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The response of the structure of the M-type barium hexaferrite (BaFe12O19) to mechanical action through high-energy milling and its impact on the magnetic behaviour of the ferrite are investigated. Due to the ability of the 57Fe Mössbauer spectroscopic technique to probe the environment of the Fe nuclei, a valuable insight on a local atomic scale i...

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... 3,4 Although there is a surge of investigations in the  eld of mechanochemistry of oxides, studies on the mechanically induced response of complex oxides possessing more than two cation sublattices, such as M-type barium hexaferrite, BaFe 12 O 19 (with  ve cation sublattices), are very scarce in the literature. 5,6 As a result of its speci  c magnetic properties, BaFe 12 O 19 is widely used in permanent magnets, magnetic recording media and microwave applications. Its derivatives are currently magnetic materials with great scienti  c and technological interests due to their relatively high Curie temperature, high coercive force and high magnetic anisotropy  eld as well as their excellent chemical stability and corro- sion resistivity. Recently, multiferroic properties have been reported for BaFe 12 O 19 ceramics. 7 In this article, for the  rst time, detailed information is obtained on the response of the local (short-range) structure of BaFe 12 O 19 to mechanical action through high-energy milling. In addition to 57 Fe Mössbauer spectroscopy, the evolution of the mechanically induced structural disorder as well as the morphology and macroscopic magnetic behaviour of the ferrite were monitored with comprehensive techniques including X-ray di ff raction (XRD), high-resolution transmission electron microscopy (TEM), and superconducting quantum interference device (SQUID) magnetometry. M-type BaFe 12 O 19 , which was used as the starting material for subsequent mechanochemical treatment, was synthesized using a chemically reliable co-precipitation route. The precursor materials of barium and iron chlorides (99.9% purity; Merck, Darmstadt, Germany) were dissolved in deionized water, and subsequently solutions of Na 2 CO 3 and NaOH were added to achieve pH 1⁄4 6. The precipitated precursor was sintered at 1173 K to obtain bulk BaFe 12 O 19 (which is further referred to as the non-treated material). Fig. 1 shows the morphology of the as-prepared bulk BaFe 12 O 19 powder, which served as a reference sample in the present study. The particles of the co-precipitated ferrite were found to be uniform in shape; the majority of them are hexagonal platelet crystals. Whereas the thickness of the ferrite platelets is in the 20 – 100 nm range, their length ranges from 300 to 400 nm. The corresponding selected area electron di ff raction (SAED) pattern of the ferrite (Fig. 1b) is dominated by the discrete di ff raction spots originating from the well crystalline hexagonal platelets. 10 grams of the bulk material were mechanically treated for various times t m (up to 8 h) in a high-energy planetary mill Pulverisette 6 (Fritsch, Idar-Oberstein, Germany) at room temperature. A grinding chamber (250 cm 3 in volume) and balls (10 mm in diameter) made of tungsten carbide were used. The ball-to- powder weight ratio was 20 : 1. The milling experiments were performed in air at 600 rpm. The XRD patterns were measured using a PW 1820 X-ray di ff ractometer (Phi- lips, Netherlands), operating in Bragg con  guration and using Cu K a radiation ( l 1⁄4 1.54056 A). ̊ The XRD scans were collected from 10 to 80 (2 Q ), using a step of 0.02 and a data collection time of 5 s. The JCPDS PDF database 9 was utilized for phase identi  cation using the STOE so  ware. The hexagonal structure of BaFe 12 O 19 was visualized using the Diamond program 10 and Java Structure Viewer so  ware. 11 The 57 Fe Mössbauer spectra were taken at 293 K in transmission geometry using a 57 Co/Rh g -ray source. Recoil spectral analysis so  ware 12 was used for the quantitative evaluation of the Mössbauer spectra. The velocity scale of the spectra was calibrated relative to 57 Fe in Rh. The morphology of the powders was studied using a combined  eld-emission (scanning) transmission electron microscope (S)TEM (JEOL JEM-2100F) with a high-resolution pole piece that provides a point resolution better than 0.19 nm at 200 kV. Prior to the TEM investigations, the powders were crushed in a mortar, dispersed in ethanol, and  xed on a copper-supported carbon grid. The magnetic measurements were performed using a SQUID magnetometer (Quantum Design MPMS-5S, USA). The samples were  lled in a small container made of polyvinyl chloride, whose diamagnetic moment was subtracted from the measured magnetization values. Magnetic hysteresis loops were recorded at 5 K in external magnetic  elds from 0 to Æ 5 T. Prior to any characterization of the structural disorder and functional properties of the mechanically treated BaFe 12 O 19 , the atomic con  guration of the non- treated bulk material has to be known. An e ff ective way to do this is by means of nuclear spectroscopic techniques such as 57 Fe Mössbauer spectroscopy, which makes possible observations on a local atomic scale. This spectroscopic method has been proven to be well suited for the investigation of the charge state, the local coordination, and the magnetic state of iron ions in various ferrites. 13 Fig. 2 illustrates the room temperature 57 Fe Mössbauer spectrum of the BaFe 12 O 19 standard sample. As can be seen, the spectrum of the material is well-  tted by a superposition of  ve magnetically split spectral components (sextets) indicating the presence of  ve di ff erent atomic environments around the iron nuclei. Three sextets with isomer shi  IS > 0.19 mm s À 1 (see Table 1) correspond to octahedrally coordinated ferric (Fe 3+ ) ions in the 4f 2 , 12k and 2a crystal sites of the hexagonal structure of the ferrite. 14 The spectral component with the lowest value of IS ( $ 0.14 mm s À 1 ) is typical for Fe 3+ cations in tetrahedral (4f 1 ) coordination of oxygen anions. 15 The sextet with a relatively large quadrupole splitting (QS $ 1.1 mm/s), indicating the presence of a large electric  eld gradient acting on the iron nuclei, corresponds to Fe 3+ ions in the trigonal bi-pyramidal (2b) sites of BaFe 12 O 19 . The Mössbauer parameters resulting from the least-squares  tting of the spectrum of bulk BaFe 12 O 19 (Table 1) are in reasonable agreement with those determined in previous work. 14 From the relative intensities of the sextets, the number of Fe 3+ cations located on the 4f 2 , 12k, 2a, 4f 1 and 2b sublattices was calculated to be 2, 6, 1, 2 and 1 per formula unit (f.u.) of BaFe 12 O 19 , respectively. Based on the present Mössbauer results, the structural formula of BaFe 12 O 19 , emphasizing the site occupancy at the atomic level, may be written as Ba [Fe 2 ] 4f2 [Fe 6 ] 12k [Fe] 2a (Fe 2 ) 4f1 {Fe} 2b O 19 , where the square brackets, parentheses and curly brackets enclose cations in sites of octahedral, tetrahedral and trigonal bi- pyramidal coordination, respectively. The hexagonal structure of BaFe 12 O 19 with the  ve di ff erent Fe nearest-neighbour con  gurations is shown in Fig. 3. The mechanically induced evolution of BaFe 12 O 19 was followed by XRD. Fig. 4 shows the XRD patterns of the ferrite milled for various times. The XRD pattern of the starting powder is characterized by sharp di ff raction peaks corresponding to BaFe 12 O 19 with the magnetoplumbite structure and space group P 6 3 / mmc (JCPDS PDF 27-1029). 9 With increasing milling time, XRD reveals a gradual decrease in the intensity and an associated broadening of the Bragg peaks of the oxide. This re  ects a continuous fragmentation of the material accompanied by the re  nement of its crystallite size ( D ) to the nanometer range; with the prolongation of t m , a monotonous reduction of the average crystallite size of the ferrite to D 1⁄4 14 nm (for t m 1⁄4 8 h) is observed, see Fig. 4. Simultaneously, the high-energy milling process leads to the formation of a broad di ff raction maximum in the range of about 30 – 40 (2 Q ) indicating a partly amorphization of the structure. The superimposition of the relatively broad di ff raction re  ections of the BaFe 12 O 19 phase on a broad di ff raction maximum in the range of about 30 – 40 (2 Q ) re  ects a typical morphology of the mechanochemically prepared nanostructured oxides 3 consisting of small crystalline regions (o  en called nanograins or nano- crystallites) surrounded/separated by structurally disordered internal interfaces (grain boundaries) and/or external surfaces (near-surface layers); the detailed TEM analysis of the mechanochemically prepared BaFe 12 O 19 nanoparticles is given below. Note that the atomic arrangement in internal interfaces/external surfaces of the mechanically treated materials may lack any long- or short-range order. 3,4 Because of the sensitivity to medium- and long-range structural order, the applied XRD technique loses much of its resolving power in such nanoscale and disordered systems. Therefore, the nature of the mechanically induced structural disorder in BaFe 12 O 19 will be analyzed concurrently with the discussion of Mössbauer data (see next paragraph). To determine the nature of the mechanically induced structural disorder in BaFe 12 O 19 , the evolution of its structure on the local atomic scale was followed by 57 Fe Mössbauer spectroscopy. The room temperature 57 Fe Mössbauer spectra of the material milled for various times are presented in Fig. 5. As can be seen, with increasing t m , the sextets corresponding to the Fe 3+ ions located on the  ve sublattices of the hexaferrite become asymmetric toward the inside of each line, slowly collapse, and are gradually replaced by a broad central doublet with the isomer shi  of about 0.21 mm s À 1 characteristic of Fe 3+ ions. It should be mentioned in this context that the central doublet is clearly visible a  er only 30 min of milling (see Fig. 5). Further milling leads to a gradual increase of its relative intensity. A  er 8 h of mechanical treatment, the sextets disappear completely and the Mössbauer spectrum of the milled BaFe 12 O 19 is dominated by the doublet. The relatively broad shape of the Mössbauer spectral lines for milled BaFe 12 O 19 , in contrast ...

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... Bereitgestellt von | KIT-Bibliothek Angemeldet Heruntergeladen am | 06.05.17 12:15 allowed for a systematic investigation of structural disorder of mechanosynthesized oxide nanoparticles at the atomic level [68,69,71,72,[74][75][76][77]. For example, the comparative 119 Sn MAS NMR and Raman spectroscopic studies of bulk and nanocrystalline Zn 2 SnO 4 enabled us to separate surface effects from bulk effects of the nanoparticles [68]. ...
... The far-from-equilibrium structural state of the mechanically prepared oxide phases has significant implications for their functional properties. This is exemplarily demonstrated in Figure 20, which compares the magnetization hysteresis loops measured at 5 K for bulk and mechanochemically prepared BaFe 12 O 19 with various crystallite sizes (D) [72]. It is found that the saturation magnetization of BaFe 12 O 19 decreases with decreasing crystallite size from ca. 69 emu g −1 (for bulk BaFe 12 O 19 with D = 220 nm) to ca. 30 emu g −1 (for nanoferrite with D = 14 nm). ...
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