Hexagonal ferrites of X-, W-, and M-type in the system Sr–Fe–O: A comparative study

• Synthesis and detailed characterization of hexagonal X-type ferrite Sr2Fe22+Fe283+O46.
• XRD and HRTEM confirm structural unit with 83.81 Å cell length in the c-direction.
• Large magnetization in agreement with Gorter model and Fe2+ on spin-up sites.
• Mössbauer and 57Fe NMR spectroscopies reveal location of Fe2+ on spin-up sublattice.
• Comparison of structure and properties with M- and W-type hexagonal Sr ferrites.

Three hexagonal ferrites from the Fe-rich part of the pseudo-binary system SrO–Fe2O3 were investigated. Besides the well-known M-type SrFe12O19 two other ferrites were found to exist at high temperatures: W-type SrFe22+Fe163+O27 and X-type Sr2Fe22+Fe283+O46 ferrites. A detailed characterization of the X-type Sr-ferrite is reported here for the first time using XRD, HRTEM, magnetization measurements, Mössbauer and 57Fe NMR spectroscopies and ab initio calculations of the electronic structure. The results are compared to those of W- and M-type Sr ferrites. Mössbauer spectra were analyzed with six Fe contributions in the case of the X-type, seven Fe-sites for the W-type and five Fe sites for the M-type in agreement with crystal structure arrangements. Based on a detailed analysis of the NMR spectra in comparison with ab initio calculations the NMR lines were assigned to individual crystal sites. A preferential occupation of ferrous ions in the S blocks of the X- and W-type ferrites was elucidated from Mössbauer and NMR data as well as ab initio calculations.

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