Enhancement of coercivity and saturation magnetization of Al3+ substituted M-type Sr-hexaferrites

Abstract Hexagonal SrFe 12-x Al x O 19 (x = 0, 0.2, 1, 2, 4) powders were prepared via mechanochemical activation and subsequently calcined at different temperatures (between 900 °C and 1300 °C). Afterwards the powders were milled by high energy milling and annealed at 1000 °C in NaCl to obtain ultrafine nano-particles. The particle size, measured by scanning electron microscopy (SEM), varied between 50 nm and few micrometers depending on Al 3+ substitution and calcination temperature. Average crystallite size, determined by X-ray diffraction (XRD), decreases from 330 nm ± 30 nm (x = 0) to 70 nm ± 10 nm (x = 4) by substitution of Fe 3+ by Al 3+ for optimized calcination temperature of 1100 °C. Furthermore coercivity measured by SQUID magnetometry increases from 420 kA/m (x = 0) to 970 kA/m (x = 4). A maximum saturation magnetization of 74 Am 2 /kg (x = 0) was observed. With substitution of Fe 3+ by Al 3+ saturation magnetization decreases monotonously to 28 Am 2 /kg (x = 4). Annealing in NaCl matrix at lower temperatures compared to calcination temperatures leads to a further increase of coercivity. At the same time saturation magnetization increases for SrFe 12-x Al x O 19 (x = 0, 1) by NaCl annealing treatment. Additionally, we discuss the initial magnetization curves of SrFe 12 O 19 and SrFe 8 Al 4 O 19 after different processing steps with respect to the specific reversal mechanism of hexaferrites. The here proposed processing route enables a simultaneous enhancement of coercivity and saturation magnetization as compared to conventional ceramic method [1] , [2] . The presented processing route can solve challenges of conventional manufacturing steps towards single domain grains in rare earth free SrFe 12-x Al x O 19 for higher coercivity and could enable an improved industrial production process.