Terbium–yttrium–iron garnet revisited

Journal of Alloys and Compounds 436 (2007) 415–420

Terbium–yttrium–iron garnet revisited Sandra da Silva Marins a , Tsuneharu Ogasawara b,∗ , Ang´elica Soares Ogasawara c a

b

Petrobr´as, Rio de Janeiro, RJ, Brazil COPPE/UFRJ-PEMM, Ilha do Fund˜ao, C.Postal 68505, CEP 21941-972 Rio de Janeiro, RJ, Brazil c EP/UFRJ-DEL, Rio de Janeiro, RJ, Brazil Received 4 May 2006; received in revised form 17 November 2006; accepted 21 November 2006 Available online 18 December 2006

Abstract An investigation was made into the synthesis and magnetic properties of Tb(3 − x) Yx Fe5 O12 garnet ferrite synthesized by coprecipitation from hydrated chlorides of rare-earth elements and ferrous sulfate. The mixed hydroxide coprecipitate was calcined at 1000 ◦ C. Ring-shaped compacts were obtained by dry pressing of finely milled calcined coprecipitate powder to which an aqueous solution containing 15 wt% of PVA was added. The compacts were dried and sintered in a 1250–1450 ◦ C temperature range. The main conclusions of this study were that saturation magnetization in TbYIG depended strongly on the sintering temperature; and the best result among tested compositions occurred for Tb2.4 Y0.6 Fe5 O12 after sintering at 1400 ◦ C. The trend of TbYIG to degrade at sintering temperatures above 1400 ◦ C was evidenced. The grain size increased along with the sintering temperature, while the coercive force reached a minimum value, increasing afterwards. This non-linear temperature dependence reflected what was predicted by the phase diagrams of the Tb–Y–Fe–O system at elevated temperatures. © 2006 Elsevier B.V. All rights reserved. PACS: 75.60.Ej; 75.90.+w; 81.40.R Keywords: Ceramics; Sintering; Magnetization; Thermal analysis

1. Introduction The study of Y3 Fe5 O12 (YIG) and rare-earth iron garnet (REIG) has been continued during last decade [1–15] due to the diversified application potentials in different association states (pure, mixed, substituted, composite [2–7]), grain or particle sizes (coarse grained, fine grained, nanosized [1,8–10]) and fields: microwave (isolator, circulator, resonator or absorber), magneto-optical, and wireless infrastructure and handheld products [5,12,13]). Differently from patents, the scientific papers have their breakthrough motivations: new fundamental knowledge [2,3,6,10,11,14], development of better or new method of synthesis and/or processing [1], better characterization techniques [15], among others, as illustrated below by a concise literature review. Microwave ferrite devices increasingly require materials whose magnetic and dielectric losses are decisive from the stand-

∗

Corresponding author. Tel.: +55 21 2562 8530; fax: +55 21 2280 7443. E-mail address: [email protected] (T. Ogasawara).

0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.11.114

point of their utilization. Among other applications, transistor oscillators can be made tunable by using a magnetically biased YIG sphere as an adjustable element in resonant load [16]. Guillot [17] published a review work on magnetic properties of ferrites wherein covered those of garnets splited in four sections: magnetic properties of the garnets, magnetic anisotropy of the garnets, magnetic structure of the rare-earth iron garnets at low temperatures, and physical properties of the iron garnets in the vicinity of the compensation temperature. Guillot et al. [18] obtained precise X-ray diffraction data at 300 and 95 K for TbIG single-crystals and concluded that this ferrimagnetic garnet retains the cubic structure down to 95 K. Greneche and Pascard [19] completely investigated two substituted garnets with the chemical composition Y3 − x Tbx Fe5 − y Scy O12 by transmission M¨ossbauer spectrometry over the 4–430 K temperature range (>TC ) and found out that the mean values of the spin reorientation transitions are centered around TR = 230 K for (x = 0.4, y = 0.75) and TR = 210 K for (x = 0.8, y = 0.85). Rodic et al. [20,21] carried out powder neutron diffraction measurements on the mixed terbium–yttrium iron garnet Tb2.5 Y0.5 Fe5 O12 at temperatures of 300 K, in the vicinity of

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Tcomp (200 K) and at 117 K, as well as measurements of the initial magnetic susceptibility of TbIG in the compensation (comp) point region. Hock et al. [22] studied the crystallographic distortion and magnetic structure of TbIG at low temperatures. With reference to physical properties of the iron garnets in the vicinity of the compensation temperature, when T > Tcomp , α is temperature independent for TbIG, and there isn’t any clear analysis of the problem below Tcomp . Similarly, Tbx Y3 − x Fe5 O12 (x = 3.0, 2.5, 1.98 and 1.0) was studied in the 80–300 K range. The Tcomp was found to be “quasi-proportional” to the terbium content and varied from 242 to 60 K [23]. Rodic et al. [24] measured the initial magnetic susceptibilities of polycrystalline samples of TbIG, at temperatures close to the compensation point: they were characterized by small steps and points of inflection at Tcomp ± (10–12 K); in TbIG a jump of the lattice parameter was unambiguously identified. To consolidate the scientific knowledge on yttrium– terbium–iron garnet it is necessary to get more precisely defined procedure for the synthesis of Tb(3 − x) Yx Fe5 O12 garnet ferrite by chemical coprecipitation method (that may provide nanocrystalline powders for diverse application purposes) and a better relationship between microstructure-magnetic properties of the sintered rings of this type of garnet and the sintering temperature. To provide a desirable experimental contribution to this scenario was the main motivation of the present work, based on Walker’s type of Hysteresisgraph.

Fig. 1. TGA and DTA of coprecipitated terbium, yttrium and iron hydroxides with composition Tb2.4 Y0.6 Fe5 O12 , in air, at a heating rate of 10 ◦ C/min. for commercial garnets. The other values used were 20 Oe and 25 Oe, plus one single test with Hm = 102.7 Oe.

3. Results and discussion Fig. 1 shows the results of TGA and DTA for Tb2.4 Y0.6 Fe5 O12 . The TGA indicated progressive water vapor loss of 18.25% from the starting mixture of terbium, yttrium and iron hydroxides in the 25–500 ◦ C range, and another 8.80% weight loss from 500 to 1000 ◦ C, totaling a 27.05% loss. The DTA curve revealed an initial dehydration peak at about 100 ◦ C, resulting on the amorphous oxide mixture, followed by three exothermic peaks: a narrow one above 200 ◦ C, a wide one at about 700 ◦ C, and a third narrow one at 974 ◦ C. These exothermic

2. Experimental procedure The material was synthesized by the coprecipitation method, starting from aqueous solutions of hydrated rare-earth chlorides and ferrous sulfate. These solutions were mixed together, resulting in a combined solution with a pH of about 2. This solution was then heated to 90 ◦ C under intensive agitation for about 30 min, followed by the addition of KOH to adjust the pH to a 9–12 range, while allowing an easy elimination of the K+ ions by washing the precipitate in distilled water. The coprecipitate was separated from the solution by vacuum filtration and dried in a desiccator for 24 h, then oven-dried at 105 ◦ C for 2 h. The dried powder was milled and subjected to thermal-gravimetric (TGA) and differential thermal analysis (DTA), which determined a 4-h holding of the coprecipitate powder at 1000 ◦ C as optimal for its calcination to yield a mixture of Tb2 O3 , Y2 O3 and Fe2 O3 . Energy-dispersive spectroscopy (EDS), scanning electron microscopy (SEM), and X-ray diffraction (XRD) using Cu K␣ radiation were used to characterize both the calcined powder and the sintered ceramic pieces. For XRD, JCPDS cards were used (10–1412, 19–1447, 21–1450, 13–534, 43–0507, 19–1325, 18–1332, 18–1300), as well as XRD patterns for TbIG and TbYIG from previous studies [20,25]. Ring-shaped compacts were obtained by uniaxially dry pressing the finely milled calcined coprecipitate powder and by adding an aqueous solution containing 15 wt% of PVA. This material was admixed as a binder, in an amount corresponding to 2 wt% polyvinyl alcohol (PVA) of the total compacted mass. The compacts were sintered at five different temperatures: 1250, 1300, 1350, 1400 and 1450 ◦ C, according to the following thermal protocol: a 10 ◦ C/min heating rate until 600 ◦ C, followed by 1 h dwell time at this temperature; another 10 ◦ C/min heating rate up to the final sintering temperature, where it stayed for 4 h; followed then by cooling the sample down to 800 ◦ C at a −10 ◦ C/min rate, and 1 h dwell time at 800 ◦ C before final cooling to room temperature. The characterization of the sintered samples included magnetic hysteresis measurements (Walker Scientific Model AMH-20). Each sintered ring-shaped sample was equipped with a varnished copper AWG29 wire coil as a solenoid, which was magnetically analyzed at a frequency of 60 Hz and a field strength of up to 30 Oe, which is an appropriate value as pointed out by Nicholas [26]

Fig. 2. (a) X-ray diffraction pattern of the terbium–yttrium–iron coprecipitate after calcination at 1000 ◦ C. F = Fe2 O3 , YF = YFeO3 , TF = TbFeO3 , T = Tb2 O3 , Y = Y2 O3 , YIG = Y3 Fe5 O12 (OR) and OR = orthorhombic. (b) X-ray diffraction pattern of Tb2.4 Y0.6 Fe5 O12 sintered at 1350 or 1400 ◦ C.

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peaks were probably related to the crystallization of terbium, yttrium and iron oxides, in pure or combined state, giving rise to crystalline Tb2 O3 , Y2 O3 , Fe2 O3 as well as YFeO3 , TbFeO3 and some Y3 Fe5 O12 , as indicated by the X-ray diffraction patterns. The XRD pattern of the material calcined at 1000 ◦ C (Fig. 2(a)) shows the formation of some orthorhombic YIG, but this calcination temperature was not enough to generate mixed terbium–yttrium–iron garnet (TbYIG); thus, sintering must be done at higher temperatures and/or for longer time durations [27]. Rodic et al. [21], for instance, used 6 h presintering at 1127 ◦ C for GdIG and TbIG, while Sztaniszlav et al. [28] pre-sintered an yttria–hematite mixture for 3–12 h in air at 800–1400 ◦ C to obtain YIG and found a maximum predominance of YFeO3 at 1100 ◦ C, while YIG formed only above 1300 ◦ C; Ings et al. [29], got success at 1300 ◦ C. Hong et al. [25] ˚ for TbIG syntheachieved lattice constant equal to 12.4364 A sized by using citrate process, in which the dried gel powder was ground and annealed at 1400 ◦ C for 3 h in air. The VSM measurements were performed in the 30–700 K temperature range, covering both the N´eel temperature (TN = 560 ± 5 K) and the compensation temperature (Tcompensation = 260 ± 5 K).

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The standard powder mixing procedure for synthesis and processing of rare-earth-iron garnet is reported by Nicolas [26]. Ferric oxide and yttrium oxide (or RE2 O3 ), taken in fine powders form, are weighed in the desired proportions and thoroughly mixed. The compound is heat treated (pre-sintered) at a temperature of approximately 1200 ◦ C. The powder obtained is milled to a grain size smaller than one micron. The milled powder, mixed with an organic binder, is granulated (e.g., by means of a spray dryer). These granules are pressure molded at a pressure 1–2 × 103 kg/cm2 . The piece thus formed is heat-treated at about 500–600 ◦ C, for elimination of the organic binders, and then sintered in an oxygen atmosphere, at about 1500 ◦ C, for 5–10 h. A longer treatment enables a much lower porosity to be obtained (less than 1%). Fig. 3(a) presents the SEM micrograph of the sample calcined at 1000 ◦ C, showing a spongy agglomerate of nanosized crystallites. Fig. 3(b) and (c) shows that ceramic rings sintered at 1250 and 1300 ◦ C (respectively) still consist of porous agglomerates of very fine crystallites similar to the microstructure of the as-calcined powder (a). Fig. 3(d) reveals a marked increase in densification at 1350 ◦ C, while the grains in Fig. 3(e) are well defined in densely sintered ceramics, indicating possible degra-

Fig. 3. SEM micrographs of Tb2.4 Y0.6 Fe2 O4 samples with the following compositions: (a) synthesized powder as-calcined at 1000 ◦ C; (b) rings sintered at 1250 ◦ C; (c) at 1300 ◦ C; (d) at 1350 ◦ C; (d) at 1400 ◦ C; (e) at 1450 ◦ C.

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Table 1 Analysis of results for Tb(3 − x) Yx Fe5 O12 sintered pieces Sintering temperature (◦ C)

x

Coercive force, Hc (Oe)

1250

0 0.6 1.5 3

2.91 – 10.08 2.15

1300

0 0.6 1.5 3

2.15 7.76 7.34 2.29

1350

0 0.6 1.5 3

2.55, 19.78a 4.95 7.63 2.24

1400

0 0.6 1.5 3

1450

0 0.6 1.5 3

a

Hysteresis loss (J m−3 )

Maximum induction, Bm (kG)

Remanent induction, Br (kG)

Grain size (␮m)

6.12 – 90.35 16.64

0.11 – 0.46 0.44

0.01 – 0.17 0.04

1.5 1.0 1.2 –

3.13 88.49 82.28 7.63

0.07 0.47 0.47 0.18

0.01 0.27 0.26 0.05

2.5 2.0 2.6 2.8

5.31, 130.16a 167.74 180.96 19.12

0.10, 0.42a 1.29 0.96 0.49

0.01, 0.10a 0.80 0.53 0.05

– 4.0 3.0 –

9.49 1.56 1.47 1.99

25.32 120.26 97.27 26.63

0.15 2.31 1.86 0.66

0.05 0.97 0.81 0.17

8.0 10.0 10.0 10.0

– 6.21 3.90 1.71

– 35.25 110.17 20.95

– 0.30 1.20 0.54

– 0.08 0.40 0.19

– – 15.0 –

Hm = 102.7 Oe, Hm = 30 Oe for all other cases.

Fig. 4. Magnetic hysteresis loops for Tb3 − x Yx Fe5 O12 ceramic rings sintered at “ST” temperatures (1300, 1350, 1400 and 1450 ◦ C): (a) x = 0; (b) x = 0.6; (c) x = 1.5; (d) x = 3.

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Table 2 Calculation of the magnetic induction saturation (Bs , Hs ) for Tb2.4 Y0.6 Fe5 O12 ring sintered at 1400 ◦ C H1 (Oe)

H2 (Oe)

H (Oe)

B1 (kG)

B2 (kG)

B (kG)

B/H

Δ(B/H)

20 25 30 35 40 45 50 50

25 30 35 40 45 50 51.3 52.5

5 5 5 5 5 5 1.3 2.5

2.11 2.22 2.31 2.38 2.43 2.46 2.47 2.47

2.22 2.31 2.38 2.43 2.46 2.47 2.48 2.47

0.11 0.09 0.07 0.05 0.03 0.01 0.005 0

0.022 0.018 0.014 0.010 0.006 0.002 0.001 0

– −0.004 −0.004 −0.004 −0.004 −0.004 −0.001 −0.002

dation phenomena at Tb2.4 Y0.6 Fe5 O12 sintering temperatures of 1400–1450 ◦ C. Fig. 2(b) presents the XRD patterns for samples (x = 0.6) sintered at 1350 and 1400 ◦ C for 4 h, showing indexed peaks of Tb2.4 Y0.6 Fe5 O12 , the unique TbYIG phase present and identified as closely fitting with Tb2.5 Y0.5 Fe5 O12 referred to in previous report [20]. There is already well-known compositiondependent restriction to the formation of mixed Tb–Y–Fe garnets, as it has been previously observed in Sm–Ho–Fe garnets [30]. Fig. 4 presents magnetic hysteresis curves showing (for x = 0.6 and 1.5) that Bmax and Bs increase with increasing sintering temperature up to a maximum at 1400 ◦ C for TbYIG, after which they drop at 1450 ◦ C, while grain size increases (see numerical details in Table 1). The hysteresis loss increases up to a maximum at 1350 ◦ C, thereafter declining, while the coercive force decreases to a minimum at 1400 ◦ C, followed by an increase at 1450 ◦ C. Hence, 1400 ◦ C appeared to be the best sintering temperature. Sintering above 1400 ◦ C is affected by the trend of rare-earth iron garnet systems to decompose into orthoferrites and liquid phase, depending on the effective partial pressure of oxygen [31,32]. It may occur at lower temperatures when the partial pressure of oxygen in the sintering atmosphere is very low. YFeO3 and YIG are predicted for high Y/Fe ratios, while hematite + YIG is predicted for the reverse ratio (a similar relationship may apply to the Tb/Fe ratio). Low partial pressure of oxygen means decomposition of Fe2 O3 to Fe3 O4 and FeO. Sirvetz and Zneimer [33] encountered this effect of partial pressure and of sintering temperature on the resistivity of rare-earth garnet: porous products sintered in air at 1250 ◦ C presented 109  cm; dense pieces sintered in air at 1400 ◦ C displayed 105  cm, and those sintered in pure oxygen at 1400 ◦ C had 108  cm. Rodic et al. [21] used 6 h sintering (at 1327 ◦ C) to prepare GdIG and TbIG, which is longer than the 4 h applied here. Fig. 4(a) and (d) presents the best hysteresis loop for Tb3 Fe5 O12 and Y3 Fe5 O12 , respectively. The other hysteresis loops were removed from these figures for the sake of clarity, once they were partially overlapping each other. In the Tb3 Fe5 O12 case (see Table 1), the sample sintered at 1400 ◦ C showed Bmax = 0.15 kG and Hmax = 30 Oe but, when sintered at 1350 ◦ C, it presented Bmax = 0.40 kG and Hmax = 102.7 Oe. This Bmax is higher than the 0.33 kG reported by Geller et al.

[35] for 4πM at 300 K. Harrison and Hodges [34] obtained a value of 4πM = 1.776 kG for pure Y3 Fe5 O12 at 300 K. In this study, Bmax = 0.66 kG was the highest value achieved, using Hmax = 30 Oe for the sample sintered at 1400 ◦ C, whose micrographs indicated the porous structure of the ceramic sample analyzed magnetically. In order to improve these results, it would require higher Hmax values and a sintered product with a denser microstructure. Fig. 4 depicts the extrapolations (see analytical procedure below) of the selected hysteresis loops, indicating that the best saturation inductions obtained here are comparable with those of the best similar garnets in the market, which have a saturation induction in the order of 1.2–1.9 kG and a coercive force of about 2.5 Oe [36]. In fact, the Tb2.4 Y0.6 Fe5 O12 sintered at 1400 ◦ C presented a saturation induction of 2.53 kG (at 64.68 Oe field strength) and a coercive force of 1.56 Oe. These results are in good agreement with those obtained for Ho2.4 Sm0.6 Fe5 O12 sintered at 1450 ◦ C for 5 h in air [30], for which Bs = 2.7 kG (1.80 emu/g), by both hysteresisgraphy and vibrating sample magnetometry, and coercive force = 3.89 Oe. An analytical support to the above-cited extrapolation is illustrated in Table 2 for Tb2.4 Y0.6 Fe5 O12 sintered at 1400 ◦ C. As pointed out in Section 2, each sintered ring was magnetically analyzed with three different magnetic field intensities (Hm ), which have supplied three couples of (Bm , Hm ) values and allowed the desired calculations, as follows. Bm /Hm = 0.11/5 in the Hm = 5 Oe from 20 to 25 Oe, and Bm /Hm = 0.09/5 for 25–30 Oe range; Δ(Bm /Hm ) = −0.02/5 kG/Oe. Applying this rate to Bm /Hm the saturation point is determined as being equal to (Bs = 2.48 kG, Hs = 51.3 Oe). 4. Conclusions For the TbYIG ceramic powder precursor synthesized by the coprecipitation method from aqueous solutions of hydrated chlorides of rare-earth elements and ferrous sulfate, followed by filtration and drying, it was found that (a) The firing of mixed hydroxide coprecipitate at 1000 ◦ C provided a mixture of Fe2 O3 , Y2 O3 , Tb2 O3 , YFeO3 , TbFeO3 and a certain amount of Y3 Fe5 O12 . (b) Sintering of dry pressed TbYIG compacts for 4 h at 1250–1400 ◦ C temperatures yielded Tb2.4 Y0.6 Fe5 O12 with

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increasing grain sizes. Both Bmax and Br increased with temperature, while coercivity decreased. (c) Tb2 .4 Y0.6 Fe5 O12 proved to be the best-tested composition for Bm , whose highest value was achieved in compacts sintered at 1400 ◦ C for 4 h. (d) Above this temperature, linear change did not prevail because the phase diagrams of the Tb–Y–Fe–O system predicts that TbYIG tends to undergo thermal decomposition due to quite low (0.21 atm) partial pressure of oxygen in the system. Acknowledgements The authors gratefully acknowledge the financial support and other forms of aid provided by CNPq, FINEP/PADCT, FUJB, CAPES, FAPERJ, Instituto Nacional de Tecnologia (INT), Instituto de F´ısica (IF/UFRJ), Instituto de Geociˆencias (IGEO/UFRJ), and Centro Brasileiro de Pesquisas F´ısicas (CBPF), all Brazilian institutions, as without them this research work could not have been done. References [1] R.J. Joseyphus, A. Narayanasamy, A.K. Nigam, R. Krishnan, J. Magn. Magn. Mater. 296 (2006) 57–64. [2] R.L. Streever, Anisotropic exchange in ErIG, J. Magn. Magn. Mater. 278 (2004) 223–230. [3] N.I. Tsidaeva, J. Alloys Compd. 374 (2004) 160–164. [4] Y. Nakata, T. Okada, M. Maeda, S. Higuchi, K. Ueda, Opt. Laser Eng. 44 (2) (2006) 147–154. [5] M. Laulajainen, P. Paturi, J. Raittila, H. Huhtinen, A.B. Abrahamsen, N.H. Andersen, R. Laiho, J. Magn. Magn. Mater. 279 (2–3) (2004) 218– 223. [6] A.S. Lagutin, G.E. Fedorov, J. Vanacken, F. Herlach, J. Magn. Magn. Mater. 195 (1999) 97–106. [7] S.A. Nikitov, J. Magn. Magn. Mater. 196–197 (1999) 400–403. [8] M. Ristic, I. Nowik, S. Popovic, I. Felner, S. Music, Mater. Lett. 57 (2003) 2584–2590. [9] S. Taketomi, C.M. Sorensen, K.J. Klabunde, J. Magn. Magn. Mater. 222 (2000) 54–64. [10] Y. Fei, M.M.C. Chou, B.H.T. Chai, J. Cryst. Growth 240 (1–2) (2002) 185–189. [11] T. Boudiar, B. Payet-Gervy, M.F. Blanc-Mignon, J.J. Rousseau, M. Le Berre, H. Joisten, J. Magn. Magn. Mater. 284 (2004) 77–85. [12] C.S. Tsai, J. Magn. Magn. Mater. 209 (1–3) (2000) 10–14.

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