Synthesis of ultrafine SrLaxFe12−xO19 particles with high coercivity and magnetization by sol–gel method

Journal of Alloys and Compounds 475 (2009) 55–59

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Synthesis of ultrafine SrLax Fe12−x O19 particles with high coercivity and magnetization by sol–gel method T.T.V. Nga, N.P. Duong ∗ , T.D. Hien International Training Institute for Materials Science (ITIMS), Hanoi University of Technology, 01 Dai Co Viet Road, Hanoi, Viet Nam

a r t i c l e

i n f o

Article history: Received 24 June 2008 Received in revised form 30 July 2008 Accepted 7 August 2008 Available online 25 September 2008 Keywords: Sol–gel synthesis Nanostructures Scanning and transmission electron microscopy X-ray diffraction Magnetic measurements

a b s t r a c t M-type hexaferrite SrLax Fe12−x O19 (x = 0 − 0.2) powders were synthesized by sol–gel method. The precursor gels were calcined at temperatures from 650 ◦ C to 1050 ◦ C in air for 2 h to obtain the SrFe12 O19 phase. The thermal decomposition of gels was investigated by DTA/TGA. Samples of substituted ferrites were characterized by various experimental techniques including X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM). A very high magnetic coercivity (i HC ) of 7.0 kOe with the magnetization at 13.5 kOe of 66 emu/g at room temperature was observed for the sample of La concentration x = 0.05 calcined at 850 ◦ C. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Amongst the various types of ferrites, hexagonal ferrites possess relatively large saturation magnetization, high coercive force and rather high magnetocrystalline anisotropy as well as excellent chemical stability and corrosion resistivity [1]. In the past decade, there has been an increasing interest in fine particles of hexaferrites because of their emerging applications in perpendicular magnetic recording media, microelectromechanical systems and ferrofluids [2]. In order to achieve highly homogenous ultrafine particles, various synthesis techniques such as chemical coprecipitation [3], hydrothermal [4], sol–gel [5,6], glass crystallization [7], microemulsion [8], citrate precursor [9] and salt melt methods [10] have been developed. On the other hand, in many studies lanthanide substitutions were employed to control the microstructure, desired phase formation and to improve the hard magnetic properties [11–15]. In this paper, we report the synthesis and characterization of Lasubstituted strontium ferrite (SrM) samples with various La3+ /Fe3+ ratios prepared by sol–gel method. We have systematically studied the influence of the heat-treatment conditions on the structure and magnetic properties of the calcined powders. The magnetic behav-

∗ Corresponding author. Tel.: +84 4 8680787; fax: ++84 4 8692963. E-mail address: [email protected] (N.P. Duong). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.08.026

iors of these materials have been compared with those of the fine SrM particles with and without La substitution reported earlier. 2. Experimental Stoichoimetric amounts of Fe(NO3 )3 ·9H2 O, Sr(NO3 )2 , La(NO3 )3 ·nH2 O were dissolved completely in deionized water. In these processes, Sr2+ /[La3+ + Fe3+ ] was fixed at 12 and xLa3+ /(12 − x)Fe3+ was varied with x = 0, 0.05, 1, 0.15 and 0.2. Each aqueous solution containing Sr2+ , Fe3+ and La3+ was poured into citric acid with [Sr2+ + La3+ + Fe3+ ]/[citric acid] = 3. Ammonium hydroxide in aqueous form was added to the mixed solutions and the pH of the solutions was adjusted to about 1. The mixtures were stirred at 1000 rpm and slowly evaporated at 80 ◦ C to form gels. These gels were dried at 100 ◦ C for 2 h and then heated in air at between 650 ◦ C and 1050 ◦ C for 2 h. The obtained powders were washed using deionized water and dried at 80 ◦ C for 12 h. The thermal decomposition of the gel precursor and formation of the M-type hexaferrite phase was investigated by differential thermal (DTA) and thermogravimetric analysis (TGA) (Universal V2960T). X-ray diffraction (XRD, Cu K␣, Siemens D-5000) was employed to identify the crystal structure. Transmission electron microscope (TEM) and scanning electron microscopy (SEM) were used to examine the particle size and morphology. The magnetic parameters and the temperature dependence of magnetization were measured using a Vibrating Sample Magnetometer (VSM) with maximum field H of 13.5 kOe and at temperatures from room temperature to 800 K.

3. Results and discussion The thermal decomposition behavior of the gel-type precursor was characterized via DTA and TGA using 6.49 mg of gel and heating rate of 5 ◦ C/min in static air. Fig. 1 shows the DTA and

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Fig. 1. DTA and TGA plots of the dried gel prepared with nominal composition SrLa0.1 Fe11.9 O19 .

TGA plots of the dried gel prepared with nominal composition SrLa0.1 Fe11.9 O19 . In the DTA curve, the first two exothermic peaks at approximately 253 ◦ C and 283 ◦ C are ascribed to the decomposition of NH4 NO3 to liberate NOy , O2 and H2 O [16] and the auto-combustion of gel [17]. The third exothermic at about 356 ◦ C shows the occurrence of the crystallization process of the main phase [16,17]. The TGA curve indicates a net weight loss of ∼78%, in the temperature range between 33 ◦ C and 600 ◦ C and can be divided into two steps. The first step is the decomposition of NH4 NO3 and the combustion of gel. The second step is the complete decomposition of remaining organics and the formation of the ferrite particles. To monitor the phase development with increasing calcination temperature, X-ray diffraction analysis for selected La-substituted SrM powders were carried out. Fig. 2a shows the XRD patterns of the SrLa0.1 Fe11.9 O19 samples fired at 650 ◦ C, 750 ◦ C, 950 ◦ C and 1050 ◦ C for 2 h. It is seen that at 650 ◦ C the desired M-type hexagonal structure was already formed. However, at calcination temperatures up to 950 ◦ C, ␣-Fe2 O3 phase and trace amount of La2 O3 are also observed. These impurity phases were diminished as the heat-treatment temperature increases and at 1050 ◦ C the obtained sample was almost single-phase. Similarly, as shown in Fig. 2b, at 950 ◦ C the XRD pattern of the sample with composition x = 0 can be identified entirely by the SrFe12 O19 structure whilst impurity phases are still observed in the samples with La substitutions. Compared to Sr2+ and Fe3+ , La3+ ions have lower mobility and their diffusion is expected to be slower. It is therefore concluded that in order to obtain the La-substituted samples of single-phase, higher calcination temperature (≥1050 ◦ C) and longer annealing time are required. Fig. 3 shows the TEM image of the SrLa0.1 Fe11.9 O19 sample calcined at 950 ◦ C. The samples consist of small crystallites with approximate size from 20 nm to 100 nm in which most of them have average diameter of 80 nm and the hexagonal-platelet shape. The SEM pictures in Fig. 4 indicate the development of grain size in the SrLa0.1 Fe11.9 O19 samples with increasing calcination temperature. The average diameter dav was estimated from 70 nm, 100 nm and up to submicron scale as the temperature increases from 850 ◦ C, 950 ◦ C and 1050 ◦ C, respectively. At 1050 ◦ C, inter-diffusion between small grains take places to form bigger ones as clearly observed in Fig. 4c. The results show that those particles are in the single-domain region.

Fig. 2. XRD patterns (a) for the SrLa0.1 Fe11.9 O19 samples calcined at 650 ◦ C, 750 ◦ C, 950 ◦ C and 1050 ◦ C for 2 h; (b) for the SrLax Fe12−x O19 samples (x = 0, 0.05, 0.1, 0.15, 0.2) calcined at 950 ◦ C for 2 h.

The magnetic properties of the samples were determined from the fool hysteresis loops measured at room temperature on compact powders. The results are gathered in Table 1. The change of the magnetization in the highest applied magnetic field of 13.5 kOe for La-substituted SrM samples for various nominal x values with increasing calcination temperatures is exhibited in Fig. 5. There is an increasing tendency of M(13.5 kOe) with calcination temperature for all the samples up to 1050 ◦ C. The increase of M(13.5 kOe) for these samples with the increase in the calcination temperature can be attributed mainly to the improvement of the crystallinity and phase homogeneity during calcination. It is also seen that with x = 0.05, 0.1 and 0.15, the magnetization is significantly enhanced at the calcination temperatures above 800 ◦ C in comparison with that of the unsubstituted sample (Table 1). The enhancement of the M(13.5 kOe) values can be assigned to the stabilization of the crystal structure with the presence of the small amount of La3+ ions which was already pointed out in previous works [18,19]. The lowest magnetization observed for the sample with La concentration x = 0.2 in the whole temperature range. This can be explained by existence of unreacted constituents (La2 O3 and ␣-Fe2 O3 ) in the sample as shown in the XRD analysis and a high temperature treatment and/or longer annealing time are expected to complete the reaction of the constituents. The maximum M(13.5 kOe) value of 72 emu/g was achieved in the sam-

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Table 1 The magnetic parameters at room temperature of the studied SrLax Fe12−x O19 particles Heat treatment for 2 h at

x=0

x = 0.05

x = 0.1

x = 0.15

x = 0.2

Magnetization M at 13.5 kOe (emu/g) 650 ◦ C 700 ◦ C 750 ◦ C 800 ◦ C 850 ◦ C 900 ◦ C 950 ◦ C 1000 ◦ C 1050 ◦ C

56 56.8 56.6 56.9 59 60 61 62 67

57 57.8 59.5 64 66 65 69 69 72

52 53.7 54.9 63 64 69 68 69 70

52 55.2 57.8 60.1 65.5 67 68 69 69.3

49 53 52 56 58 59 60 62 62

Remanent magnetization Mr (emu/g) 650 ◦ C 700 ◦ C 750 ◦ C 800 ◦ C 850 ◦ C 900 ◦ C 950 ◦ C 1000 ◦ C 1050 ◦ C

21 31.6 33 34 33 34 35.5 33 37.5

23.7 32 32.5 34.5 37 37 38 38 39

22.3 24 31.6 32 33 36 35 36 35

16 30 30 36 38 37 37 37 38

13.6 31 29.7 29 28 34 33 36 36

0.5 3 6 6.5 6.6 6.4 6.2 6 5.4

0.8 4.8 6 7 7 6.8 6.4 6.4 5.4

0.5 4.8 5.4 6.4 6.6 6.9 6.8 6 5.6

Intrinsic magnetic coercivity i HC (kOe) 650 ◦ C 700 ◦ C 750 ◦ C 800 ◦ C 850 ◦ C 900 ◦ C 950 ◦ C 1000 ◦ C 1050 ◦ C

ple with x = 0.05 calcined at 1050 ◦ C, approaching the saturation magnetization of the SrM bulk [20]. The magnetization values of the samples prepared by coprecipitation [1,15] and hydrothermal synthesis [12] are found to be around 12–30% smaller than the maximum value of the magnetization observed in this study. Due to the

Fig. 3. TEM micrograph of the SrLa0.1 Fe11.9 O19 sample calcined at 950 ◦ C for 2 h.

0.2 4.5 5.1 6.3 6.4 6.7 6.6 5.9 5.8

0.2 1 5.1 6.4 6.4 6.7 6.7 6 6

large concentration gradients of the reactants in the coprecipitation and hydrothermal processes, intermediate products such as ␣-Fe2 O3 , La2 O3 and SrFeO3 usually form [12,15] which worsen the magnetic properties including magnetization and coercivity. In the sol–gel process metal ions can be homogenously distributed in the gel matrix, therefore, impurity phases can be largely reduced. This explains the higher magnetization per mass unit obtained in these samples. It is noted that for these compact samples, the remanent magnetization Mr gains 27–60% of the M(13.5 kOe) value which depends on several factors including the density and magnetic orientation of particles. The effect of the heat-treatment temperature on the intrinsic magnetic coercivity i HC of the samples with La concentration x from 0 to 0.2 is depicted in Fig. 6. The values of i HC increase steeply with the increase of calcination temperature from 650 ◦ C to 800 ◦ C, reach the maximum values at temperature between 800 ◦ C and 950 ◦ C, and decrease strongly above 950 ◦ C. This is due to the development of grain size with increasing the calcination temperature as revealed by SEM investigation. The i HC of 7 kOe was achieved for the La-substituted SrM particles with x = 0.05 calcined at 850 ◦ C which is one of the highest values so far reported for this material [15,21], approaching theoretical limit of SrM (7.5 kOe) [20]. The i HC values observed in this work are considerably higher than those reported recently by Rezlescu et al. [12] and García-Cerda et al. [22] (3.4 and 4.2 kOe, respectively) for the SrM ferrite particles which also were prepared by the sol–gel method. The enhancement of the coercivity in these samples can be understood in the model of the magnetization process of single magnetic domain particles which is governed by the rotation process of the domain moments. In that case the coercive force can be expressed as

i HC

=a

K1 s  + b(N⊥ − N|| )MS + c MS 0 MS

(1)

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Fig. 5. Room temperature magnetization at 13.5 kOe of the SrLax Fe12−x O19 samples (x = 0, 0.05, 0.1, 0.15, and 0.2) as a function of the calcination temperature.

the shape anisotropy of the crystalline particles and the third one originates from the action of the elastic mechanical deformation. In fact, the first term plays a decisive role in creating a high coercivity, the second term can take part of several tens percents of the first term. Lastly, in the case of the ferrite particles prepared by chemical methods the third term is expected to be very small. In the La-substituted samples, La3+ ions are expected to enter the Sr2+ sites because of their compatibility in radius. As proposed by Lotgering [23], this leads to a valence change of the Fe ions from 3+ to 2+ to conserve the charge neutrality which gives a positive contribution to the magnetocrystalline anisotropy constant K1 and thereby increases the magnetic coercivity. For further study, Mössbauer spectroscopy would be a suitable technique to derive information of the site occupancy of the magnetic ions in the lattice as well as the mechanism for the magnetocrystalline anisotropy. Moreover, as revealed by TEM and SEM results, the high aspect ratio of the particles can significantly enhance the shape anisotropy of the samples (the second term in Eq. (1)).

Fig. 4. SEM images and the derived average particle diameter dav for the SrLa0.1 Fe11.9 O19 samples calcined (a) at 850 ◦ C, (b) at 950 ◦ C and (c) at 1050 ◦ C for 2 h.

where a, b, c are coefficients, N⊥ and N|| are demagnetizing factors along the directions perpendicular and parallel to the preferred magnetization direction, S is the magnetostriction and  the stress. The first term of Eq. (1) corresponds to the contribution of the magnetocrystalline anisotropy of the material, the second is given by

Fig. 6. The intrinsic magnetic coercivity at room temperature of the SrLax Fe12−x O19 samples (x = 0, 0.05, 0.1, 0.15, and 0.2) as a function of the calcination temperature.

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4. Conclusions Single-domain SrLax Fe12−x O19 (x = 0/0.2) powders were synthesized by sol–gel method followed by heat treatments in air at 750 ◦ C, 850 ◦ C, 900 ◦ C, 950 ◦ C, 1000 ◦ C and 1050 ◦ C for 2 h. The samples have average particle sizes ranging from few tens to few hundreds of nanometers. The heat-treatment temperatures and La concentrations were derived for optimizing the magnetic performance. Our work shows that followed this sol–gel method with calcination temperatures above 750 ◦ C, the magnetic properties are strongly enhanced in comparison with those prepared by different chemical preparation routes [11,12,15]. The highest magnetic coercivity of 7 kOe with magnetization at 13.5 kOe of 66 emu/g are observed in the sample with x = 0.05 calcined at 850 ◦ C. It is revealed that La3+ additives of small concentrations (x ≤ 0.15) facilitate the ferritization reaction as well as the formation of favorable microstructures and improves the quality of the hexagonal SrM crystallites. Acknowledgement This work was supported by the Ministry of Education and Training of Vietnam, code B2007-01-104. References [1] H. Kojima, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materilas, vol. 3, 1982 (Chapter 5).

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