Synthesis of nanocomposites comprising iron and barium hexaferrites

Article abstract of DOI:10.1016/S0304-8853(03)00559-6

Composites of nanometre-sized α-iron and barium hexaferrite phases, respectively, have been synthesized by the ceramic processing route. Pure barium hexaferrite (BaO·6Fe₂O₃) was first of all prepared by calcinations of the precursor oxides at a maximum temperature of 1200°C for 4 h. By subjecting the resulting powder having particle size of the order of 1 μm to a reduction treatment in the temperature range 500-650°C for a period varying from 10 to 15 min it was possible to obtain a composite consisting of nanosized barium hexaferrite and α-Fe. At reduction temperature of 650°C for a period greater than 15 min all the ferrite phase was converted to α-Fe and Ba - the particle sizes being 59. 4 and 43.6 nm, respectively. These conclusions are based on X-ray diffraction and Mossbauer studies of different samples. During reduction H⁺ ions are introduced into the hexaferrite crystallite. It is believed that due to a tensile stress the crystals are broken up into smaller dimensions and the reduction brings about the growth of nanosized α-Fe and barium, respectively, around the hexaferrite particles. Magnetic measurements show coercivity values for the reduced samples in the range 120-440 Oe and saturation magnetization varying from 158 to 53.7 emu/g. These values have been ascribed to the formation and growth of α-Fe particles as the reduction treatment is increased. By heating the nanocomposites at a temperature of 1000°C for 1 h in ordinary atmosphere it was found that they were reconverted to the barium hexaferrite phase with a particle size ～182.3 nm. The reaction described in this study is thus reversible.

Full text of DOI:10.1016/S0304-8853(03)00559-6

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 269 (2004) 42–47
Synthesis of nanocomposites comprising iron
and barium hexaferrites
a
M. Pal , S. Bid , S.K. Pradhan , B.K. Nath , D. Das , D. Chakravorty *
b
b
c
c
a,
a
Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
b
Department of Physics, University of Burdwan, Burdwan 713 104, India
Inter University Consortium for DAE Facilities, Bidhan Nagar, Kolkata 700 091, India
c
Received 3 December 2002; received in revised form 3 June 2003
Abstract
Composites of nanometre-sized a-iron and barium hexaferrite phases, respectively, have been synthesized by the
ceramic processing route. Pure barium hexaferrite (BaO ꢀ 6Fe ) was first of all prepared by calcinations of the
precursor oxides at a maximum temperature of 1200 C for 4 h. By subjecting the resulting powder having particle size
2
O
3
ꢁ
ꢁ
of the order of 1 mm to a reduction treatment in the temperature range 500–650 C for a period varying from 10 to
1
5 min it was possible to obtain a composite consisting of nanosized barium hexaferrite and a-Fe. At reduction
ꢁ
temperature of 650 C for a period greater than 15 min all the ferrite phase was converted to a-Fe and Ba—the particle
sizes being 59.4 and 43.6 nm, respectively. These conclusions are based on X-ray diffraction and Mossbauer studies of
+
different samples. During reduction H ions are introduced into the hexaferrite crystallite. It is believed that due to a
tensile stress the crystals are broken up into smaller dimensions and the reduction brings about the growth of nanosized
a-Fe and barium, respectively, around the hexaferrite particles. Magnetic measurements show coercivity values for the
reduced samples in the range 120–440 Oe and saturation magnetization varying from 158 to 53.7 emu/g. These values
have been ascribed to the formation and growth of a-Fe particles as the reduction treatment is increased. By heating the
ꢁ
nanocomposites at a temperature of 1000 C for 1 h in ordinary atmosphere it was found that they were reconverted to
the barium hexaferrite phase with a particle size B182.3 nm. The reaction described in this study is thus reversible.
r 2003 Elsevier B.V. All rights reserved.
PACS: 75.50.Tt; 81.16.Be
Keywords: Nanocomposite; Iron; Barium hexaferrite; Mossbauer spectra
1
. Introduction
ties exhibited by them and theoretical challenges
they have thrown up [1–5]. Interesting magnetic
properties have been reported in the case of
nanocrystalline ferromagnetic particles dispersed
in an insulating matrix [6,7]. Composites of
ferromagnetic nanoparticles dispersed in an anti-
ferromagnetic oxide have shown useful magnetic
properties which make them potential candidates
Nanocomposites have attracted considerable
attention in recent years because of useful proper-
*Corresponding author. 8/1, Clarke Street, Kolkata 700 026,
India. Tel.: +91-33-473-4688.
E-mail address: mlsdc@mahendra.iacs.res.in
(D. Chakravorty).
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0304-8853(03)00559-6

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M. Pal et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 42–47
43
for magnetic recording media applications [8].
Barium hexaferrites are also potential candidates
but they suffer from the drawback of possessing
low values of magnetizations. One approach is to
make composites consisting of barium hexafer-
rite—a hard magnetic material—with a soft
material like a-Fe [9]. We have been able to
prepare composites of barium hexaferrite and
a-Fe—both in nanocrystalline forms—by a con-
ventional ceramic processing method. A controlled
reduction of the polycrystalline powder of barium
hexaferrite in hydrogen brings about the forma-
tion of both the above mentioned nanocrystalline
phases. We also observe that this transformation is
reversible viz., a-Fe and barium particles can be
reconverted back to barium hexaferrite by heating
them in air at a higher temperature. The details are
reported in this paper.
diffraction peaks observed. The particle sizes of
the different phases were determined from the line
widths of the diffraction peaks using Debye
Scherrer equation [10]:
0
:9l
d ¼
;
ð1Þ
b cos y
where d is the grain diameter, b is half-intensity
width of the relevant diffraction, l is X-ray
wavelength and y the diffraction angle.
The details regarding the calculation of the
lattice parameter, rms strain and weight percen-
tage of different phases present in any sample by
Rietveld’s X-ray powder structure refinement
method have been discussed earlier [11]. Micro-
structural characterization of different samples
have been carried out by employing the Rietveld’s
whole profile fitting method [12–15] The experi-
mental profiles were fitted with the most suitable
pseudo-Voigt (pV) analytical function [14]. Simu-
lation of powder diffraction patterns was done
using the Rietveld software the LSI [15].
2
. Experimental
Analytical reagent grade BaCO3 and Fe O3
2
Mossbauer spectra of different samples were
recorded with a conventional spectrometer operat-
ing in constant acceleration mode having 1024
channels. The spectrometer was calibrated with a
pure natural iron foil of 15 mm thickness. A 10 mCi
were taken in weighed quantities such that the
compound formed after the requisite calcination
temperature had the composition BaO ꢀ 6Fe O .
2
3
The starting powder had particle size B1 mm.
5
7
BaCO and Fe O were mixed together mechani-
cally. A pellet of 1 cm diameter was prepared by
C in Rh matrixwas used as the radioactive
0
3
2
3
source. Mossbauer spectra at low temperatures
were taken by fixing the sample on the cold head
of a closed cycle refrigerator of APD, USA make.
A specially designed anti-vibration stand was used
to decouple the expander vibration from the
sample holder. The spectra were fitted to
Lorentzian lineshapes by least-squares technique.
The microstructure of some of the samples
containing ultrafine particles of the magnetic
phases was studied by a JEM 200 CX transmission
electron microscope.
ꢁ
cold pressing and it was first calcined at 600 C for
2
h. The sample was crushed and a fresh pellet was
formed with the powder so obtained. The sub-
sequent calcinations carried out were as follows:
ꢁ
ꢁ
ꢁ
1
000 C for 8 h; 1200 C for 4 h; 1200 C for 4 h. In
between every step the pellet was crushed and a
fresh pellet was prepared. The formation of
barium hexaferrite was confirmed by taking
X-ray diffractogram of the samples prepared.
The hexaferrite powder was subjected to a reduc-
tion treatment in hydrogen at temperatures in the
The magnetizations of the different samples
were measured in a vibrating sample magnet-
ometer.
ꢁ
range 500–650 C for duration varying from 10 to
2
2 min.
The X-ray powder diffraction patterns of
different samples were recorded in a Phillips
X-ray powder diffractometer PW 1710 using Ni-
filtered CuK radiation from a highly stabilized
3. Results and discussion
a
Philips X-ray generator (PW 1130). The phases
present were identified from the characteristic
Fig. 1 shows the X-ray diffractogram for barium
hexaferrite sample prepared by the ceramic

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44
M. Pal et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 42–47
Fig. 1. X-ray diffractogram for Barium hexaferrite prepared by
ceramic processing.
Fig. 2. X-ray diffractograms for specimens subjected to
ꢁ
reduction treatments at different temperatures: 550 C/12 min;
ꢁ
ꢁ
ꢁ
ꢁ
6
00 C/15 min, 650 C/10 min; 650 C/17 min; 650 C/22 min.
processing technique as described earlier. The
diffraction peaks indicated in the figure corre-
spond to pure barium hexaferrite. In Fig. 2 are
shown the X-ray diffractograms for specimens
subjected to reduction treatments at different
temperatures for various duration. It is evident
that the ferrite phase is transformed into metallic
iron and barium, respectively, even for a reduction
ꢁ
treatment at 600 C for 15 min. Table 1 sum-
marizes the phases which grow after different
reduction treatments as also the parameters
obtained from Rietveld’s structure refinement. It
is evident that the reduction treatment reduces the

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M. Pal et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 42–47
45
Table 2
Mossbauer parameters extracted for barium hexaferrite speci-
men under various stages of reduction
Specimen
Site
IS
(
QS
mm/s) (mm/s) (kOe)
Hint
BaFe120
19
12K
0.28
f1+2a 0.30
0.45
0.15
0.60
2.36
412
499
501
382
4
4
2
f2
b
0.34
0.11
BaFe12
0
19 reduced at
a-Fe
0.03
0.91
0.00
323
323
324
ꢁ
6
50 C for 10 min.
1
4
4
2
2K
1.23
0.97
0.77
0.15
f1+2a 0.46
f2
b
0.88
0.86
BaFe12
0
19 reduced at
a-Fe
0.07
0.82
0.13
ꢁ
6
50 C for 17 min
1
4
4
2
2K
1.49
1.03
0.78
2.23
f1+2a 0.38
f2
b
0.74
0.52
BaFe12
0
19 reduced at
a-Fe
0.01
0.07
ꢁ
6
50 C for 22 min
Fig. 3. (a) Mossbauer spectrum for pure Barium hexaferrite at
room temperature. (b) Mossbauer spectrum at room tempera-
ꢁ
ture of sample subjected to a reduction treatment at 650 C for
10 min. (c) Mossbauer spectrum of specimen subjected to a
reduction treatment at 600 C for 17 min. (d) Mossbauer
the five sites on which Fe atoms are distributed,
the tetrahedral (4f1) and octahedral (2a) sites have
almost identical hyperfine parameters [16]. We
therefore fitted the spectrum with four sextets. The
parameters are listed in Table 2. These are in
satisfactory agreement with those reported by
others [17].
ꢁ
spectrum of specimen subjected to reduction treatment at
ꢁ
650 C for 22 min.
particle size of barium hexaferrite and also leads to
the growth of nanometre-sized a-Fe particles. As
the reduction treatment is increased the particle
sizes of a-Fe and Ba also increase. It is believed
Fig. 3(b) shows the Mossbauer spectrum at
room temperature in the case of the specimen
which was subjected to a reduction treatment at
+
that during reduction H ions are introduced into
ꢁ
barium hexaferrite crystallites and subject the latter
to a tensile stress. This breaks down the crystallite
size of barium hexaferrite. a-Fe and Ba nanocrys-
tals then grow on the surfaces of the hexaferrite
particles. The reduction of rms strains in Ba-
Fe O and a-Fe as the above growth proceeds is
650 C for 10 min. The spectrum consists of six
sharp lines superposed on a broad absorption near
the zero velocity region. The sextet is ascribed to
a-Fe. The broad absorption is attributed to
hexaferrite nanoparticles exhibiting superpara-
magnetic relaxation. Four doublets corresponding
to four effective sites (as explained above) were
then fitted to the experimental data. The extracted
parameters are summarized in Table 2.
1
2
19
consistent with the above physical mechanism.
In Fig. 3 we show the Mossbauer spectra at
room temperature for barium hexaferrite sample
at different stages of reduction treatment.
Fig. 3(a) shows the spectrum for the pure
barium hexaferrite sample. It shows multiple lines
characteristic of barium hexaferrite [16]. Among
Fig. 3(c) gives the Mossbauer pattern obtained
in the case of the specimen subjected to a reduction
ꢁ
treatment at 650 C for 17 min. The spectrum
consists of a strong sextet and a broad central

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M. Pal et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 42–47
Fig. 4. Mossbauer spectrum of the sample corresponding to Fig. 3(b) at 60 K.
absorption. These were fitted by a-Fe sextet and
four doublets arising due to ultrafine particles of
barium hexaferrite. The extracted parameters are
listed in Table 2.
In Fig. 3(d) is shown the Mossbauer spectrum at
room temperature obtained in the case of the
specimen subjected to a reduction treatment at
ꢁ
6
50 C for 22 min. Here only a sextet is present
which has been identified as arising due to the
presence of a-Fe. The parameters extracted are
shown in Table 2.
Fig. 4 gives the Mossbauer spectrum for the
ꢁ
sample (with a reduction treatment at 650 C for
ꢁ
1
0 min) at a temperature ꢃ213 C. It shows
disappearance of the central absorption confirm-
ing superparamagnetic relaxation of hexaferrite
nanoparticles observed at room temperature. The
low temperature data show a broad sextet with an
average value of Hint equal to 450 kOe along with
a sharp sextet of natural iron with HintB328 kOe.
The broad sextet has been assigned to hexaferrite.
The individual sextets due to different sites
however could not be fitted.
Fig. 5. Magnetization vs. magnetic field for specimen subjected
ꢁ
to a reduction treatment at 650 C for 17 min.
from hexaferrite to a-Fe becomes complete at the
reduction time of 22 min.
Fig. 5 shows magnetization vs. magnetic field
data for specimen subjected to a reduction
ꢁ
treatment at 650 C for 17 min. This is typical for
By comparing the X-ray data with the Mossbauer
spectra in this investigation it is evident that the
former could not detect the ultrafine particles
of hexaferrites present in the composites prepared.
It is also to be noted that the conversion
other samples. The saturation magnetization and
coercivity as obtained from such hysteresis loops
are summarized in Table 3. The increase in the
value of saturation magnetization with increased
reduction is consistent with the growth of a-Fe as

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M. Pal et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 42–47
47
Table 3
Saturation magnetization and coercivity of samples after
different reduction treatments
magnetization values, viz., 50–160 emu/g have
been achieved by the present method of synthesis.
The reduction reaction was found to be reversible,
viz., by an oxidation treatment the original phase
of barium hexaferrite could be obtained.
Reduction
treatment
Phases
present
Saturation
magnetization
Coercivity
(Oe)
(
emu/g)
ꢁ
5
6
6
6
6
50 C/12 min
BaFe120
19
a-Fe
53.7
445
195
200
208
120
Acknowledgements
ꢁ
00 C/15 min
a-Fe
Ba
M. Pal thanks CSIR, New Delhi for a Research
Associateship. D. Chakravorty thanks CSIR for
supporting this research. The authors thank Prof.
R. Ray, Head ACMS, IIT Kanpur for extending
the magnetic measurement facilities and Shivku-
mar, ACMS, IIT Kanpur for carrying out those
measurements.
58.8
60.4
ꢁ
50 C/10 min
a-Fe
Ba
ꢁ
50 C/17 min
a-Fe
Ba
109.7
158.4
ꢁ
50 C/22 min
a-Fe
Ba
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the sample shows the presence of barium hexaferrite
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particle size of 182.3 nm was estimated.
In summary, by a controlled reduction treat-
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a-Fe and Ba was synthesized containing a small
amount of ultrafine particles of barium hexafer-
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4
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