1. Field of the Invention
This invention relates to a method of making a magnetic recording medium by an ion plating technique, more particularly, to a method of making a magnetic recording medium having excellent magnetic properties, especially excellent coercive force.
2. Description of the Prior Art
It is known that conventional .gamma.-Fe.sub.2 O.sub.3, Co-doped .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4 and CrO.sub.2 powdery ferromagnetic alloys can be dispersed into an organic binder to form a coating-layer type magnetic recording medium. However, ferromagnetic metal thin films produced by electroplating, electroless plating, sputtering, vacuum deposition or ion plating have attracted special interest as non-binder type magnetic recording media, i.e., media that do not use a binder.
As one condition required for magnetic recording media for high density recording, it has been proposed to increase the coercive force of the magnetic layer by making the magnetic layer very thin. Therefore, non-binder type magnetic recording media have been sought by the art since the thickness thereof can easily be made ten times thinner than that of binder type magnetic recording media and they have a high saturated magnetic flux density.
Ion plating is a technique which was proposed by D. M. Mattox (see Japanese Patent Publication 8328/69 and U.S. Pat. No. 3,329,601). In ion plating, both a substrate and an evaporation source are disposed in an inert gas atmosphere kept at 10.sup..sup.-3 - 10.sup..sup.-1 Torr, and a DC voltage of 0.1 - 7.0 KV is then applied therebetween, the substrate and the evaporation source being connected to the negative and positive terminals, respectively, to thereby generate a glow discharge. The inert gas ions in the glow discharge impinge upon the substrate surface to clean it. After the substrate surface is cleaned, the evaporation source is heated to evaporate the metal to be deposited on the support, and the evaporated atoms are ionized in the plasma generated by the glow discharge. The ions are accelated by a strong electric field in the dark space area of the glow discharge surrounding the substrate, and have high energy and are deposited on the substrate.
In ion plating, a considerable amount of evaporation atoms are ionized and reach the substrate surface at high kinetic energy (several tens to several KeV) and both adhere to the substrate surface and cause etching due to the bombardment of the vaporized ions during the production of the thin film, whereby strong ferromagnetic metal thin films of high adhesion are obtained.
Such thin films are suitable for magnetic recording media which are employed under severe conditions, i.e., at high relative motion against a magnetic head.
In addition, the fabrication of thin films by means of ion plating can be adapted to mass-production because the production rate of the thin films is similar to that of the vacuum deposition technique, and, further, adhesion to the substrate is high as compared to the sputtering technique by which a thin film having a quality similar to the one produced by ion plating is obtained. In addition, the production of thin films by ion plating has the advantage that waste water generated by electroplating or electroless plating techniques is not a serious problem.
However, it is difficult to obtain magnetic thin films with magnetic properties as are required for magnetic recording media, particularly in coercive force (Hc) and rectangular ratio (Br/Bm), for example, a coercive force above 300 Oe and a rectangular ratio above 0.65, by means of ion plating. Even if a metal such as Fe, Co, Ni, etc., or an alloy thereof is ion plated on a substrate to form a magnetic thin film, the coercive force is 400 Oe at most and the rectangular ratio is on the order of 0.4. Therefore, the necessary magnetic properties for use as a magnetic recording media cannot be obtained.