Magnetically hard M-type hexaferrites (AFe₁₂O₁₉, A = Ba, Sr) are oxide solids which play the dominant role on the permanent-magnet market due to their high magnetocrystalline anisotropy, chemical stability and low cost.

Monodomain submicron hexaferrite particles have excellent hard magnetic properties. One of the most efficient methods of their synthesis is crystallization of oxide glass precursor, e.g. in the SrO-Fe₂O₃-B₂O₃ system. This technique allows wide variation in chemical composition and thermal treatment conditions, thus particle size and, consequently, magnetic properties of the material can be controlled. Also the improvement of hexaferrite properties can be achieved by substitution of iron ions by aluminum. The glass crystallization is very suitable way to obtain aluminum doped monodomain hexaferrite particles at relatively low temperatures.

This work is concerned with synthesis of fine strontium hexaferrite particles by crystallization of glasses in the SrO-Fe₂O₃-Al₂O₃-B₂O₃ system.

The glasses were prepared by rapid quenching of initial reagents melt (SrCO₃, Fe₂O₃, Al₂O₃, H₃BO₃) between two steel rollers. The glass-ceramic samples were formed during heat treatment of glasses at 650-950 °C and fine hexaferrite powders were derived by dissolving of glass ceramics. The materials obtained were characterized by x-ray diffraction, electron microscopy and magnetic measurements.

The glass-ceramic materials based on SrFe_12-xAl_xO₁₉ exhibit very high coercivity values up to 10.18 Oe. The substitution rate x in the record sample equals to 1.3. The hexaferrite powder obtained from this glass-ceramic sample has the saturation magnetization value of 49.6 emu/g and corresponding coercivity. Crystal structure of the powder was refined by Rietveld method and distribution of Al atoms on Fe sites was determined. Al atoms occupy 41% of 2a sites, 14% of 12k sites and 5% of 4e(1/2) sites, while 4f sites are not affected.

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