Conventionally, .gamma.-Fe.sub.2 O.sub.3, co-adhered .gamma.Fe.sub.2 O.sub.3 and the like have been used as magnetic powders. The sizes of the above-described recently manufactured powders have been made smaller and smaller to increase S/N. Besides the above-described powders, Ba-ferrite and metal powder have been used to effect high density recording.
In order to meet the requirements of high densified magnetic recording, a spread-coating type magnetic layer, in which magnetic powder is dispersed in a binder, was first used, and then, a thin metal magnetic layer, processed by means of evaporating or sputtoring magnetic material was put into use. Next instead of the conventional horizontal recording method as above-mentioned, a vertical recording method was adopted because this method enables the demagnetizing factor to approach zero.
In the above-described vertical recording method, a vertical head as well as magnetic material has been developed and magnetic layer formation method has also been made; that is, a Co-Cr magentic layer formation method was carried out at the initial stage, however, recently, vacuum deposition, metal plating, and spread-coating methods have been developed.
Hexagonal class ferrites, whose magnetic axes are perpondicular to crystal faces have been recently used as a material for spread-coating method. The above-described hexagonal ferrites are represented by Ba ferrite of the Co-Ti-substitution type.
The combined use of hexagonal ferrite and Co-.gamma.-Fe.sub.2 O.sub.3 was proposed, in Japanese patent publication Open to Public Inspection No. 212623/1982 (hereinafter referred to as Japanese patent O.P.I. publication), so that hexagonal ferrites may increase their reproduction output. Use of Ba ferrites mixed with metal powders selected from the iron group is also proposed to make output power go higher at low frequencies because simply using Ba ferrite is insufficient for improving reproduction output at low frequencies.
It is necessary for a magnetic recording medium to have preferable magnetic properties as well as preferable anti-static properties from the viewpoint of use and handling. It is also necessary for the magnetic recording medium equipped in an instrument to exhibit high performance in light shielding when subjected to light transmitted from a sensor.
It is preferable that the electrical surface resistance of a magnetic recording medium be not more than 10.sup.10 .OMEGA./cm.sup.2 and that the transmission of light be not more than 0.1% when light having a wave length of 900 nm is applied to a magnetic layer which is 4 .mu.m thick.
The above-described .gamma.-Fe.sub.2 O.sub.3, Co-adhered .gamma.Fe.sub.2 O.sub.3, Ba ferrite, and hexagonal ferrite have poor electrical conductivity and light shielding, and magnetic recording medium consisting of the above-described magnetic powders are susceptible to electrical charges, and their light transmission ratio is high. Magnetic powders recently manufactured is very fine, which increases the above-described characteristics. Commonly, carbon black is added to a magnetic recording medium as an anti-static and a light shielding agent which lowers light transmission ratio. Using a large amount of carbon black, however, causes reduction in the percentage of magnetic powders contained in a magnetic recording medium, thereby greatly reducing its reproduction output.
There is disclosed in Japanese patent O.P.I. publication No. 124023/124023 a magnetic layer containing iron carbide as its main substance.
A magnetic layer which contains large quantities of iron carbide exhibits preferable electrical conductivity and light shielding performance. Material for iron carbide is costly because iron carbide is manufactured by carbonizing acicular iron oxide and iron oxy hydroxide, and moreover, shape of iron carbide particle is not uniform. Accordingly, a magnetic poweder consists of soley iron carbide is costly and its S/N does not attain required Bm (maximum magnetic flux density) levels.
A magnetic layer using metal powder is also low in electrical conductivity because a dense oxide film is usually formed on its surface in order to increase its chemical stability, and in addition, the size of metal powder to be used as a material for a magnetic layer is, in most case, fine.
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