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Synthesis and characterization of nanostructured strontium hexaferrite thin films by the sol–gel method
Journal of Magnetism and Magnetic Materials 324 (2012) 2239–2244
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Synthesis and characterization of nanostructured strontium hexaferrite thin films by the sol–gel method S.M. Masoudpanah n, S.A. Seyyed Ebrahimi Center of Excellence for Magnetic Materials, School of Metallurgy and Materials, University of Tehran, Tehran, Iran
a r t i c l e i n f o
abstract
Article history: Received 14 March 2011 Received in revised form 4 February 2012 Available online 6 March 2012
Nanostructured single phase strontium hexaferrite, SrFe12O19, thin films have been synthesized on the (100) silicon substrate using a spin coating sol–gel process. The thin films with various Fe/Sr molar ratios of 8–12 were calcined at different temperatures from 500 to 900 1C. The composition, microstructure and magnetic properties of the SrFe12O19 thin films were characterized using Fourier transform infrared spectroscopy, differential thermal analysis, thermogravimetry, X-ray diffraction, electron microscopy and vibrating sample magnetometer. The results showed that the optimum molar ratio for Fe/Sr was 10 at which the lowest calcination temperature to obtain the single phase strontium hexaferrite thin film was 800 1C. The magnetic measurements revealed that the sample with Fe/Sr molar ratio of 10, exhibited higher saturation magnetization (267.5 emu/cm3) and coercivity (4290 Oe) in comparison with those synthesized under other Fe/Sr molar ratios. & 2012 Elsevier B.V. All rights reserved.
Keywords: SrFe12O19 thin film Sol–gel synthesis Fe/Sr molar ratio X-ray diffraction Magnetic measurement
1. Introduction In high-density recording media, reduction in the spacing between head and media to the extent of contact and semicontact states has become unavoidable. Thus CoCrTa and CoCrPt films, which are currently used in recording media, need a protective layer to prevent crashing. Increase of media noise at high density is also a serious problem with them. Barium and strontium hexaferrite films with magnetoplumbite crystal structure are attractive candidates for high-density and overcoat free contact or semi-contact recording media [1]. On account of their superior chemical stability, mechanical hardness, excellent corrosion and wear resistance and low level of media noise, they could be applied for rigid-disk media without protective and lubricant layers. Due to the large magnetocrystalline anisotropy and strong dependence of the orientation of easy axis on the microstructure, they also have potentiality for application in both perpendicular and longitudinal magnetic recordings [2–4]. One factor that limits the performance of recording media at high areal densities is the media noise, which results from coupling between the magnetic grains. However, incoherent rotation, which results from grain interactions, has been identified as the main source of media noise in hexaferrite particulate media and films. Chen et al. [5] showed grain interactions in barium hexaferrite films are low, because of large magnetocrystalline anisotropy, and
n
Corresponding author. Tel.: þ98 21 61114132; fax: þ 98 21 8800 6076. E-mail address: [email protected] (S.M. Masoudpanah).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2012.02.109
then these films have low noise. It is also generally accepted that the grain interactions should become less significant in films with very small grain size. Therefore, in order to exploit the complete potential of hexaferrite films in high density recording, small magnetic grains (o50 nm) are desirable to realize reasonable signal to noise ratio (SNR) [6]. An important objective of this study is to synthesize strontium hexaferrite films with crystallite size below 100 nm displaying less grain interactions and low noise using a simple and economical chemical deposition method. The barium and strontium hexaferrite thin films have been prepared through various methods, including sputtering [7], pulsed laser deposition [8], metallorganic chemical vapor deposition and sol–gel [9]. Among the different chemical routes, the sol– gel method based on the Pechini type reaction [10] has received considerable attention for its relatively simple synthesis scheme [11]. In this route, coordination complexes are formed between a metallic ion and citric acid within solution and then ethylene glycol is added for enhancing the amount of polymerization reaction, which results in a high viscous gel. The pyrolysis of this gel produces a homogeneous mixed oxide. The main advantage of using the sol–gel method is the lower calcination temperature; the fact that also enables smaller crystallites to be grown. Some efforts have been carried out to modify the sol–gel process parameters such as pH [12], basic agent [13], carboxylic acid [14], and starting metal salts [15] for further decreasing of the calcination temperature and achieving the finer crystallite size. In the present work, the effects of the Fe/Sr molar ratio and calcination temperature, on the phase evolution, microstructure and magnetic properties of strontium hexaferrite thin films were
S.M. Masoudpanah, S.A. Seyyed Ebrahimi / Journal of Magnetism and Magnetic Materials 324 (2012) 2239–2244
investigated by Fourier transform infrared spectroscopy (FTIR), differential thermal analysis and thermogravimetry (DTA/TG), X-ray diffraction (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM) techniques. Transmition (%)
481
Fig. 1 shows the FTIR spectra of the citric acid solutions (a) without metal ions, (b) containing strontium ions, (c) ferric ions, and (d) strontium and ferric ions, with pH of 7 adjusted by ammonia. The spectrum of the pure citric acid solution (Fig. 1a) has a stretching band at 1625 cm 1 attributed to the CQO in the dissociated carboxylic acid, while it is 1730 cm 1 for the nondissociated acid [18]. The result confirms that the citric acid was dissociated in the solution and is ready for the formation of complexes. Compared to the spectrum of the citric acid solution without metallic ions, there are no significant changes in the spectrum of Sr–CA (Fig. 1b) which means no strong coordination of the citric acid to strontium ions. However, there are noticeable changes among the spectra of citric acid (Fig. 1a), Fe–CA (Fig. 1c) and Fe–Sr–CA–EG (Fig. 1d) solutions in which the coordination of
a
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3600
3200
2800
2400
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1600
1398
3. Results and discussion
b
1560 1625
1200
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400
Wave number (cm-1)
Fig. 1. FTIR spectra of pure citric acid (a), Sr–CA solution (b), Fe–CA solution. (c) and Fe–Sr–CA solution (d). 120
120 680 °C
100
Weight (%)
The starting materials were Fe(NO3)3 9H2O (499%), Sr(NO3)2 (499.99%), citric acid (C6H8O7) (99%), ethylene glycol (C2H4(OH)2) (499.5%) and ammonia (25 wt%). Citric acid (CA) and ethylene glycol (EG) were used as the chelating and esterification agents, respectively. The sols were prepared by dissolving the metal salts and citric acid in the deionized water at which the molar ratio of CA:total metal cations was 1:1 [14]. In these solutions, Fe/Sr molar ratios varied from 8 to 12. After homogenization, ethylene glycol was added to the solution (citric acid:ethylene glycol ratio was 2:3 in mass), and the pH was adjusted to 7 with ammonia under continuous stirring which resulted in a clear transparent Fe–Sr precursor solution [12]. The Fe–Sr precursor solution was then heated at 80 1C to obtain a desired viscosity of 15 mPa s. The sol viscosity is measured with a rotating-spindle viscometer (Brookfield viscosimeter). Finally, this homogeneous solution was spin coated onto Si (100) substrate of 12 12 mm2 at 3000 rpm for 15 s. After each coating, the film was dried in air at 200 1C for 30 min and was preheated at 450 1C for 1 h to remove the organics. The process was performed four times to achieve the desired film thickness. At the end, the films were calcined at 500– 900 1C for 1 h in air. Heating and cooling rates of 5 1C/min were applied to prevent cracking of the thin films. Spectra of the sol in the IR range of 400–4000 cm 1 were measured by FTIR spectrometer. The thermal decomposition behavior of the gel was examined by simultaneous DTA/TGA in air with the heating rate of 5 1C/min on the NETZSCH STA 409 PC/PG instrument. The structures of the calcined strontium hexaferrite thin films were characterized by a Philips (PW-1730) X-ray diffractometer using Cu Ka radiation. XRD measurements were also used for the determination of the average crystallite size. The crystallite size was determined with the Scherrer equation by applying the full-width at half-maximum value of the (114) diffraction peak [16]. The XRD patterns were also submitted to a quantitative analysis by the Rietveld method using Materials Analysis Using Diffraction (MAUD) program [17]. The morphology was studied using a scanning electron microscope (CamScan MV2300). Magnetic properties were also measured by a vibrating sample magnetometer at a maximum applied field of 10 kOe.
862 1082 1032
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511 609
2. Experimental procedure
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Fig. 2. DTA/TGA traces for the dried gel with Fe/Sr molar ratio of 10.
the citric acid to ferric ions could be distinguished. These changes can be seen by the strong split bands at 481 cm 1 which could be attributed to the stretching mode of Fe–O and confirms the chelate formation. It is worth to be mentioned that the formed complexes would be unidentate (for COO stretching mode at 1625 cm 1) and bidentate (for COO stretching mode at 1560 and 1398 cm 1) [18]. However, according to Dn ¼ (nas–ns)¼ 1625 1398¼227 cm 1, the existence of the unidentate complex is more probable [19]. A medium band at 862 cm 1 (Fig. 1d), is related to the scissor vibrations of the carboxylic ions [18]. The bands at 1031 and 1082 cm 1 (Fig. 1d) are also attributed to the organic network or the COH groups [18]. In Fig. 1d, in addition to citric acid, there is also ethylene glycol in sol for enhancing the amount of polymerization. COH groups are observed mainly in Fig. 1d which indicate the existence of polymeric chains in the sol. Other sols can also have the COH groups related to the polymerization of citrate ions, but their amount is too low to detect. It was also observed that the Fe/Sr molar ratio did not affect on the type and amount of the complexes in the Fe–Sr–CA–EG solution [18]. The TG and DTA curves for the gel after drying at 180 1C for about 30 min, with the Fe/Sr molar ratio of 10 are shown in Fig. 2. The DTA diagram shows four peaks, the endothermic peak around 100 1C with a weight loss ( 3%), could correspond to the dehydration of the absorbed water of the as burnt gel. The first exothermal peak at about 440 1C with a weight loss of 23%
S.M. Masoudpanah, S.A. Seyyed Ebrahimi / Journal of Magnetism and Magnetic Materials 324 (2012) 2239–2244
could be due to the elimination of the residual organic compounds. The second exothermal peak at 678 1C with a weight loss of 9% might be attributed to the decarboxylation of SrCO3, which has been reported to take place at 1055 1C for pure carbonate and about 800 1C for a mixture of carbonate and an iron oxide [20]. The last broadened exothermic peak at 820 1C with very small weight loss ( o0.1%) of which the temperature depends on the Fe/Sr molar ratio could be considered as a solid state reaction attributed to the gradual formation of strontium hexaferrite. However, the decomposition of unreacted starting citric acid is not significantly influenced by the Fe/Sr molar ratio. The XRD patterns of the samples calcined at 900 1C with different Fe/Sr molar ratios of 12, 11, 10, 9, and 8 are shown in Fig. 3. Mean crystallite size, lattice parameters of SrFe12O19 and weight percentage of the residual a-Fe2O3 calculated by the Rietveld method are shown in Table 1. These results show that the amount of the residual a-Fe2O3 decreases with increasing the Fe/Sr molar ratio from 8 to 10 in which the single phase strontium hexaferrite has been formed. However, with more increase in the Fe/Sr molar ratio from 10 to 12 the residual a-Fe2O3 appears again. It can be concluded that the Fe/Sr molar ratio plays an important role in the formation of the single phase strontium hexaferrite by the sol–gel method for which the required ratio is smaller than that of the stoichiometric [21]. This can be attributed to the fact that the solubility of Sr(NO3)2 decreases at the elevated temperatures, and then more Sr2 þ ions are required for formation of the strontium hexaferrite. The other reason is attributed to the
Relative intensity (arb. units)
Fe/Sr = 12
Fe/Sr = 11
Fe/Sr = 10
Fe/Sr = 9
Fe/Sr = 8
20
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2 theta Fig. 3. XRD patterns of the SrM films with different Fe/Sr molar ratios varied from 8 to 12. All films were annealed at 900 1C (’: SrFe12O19 and &: a-Fe2O3).
Table 1 Mean crystallite size, lattice parameters of SrFe12O19 and weight percentage of a-Fe2O3 of the thin films with different Fe/Sr molar ratios, calcined at 900 1C. Fe/Sr
˚ a (A)
˚ c (A)
Crystallite size (nm)
a-Fe2O3 (wt%)
12 11 10 9 8
5.8803 5.8851 5.8874 5.8872 5.8528
23.022 23.034 23.010 23.029 22.947
42 45 50 35 37
15.44 13.3 0 17.65 20.8
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increased diffusion rates in the nonstoichiometric mixtures because of the induced lattice defects which could be observed from lower lattice parameter. As a result, formation of single phase SrFe12O19 (SrM) thin film is strongly probable at lower temperatures [22]. The last important point is that the higher content of strontium nitrate in non-stoichiometric solutions results in a higher pH value than that for iron nitrate. The increasing pH of the sol assists the formation of negatively charged iron gels and the adsorption of positively charged Sr ions on iron gels. Consequently, more homogeneous solution is obtained [23], and it results in the easy formation of strontium ferrite. Hexagonal ferrites have been successfully prepared by the sol–gel technique, with excellent magnetic properties and suitable microstructure. In general, the investigations performed cover one or two aspects of the processing, which are heat treatment and Fe/Sr molar ratio. Wang et al. [23] prepared microtubules of SrM with Fe/Sr molar ratio of 11.5 calcined at 850 1C by the sol– gel method. Fu and Lin [24] showed that SrM powders with Fe/Sr value of 11.6 calcined at 1000 1C yielded the best magnetic properties with saturation magnetization of 62 emu/g (316 emu/ cm3; density, 5.10 g/cm3) and an intrinsic coercive force of 1950 Oe. However, nanocrystalline strontium hexaferrite thin films have been synthesized with the optimum molar ratio of Fe to Sr of 10 in the present work. This discrepancy of optimum values of Fe/Sr in synthesizing of the powders and thin films by the sol–gel technique can be attributed to homogeneity of cations in sol and promotion of polymerization reaction. Since a suitable amount of viscosity is required during the thin film preparation by the spin coating method, then the homogeneity of cations and polymerization in sol is slightly limited in synthesizing of the films [25]. Consequently, the crystallization of desired phase is more difficult in the thin films rather than the powders, and then, the deviation of stoichiometry for films must be more than that of powders. Table 1 implies that the crystallite size of the films dependents on the Fe/Sr molar ratio. This can be due to the different diffusion rates in the non-stoichiometric films. The increased diffusion rates caused the larger crystallite size of SrM phase. Thus, the crystallite size of the films with the Fe/Sr molar ratio of 10 calcined at 900 1C is maximum value of about 50 nm. The films with the Fe/Sr molar ratio of 10 were calcined at different temperatures, 500, 600, 700, 800, and 900 1C. The XRD patterns are shown in Fig. 4. Table 2 includes the results of Rietveld refinement (lattice parameters of SrM phase and weight percentage of a-Fe2O3) and mean crystallite size of SrM phase at different temperatures. It seems the film calcined at 500 1C was amorphous. At 600 1C, a-Fe2O3 was the main phase in the film. However, with increasing the temperature to 700 1C, the sample showed a mixture of SrFe12O19 and a-Fe2O3, whereas for the higher calcination temperatures at 800 and 900 1C, the sample was approximately single phase with a small amount of the residual a-Fe2O3. These results are consistent with the weight percentage of a-Fe2O3 presented in Table 2. Therefore, it can be concluded that the calcination at 800 1C was low enough to obtain the single phase film. It can be observed from Table 2 that the values of lattice parameters are different from those of the single ˚ c ¼23.037 A, ˚ and strain-free crystals of SrM powder (a¼ 5.886 A, according to JCPDS card no. 33-1340). This can be attributed to the induced thermal stress during calcination process originated from the difference between the thermal expansion coefficients of the SrM film and the silicon substrate. Table 2 also shows that the crystallite size of the almost single phase film with the Fe/Sr molar ratio of 10 calcined at 800 1C is 42 nm. The surface morphologies of the films calcined at 700, 800, and 900 1C are shown in Figs. 5a–c. As it is seen in these figures, the rod (acicular) shaped crystallites of SrM have formed at this
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Relative intensity (arb. units)
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2 theta Fig. 4. XRD patterns of the SrM films with Fe/Sr molar ratio of 10 calcined at 500, 600, 700, 800, and 900 1C (’: SrFe12O19 and &: a-Fe2O3).
Table 2 Mean crystallite size, lattice parameters of SrFe12O19 and weight percentage of a-Fe2O3 of the thin films with Fe/Sr molar ratio of 10, calcined at different temperatures. T (1C)
Crystallite size (nm)
˚ c (A)
˚ a (A)
a-Fe2O3 (wt%)
700 800 900
23 42 50
23.026 23.016 23.010
5.8950 5.8869 5.8874
24 1.5 0
temperature. These rod crystallites are platelet-like crystals viewed from the edge. This is also observed in sputtered films which are caused by the anisotropic growth rate [26]. With increasing the calcination temperature, the size of the rod crystallites increased as well. The rod crystallites formed at 800 1C have an average thickness of 70 nm. It is also noteworthy that Fe/Sr molar ratio did not affect on the surface morphologies of the films. Fig. 6 shows the cross section of the film with Fe/Sr molar ratio of 10 calcined at 800 1C. The thickness of the SrM film is about 900 nm. It can be also seen that there is no crack or hole at the interface between the film and Si(100) substrate. The variation of the saturation magnetization (Ms) and coercivity (Hc) due to the variation of Fe/Sr molar ratio is shown in Fig. 7. It can be observed that the coercivity and saturation magnetization for the samples first increased and then decreased with an increase in the Fe/Sr molar ratio. When the Fe/Sr molar ratio was 10, the coercivity (Hc ¼4290 Oe) and saturation magnetization (Ms ¼267.5 emu/cm3) of the sample simultaneously reached the maximum. It is believed that the chemical phases within the specimens are the main reason for the difference in saturation magnetization values. The highest saturation magnetization value of the sample with the Fe/Sr molar ratio of 10 could be a consequence of the relatively pure strontium hexaferrite phase. Accordingly, the low values of saturation magnetization at the other samples might be due to the presence of antiferromagnetic a-Fe2O3 as evidenced from the XRD results. The residual a-Fe2O3 phase has also similar effect on the variation of coercivity with Fe/Sr molar ratio. However, these results are comparable with some works on the Ba and Sr hexaferrite thin films [27]. Zi et al. [9] prepared Sr0.8La0.2Fe11.8Co0.2O19 ferrite film on a (001) sapphire substrate by chemical solution deposition. They showed that the sample calcined at 900 1C possesses magnetically anisotropy with high saturation magnetization (130 emu/cm3), high coercivity (6.9 kOe), and large squareness ratio (0.9) at room temperature. Fig. 8 shows the effect of the calcination temperature in the range of 600–1100 1C on the magnetic properties of the sample with Fe/Sr molar ratio of 10. The film exhibited very low saturation magnetization value (70 emu/cm3) at low temperature (600 1C), due to the existence of antiferromagnetic a-Fe2O3 (Fig. 4
Fig. 5. SEM micrographs of the SrM film with Fe/Sr molar ratio of 10 calcined at (a) 700 1C, (b) 800 1C, and (c) 900 1C.
S.M. Masoudpanah, S.A. Seyyed Ebrahimi / Journal of Magnetism and Magnetic Materials 324 (2012) 2239–2244
2243
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3900 3800 13
Fig. 7. The variation of saturation magnetization (Ms) and coercivity (Hc) resulting from the variation of Fe/Sr molar ratio, calcined at 800 1C.
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Fig. 9. In-plane and perpendicular hysteresis loops of the sample with Fe/Sr molar ratio of 10, calcined at 800 1C.
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Fig. 6. SEM micrographs of the cross section of the SrM film with Fe/Sr molar ratio of 10, calcined at 800 1C.
50 0 1200
Fig. 8. Dependence of the perpendicular coercivity and magnetization of the SrM film on the calcination temperature.
and Table 2). There is a sharp rise in saturation magnetization value of the samples calcined at 600 and 700 1C, indicating that partially crystallized SrFe12O19 has formed even at 700 1C. When
the samples were heat treated at relatively higher temperatures (up to 1000 1C), the values of saturation magnetization were gradually increased due to the formation of well-crystallized SrFe12O19 with relatively large grains. The coercivity, Hc, reached a maximum of 4290 Oe at 800 1C, decreasing for higher temperatures due to the grain coarsening. Magnetization curves (both in-plane and perpendicular to the applied magnetic field) of the sample with Fe/Sr molar ratio of 10 calcined at 800 1C are shown in Fig. 9. The comparison of the in-plane and out of plane magnetization curves indicates that there is no preferred c-axis orientation of SrM in the film. This could be confirmed with the XRD results (Figs. 3 and 4) in which the intensity of (107) (2y ¼32.3521) and (114) (2y ¼34.1831) peaks of SrM phase are stronger than that of (008) (2y ¼31.0491) peak related to the (0001) plane, the c-axis. The hexagonal shape cross section of the hexaferrite disks could not be seen in Fig. 5, which shows that there is not any preference along the c-axis. Cho et al. [28] observed the columnar-type grains with the c-axis perpendicular to the film plane in the film thickness less than 100 nm. In the films thicker than 100 nm, the platelet hexaferrite grains nucleated randomly on the top of the perpendicular grains. This could also be another reason for the lack of the preferred c-axis orientation of SrM disks in the film while the thickness of the films is more than 100 nm (Fig. 6).
4. Conclusions Nanostructured single phase strontium hexaferrite thin films have been successfully prepared directly on the Si (100) substrate by the sol–gel technique. Study on the effect of Fe/Sr molar ratio and calcination temperature on the formation of SrFe12O19 thin films showed that the single phase SrFe12O19 was formed at relatively low temperature, i.e. 800 1C, when molar ratio of Fe to Sr was 10. SEM study showed that the films were composed of uniformly distributed SrM rod crystallite. The crystallite sizes calculated by XRD were in the range of 20–50 nm. The study of magnetic properties also showed that the film with Fe/Sr molar ratio of 10, exhibited a good saturation magnetization (267 emu/cm3), high coercivity (4290 Oe) and relatively high remanent magnetization (134 emu/cm3). According to
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these results, these films are very suitable candidates for high density recording media. References [1] Q.H. Yang, H.W. Zhang, Y.L. Liu, Q.W. Wen, Materials Letters 63 (2009) 406–408. [2] S. Capraro, J.P. Chatelon, M. Le Berre, H. Joisten, T. Rouiller, B. Bayard, D. Barbier, J.J. Rousseau, Journal of Magnetism and Magnetic Materials 272 (2004) 1805–1806. [3] V.K. Sankaranarayanan, R.P. Pant, A.C. Rastogi, Journal of Magnetism and Magnetic Materials 220 (2000) 72–78. [4] A. Morisako, X. Liu, M. Matsumoto, Journal of Applied Physics 81 (1997) 4374–4377. [5] Y. Chen, M. Rao, D.E. Laughlin, M.H. Kryder, IEEE Transactions on Magnetics 32 (1996) 3608–3610. [6] M. Matsumoto, A. Morisako, S. Takei, Journal of Alloys and Compounds 326 (2001) 215–220. [7] A. Kaewrawang, A. Ghasemi, X. Liu, A. Morisako, Thin Solid Films 518 (2010) 7059–7063. [8] X.Y. Zhang, C.K. Ong, S.Y. Xu, Z. Yang, Journal of Magnetism and Magnetic Materials 190 (1998) 171–175. [9] Z.F. Zi, H.Y. Liu, Y.N. Liu, L. Fang, Q.C. Liu, J.M. Dai, X.B. Zhu, Y.P. Sun, Journal of Magnetism and Magnetic Materials 322 (2010) 3638–3641. [10] M.P. Pechini, US Patent no. 3,330,697, 11 July 1967. [11] H. Tang, W. Zhang, B. Peng, W. Zhang, Thin Solid Films 518 (2010) 3342–3344. [12] S.M. Masoudpanah, S.A. Seyyed Ebrahimi, Journal of Magnetism and Magnetic Materials 323 (2011) 2643–2647.
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Synthesis and characterization of nanostructured strontium hexaferrite thin films by the sol–gel method S.M. Masoudpanah n, S.A. Seyyed Ebrahimi Center of Excellence for Magnetic Materials, School of Metallurgy and Materials, University of Tehran, Tehran, Iran
a r t i c l e i n f o
abstract
Article history: Received 14 March 2011 Received in revised form 4 February 2012 Available online 6 March 2012
Nanostructured single phase strontium hexaferrite, SrFe12O19, thin films have been synthesized on the (100) silicon substrate using a spin coating sol–gel process. The thin films with various Fe/Sr molar ratios of 8–12 were calcined at different temperatures from 500 to 900 1C. The composition, microstructure and magnetic properties of the SrFe12O19 thin films were characterized using Fourier transform infrared spectroscopy, differential thermal analysis, thermogravimetry, X-ray diffraction, electron microscopy and vibrating sample magnetometer. The results showed that the optimum molar ratio for Fe/Sr was 10 at which the lowest calcination temperature to obtain the single phase strontium hexaferrite thin film was 800 1C. The magnetic measurements revealed that the sample with Fe/Sr molar ratio of 10, exhibited higher saturation magnetization (267.5 emu/cm3) and coercivity (4290 Oe) in comparison with those synthesized under other Fe/Sr molar ratios. & 2012 Elsevier B.V. All rights reserved.
Keywords: SrFe12O19 thin film Sol–gel synthesis Fe/Sr molar ratio X-ray diffraction Magnetic measurement
1. Introduction In high-density recording media, reduction in the spacing between head and media to the extent of contact and semicontact states has become unavoidable. Thus CoCrTa and CoCrPt films, which are currently used in recording media, need a protective layer to prevent crashing. Increase of media noise at high density is also a serious problem with them. Barium and strontium hexaferrite films with magnetoplumbite crystal structure are attractive candidates for high-density and overcoat free contact or semi-contact recording media [1]. On account of their superior chemical stability, mechanical hardness, excellent corrosion and wear resistance and low level of media noise, they could be applied for rigid-disk media without protective and lubricant layers. Due to the large magnetocrystalline anisotropy and strong dependence of the orientation of easy axis on the microstructure, they also have potentiality for application in both perpendicular and longitudinal magnetic recordings [2–4]. One factor that limits the performance of recording media at high areal densities is the media noise, which results from coupling between the magnetic grains. However, incoherent rotation, which results from grain interactions, has been identified as the main source of media noise in hexaferrite particulate media and films. Chen et al. [5] showed grain interactions in barium hexaferrite films are low, because of large magnetocrystalline anisotropy, and
n
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0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2012.02.109
then these films have low noise. It is also generally accepted that the grain interactions should become less significant in films with very small grain size. Therefore, in order to exploit the complete potential of hexaferrite films in high density recording, small magnetic grains (o50 nm) are desirable to realize reasonable signal to noise ratio (SNR) [6]. An important objective of this study is to synthesize strontium hexaferrite films with crystallite size below 100 nm displaying less grain interactions and low noise using a simple and economical chemical deposition method. The barium and strontium hexaferrite thin films have been prepared through various methods, including sputtering [7], pulsed laser deposition [8], metallorganic chemical vapor deposition and sol–gel [9]. Among the different chemical routes, the sol– gel method based on the Pechini type reaction [10] has received considerable attention for its relatively simple synthesis scheme [11]. In this route, coordination complexes are formed between a metallic ion and citric acid within solution and then ethylene glycol is added for enhancing the amount of polymerization reaction, which results in a high viscous gel. The pyrolysis of this gel produces a homogeneous mixed oxide. The main advantage of using the sol–gel method is the lower calcination temperature; the fact that also enables smaller crystallites to be grown. Some efforts have been carried out to modify the sol–gel process parameters such as pH [12], basic agent [13], carboxylic acid [14], and starting metal salts [15] for further decreasing of the calcination temperature and achieving the finer crystallite size. In the present work, the effects of the Fe/Sr molar ratio and calcination temperature, on the phase evolution, microstructure and magnetic properties of strontium hexaferrite thin films were
S.M. Masoudpanah, S.A. Seyyed Ebrahimi / Journal of Magnetism and Magnetic Materials 324 (2012) 2239–2244
investigated by Fourier transform infrared spectroscopy (FTIR), differential thermal analysis and thermogravimetry (DTA/TG), X-ray diffraction (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM) techniques. Transmition (%)
481
Fig. 1 shows the FTIR spectra of the citric acid solutions (a) without metal ions, (b) containing strontium ions, (c) ferric ions, and (d) strontium and ferric ions, with pH of 7 adjusted by ammonia. The spectrum of the pure citric acid solution (Fig. 1a) has a stretching band at 1625 cm 1 attributed to the CQO in the dissociated carboxylic acid, while it is 1730 cm 1 for the nondissociated acid [18]. The result confirms that the citric acid was dissociated in the solution and is ready for the formation of complexes. Compared to the spectrum of the citric acid solution without metallic ions, there are no significant changes in the spectrum of Sr–CA (Fig. 1b) which means no strong coordination of the citric acid to strontium ions. However, there are noticeable changes among the spectra of citric acid (Fig. 1a), Fe–CA (Fig. 1c) and Fe–Sr–CA–EG (Fig. 1d) solutions in which the coordination of
a
4000
3600
3200
2800
2400
2000
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1398
3. Results and discussion
b
1560 1625
1200
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400
Wave number (cm-1)
Fig. 1. FTIR spectra of pure citric acid (a), Sr–CA solution (b), Fe–CA solution. (c) and Fe–Sr–CA solution (d). 120
120 680 °C
100
Weight (%)
The starting materials were Fe(NO3)3 9H2O (499%), Sr(NO3)2 (499.99%), citric acid (C6H8O7) (99%), ethylene glycol (C2H4(OH)2) (499.5%) and ammonia (25 wt%). Citric acid (CA) and ethylene glycol (EG) were used as the chelating and esterification agents, respectively. The sols were prepared by dissolving the metal salts and citric acid in the deionized water at which the molar ratio of CA:total metal cations was 1:1 [14]. In these solutions, Fe/Sr molar ratios varied from 8 to 12. After homogenization, ethylene glycol was added to the solution (citric acid:ethylene glycol ratio was 2:3 in mass), and the pH was adjusted to 7 with ammonia under continuous stirring which resulted in a clear transparent Fe–Sr precursor solution [12]. The Fe–Sr precursor solution was then heated at 80 1C to obtain a desired viscosity of 15 mPa s. The sol viscosity is measured with a rotating-spindle viscometer (Brookfield viscosimeter). Finally, this homogeneous solution was spin coated onto Si (100) substrate of 12 12 mm2 at 3000 rpm for 15 s. After each coating, the film was dried in air at 200 1C for 30 min and was preheated at 450 1C for 1 h to remove the organics. The process was performed four times to achieve the desired film thickness. At the end, the films were calcined at 500– 900 1C for 1 h in air. Heating and cooling rates of 5 1C/min were applied to prevent cracking of the thin films. Spectra of the sol in the IR range of 400–4000 cm 1 were measured by FTIR spectrometer. The thermal decomposition behavior of the gel was examined by simultaneous DTA/TGA in air with the heating rate of 5 1C/min on the NETZSCH STA 409 PC/PG instrument. The structures of the calcined strontium hexaferrite thin films were characterized by a Philips (PW-1730) X-ray diffractometer using Cu Ka radiation. XRD measurements were also used for the determination of the average crystallite size. The crystallite size was determined with the Scherrer equation by applying the full-width at half-maximum value of the (114) diffraction peak [16]. The XRD patterns were also submitted to a quantitative analysis by the Rietveld method using Materials Analysis Using Diffraction (MAUD) program [17]. The morphology was studied using a scanning electron microscope (CamScan MV2300). Magnetic properties were also measured by a vibrating sample magnetometer at a maximum applied field of 10 kOe.
862 1082 1032
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511 609
2. Experimental procedure
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Fig. 2. DTA/TGA traces for the dried gel with Fe/Sr molar ratio of 10.
the citric acid to ferric ions could be distinguished. These changes can be seen by the strong split bands at 481 cm 1 which could be attributed to the stretching mode of Fe–O and confirms the chelate formation. It is worth to be mentioned that the formed complexes would be unidentate (for COO stretching mode at 1625 cm 1) and bidentate (for COO stretching mode at 1560 and 1398 cm 1) [18]. However, according to Dn ¼ (nas–ns)¼ 1625 1398¼227 cm 1, the existence of the unidentate complex is more probable [19]. A medium band at 862 cm 1 (Fig. 1d), is related to the scissor vibrations of the carboxylic ions [18]. The bands at 1031 and 1082 cm 1 (Fig. 1d) are also attributed to the organic network or the COH groups [18]. In Fig. 1d, in addition to citric acid, there is also ethylene glycol in sol for enhancing the amount of polymerization. COH groups are observed mainly in Fig. 1d which indicate the existence of polymeric chains in the sol. Other sols can also have the COH groups related to the polymerization of citrate ions, but their amount is too low to detect. It was also observed that the Fe/Sr molar ratio did not affect on the type and amount of the complexes in the Fe–Sr–CA–EG solution [18]. The TG and DTA curves for the gel after drying at 180 1C for about 30 min, with the Fe/Sr molar ratio of 10 are shown in Fig. 2. The DTA diagram shows four peaks, the endothermic peak around 100 1C with a weight loss ( 3%), could correspond to the dehydration of the absorbed water of the as burnt gel. The first exothermal peak at about 440 1C with a weight loss of 23%
S.M. Masoudpanah, S.A. Seyyed Ebrahimi / Journal of Magnetism and Magnetic Materials 324 (2012) 2239–2244
could be due to the elimination of the residual organic compounds. The second exothermal peak at 678 1C with a weight loss of 9% might be attributed to the decarboxylation of SrCO3, which has been reported to take place at 1055 1C for pure carbonate and about 800 1C for a mixture of carbonate and an iron oxide [20]. The last broadened exothermic peak at 820 1C with very small weight loss ( o0.1%) of which the temperature depends on the Fe/Sr molar ratio could be considered as a solid state reaction attributed to the gradual formation of strontium hexaferrite. However, the decomposition of unreacted starting citric acid is not significantly influenced by the Fe/Sr molar ratio. The XRD patterns of the samples calcined at 900 1C with different Fe/Sr molar ratios of 12, 11, 10, 9, and 8 are shown in Fig. 3. Mean crystallite size, lattice parameters of SrFe12O19 and weight percentage of the residual a-Fe2O3 calculated by the Rietveld method are shown in Table 1. These results show that the amount of the residual a-Fe2O3 decreases with increasing the Fe/Sr molar ratio from 8 to 10 in which the single phase strontium hexaferrite has been formed. However, with more increase in the Fe/Sr molar ratio from 10 to 12 the residual a-Fe2O3 appears again. It can be concluded that the Fe/Sr molar ratio plays an important role in the formation of the single phase strontium hexaferrite by the sol–gel method for which the required ratio is smaller than that of the stoichiometric [21]. This can be attributed to the fact that the solubility of Sr(NO3)2 decreases at the elevated temperatures, and then more Sr2 þ ions are required for formation of the strontium hexaferrite. The other reason is attributed to the
Relative intensity (arb. units)
Fe/Sr = 12
Fe/Sr = 11
Fe/Sr = 10
Fe/Sr = 9
Fe/Sr = 8
20
30
40
50
60
70
2 theta Fig. 3. XRD patterns of the SrM films with different Fe/Sr molar ratios varied from 8 to 12. All films were annealed at 900 1C (’: SrFe12O19 and &: a-Fe2O3).
Table 1 Mean crystallite size, lattice parameters of SrFe12O19 and weight percentage of a-Fe2O3 of the thin films with different Fe/Sr molar ratios, calcined at 900 1C. Fe/Sr
˚ a (A)
˚ c (A)
Crystallite size (nm)
a-Fe2O3 (wt%)
12 11 10 9 8
5.8803 5.8851 5.8874 5.8872 5.8528
23.022 23.034 23.010 23.029 22.947
42 45 50 35 37
15.44 13.3 0 17.65 20.8
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increased diffusion rates in the nonstoichiometric mixtures because of the induced lattice defects which could be observed from lower lattice parameter. As a result, formation of single phase SrFe12O19 (SrM) thin film is strongly probable at lower temperatures [22]. The last important point is that the higher content of strontium nitrate in non-stoichiometric solutions results in a higher pH value than that for iron nitrate. The increasing pH of the sol assists the formation of negatively charged iron gels and the adsorption of positively charged Sr ions on iron gels. Consequently, more homogeneous solution is obtained [23], and it results in the easy formation of strontium ferrite. Hexagonal ferrites have been successfully prepared by the sol–gel technique, with excellent magnetic properties and suitable microstructure. In general, the investigations performed cover one or two aspects of the processing, which are heat treatment and Fe/Sr molar ratio. Wang et al. [23] prepared microtubules of SrM with Fe/Sr molar ratio of 11.5 calcined at 850 1C by the sol– gel method. Fu and Lin [24] showed that SrM powders with Fe/Sr value of 11.6 calcined at 1000 1C yielded the best magnetic properties with saturation magnetization of 62 emu/g (316 emu/ cm3; density, 5.10 g/cm3) and an intrinsic coercive force of 1950 Oe. However, nanocrystalline strontium hexaferrite thin films have been synthesized with the optimum molar ratio of Fe to Sr of 10 in the present work. This discrepancy of optimum values of Fe/Sr in synthesizing of the powders and thin films by the sol–gel technique can be attributed to homogeneity of cations in sol and promotion of polymerization reaction. Since a suitable amount of viscosity is required during the thin film preparation by the spin coating method, then the homogeneity of cations and polymerization in sol is slightly limited in synthesizing of the films [25]. Consequently, the crystallization of desired phase is more difficult in the thin films rather than the powders, and then, the deviation of stoichiometry for films must be more than that of powders. Table 1 implies that the crystallite size of the films dependents on the Fe/Sr molar ratio. This can be due to the different diffusion rates in the non-stoichiometric films. The increased diffusion rates caused the larger crystallite size of SrM phase. Thus, the crystallite size of the films with the Fe/Sr molar ratio of 10 calcined at 900 1C is maximum value of about 50 nm. The films with the Fe/Sr molar ratio of 10 were calcined at different temperatures, 500, 600, 700, 800, and 900 1C. The XRD patterns are shown in Fig. 4. Table 2 includes the results of Rietveld refinement (lattice parameters of SrM phase and weight percentage of a-Fe2O3) and mean crystallite size of SrM phase at different temperatures. It seems the film calcined at 500 1C was amorphous. At 600 1C, a-Fe2O3 was the main phase in the film. However, with increasing the temperature to 700 1C, the sample showed a mixture of SrFe12O19 and a-Fe2O3, whereas for the higher calcination temperatures at 800 and 900 1C, the sample was approximately single phase with a small amount of the residual a-Fe2O3. These results are consistent with the weight percentage of a-Fe2O3 presented in Table 2. Therefore, it can be concluded that the calcination at 800 1C was low enough to obtain the single phase film. It can be observed from Table 2 that the values of lattice parameters are different from those of the single ˚ c ¼23.037 A, ˚ and strain-free crystals of SrM powder (a¼ 5.886 A, according to JCPDS card no. 33-1340). This can be attributed to the induced thermal stress during calcination process originated from the difference between the thermal expansion coefficients of the SrM film and the silicon substrate. Table 2 also shows that the crystallite size of the almost single phase film with the Fe/Sr molar ratio of 10 calcined at 800 1C is 42 nm. The surface morphologies of the films calcined at 700, 800, and 900 1C are shown in Figs. 5a–c. As it is seen in these figures, the rod (acicular) shaped crystallites of SrM have formed at this
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Relative intensity (arb. units)
900 °C
800 °C
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500 °C
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2 theta Fig. 4. XRD patterns of the SrM films with Fe/Sr molar ratio of 10 calcined at 500, 600, 700, 800, and 900 1C (’: SrFe12O19 and &: a-Fe2O3).
Table 2 Mean crystallite size, lattice parameters of SrFe12O19 and weight percentage of a-Fe2O3 of the thin films with Fe/Sr molar ratio of 10, calcined at different temperatures. T (1C)
Crystallite size (nm)
˚ c (A)
˚ a (A)
a-Fe2O3 (wt%)
700 800 900
23 42 50
23.026 23.016 23.010
5.8950 5.8869 5.8874
24 1.5 0
temperature. These rod crystallites are platelet-like crystals viewed from the edge. This is also observed in sputtered films which are caused by the anisotropic growth rate [26]. With increasing the calcination temperature, the size of the rod crystallites increased as well. The rod crystallites formed at 800 1C have an average thickness of 70 nm. It is also noteworthy that Fe/Sr molar ratio did not affect on the surface morphologies of the films. Fig. 6 shows the cross section of the film with Fe/Sr molar ratio of 10 calcined at 800 1C. The thickness of the SrM film is about 900 nm. It can be also seen that there is no crack or hole at the interface between the film and Si(100) substrate. The variation of the saturation magnetization (Ms) and coercivity (Hc) due to the variation of Fe/Sr molar ratio is shown in Fig. 7. It can be observed that the coercivity and saturation magnetization for the samples first increased and then decreased with an increase in the Fe/Sr molar ratio. When the Fe/Sr molar ratio was 10, the coercivity (Hc ¼4290 Oe) and saturation magnetization (Ms ¼267.5 emu/cm3) of the sample simultaneously reached the maximum. It is believed that the chemical phases within the specimens are the main reason for the difference in saturation magnetization values. The highest saturation magnetization value of the sample with the Fe/Sr molar ratio of 10 could be a consequence of the relatively pure strontium hexaferrite phase. Accordingly, the low values of saturation magnetization at the other samples might be due to the presence of antiferromagnetic a-Fe2O3 as evidenced from the XRD results. The residual a-Fe2O3 phase has also similar effect on the variation of coercivity with Fe/Sr molar ratio. However, these results are comparable with some works on the Ba and Sr hexaferrite thin films [27]. Zi et al. [9] prepared Sr0.8La0.2Fe11.8Co0.2O19 ferrite film on a (001) sapphire substrate by chemical solution deposition. They showed that the sample calcined at 900 1C possesses magnetically anisotropy with high saturation magnetization (130 emu/cm3), high coercivity (6.9 kOe), and large squareness ratio (0.9) at room temperature. Fig. 8 shows the effect of the calcination temperature in the range of 600–1100 1C on the magnetic properties of the sample with Fe/Sr molar ratio of 10. The film exhibited very low saturation magnetization value (70 emu/cm3) at low temperature (600 1C), due to the existence of antiferromagnetic a-Fe2O3 (Fig. 4
Fig. 5. SEM micrographs of the SrM film with Fe/Sr molar ratio of 10 calcined at (a) 700 1C, (b) 800 1C, and (c) 900 1C.
S.M. Masoudpanah, S.A. Seyyed Ebrahimi / Journal of Magnetism and Magnetic Materials 324 (2012) 2239–2244
2243
300
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3900 3800 13
Fig. 7. The variation of saturation magnetization (Ms) and coercivity (Hc) resulting from the variation of Fe/Sr molar ratio, calcined at 800 1C.
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Fig. 9. In-plane and perpendicular hysteresis loops of the sample with Fe/Sr molar ratio of 10, calcined at 800 1C.
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Fig. 6. SEM micrographs of the cross section of the SrM film with Fe/Sr molar ratio of 10, calcined at 800 1C.
50 0 1200
Fig. 8. Dependence of the perpendicular coercivity and magnetization of the SrM film on the calcination temperature.
and Table 2). There is a sharp rise in saturation magnetization value of the samples calcined at 600 and 700 1C, indicating that partially crystallized SrFe12O19 has formed even at 700 1C. When
the samples were heat treated at relatively higher temperatures (up to 1000 1C), the values of saturation magnetization were gradually increased due to the formation of well-crystallized SrFe12O19 with relatively large grains. The coercivity, Hc, reached a maximum of 4290 Oe at 800 1C, decreasing for higher temperatures due to the grain coarsening. Magnetization curves (both in-plane and perpendicular to the applied magnetic field) of the sample with Fe/Sr molar ratio of 10 calcined at 800 1C are shown in Fig. 9. The comparison of the in-plane and out of plane magnetization curves indicates that there is no preferred c-axis orientation of SrM in the film. This could be confirmed with the XRD results (Figs. 3 and 4) in which the intensity of (107) (2y ¼32.3521) and (114) (2y ¼34.1831) peaks of SrM phase are stronger than that of (008) (2y ¼31.0491) peak related to the (0001) plane, the c-axis. The hexagonal shape cross section of the hexaferrite disks could not be seen in Fig. 5, which shows that there is not any preference along the c-axis. Cho et al. [28] observed the columnar-type grains with the c-axis perpendicular to the film plane in the film thickness less than 100 nm. In the films thicker than 100 nm, the platelet hexaferrite grains nucleated randomly on the top of the perpendicular grains. This could also be another reason for the lack of the preferred c-axis orientation of SrM disks in the film while the thickness of the films is more than 100 nm (Fig. 6).
4. Conclusions Nanostructured single phase strontium hexaferrite thin films have been successfully prepared directly on the Si (100) substrate by the sol–gel technique. Study on the effect of Fe/Sr molar ratio and calcination temperature on the formation of SrFe12O19 thin films showed that the single phase SrFe12O19 was formed at relatively low temperature, i.e. 800 1C, when molar ratio of Fe to Sr was 10. SEM study showed that the films were composed of uniformly distributed SrM rod crystallite. The crystallite sizes calculated by XRD were in the range of 20–50 nm. The study of magnetic properties also showed that the film with Fe/Sr molar ratio of 10, exhibited a good saturation magnetization (267 emu/cm3), high coercivity (4290 Oe) and relatively high remanent magnetization (134 emu/cm3). According to
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