Synthesis of hierarchical sword-like cobalt particles and their microwave absorption properties

Article abstract of DOI:10.1039/c4ra05716c

In this paper, we reported synthesis of the cobalt particles with a hierarchical sword-like structure through a liquid reduction method. We have investigated the electromagnetic properties of the cobalt particles in the microwave frequency range of 2-18 GHz. The cobalt particles presented multiple dielectric and magnetic resonance peaks, which are related to their unique structure. We have predicted the microwave absorption properties of the cobalt particles according to the transmit line theory. The maximum reflection loss (RL) of the cobalt particles reaches -60.13 dB, and the effective absorption bandwidth (RL ≤ -10 dB) is 5.42 GHz corresponding to a thickness of 1.4 mm, revealing better microwave absorption than previously reported materials. The superior microwave absorption properties indicate that the as-prepared cobalt particles have potential application as candidates for microwave absorption. This journal is

Full text of DOI:10.1039/c4ra05716c

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Synthesis of hierarchical sword-like cobalt particles
and their microwave absorption properties
Cite this: RSC Adv., 2014, 4, 40456
*
Shulai Wen, Xiuchen Zhao, Ying Liu, Jingwei Cheng and Hong Li
In this paper, we reported synthesis of the cobalt particles with a hierarchical sword-like structure through a
liquid reduction method. We have investigated the electromagnetic properties of the cobalt particles in the
microwave frequency range of 2–18 GHz. The cobalt particles presented multiple dielectric and magnetic
resonance peaks, which are related to their unique structure. We have predicted the microwave absorption
properties of the cobalt particles according to the transmit line theory. The maximum reflection loss (RL) of
the cobalt particles reaches ꢀ60.13 dB, and the effective absorption bandwidth (RL # ꢀ10 dB) is 5.42 GHz
corresponding to a thickness of 1.4 mm, revealing better microwave absorption than previously reported
materials. The superior microwave absorption properties indicate that the as-prepared cobalt particles
have potential application as candidates for microwave absorption.
Received 13th June 2014
Accepted 22nd August 2014
DOI: 10.1039/c4ra05716c
www.rsc.org/advances
multiple crystal structures to study microwave performance,
such as hcp, fcc and bcc,17 and many interesting physical
1 Introduction
properties related to crystal structure could be exhibited.
Therefore, there are wide academic and practical values to study
the microwave performance and absorption for cobalt.
In this work, we synthesized cobalt particles, with plate-,
sphere-chain, spherical and sword-like morphologies, through
liquid phase reduction method. In this method, CoSO4$7H2O
acted as cobalt source, N2H4$H2O acted as reducing agent,
Due to intense demands for reduction of electromagnetic
pollution/radiation and shielding interference in electric
communication eld, increasing attentions have been focused
on microwave absorption materials in the GHz range.1–6 For
magnetic metallic materials, the higher permeability could be
obtained due to higher saturation magnetization according to
Snoek's limit.7–11 Nevertheless, due to eddy current loss induced
by the electromagnetic wave in GHz range, the permeability
would decrease. The method to overcome eddy current loss is to
make the magnetic materials be isolated in the insulator.
Therefore, many nanocomposites with core@shell structure
were synthesized, such as Ni@C,12 Fe@C,13 Fe@SiO2,14 Ni–Co–
P@SiO2,15 FeCoNi@C,16 and so on. It is well known that the
saturation magnetization of magnetic metallic particles
decrease with the size decreasing, and the nanocomposites
possess lower permeability due to their size in nanoscale.
Another effective way is to synthesize magnetic metallic parti-
cles with complex structure, which could avoid the eddy current
loss.
C
16H33(CH3)3NBr acted as surfactant agent, C4O6H4KNa$4H2O
acted as complex agent, and NaOH acted as pH adjusting agent.
We systematically investigated the effects of temperature, NaOH
concentration and [C4H4O6]2ꢀ/Co2+ ratio on the morphologies
of cobalt particles. The electromagnetic and microwave
absorption properties for sword-like cobalt particles were
investigated in the microwave frequency range of 1–18 GHz.
2 Experimental
2.1 Preparation of cobalt particles
As a typical magnetic metallic material, cobalt has unique
physical properties for microwave absorption. Cobalt possesses
high saturation magnetization, and high permeability could
therefore be achieved according Snoek's limit. Moreover, cobalt
has high Curie temperature, which results in cobalt used as
candidate for microwave absorption in a wide temperature
range. It is well known that microwave performance and
absorption are related to crystal structures, and cobalt has
All chemicals used in this work were of analytical grade and
were used as received without further purication. The
synthesis of cobalt particles was carried out in a 3-necked ask
equipped with an ultrasound reactor. In a typical synthesis, 2.8
g CoSO4$7H2O and 2.8 g C4O6H4KNa were dissolved in 150 mL
ꢁ
deionized water under vigorously stirring for 40 min at 40 C,
followed by 14 g NaOH. Then 10 mL N2H4$H2O (80 wt%) was
added to the above solution. The reaction temperature was
80 ꢁC, the power was 1400 W, and the ultrasonic frequency was
40 kHz. The dark grey precipitates were separated, washed with
deionized water and absolute ethanol three times, and dried
School of Materials Science and Engineering, Beijing Institute of Technology, Beijing
100081, People's Republic of China. E-mail: yingliu@bit.edu.cn; abc.1984816@163.
com
ꢁ
under vacuum at 40 C for 24 h.
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2.2 Instruments and measurements
The X-Ray diffraction (XRD) patterns were recorded on a Bruker
˚
D8 Advance diffractometer in Mo Ka radiation (l ¼ 0.7093 A)
operated at 50 kV and 30 mA. The scanning electron microscopy
(SEM) images were obtained using a QUANTA600 scanning
electron microscope operated at 25 kV. Transmission electron
microscopy (TEM) images and the corresponding selected-area
electron diffraction (SAED) pattern were obtained with a JEOL
JEM-2010 microscope at 200 kV. The magnetization measure-
ment was performed by vibrating sample magnetometer (VSM).
The complex permittivity (3r ¼ 30 ꢀ j300) and relative complex
permeability (mr ¼ m0 ꢀ jm00) were determined using the T/R
coaxial line method in the range of 1–18 GHz with an O-ring
shaped sample (i.d. ¼ 3 mm, o.d. ¼ 7 mm and thickness d ¼ 2
mm) using an HP8722ESS vector network analyzer.
3 Results and discussion
Fig. 2 The SEM and TEM images of the cobalt particles. (a) SEM image
of the cobalt particles (2.5k); (b) SEM image of the cobalt particles (5k);
(c) TEM image of the cobalt particles; (d) the selected-area diffraction
(SAED) pattern of the circles area in (c).
Fig. 1 shows the XRD pattern of as-prepared samples. The peaks
at 2q ¼ 18.86ꢁ, 21.40ꢁ and 35.96ꢁ match very well with the (100),
(101) and (103) planes of hcp-cobalt. The peaks at 2q ¼ 23.09ꢁ
and 40.56ꢁ can be well indexed to the (200) and (222) planes of
fcc-cobalt. The remaining are overlap peaks of hcp and fcc-
cobalt. The result demonstrates that the as-synthesized samples
are a mixture of hcp and fcc-cobalt. No impurity peaks were
detected in the XRD pattern, such as Co(OH)2, CoO and so on,
which indicated the as-prepared samples were pure.
Fig. 2 shows the morphology of cobalt particles. Hierarchic
sword-like cobalt particles, assembled by sword-like akes, were
observed. The size of cobalt particles is ꢂ8 mm, and the thick-
ness of sword-like akes is ꢂ1 mm. Further observation that
each leaf seems to grow from the same center, and each sword-
like ake consists of a long central main branch and very blunt
secondary branches. The secondary branches preferentially
grow along two denite directions rather than exhibit randomly
ramied grown, as shown in Fig. 2(a and b). The SAED pattern
(Fig. 2(d)), corresponding to the circles area in Fig. 2(c), illus-
trates a perfect single crystal nature for sword-like akes. As
shown in Fig. 2(d), the [001] direction corresponding to the
(002) plane of hcp-cobalt is parallel to the grown direction of the
main branch, as displayed by the arrow in Fig. 2(c). However,
the [101] direction is parallel to the grown direction of
secondary branches. These indicate that the growths of the
main branch and secondary branches are along [001] and [101]
directions, respectively. The XRD pattern shows that main
phase for as-synthesized sample is fcc-cobalt, not agreeing with
the SAED pattern. The reason may be that at the early stage, the
formation of fcc-cobalt occurs due to high deposition rate of
cobalt atoms, and then the deposition rate of cobalt atoms
become slower at the later stage, causing hcp-cobalt formed. So
the SAED pattern on the edge of leaf-like ake illustrates hcp-
cobalt structure.
Fig. 3 shows effect of temperature and concentration of
NaOH on the morphology of cobalt particles. From le to right,
when the concentration of NaOH remains the same, with the
reaction temperature increasing, the cobalt particles tend to
fractal grown. From bottom to top, improving the concentration
of NaOH is benecial to fractal grown in the cobalt particles
when the temperature is equal. Cobalt particles with different
morphologies are obtained by adjusting the reaction tempera-
ture and concentration of NaOH, respectively. In particular, it is
interesting that cobalt particles with ball-chain structure were
achieved when the reaction temperature is 40 ꢁC, and the
concentration is 0.5 mol Lꢀ1 and 1 mol Lꢀ1, respectively, as
shown in Fig. 3(g and j), which is related to magnetic interaction
between cobalt particles.
Fig. 4 shows effect of temperature and [C4H4O6]2ꢀ/Co2+ ratio
on the morphology of cobalt particles. From le to right, it is
benecial to fractal grown by improving the temperature when
[C4H4O6]2ꢀ/Co2+ ratio is equal. From bottom to top, the
morphology of cobalt particles tends to fractal grown by
decreasing the [C4H4O6]2ꢀ/Co2+ ratio when the reaction
temperature remains the same. Further observed that cobalt
particles with ball-chain structure were also obtained when
Fig. 1 The XRD pattern of the as-synthesized samples.
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Fig. 3 Effect ꢀo1f temperature and ꢀc1oncentration of NaOH on the morphꢀo1logy of cobalt partꢀic1les (a) 40 ꢁC, 7 mol Lꢀ1; (b) 65 ꢁC, 7 mol Lꢀ1; (c)
80 ꢁC, 7 mol L ; (d) 40 ꢁC, 4 molꢀL1 ; (e) 65 ꢁC, 4 mol Lꢀ1;ꢀ(f1) 80 ꢁC, 4mol L ; (g) 40 ꢁC, 1 mol L ; (h) 65 ꢁC, 1 mol Lꢀ1; (i) 80 ꢁC, 1 mol Lꢀ1; (j) 40 ꢁC,
0.5 mol Lꢀ1; (k) 65 ꢁC, 0.5 mol L ; (m) 80 ꢁC, 0.5 mol L
.
temperature was 40 C and the [C4H4O6]2ꢀ/Co2+ ratio was 3, as
shown in Fig. 4(d).
ꢁ
[Co(N2H4)3]2+ + N2H4 + OHꢀ
CoY + N2[ + H2[ + NH3[ + H2O
/
(3)
In general, the morphology of cobalt particles is ascribed to
the formation rate of cobalt atoms. A fast reduction rate of Co2+
results in a high cobalt atom concentration, which is benecial
to promote one-dimensional grown of cobalt. The faster the
reduction rate of Co2+ is, the more obvious the branching ratio
in the cobalt particles. Conversely, a slower reduction rate
promotes the formation of ball structure of cobalt particles.
According to the following chemical reactions:18–20
According to the chemical reaction (3) and (4), the reduction
rate of Co2+ could be promoted though decreasing the
[C4H4O6]2ꢀ/Co2+ ratio or improving the concentration of NaOH,
being benecial to the fractal grown in the cobalt particles.
Meanwhile, the reduction ability of N2H4$H2O is directly
determined by the concentration of NaOH or [C4H4O6]2ꢀ/Co2+
ratio, controlling formation rate of cobalt atoms. As well known,
improving the reaction temperature could also promote the
reduction rate of Co2+, and tend to the fractal grown. Therefore,
the hierarchic sword-like morphology of cobalt particles is the
result of comprehensive action of the reaction factors.
2ꢀ2n
Co2+ + n(C4O6H4)2ꢀ 4 [Co(C4H6O4)]n
(1)
[Co(C4O6H4)]n2ꢀ2n + 3N2H4 /
[Co(N2H4)3]2+ + n(C4O6H4)2+
(2)
Fig. 5 shows magnetic hysteresis loop for cobalt particles at
room temperature. We can see that the saturation
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Fig. 4 Effect of temperature and [C4H4O6]2ꢀ/Co2+ ratio on the morphology of the cobalt particles (a) 40 ꢁC, [C4H4O6]2ꢀ/Co2+ ¼ 6; (b) 65 ꢁC,
[C4H4O6]2ꢀ/Co2+ ¼ 6; (c) 80 ꢁC, [C4H4O6]2ꢀ/Co2+ ¼ 6; (d) 40 ꢁC, [C4H4O6]2ꢀ/Co2+ ¼ 3; (e) 65 ꢁC, [C4H4O6]2ꢀ/Co2+ ¼ 3; (f) 80 ꢁC, [C4H4O6]2ꢀ
/
Co2+ ¼ 3; (g) 40 ꢁC, [C4H4O6]2ꢀ/Co2+ ¼ 1; (h) 65 ꢁC, [C4H4O6]2ꢀ/Co2+ ¼ 1; (i) 80 ꢁC, [C4H4O6]2ꢀ/Co2+ ¼ 1; (j) 40 ꢁC, [C4H4O6]2ꢀ/Co2+ ¼ 0; (k)
65 ꢁC, [C4H4O6]2ꢀ/Co2+ ¼ 0; (m) 80 ꢁC, [C4H4O6]2ꢀ/Co2+ ¼ 0.
magnetization (Ms) of cobalt particles is 150.17 emu gꢀ1, less particles, and Ln, between each odd-numbered and even-
7% than that of bulk cobalt (162 emu gꢀ1).21 The reduction in Ms numbered pairs. The number, n, is for the sphere numbering in
for the cobalt particles is ascribed to formation of oxides on the the chain. According to the equation, Hc is inversely propor-
particle's surface and a decrease in crystallinity. The former tional to R3, so Hc of sword-like cobalt particles is larger due to
introduces paramagnetism and instead the latter introduces cobalt particles in micron scale. Moreover, hcp-cobalt in cobalt
superparamagnetism. The coercivity (Hc) for samples is 100.34 particles could improve coercivity of cobalt particles due to
Oe, much larger than that of bulk cobalt (10 Oe).22,23 Hc can be larger magnetocrystalline anisotropy energy. The stacking fault
expressed as:24,25
could also improve the coercivity for cobalt particles.26 There-
fore, enhanced coercivity for sword-like cobalt particles is a
comprehensive consequence of size, crystal phase and stacking
fault.
The reection loss (RL) for sword-like cobalt particles was
calculated using the complex permittivity and permeability at
given frequency and thickness layer according to the transmit
line theory, which is summarized as the following equations:27,28
m
R3
Hc;n
¼
ð6Kn ꢀ 4LnÞ
(4)
where m and R are the dipole moment and the diameter of each
sphere. The expression takes into account the dipolar interac-
tion between the magnetic spheres in the chain based on the
assumption of fanning mode. In particular, Kn accounts for the
dipolar interaction between each every pair of the magnetic
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thickness is in the range of 1.01–4.73 mm, and an absorption
exceeding ꢀ20 dB is obtained for the thickness of 1.11–1.97
mm. Table 1 shows the comparison of the cobalt particles and
other representative materials in earlier literatures. We can see
that the as-prepared cobalt particles have potential application
as candidate for microwave absorption.
To investigate the possible microwave absorption mecha-
nism for sword-like cobalt particle, complex permittivity and
permeability behaviors have been studied as shown in Fig. 7.
Three resonance peaks were achieved for real part of permit-
tivity at 7.43, 13.27, 17.30 GHz, respectively. Lagging behind
resonance peaks for real part, both imaginary part and dielec-
tric loss exhibit four resonance peaks at 3.40, 9.08, 13.48 and
17.63 GHz. Further observed that permittivity and dielectric loss
increase with frequency increasing in the range of 1–18 GHz as
shown in Fig. 7(a), which may be due to the defect in the cobalt
particles. In general, permittivity usually stems from orientation
polarization, atomic polarization and electronic polariza-
tion.33,34 Normally, the resonance caused by vacancy or pores
usually dominates in the low-frequency regions. Atomic and
electronic polarization is contributed to high-frequency reso-
nance. Therefore, multi-nonlinear dielectric resonance is a
comprehensive result of atomic and electronic polarization.
Fig. 7(b) shows the real part of permeability (m0) deceases
Fig. 5 The hysteresis loop of the sword-like cobalt particles (at room
temperature).
ꢀ
ꢁ
rffiffiffiffi
mr
2p
pffiffiffiffiffiffiffiffi
Zin ¼ Z0
tanh j
mr3rfd
(5)
(6)
3r
c
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
Zin ꢀ Z0
RLðdBÞ ¼ Z0 log
Zin þ Z0
where Z0 is the impedance of the free space, Zin is the input
impedance, mr and 3r are the relative permeability and permit- from 1.87 to 0.39 with frequency increasing. Multiple resonance
tivity of the absorber, respectively, f is the frequency of the peaks were exhibited for imaginary part (m00) of permeability at
electromagnetic wave, d is the thickness of the absorber, and c is 5.73, 11.60 and 15.26 GHz. For magnetic materials, the
the velocity of the light in free space. The complex permeability magnetic loss mainly stems from magnetic hysteresis, domain
and permittivity were tested on a network analyzer with the wall resonance, natural resonance35 and exchange resonance.36
frequency of 1–18 GHz. The calculated reection loss for the Magnetic hysteresis originates from irreversible magnetization
paraffin-based composite with 70 wt% cobalt particles is shown occurs in a highly applied eld, while domain wall movement
in Fig. 6. A maximum reection loss of ꢀ60.13 dB at 13.72 GHz derived from multi-domain materials occurs in the low
was obtained for cobalt particles corresponding to a thickness frequency (<2 GHz). However, resonance resulting from spin
of 1.4 mm, and the absorption bandwidth (RL # ꢀ10 dB) is 5.42 rotational component occurs at high-frequency range.37 There-
GHz in Fig. 6(a). Moreover, the effect absorption bandwidth (RL fore magnetic loss for sword-like cobalt particles could be
# ꢀ20 dB) of 1.75 GHz was achieved. Strong and high-efficiency ascribed to natural resonance and exchange resonance.
microwave absorption property for cobalt particles has been
exhibited, and it is expected to be used as absorbers for elec-
tromagnetic microwave absorption and shielding elds.
Fig. 6(b) shows the dependence of reection loss on the thick-
ness in the range of 1–18 GHz. It can be seen that microwave
absorption below ꢀ10 dB can be achieved as long as the
To investigate the magnetic loss, we introduced the equa-
2
tion, m00ðm0Þꢀ2f ꢀ1
¼
pm0d2s; 38,39 where m0 is the vacuum
3
permeability, m0 is the real part of permeability, m00 is the
imaginary part of permeability, s is the conductivity, f is the
frequency, d is the thickness of the sample. According to the
Fig. 6 Frequency dependence of reflection loss of cobalt particles in the range of 1–18 GHz.
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Table 1 The comparison of microwave absorption between as-prepared cobalt particles and other representative materials
dm (mm)
Frequency (GHz)
Sample
RLmax (dB)
(RL # ꢀ20 dB)
(RL # ꢀ20 dB)
Percentage (wt%)
Reference
Fe/SmO
Fe/Y2O3
Fe3Al/Al2O3
C/Co
MWCNTs/Co
Sword-like Co
ꢀ52.00
ꢀ36.00
ꢀ45.00
ꢀ40.00
ꢀ37.00
ꢀ60.13
7.9–13.1
3–5
1.5–2.5
5
4.18–6.82
1.1–1.92
0.73–1.3
2–3.5
7.2–17.4
4.2
2.35–3.51
11.87–17.85
80
80
75
30
60
70
29
30
31
32
16
This work
equation, if the values of m00(m0)
f
ꢀ2 ꢀ1 is constant with frequency
The two resonance peaks at 11.60 and 15.26 GHz is
changing, magnetic loss only stems from the eddy current. If ascribed to exchange resonance. Multiple resonance problem
not, the magnetic loss is ascribed to natural resonance and is an interesting subject which is widely discussed in the
exchange resonance. Fig. 8 shows frequency dependence of reported literatures. And the famous and successful idea is
m00(m0)ꢀ2 ꢀ1 in the range of 1–18 GHz. We can see that the value exchange resonance mode developed by Aharoni.44 When the
f
of m00(m0)ꢀ2fꢀ1 deceases with frequency increasing, indicating magnetic particles size is in nanoscale, the exchange energy
that magnetic loss of sword-like cobalt particles resulted from a among magnetic particles becomes important compared
consequence of natural resonance and exchange resonance.
with magnetostatic energy. The exchange energy would
The rst resonance peak at 5.73 GHz is due to natural contribute to multiple resonance,45 and according to the
resonance.40 When the frequency of incident microwave accords exchange resonance mode, the exchange resonance
with the intrinsic frequency of the magnetization spinning frequency is given by u/g0 ¼ Cmkn2/R2Ms + Hz, where C is the
oscillation along the effective anisotropic eld, the natural exchange constant (fcc-Co, 6.6 ꢃ 10ꢀ11 J mꢀ3, hcp-Co, 6 ꢃ
resonance would occur in magnetic particles. The natural 10ꢀ11 J mꢀ3), R is the radius of the sphere particles, Hz is the
resonance frequency is given by fr ¼ (g0/2p) ꢃ He,40,41 with g0 applied dc eld. The eigen values mkn are the roots of the
the gyromagnetic ratio, 2.6 GHz/T (ref. 42) and He the effective equation, [djn(mknr/R)/dr]r¼R ¼ 0, with jn the spherical Bessel
anisotropic eld. The anisotropic eld depends on the crystal- functions. The three rst mkn roots are m11 ¼ 2.08; m12 ¼ 3.34;
line anisotropy, particles morphology and the magnetic inter- m13 ¼ 4.51. Further, the exchange energy has been evidenced
action between particles. For bulk cobalt, the effective to be important for nanocrystalline alloy particles,45 and we
anisotropy eld is only ascribed to its crystalline anisotropy. The therefore assumes that the exchange resonance mode is
4K1
3m0Ms
suitable for our cobalt particle case. Table 2 shows the reso-
nance parameters of the cobalt superstructure are compar-
ison to the other magnetic materials in earlier reported
literatures. It has been demonstrated that the resonance
frequency of magnetic materials in GHz range is determined
by particle size, particle morphology and the composition of
the magnetic particles. Anyhow, the coexisting of the
magnetic resonance, including natural resonance and
exchange resonance, is benecial to widen the bandwidth of
microwave absorption.
magnetocrystalline anisotropy eld Hk ¼ ꢀ
(fcc-Co, K1 ¼
ꢀ6.5 ꢃ 104 J mꢀ3, hcp-Co, K1 ¼ 5.2 ꢃ 105 J mꢀ3). However, the
as-synthesized cobalt particles are sword-like structure, and so
the particles morphology also inuences the effective anisot-
ropy eld through its demagnetization eld. Therefore, the
natural resonance frequency is given by fr ¼ (g0/2p) ꢃ (Hc + Hk),
where Hc is the shape anisotropy eld. The broader resonance
peak is attributed to the particles size distributions, disordered
particles orientation or the larger intrinsic damping parameter
of the cobalt particles.43,44
Fig. 7 Frequency dependence of electromagnetic parameters of cobalt particles in the range of 1–18 GHz. (a) Permittivity and dielectric loss; (b)
permeability and magnetic loss.
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particles have potential application as candidate for microwave
absorption.
References
1 J. Liu, J. Cheng, R. Che, J. Xu, M. Liu and Z. Liu, ACS Appl.
Mater. Interfaces, 2013, 5, 2503–2509.
2 Y. Tang, Y. Shao, K. Yao and Y. Zhong, Nanotechnology, 2014,
25, 035704.
3 L.-S. Fu, J.-T. Jiang, L. Zhen and W.-Z. Shao, Mater. Sci. Eng.,
B, 2013, 178, 225–230.
4 H. Qin, Q. Liao, G. Zhang, Y. Huang and Y. Zhang, Appl. Surf.
Sci., 2013, 286, 7–11.
Fig. 8 Frequency dependence of m00(m0)ꢀ2fꢀ1 of cobalt particles in the
range of 1–18 GHz.
5 C. Yin, Y. Cao, J. Fan, L. Bai, F. Ding and F. Yuan, Appl. Surf.
Sci., 2013, 270, 432–438.
6 A. Porch, D. Slocombe and P. P. Edwards, Phys. Chem. Chem.
Phys., 2013, 15, 2757–2763.
Table 2 The comparison of resonance parameters of cobalt super-
structure with other reported magnetic particles
7 J. Snoek, Physica, 1948, 14, 207–217.
Materials
fnat (GHz) fexch-1 (GHz) fexch-2 (GHz) Reference
8 O. Acher and S. Dubourg, Phys. Rev. B: Condens. Matter Mater.
Phys., 2008, 77, 104440.
FeCoNi ake
FeCo nanoplates
Ni nanocone
Co nanoplatelets
Co nanochains
Co nanoakes
Co particles
7.91
4.80
4.40
3.30
12.30
5.74
5.73
14.87
15.26
12.00
11.60
14.50
7.79
—
—
—
—
—
12.00
15.26
46
47
48
49
50
41
´
9 P. Toneguzzo, G. Viau, O. Acher, F. Fievet-Vincent and
´
F. Fievet, Adv. Mater., 1998, 10, 1032–1035.
10 S.-S. Kim, S.-T. Kim, Y.-C. Yoon and K.-S. Lee, J. Appl. Phys.,
2005, 97, 10F905.
11 L. Wu, J. Ding, H. Jiang, C. Neo, L. Chen and C. Ong, J. Appl.
Phys., 2006, 99, 083905.
11.60
This work
12 X. Zhang, X. Dong, H. Huang, Y. Liu, W. Wang, X. Zhu, B. Lv,
J. Lei and C. Lee, Appl. Phys. Lett., 2006, 89, 053115.
13 X. F. Zhang, X. L. Dong, H. Huang, B. Lv, J. P. Lei and
C. J. Choi, J. Phys. D: Appl. Phys., 2007, 40, 5383–5387.
14 L. Yan, J. Wang, X. Han, Y. Ren, Q. Liu and F. Li,
Nanotechnology, 2010, 21, 095708.
15 Z. Ma, J. Wang, Q. Liu and J. Yuan, Appl. Surf. Sci., 2009, 255,
6629–6633.
16 F. Wen, F. Zhang and Z. Liu, J. Phys. Chem. C, 2011, 115,
14025–14030.
17 G. Tong, J. Yuan, W. Wu, Q. Hu, H. Qian, L. Li and J. Shen,
CrystEngComm, 2012, 14, 2071–2079.
18 L. Guo, F. Liang, N. Wang, D. Kong, S. Wang, L. He, C. Chen,
X. Meng and Z. Wu, Chem. Mater., 2008, 20, 5163–5168.
19 L.-P. Zhu, H.-M. Xiao, W.-D. Zhang, G. Yang and S.-Y. Fu,
Cryst. Growth Des., 2008, 8, 957–963.
It is well known that to obtain the materials with high-effi-
cient microwave absorption, two problems must be focused on:
impedance matching between air and absorbent, attenuation in
the interior of absorbent. To meet the impedance matching, the
permittivity should be equal to permeability, i.e. 30 ¼ m0, 300 ¼ m00.
However, for the magnetic metallic materials, the real part of
permeability is smaller than that of permittivity. Therefore, the
real part of permittivity decreases to satisfy the impedance
matching. The high attenuation in the interior of absorbent
needs higher dielectric and magnetic loss. In a word, tailoring
the electromagnetic parameters of the magnetic materials is
necessary to achieve superior microwave absorption. In the
present case, the superior microwave absorption of the cobalt
particles is due to the optimized electromagnetic parameters.
No doubt that the multiple dielectric and magnetic resonance is
benecial to widen the microwave absorption bandwidth.
20 G. Dumpich, T. Krome and B. Hausmanns, J. Magn. Magn.
Mater., 2002, 248, 241–247.
21 J. B. Tracy, D. N. Weiss, D. P. Dinega and M. G. Bawendi,
Phys. Rev. B: Condens. Matter Mater. Phys., 2005, 72, 064404.
4 Conclusions
In summary, the cobalt particles with hierarchic sword-like 22 S. Chikazumi and S. H. Charap, Physics of magnetism, RE
structure were synthesized through the liquid reduction Krieger Publishing Company, Malabar, FL, 1978.
method. Multiple dielectric and magnetic resonance peaks were 23 Y.-J. Zhang, Q. Yao, Y. Zhang, T.-Y. Cui, D. Li, W. Liu,
exhibited in the microwave range of 1–18 GHz due to atomic
and electronic polarization. One natural resonance and two
W. Lawrence and Z.-D. Zhang, Cryst. Growth Des., 2008, 8,
3206–3212.
exchange resonance were obtained, contributing to widen the 24 I. Jacobs and C. Bean, Phys. Rev., 1955, 100, 1060.
effective microwave absorption bandwidth. A maximum reec- 25 W. Zhou, K. Zheng, L. He, R. Wang, L. Guo, C. Chen, X. Han
tion loss of ꢀ60.13 dB at 13.72 GHz for cobalt particles was
and Z. Zhang, Nano Lett., 2008, 8, 1147–1152.
´
˜
˜
´
obtained with a thickness of 1.4 mm, and the effective absorp- 26 J. Sort, J. Nogues, S. Surinach, J. Munoz and M. Baro, Mater.
tion bandwidth (RL # ꢀ10 dB) is 5.42 GHz, indicating the cobalt
40462 | RSC Adv., 2014, 4, 40456–40463
Sci. Eng., A, 2004, 375, 869–873.
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Paper
RSC Advances
27 E. Michielssen, J.-M. Sajer, S. Ranjithan and R. Mittra, IEEE 39 M. Wu, Y. Zhang, S. Hui, T. Xiao, S. Ge, W. Hines, J. Budnick
Trans. Microwave Theory Tech., 1993, 41, 1024–1031. and G. Taylor, Appl. Phys. Lett., 2002, 80, 4404–4406.
28 M. Zhou, X. Zhang, J. Wei, S. Zhao, L. Wang and B. Feng, 40 C. Kittel, Phys. Rev., 1948, 73, 155.
J. Phys. Chem. C, 2010, 115, 1398–1402.
41 F. Ma, Y. Qin and Y.-Z. Li, Appl. Phys. Lett., 2010, 96, 202507.
29 S. Sugimoto, T. Maeda, D. Book, T. Kagotani, K. Inomata, 42 X. Liu, M. M. Steiner, R. Sooryakumar, G. A. Prinz,
M. Homma, H. Ota, Y. Houjou and R. Sato, J. Alloys
Compd., 2002, 330–332, 301–306.
R. F. C. Farrow and G. Harp, Phys. Rev. B: Condens. Matter
Mater. Phys., 1996, 53, 12166–12172.
30 J. R. Liu, M. Itoh and K.-i. Machida, Chem. Lett., 2003, 394– 43 R. Ramprasad, P. Zurcher, M. Petras, M. Miller and
395. P. Renaud, J. Appl. Phys., 2004, 96, 519–529.
31 W. Yang, L. Qiao, J. Wei, Z. Zhang, T. Wang and F. Li, J. Appl. 44 K. Rozanov, Z. Li, L. Chen and M. Koledintseva, J. Appl. Phys.,
Phys., 2010, 107, 033913. 2005, 97, 013905.
32 Q. Liu, D. Zhang and T. Fan, Appl. Phys. Lett., 2008, 93, 45 W. Zhong, C. Q. Sun, S. Li, H. Bai and E. Jiang, Acta Mater.,
013110–013110-3. 2005, 53, 3207–3214.
33 Z. Li, B. Shen, Y. Deng, L. Liu and W. Hu, Appl. Surf. Sci., 46 L. J. Deng, P. H. Zhou, J. L. Xie and L. Zhang, J. Appl. Phys.,
2009, 255, 4542–4546. 2007, 101.
34 Z. Li, Y. Deng, B. Shen and W. Hu, Mater. Sci. Eng., B, 2009, 47 Y. Yang, C. Xu, Y. Xia, T. Wang and F. Li, J. Alloys Compd.,
164, 112–115.
2010, 493, 549–552.
35 G. Bertotti, J. Appl. Phys., 1985, 57, 2110–2117.
36 A. Aharoni, J. Appl. Phys., 1991, 69, 7762–7764.
48 F. Ma, Y. Qin, F. Wang and D. Xue, Scr. Mater., 2010, 63,
1145–1148.
37 G. Tong, W. Wu, R. Qiao, J. Yuan, J. Guan and H. Qian, 49 J. Li, J. Huang, Y. Qin and F. Ma, Mater. Sci. Eng., B, 2007,
J. Mater. Res., 2011, 26, 1639. 138, 199–204.
38 F. Wen, H. Yi, L. Qiao, H. Zheng, D. Zhou and F. Li, Appl. 50 X.-L. Shi, M.-S. Cao, J. Yuan and X.-Y. Fang, Appl. Phys. Lett.,
Phys. Lett., 2008, 92, 042507.
2009, 95, 163108.
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