Low oxygen potential synthesis of W-type strontium ferrite SrFe₁₈O₂₇ in a closed atmosphere

Abstract

W-type strontium ferrite SrFe₁₈O₂₇ (SrW) has the largest room-temperature (RT) magnetization among iron-based non-substituted hexagonal ferrites and is a candidate for high-magnetization materials. However, the expected properties have not been achieved because, in most cases, the synthesized samples contain secondary phases or have non-stoichiometric compositions, indicating that the phase stability of SrW is not well understood. Single-phase SrW is known to be stable only at low oxygen potentials, but the stable temperature range remains unknown. In this study, we applied the sealed-tube technique to investigate the phase stability and decomposition reaction of SrW and found that SrW was formed as the predominant phase (the ratio of 97–98 wt %) when synthesized in a closed low oxygen potential atmosphere at temperatures only above 1448 K, below which M-type ferrite (SrFe₁₂O₁₉) and magnetite (Fe₃O₄) were formed instead. In contrast, once formed, SrW does not decompose by post-annealing at temperatures below the phase boundary temperature for a sufficiently long time, indicating that the decomposition reaction of SrW as a metastable phase is slow enough to ensure the availability of relatively pure SrW at RT using the sealed-tube technique.