Crystallization of M-type hexagonal ferrites from mechanically activated mixtures of barium carbonate and goethite

Bull. Mater. Sci., Vol. 29, No. 5, October 2006, pp. 457–460. © Indian Academy of Sciences.
Crystallization of M-type hexagonal ferrites from mechanically
activated mixtures of barium carbonate and goethite
J TEMUUJIN*,†, M AOYAMA†, M SENNA†, T MASUKO††, C ANDO††, H KISHI†† and
A MINJIGMAA
*Institute of Chemistry and Chemical Technology, Mongolian Academy of Sciences, Ulaanbaatar 51, Mongolia
†Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi,
Yokohama 223-8522, Japan
††Materials Research and Development Division, General Research and Development Laboratories, Taiyo Yuden
Co. Ltd., 5607-2 Nakamuroda, Gunma 370-3347, Japan
MS received 16 May 2006
Abstract. M-type hexagonal ferrite precursor was prepared by a soft mechanochemical treatment of BaCO3 and
?-FeOOH mixtures. The effect of milling on its structure and thermal behaviour was examined by XRD, SEM
and FTIR. Well crystallized M-type hexagonal ferrite was formed from just 1 h milled precursors at 800°C.
The beneficial effect of milling was explained in terms of increased homogeneity with simultaneous hetero
bridging bond formation between powder constituents.

Keywords. M-type hexagonal ferrite; mechanical activation; powder; crystallization.
1. Introduction
muujin et al 2004). We found that the existence of the
hydroxyl containing compounds in the raw materials does
Hexagonal ferrite, BaFe12O19 (M-phase), is widely studied
not guarantee an improved solid state reaction rate of the
as a permanent magnet and more recently for high-density
powder constituents. Soft-mechanochemical reaction takes
magnetic recording media. Many synthesis techniques,
place during the milling of water or hydroxyl containing
such as solid state reaction (Steier et al 1999), co-precipi-
compounds because of their easy polarization of the surface
tation (Ogasawara and Oliveira 2000; Janasi et al 2002),
hydroxyl groups and allows obtaining of precursors con-
combustion (Huang et al 2003), have been exploited.
taining hetero bridging bonds between the metallic species
Mechanochemical method is one of the interesting methods
by relatively mild mechanical stresses. The aim of the
for the preparation of M-phase. There are several reports on
present research is to characterize the reaction mixture of
this subject (Subrt and Tlaskal 1993; Ding et al 1998; Liu
the mechanically treated hydroxyl containing compounds
et al 2000; Mendoza-Suarez et al 2001). From mechani-
and to evaluate its crystallization behaviour during heat
cally activated mixtures, crystallization of M-phase usually
treatment.
occurs between 800 and 1200°C and shows nano or submi-
cron size. However, mechanical milling is a high energy
2. Experimental
consuming process, which requires additional equipment
such as high energy shaker mill (Ding et al 1998; Liu et al
The starting reagents were goethite ?-FeOOH (GNA-85N,
2000).
with 0?2 µm long needle like morphology) from Toda
Subrt and Tlaskal (1993) have described that if starting
Kogyo Corp. and BaCO
mixture contains hydroxyl containing compounds the
3 (0?2 µm long rod like morpho-
logy), both with analytical grade purity. A stoichiometric
crystallization of M-phase occurs at 800°C. They have
mixture for M-phase was activated with multi-ring type
attributed this reaction to the appearance of a highly re-
®
mill, Mechano Micros (Nara Machinery) for 1 and 4 h. The
active iron oxide formed during dehydration of iron hy-
rates of revolution and counter-revolution of the rotor and
droxides.
the vessel were fixed at 1250 and 250 min–1, respectively.
We have previously reported that a soft-mechano-
A 30 g of the powder, corresponding to 15% of the effective
chemical reaction has a great potential for the synthesis of
volume of the vessel, was charged for activation. After
hexagonal ferrites such as Y-phase (Ba2Co2Fe12O22) (Te-
milling, samples were calcined at 600, 800 and 1000°C in air.
Samples were characterized by a powder X-ray diffracto-
metry (Rigaku RINT 2000), FTIR (Bio-Rad Win-IR) and

*Author for correspondence (jtemuujin@yahoo.com)
FE-SEM (Hitachi S-4000).

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J Temuujin et al


Figure 1. XRD patterns of the raw materials and the mixture
Figure 2. FTIR spectra of the raw materials and the mixture
(M-) milled for 1 and 4 h.
(M-) milled for 1 and 4 h.




Figure 3. FE–SEM micrographs of the samples milled for 1 h (a) and 4 h (b).


3. Results and discussion
samples was negligible, indicating its reduction of crystal-
lite size and accumulation of microstrain to be insignifi-
The XRD patterns of the raw materials and samples
cant. Structural changes caused by milling were also
milled for 1 and 4 h are shown in figure 1. Milling signi-
examined by FT–IR spectra. As shown in figure 2, barium
ficantly reduced diffraction intensities of barium carbonate
carbonate is severely affected by milling, as evidenced by
and goethite peaks. However, the mixture was not fully
the disappearance or reduction of the absorption band
amorphized even after 4 h milling. There is a small change
intensities at 1450, 1059 and 856 cm–1, being characteristic
in terms of diffraction intensity between the samples
of the carbonate groups in BaCO3. Goethite sample
milled for 1 and 4 h. Diffraction line broadening of the milled
shows absorption bands at 3195, 890, 796 and 3440 cm–1,

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Crystallization of M-type hexagonal ferrites
459
due to OH and H2O vibrations and a band at 630 cm–1 due
The first reaction starts between 600 and 750°C and the sec-
to lattice vibration of FeO6 octahedra as reported previously
ond between 720 and 900°C (Schoeps 1979). Completion
(Temuujin et al 2004). Most of these bands are present in
of the overall reaction takes place at about 1000–1100°C
the milled samples. However, a stretching vibration of the
(Schoeps 1979; Steier et al 1999). Therefore, we can suggest
raw goethite, ?(FeO6), appearing at 630 cm–1 became flat
that mechanical activation drastically improved the rate
by milling for 1 h and broader and centred at 565 cm–1
of solid state reaction to form barium hexaferrite and the
after milling for 4 h. Similar changes also occurred in the
first step was completed at below 600°C. In the present
milled Y-phase hexaferrite composition and we have attri-
case, the overall reaction was completed at 800°C without
buted it to substitution of Co cations into octahedral lat-
any other intermediates. Just 1 h milling was sufficient to
tice of goethite or partial decomposition of goethite into
cause formation of barium hexaferrite at 800°C. An increase
hematite like structure (Temuujin et al 2004). However,

M-phase hexaferrite does not have Co atoms in its struc-
ture. Therefore, the above mentioned change of ?(FeO6)
vibration is probably related with an appearance of the
hematite like structure. Weakening of the carbonate bands
of the BaCO3 and appearance of the hematite like structure
may also indicate a chemical bond formation between
iron and barium constituents with partial decarbonation.
Scanning electron micrographs of the samples milled
for 1 and 4 h are shown in figure 3. The sample milled for
1 h comprises spherical particles with their average particle
size between 50 and 60 nm, indicating improved homo-
geneity and particle size reduction of the starting powders.
Increasing the milling time causes some agglomeration
with change of morphology from spherical to rod like.
Figure 4 shows XRD patterns of the milled samples
after calcining at different temperatures. Both samples
calcined at 600°C contain BaFe2O4 and ?-Fe2O3 phases.
Solid state reaction for the formation of barium hexaferrite
from hematite and barium carbonate usually occurs via 2
steps (Schoeps 1979)

BaCO3 + Fe2O3 ? BaFe2O4 + CO2, (1)
Figure 4. XRD patterns of the milled samples after calcining
BaFe2O4 + 5Fe2O3 ? BaO?6Fe2O3. (2) at different temperatures.



Figure 5. FE–SEM micrographs of the 1 h milled sample after calcining at 800 (a) and 1000°C (b).

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J Temuujin et al
in the milling time or calcination temperature does not
Acknowledgement
exhibit any considerable effect on the crystallization beha-
viour of the M-phase. As discussed above, the mechanical
This research was partially supported by the Ministry of
activation results in homogenization and possible hetero
Agriculture and Science and Technological Foundation of
bridging bond formation between powder constituents during
Mongolia under the project “Technological investigation
milling and that could be the reason of the improved solid
on the preparation of the ecologically pure activated
state reactivity.
phosphorous fertilizer from natural phosphate rock”.
In the micrograph shown in figure 5 for 1 h milled sample
(a), we observe that the barium hexaferrite obtained by
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