Effect of Ca and La substitution on the structure and magnetic properties of M-type Sr-hexaferrites

Journal of Alloys and Compounds 771 (2019) 350e355

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Effect of Ca and La substitution on the structure and magnetic properties of M-type Sr-hexaferrites Kyoung-Seok Moon a, Eun-Soo Lim b, Young-Min Kang b, * a School of Materials Science and Engineering and Research Center for Aerospace Parts Technology, Gyeongsang National University, Jinju, 52828, Republic of Korea b Department of Materials Science and Engineering, Korea National University of Transportation, Chungju, 27469, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 April 2018 Received in revised form 7 August 2018 Accepted 30 August 2018 Available online 31 August 2018

Ca- and La-substituted M-type Sr-hexaferrites with the formulae Sr1-xCaxFe12O19, SrFe12-xCaxO19, Sr1xLaxFe12O19, and SrFe12-xLaxO19 (x ¼ 0, 0.1, 0.2, 0.3, and 0.4) were prepared by conventional solid-state reaction routes. Based on the changes in the lattice parameter with different x, it can be concluded that Ca substitutes into both Sr and Fe sites, whereas La substitutes only into the Sr site in the M-type hexaferrite structure. Ca substitution promotes the densification of the hexaferrites with the growth of large grains at the doping level of x  0.3 for Sr1-xCaxFe12O19 and x  0.1 for SrFe12-xCaxO19 samples. La substitution into the Sr site in the Sr1-xLaxFe12O19 samples (x  0.1) was effective in controlling grain growth in the samples. The saturation magnetization value decreased with increasing x in the SrFe12xCaxO19, Sr1-xLaxFe12O19, and SrFe12-xLaxO19 samples, whereas it remained almost constant in the Sr1xCaxFe12O19 samples. © 2018 Elsevier B.V. All rights reserved.

Keywords: M-type hexaferrite Cation substitution Magnetic properties

1. Introduction Hexaferrites have attracted considerable attention due to their wide applicability, such as in permanent magnets, magnetic recording, and RF and microwave applications [1]. They are classified into six types namely, M, U, W, X, Y, and Z, according to different stacking sequences of the three basic blocks S, R, and T in the crystal structure. Among the six types of hexaferrites, M-type Ba- or Sr-hexaferrites, BaFe12O19 (BaM) or SrFe12O19 (SrM), are the most utilized materials as permanent magnets, owing to their low price and good chemical stability [2e5]. Recently, considerable research efforts have focused on La-, Ca-, and Co-substituted Mtype Sr-hexaferrites, because the co-substitution of Ca, La, and Co into SrM has proven to be the most successful approach which significantly enhanced the hard magnetic properties of the hexaferrites [4e6,15]. It is believed that the co-substitution of La and Co significantly increases the magnetocrystalline anisotropy of the Mtype hexaferrite, and the co-substitution of Ca enhances the solubility of La and Co. Although many studies on the Ca-substituted [7e9] or Lasubstituted [10e14] SrM or BaM have been reported, the

* Corresponding author. E-mail address: [email protected] (Y.-M. Kang). https://doi.org/10.1016/j.jallcom.2018.08.306 0925-8388/© 2018 Elsevier B.V. All rights reserved.

substitutions are only considered on either Sr site or Fe site of the M-type structure. In most previous reports, it is basically assumed that La and Ca ions can substitute into Sr (or Ba) sites of the M-type structure. There have been few researches reported on the substitution of Fe by La in the M-type hexaferrites, prepared by sol-gel method [13,14]. The effect of substitution of these elements on the lattice parameters and on the magnetic properties are also significantly different among the studies depending on the sample preparation methods and processes. Possible sites of substitution by Ca, La ions would be either of Sr and Fe sites or both of these sites in the SrM structure. In this study, Ca-substituted and La-substituted SrM were synthesized, respectively, by assuming that these elements might substitute both Fe and Sr sites. The substitution site occupied by Ca and La in the SrM lattice could be determined from the cell volume change and the ionic radii of the cations La, Sr, Ca, and Fe. In addition, the effect of Ca and La substitution into the SrM lattice on the magnetic properties and microstructure of the hexaferrites was elucidated. 2. Experimental procedures The hexaferrite samples of stoichiometric SrFe12O19 (SrM) and the Ca-substituted SrM with formulae Sr1-xCaxFe12O19 (x ¼ 0.1, 0.2, 0.3, 0.4) and SrFe12-xCaxO19 (x ¼ 0.1, 0.2, 0.3, 0.4), and La-substituted

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SrM with formulae Sr1-xLaxFe12O19 (x ¼ 0.1, 0.2, 0.3, 0.4) and SrFe12(x ¼ 0.1, 0.2, 0.3, 0.4) were prepared by conventional solidstate reaction processes. Stoichiometric quantities of the precursor powders of Fe2O3 (99%), SrCO3 (99.5%), CaCO3 (99.9%), and La2O3 (99.9%) were weighed and ball-milled in water, dried, and precalcined in air at 1100  C for 4 h. The calcined powders were then grinded, ball-milled again in water, dried, pelletized, and sintered at temperatures of 1250 and 1300  C for 2 h in air. Phase analysis was performed using X-ray diffraction (XRD, D8 Advance, Bruker) with a Cu Ka radiation source (l ¼ 0.154056 nm). For XRD analysis, all the powder samples were mixed with 30 wt% Si powder for correcting the error in peak positions due to instrumental artefacts. Microstructural observation of the fractured surface of the sintered samples was performed by field emission scanning electron microscopy (FE-SEM, JSM-7610F, JEOL). Magnetization curves were measured using the vibrating sample magnetometer (VSM) of the physical property measurement system (PPMS, Quantum Design Dynacool) at 300 K with a sweeping magnetic field (H) within ±50 k Oe.

xLaxO19

3. Results and discussion Fig. 1(a) and (b) shows the XRD patterns of the calcined powders of Sr1-xCaxFe12O19 (0  x  0.4) and SrFe12-xCaxO19 (0  x  0.4), respectively. The diffraction peaks of the SrM samples were indexed to those of the hexagonal magnetoplumbite crystal structure with

Fig. 2. XRD patterns of the hexaferrite powders of (a) Sr1-xLaxFe12O19 (0  x  0.4) and (b) SrFe12-xLaxO19 (0  x  0.4) calcined at 1300  C.

space group P63/mmc. The XRD patterns of Sr1-xCaxFe12O19 samples with x ¼ 0 in Fig. 1(a) show pure M-type phases without any second phase peaks. A low-intensity Fe2O3 peak is present in other samples corresponding to x ¼ 0.1, 0.2, 0.3, and 0.4. Meanwhile, from the XRD patterns of the SrFe12-xCaxO19 samples in Fig. 1(b), an almost pure M-type structure without the second phase peaks of Fe2O3 was observed. For the series of Sr1-xCaxFe12O19 and SrFe12-xCaxO19 samples (0  x  0.4), the calculated lattice parameters a and c, and the cell volume, are listed in Table 1. Lattice parameters a and c were calculated from the dhkl values corresponding to the (107) and (114) peaks as follows:

dhkl

Fig. 1. XRD patterns of the hexaferrite powders of (a) Sr1-xCaxFe12O19 (0  x  0.4) and (b) SrFe12-xCaxO19 (0  x  0.4) calcined at 1250  C.

(  )1=2  4 h2 þ hk þ k2 l2 ¼ þ 2 c 3a2

(1)

where dhkl is the interplanar spacing, and h, k, and l are the Miller indices. The peak shift introduced by Ca substitution into the SrM lattice is presented in the inset of each figure for the (114) peaks. A peak shift toward the right can be observed in the Sr1-xCaxFe12O19 samples (0.1  x  0.4), and toward the left in the SrFe12-xCaxO19 samples (0.1  x  0.4), compared to the peak of non-doped SrM (x ¼ 0). The variations of cell volume according to the substitution level, x, are shown in Fig. 3. Although there is no clear trend in the plots of cell volume vs. x for both Sr1-xCaxFe12O19 and SrFe12-

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Table 1 Sample composition, cell parameters (a, c), cell volume, and MS and HC values of the Ca-substituted hexaferrites. Composition

x



Sr1-xCaxFe12O19 (1300 C)

SrFe12-xCaxO19 (1250  C)

0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4

Cell parameters

Magnetic properties

a

c

volume

MS (emu/g)

HC (Oe)

5.879 5.878 5.878 5.877 5.878 5.879 5.879 5.881 5.879 5.879

23.032 23.028 23.026 23.010 23.018 23.030 23.031 23.034 23.029 23.038

689.297 689.102 688.924 688.247 688.813 689.281 689.444 690.020 689.352 689.495

72.6 70.9 70.5 70.8 70.5 72.2 71.6 70.2 68.3 66.7

2575 2388 2288 1885 1549 3760 2436 2501 2586 2733

Fig. 3. (a) Plots of cell volume variation for Ca substitution, x, in Sr1-xCaxFe12O19 (0  x  0.4) and SrFe12-xCaxO19 (0  x  0.4) and (b) plots of cell volume variation for La substitution, x, in SrFe12-xLaxO19 (0  x  0.4) and SrFe12-xLaxO19 (0  x  0.4); and (c), (d) plots of MS variation for x in those samples of Fig. 3(a) and (b).

xCaxO19 samples, the cell volumes of Sr1-xCaxFe12O19 (x ¼ 0.1, 0.2, 0.3, 0.4) were smaller than that of the non-substituted SrM (x ¼ 0). On the other hand, the cell volumes of SrFe12-xCaxO19 (x ¼ 0.1, 0.2, 0.3, 0.4) were larger than that of the non-substituted SrM (x ¼ 0). This variation is understandable considering the relative ion sizes (Sr2þ (0.113 nm), Ca2þ (0.099 nm), and Fe3þ (0.064 nm)). On the basic assumption that the cell volume changes depending on their relative ionic radius, a decrease of the cell volume in Sr12þ (0.099 nm) ions substixCaxFe12O19 implies that the smaller Ca tute the Sr2þ (0.113 nm) sites. In the same way, an increase of cell volume in SrFe12-xCaxO19 can be attributed to the substitution of the larger Ca2þ (0.099 nm) ions into the Fe3þ (0.064 nm) sites. Overall, Ca can be substituted into both Sr and Fe sites in the SrM structure. A previous study [15] on the occupation sites of Ca in the Ca-La-Co-substituted M-type hexaferrite reported that the Ca ion occupied not only the Sr site, but also the Fe site; this was verified by extended X-ray absorption fine structure (EXAFS) and X-ray magnetic circular dichroism (XMCD) analysis.

XRD analysis was also performed for the La-substituted samples, Sr1-xLaxFe12O19 (0  x  0.4) and SrFe12-xLaxO19 (0  x  0.4), the patterns of which are shown in Fig. 2(a) and (b). A small amount of the Fe2O3 phase was detected in the XRD patterns of Sr1xLaxFe12O19 (x ¼ 0.1, 0.2, 0.3, 0.4), and an almost-pure M-type phase could be detected in the SrFe12-xLaxO19 samples. The continuous peak shift due to La substitution can be observed in the inset of Fig. 2(a). The calculated cell volume of these samples decreases gradually with x, as shown in Fig. 3(b). This tendency has been also reported in the Sr1-xLaxFe12O19 samples prepared by the sol-gel auto combustion process [16]. Meanwhile, in the SrFe12xLaxO19 (0  x  0.4) samples, the cell volume decreases for x ¼ 0.1 and then remains almost constant with the increase of x from 0.2 to 0.4 (Fig. 3(b)). Considering the relative ionic radius of La3þ(0.106 nm), Sr2þ (0.113 nm), and Fe3þ (0.064 nm), the initial decrease in cell volume for x ¼ 0.1 is possibly due to La substitution into the Sr site, although the formula SrFe12-xLaxO19 originally implies La substitution into the Fe site. Thus it is understandable that

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La can substitute into the Sr site only and it cannot substitute into the Fe site. Abnormal increases in the peak intensity at 2q ¼ ~31, which corresponds to the (008) plane of SrM, are observed for the samples with x ¼ 0.1. This can be attributed to issues with the preparation of the XRD powder samples, for which the 〈00l〉 direction of the plate-shaped hexaferrite grains are being perpendicular to the XRD specimen holder surface during the XRD powder sample packing. This is more likely to occur in samples exhibiting abnormal grain growth [17,18]. The effect of Ca and La substitution on the microstructure is revealed in the SEM micrographs in Figs. 4 and 5, respectively. Fig. 4(a) e (e) shows SEM micrographs of the Sr1-xCaxFe12O19 (0  x  0.4) samples sintered at 1300  C in air. The microstructure is porous at x ¼ 0.1, and then becomes denser with increasing x. Fe2O3 could have suppressed the migration of the grain boundary at x ¼ 0.1, and the number of large grains increased with increasing x above x ¼ 0.2, thereby contributing to the dense microstructure [17]. A denser microstructure with a larger grain size can be observed in samples with x  0.3. Fig. 4(f) e (j) shows the microstructures of the SrFe12-xCaxO19 (0  x  0.4) samples sintered at 1250  C (In this case, the samples with x  0.1 partially melted at the sintering temperature of 1300  C). When Ca was doped instead of Fe, a much denser microstructure was obtained in all the doped samples (x  0.1). For the SrFe12-xCaxO19 (0  x  0.4) samples, grain growth occurred more intensively with increasing x, without suppression of grain boundary migration by the formation of Fe2O3. The less content of Fe is more favorable for the grain growth during

353

sintering process of the hexaferrites [19,20]. Fig. 5 (a) e (d) shows SEM images of the Sr1-xLaxFe12O19 (0.1  x  0.4) samples sintered at 1300  C. Compared to the nondoped sample (x ¼ 0) (Fig. 4(a)), one can observe that the microstructure becomes somewhat porous by La doping for x ¼ 0.1 and 0.2, with smaller grain sizes. It then becomes denser at x ¼ 0.3 and x ¼ 0.4. The average grain size is not sensitive to the La doping amount (0.1  x  0.4). The microstructures of the SrFe12-xLaxO19 (0.1  x  0.4) samples presented in Fig. 5(e) e (h) are quite different from those in Fig. 5 (a) e (d), and show larger grain sizes. While the substitution of Sr by La with a fixed content of Fe in Sr1xLaxFe12O19 (0.1  x  0.4) does not give significant effect on the microstructural change, the less content of Fe than stoichiometry with addition of La in SrFe12-xLaxO19 (0.1  x  0.4) promotes grain growth and densification. This is possibly owing to the existence of excess Sr which has a character to promote grain growth during sintering process. Note that La cannot substitute into the Fe site, as already discussed in former part of this research. Abnormally grown very large grains (>100 mm) were formed in the sample with x ¼ 0.1 (Fig. 5(e)). Generally, grain growth mechanisms in the ceramic processing can be classified into two types, either normal or abnormal (or exaggerated) grain growth. Abnormal grain growth occurs by the formation of some exceptionally large grains when a few large grains grow faster than the fine grains [21]. The occurrence of the abnormal grain growth depends not only on sintering temperature but composition of the sample [17]. Generally, La doping into the M-type SrCaCo-hexaferrite are effective to obtain

Fig. 4. (a) - 4(e): SEM micrographs of the Sr1-xCaxFe12O19 (0  x  0.4) samples sintered at 1300  C and 4(f) - 4(j): those of SrFe12-xCaxO19 (0  x  0.4) sintered at 1250  C.

Fig. 5. (a)-5(e): SEM micrographs of the Sr1-xLaxFe12O19 (0.1  x  0.4) samples sintered at 1300  C and 5(f) - 5(j): those of SrFe12-xLaxO19 (0.1  x  0.4) sintered at 1300  C. Refer to Fig. 4(a) for micrograph of the x ¼ 0 sample sintered under the same condition.

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K.-S. Moon et al. / Journal of Alloys and Compounds 771 (2019) 350e355

fine microstructure. However, it is found that a limited amount of La (x  0.1) with a less Fe content than stoichiometry in the SrM causes intensive abnormal grain growth. A similar tendency of doping element effect on the microstructure has been reported in Nb-doped SrTiO3 [22] although not reported in the La-doped SrM. Except for the sample with x ¼ 0.1, wherein abnormal grain growth occurred, other samples with x ¼ 0.2, 0.3, and 0.4 ((Fig. 5(f) e (h)) show a similar microstructure. Fig. 6 (a) and (b) shows the M-H curves of the Ca-substituted SrM samples in Fig. 4(a) e (j). The MS and HC values of these samples are also presented in Table 1. MS was defined as the magnetization value at the applied magnetic field of H ¼ 50 kOe. The plots of MS vs. x of Ca-substituted hexaferrites are presented in Fig. 3 (c). When it is compared with the plots of cell volume variation with x in Fig. 3 (a), no clear relationship is shown between the two graphs. In the Sr1-xCaxFe12O19 samples, the MS value decreases once for x ¼ 0.1. However, it remains almost constant at ~70.5 emu/ g from x ¼ 0.2 to x ¼ 0.4. It is understandable that the replacement of Sr by Ca does not give significant effect on the MS change. The slight decrease of MS in the Ca-substituted samples (x  0.1) is due to the existence of unreacted Fe2O3 as shown in Fig. 1(a). The smaller HC at x ¼ 0.3 and x ¼ 0.4 is due to the large-grain growth, as shown in Fig. 4(d) and (e). For the SrFe12-xCaxO19 samples, MS decreases gradually from 72.2 emu/g to 66.7 emu/g with increasing x from 0 to 0.4. MS decreases significantly as the amount of nonmagnetic Ca substitution into the Fe site increases over x ¼ 0.3. HC does not vary significantly with x at 0.1  x  0.4, just like the microstructure of these samples seems similar in Fig. 4(g) e (j). The relation between grain size and HC in the hexaferrites has been

well-characterized in previous studies [23,24]. The M-H curves of La-substituted SrM are presented in Fig. 6(c) and (d), whose microstructures are shown in Figs. 4(a) and 5 (a) e (h). The MS and HC values of these samples are presented in Table 2. The plots of MS vs. x are presented in Fig. 3 (d). In the Sr1xLaxFe12O19 samples, MS gradually decreases with increasing x, whereas HC does not vary as much with x. The gradual decrease of MS is possibly due to the gradual substitution of La into Sr site as shown in Fig. 3 (b). This trend is different from that observed in the Ca-substituted SrM samples of Sr1-xCaxFe12O19, where the MS value is almost constant for 0.1  x  0.4. The decrease of MS in Sr1xLaxFe12O19 is probably due to the charge imbalance caused by the substitution of La3þ into the Fe3þ site of SrM, which causes the Fe ion oxidation state to change from Fe3þ to Fe2þ. In the SrFe12xLaxO19 samples, the highest MS of 73.1 emu/g and very small HC of ~159 Oe are obtained at x ¼ 0.1. This extraordinary magnetic behavior, deviating from the trends, both in MS and HC, is attributed to abnormal grain growth occurring in this composition. This result was reproducible in the x ¼ 0.1 samples as well. Because it is believable that La cannot be substituted into the hexaferrite matrix when x is over than 0.1 (see Fig. 3 (b)), the decrease of MS at x  0.2 is possibly attributed to the increase of non-magnetic second phase (La-rich) although the second phase is not detected in the XRD patterns of Fig. 2(b). 4. Summary Ca- and La-substituted SrM samples were prepared by conventional solid-state reaction processes. The cell parameters and

Fig. 6. M-H curves of the hexaferrites with varying substitution levels (0  x  0.4) in (a) Sr1-xCaxFe12O19, (b)SrFe12-xCaxO19, (c) SrFe12-xLaxO19, and (d) SrFe12-xLaxO19 samples. The calcination temperature is 1300  C for the samples in (a), (c), and (d), and 1250  C for the samples in (b).

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Table 2 Sample composition, cell parameters (a, c), cell volume, and MS and HC values of the La-substituted hexaferrites. Composition

x



Sr1-xLaxFe12O19 (1300 C)

SrFe12-xLaxO19 (1300  C)

0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4

Cell parameters

Magnetic properties

a

c

volume

MS (emu/g)

HC (Oe)

5.879 5.878 5.875 5.873 5.872 5.879 5.876 5.876 5.876 5.875

23.030 22.996 23.011 22.988 22.974 23.030 23.011 23.012 23.012 23.017

689.281 688.146 687.823 686.702 685.914 689.281 688.160 688.176 688.132 688.005

72.6 71.6 70.2 68.3 66.7 72.6 73.1 70.1 68.6 67.4

2575 2436 2501 2586 2733 2575 159 2169 2200 2703

cell volumes of the samples were calculated from the XRD patterns. The cell volumes of the Sr1-xCaxFe12O19 (x ¼ 0.1, 0.2, 0.3, 0.4) samples were lesser than that of non-substituted SrM (x ¼ 0), whereas those of the SrFe12-xCaxO19 (x ¼ 0.1, 0.2, 0.3, 0.4) samples were greater than that of non-substituted SrM (x ¼ 0). This implies that the Ca ions possibly substitute both Sr and Fe sites in the SrM structure. For the Sr1-xLaxFe12O19 samples, the cell volume gradually decreased with increasing x. On the other hand, for the SrFe12xLaxO19 samples, the cell volume first decreased at x ¼ 0.1 and then remained almost constant with increase of x from 0.2 to 0.4. These results indicate that La only substitutes into the Sr site in SrM. Sample densification with large-grain growth could be achieved by Ca substitution with a doping level of x  0.3 for Sr1-xCaxFe12O19 and x  0.1 for SrFe12-xCaxO19. La substitution into the Sr site (x  0.1) was effective in controlling the grain growth of the samples. MS decreased with increasing x in the SrFe12-xCaxO19, Sr1xLaxFe12O19, and SrFe12-xLaxO19 samples (x  0.4), whereas it was almost constant in the Sr1-xCaxFe12O19 samples (x  0.4). The HC values were more dependent on the microstructure than on the substitution level (x). Acknowledgment This work was supported by the 2017 Research Fund of Basic Science Research Program, through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT & Future Planning (2017R1C1B2002394). References

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