Hexagonal ferrite magnetic powder, method for producing the same and magnetic recording medium

A method for producing a hexagonal ferrite magnetic powder having an average tabular diameter of from 15 to 30 nm, a coercive force (Hc) of from 2,000 to 5,000 Oe (from 160 to 400 kA/m) and a saturated magnetization (σs) of equal to or more than [the average tabular diameter (nm)×0.37+45] A·m2/kg, and the method comprising:

• melting a starting material containing a material that has a composition within a composition region surrounded by four points of a, b, c and d in a triangular phase diagram wherein each of AO, B2O3 and Fe2O3 constitutes an apex, and wherein A represents at least one selected from Ba, Sr, Ca and Pb, and a, b, c and d each represents:
• (a) B2O3=50, AO=40, Fe2O3=10 mol %;
• (b) B2O3=45, AO=45, Fe2O3=10 mol %;
• (c) B2O3=25, AO=25, Fe2O3=50 mol %;
• and
• (d) B2O3=30, AO=20, Fe2O3=50 mol %, so as to form a melted starting material, and quenching the melted starting material to obtain an amorphous product;
• and
• subjecting the amorphous product to a thermal treatment to precipitate a hexagonal ferrite.