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Phase formation, magnetic properties, and phase stability in reducing atmosphere of M-type strontium hexaferrite nanoparticles synthesized via a modified citrate process

Bohlender, Carmen, Kahnes, Marcel, Müller, Robert, Töpfer, Jörg
Journal of materials science 2019 v.54 no.2 pp. 1136-1146
ambient temperature, annealing, carbonates, citrates, ferrimagnetic materials, iron, maghemite, magnetic properties, magnetite, nanocomposites, nanoparticles, strontium, thermal degradation, thermal stability
Nanosize Sr-hexaferrite particles (SrM) were synthesized via a citrate-based sol–gel route, and the details of the calcination reaction conditions were investigated. Thermal decomposition of a citrate precursor proceeds in a two-step process: at low temperature T₁ the precursor decomposes into maghemite and Sr carbonate, and transforms into hexaferrite upon a second treatment at another temperature T₂. A synthesis protocol with T₁ = 350 °C and T₂ = 650 °C gives hexaferrite particles with size of below 100 nm. A systematic study of reaction conditions revealed that the formation of a hematite-free decomposition product at T₁ is the prerequisite for the synthesis of single-phase hexaferrite nanosize particles. The hexaferrite particles exhibit a saturation magnetization at room temperature of Mₛ = 58 emu/g with a coercivity of Hc = 3.7 kOe. Further fine-milling of the as-synthesized ferrite in aqueous media gives particles below 50 nm in size with Mₛ = 48–54 emu/g and Hc = 4.2–5.5 kOe under preservation of the M-type structure. The thermal stability of SrM particles under reducing conditions at moderate temperature was also studied. Annealing of ferrite particles in Ar/5%H₂ atmosphere at 350 °C results in magnetite formation; iron is formed at T ≥ 450 °C after complete hexaferrite decomposition; hence, SrM@Fe nanocomposites are not accessible via particle reduction of SrM particles.