Magnetic powder for magnetic recording medium

Abstract

A magnetic powder for magnetic recording medium comprises acicular particles constituted primarily of Fe, wherein the powder contains Co in an amount such that the Co/Fe ratio is 50 at. % or less and the Co is contained in a manner such that the surface portion has a higher concentration than the core portion of the particles, and upon subjecting the magnetic powder for magnetic recording medium to TG measurement, the powder exhibits at least two oxidation starting points: a low-temperature side oxidation starting point and a high-temperature side oxidation starting point. The magnetic powder achieves improved resistance to oxidation without sacrificing magnetic characteristics.

1 . A magnetic powder for magnetic recording medium comprising acicular particles constituted primarily of Fe, wherein, upon subjecting the magnetic powder for magnetic recording medium to thermogravimetric (TG) measurement, the powder exhibits at least two oxidation starting points: a low-temperature side oxidation starting point and a high-temperature side oxidation starting point; here, TG measurement is the measurement of the weight change of a 10 mg sample of powder in air when the temperature is raised at a heating rate of 10° C./minute.  
     
     
         2 . The magnetic powder for magnetic recording medium according to  claim 1 , wherein the relationship between the low-temperature side oxidation starting point [called the apparent ignition temperature (IT (i) )] and the high-temperature side oxidation starting point [called the actual ignition temperature (IT (ii) )] is as follows:  
           IT   (ii)   −IT   (i) >10° C.  (1)  
     
     
         3 . The magnetic powder for magnetic recording medium according to  claim 1 , wherein the weight gain in the TG measurement from the start of measurement until 300° C. is 10 wt. % or greater.  
     
     
         4 . The magnetic powder for magnetic recording medium according to  claim 1 , wherein the drop in saturation magnetization Δσ s  when stored for one week under constant temperature and constant humidity at a temperature of 60° C. and humidity of 90% RH is less than 15%; provided that when the saturation magnetization prior to the storage under constant temperature and constant humidity is given as σ s(i)  and the saturation magnetization after one week of storage is given as σ s(ii) , Δσ s  is defined as:  
         Δσ s =100[σ s(i) −σ s(ii) ]/σ s(i) .  
     
     
         5 . The magnetic powder for magnetic recording medium according to  claim 1 , wherein the powder contains Co in an amount such that the Co/Fe ratio is 50 at. % or less and the Co is contained in a manner such that the surface portion of the particles has a higher concentration than the core portion of the particles.  
     
     
         6 . The magnetic powder for magnetic recording medium according to  claim 1 , wherein the average length of the major axis is 10-200 nm.  
     
     
         7 . Magnetic recording medium that uses the magnetic powder according to  claim 1  as magnetic powder constituting a magnetic layer.