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The magnetic properties of strontium hexaferrites with La–Cu substitution prepared by SHS method
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Journal of Magnetism and Magnetic Materials 318 (2007) 74–78 www.elsevier.com/locate/jmmm
The magnetic properties of strontium hexaferrites with La–Cu substitution prepared by SHS method Liang Qiao, Lishun You, Jingwu Zheng, Liqiang Jiang, Jiawei Sheng College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310014, People’s Republic of China Received 17 July 2006; received in revised form 7 March 2007 Available online 5 May 2007
Abstract La–Cu substituted strontium hexaferrites with the chemical composition of Sr1xLaxFe12xCuxO19 were prepared by self-propagating high-temperature synthesis. The effects of La–Cu substitution on the microstructure and magnetic properties of Sr-ferrites were studied. The XRD results show that all the samples are single SrM-type phase for xo0.4. Compared with the samples without La–Cu substitution, the magnetic properties of the samples with the composition of Sr1xLaxFe12xCuxO19 are remarkably improved for xo0.4. The possibility that the substitution of Sr2+ by La3+ in the Sr-layer makes the Cu2+ preferably substitutes the Fe3+ in 4f2 sites is predicted to be associated with the improvement of the magnetic properties of La–Cu substituted samples. r 2007 Elsevier B.V. All rights reserved. PACS: 74.62.Bf; 75.50.Dd; 74.62.Dh; 75.60.d Keywords: SHS; Ferrite; Substitute; Magnetic property
1. Introduction Since its discovery in 1950s, M-type ferrite AFe12O19 (A stands for Ba, Sr and Pb) has attracted considerable interests because of their applications as hard magnetic materials and high-density magnetic record media derived from their good magnetic properties and low cost [1]. M-type ferrites have a hexagonal structure with 64 ions per unit cell over 11 different symmetry sites. The 24 Fe3+ ions are distributed over three octahedral sites (2a, 12k and 4f2), one tetrahedral site (4f1) and one bipyramidal site (2b). The three parallel (2a, 12k and 2b) and two antiparallel (4f1 and 4f2) sub-lattices form the ferrimagnetic structure [2,3]. The intrinsic magnetic properties can be significantly improved by substituting Fe3+ in different sites with other suitable ions. Sr-ferrites with different ion substitution such as La–Co [2–6], La–Zn [7], Zn–Sn [8], Zn–Ti [9], La [10], Nd [11], Cr [12], etc. were synthesized by sol–gel and conventional firing methods. As known, the substitution with La–Co Corresponding author. Tel./fax: +86 571 88320142.
E-mail address: [email protected] (L. Jiang). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.04.028
benefits achieving the high intrinsic magnetic properties [2–6]. For example, the intrinsic coercivity is usually above 300 kA/m. Considering Cu2+ has the similar ion radius and the substituted ability as Co2+ but low cost, strontium hexagonal ferrites with the chemical composition of Sr1xLaxFe12xCuxO19 were synthesized by SHS method and the effects of the La–Cu substitution on the magnetic properties were also studied in this paper. 2. Experimental procedure Powders of Sr1xLaxFe12xCuxO19 with 0pxp0.6 were synthesized using a SHS process. The starting materials used in the study were Fe, Fe2O3, SrCO3, La2O3 and CuO with the mole ratios satisfying the chemical composition. NaClO4 was also added as the oxidizer. The red mixture containing the reagents was dried in an oven and then deposited into a self-made SHS device. After ignited by a Ni–Cr resistance wire, the SHS reaction happened and the red mixture turned black. The as-received samples were broken up and ball-milled for 5 h into the fine particles with the size of 0.8–2 mm. After cleaned by distilled water to eliminate the by-product of NaCl from the decomposition
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of NaClO4, the powders were then annealed at 1000 1C for 1 h. In comparison, the samples with only Cu substitution also were prepared using the same process. The phase compositions of different samples were investigated using X-ray diffraction (Thermo ARL X’TRA XRD) in continuous scanning mode with CuKa radiation (l ¼ 1.5406 A˚). SEM (Hitachi S-4700) was used to observe the particle morphology of the as-received powders. To achieve the magnetic properties of the isotropic samples, the annealed powders at 1000 1C with 25 wt% PA6 (1013B, UBE nylon) as binder were injected into a cylinder with the size of + 10 mm 10 mm. The magnetic properties including Br, Hcb and (BH)m of the powder were calculated according to the volume content of the powder in the cylinder and the magnetic properties of the cylinder were measured using permanent magnetic measurement equipment (HT610, made by Shanghai Hengtong Magnetoelectric Technology Ltd.). 3. Results and discussion Fig. 1 shows the XRD patterns of the samples with different x. There is only one magnetoplumbite phase (SrM) in the three samples (x ¼ 0.15, 0.25 and 0.3) as shown in Fig. 1, which implies that Cu2+ and La3+ enter the magnetoplumbite lattice and no second phases containing them form. However, another phase of LaFeO3 begins to occur when x increases to 0.4. The weak peaks of LaFeO3 are observed in the samples with x ¼ 0.4, 0.5 and 0.6 as shown in Fig. 1. In general, FeO, Fe3O4, Fe2O3 and SrFe10O22 are the impurities formed in the samples prepared by SHS method [13]. However, no XRD peaks of Fe2O3 and other impurities were observed in the achieved samples when xo0.4. This also indicates that the single SrM-type ferrites can be achieved under the experimental conditions by SHS process. The qualitative information about La–Cu ions into the SrM lattice can be obtained by the careful determination of the Sr1xLaxFe12xCuxO19 lattice parameters using XRD method through a slow rate of 0.11/min. Fig. 2 shows the shift of the XRD peaks of the samples with different
Fig. 1. XRD patterns of the samples with different x.
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Fig. 2. The shift of the XRD peaks of the samples with different x.
Table 1 The lattice parameters calculated of the different samples x
a (A˚)
c (A˚)
a/c
0 0.3 0.6
5.889 5.887 5.886
23.011 23.038 23.034
0.2559 0.2555 0.2554
substituted amount x. It can be seen that the XRD peaks of SrM-type phase in the samples with x ¼ 0.3 and 0.6 have a slight shift toward lower 2y in comparison with the sample with x ¼ 0, which indicates that Cu2+ and La3+ enter the magnetoplumbite lattice and change the lattice size. The lattice parameters of different samples are indicated in Table 1. The length of a-axis has no obvious change with increasing x, which is agreement with the previous results where the substitution of ions such as La3+ has no influence on the a-axis length of SrM lattice [10]. However, the c-axis length of the sample with x ¼ 0.3 increases from 23.011 to 23.038 A˚ compared with the sample with x ¼ 0. According to the decrease of the c-axis length in the Sr1xLaxFe12O19 lattice with increasing x due to the smaller radius of La3+ (1.22 A˚) in comparison with that of Sr2+ (1.27 A˚) [10], the length of c-axis is expected to increase because of the larger radius of Cu2+ (0.78 A˚) than that of Fe3+ (0.67 A˚) when Cu ions enters the SrM lattice and substitutes the Fe ions. The increase of c-axis length of the sample with x ¼ 0.3 also implies that the contribution of Cu2+ substitution on increasing the SrM lattice size exceeds the contrary action of Sr2+ substituted by La3+. When x ¼ 0.6, the formation of LaFeO3 changes the La/Cu ratio, which may result in forming the oxygen vacancies for electrovalent balance since no other phases containing Cu are detected in the sample. The oxygen vacancies can restrain the continuous increase of the lattice parameters and consequently produce the similar a/c and c-axis length as the sample with x ¼ 0.3. Fig. 3 shows the change of the remanent inductions (Br) of samples with the different substitution amount x. As shown in Fig. 3, the Br of Cu-substituted samples slightly decrease compared with the sample with no substitution.
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Fig. 3. Br dependence on the substituted amount x of the different samples.
However, they increase by about 10% from 0.220 T (x ¼ 0) to 0.243 T (x ¼ 0.3) for the La–Cu substituted samples. In the oxygen atmosphere, the oxides containing Cu can exist in the form of Cu2O and CuO at the high temperature. According to the abilities of different ions into the octahedral site: Fe3+ oCu+ oFe2+ oCu2+, it is difficult that Cu+ substitutes Fe3+ since it is in the middle of Fe3+ and Fe2+. In addition, the radius of Cu+ is 0.96 A˚, which is greatly larger than that of Fe3+ (0.67 A˚). Therefore, it is believed that Cu2+ has a large possibility to substitute Fe3+ in the octahedral sites of the SrM lattice before it could change into Cu+. Because of the same possibility that Cu2+ substitutes Fe3+ in every octahedral site of 12k, 4f2 and 2a in a Sr-ferrite lattice, the Br of the Cu-substituted samples slightly decreases since the number of Fe3+ in parallel sub-lattice (12k and 2a sites) is more than that in antiparallel sub-lattice (4f2). In the La–Cu substituted samples, Cu2+ probably prefers to substitute the Fe3+ in the 4f2 sites considering that the 4f2 site is closer to the Sr-layer (shown in Fig. 1 of Ref. [4] and Fig. 8 of Ref. [14]) than other octahedral sites (12k and 2a) for electrovalent balance since the substitution of the Sr2+ by La3+ occurs in the Sr-layer. In addition, the occupation of Cu2+ in the 4f2 site close to the Sr-layer causes the small lattice change as a compensation of the substitution of Sr2+ by La3+. There is only a 0.027 A˚ enhancement of c-axis length for the sample with x ¼ 0.3 as shown in Table 1. Thus, the magnetic moment in antiparallel direction (4f1 and 4f2) is weakened and the whole magnetic moment increases. This greatly improves the Br of samples with La–Cu substitution. As investigated by Wiesinger et al. [4], Co2+ preferably enters the 4f2 sub-lattice in the substituted Sr1xLaxFe12xCoxO19 ferrites using 57Fe Mo¨ssbauer and 57Fe NMR. Therefore, it is possible that Cu2+ also prefers to enter the 4f2 sub-lattice like Co2+ in the Sr1xLaxFe12xCuxO19 since Cu2+ has the similar radius (0.78 A˚) and ability to occupy the octahedral site with Co2+ (0.82 A˚). Liu et al. [10] studied the change of hyperfine parameters of Sr1xLaxFe12O19 using Mo¨ssbauer spectra of 57Fe. The results show that the substitution of Sr2+ by La3+ is associated with a valence change of Fe3+ to Fe2+ at 2a or 4f2
site. The decreases of c-axis enlarge the bond angle of (2b)–O2–(4f2) and then increase the hyperfine field contribution, which is supertransferred from the Fe-spins at the 4f2 sites onto the 57Fe nuclei at the 2b site. As shown in Table 1, if the slight increase of c-axis reduces the bond angle of (2b)–O2(4f2), the antiparallel moments may slightly fall down. However, as mentioned by Liu et al. [10], it is difficult to directly predict the definitely change of the hyperfine field at the 4f2 site because of the superexchange (4f2)–O2–(4f2) and the direct exchange (4f2)–(4f2) which is due to the overlap of the 3d-orbitals of the Fe3+ at 4f2 acts on the spins at the 4f2 site. In the study by Wiesinger et al. [4], the possibility that Co2+ occupies the 2a and 4f1 sites cannot also be excluded because of the too weak Fe(2a) resonance in the Mo¨ssbauer spectra. Consequently, the direct experimentally proof is lacking here about Cu2+ in the 4f2 sites due to the overlap of the spectra in the 4f2 site. It can be predicted that the Br enhancement of the samples with La–Cu substitution may be associated with a preferred Cu2+ occupation in the 4f2 sites close to the Sr-layer for the consideration of electrovalent balance. It also can be seen in Fig. 3 that the Br decreases after x40.3 and is only 0.20 T when x ¼ 0.6. This decrease either comes from the formation of LaFeO3 phase and the oxygen vacancies from the XRD patterns and the calculated results of lattice parameters or the decrease of coercivity (Hcj) due to the different particle morphologies as indicated behind. The coercivity (Hcb) and maximum magnetic product of Sr1xLaxFe12xCuxO19 dependence on different x are displayed in Fig. 4. Hcb and (BH)m have the same feature as Br. This can be explained from the magnetic hysteresis loop (especially the demagnetizing curve in the second quadrant) of the samples. Fig. 5 illustrates the magnetic hysteresis loop of a cylinder prepared from the sample (x ¼ 0.3) with 25 wt% PA6 as binder. It can be seen that there is a near linear relationship between J and H when H o Hcb5Hcj. Thus, B–H curve is almost a line when HoHcb according to the expression: B ¼ m0(H+M). The measured experimental data of Mr and Hcb of different samples were shown in Fig. 6. A near linear relationship
Fig. 4. Hcb and (BH)m dependence on the substituted amount x of La–Cu substituted samples.
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Fig. 5. The magnetic hysteresis loop of a cylinder prepared using the asreceived powder (x ¼ 0.3) with 25 wt% PA6 as binder.
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Fig. 7. Hcj dependence on the substituted amount x of La–Cu substituted samples.
The intrinsic coercivity (Hcj) of samples dependence on different x is shown in Fig. 7. It can be seen in this figure that Hcj gradually decreases with the increase of x. For example, Hcj decreases from 341 to 273 kA/m while x increases from 0.1 to 0.6. Many factors have the effects on the Hcj of the samples such as the crystal structure, morphological features and the elemental compositions of the particles. In general, Hcj can be expressed as Hc ¼ Fig. 6. The linear relationship between Br and Hcb of the La–Cu substituted samples.
between Mr(or Br) and Hcb with the fitting factor of 0.9967 occurs in this figure with the fitting expression as follows: M r ¼ 24:78 þ 1:026H cb .
(1)
So, the following expression M r M cb 24:78 ¼ þ 0:026 H cb H cb
(2)
can be gotten according to Mcb ¼ Hcb, where Mcb is the magnetism for H ¼ Hcb. This indicates the increase of the decline slope when HoHcb in the second quadrant of the hysteresis loop with the decrease of Br and Hcb. For the same demagnetizing field intensity H (0oHoHcb), the Mr–MH would be smaller for the sample with the larger Hcb and Br. This is agreement with the demagnetizing change mainly controlled by domain rotation of the single domain particles in the SrM-type ferrites. Therefore, the result that Hcb increases with increasing Br also implies that La–Cu substitution with different x does not produce the different demagnetizing mechanism but only the quantities of the critical parameters. The different x only changes the size of SrM lattice and does not result in the formation of samples of low Hcb and Hcj together with high Br. Likewise, the maximum magnetic products of the samples determined by Br and Hcb has same feature as Br as shown in Fig. 3. In comparison with the sample with no substitution, the (BH)m of the sample with x ¼ 0.3 increases by 14% and reaches 10 kJ/m3.
aH A NðM r þ M s Þ , m0
(3)
where a is the microstructure factor which increases with the decreasing grain size and N is the demagnetization factor determined by many parameters one of which is the aspect ratio [15]. This expression can also be used to describe the effects of magnetocrystalline anisotropy and microstructure on the intrinsic coercivity of the samples. The typical SEM images of the as-prepared sample with different x are shown in Fig. 8. The particle sizes slowly grow with increasing x in the compositional range x ¼ 0.1–0.3 and most of them are lower than 1.5 mm as shown in Figs. 8(a–c). However, it is obviously large for the sample with x ¼ 0.6 as shown in Fig. 8(d). This can account for the decline of the Hcj due to the decreasing microstructure factor a with the increasing particle size. It can also be seen from Fig. 8(d) that most particles have the size lower than 2 mm, which generally fills the requirement of bonded magnets. That is why the Hcj is quite high exceeding 238 kA/m (3 kOe) for the sample with x ¼ 0.6 even after the obvious decrease in comparison with the sample with x ¼ 0.1. The similar Hcj of the samples with x ¼ 0.2 and 0.3 shown in Fig. 7 is associated with the similar particle morphologies and sizes as shown in Figs. 8(b) and (c). Since the particle morphology and size are influenced by the La–Cu substituted amount, it is possible to control the microstructure of the sample and obtain the good magnetic properties by optimizing the SHS process. 4. Conclusions Strontium hexagonal ferrites with La3+ and Cu2+ substitution were synthesized by the SHS method. There
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Fig. 8. The SEM images of the samples with (a) x ¼ 0.1, (b) x ¼ 0.2, (c) x ¼ 0.3 and (d) x ¼ 0.6.
is no obvious improvement in the magnetic properties of the samples with only Cu2+ substitution. However, the samples substituted by La–Cu ions show a remarkable enhancement in the magnetic properties such as Br ¼ 0.243 T, Hcb ¼ 164 kA/m, Hcj ¼ 311 kA/m and (BH)m ¼ 10 kJ/m3 for the Sr1xLaxFe12xCuxO19 sample with x ¼ 0.3. The maximum improvement in the Br and (BH)m are 10% and 14%, respectively, in comparison with the sample with no substitution. For the sample with x ¼ 0.1, the achieved Hcj is high to 341 kA/m due to the good hexagonal plate-like morphology and small particle size. The lattice sizes change in the samples with La–Cu substitution, which indicates the ions enter the SrM lattice and have obvious effects on the magnetic properties of the samples. The improvement of the magnetic properties may be the result that the substitution of Sr2+ by La3+ in the Sr-layer makes the Cu2+ preferably substitute the Fe3+ in 4f2 sites. The detailed mechanism needs be further studied in the future. Acknowledgment This work was supported by the Key Project for Science and Technology of Zhejiang Province.
References [1] D.H. Choi, S.W. Lee, I. Shim, C.S. Kim, J. Magn. Magn. Mater. 304 (2005) e243. [2] M. Pieper, A. Morel, F. Kools, J. Magn. Magn. Mater. 242–245 (2002) 1408. [3] A. Morel, J.M. Le Breton, J. Kreisel, G. Wiesinger, F. Kools, P. Tenaud, J. Magn. Magn. Mater. 242–245 (2002) 1405. [4] G. Wiesinger, M. Muller, R. Grossinger, M. Pieper, A. Morel, F. Kools, P. Tenaud, J.M. LE Breton, J. Kreisel, Phys. Status Solidi (a) 189 (2) (2002) 499. [5] P. Tenaud, A. Morel, F. Kools, J.M.L. Breton, J. Alloys Compds. 370 (2004) 331. [6] X. Liu, P. Herna´ndez-Go´mez, K. Huang, S. Zhou, Y. Wang, X. Cai, H. Sun, B. Ma, J. Magn. Magn. Mater. 305 (2006) 524. [7] J.C. Corral-Huacuz, G. Mendoza-Suarez, J. Magn. Magn. Mater. 242–245 (2002) 430. [8] C.K. Ong, H.C. Fang, Z. Yang, Y. Li, J. Magn. Magn. Mater. 213 (2000) 413. [9] Q. Fang, Y. Liu, P. Yin, X. Li, J. Magn. Magn. Mater. 234 (2001) 366. [10] X. Liu, W. Zhong, S. Yang, Z. Yu, B. Gu, Y. Du, J. Magn. Magn. Mater. 238 (2002) 207. [11] H. Mocuta, L. Lechevallier, J.M.L. Breton, J.F. Wang, I.R. Harris, J. Alloys Compds. 364 (2004) 48. [12] S. Ounnunkad, P. Winotai, J. Magn. Magn. Mater. 301 (2006) 292. [13] K.S. Martirosyan, P.B. Avakyan, Inorg. Mater. 38 (2002) 400. [14] X. Zhang, Y. Duan, H. Guan, S. Liu, B. Wen, J. Magn. Magn. Mater. (2006). [15] F. Kools, J. Phys. 46 (1985) 349.
Journal of Magnetism and Magnetic Materials 318 (2007) 74–78 www.elsevier.com/locate/jmmm
The magnetic properties of strontium hexaferrites with La–Cu substitution prepared by SHS method Liang Qiao, Lishun You, Jingwu Zheng, Liqiang Jiang, Jiawei Sheng College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310014, People’s Republic of China Received 17 July 2006; received in revised form 7 March 2007 Available online 5 May 2007
Abstract La–Cu substituted strontium hexaferrites with the chemical composition of Sr1xLaxFe12xCuxO19 were prepared by self-propagating high-temperature synthesis. The effects of La–Cu substitution on the microstructure and magnetic properties of Sr-ferrites were studied. The XRD results show that all the samples are single SrM-type phase for xo0.4. Compared with the samples without La–Cu substitution, the magnetic properties of the samples with the composition of Sr1xLaxFe12xCuxO19 are remarkably improved for xo0.4. The possibility that the substitution of Sr2+ by La3+ in the Sr-layer makes the Cu2+ preferably substitutes the Fe3+ in 4f2 sites is predicted to be associated with the improvement of the magnetic properties of La–Cu substituted samples. r 2007 Elsevier B.V. All rights reserved. PACS: 74.62.Bf; 75.50.Dd; 74.62.Dh; 75.60.d Keywords: SHS; Ferrite; Substitute; Magnetic property
1. Introduction Since its discovery in 1950s, M-type ferrite AFe12O19 (A stands for Ba, Sr and Pb) has attracted considerable interests because of their applications as hard magnetic materials and high-density magnetic record media derived from their good magnetic properties and low cost [1]. M-type ferrites have a hexagonal structure with 64 ions per unit cell over 11 different symmetry sites. The 24 Fe3+ ions are distributed over three octahedral sites (2a, 12k and 4f2), one tetrahedral site (4f1) and one bipyramidal site (2b). The three parallel (2a, 12k and 2b) and two antiparallel (4f1 and 4f2) sub-lattices form the ferrimagnetic structure [2,3]. The intrinsic magnetic properties can be significantly improved by substituting Fe3+ in different sites with other suitable ions. Sr-ferrites with different ion substitution such as La–Co [2–6], La–Zn [7], Zn–Sn [8], Zn–Ti [9], La [10], Nd [11], Cr [12], etc. were synthesized by sol–gel and conventional firing methods. As known, the substitution with La–Co Corresponding author. Tel./fax: +86 571 88320142.
E-mail address: [email protected] (L. Jiang). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.04.028
benefits achieving the high intrinsic magnetic properties [2–6]. For example, the intrinsic coercivity is usually above 300 kA/m. Considering Cu2+ has the similar ion radius and the substituted ability as Co2+ but low cost, strontium hexagonal ferrites with the chemical composition of Sr1xLaxFe12xCuxO19 were synthesized by SHS method and the effects of the La–Cu substitution on the magnetic properties were also studied in this paper. 2. Experimental procedure Powders of Sr1xLaxFe12xCuxO19 with 0pxp0.6 were synthesized using a SHS process. The starting materials used in the study were Fe, Fe2O3, SrCO3, La2O3 and CuO with the mole ratios satisfying the chemical composition. NaClO4 was also added as the oxidizer. The red mixture containing the reagents was dried in an oven and then deposited into a self-made SHS device. After ignited by a Ni–Cr resistance wire, the SHS reaction happened and the red mixture turned black. The as-received samples were broken up and ball-milled for 5 h into the fine particles with the size of 0.8–2 mm. After cleaned by distilled water to eliminate the by-product of NaCl from the decomposition
ARTICLE IN PRESS L. Qiao et al. / Journal of Magnetism and Magnetic Materials 318 (2007) 74–78
of NaClO4, the powders were then annealed at 1000 1C for 1 h. In comparison, the samples with only Cu substitution also were prepared using the same process. The phase compositions of different samples were investigated using X-ray diffraction (Thermo ARL X’TRA XRD) in continuous scanning mode with CuKa radiation (l ¼ 1.5406 A˚). SEM (Hitachi S-4700) was used to observe the particle morphology of the as-received powders. To achieve the magnetic properties of the isotropic samples, the annealed powders at 1000 1C with 25 wt% PA6 (1013B, UBE nylon) as binder were injected into a cylinder with the size of + 10 mm 10 mm. The magnetic properties including Br, Hcb and (BH)m of the powder were calculated according to the volume content of the powder in the cylinder and the magnetic properties of the cylinder were measured using permanent magnetic measurement equipment (HT610, made by Shanghai Hengtong Magnetoelectric Technology Ltd.). 3. Results and discussion Fig. 1 shows the XRD patterns of the samples with different x. There is only one magnetoplumbite phase (SrM) in the three samples (x ¼ 0.15, 0.25 and 0.3) as shown in Fig. 1, which implies that Cu2+ and La3+ enter the magnetoplumbite lattice and no second phases containing them form. However, another phase of LaFeO3 begins to occur when x increases to 0.4. The weak peaks of LaFeO3 are observed in the samples with x ¼ 0.4, 0.5 and 0.6 as shown in Fig. 1. In general, FeO, Fe3O4, Fe2O3 and SrFe10O22 are the impurities formed in the samples prepared by SHS method [13]. However, no XRD peaks of Fe2O3 and other impurities were observed in the achieved samples when xo0.4. This also indicates that the single SrM-type ferrites can be achieved under the experimental conditions by SHS process. The qualitative information about La–Cu ions into the SrM lattice can be obtained by the careful determination of the Sr1xLaxFe12xCuxO19 lattice parameters using XRD method through a slow rate of 0.11/min. Fig. 2 shows the shift of the XRD peaks of the samples with different
Fig. 1. XRD patterns of the samples with different x.
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Fig. 2. The shift of the XRD peaks of the samples with different x.
Table 1 The lattice parameters calculated of the different samples x
a (A˚)
c (A˚)
a/c
0 0.3 0.6
5.889 5.887 5.886
23.011 23.038 23.034
0.2559 0.2555 0.2554
substituted amount x. It can be seen that the XRD peaks of SrM-type phase in the samples with x ¼ 0.3 and 0.6 have a slight shift toward lower 2y in comparison with the sample with x ¼ 0, which indicates that Cu2+ and La3+ enter the magnetoplumbite lattice and change the lattice size. The lattice parameters of different samples are indicated in Table 1. The length of a-axis has no obvious change with increasing x, which is agreement with the previous results where the substitution of ions such as La3+ has no influence on the a-axis length of SrM lattice [10]. However, the c-axis length of the sample with x ¼ 0.3 increases from 23.011 to 23.038 A˚ compared with the sample with x ¼ 0. According to the decrease of the c-axis length in the Sr1xLaxFe12O19 lattice with increasing x due to the smaller radius of La3+ (1.22 A˚) in comparison with that of Sr2+ (1.27 A˚) [10], the length of c-axis is expected to increase because of the larger radius of Cu2+ (0.78 A˚) than that of Fe3+ (0.67 A˚) when Cu ions enters the SrM lattice and substitutes the Fe ions. The increase of c-axis length of the sample with x ¼ 0.3 also implies that the contribution of Cu2+ substitution on increasing the SrM lattice size exceeds the contrary action of Sr2+ substituted by La3+. When x ¼ 0.6, the formation of LaFeO3 changes the La/Cu ratio, which may result in forming the oxygen vacancies for electrovalent balance since no other phases containing Cu are detected in the sample. The oxygen vacancies can restrain the continuous increase of the lattice parameters and consequently produce the similar a/c and c-axis length as the sample with x ¼ 0.3. Fig. 3 shows the change of the remanent inductions (Br) of samples with the different substitution amount x. As shown in Fig. 3, the Br of Cu-substituted samples slightly decrease compared with the sample with no substitution.
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Fig. 3. Br dependence on the substituted amount x of the different samples.
However, they increase by about 10% from 0.220 T (x ¼ 0) to 0.243 T (x ¼ 0.3) for the La–Cu substituted samples. In the oxygen atmosphere, the oxides containing Cu can exist in the form of Cu2O and CuO at the high temperature. According to the abilities of different ions into the octahedral site: Fe3+ oCu+ oFe2+ oCu2+, it is difficult that Cu+ substitutes Fe3+ since it is in the middle of Fe3+ and Fe2+. In addition, the radius of Cu+ is 0.96 A˚, which is greatly larger than that of Fe3+ (0.67 A˚). Therefore, it is believed that Cu2+ has a large possibility to substitute Fe3+ in the octahedral sites of the SrM lattice before it could change into Cu+. Because of the same possibility that Cu2+ substitutes Fe3+ in every octahedral site of 12k, 4f2 and 2a in a Sr-ferrite lattice, the Br of the Cu-substituted samples slightly decreases since the number of Fe3+ in parallel sub-lattice (12k and 2a sites) is more than that in antiparallel sub-lattice (4f2). In the La–Cu substituted samples, Cu2+ probably prefers to substitute the Fe3+ in the 4f2 sites considering that the 4f2 site is closer to the Sr-layer (shown in Fig. 1 of Ref. [4] and Fig. 8 of Ref. [14]) than other octahedral sites (12k and 2a) for electrovalent balance since the substitution of the Sr2+ by La3+ occurs in the Sr-layer. In addition, the occupation of Cu2+ in the 4f2 site close to the Sr-layer causes the small lattice change as a compensation of the substitution of Sr2+ by La3+. There is only a 0.027 A˚ enhancement of c-axis length for the sample with x ¼ 0.3 as shown in Table 1. Thus, the magnetic moment in antiparallel direction (4f1 and 4f2) is weakened and the whole magnetic moment increases. This greatly improves the Br of samples with La–Cu substitution. As investigated by Wiesinger et al. [4], Co2+ preferably enters the 4f2 sub-lattice in the substituted Sr1xLaxFe12xCoxO19 ferrites using 57Fe Mo¨ssbauer and 57Fe NMR. Therefore, it is possible that Cu2+ also prefers to enter the 4f2 sub-lattice like Co2+ in the Sr1xLaxFe12xCuxO19 since Cu2+ has the similar radius (0.78 A˚) and ability to occupy the octahedral site with Co2+ (0.82 A˚). Liu et al. [10] studied the change of hyperfine parameters of Sr1xLaxFe12O19 using Mo¨ssbauer spectra of 57Fe. The results show that the substitution of Sr2+ by La3+ is associated with a valence change of Fe3+ to Fe2+ at 2a or 4f2
site. The decreases of c-axis enlarge the bond angle of (2b)–O2–(4f2) and then increase the hyperfine field contribution, which is supertransferred from the Fe-spins at the 4f2 sites onto the 57Fe nuclei at the 2b site. As shown in Table 1, if the slight increase of c-axis reduces the bond angle of (2b)–O2(4f2), the antiparallel moments may slightly fall down. However, as mentioned by Liu et al. [10], it is difficult to directly predict the definitely change of the hyperfine field at the 4f2 site because of the superexchange (4f2)–O2–(4f2) and the direct exchange (4f2)–(4f2) which is due to the overlap of the 3d-orbitals of the Fe3+ at 4f2 acts on the spins at the 4f2 site. In the study by Wiesinger et al. [4], the possibility that Co2+ occupies the 2a and 4f1 sites cannot also be excluded because of the too weak Fe(2a) resonance in the Mo¨ssbauer spectra. Consequently, the direct experimentally proof is lacking here about Cu2+ in the 4f2 sites due to the overlap of the spectra in the 4f2 site. It can be predicted that the Br enhancement of the samples with La–Cu substitution may be associated with a preferred Cu2+ occupation in the 4f2 sites close to the Sr-layer for the consideration of electrovalent balance. It also can be seen in Fig. 3 that the Br decreases after x40.3 and is only 0.20 T when x ¼ 0.6. This decrease either comes from the formation of LaFeO3 phase and the oxygen vacancies from the XRD patterns and the calculated results of lattice parameters or the decrease of coercivity (Hcj) due to the different particle morphologies as indicated behind. The coercivity (Hcb) and maximum magnetic product of Sr1xLaxFe12xCuxO19 dependence on different x are displayed in Fig. 4. Hcb and (BH)m have the same feature as Br. This can be explained from the magnetic hysteresis loop (especially the demagnetizing curve in the second quadrant) of the samples. Fig. 5 illustrates the magnetic hysteresis loop of a cylinder prepared from the sample (x ¼ 0.3) with 25 wt% PA6 as binder. It can be seen that there is a near linear relationship between J and H when H o Hcb5Hcj. Thus, B–H curve is almost a line when HoHcb according to the expression: B ¼ m0(H+M). The measured experimental data of Mr and Hcb of different samples were shown in Fig. 6. A near linear relationship
Fig. 4. Hcb and (BH)m dependence on the substituted amount x of La–Cu substituted samples.
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Fig. 5. The magnetic hysteresis loop of a cylinder prepared using the asreceived powder (x ¼ 0.3) with 25 wt% PA6 as binder.
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Fig. 7. Hcj dependence on the substituted amount x of La–Cu substituted samples.
The intrinsic coercivity (Hcj) of samples dependence on different x is shown in Fig. 7. It can be seen in this figure that Hcj gradually decreases with the increase of x. For example, Hcj decreases from 341 to 273 kA/m while x increases from 0.1 to 0.6. Many factors have the effects on the Hcj of the samples such as the crystal structure, morphological features and the elemental compositions of the particles. In general, Hcj can be expressed as Hc ¼ Fig. 6. The linear relationship between Br and Hcb of the La–Cu substituted samples.
between Mr(or Br) and Hcb with the fitting factor of 0.9967 occurs in this figure with the fitting expression as follows: M r ¼ 24:78 þ 1:026H cb .
(1)
So, the following expression M r M cb 24:78 ¼ þ 0:026 H cb H cb
(2)
can be gotten according to Mcb ¼ Hcb, where Mcb is the magnetism for H ¼ Hcb. This indicates the increase of the decline slope when HoHcb in the second quadrant of the hysteresis loop with the decrease of Br and Hcb. For the same demagnetizing field intensity H (0oHoHcb), the Mr–MH would be smaller for the sample with the larger Hcb and Br. This is agreement with the demagnetizing change mainly controlled by domain rotation of the single domain particles in the SrM-type ferrites. Therefore, the result that Hcb increases with increasing Br also implies that La–Cu substitution with different x does not produce the different demagnetizing mechanism but only the quantities of the critical parameters. The different x only changes the size of SrM lattice and does not result in the formation of samples of low Hcb and Hcj together with high Br. Likewise, the maximum magnetic products of the samples determined by Br and Hcb has same feature as Br as shown in Fig. 3. In comparison with the sample with no substitution, the (BH)m of the sample with x ¼ 0.3 increases by 14% and reaches 10 kJ/m3.
aH A NðM r þ M s Þ , m0
(3)
where a is the microstructure factor which increases with the decreasing grain size and N is the demagnetization factor determined by many parameters one of which is the aspect ratio [15]. This expression can also be used to describe the effects of magnetocrystalline anisotropy and microstructure on the intrinsic coercivity of the samples. The typical SEM images of the as-prepared sample with different x are shown in Fig. 8. The particle sizes slowly grow with increasing x in the compositional range x ¼ 0.1–0.3 and most of them are lower than 1.5 mm as shown in Figs. 8(a–c). However, it is obviously large for the sample with x ¼ 0.6 as shown in Fig. 8(d). This can account for the decline of the Hcj due to the decreasing microstructure factor a with the increasing particle size. It can also be seen from Fig. 8(d) that most particles have the size lower than 2 mm, which generally fills the requirement of bonded magnets. That is why the Hcj is quite high exceeding 238 kA/m (3 kOe) for the sample with x ¼ 0.6 even after the obvious decrease in comparison with the sample with x ¼ 0.1. The similar Hcj of the samples with x ¼ 0.2 and 0.3 shown in Fig. 7 is associated with the similar particle morphologies and sizes as shown in Figs. 8(b) and (c). Since the particle morphology and size are influenced by the La–Cu substituted amount, it is possible to control the microstructure of the sample and obtain the good magnetic properties by optimizing the SHS process. 4. Conclusions Strontium hexagonal ferrites with La3+ and Cu2+ substitution were synthesized by the SHS method. There
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Fig. 8. The SEM images of the samples with (a) x ¼ 0.1, (b) x ¼ 0.2, (c) x ¼ 0.3 and (d) x ¼ 0.6.
is no obvious improvement in the magnetic properties of the samples with only Cu2+ substitution. However, the samples substituted by La–Cu ions show a remarkable enhancement in the magnetic properties such as Br ¼ 0.243 T, Hcb ¼ 164 kA/m, Hcj ¼ 311 kA/m and (BH)m ¼ 10 kJ/m3 for the Sr1xLaxFe12xCuxO19 sample with x ¼ 0.3. The maximum improvement in the Br and (BH)m are 10% and 14%, respectively, in comparison with the sample with no substitution. For the sample with x ¼ 0.1, the achieved Hcj is high to 341 kA/m due to the good hexagonal plate-like morphology and small particle size. The lattice sizes change in the samples with La–Cu substitution, which indicates the ions enter the SrM lattice and have obvious effects on the magnetic properties of the samples. The improvement of the magnetic properties may be the result that the substitution of Sr2+ by La3+ in the Sr-layer makes the Cu2+ preferably substitute the Fe3+ in 4f2 sites. The detailed mechanism needs be further studied in the future. Acknowledgment This work was supported by the Key Project for Science and Technology of Zhejiang Province.
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