Structural, magnetic and electrical properties of CoSi ferrites synthesized by sol-gel self-propagating method

Article abstract of DOI:10.1016/j.physb.2020.412655

CoFe₂O₄ is mainly used in magnetic strain sensor, magnetic resonance imaging, supercapacitor, cryogen, high density magnetic recording medium and magneto-optical devices and so on. In this study, Si_xCo_1-xFe

Full text of DOI:10.1016/j.physb.2020.412655

Physica B xxx (xxxx) xxx
Contents lists available at ScienceDirect
Physica B: Physics of Condensed Matter
journal homepage: http://www.elsevier.com/locate/physb
Structural, magnetic and electrical properties of CoSi ferrites synthesized by
sol-gel self-propagating method
Zheng-Xiong Tao, Le-Zhong Li*, Xiao-Hui Wu, Xiao-Xi Zhong, Zi-Chen Zhong
Sichuan Province Key Laboratory of Information Materials and Devices Application, College of Optoelectronic Engineering, Chengdu University of Information
Technology, Chengdu, 610225, Sichuan, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords:
CoFe2O4 is mainly used in magnetic strain sensor, magnetic resonance imaging, supercapacitor, cryogen, high
density magnetic recording medium and magneto-optical devices and so on. In this study, SixCo1-xFe2O4 (where
x = 0, 0.05, 0.10, 0.15, 0.30, 0.40, 0.50) ferrites were prepared by sol-gel self-propagating method, and the
effects of different amount of silicon substitution on the properties of cobalt ferrite were studied. The X-ray
diffraction spectrums demonstrate that the secondary phase Fe2O3, Fe2⋅56Si0⋅44O4 and FeO appear when x ≥ 0.15.
The SEM images of the samples show that with the increase of silicon ion content, the porosity of cobalt ferrite
increases and the sintering density decreases. The substitution of silicon significantly changed the electromag-
netic properties of Co ferrite samples. Fortunately, the coercive force (Hc) increased. Moreover, the dielectric
constant raised initially when 0 ≤ x ≤ 0.30 and then reduced when 0.30 < x ≤ 0.50 with the increasing Si
substitution at low frequency, which achieves the maximum at x = 0.30.
Co ferrites
Si substitution
Electromagnetic properties
1. Introduction
MeFe2O4, where Me is a divalent cationic metals, like Ni, Zn, Mn, Co, Cu
[14–19]. Co ferrite has an inverse spinel structure, and the degree of
As a permanent magnet material, cobalt ferrite has been widely
studied for its high Curie temperature, high coercivity, high resistivity,
high magnetocrystalline anisotropy, medium saturated magnetization,
excellent chemical stability as well as good mechanical and thermal
solidity [1–6]. This ferrite is mainly used in magnetic strain sensor,
magnetic resonance imaging, supercapacitor, cryogen, high density
magnetic recording medium and magneto-optical devices and so on
[7–11]. In recent years, with the rapid improvement of software and
hardware technology in the world, the properties of present materials is
difficult to fulfill the needs of the development of electronic equipment,
so it is urgent to improve the electromagnetic properties of existing
materials to make up for the defects of current materials. Researchers
found that metal cation substitution can effectively enhance the elec-
tromagnetic properties of ferrites. In many methods of preparing ferrite
nanopowders, sol-gel method can produce nanopowders with high pu-
rity at lower temperature, and its cost is low [12,13]. Therefore, we
prepared the silicon substituted CoFe2O4 by Sol-gel self-propagating
method to improve the electrical and magnetic properties of the
material.
reversal is decided by the heat-treat condition [20]. Moreover, Co2+ is
mainly distributed in B sites, Fe3+ occupied A (tetrahedral) and B
(octahedral) positions, and the number of Fe3+ in A positions is almost
the same as that in B sites [21]. Generally speaking, the performance of
spinel ferrite is mainly decided by the inherent cation occupation be-
tween A and B sites, the lattice strain caused by substitution/doping and
the properties of substitution/doping elements [1,14]. CoFe2O4 is
anti-spinel structure [11]. Therefore, scientists often use different metal
cations substitution to improve the microstructure and electromagnetic
properties of cobalt ferrite. In recent years, there are two main types of
substitution studies, one is single cation substitution, such as
Ni-substituted Co–Mn ferrite [1], Ni-substituted Co ferrite [22],
In-substituted NiFe2O4 ferrite [23], Co replace Fe in CoFe2O4 [24], Al
substituted for Fe in NiFe2O4 ferrite [19]. And the other is the joint
substitution of bimetallic cations, such as the joint substitution of Si4+
and Co2+ for Fe3+ [18], the joint substitution of Ge4+ and Co2+ for Fe3+
[25], the joint substitution of Ti4+ and Co2+ for Fe3+ [21].
Some studies have shown that the addition of different silicon con-
tent to ferrite samples will cause different changes in its electrical or
magnetic properties [17,26,27]. In addition, Si4+ and Ge4+ are both
As everyone knows, the chemical formula of spinel ferrite is
* Corresponding author.
E-mail address: lezhongli@cuit.edu.cn (L.-Z. Li).
https://doi.org/10.1016/j.physb.2020.412655
Received 13 August 2020; Received in revised form 19 October 2020; Accepted 20 October 2020
Available online 5 November 2020
0921-4526/© 2020 Elsevier B.V. All rights reserved.
Please cite this article as: Zheng-Xiong Tao, Physica B, https://doi.org/10.1016/j.physb.2020.412655

Z.-X. Tao et al.
Physica B: Physics of Condensed Matter xxx (xxxx) xxx
tetravalent ions, and the behavior of semiconductors is quite similar.
Both Si4+ and Ge4+ tend to occupy A (tetrahedral) positions in spinel
ferrite [18,25,28]. The study of Ge4+ substituted cobalt ferrite has been
reported [29], but no one has ever done the study of Si4+ substituted
Co2+ for CoFe2O4 ferrite. Therefore, for the purpose of researching how
different Si4+ content affects the microstructure and physical perfor-
mance of CoFe2O4 ferrite, we produced Si displaced Co ferrite by sol-gel
self-propagating method. Subsequently, the microstructure and elec-
tromagnetic properties of these samples were tested.
h k and l are the Miller indices and dhkl is the inter-planar spacing in this
calculation formula.
The X-ray density can be counted by the following calculation for-
mula [31,32]:
8Mm
dx =
(2)
NAa3
where Mm is the relative molecular mass; NA is the Avogadro’s number
and a is the lattice constant.
In addition, the sintering density (d) was measured by Archimedes
drainage method, and the porosity percentage (P) was computed on the
basis of the formula:
2. Experimental process
2.1. Production of silicon substituted Co ferrite
[
]
d
dx
P = 1 ꢀ
× 100%
(3)
The sample of SixCo1-xFe2O4 (where x = 0, 0.05, 0.10, 0.15, 0.30,
0.40 and 0.50) ferrite nanoparticles were prepared by means of sol-gel
self-propagation method. The raw materials used are Co(NO3)2⋅6H2O,
Fe(NO3)3⋅9H2O, Silica powder and citric acid. The molar ratio of citric
acid to cation is 3:1. The raw materials were dissolved in a beaker with
deionized water and heated to 80 ◦C in a water bath. So, 500 ml of 0.1
mol/L mixed solution was prepared. The sol precursor was formed after
stirring in water bath for 3 h. After mixing, slowly add ammonia to the
solution when it cools to normal atmospheric temperature so that
According to Brown’s relation coercive force (Hc) combined with
anisotropy constant (K1) and saturated magnetization (Ms) though the
following computed fomula [31]:
2K1
μ0MS
Hc =
(4)
The real part of the permittivity is calculated from the measured
capacitance data according to the following formula [33]:
◦
regulate the pH to 7. The sol precursor was dried at 80 C to obtain
xerogel. Finally, a small amount of ethanol was poured in as a com-
bustion supporting agent to ignite the xerogel. Then, the self-
propagation combustion was used to obtain ferrite nanopowders. Add-
ing a small amount of 7 wt% poly-vinylalcohol (PVA) to the prepared
ferrite nano powder to grind it into particles. The final sample is ob-
tained by pressing the powder particles into a round piece and sintering
in muffle furnace for 4 h at 1050 ◦C. Table 1 list the chemical reagents
dosage used in this experiment.
Cd
ε′
=
(5)
ε0A
Here, C is capacitance, d is the thick of the pellet, ε0 is the vacuum
dielectric constant, A is the circular area of the sample.
The saturation magnetization of cubic spinel ferrite can be calculated
by the following formula [34]:
8M
Ms =
(6)
(7)
a3
2.2. Structure and performance measurement
M = |MB ꢀ MA|
The main crystal phase and impurity phase were determined by DX
2700 X-ray diffraction instrument (XRD) (Cu target, Kα irradiation, λ =
1.5406 Å) at normal atmospheric temperature. The microstructure of
the cross section of the sample was surveyed by the JEOL JSM-6490L
scanning electron microscope (SEM). The hysteresis loop of the sam-
ple was measured by a Lake Shore 8604 vibrating sample magnetometer
where MA, MB are the magnetization of A and B atoms, respectively, a is
lattice constant and M is the net magnetic moment.
3. Results and analysis
3.1. Microstructure properties
(VSM) at normal atmospheric temperature. The dielectric constant (ε’)
of the sample at normal atmospheric temperature was measured with
WK 6500P LCR meter bridge in the frequency scale from 100 Hz to 1
MHz. The impedance temperature spectrum of the samples from 30 ◦C to
300 ◦C was measured by high temperature impedance analyzer.
The XRD diffraction spectrums of SixCo1-xFe2O4 (x = 0, 0.05, 0.10,
2.3. Formulas
The cell parameter (a) of the samples is computed using the
following calculation formula [30]:
dhkl
√̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
a =
(1)
h2 + k2 + l2
Table 1
Dosage of chemical reagent.
SixCo1-
xFe2O4
Si
Co(NO3)2⋅6H2O
Fe(NO3)3⋅9H2O
C6H8O7⋅H2O
(mol)
(mol)
(mol)
(mol)
x = 0.00
x = 0.05
x = 0.10
x = 0.15
x = 0.30
x = 0.40
x = 0.50
0
0.05
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.45
0.45
0.45
0.45
0.45
0.45
0.45
0.0025
0.005
0.0075
0.015
0.02
0.0475
0.045
0.0425
0.035
0.03
0.025
0.025
Fig. 1. XRD spectrum of SixCo1-xFe2O4 ferrites.
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Z.-X. Tao et al.
Physica B: Physics of Condensed Matter xxx (xxxx) xxx
0.15, 0.30, 0.40, 0.50) ferrite with x substitution are depicted in Fig. 1.
As shown in Fig. 1, the inflection peak matches well with that of spinel
within the error range when 0 ≤ x < 0.15; When 0.15 ≤ x ≤ 0.50,
Fe2⋅56Si0⋅44O4, Fe2O3 and FeO heterophases appear in the diffraction
pattern. This may be due to the balance of chemical valence, the
replacement of Si4+ ions make a part of the Fe3+ ions change into Fe2+
ions. Furthermore, the excessive substitution of Si4+ makes Si4+ ions fail
to enter the spinel structure completely, thus forming impurity phases.
Table 2 shows the change of lattice parameters, theoretical density,
X-ray density and porosity of the SixCo1-xFe2O4 ferrite with the substi-
tution amount of silicon atom. The relationship among these parameters
is given by formula (1), (2) and (3) in the experimental part. Derya
Erdem et al. reports that Si4+ has a strong preference for A-sites [7], so
when Si4+ takes the place of Co2+, Si4+ will occupy A-sites first, and
squeeze the original Fe3+ on A-positions to B-positions. Due to the
equilibrium of chemical valence, part of Fe3+ will change into Fe2+
when Si takes the place of Co2+. The radii of Si4+, Co2+, Fe3+ and Fe2+
are 0.4 Å, 0.82 Å, 0.67 Å, 0.83 Å, respectively. Therefore, as we can see
from Table 2, the increase of lattice parameter (a) when 0≤x ≤ 0.05 may
be due to the fact that a small amount of Si4+ entered the lattice, which
results in part of the small radius Fe3+ transformed into the large radius
Fe2+. However, the lattice constant decreased when 0.05 < x ≤ 0.15
may be because the small radius Si4+ (0.4 Å) replaces the large radius
Co2+ (0.82 Å), the Si4+ enters the tetrahedral position. When 0.15 < x ≤
0.30, the lattice constant increased again due to the formation of
Fe2⋅56Si0⋅44O4 heterophases at the grain boundary. Si4+ does not enter
the lattice completely, while Fe2+ continues to increase, so the lattice
continues to expand. Finally, when 0.30 < x ≤ 0.50, the decrease of
lattice constant may be due to the excessive Si4+ enters octahedral po-
sition in large quantities.
3.3. Magnetic performances
Fig. 3. (a) is the hysteresis loop diagram of SixCo1-xFe2O4 ferrites
measured at normal atmospheric temperature. The variations of coer-
cive force (Hc) and saturation magnetization (Ms) with the substitution
amount of Si4+ ions are shown in Fig. 3. (b). As is shown in Fig. 3. (b), the
saturation magnetization (Ms) initially reduces when x ≤ 0.40, and then
improves when x = 0.50 with the improvement of Si substitution. On the
other hand, the coercive force increases with the increasing substitution
amount of Si4+ ions on the whole.
Since cobalt ferrite is anti-spinel structure [11] and Si4+ ions prefer
to occupy A site [7], the metal ion distribution of SixCo1-xFe2O4 can be
written in the following form:
ꢀ
)[
]
3+
2+
Si4x+Fe
Fe2x+Co Fe3+ O4
1ꢀ x
1ꢀ x
The magnetic moments of Si4+, Co2+, Fe2+ and Fe3+ ions are 0, 3, 4
and 5 μB. So, according to formula (6) and (7), the molecular magnetic
moment of the sample can be calculated: M = |6x + 3|, and Ms∝M,
Therefore, theoretically, Ms should increase with the increase of x. This
is just in contradiction with the Ms curve shown in Fig. 3 (b). The reason
for this phenomenon may be the influence of porosity and impurities on
the sample is the main factor.
Research findings that the magnetic performances of ferrite are
closely related to its structure, composition, defect, internal stress and
cation distribution [37]. In addition, the grain size and phase pureness of
the sample also play an important role in the magnetic performances
[38]. It may be noted that the saturated magnetization (Ms) and coercive
force (Hc) of CoFe2O4 ferrite are largely decided by the cations distri-
bution, particle size and shape of the grains [39].
When 0 ≤ x < 0.10, the saturated magnetization (Ms) is inversely
proportional to the silicon content. This phenomenon can be explained
from the following two aspects. For one thing, the superexchange
interaction between atoms in A-B position is weakened as Si4+ enters A-
position, which result in the reduction of saturation magnetization (Ms);
For another, because of the inverse relationship between porosity and
magnetization of ferrite, the decrease of density and the increase of
porosity also result in the reduction of saturation magnetization (Ms)
[40]. When 0.10 < x ≤ 0.40, the reduction of saturation magnetization
(Ms) can also be explicated by the interaction between A-B positions and
porosity. Besides, the formation of impurity phase (Fe2O3 and
Fe2⋅56Si0⋅44O4) is also an important account for the decrease of satura-
tion magnetization (Ms). Because the formation of secondary phase will
increase the internal stress of the material, which will result in the
reduce of saturation magnetization (Ms). when 0.40 < x ≤ 0.50, the
change of saturation magnetization (Ms) is not obvious.
According to formula (2), with the increase of silicon substitution,
the X-ray density reduced from 5.29 g/cm3 to 4.94 g/cm3. This phe-
nomenon is largely thanks to the substitution of Si4+ ions with lower
relative atomic weight (28) for Co2+ ions with higher relative atomic
weight (58.93), resulting in the decrease of relative molecular weight,
and it caused the X-ray density decrease.
3.2. Microstructure properties
The fracture surface morphologies of SixCo1-xFe2O4 ferrites with the
different value of x is depicted in Fig. 2. From (a) to (g), the porosity of
the samples increased gradually, the grain size also increased slightly,
besides the agglomeration between particles is deepened. This phe-
nomenon indicates that the addition of silicon changed the micro-
structure of the samples. As far as we know, this kind of agglomeration
may be thanks to changing distribution of magnetic ions at A and B
positions by the addition of Si4+, which is caused by the interaction
among the magnetic ions [35]. Moreover, with the substitution of Co2+
by Si4+, in order to keep the charge equilibrium, the amount of metal ion
vacancy closed to the grain boundary increases, which promotes the
crystal boundary move, so the grain size increases [36].
The coercive force (Hc) of SixCo1-xFe2O4 ferrites as a variable of Si4+
substitution (x) is shown in the blue line in Fig. 3. (b), which suggests
that Hc increases in general with the improvement of the Si content.
When 0 ≤ x ≤ 0.10, the coercive force (Hc) of the samples improved
sharply with the increase of Si substitution.
According to formula (4), the improvement of Hc is mainly thanks to
the reduce of MS. In addition, according to the principle of magneto-
crystalline anisotropy model, stress anisotropy and impurity model
under multi-domain wall structure, Hc improved with the improvement
of magneto-crystalline anisotropy energy, stress, impurities contents
and the reduce of Ms [31]. Subsequently, the coercive force (Hc) increase
slowly when 0.10 < x ≤ 0.50, which may be due to the formation of
impurity phases. These newly formed phases increase the internal stress
of the material. Fig. 2 indicates that the porosity of the samples is
improving with the raise of Si concentration when 0.10 < x ≤ 0.50.
Therefore, the increase of porosity is another account for the increasing
coercive force.
As we can see from Table 2, the sintering density of the samples
decreased with the improvement of Si4+ ions substitution, which could
result in the increase of porosity according formula (3). This is consistent
with the increasing trend of porosity observed in Fig. 2.
Table 2
Structure parameters of SixCo1-xFe2O4 ferrites with x.
x
a (nm)
d (g/cm3)
dx (g/cm3)
P (%)
0.00
0.05
0.10
0.15
0.30
0.40
0.50
0.8381
0.8401
0.8382
0.8369
0.8423
0.8395
0.8382
4.82
4.65
4.32
4.52
3.72
3.62
3.42
5.29
5.22
5.22
5.21
5.01
4.99
4.94
8.91
10.90
17.37
13.34
25.72
27.48
30.78
3

Z.-X. Tao et al.
Physica B: Physics of Condensed Matter xxx (xxxx) xxx
Fig. 2. SEM images of SixCo1-xFe2O4 ferrites.
Fig. 3. (a) Hysteresis loops of SixCo1-xFe2O4 ferrites and (b) Ms and Hc of SixCo1-xFe2O4 ferrites.
3.4. Electrical performances
and the amount of Si substitution (x). We can see from this graph that the
activation energy initially reduced and then improved with the raise of
x, and there is a minimum when x = 0.30.
3.4.1. The electrical conductivity
Fig. 4 (a-b) demonstrates the change curve of direct-current re-
sistivity (ρd) with temperature for SixCo1-xFe2O4 ferrites. Fig. 4. (c) de-
picts the relationship between the activation energy (Eρ) of the sample
As a whole, the resistivity of the investigated samples reduces
exponentially with the improvement of temperature expect the sample
x = 0.10, which indicates they showed semiconductor characteristics.
4

Z.-X. Tao et al.
Physica B: Physics of Condensed Matter xxx (xxxx) xxx
Fig. 4. (a) ρd of the SixCo1-xFe2O4 ferrites (x = 0, 0.05, 0.10, 0.15, 0.30, 0.40, 0.50), (b) ρd of the SixCo1-xFe2O4 ferrites (x = 0.30, 0.40, 0.50) and (c) Eρ of the SixCo1-
xFe2O4 ferrites.
The sample with x = 0.10 indicats the metal-semiconductor transition
behavior, it has a transition temperature Tp. When the temperature is
below Tp, ρd shows the metallic conduction performance and improved
with the temperature; When the temperature is above Tp, ρd displays the
semi-conduction feature. The mechanism of metal conduction and
semiconductor conduction as well as the transition temperature of
ferrite have been studied [41,42]. The conductivity of ferrite is largely
contributed to the diversion of electrons between Fe2+ and Fe3+ ions
(Fe2+ ↔ Fe3+ + eꢀ ) [43]. The electron diversions are decided by the
activation energy [44]. The temperature dependence of direct-current
energy when 0.30 < x ≤ 0.50, the potential barrier will increase, which
will make it difficult to transfer electrons, increase the dc resistivity from
Fig. 4. (c). So, the resistivity of the sample increased when 0.30 < x ≤
0.50.
3.4.2. The dielectric performances
Fig. 5 (a) and (b) depicts the dependence of the dielectric constant
(ε’) of SixCo1-xFe2O4 ferrites in a frequency scale from 100 Hz to 1 MHz
at normal atmospheric temperature.
Fig. 5 indicates that the dielectric constant (ε′) of all of the SixCo1-
xFe2O4 ferrites decrease exponentially with the increase of frequency
approximately. Ferrite materials are composed of grains with good
conductivity, and there are grain boundaries with poor conductivity
between the grains [47]. The large space charge polarization will be
produced due to electrons gather at the grain boundary with high
resistance [48]. Therefore, the dielectric constant value is large at low
frequency. When the frequency exceeds a certain value, the electron
diversion between Fe2+ and Fe3+ can’t catch up with the change of
resistivity
(ρd) could be characterized by the formula: ρd =
ρ0exp(Eρ /kT) [45]. Where ρ0 is the resistivity at at infinite temperature,
k is the Boltzman’s constant. The activation energy (Eρ) of ferrite with
semi-conducting behavior can be concluded from the formula [46]. We
can see that ρd initially decreases when 0 ≤ x ≤ 0.30 from Fig. 4 (a-b). As
has been said above, Fe2+ ions must be improving with the rise of Si4+
contents. So the electron transfer between Fe2+ and Fe3+ is intensified
and then it caused the decrease of ρd. Due to the increase of activation
Fig. 5. (a) Dielectric constant of all the investigated SixCo1-xFe2O4 ferrites (x = 0, 0.05, 0.10, 0.15, 0.30, 0.40, 0.50) and (b) Dielectric constant SixCo1-xFe2O4 ferrites
(x = 0, 0.05, 0.10, 0.50).
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Physica B: Physics of Condensed Matter xxx (xxxx) xxx
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zation will decrease dramatically. This is consistent with the fact that the
actual permittivity is a low constant at high frequencies.
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and then decreases with the amount of Si substitution, and reaching the
peak when x = 0.30. The dielectric constant (ε′) increases with the in-
crease of x due to the increase of Fe2+ in octahedral position when 0 ≤ x
< 0.30. Because some research shows that the polarizability of ferrite
depends on the local displacement of electrons between Fe2+ and Fe3+ in
the outfield direction [37,50]. It is pointed out above that due to the
addition of Si, in order to maintain the chemical valence balance, part of
Fe3+ will be converted to Fe2+. Therefore, the number of Fe2+ on
octahedral position increases. When 0.30 < x ≤ 0.50, the dielectric
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4. Conclusion
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In this paper, the effect of silicon substitution on the electromagnetic
properties of cobalt ferrite (SixCo1-xFe2O4 where x = 0, 0.05, 0.10, 0.15,
0.30, 0.40, 0.50) was studied. Among them, the substitution amount of
silicon has an important influence on the microstructure and electro-
magnetic performance of the samples. According to XRD, the structure
of the prepared sample is single-phase when x < 0.15, while the impu-
rity phase Fe2O3, Fe2⋅56Si0⋅44O4 and FeO appeared in the prepared
sample when x ≥ 0.15. According to SEM, the grain size and porosity of
the samples increase with the increase of Si4+ substitution, which shows
that the addition of Si4+ improves the microstructure of cobalt ferrite.
According to VSM, the addition of silicon decreases the saturated
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Declaration of competing interest
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The authors declare no conflict of interest.
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Acknowledgments
This work was supported by the Project of Science and Technology
Supporting Plan in Sichuan Province of China (2019YJ0354,
2019YJ0364), the National Natural Science Foundation of China Grant
(51701025) and Research Found for Young Academic Leaders of CUIT
Grant (J201710).
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structural and magnetic properties of manganese substituted NiFe2O4
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