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

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.