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EUROPEAN JOURNAL OF MATERIALS SCIENCE AND
ENGINEERING
Volume 2, Issue 2-3, 2017: 41-50 | www.ejmse.tuiasi.ro | ISSN:
2537-4338
STRUCTURAL AND MAGNETIC CHARACTERIZATIONS OF
BARIUM-STRONTIUM-FERRITES USING HEMATITE OF
ANALYTICAL GRADE AND MAGNETITE FROM COX’S BAZAR
BEACH SAND MINERAL
Ummay HABIBA1, Hasan Khaled ROUF1*, Sheikh Manjura HOQUE2
1 Department of Electrical and Electronic Engineering, University of Chittagong, Chittagong – 4331, Bangladesh
2 Material Science Division, Bangladesh Atomic Energy Centre, Ramna, Dhaka, Bangladesh
Abstract
This article presents the characterizations of BaO.SrO.xFe2O3 and BaO.SrO.xFe3O4 (x= 5.6,
5.8, 6) hexaferrites. We have used hematite of analytical grade and magnetite from Cox’s
Bazar beach sand mineral to prepare these samples. Structural characteristics of the
compositions have been revealed by the Scanning Electron Microscopic (SEM) tests.
Magnetic characterizations have been performed by the Vibrating Sample Magnetometer
(VSM) test and Mossbauer spectroscopy. From VSM test, we have determined the hysteresis
parameters of the samples. The Curie temperatures of those hexaferrites have been obtained
from the temperature dependent magnetic moments. The Mossbauer study indicates that the
samples are in the ferromagnetic state and they are hexagonal ferrites.
Keywords: hexaferrite, magnetism, curie temperature, ferromagnet, hardmagnet.
Introduction
With the rapid development of technology, there is an increasing demand for various
permanent magnets. Magnetic materials include a wide variety of materials, which are used in
versatile range of applications. Ferrite materials dominate over other magnetic materials due to
their low cost, high stability and versatile applications in the development of the current
technology [1]. Ferrites are generally obtained from wide range of minerals and synthetic
materials, and they have been used in different fields of interests for a long time [2]. M-type
hexaferrites containing the Me ions have been widely used as permanent magnet. The
ferromagnetic oxides with hexagonal crystal structure are called hexagonal ferrites [3]. The
general formula for representing hexagonal ferrites is m(MeO).n(Fe2O3), where Me = Ba, Sr, or
Pb. The two formula units of MeFe12O19 per unit cell and the Fe+3 ions distributed over five non-
equivalent crystallographic positions are responsible for building blocks of hexagonal crystal [4].
Hexagonal ferrites exhibit a hard and brittle magnetic behavior, which includes high
coercive field, and high permeability in one plane and low permeability in other directions.
These result in their high reactivity and subsequently low temperature processing to produce a
better material in terms of activity parameters [5]. Although barium ferrite is the most common
hexaferrite, nowadays strontium ferrites have become an excellent supplement to the former
owing to its higher coercive force [6].
Corresponding author: hasan.rouf@cu.ac.bd
U. HABIBA et al.
In recent years, there have been many works on the synthesis and characterization of
hexagonal ferrites. The influence of the admixture of group III elements upon magnetic
properties of the Ba-Sr-ferrite has been investigated by Wlodzimierz [7]. Admixture of B2O3 did
not favour the synthesis. For low concentration of B2O3 a slight increase in saturation
magnetization was found while for high concentration of B2O3 the saturation magnetization was
decreased. Nanocrystalline particles of barium hexaferrite were synthesized by a sol-gel
combustion route in [8].The effect of addition of polyethylene glycol (PEG) solutions with
different molecular weights on magneto-structural properties of barium hexaferrite was also
studied there. In [4] strontium hexaferrite nanoparticles had been prepared by co-precipitation
in aqueous solutions and precipitation in microemulsion system water/SDS/n-butanol/
cyclohexane. Iron and strontium nitrates in different molar ratios were used as the starting
materials. The precursors of nanocrystalline particles of SrFe12O19 with average particle size of
around 30 nm and relatively high specific magnetization were successfully prepared. In [9]
strontium hexaferrite (SrFe12O19-SrF) powders have been prepared by the sol-gel process and
magnetic properties of conventional and microwave calcined strontium hexaferrite powders have
been studied. The advantages and disadvantages of Ba and Sr ferrite magnets as well as
applications of ferrites in high-efficiency motors are discussed in [6]. Reference [10] investigated
the magnetic and structural properties of copper substituted Barium ferrite powder particles via
co-precipitation method. Copper-doped barium ferrite increases the coercivity and the magnetic
storage capacity of the permanent magnet. In this work, Barium-Strontium-Ferrites have been
prepared using Hematite of Analytical Grade and Magnetite from Cox’s Bazar Beach Sand
Minerals to characterize their structural and magnetic properties for permanent magnet
applications. BaO.SrO.xFe2O3 and BaO.SrO.xFe3O4 (x=5.6, 5.8, 6) samples have been prepared
by solid-state method. These Barium-Strontium-Ferrites were prepared using the same procedure
as described in [11]. In [11] the X-ray diffraction measurement was performed to reveal their
structural properties. In our present study, we have performed the Scanning Electron Microscopy
(SEM) to determine the structural properties of those samples. Mossbauer spectroscopy as well
as magnetic hysteresis measurements and temperature dependence of permeability measurements
have been performed to determine magnetic properties.
Experimental Details
Sample preparation
Sample preparation technique is an important part for ferrites processing. The usual
method of preparing ferrite comprises of conventional ceramic method i.e. solid state reaction
method involving milling of reactions followed by sintering. To synthesize the sample we
followed this method. BaCO3, SrCO3 and hematite were used to prepare BaO.SrO.xFe2O3 (x=5.6,
5.8, 6) while BaCO3, SrCO3 and magnetite were used to prepare BaO.SrO.xFe3O4 (x=5.6, 5.8, 6).
Intimate mixing of the materials was carried out using agate mortar (hand milled) for 4 hours for
fine homogeneous mixing. Then the mixed samples were pre-sintered at a temperature between
8500c to 9000c for 5 hours to form ferrite through chemical reaction. The pre-sintered materials
were milled for another 4 hours in distilled water to reduce them to small crystallites of uniform
size. The mixtures were then dried and a small amount of saturated solution of polyvinyl alcohol
was added as a binder. The resulting powder were pressed uniaxially under a pressure of 15-20
KN.cm-2 in a stainless steel die to make pellets, rods and toroids. Then the pressed pellet, rod and
toroid shaped samples were finally sintered at 12500c temperature for 4 hours and then cooled in
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STRUCTURAL AND MAGNETIC CHARACTERIZATIONS OF BARIUM-STRONTIUM-FERRITES
the furnace. The XRD patterns confirmed that the samples were phase pure. Small pieces were
cut from the sintered pellets with weight 0.005– 0.01 g for different characterizations.
Physiochemical characterization
The ferrite materials were characterized to evaluate their compositional and magnetic
properties. We used Scanning Electron Microscopy (SEM) to determine the grain size of the
materials and to compare the size with the change of compositions of the ferrite contents. The
hysteresis loop has been measured for each sample using Vibrating Sample Magnetometer
(VSM). From the VSM tests, we have also obtained the saturation magnetization, remanent
magnetization and coercive field of the samples. The saturation magnetization is the maximum
value after which the magnetization remains constant with the increase of the magnetic field.
And the remanent magnetization is the magnetization which is obtained when the applied
magnetic field is zero. The coercive field is the applied external field responsible for obtaining
zero magnetization. The Curie temperature for each sample has been determined by moment vs.
temperature (M-T) curves and dM/dT vs. T curves. Mossbauer analysis was performed to
observe the Mossbauer spectra and to determine the magnetic hyperfine field, the isomer shift
and the electrical quadrupole splitting and for each sample.
Results and Discussions
Scanning Electron Microscopy (SEM)
We used scanning electron microscopic (SEM) tests to determine the structural
characteristics of the composition. The grain-size of the materials were determined and shown in
Fig. 1 and Table 1.
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19
U. HABIBA et al.
Fig.1. Grain size of BaO.SrO.xFe2O3 for (a) x=5.6, (b) x=5.8, (c) x=6 and BaO.SrO.xFe3O4 for (d) x=5.6, (e) x=5.8, (f)
x=6.
Grain-sizes of both BaO.SrO.xFe2O3 and BaO.SrO.xFe3O4 for different values of x are
shown. Both Fig. 1 and Table 1 show that the average grain size depends on the value of x. With
the increase of x, the average grain size also increases. Fig. 2 graphically shows how the grain-
size is changed with the values of x. Here the average grain sizes are plotted against the
concentration of the ferrite elements. We observe that the average grain size of the
BaO.SrO.xFe2O3 is greater than that of the BaO.SrO.xFe3O4.
Table 1. Average grain sizes of the samples
Grain size (µm) for x=5.6 Grain size (µm) for x =5.8 Grain size (µm) for x=6
Fe2O31.94 2.5 3.7
Fe3O41.73073 2.0046 2.11407
Fig.2. Change of grain size of the materials with x.
Vibrating Sample Magnetometer (VSM)
The hysteresis parameters of the compositions have been studied by Vibrating Sample
Magnetometer (VSM) tests. Fig. 3 shows the hysteresis loops for Ba-Sr-ferrites using hematite
and magnetite. The hysteresis loops indicate that the samples are of ferromagnetic type. Next we
studied the temperature dependence of magnetic moments of BaO.SrO.xFe2O3 and
BaO.SrO.xFe3O4. Fig. 4 shows the magnetic moment vs. temperature (M-T) curves for all of the
compositions. The Curie temperatures of different compositions can be obtained from such
study. We also observed the dM/dT versus T curve as shown in Fig. 5. All the magnetic
parameters obtained from the VSM tests are summarized in Table 2.
The coercive fields, remanent magnetizations (Mr), saturation magnetization (Ms) and
Curie temperature (Tc) for different values of x are shown in Table 2. All the samples have very
high coercive fields, remanent magnetizations (Mr), saturation magnetization (Ms) and Curie
temperature (Tc) which further confirms their strong ferromagnetic nature. We observe that the
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STRUCTURAL AND MAGNETIC CHARACTERIZATIONS OF BARIUM-STRONTIUM-FERRITES
saturation magnetization of both Ba-Sr-ferrites decreases with the decreasing values of x. The
coercive fields for the samples using hematite (Fe2O3) are higher than that for the samples using
magnetite (Fe3O4). Again the Curie temperatures for samples using hematite are higher than
those of the samples using magnetite. All these results demonstrate that, the stability of
magnetism is greater in BaO.SrO.xFe2O3 than in BaO.SrO.xFe3O4.
Fig.3. Hysteresis loop of Ba-Sr-ferrites (a) using Hematite and (b) using Magnetite.
Fig. 4. M-T curves of Ba-Sr-ferrites (a) using Hematite and (b) using Magnetite.
Fig.5. dM/dT versus T curves of Ba-Sr-ferrites (a) using Hematite and (b) using Magnetite.
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U. HABIBA et al.
Table 2. Magnetic parameters from VSM test
x Coercive field
(oe)
Remanent magnetization, Mr
(emu)
Saturation magnetization, Ms
(emu) Tc(0c)
Fe2O3
6 2468.15 3.18E+01 7.25E+01 460
5.8 2476.096 3.35E+01 7.13E+01 460
5.6 1810.477 3.98E+01 6.46E+01 450
Fe3O4
6 1897.688 3.55E+01 7.31E+01 420
5.8 1931.297 3.76E+01 6.84E+01 420
5.6 1588.256 3.31E+01 6.72E+01 420
Mossbauer spectroscopy
To estimate the distribution of iron (Fe3+) in the ferrite, 57Fe-Mossbauer spectra were
recorded. Figure 6 shows the Mossbauer spectra for BaO.SrO.xFe2O3 and BaO.SrO.xFe3O4 (x=
5.6, 5.8 &6). Both the theoretical data and fitted curves by least square method are shown in Fig.
6. Small circles indicates the theoritical data. The calculated spectrum of each individual
component is represented by a continuous line. The hyperfine parameters of Mossbauer
spectroscopy are shown in Table 3 and Table 4 for BaO.SrO.xFe2O3 and BaO.SrO.xFe3O4
respectively.
Fig. 6. Spectrum of Mossbauer Spectroscopy for BaO.SrO.xFe2O3 with (a) x=5.6, (b) x=5.8, (c) x=6and
BaO.SrO.xFe3O4with (d) x=5.6, (e) x=5.8, (f) x=6. Circles (o) represent the theortical data while the continuous lines
correspond to least square fits.
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STRUCTURAL AND MAGNETIC CHARACTERIZATIONS OF BARIUM-STRONTIUM-FERRITES
Table 3. Hyperfine parameters for Samples using hematite
Fe2O3
Sub-species Chemical Shift ()
mm/sec
Quadruple splitting (dEq)
mm/sec
Hyperfine field (Hint)
kG Relative Area
x=5.6
Ia 0.38 0.383 -517.002 0.202
Ib 0.354 0.414 -412.295 0.497
II 0.786 1.284 -405.349 0.0482
IIIa 0.201 0.966 -337.461 0.0017
IIIb 0.276 0.2 -494.18 0.268
x=5.8
Ia 0.376 0.18 -516.877 0.225
Ib 0.275 0.202 -493.893 0.256
II 0.752 1.302 -404.71 0.0550
IIIa 0.375 0.454 -412.218 0.315
IIIb 0.313 0.328 -412.513 0.152
x=6
Ia 0.155 1.25 -424.731 0
Ib 0.381 0.529 -518.742 0.278
II 0.805 1.5 -402.103 0.0515
IIIa 0.355 0.416 -415.11 0.437
IIIb 0.275 0.562 -495.871 0.237
The mossbauer spectra of all the samples indicate that the samples are hexagonal ferrites
consisting of 5 lattice sites. The Mossbauer spectra of those hexagonal samples are the results of
superposition of spectra of those 5 sites for each sample. The line broadening of the spectrum
increases with the increase of impurity in the sample. The broad spectra indicate the wide
distribution of magnetic field on iron ions in the neighborhood of impurity atoms. The magnitude
of the hyperfine field is a measure of the magnetic moment on the atoms in ferromagnetic alloys.
The hyperfine magnetic field at the Iron nucleus is proportional to the magnetization of the sub-
lattice [12-18].
Table 4. Hyperfine parameters for samples using magnetite
Fe3O4
Sub-species Chemical Shift ()
mm/sec
Quadruple splitting (dEq)
mm/sec Hyperfine field (Hint) kG RelativeArea
x=5.6
Ia 0.102 0 -400 0.0512
Ib 0.376 0.512 -509.518 0.196
II 0.549 1.5 -371.382 0.0706
IIIa 0.374 0.424 -414.093 0.404
IIIb 0.273 0.411 -484.635 0.308
x=5.8
Ia 0.279 0.192 -485.202 0.27
Ib 0.382 0.161 -509.425 0.235
II 0.446 1.275 -432.142 0.0574
IIIa 0.455 0.637 -366.926 0.16
IIIb 0.367 0.439 -412.806 0.369
x=6
Ia 5.11e-18 2.50e-11 -548.347 0.000
Ib 0.381 0.308 -507.379 0.278
II 0.408 0.450 -363.285 0.105
IIIa 0.410 0.484 -413.436 0.211
IIIb 0.252 0.341 -415.854 0.148
From the above hyperfine parameters, it is clear that those samples are ferromagnetic
materials, because the hyperfine fields were obtained without applying any external magnetic
field. The average hyperfine field of each ferrite increases with the increase of x. So the
ferromagnetism of the materials increases with the values of x. Again the average HF field of
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U. HABIBA et al.
BaO.SrO.xFe3O4 is larger than BaO.SrO.xFe2O3. So the ferromagnetism of BaO.SrO.xFe3O4 is
higher than that of BaO.SrO.xFe2O3. The isomer shifts tend to be small, when HMFs usually
provide large and distinct shifts of Mossbauer peak. They change with the valence of the iron
ions. The quadrupole interaction is caused by an electric quadrupole moment in the atomic
crystal lattice.
Conclusions
The structural as well as magnetic characterizations of BaO.SrO.xFe2O3 and
BaO.SrO.xFe3O4 have been extensively studied in this work. Hematite from the analytical grades
and magnetite from Cox’s Bazar beach sand mineral have been used as ferrite elements. SEM
tests showed that the average grain size is larger for BaO.SrO.xFe2O3 than that of
BaO.SrO.xFe3O4. We also found that the grain size increases with the increase of x for both
compositions. Studies of magnetic characterizations were performed by VSM tests and
Mossbauer spectroscopy. The hysteresis loops indicate that all the samples are in ferromagnetic
state. From the hysteresis parameters, we observed that the saturation magnetization increases
with x for both of the compositions. BaO.SrO.xFe2O3 was found to have higher magnetic
stability than BaO.SrO.xFe3O4 as the former has higher coercive field and higher Curie
temperature. The Mossbauer spectra indicate that the ferrites are in hexagonal state. According to
the hyperfine field of the samples, the ferromagnetism of the hexaferrites increases with the
values of x and the ferromagnetism of BaO.SrO.xFe3O4 is higher than that of BaO.SrO.xFe2O3.
We can say by correlating the results of both structural and magnetic characterizations that the
magnetization is related to the grain size as well as concentration of ferrite contents and
magnetization and magnetic stability increase with the grain size. But the smaller grain size is
desirable to get smaller porosity. From the above results, we can conclude that, the hexaferrite
materials using magnetite from the Cox’s Bazar beach sand mineral are more suitable for
ferromagnetic applications (for example, magnetic refrigeration) than the hexaferrites from
hematite of analytical grade though the later has larger Curie temperature.
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