Tailored magnetic properties of Sm(Zn) substituted nanocrystalline barium hexaferrites

Tailored magnetic properties of Sm(Zn) substituted nanocrystalline barium hexaferrites

Journal of Alloys and Compounds 486 (2009) 348–351 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.e...

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Journal of Alloys and Compounds 486 (2009) 348–351

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom

Tailored magnetic properties of Sm(Zn) substituted nanocrystalline barium hexaferrites Han Yanbing a,b , Sha Jian a,∗ , Sun Lina b , Tang Quan b , Lu Qin b , Jin Hongxiao b , Jin Dingfeng b , Bo hong b , Ge Hongliang b , Wang Xinqing b,∗ a b

Department of Physics, Zhejiang University, Hangzhou 310027, PR China College of Materials Science and Engineering, Zhejiang Province Key Laboratory of Magnetism, China Jiliang University, Hangzhou, 310018, PR China

a r t i c l e

i n f o

Article history: Received 4 March 2009 Received in revised form 20 June 2009 Accepted 22 June 2009 Available online 30 June 2009 Keywords: Barium hexaferrite Sol–gel synthesis Magnetic properties

a b s t r a c t Sm and SmZn substituted nanocrystalline barium hexaferrites (Ba1−x Smx Fe12 O19 and Ba1−x Smx Fe12−x Znx O19 , x = 0–0.6) were prepared by the sol–gel autocombustion process. X-ray diffraction (XRD) and vibrating sample magnetometer (VSM) were used to characterize the phase composition, crystal structure and magnetic properties of the as-prepared barium hexaferrites. All results indicated that the substitution content (x) critically influenced on the phase composition and magnetic properties. When the substitution content x > 0.2, impurity phases such as ˛-Fe2 O3 , SmFeO3 and ZnFe2 O4 were detected, which diluted and weakened saturation magnetization (Ms ). For the Sm-doped samples, owing to the hyperfine field, canting spin, magnetic dilution and impurity phases, Ms increased with x firstly, and then decreased when x > 0.03. On the other hand, the coercivity (Hc) continuously increased with x. Nevertheless, for the SmZn-doped samples, Ms reached maximum when x = 0.06 and Hc almost unchanged with x. Compared the magnetic properties of Sm- and SmZn-doped samples, it was proved that Zn2+ substituted Fe3+ at 4f2 sites and Sm3+ substituted Ba2+ , which changed Fe3+ to Fe2+ at 2a sites in order to satisfy the electroneutrality principle. © 2009 Elsevier B.V. All rights reserved.

1. Introduction M-type magnetoplumbite barium hexaferrites (BaFe12 O19 ) have been intensively studied recently owing to the excellent characters such as superior chemical stability, powerful corrosion, good mechanical hardness, high level of signal-to-noise ratio, high resistance, large single-axis anisotropy, large coercivity and magnetic energy product and the highest performance-to-price ratio [1–5]. With the development of nanotechnology synthesizing, nanocrystalline hexaferrites could be prepared by many methods [6–9], such as sol–gel autocombustion, chemical co-precipitation, solid-state reaction, mechanical activation, low-temperature combustion, micro emulsion and reverse micro emulsion, ammonium nitrate melt technique, citrate-EDTA complexing method and so on. Due to the simple process and the controllable stoichiometric amounts, the sol–gel autocombustion method [10–12] was widely used to prepare nanocrystalline barium hexaferrites.

∗ Corresponding author at: College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China. Tel.: +86 571 8687 5609; fax: +86 571 8691 4452. E-mail addresses: [email protected] (J. Sha), [email protected], [email protected] (X. Wang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.06.143

In order to improve the magnetic properties such as saturation magnetization (Ms ) and coercivity (Hc), many studies have focused on the cationic substitution for nanocrystalline barium hexaferrites [10–13]. In M-type hexaferrites, the Fe3+ occupies five different sites: the octahedral sites 2a, 12k, 4f2 and the tetrahedral sites 2b, 4f. The 2a, 2b and 12k sites have their spins aligned parallel to the crystallographic c-axis, whereas those of 4f1 and 4f2 point in the opposite direction. In order not to change the magnetoplumbite crystalline structure greatly in the cationic substitution, the valences balance and the diameter of cations must be taken into account. Teh et al. had prepared Co2+ and Co3+ ions substituted nanocrystalline barium hexaferrites by the sol–gel method, and found that Co3+ ion substitution could make Ms be enlarged and Hc declined [10]. Zhou et al. had synthesized the Co–Zn ions doped nanocrystalline barium hexaferrites, and all results indicated that Hc increased continuously with the substitution content, while Ms increased firstly and then decreased [11]. Ounnunkad had proved that substitution of La or Pr ions into the barium hexaferrites could be utilized for improvement of magnetic characters such as Ms and Hc [13]. In this paper, the sol–gel autocombustion method was used to prepare Sm- and SmZn-substituted M-type nanocrystalline barium hexaferrites. The phase composition, microstructure and magnetic properties of the as-prepared samples were studied by X-ray diffraction (XRD) and vibrating sample magnetometer (VSM).

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Furthermore, the influence of the substitution of Sm and SmZn on phase composition, crystal structure, and magnetic properties were investigated in detail by taking into account of the preferential occupancy sites. 2. Experimental The chemicals were all reagent purity. Stoichiometric amounts of nitrates (Sm(NO3 )2 ·6H2 O, Ba(NO3 )2 , and Fe(NO3 )3 ·9H2 O) and citric acid (C6 H8 O7 ·H2 O) in the molar ratio 2:3 were put into deionized water, in which the Ba:Fe:Sm = 1 − x:12:x and Ba:Fe:Sm:Zn = 1 − x:12-x:x:x for Sm- and SmZn-substitution samples (0 < x < 0.6), respectively. After agitating 4 h, the diluted ammonia was dropped to adjust the pH value to about 7. And then the sol solution was evaporate at 80 ◦ C till the viscous wet gel was obtained. After dried at 250 ◦ C for 12 h, the powders were calcined at the different temperature of 900 ◦ C, 1000 ◦ C and 1100 ◦ C for 3 h. XRD (X’Pert MPD, Philips, Holland, CuK␣ ,  = 0.154056 nm) and VSM (Model 7407, Lakeshaore, USA) were used to characterize the phase composition, crystal structure and magnetic properties of the Sm and SmZn doped nanocrystalline barium hexaferrites.

3. Results and discussion Owing to their similar radii, Sm3+ (0.96 Å) ions preferred to substitute partly Ba2+ (1.43 Å) ions, and Zn2+ (0.74 Å) preferred to substitute Fe3+ (0.64 Å). XRD was used to characterize the phase composition and microstructure of Sm and SmZn doped nanocrystalline barium hexaferrites prepared at different calcining temperature (T) of 900 ◦ C, 1000 ◦ C and 1100 ◦ C, as shown in Fig. 1. The XRD patterns of all samples were normalized according to the (114) peak. The results in Fig. 1a indicated that the nanocrystalline Ba0.97 Sm0.03 Fe12 O19 presented the single magnetoplumbite phase structure with space group P63/mmc (194), and the magnetoplumbite phase structure hardly changed with the calcining temperature. Furthermore, the crystallization of the as-prepared samples improved when T < 1000 ◦ C, and hardly changed when T > 1000 ◦ C. According to Scherrer formula, the average particle size of nanocrystalline Ba0.97 Sm0.03 Fe12 O19 was about 45 nm, which was much less than that of the single domain of barium hexaferrites. The similar conclusions could be obtained for the nanocrystalline Ba0.97 Sm0.03 Fe0.97 Zn0.03 samples as shown in Fig. 1b. For the nanocrystalline Ba0.97 Sm0.03 Fe12 O19 and Ba0.97 Sm0.03 Fe0.97 Zn0.03 O19 samples, VSM was used to study the influence of the calcining temperature on Ms and Hc, which the detail results were shown in Fig. 2. Usually, the higher the calcining temperature was, the larger the density and the average particle size would be, which caused the improvement of the crystallization. As the result, Ms increased and Hc decreased with the calcining temperature. When the calcining temperature was over to 1000 ◦ C, Ms remained constant, which should be attributed to the balance of magnetocrystalline anisotropy energy and domain wall energy. Considering XRD and VSM results, the optimum calcining temperature for the Sm and SmZn doped nanocrystalline barium hexaferrites was confirmed at 1000 ◦ C. The microstructure and the phase structure of the as-prepared samples were characterized by XRD as shown in Fig. 3, which all samples presented the magnetoplumbite BaFe12 O19 major phase structure. For the Ba1−x Smx Fe12 O19 samples in Fig. 3a, the other phases such as ˛-Fe2 O3 and SmFeO3 were detected when x > 0.2, which should be attributed to the valence in-balance of Ba1−x Smx Fe12 O19 primitive cell. When Sm3+ substituted Ba2+ , Fe3+ at 2a sites changed to Fe2+ in order to satisfy the principle of electroneutrality. The percent of the number of 2a sites in one primitive cell is only 8.3%. When x > 0.1, all 2a sites would be occupied by Fe2+ . Therefore, the surplus Sm3+ and Fe3+ precipitated to form ˛-Fe2 O3 and SmFeO3 at x = 0.1. Furthermore, the effect of the SmZn substitution on the Ba1−x Smx Fe12−x Znx O19 phase structure was shown in Fig. 3b, a similar result could be obtained: the ˛-Fe2 O3 , SmFeO3 and ZnFe2 O4 phases were detected when x > 0.3.

Fig. 1. XRD patterns of Sm (a) and SmZn (b) doped nanocrystalline barium hexaferrites prepared at the different calcining temperature.

The moments came from Fe3+ in barium hexaferrites since the moment of Ba2+ is zero. It was well known that Fe3+ cations in barium hexaferrites can occupy five kinds of sites: 2a, 4f2 , 12k (octahedral), 4f1 (tetrahedral) and 2b (hexahedral). The magnetic moments of sublattices at 2a, 2b and 12k sites are antiparallel to that at 4f1 and 4f2 sites owing to their ultra-exchange interactions. Therefore, the magnetic properties of barium hexaferrites could be improved by substituting Fe3+ with other ions. Owning to the varied 4f electron shell, orbital and spin magnetic moments, rare-earth elements were usually selected as the substitution elements. Herein, Sm was introduced to discuss the magnetic properties of the Sm doped nanocrystalline barium hexaferrites, and the detailed results of hysteresis loops were given in Fig. 4. For the Sm doped barium hexaferrites (see Fig. 4a), Ms increased when x < 0.03, and then decreased. As mentioned above, the substitution

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Fig. 2. VSM curves of Sm (a) and SmZn (b) doped nanocrystalline barium hexaferrites prepared at the different calcining temperature (x = 0.03).

of Sm3+ → Ba2+ led to the change of Fe3+ to Fe2+ at 2a sites. As the result, the enhancement of the Fe3+ –O–Fe3+ super-exchange interaction increased the hyperfine field at 12k and 2b sites, which led to the increase of Ms with the lower doped content. In order to keep the electroneutrality of barium hexaferrites cell, Fe3+ (5 ␮B) changed to Fe2+ (4 ␮B), which caused the magnetic dilution. On the other hand, Sm3+ cations produced the canting spin themselves, which also decreased Fe3+ –O–Fe3+ super-exchange interaction of barium hexaferrites. When x > 0.2, Ms decreased sharply owing to the appearance of ˛-Fe2 O3 and SmFeO3 phases. On the contrary, Hc decreased firstly, and then increased with x, which resulted from the difference of the magnetocrystalline anisotropy for the radii of Sm3+ (0.96 Å) and Ba2+ (1.43 Å) and the crystallization with the different x. Usually, the magnetocrystalline anisotropy of barium hexaferrites increased with the Sm3+ -doped, Hc should increase to some extent. But in our samples, owning to the preparation conditions, the crystallization enhanced with the lower substitution

Fig. 3. XRD patterns of Sm (a) and SmZn (b) doped nanocrystalline barium hexaferrites x.

content (x = 0.03 and 0.06). With the increasing x, the magnetocrystalline anisotropy predominated, so Hc increased when x > 0.06. Ms reached the maximum value of 65.4emu/g with Hc = 4778Oe at x = 0.03, while Hc reached the maximum value 5762Oe with Ms = 33.9emu/g at x = 0.5. Considering both Ms and Hc, the magnetic properties could be improved when x was about 0.03–0.1. In addition, the similar method was used to study the magnetic properties of the SmZn-doped nanocrystalline barium hexaferrites in Fig. 4b. Owing to the combined effect of the hyperfine field, the canting spin and the magnetic dilution of the impurity phases of ˛-Fe2 O3 , SmFeO3 and ZnFe2 O4 , Ms of Ba1−x Smx Fe12−x Znx O19 was similar to that of Ba1−x Smx Fe12 O19 , which increased firstly, and then decreased. But, Hc retained constant with the value of about 4600 Oe.

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Fig. 5. Ms and Hc curves versus x for Sm (a) and SmZn (b) doped nanocrystalline barium hexaferrites.

4. Conclusions Sm and SmZn substituted nanocrystalline barium hexaferrites were prepared, and then XRD and VSM were used to characterize the phase composition and the magnetic properties of the asprepared nanocrystalline barium hexaferrites. All results indicated that x greatly affected the phase composition and the magnetic properties. When x > 0.2, ˛-Fe2 O3 , SmFeO3 and ZnFe2 O4 phases were detected, which affected the magnetic properties. Owing to the hyperfine field, the canting spin, the magnetic dilution and impurity phases, the substitution of Sm led to the increase of Ms firstly, and then decreased, but Hc increased with x. The magnetic properties indicated the doped-Zn2+ substituted Fe3+ at 4f2 site, which made Ms enlarge and Hc decline. In conclusion, the magnetic properties of nanocrystalline barium hexaferrites could be tailored by controlling the cationic substitution. Acknowledgements The research is funded by National Natural Science Foundation of China (20801050), Zhejiang Province Science Foundation (Y4080114) and Scientific and Technological Project of Zhejiang Province (2007C31026). Fig. 4. VSM curves of Sm (a) and SmZn (b) doped nanocrystalline barium hexaferrites with x.

Generally, Zn2+ , substituted Fe3+ , could occupy 4f1 and 4f2 sites that induced the enhancement of Ms , and also could occupy 2b sites that led to the abasement of Ms [14]. The magnetocrystalline anisotropy of barium hexaferrites mainly rooted in the Fe3+ cations at 4f2 and 2b sites. If Zn2+ occupied 4f2 or 2b sites, Hc would decrease greatly. Compared the magnetic parameters of the asprepared Ba1−x Smx Fe12−x Znx O19 and Ba1−x Smx Fe12 O19 in Fig. 5, Ms of SmZn-doped samples was larger than that of Sm-doped with the same x, while Hc was smaller. It’s attributed to the substitution of Zn2+ → Fe3+ at 4f2 site, that made Ms enlarge and Hc decline. Both considering the influence of the substitution on the magnetocrystalline anisotropy, Hc retained constant. Therefore, the optimum x for SmZn-doped samples was x = 0.06 with Ms = 66.15 emu/g and Hc = 4448 Oe.

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