Spinel-type ferrimagnetic particles process for producing the same, and magnetic recording medium using the same

Abstract

Spinel-type ferrimagnetic particles having a composition represented by the formula: (MO). n /2(Fe 2 O 3 ) wherein M is a divalent metal; and n is a molar ratio of Fe to M (n=Fe/M) which is from more than 2.05 to less than 2.5 (2.05<n<2.5), containing a superparamagnetic component in an amount of not more than 2% by mass, and having an average particle diameter of 5 to 30 nm, said spinel-type ferrimagnetic particles being respectively coated on surface thereof with a hydroxide of at least one metal selected from the group consisting of Si, Al, P and Zn in an amount of not more than 10% by mass, calculated as the metal, as well as a magnetic recording medium containing the spinel-type ferrimagnetic particles. The spinel-type ferrimagnetic particles can exhibit a high coercive force irrespective of fine particles, and are excellent in dispersibility and chemical stability, as well as the magnetic recording medium for high-density recording can exhibit not only excellent frequency characteristics and high output characteristics, but also an excellent weather resistance and a high reliability.

1 . Spinel-type ferrimagnetic particles having a composition represented by the formula:  
         (MO). n/ 2(Fe 2 O 3 )  
       wherein M is a divalent metal; and n is a molar ratio of Fe to M, n=Fe/M, which is from more than 2.05 to less than 2.5, 2.05<n<2.5, 
 containing a superparamagnetic component in an amount of not more than 2% by mass, and having an average particle diameter of 5 to 30 nm,  
 said spinel-type ferrimagnetic particles being respectively coated on surface thereof with a hydroxide of at least one metal selected from the group consisting of Si, Al, P and Zn in an amount of not more than 10% by mass, calculated as the metal.  
 
     
     
         2 . Spinel-type ferrimagnetic particles according to  claim 1 , wherein said M is at least two metals selected from the group consisting of Co, Ni, Zn and Mn.  
     
     
         3 . Spinel-type ferrimagnetic particles according to  claim 1 , wherein said spinel-type ferrimagnetic particles have a coercive force of 159.2 to 318.4 kA/m, 2000 to 4000 Oe, and a saturation magnetization value σs of 40 to 70 Am 2 /kg, 40 to 70 emu/g.  
     
     
         4 . Spinel-type ferrimagnetic particles according to  claim 1 , wherein said spinel-type ferrimagnetic particles have a soluble Na content of not more than 100 ppm and a soluble Ca content of not more than 100 ppm.  
     
     
         5 . Spinel-type ferrimagnetic particles according to  claim 1 , wherein said spinel-type ferrimagnetic particles has a composition represented by the formula:  
         (CoO) x (NiO) y (MnO) z   ·n/ 2 (Fe 2 O 3 )  
       wherein n is a molar ratio of Fe to M, n=Fe/M, which is from more than 2.05 to less than 2.5, 2.05<n<2.5; x is a number of from more than 0.4 to less than 0.6, 0.4<x<0.6; y is a number of from more than 0.3 to less than 0.5, 0.3<y<0.5; and z is a number of from more than 0.02 to less than 0.2, 0.02<z<0.2, with the proviso that a sum of x, y and z is 1, x+Y+z=1.  
     
     
         6 . A process for producing the spinel-type ferrimagnetic particles as defined in  claim 1 , comprising the steps of: 
 a preparation step for a raw aqueous solution of mixing an aqueous iron (Fe)-containing solution and an aqueous divalent metal (M)-containing solution with each other such that a molar ratio of Fe to M is in the range of from more than 2.05 to less than 2.5, 2.05<Fe/M<2.5;    a production step for a co-precipitate-containing solution of adding an aqueous alkali solution to the raw aqueous solution at a temperature of from 60° C. to less than 110° C. so as to adjust a concentration of alkali in the raw aqueous solution to 0.05 to 10 M;    a production step for black particles of heat-treating the co-precipitate-containing solution at a temperature of 80 to 120° C.;    an etching step for the black particles of washing the black particles to remove the alkali therefrom, and adding an acid thereto to remove a superparamagnetic component therefrom; and    a coating step for a hydroxide of the metal of subjecting the resultant solution containing the etched black particles to filtration and washing, and then adding an aqueous solution containing at least one metal selected from the group consisting of Si, Al, P and Zn to the resultant solution containing the washed black particles.    
     
     
         7 . A magnetic recording medium comprising a non-magnetic substrate and a magnetic layer formed on the non-magnetic substrate, said magnetic layer comprising the spinel-type ferrimagnetic particles as defined in any of  claims 1  to  5 , and a binder.