Structural, morphological, optical, cation distribution and Mössbauer analysis of Bi3+ substituted strontium hexaferrite
Introduction
Microcrystalline ceramic magnets or ferrite magnets form the basis of materials currently used for magnetic information recording and storage. To increase the recorded information density, it seems reasonable to obtain nanocrystalline ferrite and to prepare magnetic carriers on them [1]. Hexaferrite with magnetoplumbite structure can be divided into six categories: M-type (AFe12O19), W-type (AB2Fe16O27), X-type (A2B2Fe20O46), Y-type (A2B2Fe12O22), Z-type (A3B2Fe24O41) and U-type (A4B2Fe36O60), where A may be Ba, Sr, or Pb and B may be bivalent cations of transition metal such as Ni, Co, Zn [2]. M-type hexaferrites gained more attention of researchers recently, this is obviously due to their high saturation magnetization, large magnetocrystalline anisotropy, tunable coercivity, high resistivity, chemical stability and corrosion resistance [3], [4]. Due to development of new signal processing devices, radars, microwave darkrooms, anti-electromagnetic interference coating, etc, above properties are very essential tools in magnetic recording devices, telecommunication and fabrication of multilayer chip [5]. In recent years, there is focused attention on microwave absorbers for which M-type hexaferrite have become suitable candidates as they operate in GHz frequency range [6]. Strontium hexaferrite demonstrates better magnetic properties then its barium counterpart [7].
A typical crystalline structure of M-type hexaferrites unit cell is made of two molecular units with the sum of 64 ions, which crystallizes in P63/mmc space group. In each molecular unit, there is cubical S and hexagonal R blocks in the form SRS*R* over lapping one another. Two out of 64 ions are either cations of Ba2+, Sr2+ or Pb2+, 38 are O2− anions and 24 are Fe3+ disposed over five symmetry sites: 3 octahedral (12k, 2a and 4f2), 1 tetrahedral (4f1) and 1 bipyramidal (2b) [8]. Because it is possible to replace Fe or Sr cations with another of similar ionic radii, researchers are trying to improve the intrinsic magnetic properties of SrM by doping with variety of cations such as Ca2+, Pb2+, La3+, Ni2+-Pr3+, Zn2+, Co2+, Mn2+, Cu2+, Sm3+, Bi3+, La3+ etc. [9], [10], [11], [12], [14].
Mössbauer spectroscopy, X-ray absorption fine spectroscopy (XAFS) and X-ray photon electron spectroscopy (XPS) are examples of characterization techniques used to obtain information about site preference of doped cation in the five Fe3+ crystallographic sub lattice and how this relate to magnetic property of a particular sample. Mössbauer analysis was employed by Chawla et al. in their study of doped Co2+ and Zn2+ in Fe3+ site of SrM, they found that the concentration of dopant ions controls site preferences in the crystal lattice and Hc was greatly decrease from 6082 Oe to 1104 Oe while Ms value was not largely influenced by the dopants [13]. Thakur et al. substituted La3+ in SrM and found that saturation magnetization for La3+ doped ferrite decreased with La3+ content, reduction in super exchange interaction and abundance of Fe2+ ions (spin canted structure) are liable for same. While coercivity on the other hand has been improved by substitution of La3+, and a qualitative explanation for this enhancement was given on the basis of strong magnetocrystalline anisotropy effect of Fe at 2a site [10].
Bi3+ incorporation in ferrite is poorly studied. Ram [14] however, in his article applied conventional solid state route and synthesized BaBixFe12−xO19 with x≤0.35. The ultimate Ms value obtained for pure and doped samples were 14.50 and 12.72 μB respectively. The decrease was due to total dissolution of Bi ion in solid solution of BaM. The ionic size of Bi3+ (0.69 Å) is reasonably large and expectedly would prefer Fe3+ octahedral site (0.65 Å) then tetrahedral (0.49 Å). The dielectric permittivity (ε) was enhanced as much as 105 by doped sample which was believed to be due to extended wave function of Bi3+ mixed with Fe3+ and ligand field resulting in Bi3+–Fe3+ couple pairs [15]. Zhang et al. [16] stated that the substitution of Bi3+ on Ba2+ in Y-type and Z-type hexagonal ferrite was also used to promote the low-firing character successfully, and excellent magnetic properties were obtained [17] (Bi substitution remarkably improves the soft magnetic properties in very high frequency (VHF)) and also improve planar anisotropy [18]. Moreover, Bi substitution promotes the liquid-phase mass transfer in sintering process [19].
In this study, we present Mössbauer analysis, cation distribution and optical properties of Bi3+ substituted strontium hexaferrites (SrBixFe12−xO19 (0.0≤x≤1.0) synthesized by co-precipitation assisted ceramic method.
Section snippets
Chemicals and instrumentation
The starting materials used for the syntheses are analytical grade purchased from mark and were applied directly without further purification, they include: strontium nitrate anhydrous (Sr(NO3)2), iron (III) nitrate nonahydrate (Fe(NO3)3·9H2O), bismuth oxide (Bi2O3), tetramethylammonium hydroxide solution 25.5% (TIMAH) (C4H13 NO), nitric acid solution 65% (HNO3) and ammonia solution 25% (NH3).
The phase compositions of the samples were determined using an X-ray diffractometer (XRD, DX-2700,
XRD analysis
Phase analysis of as synthesized powder was carried out using X-ray diffraction powder and presented in Fig. 1. All the peaks obtained are in crystalline phase. The peaks of the XRD patterns with the hkl values of (006), (110), (008), (107), (114), (108), (203), (205), (206), (209), (300), (217), (2011), (220) and (2014) presented in Fig. 1, were successfully indexed with the hexagonal structure of M-type SrFe12O19 hexaferrite (ICDD file no. 39-1433) belonging to the P63/mmc space group. The
Conclusion
Bismuth substituted strontium ferrites have been prepared using co-precipitation method. All the ferrites are found to have the magnetoplumbite structure. FT-IR measurements showed that Sr–O and Fe–O bonds formed in the nanoparticles. Mössbauer spectrometry showed corresponding changes in the spectra when Bi3+ ions entered the lattice of M-type strontium ferrite and indicated their positions in the magnetic structure of Sr–hexaferrite. According to the Mössbauer results, the average hyperfine
Acknowledgments
This study was supported by Fatih University Research Office with the Project number: P50031504_B (10790).
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