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Journal of Materials Processing Technology 181 (2007) 106–109

Synthesis and characterization of barium hexaferrite nanoparticles

M. Radwan ∗, M.M. Rashad, M.M. HessienElectronic Materials Laboratory, Advanced Materials Department,

Central Metallurgical Research and Development Institute (CMRDI) P.O. Box 87 Helwan, Cairo 11421, Egypt

bstract

This investigation dealt with the synthesis of nanocrystalline barium hexaferrite (BaFe12O19) powders through the co-precipitation–calcinationoute. The ferrite precursors were obtained from aqueous mixtures of barium and ferric chlorides by co-precipitation of barium and iron ions usingM sodium hydroxide solution at pH 10 in room temperature. These precursors were calcined at temperatures of 800–1200 ◦C for constant 2 h

n a static air atmosphere. The effect of Fe3+/Ba2+ mole ratio and addition of surface active agents during co-precipitation step on the structuralnd magnetic properties of produced ferrite powders were studied. It is found that the formation of single phase BaFe O powders was achieved

12 19

y decreasing the Fe3+/Ba2+ molar ratio from the stoichiometric value 12–8 and increasing the calcination temperature ≥1000 ◦C. In addition,he Fe3+/Ba2+ mole ratio of 8 the surface active agents promoted the formation of homogeneous nanopowders (ca. 113 nm) of BaFe12O19 at aow-temperature of 800 ◦C with resultant good magnetic saturations (50.02 emu/g) and wide intrinsic coercivities (642.4–4580 Oe).

2006 Elsevier B.V. All rights reserved.

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eywords: M-type ferrite; Co-precipitation; Magnetic properties; Surfactants;

. Introduction

Barium hexaferrite, BaFe12O19, is a well-known permanentagnet with great technical importance attracted an exten-

ive attention for the last few decades. BaFe12O19 has aagnetoplumbite-type structure (hexagonal) with fairly large

rystal anisotropy along c-axis. It has high intrinsic coerciv-ty (6700 Oe), large saturation magnetization (72 emu/g) andigh Curie temperature (450 ◦C) [1,2]. It was widely used inhe fabrication of commercial permanent magnets, computerata storage, high-density perpendicular magnetic and magneto-ptic recording, magnetic fluids and certain microwave devices1,2]. For ideal performance, ultrafine barium hexaferrite pow-er (∼ 0.1 �m) with homogeneous particle size distribution andontrolled magnetic properties is important [3].

It is difficult to obtain ultrafine and monodispersed parti-les by the commercial ceramic method (solid-state reaction)hich involves the firing of stoichiometric mixture of bar-

um carbonate and �-iron oxide at high temperatures (about200 ◦C) [4]. In this respect, several low-temperatures chem-cal methods were investigated for the formation of ultrafine

∗ Correspondence to: Joining and Welding Research Institute, Osaka Univer-ity, 11-1 Mihogaoka Ibaraki, Osaka 567-0047, Japan. Tel.: +81 6 6879 8693;ax: +81 6 6879 8693.

E-mail address: radwan72001@yahoo.com (M. Radwan).

(Tsc1ttf(a

924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2006.03.015

ne powder

aFe12O19 particles. These methods comprised co-precipitation5–8], hydrothermal [9–11], sol–gel [12–14], microemulsion2], citrate precursor [15], glass crystallization [16], sono-hemical [17] and mechano-chemical activation [18]. The chem-cal co-precipitation method is a low-cost technique suitable forhe mass production compared to the other mentioned methods.he main drawback is that the particle size is not small andonodispersed enough for applications like recording media

pplications. This paper studies the effect of Fe3+/Ba2+ moleatio and the addition of surfactants on the synthesis of nanocrys-alline barium hexaferrite powder by chemical co-precipitation

ethod. Such a study is important to understand how to controlhe synthesis of single phase barium hexaferrite powder withomogeneous ultrafine size at low calcination temperature.

. Experimental

Guaranteed chemically grade ferric chloride (FeCl3·6H2O), barium chlorideBaCl2·2H2O) and sodium hydroxide (NaOH) were used as starting materials.hree surfactants; cetyltrimethyl ammonium bromide (CTAB), sodium dodecylulfate (SDS) and Triton X-100 were used. A series of ferric and bariumhlorides solutions in distilled water with various Fe3+/Ba2+ molar ratios (12,0.9, 9.23 and 8) were prepared. The ferrite precursors were precipitated from

hese mixtures by adding gradually sodium hydroxide (5 M) solution at roomemperature to pH 10. The aqueous suspensions were stirred at constant 500 rpmor 15 min to achieve good homogeneity and attain a stable pH condition. Thebrownish) co-precipitates were filtered off, washed with water and dried inn oven at 100 ◦C overnight. In order to form the hexaferrite phase, the dry

M. Radwan et al. / Journal of Materials Processing Technology 181 (2007) 106–109 107

Table 1XRD results of products obtained from precursors with different Fe3+/Ba2+ mole ratio and calcined for 2 h at 800–1200 ◦C (no surfactants used)

Fe3+/Ba2+ Phases obtained as deduced by XRD

800 ◦C 1000 ◦C 1200 ◦C

12 BaFe0.24Fe0.76O2.88 > BaFe12O19 > Fe2O3 (122.1)a BaFe12O19 > Fe2O3 � BaFe0.24Fe0.76O2.88 (140) BaFe12O19 � Fe2O3 (191.2)10.9 BaFe12O19 > Fe2O3 > BaFe0.24Fe0.76O2.88 (143.8) BaFe12O19 � Fe2O3 � BaO2 (185) BaFe12O19 (191.1)

9.23 Fe2O3 > BaFe12O19 > Ba2Fe6O11 > BaFe0.24Fe0.76O2.88 BaFe12O19 > Fe2O3 � BaO2 � Ba2Fe6O11 (174.7) BaFe12O19 � Ba2Fe6O11 (221.4)

12O19

ing c

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oamfX1rpafstotciand formation of nanocrystalline single phase barium hexaferritepowders.

The SEM micrographs of different powders obtained by eachsurfactant are given in Fig. 4(a–d). It can be observed that the

(98.8)8 BaFe12O19 � Fe2O3 (153.1) BaFe

a Crystallite size (nm) of BaFe12O19 phase as calculated from XRD results us

recursors (reddish solids) were heated (calcined) at a rate of 10 ◦C/min in staticir atmosphere up to different temperatures (800, 1000 and 1200 ◦C) where theyere maintained for 2 h. The effect of surface active agents on the formation ofaFe12O19 was studied by addition of 1000 ppm of the surfactant (CTAB, SDSr Triton X-100) to the mixed Fe/Ba solution before the precipitation step.

The calcined powders were ground gently by agate mortar prior to theirharacterization. The crystalline phases present in the different calcined samplesere identified by X-ray diffraction (XRD) on a Bruker axis D8 diffractometersing Cu K� radiation. The average crystallite size of the powders was estimatedutomatically from corresponding XRD data (using X-ray line-broadening tech-ique employing the classical Scherrer formula). The particles morphologiesere observed by scanning electron microscopy (SEM, JSM-5400). The mag-etic properties of different calcined powders were measured at room temper-ture on a vibrating sample magnetometer (VSM, model LDJ 9600-1, USA)n a maximum applied field of 15 kOe. From the obtained hysteresis loops, theaturation magnetization (Ms) and coercivity (Hc) were determined.

. Results and discussion

The effect of Fe3+/Ba2+ mole ratio and calcination temper-ture on the phase composition of products has been followedy the XRD analyses. The results obtained are listed in Table 1.t can be observed that the stoichiometric ratio was not appro-riate to result in a single phase barium hexaferrite powder andecreasing the Fe3+/Ba2+ mole ratio (i.e. increasing the Ba con-entration) promotes the formation of barium hexaferrite phaset the various thermal treatments. At low calcination temperaturef 800 ◦C, the hematite Fe2O3 phase impurity was found in allamples. The intermediate BaFe0.24Fe0.76O2.88 phase appearss a major phase at the stoichiometric sample and its concen-ration decreases as the Fe3+/Ba2+ mole ratio decreases to 9.23nd disappears at 8. The phases obtained from precursors withe3+/Ba2+ mole ratio of 9.23 were rather complicated in con-

rast to other starting ratios, the hematite became the major phasend another stable ferrite phase (Ba2Fe6O11) was remained inhe calcined powders even after heating at 1000 and 1200 ◦C.

Increasing the calcination temperature to 1000 ◦C enhancedhe formation of barium hexaferrite phase and a single phaseaFe12O19 powder was formed at the Fe3+/Ba2+ mole ratio of. The intermediate hematite phase was obtained in other start-ng ratios. But the BaFe0.24Fe0.76O2.88 phase disappeared byecreasing the Fe3+/Ba2+ mole ratio below the stoichiometricalue. An intermediate barium oxide (BaO2) phase was detectedn the powders with the starting ratios of 10.9 and 9.23.

At the high calcination temperature (1200 ◦C), Fe2O3 wasbtained only in the stoichiometric mixture and single phasearium hexaferrite powders were obtained from precursors withtarting mole ratios of 10.9 and 8.

F(

(151) BaFe12O19 (200.6)

lassic Scherrer formula.

The crystallite size of formed single phase barium hexaferriteowders as calculated from XRD analyses using Debye-Scherrerormula was in the range of 151–200 nm.

The SEM micrograph of single phase barium hexaferriteowder obtained from starting mixture with the ratio of 8 andeating at 1000 ◦C is shown in Fig. 1. The powder consistsf micro-aggregates with ultrafine particles mostly of platelethape, more longitudinal, with mean diameters (≤200 nm).

The effect of surfactants on the formation and propertiesf barium hexaferrite powders was studied by the addition of1000 ppm surfactant to the cations mixture (with Fe3+/Ba2+

ole ratio of 8) prior to the co-precipitation step. Three dif-erent surfactants were experimented; CTAB, SDS and Triton-100. The dry precursors were then calcined at 800, 1000 and200 ◦C for 2 h. The XRD analyses of various calcined samplesevealed the formation of only single phase barium hexaferriteowders. Fig. 2 gives the XRD pattern of Bafe12O19 obtainedfter calcination at 800 ◦C when CTAB was used as the sur-actant. The pattern shows well-defined Bragg peaks which isignificant of a good crystalline state of the sample. Fig. 3 showshe effect of various surfactants and calcination temperaturesn the crystallite size of obtained powders. It can be observedhat the addition of surfactants before the co-precipitation stepould prevent agglomeration of particles and grain growth dur-ng calcination course, which leads to the decrease of grain size

ig. 1. SEM micrograph of obtained single phase BaFe12O19 powderFe3+/Ba2+ = 8, calcination temperature = 1000 ◦C).

108 M. Radwan et al. / Journal of Materials Processing Technology 181 (2007) 106–109

Fm

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Fo

tipfoiacalcination temperature (to 1200 ◦C) led to the coarsening of

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ig. 2. XRD pattern of BaFe12O19 obtained from a precursor with Fe3+/Ba2+

ole ratio of 8 and calcined for 2 h at 800 ◦C (with CTAB surfactant).

urfactants can control the microstructure of formed barium hex-ferrite powders. When the anionic surfactant SDS was used,he 800 ◦C calcined hexaferrite powders (Fig. 4a) had well-rranged grains with homogeneous nanometer size (∼100 nm).e think that some kind of repulsion between negative surfac-

ant anions and molecules of Ba and Fe chlorides or hydroxidesxisted which prevented the agglomeration of particles dur-

ng the heating step. The nucleation of hexaferrite particlesas homogeneous and the grain growth was hindered which

esulted in good size homogeneity. The cationic CTAB surfac-

tXe

ig. 4. Effect of surfactants additions on microstructure of synthesized BaFe12O19 pnd calcined at 800 ◦C, (b) using CTAB and calcined at 800 ◦C, (c) using CTAB and

ig. 3. Effect of surfactants and calcination temperature on crystallite size (nm)f formed BaFe12O19 obtained from precursors with Fe3+/Ba2+ = 8.

ant was found to make the barium hexaferrite particles arrangedn laminates-structure like or clusters which include very smallarticles of nanometer size (Fig. 4b). We think that an attractionorces existed between CTAB cationic surfactant and moleculesf barium and ferric chlorides. When NaOH was added, the bar-um and ferric hydroxides molecules were co-precipitated andrranged in some way in laminates like structure. Increasing the

hese laminates to few tenths of microns (Fig. 4c). Since Triton-100 is non-ionic surfactant, such electrostatic forces are not

xist and less localization of surfactant on interfacial surfaces of

owders obtained from precursors with Fe3+/Ba2+ mole ratio = 8: (a) using SDScalcined at 1200 ◦C and (d) using X-100 and calcined at 800 ◦C.

M. Radwan et al. / Journal of Materials Proce

Table 2Magnetic properties of single phase BaFe12O19 powders (Hc and Bs were mea-sured at room temperature under Hmax = 15 kOe)

Preparation conditions Magnetic properties

Fe3+/Ba2+ Surfactant Calcinationtemperature (◦C)

Hc (Oe) Ms (emu/g)

10.9 – 1200 2556 44.98 – 1000 3923 28.88 – 1200 1670 44.128 CTAB 800 4347 45.138 CTAB 1000 4358 46.658 CTAB 1200 669.9 49.858 SDS 800 4518 46.668 SDS 1000 4327 45.298 SDS 1200 976.6 50.028 X-100 800 2050 47.22

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arium and ferric molecules was found. On the calcination, theucleation and growth of hexaferrite particles were inhomoge-eous which led to a microstructure with various morphologiesf irregular shapes (Fig. 4d).

The magnetization curves of different powders of singlehase barium hexaferrite were measured at room temperaturender an applied field of 15 kOe. The results of magnetic prop-rties Hc and Ms are summarized in Table 2. In general, asommonly observed in the case of ultrafine particles, the sat-ration magnetization values are lower than the theoretical sat-ration magnetization for single crystals of barium hexaferriteBs = 72 emu/g) as reported by Shirk and Buessem. Also, the val-es of intrinsic coercivity Hc obtained for our samples are lowerhan theoretical calculations (Hc ≈ 6700 Oe) using the Stonernd Wohlfarth model of single-domain particles [2]. Variousheories, including surface area, spin canting and sample inho-

ogeneity have been proposed to account for the relatively lowagnetization in fine particles [15]. It may also be due the pres-

nce of superparamagnetic fractions of very fine particles in theormed ferrite nanopowders.

. Conclusions

The observations from the XRD, SEM and VSM studies areummarized as follows:

(i) A barium surplus is important to synthesize single phaseBaFe12O19 powder by chemical co-precipitation method.Pure ultrafine barium hexaferrite powders (150–200 nm)were obtained from precursors with Fe3+/Ba2+ mole ratio

[[

ssing Technology 181 (2007) 106–109 109

of 8 after calcination at temperatures ≥1000 ◦C for 2 h instatic air atmosphere.

(ii) The addition of surfactants to the starting solution of bariumand ferric cations (Fe3+/Ba2+ = 8) before co-precipitationstep was found to enhance the formation of single phasebarium hexaferrite powders at low calcination temperatureof 800 ◦C.

iii) The surfactant addition controls the microstructure offormed barium hexaferrite powders. The anionic SDS sur-factant led to the formation of well-arranged barium hex-aferrite grains with homogeneous nanometer sizes. Thecationic CTAB surfactant resulted in micrometer sized lam-inates or clusters like containing ultrafine individual bariumhexaferrite particles. The non-ionic Triton X-100 surfac-tant produced nano-size hexaferrite particles with irregularshapes.

iv) The formed nano-sized barium hexaferrite powders hadgood magnetic saturations (50.02 emu/g) and wide intrinsiccoercivities (642.4–4580 Oe).

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