X-ray absorption and Mossbauer spectra, and microwave absorption of (Co, Mn)-doped SrFe12O19 hexaferrites

Polycrystalline SrCo1.2Fe10.8O19 (SCFO) and SrMn1.2Fe10.8O19 (SMFO) samples were prepared by the normal ceramic technique. Powder X-ray diffraction patterns indicated the crystallization in the M-type hexaferrite structure of the samples. X-ray absorption analyses proved Co2+ and Mn-3+,Mn-4+ ions existing in SCFO and SMFO, respectively. Though Fe3+ is dominant in both the compounds, small amounts of Fe2+ and Fe4+ are present in SCFO and SMFO, respectively. Mossbauer spectra analyses also confirmed the dominance of Fe3+ in both the samples. Additionally, Fe3+ ions in the 12 k, 4f(1), 4f(2), and 2a sites could be attributed to the high-spin state (S = 5/2). A large QS value for the sextet assigned to the 2b site indicated that in this case Fe3+ could be in the lowspin state (S = 1/2) or there was the addition of Fe2+ (or Fe4+) when Fe in SrFe12O19 was partially replaced by Co2+ (or Mn-3+,Mn-4+). The mixture of transition-metal ions with different oxidation states influenced the magnetic properties and microwave shielding ability. At frequencies f = 10-18 GHz, we have found SCFO showing the lowest reflection loss with RL = 11.1 dB at 13.2 GHz and the thickness t = 2.2 mm. With an absorption bandwidth of 0.7 GHz, and about 92% microwave energy being absorbed, SCFO is considered as a potential candidate for microwave shielding applications.