Influence of charge disproportionation on microwave characteristics of Zn–Nd substituted Sr-hexaferrites

Sr(Zn,Nd)_xFe_12−xO₁₉ (0.0 ≤ x ≤ 1.0) nanohexaferrites (NHFs) were obtained using citrate sol–gel approach. XRD and FE-SEM analysis were performed to confirm the formation of Sr M-type hexaferrite. Main electrodynamics characteristics (frequency dependences of real and imaginary parts of the permittivity and permeability) were measured in the range 5–18 GHz (region of the natural ferromagnetic resonance). Permittivity and permeability data correlates well. All samples were characterized by peak on the frequency dependences due to electrical and magnetic losses respectively. However, for compounds with x = 0.1 and 0.9 we observed second peak (at 14–15 GHz). This was explained by double oxidation state for iron ions (Fe³⁺/Fe⁴⁺). Presence of the Fe⁴⁺ ions was due to heterovalent Zn²⁺ substitution. Absence of the second peak for compounds x = 0.3; 0.5 and 0.7 was explained by charge disproportionation (according to the following scheme: 2Fe⁴⁺ = Fe³⁺ + Fe⁵⁺) under the influence of the crystal field energy. It was demonstrated that modification of crystal and magnetic structures by heterovalent substitution leads to transformation the microwave characteristics (reflection losses).