W-Type Hexaferrites Made in Seconds—An In Situ Powder Diffraction Study

The formation of W-type hexaferrite magnets was investigated using in situ synchrotron X-ray powder diffraction. Both the holding time and precursor composition were investigated to evaluate the effect on phase purity and crystallite size using Rietveld refinements. Based on the refinements, it was found that the formation of W-type hexaferrites initiates shortly after reaching 1200 °C and happens within a few seconds. M-type hexaferrite was formed at a lower temperature before being transformed into W-type hexaferrite. Despite the short holding times, at elevated temperatures, the crystallite sizes along the a,b-axis exceeded the detection limit of the powder diffraction data, as the peak widths associated with the a,b-planes became too narrow to resolve changes as function of time. Magnetization data recorded from the in situ prepared samples revealed a higher saturation magnetization in the W-type hexaferrites relative to conventional M-type hexaferrites. No appreciable coercivity was found, which can be attributed to different effects: (1) reduced anisotropy constant, (2) large crystallite growth resulting in multi-domain crystallites, or (3) exchange-coupling with soft spinel ferrite found in the sample. Based on this study, we conclude that W-type hexaferrites can be formed after a few seconds at 1200 °C and that crystallite growth happens subsequently after the formation of the W-type hexaferrite structure.