Dynamic magnetic information storage or retrieval – Record medium – In container
Reexamination Certificate
2000-09-13
2003-02-25
Klimowicz, William (Department: 2652)
Dynamic magnetic information storage or retrieval
Record medium
In container
Reexamination Certificate
active
06525908
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a floppy disk, and in particular, to a floppy disk type magnetic recording medium having high surface recording density and good storage property.
In a magnetic recording medium such as magnetic tape, hard disk, etc., a vacuum-deposited tape or a metal thin film type hard disk, etc. are used. Using a ferromagnetic metal thin film prepared by vacuum film forming method, i.e. sputtering method, vacuum evaporation method, etc., this magnetic recording medium is used as a recording layer in practical application. In this type of magnetic recording medium, high magnetic energy can be easily obtained, and flat surface can be easily achieved by smoothening the surface of non-magnetic substrate. In this respect, it is characterized in that spacing loss is low and high electromagnetic transfer characteristics can be attained, and this is suitable for a method to manufacture a high-density recording material. In particular, the sputtering method can increase magnetic energy than the vacuum deposition method, and this is adopted in the magnetic recording medium such as hard disk, which must provide high surface recording density.
On the other hand, compared with the hard disk, a floppy disk type magnetic recording medium has higher impact resistance and can be produced at lower cost, and it is widely used in 2HD class products. Further, in recent years, a high-density magnetic recording medium represented by Zip (Iomega Inc.) based thin layer coating technique has been introduced to practical use. In the magnetic recording medium as described above, recording and reproduction are performed at high speed of about 3000 rpm, and high transfer rate closer to that of hard disk can be achieved. However, its recording density is still less than {fraction (1/10)} of that of the hard disk. This is attributable to the fact that a floppy disk type magnetic recording medium which has the magnetic layer produced by sputtering method as in case of the hard disk is not yet suitable for practical application.
There are various reasons for this. One of the reasons is that, in the magnetic recording mode using such a floppy disk, it is difficult to attain high reliability including storage property for practical use.
In a floppy disk produced by conventional coating method, oxide film is present on the surface of a magnetic metal particles in a magnetic film, and the magnetic film further contains bonding resin, hard particles, or lubricant. As a result, water, oxygen, corrosive gas, etc. to cause corrosion are generally not brought into direct contact with the magnetic metal particles. Hence, it has high storage property. On the other hand, in a floppy disk, for which the magnetic film is produced by the sputtering method, the magnetic film is a metal thin film, and a protective film comprising carbon and the like and a lubricant film are laminated on it. When storage test is performed under corrosive condition, corrosion develops quicker than the case of the coating type floppy disk.
The corrosion of this metal thin film type floppy disk is caused by pinholes and other defects on the protective film. Through the pinholes, corrosive gas or moisture come into contact with the magnetic film, and corrosion occurs. When film is formed on a mirror-surface substrate, pinholes are not developed on the protective film almost at all. In the floppy disk actually in use, however, it is necessary to provide micro-size and low-profile projections on the surface, and pinholes are generated around the micro-size projections. Similar phenomenon is observed on a hard disk or vacuum deposited tape where the magnetic film is made of metal thin film. Hard disk is stored in relatively clean environment and vacuum deposited tape is stored in rolled-up state. The metal thin film type floppy disk must have higher corrosion resistance than the hard disk or the vacuum deposited tape.
When a magnetic recording medium using metal thin film as the magnetic film is used under general environmental condition, corrosion becomes an important issue, and there are two different mechanisms in the corrosion. The first is the corrosion which occurs when the medium is stored for long time under high temperature and high humidity conditions. In this case, moisture is brought into contact with the magnetic film through pinholes or cracks on the protective film. Corrosive reaction takes place between water and magnetic metal, and a hydroxide of metal such as cobalt hydroxide is generated. The second is the corrosion when the medium is stored for long time under high temperature and high humidity condition where sulfur oxide gas such as sulfur dioxide, or corrosive gas such as hydrogen sulfide gas is present. In this case, corrosive gas is taken up into the moisture, which is present in pinholes and cracks on the protective film. As a result, corrosive reaction occurs between the components derived from the gas and the magnetic metal, and, for example, a metal salt such as cobalt sulfate is generated.
In these corrosive reactions, the presence of water is very important, and the speed or rate of the reaction greatly depends upon environmental humidity, and the reaction is very likely to occur under high humidity condition.
In particular, under general conditions in urban area, it is known that the influence of corrosive gas is strong, and the most controversial corrosive gas is sulfur dioxide. It is also known that the level of corrosion resistance to sulfur dioxide approximately agrees with the level of corrosion resistance to hydrogen sulfide and the like.
In the corrosive reaction as described above, the important action to be taken is to shield and protect the magnetic particles from water and corrosive gas although the corrosive reaction is different in each case. Therefore, the following conditions are required to ensure excellent storage property in the metal thin film type floppy disk:
(1) to use a protective film where pinholes are not present;
(2) to use a lubricant film having high performance characteristics to shield from gas;
(3) to simultaneously use rust preventive agent on the lubricant film; and
(4) to prepare a magnetic film, which is very likely to corrode.
However, in case micro-size projections are present on the surface, it is very difficult to completely eliminate the pinholes on the protective film even when CVD method is used, which is considered to have high coverage rate. Further, when it becomes necessary to decrease the thickness of the protective film to improve the recording density, the frequency to develop pinholes is increased further. Also, the protective film is not formed on edges on inner and outer peripheries of the disk, and it is not possible to prevent the development of corrosion from these edge portions.
In the lubricant film, the lubricant of perfluoropolyether type as generally used for hard disk or the like has low performance characteristics to shield from gas, and it is very difficult to improve corrosion resistance. As a lubricant film having high shielding performance, an orientation film comprising adsorptive film of a compound having long-chain alkyl group such as stearic acid may be considered. The lubricant film of this type has high effect to improve corrosion resistance, while fluid lubricating property is lower than perfluoropolyether, and durability is decreased. When the method to simultaneously use the alkyl type lubricant with perfluoropolyether is used, both durability and corrosion resistance can be attained to some extent, but performance characteristics are not good enough. The lubricant film of this type is worn out through sliding on the head. When the protective film is worn out or cracks occur on the protective film, corrosion develops around the worn or cracked portions. Further, corrosion of the edges cannot be prevented.
A magnetic recording medium containing a rust preventive agent in a magnetic layer or in a protective layer is described in each of JP-A-61-41048, JP-A-6-76272, or JP-B-2650224, or JP-B-3-55886.
Fuji Photo Film Co. , Ltd.
Klimowicz William
Sughrue & Mion, PLLC
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