Samarium-iron garnet nanopowder obtained by co-precipitation

V. T. Caffarena^1,4, T. Ogasawara^2,5, M. S. Pinho^3,6 and J. L. Capitaneo^2,7

¹ Brazilian Physical Research Center - CBPF, Rua Dr. Xavier Sigaud, 150, Urca,
² PEMM/COPPE, UFRJ, PO Box 68505, ZIP CODE 21941-972, R J, Brazil,
³ Brazilian Navy Research Institute (IPqM), Rua Ipiru n^o 2, Praia da Bica, Ilha do Governador, Rio de Janeiro, R J, Brazil, CEP 21931-090, Fax number: 005521-3386-2788
⁴ valeska@cbpf.br, ⁵ ogasawat@metalmat.ufrj.br, ⁶ magalipinho@yahoo.com.br, ⁷ jeff@metalmat.ufrj.br

Abstract — In this work, samarium-iron garnet nanopowder (Sm₃Fe₅O₁₂) was obtained and characterized by TG/DTA, XRD, SEM and EDS. The synthesis of this magnetic material was carried out by the co-precipitation method, using hydrated chlorides of the rare-earth elements and ferrous sulfate. PVA was added to the calcined powder in order to facilitate the production of a toroidal compact specimen, which was submitted to sintering in the range of 1200 ^oC to 1400 ^oC, to stablish a correlation of its magnetic properties with morphology.

Keywords — Garnet. Co-precipitation. Magnetic Properties. Nanomaterial.

I. INTRODUCTION

Billions of soft ferrites are used in every conceivable electronic devices. Rare-earth garnet-structure ferrites, R₃Fe₅0₁₂ (where R is yttrium or rare-earth cation), have attracte great attention for applications such as both microwave devices and magnetic recording media (Sugimoto, 1980; Rodic et al., 1999).

Recently, garnets are widely used in microwave communication through mobile and satellite communications and are of significant interest for numerous applications, including magnetic materials, lasers, phosphorescent sources, and electrochemical devices (Waerenborgh et al., 2004).

The garnet materials possess unique optical, thermophysical and mechanical properties, in particular, excellent creep and radiation damage resistance, fracture toughness, moderate thermal expansion coefficients, high thermal conductivity and energy-transfer efficiency (Waerenborgh et al., 2004).

Samarium-iron garnet (Sm₃Fe₅O₁₂) is of considerable interest for its potential use as a broadband material in microwave application (Cunningham and Anderson, 1960).

In this work, this material was synthesized by co-precipitation method and its magnetic properties were studied. The resulted garnet shows nanometric size and better magnetic properties than others prepared by traditional methods.

Co-precipitation is a chemical route which plays a crucial role in preparing the final product by minimizing problems associated with diffusion, impurities and agglomeration. In solid state methods, mechanical mixing of oxides followed by calcination, result in final products with worst electrical, mechanical and magnetic properties (Lax and Button, 1962; Horvath, 2000; Dionne, 1971; Adair, 1991).

II. EXPERIMENTAL PROCEDURE

The synthesis of the material was carried out by the co-precipitation method, using as reagents the rare-earth element hydrated chlorides (SmCl₃.6H₂O, purity 99%, Aldrich) and ferrous sulfate (FeSO₄.7H₂O, purity 99%, Reagen). These solutions were all mixed together in a 500 mL recipient and the pH of the total solution was kept in the range 2 - 3.

This solution was heated up to 6 ^oC under intensive agitation for about 30 minutes, followed by KOH addition to increase the pH value in 10 - 11 range, while allowing easy elimination of the K⁺ ions, by simple washing of the precipitate with distilled water (Adair, 1991).

Thermodynamic analysis (Caffarena and Ogasawara, 1999) shows that the final pH for co-precipitation method shall be in the range 9 - 12, so the synthesis has been repeated three times with pH values from 10 to 11.

The co-precipitated solids were separated from the initial solution by vacuum filtration, after several washings with distilled water, until Cl^- and SO₄^2- anions were no more detectable by test reactions with AgCl and BaSO₄, respectively (Morita and Assumpção, 1972).

The drying. of the precipitate was carried out according to Reed's recommendation (Reed, 1991) with initial drying inside a desiccator for 24 hours, followed by heating at 75 ^oC for 4 hours. The dried co-precipitate was submitted to thermal analyses using Shimadzu 50H Differential Thermal Analyzer and Shimadzu TGA-50 Thermogravirnetric Analyzer (Sorrel, 1991).

Subsequent calcination (Halloran, 1991) of co-precipitates was carried out under conditions suggested by thermal analysis, corresponding to heating up to 1000^oC (rate of 5 ^oC/minute); holding time of 4 hours, followed by natural cooling down inside the furnace.

The powder was characterized (JCPDS, 1977) by X-ray fluorescence (Philips model PW2400), scanning electron microscopy (Zeiss SEM model DSM 940A and Oxford-Link EXL II EDS Module), X-ray diffraction (Philips PW3170 X-Ray Diffractometer, copper K_α [λ = 1.542 Å] radiation, generator at 40 kV and 40 mA, scanning 2θ angles from 10 100 ^o).

Ring shaped compacts (2.0 cm outer diameter, 0.8 cm inner diameter and 0.4 cm height) were produced by dry pressing of the calcined powder, to which was added as a binder an aqueous solution containing 15 wt % of PVA (polyvinyl alcohol), in a suitable amount corresponding to 2 wt% of the total compacted mass. A total of 50 pieces were produced for each batch.

Ten compacted pieces were sintered at 6 different firing temperatures (1200 ^oC, 1250 ^oC, 1300 ^oC, 1350 ^oC, 1400 ^oC and 1450 ^oC) in order to promote formation of the desired samarium-iron garnet, according to the reaction 1:

3 Sm₂O₃ + 5 Fe₂O₃ → Sm₆Fe₁₀O₂₅ (1)

The firing was performed in the following heating schedule: 4 ^oC/minute up to 400 ^oC, a 400^oC plateau for 1 hour, 8^oC/minute up to the final sintering temperature, where the sample was kept during 5 hours, in air. The cooling down inside the furnace was carried out at a rate of 8 ^oC/minute down to 800 ^oC, where the sample was kept during 1 hour, before the final cooling down to the room temperature, at the same rate. Figure 1 shows the thermal profile of the ceramic firing for different sintering temperatures.

Figure 1. Thermal background for sintering ceramic rings at 1400 ^oC.

Magnetic properties of the sintered samples were determined by using magnetic hysteresisgrapher (Walker Scientific model AMH-20): each ceramic ring received a varnished copper AWG 29 wire winding, to provide a solenoid (magnetically analyzed under 60 Hz frequency and 25 Oe maximum magnetic field). The microstructure of the sintered toroids (rings) was analyzed by scanning electron microscopy (SEM). They were mounted on aluminum supports and coated with a film of gold.

III. RESULTS AND DISCUSSION

Results of thermogravimetric (TGA) and differential thermal analysis (DTA) are shown in Fig. 2. There is not any DTA peak below 500 ^oC and the TGA information reveals the formation of mixed oxide, firstly as an amorphous phase that converts to crystalline phases during the latter heating. DTA revealed an exothermic peak at 946 ^oC associated to the transformation of the samarium and iron (III), amorphous mixed oxides into the crystalline ones.

Figure 2. TGA and DTA of co-precipitated samarium and iron hydroxides, in air, with heating rate of 10 ^oC/min.

The powder was calcined at 1000^oC and the X-ray diffraction pattern of the product confirmed this transformation (Fig. 3), giving rise to intermediate compounds (SmFeO₃, Fe₂O₃ and Sm₂O₃), a feature meaning that greater time and temperature are needed, in order to achieve full conversion of the material to true iron garnet (Forterre, 1991; Gasgnier et al., 1998).

Figure 3. X-ray diffraction pattern of the ceramic pieces, corresponding to samarium-iron garnet formation, where: Δ SmFeO₃, + Fe₂O₃ and O Sm₂O₃.

Figure 4 shows the X-ray diffraction pattern of the mixed oxide sintered at different temperatures in the 1200-1450^oC range. At 1200 ^oC (Fig. 4-a), 1250 ^oC (Fig. 4-b) and 1300^oC (Fig. 4-c) remained samarium orthoferrite, Sm₂O₃ and Fe₂O₃ phases. At 1350 ^oC (Fig. 4-d), the XRD pattern indicate the presence of Sm₃Fe₅0i₂, Sm₂O₃ and orthoferrite. It is seen that complete formation of the samarium-iron garnet only occurred at elevated temperatures at 1400^oC (Fig. 4-e) according to JCPDS 23-526 data.

Figure 4. X-ray diffraction pattern of the ceramic pieces sintered at different temperatures in the 1200-1450^oC range, where √ Sm₃Fe₅0₁₂, Sm₂O₃, and SmFeO₃ and + Fe₂O₃.

In this work, 50 ceramic pieces were produced (10 at 1200 ^oC , 10 at 1250 ^oC, 10 at 1300 ^oC, 10 at 1350 ^oC and 10 at 1400 ^oC), and each group showed similar results.

The morphologies of powder calcined at 1000 ^oC are illustrated in Fig. 5 (a and b). As can be seen, the particles are more or less uniform in size (about 150 nm) and in shape.

(a)

(b)
Figure 5. SEM photomicrographs of powder calcined at 1000^oC/4 horas: (a) 10000 X; (b) 20000 X.

X-ray Fluorescence (Table 1) and Energy Dispersive X-ray Spectroscopy (Fig. 6) results confirmed that the synthesized powders of the present study reached the intended stoichiometry.

Table 1. XRF results for ferrite nanopowder.

Figure 6. EDS for the calcined powder.

Table 2 summarizes the main data of the sintered ceramics. The hysteresis loops obtained are so good as those from the best similar garnets in the market, with saturation magnetization in the order of 1.2 - 1.9 kG and coercive force of about 2.5 Oe (Lax and Button, 1962; Globus, 1977).

Table 2. Analysis of results for Sm₃Fe₅O₁₂ sintered pieces.

Where: ST = Sintering Temperature; GS = Grain Size; MF = Maximum Magnetic Field; H_c = Coercive Force; HL = Hysteresis Loss; B_max = Maximum Induction and B_r = Remanent Induction.

The coercive force decreased with the increase of sintering temperature, as result produced by an increase in the grain-growth of the ceramic pieces, during sintering at higher temperature. In fact, it could be easily seen that the increase of the sintering temperature produced progressive grain-growth.

Initial permeability, coercive force, switching time, effective linewidth and spinwave linewidth are well-know grain-size dependent magnetic parameters. According to Globus (1977), who studied the relationship between the hysteresis loop and the microstructure parameters (such as the grain size and the intragranular porosity) for the yttrium iron garnet (YIG), the coercive force is inversely proportional to the average grain size, which is confirmed to Sm₃Fe₅O₁₂ ferrite.

IV. CONCLUSIONS

The most important results of the work are those related to formation of the samarium-iron-garnet nano powder (150 nm) by co-precipitation method, according to the previous studied thermodinamic analysis.

The magnetic properties of this soft ferrite can be associate to this microstructure and the following correlations were observed:

(a) The grain-size increased from 2.0 u.m to 4.2 urn in the 1200-1400 ^oC range;

(b) The coercive force decreased from 7.16 Oe to 4.72 Oe in the 1200 - 1400^oC range, as the result of the grain growth as expected;

(c) Hysteresis losses, maximum magnetization and the magnetic remanence presented maximum at 1350^oC.

ACKNOWLEDGEMENTS
The authors thank to CNP_q, FAPERJ, IPqM, IF/UFRJ, IGEO/UFRJ and CBPF for the financial support and other aids to the development of the work done.

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Received: August 20, 2005.
Accepted: February 6, 2006.
Recommended by Subject Editor R. Gomez.