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Issue 3, Vol. 1 January 2014
ISSN 2319 – 7277
Preperation ,Charactrizationand Electrical properties of
SrFe12O19 (Strontiumhexaferrite) Ceramics.
SunitaShrivastava( GGCT, Jabalpur) 1 ,
Dr.KirtiVishwakarma (GGCT, Jabalpur) 2
Abstract.
Strontium hexaferrite (SrFe12O 19) Ceramics have been prepared with Citrate precursor
method in deionized water. In chemical based citrate precursor method, generally nitrate of
metal is taken because nitrates of most metals is highly soluble in water. Also nitrate of iron
and strontium and citric acid are taken in stoichiometr proportions and then dissolved in
deionized water. and mixed together and magnetic stirred at 600 C to 800 C temperature for 2
hours . Brown Slurry so formed is known as precursor. Then the precursor was dried in an
oven at a temperature of 800C. This dried material is the citrate precursor . Then Citrate
precursor was annealed at 7000 C in a temperature controlled muffle furnace .
These samples were crushed in a crucible and the powdered samples were stored. After that,
these samples were characterized through Impedance analyzer.
I INTRODUCTION
Strontium hexaferritewith chemical formula of SrFe12O19 have been widely used as
permanent magnet.. It was widely used in the fabrication of computer data storage, highdensityperpendicular magnetic and magneto-optic recording, magnetic fluids and certain
microwave devices.For ideal performance, ultrafine strontium hexaferrite powder (≈0.1 μm)
with homogeneous particle size distribution and controlled magnetic properties is important .
It is difficult to obtainultrafine and monodispersed particles by the Citrate precursor method
which involves the stoichiometric mixture of strontiunitrat,Ironnitrate and citric acid at
temperatures (about 80 °C) .
In order to achieve highly homogeneous ultrafine particles of Strontiumhexaferrite, various
techniques were investigated, such as
 Citrate precursor method
 Ceramic method
 Co-precipitation method
 Sol-gel method
 Combustion system
 Hydrothermal precipitation
 Spray drying
 Freeze random
 Freeze drying
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The objective of the present work is to give possible explanation and to investigate the
influence ofSr2+/Fe3+ molar ration and the addition of surfactants on the synthesis of
nanocrystalline strontium hexaferrite particles by Citrate precursor methods using nitrate
precursors.
II TECHNIQUES AND EXPERIMENTAL PROCEDURE
Impedance analysis:
Complex impedance spectroscopy is a unique, non destructive and powerful
technique to study the electrical properties of crystalline materials over a wide range of
frequency and temperature. The frequency dependence of the electrical properties of
materials is represented in terms of complex impedance (Z*) and electric modulus (M*).
Complex impedance is the ratio of complex voltage and current which is given by,
Z*=(Z'-jZ")=RS–(j/ωCS)
(1.1)
where RS is the series resistance, ω is the angular frequency and CS is the capacitance in
series.
Electric modulus
M*=1/ε*=M'+jM=jωC0Z
(1.2)
Complex admittance is the inverse of complex impedance which is given by,
Y*=Y'+jY"=(1/RP)+jωCP
(1.3)
Relativepermittivity=ε*=1/M*=ε'-jε"
(1.4)
ε' is the real part and ε" is the imaginary part of relative permittivity. ε" is related to dielectric
loss which is given by
Loss tangent = tan δ = ε"/ε = M"/M' = -Z'/Z" = Y"/Y'
(1.5)
Impedance Analyzer:
Impedance analysis is a non destructive method to study dielectric properties of crystalline
and ionic materials over a wide frequency and temperature range. This is very useful for
understanding the contribution of bulk, grain boundary and electrode material. Pellets of the
synthesized nanoparticles are prepared by using 10 mm diameter pelletizer. Then thin layer of
silver paste is used on both side of the pellet to make it conductive. The instrument is
calibrated to remove any background noise. A programmable furnace is also used for taking
the temperature variation of dielectric constant. The measurement is taken in the frequency
range of 100 Hz to 10 MHz. All the measurements were carried out by this Impedance
Analyzer (NumetriQ PSM1735) which is shown in figure 1.
Complex impedance spectroscopy is a powerful technique to characterize the electrical
properties of materials in the frequency domain. This technique enables us to separate the
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grain, grain boundary and material electrode interface contribution from the total conductivity
of the sample. In this work, detailed electrical investigations are conducted in the frequency
range of 100 Hz -10 MHz and temperature range between 40⁰C and 250⁰C.
Here the frequency dependence of the electrical properties of materials is represented
in terms of complex impedance (Z*) and electric modulus (M*).
Complex impedance is the ratio of complex voltage and current which is given by,
Z* = (Z'- jZ") = RS – (j/ωCS)
(4.1)
where RS is the series resistance, ω is the angular frequency and CS is the capacitance in
series. Electric modulus is given by,
M* = 1/ε* = M' + jM" = jωC0Z*
(4.2)
Generally, polycrystalline dielectric materials have grains separated by interfacial
boundary layers (grain boundaries). The corresponding features can be seen with two semicircular arcs in the complex impedance plots. Each semi-circular arc can be modeled in terms
of electrical equivalent circuit consisting of a parallel combination of a resistor R and a
capacitor (or constant phase element CPE). The impedance of the resultant circuit can be
expressed as
𝑍′ =
𝑅𝑔
𝑅𝑔𝑏
𝑛𝑔 +
1 + (𝑗𝜔𝑅𝑔 𝐶𝑔 )
1 + (𝑗𝜔𝑅𝑔𝑏 𝐶𝑔𝑏 )𝑛 𝑔𝑏
DC conductivity of the compounds were calculated from the equation
𝑡
𝐴𝑅
wheret is the thickness of the pellet, R is the resistance (of the grain or grain boundary) of the
𝜎𝑑𝑐 =
compound and A is the area of the electrode.
Fig-1 Image of Impedance Analyzer (NumetriQ PSM1735).
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Dielectric Constant analysis:
Figure 2 shows the variation of dielectric constant (ε’) with frequencies at room
temperature. In general, the dielectric constant is because of the four types of polarizations
viz. dipolar polarization, electronic polarization, ionic polarization and interfacial
polarization. At low frequencies, interfacial polarization and dipolar polarization are known
to play the most significant role and both of these polarizations are temperature dependent.
Interfacial polarizations are because of the build up of charges at the grain boundary. Further,
the results show that the dielectric constant is decreasing with an increase in frequency. The
dielectric structure is composed of well conducting grains separated by the poorly conducting
grain boundaries33 and the electronic exchange between the Fe2+ and Fe3+ ions results in the
local displacements of electrons along the direction of applied external field. These
displacements determine the polarization as well as the dielectric properties. The observed
decrease in dielectric constant with an increase in frequency is due to the fact that above
certain frequency, the electronic exchange between the ferrous and ferric ions does not follow
the applied field. Figure (3.16) shows the variations of dielectric loss with temperature at
different frequencies. The energy losses in the dielectrics are because of the electric
conductivity of the materials and the relaxation effect to the orientation of the dipole. Further,
the results indicate a maximum in dielectric loss and can be explained on the basis of Koop’s
model34. According to Koop’s model the solid is composed of grains and grain boundaries.
The grains have low resistivity and large thickness while the grain boundaries have high
resistivity and small thickness. Moreover, it was assumed that each of the grain and grain
boundaries has its characteristic peak.
The conduction mechanism in ferrites is considered due to hopping of electrons
between Fe2+ and Fe3+ ions. The dielectric loss (tan δ) decreases rapidly in the low frequency
region, while the rate of decrease is slow in high frequency region where it becomes almost
frequency independent. Such a behavior can be explained on the basis that in low frequency
region, which corresponds to low conductivity of grain boundary, more energy is required for
electron exchange between Fe2+ and Fe3+ ions, as a result the loss is high. In high frequency
region, which corresponds to high conductivity of grain, a small energy is required for
electron transfer between the Fe2+ and Fe3+ ions at the octahedral site. As the dielectric loss
arises if the polarization lags behind the applied altering field and is caused by the presence
of impurities and structural inhomogeneities thus the low dielectric loss values obtained in
the present work are therefore attributed to more structurally perfect and homogeneous
ferrites processed by the citrate precursor technique.
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o
40 C
1200
o
100 C
o
150 C
1000
o
200 C
o
250 C
K'
800
600
400
200
0
0
10
10
1
2
3
10
10
10
Frequency (Hz)
4
10
5
10
6
Fig 2: Room temperature dielectric constant versus frequency (plotted at log scale) plot of
SrFe12O19 ceramics.
o
40 C
o
100 C
180
o
150 C
o
200 C
o
K''
250 C
160
10
2
10
3
4
10
10
Frequency (Hz)
5
10
6
Fig 3 Room temperature dielectric constant versus frequency (plotted at log scale) plot of
SrFe12O19 ceramics.
4.1. Variation of Z' and Z'' with frequency:
Frequency variations of real part of impedance at different temperatures are shown in
Figs 4 & 5. Presence of dielectric relaxation behaviour was evident from the dispersion
behavior of Z' in the low frequency regions. Decrement in Z' with frequency was due to the
increase of hopping of charge carrier which leads increased ac conductivity. Negative
temperature coefficient of resistance (NTCR) character could be accounted in terms of
decrement in Z' with temperature. Increase in Z' with Al substitution indicated that the bulk
resistance was improved. The Z' versus frequency plot indicates a crossover from low
frequency relaxation behavior to high frequency dispersion phenomenon. The segment of
nearly constant real impedance becomes predominant with increasing temperature, which
suggests a strong relaxation behavior. For the measured temperature above 100⁰C, step like
impedance behavior is arising from the low frequency side, which pushes the previous high
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frequency dispersive behavior. The strong relaxation found at high temperature is due to
thermal activation mechanism.
Fig 5 shows frequency dependence of the imaginary part of impedance (Z'') of
SrFe12O19 ceramics. Above 100 ⁰C, Z'' data is characterized by (i) appearance of a peak with
a maximum value (Z''max) at a certain frequency, (ii) asymmetrical broadening and (iii)
shifting of Z''max value to higher frequency with the increase in temperature. Appearance of
peak indicates the presence of electrical relaxation in the compounds. Asymmetric
broadening suggests that there is a distribution of relaxation times. Shifting of Z''max value to
higher frequency with temperature indicates that these relaxations are temperature dependent.
Z'' peak became weak which is due to the presence of indistinguishable relaxation times
corresponding to grain and grain boundaries.
o
o
40 C
40
150 C
o
200 C
o
250 C
6
o
100 C
Z' (M)
5
Z' (M)
30
20
4
3
2
1
10
0
10
2
10
3
4
5
10
10
10
6
Frequency (Hz)
0
10
1
10
2
10
3
10
4
10
5
10
6
Frequency (Hz)
Fig 4: Frequency dependence of real impedance for SrFe12O19ceramics .
o
40 C
140
3.5
o
100 C
Z'' (M)
Z'' (M)
120
100
80
o
150 C
o
200 C
o
250 C
3.0
2.5
2.0
1.5
1.0
60
0.5
40
0.0
10
20
1
10
2
10
3
4
5
10
10
10
6
Frequency (Hz)
0
10
1
10
2
10
3
10
4
10
5
10
6
Frequency (Hz)
Fig 5: Frequency dependence of imaginary part of impedance for SrFe12O19 ceramics.
(4.4) Variation of electric modulus with frequency:
Complex electric modulus formalism is a powerful technique to study the electrical response
of the material. The electric modulus (MÃ) is expressed as,
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M* = M' + iM'' = iωC0Z* = iωC0 (Z' - iZ'') = ωC0Z'' + iωC0Z'
where ω is the frequency of applied electric field and C0 is the capacitance in vacuum.
Frequency variations of real part of electric modulus plots at different temperatures are
shown in Figs.
The value of electric modulus (M'') is calculated using the formula
M'' = ωC0Z'
Frequency variation of M'' curves are characterized by (i) appearance of peak at unique
frequency, at a given temperature, (ii) significant broadening in the peak which indicates the
presence of distribution of relaxation times and hence the relaxation is of non-Debye type and
(iii) shifting of peak position towards high frequency region with the rise of temperature
which indicates the relaxation process is thermally activated.
o
0.010
40 C
o
100 C
o
150 C
M'
0.008
o
200 C
o
250 C
0.006
0.004
0.002
0.000
0
10
10
1
2
3
4
10
10
10
10
Frequency (Hz)
5
10
6
Fig 6: Frequency dependence of real electrical modulus for SrFe12O19 ceramics.
Fig 7: Frequency dependence of imaginary electrical modulus for SrFe12O19 ceramics.
(4.5) Variation of Z'' with Z':
Fig 8 shows the Cole-Cole plot for the SrFe12O19 ceramics. As the temperature increases the
arc becomes semicircle which indicates single relaxation process. In general, whether a full,
partial or no semicircle is observed depends on the strength of relaxation and frequencies.
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The presence of non-semicircle or asymmetric/distorted semicircle suggests the existence of
non-Debye type of relaxation in the samples. For an ideal Debye-type relaxation, a perfect
semicircle with its center at Z'-axis is observed. The observations of the Cole-Cole plot given
in Fig. 8 clearly suggest that the Debye-type of relaxation does not exist in the studied
materials. A minor deviation from semicircle reveals i.e., non-Debye-type response. Areas
enclosed by semicircle decreases with increasing temperature which suggests the NTCR
(negative temperature coefficient of resistance) properties as of semiconducting
characteristics.
Measured
Calculated
15
Z''(M)
0
150 C
10
0
200 C
5
0
250 C
0
0
5
Z'(M)
Fig 8: Typical Niquist Plot of Z'' with Z' for SrFe12O19 ceramics.
(4.6) Variation of ac conductivity with frequency:
Fig 9 shows the frequency dependence of ac conductivity at different temperatures. The
response of the material to the applied electric field is described by the ac conductivity. These
studies will be useful to investigate the nature of transport process in the compounds. The ac
electrical conductivity was calculated by using the relation
𝜎𝑎𝑐 = 𝜀0 𝜀𝑟 𝜔 𝑡𝑎𝑛𝛿
The frequency independent plateau-like region observed in the low frequency regime can be
attributed to the frequency independent conductivity. The dc conductivity increases with the
increase in the temperature. It becomes predominant over large frequency range at high
temperature. It also increases by order of magnitude with the increase in the temperature. It is
important to observe that the ac conductivity decreases with the increase in the substituent
percentage.
In the present study it has been observed that the AC conductivity is increasing with an
increase in frequency. The increase in AC conductivity with an applied field frequency can
be explained on the basis that the pumping force of the applied field helps in transferring the
charge carriers between the different localized states as well as liberate the trapped charges
from the different trapping centers [22]. These charge carriers will take part in the conduction
process all together with the electrons produced from the valence exchange between the
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different metal ions. In other words, increase in frequency will increase the electron hopping
frequency between the charge carriers resulting thereby in an increase in conductivity. The
room temperature resistivity is found to be decreasing with an increase in temperature. AC
conductivity is found to increase with an increase in temperature which is due to thermally
enhanced drift mobility of charge carriers.
-4
1.4x10
-1
a.c()(m)
0
250 C
Measured
Calculated
-4
1.2x10
-4
1.0x10
-5
8.0x10
0
200 C
-5
6.0x10
-5
4.0x10
-5
0
2.0x10
150 C
0
5
1x10
5
2x10
5
3x10
5
4x10
Frequency (Hz)
Fig 9: Frequency dependence of ac conductivity for SrFe12O19 ceramics.
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