Materials Sciences and Applicatio ns, 2011, 2, 638-642
doi:10.4236/msa.2011.26087 Published Online June 2011 (http://www.SciRP.org/journal/msa)
Synthesis and Magnetic Properties of Conventional
and Microwave Calcined Strontium Hexaferrite
Powder
Kanagesan Samikannu1, Jesurani Sinnappan1,2, Sivakumar Mannarswamy1, Thirupathi Cinnasamy1,
Kalaivani Thirunavukarasu1
1Center for Material Science and Nano Devices Department of Physics, SRM University, Kattankulathur, India; 2Department of
Physics, Jeyaraj Annapackium College for Women, Periyakulam, India.
Received December 24th, 2010; revised March 21st, 2011; accepted May 17th, 2011.
ABSTRACT
Powders of strontium hexaferrite (SrFe12O19-SrF) have been prepared by the sol-gel process. The prepared precursor
was calcined in two different calcination techniques, using conventional furnace and microwave furnace. Thermal
analysis studies showed exothermic and endothermic reaction peak at room temperature to 1200˚C. An investigation of
SrFe12O19 crystalline powder from the structural and magnetic aspect is performed using X-ray diffraction (XRD), high
resolution scanning electron microscopy (HR-SEM) and vibrating sample magnetometer (VSM). The average particle
diagonal size of SrFe12O19 powder was 80 - 100 nm in conventional and 40 - 70 nm in microwave calcinations respec-
tively. XRD result showed the formation of SrFe12O19 of the sample calcined at 900˚C with Fe/Sr: D-Fructose ratio = 12.
Keywords: Sol-Gel, Strontium Hexaferrite, D-Fructose, Magnetization
1. Introduction
The M-type Strontium Hexaferrite-SrFe12O19 (SrF) is a
hard magnetic material due to its high coercivity.
SrFe12O19 crystallize with 64 ions per unit cell on 11 dif-
ferent symmetry sites, the magnetic structure given by
Gorter model, 24 Fe3+ atoms are arranged over five dis-
tinct sites: three octahedral sites and two tetrahedral sites.
These five sites are coupled via ferromagnetic superex-
change interaction through O2− ions [1]. Recently, Wang
et al., reported a correlation between magnetic properties
and particles morphology of SrFe12O19 [2]. By control-
ling the microstructure, morphology and chemical com-
position and particle size, the magnetic properties of the
material can be improved. Ferrites, typically spinel ferrite
and magnetoplumbite ferrite, can be used as recording
materials, microwave devices, humidity sensors, pig-
ments etc. Compared with spinel ferrite, magnetoplum-
bite ferrites, strontium ferrite have attracted more scien-
tific research in recent years due to their high uniaxial
magnetic anisotropy, high saturation magnetization and
high coercivity [3,4]. In order to get homogeneous ferrite,
several techniques have been used in the preparation of
Sr hexaferrite, such as the chemical co precipitation [5],
hydrothermal [6], sol-gel [7,8], micro emulsion [9], and
citrate precursor [10] have been developed. Therefore,
the preparation of SrFe12O19 having high purity, ultrafine
size, good dispersion and excellent magnetism has been
the focus of recent research [7,11]. The growing interest
during the past decade is essentially due to the fact that
microwave heat treatment could influence the micro-
structure can improve the material properties. Conven-
tional furnace heating samples by the surface heating
mechanism and depending on the rate of heating, a large
thermal gradient from the surface to the centre can be
generated within a sample, particularly with materials
heaving a poor thermal conductivity. Microwave heating
would be promising because it is a self-heating process
of absorbing the electromagnetic energy. As a result of
the internal and volumetric heating at high heating rate
may lead to reduction in manufacturing cost on account
of energy savings, shorter processing times and improved
product uniformity and yields, limited grain growth
throughout the ceramic body. It is well known that the
heating rate and thermal efficiency of the microwave
heating is higher than those of conventional method [12,
13]. In this paper, we report the preparation of SrFe12O19

Synthesis and Magnetic Properties of Conventional and Microwave Calcined Strontium Hexaferrite Powder639
by sol-gel process and important hysteresis parameters of
the powders with influence of convention and microwave
calcinations.
2. Experimental
The powder was prepared by sol-gel technique using
D-Fructose as the fuel. Analytical grade Ba(NO3)2,
Fe(NO3)3·9H2O and D-Fructose were used as starting
materials. Nitrate and fuel ratio is 1:1. Stoichiometric
amount of metal nitrates and fuel were taken, dissolved
in distilled water and stirred by magnetic stirrer for 2 h to
get a solution. Sol was heated at 80˚C with stirring con-
tinuously, finally it changed in to sticky liquid gel and it
was preheated at 130˚C in a hot air oven for two days to
get precursor. The precursor was calcined (Conventional
and microwave) at 900˚C, to get crystalline barium
hexaferrite powder.
2.1. Characterization
Thermo gravimetric analysis of the mixture composed of
barium nitrate, iron nitrate and D-Fructose (precursor)
were carried out between 28˚C and 1200˚C on NETZSCH
STA 409 C/CD in the static air atmosphere at the heating
rate of 10˚C per minute. The DTA analyses of same
mixtures were also carried out on the same instrument at
same condition. The crystalline phases were identified by
means of X-ray diffraction (XRD) measurements (PANa-
lytical X’pert pro) CuKα radiation at 45 kV and 40 mA
(λ = 0.15406 nm) in a wide range of 2θ (10˚ < 2θ < 80˚).
The surface morphology and size of the ferrite particles
were studied by using FEI Quanta FEG 200-High resolu-
tion scanning electron microscope (HR-SEM) and Mag-
netization measurements at room temperature were car-
ried out on Lakeshore Vibrating Sample Magnetometer
(VSM) at a maximum applied field of 15,000 Gauss at
room temperature.
3. Results and Discussion
The thermogram of the precursor of barium hexaferrite
derived by mixing of barium nitrate, ferric nitrate and
D-Fructose as shown in Figure 1. TGA shows the initial
weight loss from 28˚C to 185˚C due to the loss of ab-
sorbed water [14]. The subsequent loss up to 400˚C is
associated mainly to the decomposition of the D-fructose.
In order to verify this, separate TGA was undertaken for
D-fructose, thermogram is shown in Figure 2. It shows
the major weight loss between 200˚C and 400˚C thus
supporting our assignment. Therefore, D-Fructose pro-
vides self heat to promote the reaction and to reduce the
crystallization temperature of the hexaferrite. The stage
of decomposition between 400˚C and 775˚C is due to
decomposition of nitrates and starting formation of
hexaferrite. There is no considerable weight loss above
Figure 1. TG-DTA curves for the precursor.
Figure 2. TG-DTA curves for D-fructose.
900˚C, confirming the formation of the stable Strontium
hexaferrite this analysis, therefore illustrates the optimum
calcinations temperature for Strontium hexaferrite is
around 900˚C.
The sequences of reaction taking place is shown in the
following steps
200C - 500C
23
PrecursorFe O+SrO

250C - 750C
23 24
Fe OSrOSrFe O 

above 750 C
232412 19
5Fe OSrFe OSrFeO 

Figure 3 shows the XRD patterns of the powders con-
ventionally calcined at temperatures 500˚C, 750˚C and
900˚C for 3 h in air, respectively. The precursor is cal-
cined at 500˚C; the powders can be described as Fe2O3
and SrO and then the phase of SrFe2O4 and hexagonal
SrFe12O19 can been detected for samples calcined at
750˚C. Clear diffraction peak of SrFe12O19 can be ob-
tained at 900˚C, which coincides with the JCPDS file

Synthesis and Magnetic Properties of Conventional and Microwave Calcined Strontium Hexaferrite Powder 640
number: 84-1531. Calcination temperature and interme-
diate Fe2O3 plays an important role in the formation of
Strontium Hexaferrite. The phase development micro-
wave calcined powder at different temperatures, 500˚C,
750˚C and 900˚C for 10 minutes, the peaks correspond-
ing to the standard diffraction pattern of SrO, SrFe2O4
and SrFe12O19 and is shown in Figure 4.
In order to visualize the conventional calcined powder
are in elongated hexagonal like structure, diagonal size
vary in the range of 80 - 100 nm and it is not well de-
fined shape (Figure 5). The HR-SEM micrograph for the
microwave calcined powder at 900˚C for 10 minutes is
shown in Figure 6. The particles are hexagonal platelets
and well crystalline strontium ferrite, diameters are in the
range of 40 nm to 70 nm. This type of shape is usually
observed for BaFe12O19 or SrFe12O19 obtained by sol-gel
process [4,15]. The morphology of the microwave cal-
cined powder samples reveal smaller particles compared
Figure 3. XRD patterns of the powders calcined at different
temperatures: (a) 500˚C, (b) 750˚C, and (c) 900˚C for 3 h.
Figure 4. XRD patterns of the powders microwave calcined
at different temperatures: (a) 500˚C, (b) 750˚C, and (c)
900˚C for 10 minutes.
Figure 5. HR-SEM image of conventionally calcined pow-
der at 900˚C for 3 h.
Figure 6. HR-SEM image of microwave calcined powder at
900˚C for 10 minutes.
to the conventionally calcined powder. Hard magnetic
materials with hexagonal structure is mainly due to the
microwave energy coupled through polarization, elec-
tronic and ionic conductivity loss therefore a smaller
particle size resulted from the enhanced diffusion and
accelerated densification [16].
Figure 7 shows the magnetization versus applied field
for conventional and microwave treated samples at room
temperature. The reduction in Ms in microwave calcined
powder can be attributed to the decrease in the size of the
particles. The observed value of saturation magnetization
47 A·m2/kg for the sample conventionally calcinated at
900˚C are far from the theoretical Ms value of 74.3

Synthesis and Magnetic Properties of Conventional and Microwave Calcined Strontium Hexaferrite Powder641
Figure 7. Magnetization curve of SrFe12O19 (a) powder
conventionally calcined at 900˚C for 3 h; (b) powder mi-
crowave calcined at 900˚C for 10 minutes.
A.m2/kg and the coercivity 6,709 Gauss very close to the
theoretical Hc. Observed magnetization values are close
to those observed in other methods of preparation (50 -
60 Am2/kg) [5,6,10,17]. The value of Mr (26.58 Am2/kg)
is approximately 59% of Ms it has maximum coercivity
of 6,708 Gauss for microwave calcined powder is lower
than those of the literature and of the theoretical limit
(7,500 Gauss) [18]. The samples calcined conventionally
and microwave at 900˚C shows smooth hysteresis loop,
which confirms the formation of pure strontium hexafer-
rite [18,19].
4. Conclusions
The effective influence of conventional and microwave
on the structure and magnetic properties crystalline
SrFe12O19 are discussed. The samples were subjected to
two different heat treatments. From the analysis of vari-
ous characterization techniques such as XRD, HR-SEM
and VSM, we observe that the structure remained intact
with different heating treatment process. The possibility
of lowering the synthesis temperature and get a pure SrF
powder, microwaves allows the reduction of particle size
in the hexaferrite. The external diameters of the obtained
different method of calcined SrFe12O19 particles range
between 40 to 100 nm. The results indicate that calcina-
tions method has significant effect on the saturation
magnetization (Ms). These magnetic materials can poten-
tially be used in micro/nano electronic devices, gas sen-
sors and catalysts.
5. Acknowledgements
The authors thank SRM University for providing the
facilities available in Nanotechnology center.
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