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

Abstract

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·m 2 /kg. This magnetic powder is obtained by melting a starting material containing a material which has a composition within the hatched region (1) in the triangular phase diagram shown in FIG. 1 and quenching the molten product to obtain an amorphous product, subjecting the amorphous product to a thermal treatment, acid treatment, and washing. Also, a magnetic recording medium is obtained by adding this magnetic powder to the magnetic layer and coating it on the support.

1 . (canceled)  
     
     
         2 . 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·m 2 /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, B 2 O 3  and Fe 2 O 3  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) B 2 O 3 =50, AO=40, Fe 2 O 3 =10 mol %;    (b) B 2 O 3 =45, AO=45, Fe 2 O 3 =10 mol %;    (c) B 2 O 3 =25, AO=25, Fe 2 O 3 =50 mol %; and    (d) B 2 O 3 =30, AO=20, Fe 2 O 3 =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.    
     
     
         3 . (canceled)  
     
     
         4 . (canceled)