Fine strontium hexaferrite particles were prepared by the low temperature auto-combustion method. Aqueous solution of the corresponding metal salts and a combustible co-reactant compound -glycine were used as initial precursor. The resulting powders were calcined at various temperatures from 750 • C up to 1100 • C. The microstructure and magnetic properties were investigated by the Mössbauer spectroscopy, thermomagnetic analysis, scanning electron microscopy, and vibrating magnetometer

A Grusková

J Sláma

M Ušáková

V Jančárik

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Vol. 113 (2008) ACTA PHYSICA POLONICA A No. 1
Proceedings of the CSMAG’07 Conference, Košice, July 9–12, 2007
Optimization of Magnetic and Structural
Properties of Sr Ferrite Prepared
by Low-Temperature Combustion
A. Grusková, V. Jančárik, J. Sláma and M. Ušáková
Faculty of Electrical Engineering and Information Technology
Slovak University of Technology
Ilkovičova 3, 812 19 Bratislava, Slovakia
Fine strontium hexaferrite particles were prepared by the low tempe-
rature auto-combustion method. Aqueous solution of the corresponding
metal salts and a combustible co-reactant compound — glycine were used
as initial precursor. The resulting powders were calcined at various tem-
peratures from 750◦C up to 1100◦C. The microstructure and magnetic
properties were investigated by the Mössbauer spectroscopy, thermomag-
netic analysis, scanning electron microscopy, and vibrating magnetometer.
The particles of sizes less than 1 µm have monophasic hexagonal struc-
ture. Coercivity Hc ≈ 400 kA/m, specific saturation polarization Js−m ≈
72 × 10−6 Tm3 kg−1 and remanence polarization Js−r ≈ 43 × 10−6 T m3
kg−1 were achieved for the Sr ferrite annealed at 850◦C for 6 h.
PACS numbers: 75.50.–y, 75.50.Ww, 74.25.Ha, 33.45.+x
1. Introduction
The M-type strontium hexaferrite, which is a hard magnetic material, has
been extensively used as a permanent magnet due to its low cost, relatively high
coercivity, corrosion resistance, and chemical stability [1]. Magnetic properties of
Sr or Ba ferrite powders strongly depend on the microstructure [2]. In order to
get pure crystalline monodomain particles, various synthesis techniques have been
developed, such as sol-gel technique [3], chemical coprecipitation [4], etc. A novel
low-temperature combustion synthesis (LCS) is presented in this work, greater
amount of the product can be obtained by this way.
Mixed solutions of metal nitrates and glycine were used to prepare
(SrFe12O19). The effects of processing parameters, such as Sr/Fe ratio, ni-
trates/glycine ratio, reaction temperature and magnetic properties of the resultant
Sr ferrite powders were investigated. Four samples prepared by various procedures
were compared. The reproducibility of magnetic parameters was also an important
criterion for procedure evaluation.
2. Experimental
Sr(NO3)2, Fe(NO3)3 × 9H2O analytical reagent grade and HOOCCH2–NH2
(glycine) were used as raw components. Sufficient water was added to dissolve the
(557)
558 A. Grusková et al.
metal salts and glycine. Nitrate/glycine ratio considerably affects the combustion
process and the grain size of prepared ferrite powder. The best product usually
results when the glycine-to-cation molar ratio is adjusted to produce the largest
bulk volume of ash [5]. The precursor mixture is processed by evaporating wa-
ter into a glass-like substance. The auto-ignition was reached within a minute,
yielding the brown-colored ash as a precursor. The ferrite particles generally grow
in size during the calcination process in the laboratory muffle. Annealing tem-
peratures 750, 800, 850, 900, 950, 1000, 1050, and 1100◦ were used for 6 hours.
Ferrite particles calcined at 850◦ had typically submicron size with a narrow size
distribution. In order to obtain a single phase Sr ferrite, optimal atomic ratio of Sr
to Fe was chosen and appropriate amount of NH4NO3 and diethylglycole (DEG)
into the precursor was added.
The microstructure and magnetic properties can be optimized by adding
these modifiers and by correction of the calcination temperature.
The samples A and B were prepared in an amount of 10g of the final
SrFe12O19 product. The molar ratio of sample A was chosen as follows:
Sr(NO3)2 : Fe(NO3)3 × 9H2O : glycine
1 : 10.5 : 30
Other ingredients except glycine was added into sample B in the following molar
ratios:
Sr(NO3)2 : Fe(NO3)3 × 9H2O : NH4NO3 : DEG : glycine
1 : 10.5 : 10.5 : 1 : 30
The samples C and D were prepared in greater amount (approximately 100 g) in
composition as sample B. The Mössbauer spectroscopy was carried out to study
the cationic distribution on the hexagonal structure using a spectrometer with
conventional constant acceleration mode and 57Co/Rh γ-ray source. The spectra
were fitted by NORMOS software. Temperature dependences of magnetic suscep-
tibility χ(T ) were measured by bridge method in an alternating magnetic field of
421 A/m at 920 Hz to obtain the Curie temperature Tc. The shape of these depen-
dences gives also an information about the phase purity, presence of Hopkinson’s
peak gives an information about anisotropy and size of the particles. The magnetic
properties of the ferrite powders were measured at room temperature using a vi-
brating sample magnetometer with an applied field of up to 755 kA/m. Scanning
electron microscope (SEM) was used to examine the grain size and morphology.
3. Results
The Mössbauer spectra were analysed using five sextets. The Fe3+ ions
in positions 4f2, 2a and 12k have octahedral locations, 4f1 tetrahedral and 2b
occupy trigonal bipyramide. Hyperfine parameters of the Mössbauer spectra (Bhf
and relative area RA) at room temperature for the sample D annealed at 850 and
1050◦C are compared with those of bulk sample (theoretical values [6]) in Table I.
The relative area of each subspectrum is proportional to the number of ferric ions
Optimization of Magnetic and Structural Properties . . . 559
on the corresponding sublattice. The magnetic properties of the samples A, B, C,
D calcined at 850◦C for 6 h were measured by a method described in [7], results
are shown in Table II. All samples have promising values of magnetic parameters.
TABLE I
Influence of annealing temperature on hyperfine interaction pa-
rameters of the sample D.
Site Bhf [T] RA [%]
850◦C 1050◦C Theor. 850◦C 1050◦C Theor.
4f2 51.60 51.58 51.8 15.8 14.5 16.7
2a 50.86 50.85 50.6 7.4 9.2 8.3
4f1 48.96 49.13 49.2 19.1 20.5 16.7
12k 41.33 41.35 41.3 50.0 48.9 50.0
2b 40.85 40.80 41.0 7.7 6.9 8.3
TABLE II
Magnetic properties of Sr ferrite powders.
Sample Js−m [10−6 T m3 kg−1] Js−r [10−6 T m3 kg−1] Hc [kA m−1]
A 72.59 43.41 434
B 71.92 42.53 404
C 69.00 41.66 400
D 73.45 43.74 370
Fig. 1. Temperature dependence of magnetic susceptibility χ(T ) of sample D – pre-
cursor (a), annealed at 850 and 1050◦C (b).
Temperature dependence of magnetic susceptibility χ(T ) of the sample D
before calcination process (precursor) is shown in Fig. 1a. This dependence cor-
responds with the maghematized magnetite. The dependences for sample D an-
nealed at 850 and 1050◦C are shown in Fig. 1b. The χ(T ) dependences exhibit a
Hopkinson peak in the vicinity of the Curie temperature and confirms phase purity
of ferrite powders. The height of the Hopkinson peak decreases with increasing
annealing temperature, which is connected with grain size rising.
560 A. Grusková et al.
The morphology and grain size for sample D annealed at 850◦C/6 h and
1050◦C/6 h are shown in Fig. 2. The particle size of sample annealed at 850◦C is
much below 1 µm, while annealing at 1050◦C yields particles of size over 1 µm of
nearly hexagonal platelet shape.
Fig. 2. Micrograph of particles size and morphology of sample D, annealed at 850◦C
for 6 h (a) and 1050◦C for 6 h (b).
Fine strontium hexaferrite powders have been prepared at a relatively low
calcination temperature by combustion synthesis also in greater amount of the
product (over hundreds of grams). The initial nitrate/glycine ratio such as Fe/Sr
ratio have a strong influence on the combustion process and the grain size of the
product. The well–shaped SrFe12O19 particles were obtained at relatively low
temperature 850◦C for 6 h. It improved the magnetic properties coercivity around
Hc ≈ 400 kA/m for all samples (A, B, C, D). It is possible to use these materials
as fillers in magnetocomposites.
Acknowledgments
This work was supported by the VEGA of the Slovak Republic, the projects
No. G-1/3096/06 and G-1/3189/06, respectively.
References
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[3] G. Mendoza-Suárez, M.C. Cisneros-Morales, M.M. Cisneros-Guerrero, K.K. Johal,
H. Mancha-Molinar, O.E. Ayala-Valenzuela, J.I. Escalante-Garcia, Mater. Chem.
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[4] K. Haneda, A.H. Morrish, IEEE Trans. on Magn. 25, 2597 (2089).
[5] L.R. Pederson, L.A. Chick, G.J. Exarhos, U.S. Patent, 5,114,702 (5/1992).
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Optimization of Magnetic and Structural Properties of Sr Ferrite Prepared by Low-Temperature Combustion

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Optimization of Magnetic and Structural Properties of Sr Ferrite Prepared by Low-Temperature Combustion

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