the first peak exhibiting a value of
0.78 at approximately 4.5 GHz and the second peak ex-
hibiting a value of 0.82 at approximately 8 GHz; these
values are consistent with the peaks of
μ
′′ shown in
Figure 6. The peak position of the microwave reflectiv-
ity is predicted to be between the two magnetic loss tan-
gent peaks, though closer to the second peak, which is
consistent with the data shown in Figure 4.
The magnetic loss primarily results from domain wall
displacement, domain turning, domain wall resonance
and natural resonance. As shown in Figure 7, the first
loss tangent peak originates from domain wall resonance,
and the second peak originates from natural resonance,
while magnetization relaxation is present over the entire
frequency range. Doping with MnZn can result in the
effect of domain wall pinning and the change of mag-
netic anisotropy, so the damping of domain turning and
domain wall displacement increases, thus leading to
resonance enhancement and resonance frequency-posi-
tion shift. However, an excessive amount of doping may
result in weak microwave absorption, due to the increase
in electric resistivity from the enhancement of electron
scattering and the increase of the lattice defects. Conse-
quently, Ohmic loss is increased, and the microwave
absorption is not conducive to impedance matching. As a
result, a suitable amount of MnZn-doping is required to
improve the microwave absorption of W-type barium-
cobalt ferrites (as in Figure 4). In addition, because the
powder morphology was hexagonal flaky, this shape
anisotropy can result in magnetocrystalline anisotropy,
which increases the microwave scattering in the medium,
thereby enhancing the magnetic loss.
4. Conclusion
The crystal structure of MnZn-doped barium-cobalt fer-
rite Ba(MnZn)xCo2(1−x)Fe16O27 (x = 0.1, 0.2, 0.3, 0.4 and
0.5) prepared using a sol-gel process was hexagonal
W-type phase, and the samples exhibited a soft magnet-
ism. Incorporating MnZn into the structure resulted in
nearly no change in the structure with different doping
amounts, and the morphology of the powders was hex-
agonal flaky with an average particle size of 5 μm. As a
result, Ba(MnZn)xCo2(1−x)Fe16O27 ferrite exhibits the ex-
cellent microwave absorption properties of wide band-
width and strong loss over the frequency range of 2 - 18
GHz. The amount of MnZn doping will greatly influence
the absorption properties of Ba(MnZn)xCo2(1−x)Fe16O27.
The reflection loss peak exhibited the characteristics of a
maximum loss of −40.7 dB at a frequency of 7.3 GHz,
with an effective bandwidth of 6.6 GHz for losses less
than −10 dB when the thickness of the sample was 2.8
mm and x = 0.4. The microwave absorption of
Ba(MnZn)xCo2(1−x)Fe16O27 ferrite originated from weak

X. M. QIN ET AL. 12
246810 12 14 16 18
0
1
2
3
4
5
6
7
ε
'
ε
'
'
f / GHz
ε '
ε ''
(a)
246810 12 14 1618
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
μ
'
μ
'
'
f / GHz
μ '
μ ''
(b)
Figure 6. Spectra of the complex permittivity and permeability of Ba(MnZn)0.4Co1.2Fe16O27. (a) permittivity: real part
and imaginary part ; (b) permeability: real part and imaginary part .
ε'
′′
ε′ μ′′ μ
246810 12 14 16 18
0.0
0.2
0.4
0.6
0.8
tan δ
e
tan δ
m
tan
δ
e & tan
δ
m
f / GHz
Figure 7. Variation of the dielectric loss tan δ
e and the mag-
netic loss tan
δ
m with frequency for Ba(MnZn)0.4Co1.2Fe16O27
ferrites.
dielectric loss and strong magnetic loss. The dielectric
loss was caused by the polarization relaxation of dipoles,
and the magnetic loss was caused by magnetization re-
laxation, domain wall and natural resonances. Both the
suitable amount of doping of MnZn and the hexagonal
flaky morphology of the powder particles are helpful for
enhancing the electromagnetic loss of the ferrites.
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