This invention relates to barium ferrite magnetic powder, and more specifically to barium ferrite magnetic powder having fine particle sizes with narrow particle size distribution and an effectiveness for high-density recording, and also to a recording medium employing the same.
Magnetic recording media which have hitherto been used in video recording, digital recording, etc. were those obtained by coating acicular magnetic particles such as .gamma.-Fe.sub.2 O.sub.3, CrO.sub.2 or the like on a substrate and then orienting the same. Such a process requires that the particle size of magnetic particles be significantly smaller than a minimum recording unit in order to obtain a sufficiently high S/N ratio. In the case of the presently available video recording for example, acicular magnetic powder having a length of about 0.3 .mu.m is used for the shortest recording wavelength of about 1 .mu.m. It is desired in recent years to improve the recording density further. Reflecting such a demand, it is strongly desired to obtain magnetic powder having a still finer particle size than the currently available acicular magnetic powder.
Magnetic powder having a uniaxial easy axis of magnetization is preferred as magnetic powder for magnetic recording. Namely, in current recording, uniaxial anisotropy has been imparted to the magnetic recording layer and signals are recorded in the direction of the easy axis of magnetization.
For the reasons mentioned above, recording media are generally used which have individually been obtained by dispersing and orienting magnetic powder having a uniaxial easy axis of magnetization in such a way that the recording direction becomes parallel to the direction of the easy axis.
As such magnetic powder, hexagonal ferrites led by barium ferrite are attracting attention in place of such acicular magnetic powde as .gamma.-Fe.sub.2 O.sub.3 and CrO.sub.2. These hexagonal ferrites may be considered as magnetic powder inherently suitable for high-density recording, because each of the hexagonal ferrites has a hexagonal, plate-like shape with a uniaxial easy axis of magnetization extending in a direction perpendicular to the plane of the plate-like shape and hence permits the perpendicular magnetization and recording.
As a method for production of such magnetic barium ferrite powder, the glass crystallization method is known.
This method comprises melting the oxides and/or carbonates of various elements required for an intended barium ferrite together with a glass-forming material such as boric acid for example, quenching the resulting melt to form an oxide glass, subjecting the oxide glass to a heat treatment at predetermined temperatures to allow powdery crystals of the intended barium ferrite to precipitate and finally removing the glass component in an acid bath (as disclosed in U.S. Pat. No. 4,341,648).
In general, when producing barrium ferrite, BaCO.sub.3, BaCl.sub.2 and the like are used as barium (Ba) sources. Strontium (Sr) and calcium (Ca), which belong to the same Group as Ba, are contained in such Ba sources although their concentrations are low. In usual cases, these elements are contained in amounts of 2 to 5 wt.% as SrO and CaO, based on the principal component BaO, unless they are removed intentionally. When such a material whose SrO and CaO contents are not controlled is used to produce the barrium ferrite, Sr and Ca are inevitably contained in it in amounts of about 0.3 to about 0.7 wt.% as SrO and CaO, respectively.
There has been no report that inclusion of such inevitable elements will impair the magnetic characteristics of the barrium ferrite, so long as the particle sizes of the barium ferrite magnetic powder are on such an order of micrometers as used in a usual barrium ferrite sintered body or in a low-density recording. However, as a result of intensive studies by the present inventors, it was found that when the barrium ferrite powder is used in the mean particle size of 0.3 .mu.m or less for the purpose of high-density recording, the Sr and Ca components contained as impurities extremely impair the characteristics of the barrium ferrite.
Furthermore, silicon dioxide (SiO.sub.2) is generally caused to mix in the barrium ferrite as an impurity from a starting Fe.sub.2 O.sub.3 material or as an impurity from a melting crucible to be used when melting raw materials. This SiO.sub.2 is a component generally capable of increasing the sintered density in a usual sintered body. In the case of magnetic powder on the other hand, an inclusion of SiO.sub.2 in an amount of 0.5 to 1 wt.% or so makes crystalline particles smaller and hence increases the coercive force, and thus it is used positively as a useful additive.
However, where the mean particle sizes of magnetic powder are 0.3 .mu.m or smaller, which is required to render the magnetic powder useful for high-density magnetic recording, an inclusion of SiO.sub.2 will, different from the case of sintered bodies, make the particle sizes of crystals of the magnetic powder coarser and at the same time, will result in a reduction to the saturation magnetization even if its content is of the impurity level.