Previously, coating type magnetic recording media have been prepared by coating non-magnetic supports with a coating comprising organic polymer binders and magnetic materials of ferromagnetic powders dispersed therein. In recent years, with the increased demand for high density recording, magnetic recording media of the so-called thin metal film type which are free of binders and which have thin ferromagnetic metal films formed by paper deposition methods (such as vacuum deposition, sputtering and ion plating), or plating methods (such as electroplating and electroless plating) as magnetic layers, have attracted attention and partly utilized.
In particular, the vacuum deposition method does net require waste fluid treatment which is necessary in the plating method. Further, the manufacturing processes involved in vacuum deposition are simple, and the deposition rate of films is high. This method therefore has merit. Methods for producing magnetic films, having a coercive force and a squareness which are desirable for magnetic recording media, by the vacuum deposition and oblique incidence vapor deposition methods are described in U.S. Pat. Nos. 3,342,632 and 3,342,633.
Major problems concerning magnetic recording media having thin ferromagnetic metal films include weather resistance, running properties and durability.
Magnetic recording media move at a high speed compared to the magnetic head during recording, reproducing and erasing of magnetic signals. Running the media must be carried out smoothly and stably, and at the same time, abrasion or breakage due to contact with the head must not occur. In view of these problems, there is a need for lubricating layers or protective layers on the thin ferromagnetic metal films as means for improving their running properties and durability.
The protective layers of the magnetic recording media of the thin metal film type include layers formed by applying thermoplastic resins, thermosetting resins, fatty acids, metal salts of fatty acids, fatty acid esters or alkyl phosphates dissolved in organic solvents. For example, such protective layers are disclosed in JP-A-60-69824 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-60-85427.
Recently, a technique of improving the durability of the media by using compounds having branched perfluoroalkenyl groups has been developed (JP-A-61-107528).
Investigations of the application of perfluoroalkyl polyether compounds have also been extensively conducted, and U.S. Pat. No. 3,778,308 proposes providing a perfluoropolyether on a thin ferromagnetic metal film. Further, JP-B-60-10368 (the term "JP-B" as used herein means an "examined Japanese patent publication") describes a perfluoropolyether of the terminal modification type in which one or both termini of the perfluoropolyether chain are modified with polar groups such as carboxyl groups.
Furthermore, there is the report that a multi-chain type perfluoropolyetheramide obtained by dehydration condensation of a compound having a plurality of amine groups and a perfluoropolyether chain having a carboxyl group at one terminus thereof was tested as a lubricating agent for disk-like magnetic recording media of the thin metal film type (Sugiyama et al., the 34th National Meeting, the Lubrication Society of Japan, B.multidot.28; Preliminary Reports issued on October, 1989, p. 425). This compound results in fairly good running properties and durability, but has the undesirable effect of reducing the coefficient of static friction. Improvements have been therefore desired.
As described above, the magnetic recording media of the thin metal film type shown in the prior art have the problems that the running properties and durability are insufficient under severe conditions of high or low humidity, and that the electromagnetic characteristics deteriorate due to spacing loss between the head and the recording medium caused by the thickness of the protective or lubricating layer. These problems have limited the practical applications of the magnetic recording media of the thin metal film type.