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

Hexagonal SrFe12-xAlxO19 (x = 0, 0.2,1, 2, 4) powders were prepared via mechanochemical activation and subsequently calcined at different temperatures (between 900 degrees C and 1300 degrees C). Afterwards the powders were milled by high energy milling and annealed at 1000 degrees 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 Al3+ 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 Fe3+ by Al3+ 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 Fe3+ by Al3+ 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 SrFe12-xAlxO19 (x = 0, 1) by NaCl annealing treatment. Additionally, we discuss the initial magnetization curves of SrFe12O19 and SrFe8Al4O19 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 SrFe12-xAlxO19 for higher coercivity and could enable an improved industrial production process. (C) 2016 Elsevier B.V. All rights reserved.