JP2008159199A - Transfer tool, manufacturing method of glass substrate for recording medium, glass substrate for recording medium and recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a transfer tool by which a glass substrate for a recording medium can be stably transferred and the glass substrate is not damaged, to provide a manufacturing method of the glass substrate for the recording medium using the transfer tool and to provide the glass substrate for the recording medium and the recording medium. <P>SOLUTION: In the transfer tool 5 transferring the glass substrate having an opening part in the center part thereof, the transfer tool 5 has a bar-shaped transfer part 50 and a grip part 51 and the opening part is supported by two supporting parts P1 and P2 of recessed parts of the transfer part 50. The transfer tool is characterized in that the transfer part 50 has a bar-shaped core material 502 and a pair of plate materials 501 with recessed parts fixed and held to side surfaces of the core material, the bar-shaped core material 502 is made of metal and the pair of plate materials 501 are made of resin. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a transfer jig, a method for manufacturing a glass substrate for a recording medium using the transfer jig, a glass substrate for a recording medium, and a recording medium.

  Conventionally, as a recording medium substrate, aluminum alloys are used for stationary information devices such as desktop computers and servers, and glass substrates are generally used for portable information devices such as notebook computers and mobile computers. It had been. Since the aluminum alloy is easily deformed and has insufficient hardness, the smoothness of the substrate surface after polishing cannot be said to be sufficient. Further, when the recording head mechanically contacts the magnetic disk, there is a problem that the magnetic film is easily peeled off from the substrate. Therefore, glass substrates with little deformation, good smoothness, and high mechanical strength will be widely used not only for portable devices but also for home-use devices such as stationary information devices and other televisions in the future. It is predicted.

  In order to improve the mechanical strength of the glass substrate for recording media, chemical strengthening treatment has been widely performed. In this chemical strengthening treatment, a glass substrate is immersed in a chemical strengthening solution stored in a chemical strengthening treatment tank, and alkali metal ions on the surface of the glass substrate are replaced with alkali metal ions having an ion diameter larger than the alkali metal ions. Thus, compressive strain is generated and mechanical strength is improved.

  When chemically strengthening the glass substrate, for example, the following method is used. A holding member holding a plurality of glass substrates is heated to 300 ° C. in advance and immersed in a chemical strengthening treatment tank containing a chemical strengthening treatment liquid at 400 ° C. for about 3 hours. Thereafter, the glass substrate surface can be chemically strengthened by taking out the holding member holding the glass substrate from the chemical strengthening treatment liquid, immersing it in a 20 ° C. water bath, rapidly cooling it, and maintaining it for about 10 minutes.

  For this reason, until the chemical strengthening process is performed, a light and portable holding member (hereinafter referred to as a process cassette) such as plastic is used to transfer between manufacturing processes. The holding member is generally made of a metal that can be immersed in a high-temperature chemical strengthening treatment tank. For example, what is described in Patent Document 1 is known.

  As shown in FIG. 1, the holding member 30 has a plate-like side plate 36 in which three holes are formed, and three shaft-like bodies 32 having a plurality of abacus bead-like projections fixed thereto. . Valley portions are formed between the protrusions formed on these shafts 32, and these valley portions can support the outer peripheral portion of the glass substrate 31. And these shaft-shaped bodies 32 can hold | maintain the several glass substrate 31 so that the side surface of the glass substrate 31 may orthogonally cross the axial direction of the shaft-shaped body 32 by supporting three places of the outer peripheral part of the glass substrate 31. It has become.

For this reason, it is necessary to transfer the glass substrate 31 on the plastic process cassette used before the chemical strengthening process to the holding member 30 for the chemical strengthening process. As this transfer jig, conventionally, a transfer jig 4 having a grip portion 41 attached to a shaft 40 having a plurality of Soroban ball-shaped projections as shown in FIG. 2 is inserted into the opening of the glass substrate 31, It was lifted and transferred. In addition, after the chemical treatment step, the transfer jig is used to transfer and carry the chemical treatment step holding member 30 to a plastic process cassette having good portability.
JP-A-7-176045

  However, in the conventional transfer jig 4 as shown in FIG. 2, there is only one support point P when viewed from the cross section, and the glass substrate 31 is likely to swing around the shaft 40, and the transfer operation is performed slowly and carefully. Otherwise, the glass substrate 31 may be applied to the holding member 30 or the like and may be damaged.

  Therefore, the technical problem to be solved by the present invention is that a glass substrate can be stably transferred, a transfer jig that does not damage the glass substrate, and a glass for a recording medium using the transfer jig It is providing the manufacturing method of a board | substrate, the glass substrate for recording media, and a recording medium.

  In order to solve the above problems, the present invention has the following features.

1.
In a transfer jig for transferring a glass substrate having an opening in the center,
The transfer jig has a rod-shaped transfer part having a recess, and a grip part,
A transfer jig having support portions for supporting openings of the glass substrate at two locations of the recess.

2.
2. The transfer jig according to 1, wherein the transfer portion includes a rod-shaped core member and a pair of plate members having a concave portion fixedly held on a side surface of the core member.

3.
3. The transfer jig according to 2, wherein the rod-shaped core member is a metal, and the pair of plate members are made of resin.

4).
In the manufacturing method of the glass substrate for recording media which has the process of immersing a glass substrate in a chemical strengthening liquid, and performing a chemical strengthening, the jig | tool for transfer of any one of 1 thru | or 3 is used for recording media characterized by the above-mentioned. A method for producing a glass substrate.

5.
A glass substrate for a recording medium, which is produced using the method for producing a glass substrate for a recording medium according to 4.

6).
6. A recording medium comprising a magnetic film on the surface of the glass substrate for recording medium according to 5.

  According to the present invention, since the opening portion of the glass substrate having the opening portion in the center is configured to have the support portion that supports the two recess portions of the transfer portion of the transfer jig, the glass substrate can be stably provided. A transfer jig that can be transferred and does not damage the glass substrate, a method for manufacturing a glass substrate for a recording medium without using the transfer jig, a glass substrate for a recording medium, and a recording medium Can be provided.

  Although the present invention will be described based on the illustrated embodiment, the present invention is not limited to the embodiment.

  FIG. 3 shows an embodiment of the transfer jig according to the present invention. The transfer jig 5 includes a transfer portion 50 that holds and holds a pair of plate members 501 having recesses on the side surfaces of a rod-shaped core member 502, a grip portion 51 that is attached to an end portion of the core member 502, and the like. The

  This transfer jig 5 is used in a process cassette for simultaneously holding and moving a plurality of glass substrates 31 in a glass substrate manufacturing process, and in a chemical strengthening treatment process for simultaneously chemically strengthening a plurality of glass substrates. A jig used to transfer the glass substrate to and from the holding member 30, and can be transferred in a stable state so that the glass substrates do not come in contact with each other at the time of transfer and are not damaged or broken. Is.

  4A and 4B are a top view (a) and a front view (b) of the process cassette 6 made of plastic capable of holding a plurality of glass substrates 31. FIG. The process cassette 6 is used until the cleaning step before the chemical strengthening step for chemically strengthening the glass substrate 31 after the molten glass is press-molded in the glass substrate manufacturing step and then placed on the process cassette to move each step. It is done. After finishing the cleaning process, the glass substrate 31 is transferred to the chemical strengthening holding member 30 shown in FIG.

  At this time, the transfer jig 5 shown in FIG. 3 is used to transfer the glass substrate 31 from the process cassette 6 to the holding member 30. The transfer jig 5 is inserted into an opening opened in the center of the glass substrate 31 placed on the process cassette 6, and the glass substrate 31 is supported by a recess provided in the transfer jig 5. The cassette 6 is pulled up and taken out, and transferred to the chemical strengthening holding member 30 in this state. The glass substrate 31 supported by the concave portion of the transfer jig 5 is a pair of plate members at two support portions (P1 and P2 in FIG. 3B) of the opening portion opened in the central portion of the glass substrate 31. By contacting 501, it is possible to transfer stably without causing rattling or swinging around a supported point. In addition, in FIG.3 (b), the space | interval of P1 and P2 of a pair of board | plate material 501 which forms a recessed part is a value of 1/4-3/4 of the internal diameter D2 opened in the glass substrate 31 to be transferred. Is preferred. By setting this range, the transfer jig 5 can be easily inserted into the opening of the glass substrate 31, and the glass substrate 31 can be more stably supported and transferred. If it is smaller than this range, it tends to be unstable, and if it is larger than this range, it becomes difficult to insert.

  3 is an example of a V-shaped groove with a flat bottom surface, but may be V-shaped or R-shaped instead of flat, and may be a rod-shaped transfer jig 5. What is necessary is just the structure supported by the two places of the both sides of the transfer part 50 of this. In the case of a flat surface, line contact is made at two support portions, and in the case of V-shape or R-shape, contact is made at two points on each of the left and right side portions. Thus, by supporting at two places on both sides of the transfer part 50, the glass substrate can be stably transferred without shaking during the transfer.

  Further, the rod-shaped core member 502 and the pair of plate members 501 having the recesses may be configured as an integral member. By separating the core material 502 and the plate material 501, the core material 502 can be made of a strong material such as metal, and the plate material 501 can be made of a resin material that does not damage the glass substrate 31. Further, as shown in FIG. 5, the core member 502 may be covered with an outer peripheral member 503 made of resin to form a groove 504. The groove 504 may be V-shaped or R-shaped (arc-shaped), and may be flat.

  Next, along with the manufacturing process of a glass substrate, the usage example of the transfer jig 5 according to the present invention will be specifically described.

(Manufacturing process)
The manufacturing method of the glass substrate for recording media is demonstrated. FIG. 6 is a flowchart showing an example of the manufacturing process of the recording medium glass substrate. First, a glass material is melted (glass melting process), molten glass is poured into a lower mold, and press molding is performed with an upper mold to obtain a disk-shaped glass substrate precursor (press molding process). Note that the disk-shaped glass substrate precursor may be produced by cutting a sheet glass formed by, for example, a downdraw method or a float method with a grinding stone, without using press molding.

  The press-molded glass substrate precursor is opened at the center with a core drill or the like to become the glass substrate 31 (coring step). Then, both surfaces of the glass substrate 31 are polished, and the overall shape of the glass substrate, that is, the parallelism, flatness and thickness of the glass substrate are preliminarily adjusted (first lapping step).

  The glass substrates 31 that have finished the first lapping step are set one by one in the process cassette 6 shown in FIG. 4 and are used in the next chemical strengthening step while being held so that the substrate surface is not damaged. Move between each process until it is transferred.

  After the first lapping step, the outer peripheral end surface and the inner peripheral end surface of the glass substrate 31 are ground and chamfered, and the outer diameter size and roundness of the glass substrate 31, the inner diameter size of the hole, and the concentricity between the glass substrate 31 and the hole. The degree is finely adjusted (inner / outer diameter machining process). After the fine adjustment, as the next step, the inner peripheral end surface of the glass substrate 31 is polished to remove fine scratches and the like (inner peripheral end surface processing step).

  Next, both surfaces of the glass substrate 31 are polished again, and the parallelism, flatness, and thickness of the glass substrate 31 are finely adjusted (second lapping step). And the outer peripheral end surface of the glass substrate 31 is grind | polished and a fine crack etc. are removed (outer peripheral end surface processing process).

  Next, after the glass substrate 31 is cleaned (cleaning step), the glass substrate 31 placed on the process cassette 6 is transferred to the holding member 30 shown in FIG. At the time of the transfer, the transfer jig 5 according to the present invention is inserted into the central opening of the glass substrate held by the process cassette 6, and the glass substrate 31 is supported in the recess of the transfer jig 5. Then, the process cassette 6 is pulled up and taken out. Next, the glass substrate 31 supported by the transfer jig 5 is brought to a position corresponding to the concave portion of the holding member 30 for chemical strengthening treatment, and transfer is performed.

  As described above, by using the transfer jig 5 according to the present invention, the glass substrate 31 is not shaken during the transfer and can be transferred in a stable state, so that the glass substrate 31 is not damaged. Also, work efficiency is improved.

  After the glass substrate 31 is transferred to the holding member 30 for chemical strengthening treatment, the glass substrate 31 is immersed in a chemical strengthening solution to form a chemical strengthening layer on the glass substrate 31 (chemical strengthening step). Thereafter, the transfer jig 5 is used again to transfer to the process cassette 6 to be used as a transfer cassette between subsequent processes.

  As the next step, a polishing process for precisely finishing the surface of the glass substrate 31 is performed (polishing step). Then, cleaning (cleaning process) and inspection (inspection process) are performed, and the recording medium glass substrate 31 as a product is completed.

  The reason why the glass substrate 31 is transferred from the process cassette 6 to the chemical strengthening holding member 30 before and after the chemical strengthening step is that the holding member 30 is exposed to a high temperature in the chemical strengthening step.

  The contents of the chemical strengthening process are shown in the flowchart of FIG. The cleaned glass substrate 31 is preheated (preheating step) and then immersed in a chemical strengthening solution (chemical strengthening solution soaking step). The glass substrate 31 taken out from the chemical strengthening solution is washed with water (water immersion process) and dried (drying process).

  In the chemical strengthening process, when each process from the series of preheating process to drying process is actually performed, for example, the following process is performed. First, a holding member 30 holding a plurality of glass substrates 31 is prepared, the glass substrates 31 together with the holding member 30 are sequentially put into a preheating furnace, immersed in a chemical strengthening liquid tank, immersed in a cleaning tank, and a drying furnace. It is possible to perform a series of chemical strengthening processes. The temperature applied to the glass substrate 31 and the holding member 30 that holds the glass substrate 31 in the chemical strengthening step varies depending on the glass substrate material, the chemical strengthening solution, and the like, but is approximately the following, for example. 200 ° C to 600 ° C in the preheating furnace in the preheating step, 280 ° C to 660 ° C in the chemical chemical strengthening solution bath in the chemical strengthening solution immersion step, 35 ° C to 100 ° C in the cleaning bath in the water soaking step, and drying In a furnace, it is 100 to 150 ° C. Moreover, between each process, it is necessary to move the holding member 30 in the air at room temperature between the furnaces or tanks. Therefore, the glass substrate and the holding member 30 are exposed to a large temperature difference such as room temperature and about 600 ° C. from the above example.

  The holding member 30 exposed to a large temperature difference as described above cannot be made of a light and portable material made of resin used for a process cassette, and is mainly made of a metal or ceramic material that can withstand high temperatures. For this reason, if such a holding member 30 is used in a process other than the chemical strengthening process, it is heavy and the working efficiency is deteriorated. For this reason, it is necessary to transfer between the process cassette 6 with good portability and the holding member 30 with good heat resistance.

  The material of the transfer jig 5 is not particularly limited, but a metal having a high mechanical strength is used for the core material 502, and a resin is used for the support portion that contacts the glass substrate. It is preferable from the point of not scratching. For example, iron, aluminum, SUS, or the like can be used as the core material, and a resin material such as polycarbonate, acrylic, polyester, or polyacetal can be used as the plate material or the outer peripheral member.

(Chemical strengthening liquid immersion process)
In the chemical strengthening solution immersion step, the glass substrate is immersed in a chemical strengthening solution in which the chemical strengthening agent is melted, and the alkali metal ions on the surface of the glass substrate are ion-exchanged with the alkali metal ions of the chemical strengthening solution.

As the chemical strengthening agent, conventionally known ones can be used, and examples thereof include potassium nitrate (KNO ₃ ), sodium nitrate (NaNO ₃ ), potassium carbonate (K ₂ CO ₃ ) and the like. These can be used alone or in combination of two or more. To use.

  The chemical strengthening agent is charged into a chemical strengthening treatment tank in a predetermined amount and heated to melt and become a chemical strengthening liquid. The heating temperature of the chemical strengthening solution is preferably in the range of 280 ° C. to 660 ° C., more preferably in the range of 300 ° C. to 450 ° C., from the viewpoint of the rate of ion exchange and the glass transition temperature Tg of the glass substrate. By setting this high temperature side (upper limit) to a range of 300 ° C. to 450 ° C. lower than the glass transition temperature Tg, the reaction rate of ion exchange is not too slow, and the shape of the glass substrate is not affected.

  The time for immersing the glass substrate in the chemical strengthening solution is preferably in the range of 0.1 hour to several tens of hours. Moreover, as shown in this example, it is preferable to preheat the glass substrate before immersing it in the chemical strengthening solution. When the glass substrate is heated in advance, the temperature of the chemical strengthening solution does not decrease excessively when immersed in the chemical strengthening solution, and chemical strengthening can be performed efficiently.

  The thickness of the chemically strengthened layer is preferably in the range of about 5 μm to 15 μm in view of improving the strength of the glass substrate and shortening the time of the polishing process.

(Water immersion process)
After immersing the glass substrate in the chemical strengthening solution, the glass substrate is continuously immersed in water in order to remove the chemical strengthening solution on the surface of the glass substrate evenly. By immersing the entire glass substrate in water, the chemical strengthening liquid does not partially exist on the glass substrate, and chemical strengthening does not proceed partially. For this reason, since there is no unevenness in chemical strengthening, the glass substrate can have a uniform strength.

  The chemical strengthening liquid or the crystalline substance of the salt forming the chemical strengthening liquid can be efficiently removed from the surface of the glass substrate in a shorter time as the temperature of the immersion water is increased. The temperature of such water is preferably 35 ° C. to 100 ° C. under atmospheric pressure. The time for immersing the glass substrate in water is preferably 1 second or longer. If it is less than 1 second, the chemical strengthening solution on the glass substrate cannot be sufficiently removed, so that the chemical strengthening solution remains on the glass substrate and unevenness of strengthening occurs. The upper limit of the time for immersing in water is not particularly limited, and may be appropriately determined in consideration of productivity.

  In addition, in order to reduce the temperature difference so that the glass substrate does not crack or crack in the water immersion process due to the temperature difference between the water temperature and the chemical strengthening liquid, between the chemical strengthening liquid immersion process and the water immersion process. A cooling step for cooling the glass substrate may be provided.

(Glass substrate)
The glass substrate to be chemically strengthened is not particularly limited, but soda lime glass mainly composed of silicon dioxide, sodium oxide, calcium oxide; silicon dioxide, aluminum oxide, R2O (R = K, Na, Li) as main components. Aluminosilicate glass; borosilicate glass; lithium oxide-silicon dioxide glass; lithium oxide-aluminum oxide-silicon dioxide glass; R'O-aluminum oxide-silicon dioxide glass (R '= Mg, Ca, Sr or Ba) can be used, and these glass materials may be added with zirconium oxide, titanium oxide or the like.

  The size of the glass substrate is not limited, and the method of the present invention can be applied to a small-diameter disk of 2.5 inches, 1.8 inches, 1 inch, 0.85 inches or less, and the thickness thereof. Can be applied to a thin type such as 2 mm, 1 mm, 0.63 mm, or less.

  In the glass substrate provided for the chemical strengthening step, the roughness of the main surface and the end face portion is not particularly limited, but the surface roughness of the main surface of the glass substrate is Rmax (maximum height) of 10 nm or less, Ra (center) The line average roughness) is preferably 1.0 nm or less. The surface roughness of the end face is preferably in the range of Rmax from 0.01 μm to 1 μm and Ra in the range of 0.001 μm to 0.8 μm. When the surface-polished glass substrate is chemically strengthened, the reinforcing layer can be formed uniformly.

(recoding media)
Next, a recording medium using the recording medium glass substrate described so far will be described. When this recording medium glass substrate is used, durability and high recording density are realized. Hereinafter, a recording medium will be described with reference to the drawings.

  FIG. 8 is a perspective view of the magnetic disk. In this magnetic disk DI, a magnetic film 2 is directly formed on the surface of a circular recording medium glass substrate 1. As a method for forming the magnetic film 2, a conventionally known method can be used. For example, a method in which a thermosetting resin in which magnetic particles are dispersed is spin-coated on a substrate, or a method by sputtering or electroless plating is used. A method is mentioned. The film thickness by spin coating is about 0.3 μm to 1.2 μm, the film thickness by sputtering is about 0.04 μm to 0.08 μm, and the film thickness by electroless plating is 0.05 μm to 0.1 μm. From the viewpoint of thinning and densification, film formation by sputtering and electroless plating is preferable.

  The magnetic material used for the magnetic film is not particularly limited, and a conventionally known material can be used. However, in order to obtain a high coercive force, Ni having a high crystal anisotropy is basically used, and Ni or A Co-based alloy to which Cr is added is suitable. Specific examples include CoPt, CoCr, CoNi, CoNiCr, CoCrTa, CoPtCr, and CoNiPt containing Co as a main component, CoNiCrPt, CoNiCrTa, CoCrPtTa, CoCrPtB, and CoCrPtSiO. The magnetic film may have a multilayer structure (for example, CoPtCr / CrMo / CoPtCr, CoCrPtTa / CrMo / CoCrPtTa) that is divided by a nonmagnetic film (for example, Cr, CrMo, CrV, etc.) to reduce noise. In addition to the magnetic materials described above, ferrite, iron-rare earth, and granular materials having a structure in which magnetic particles such as Fe, Co, FeCo, CoNiPt are dispersed in a nonmagnetic film made of SiO2, BN, etc. Good. Further, the magnetic film may be either an inner surface type or a vertical type recording format.

  In addition, a lubricant may be thinly coated on the surface of the magnetic film in order to improve the sliding of the magnetic head. Examples of the lubricant include those obtained by diluting perfluoropolyether (PFPE), which is a liquid lubricant, with a freon-based solvent.

  Furthermore, you may provide a base layer and a protective layer as needed. The underlayer in the magnetic disk is selected according to the magnetic film. Examples of the material for the underlayer include at least one material selected from nonmagnetic metals such as Cr, Mo, Ta, Ti, W, V, B, Al, and Ni. In the case of a magnetic film containing Co as a main component, Cr alone or a Cr alloy is preferable from the viewpoint of improving magnetic characteristics. Further, the underlayer is not limited to a single layer, and may have a multi-layer structure in which the same or different layers are stacked. For example, a multilayer underlayer such as Cr / Cr, Cr / CrMo, Cr / CrV, NiAl / Cr, NiAl / CrMo, or NiAl / CrV may be used.

  Examples of the protective layer that prevents wear and corrosion of the magnetic film include a Cr layer, a Cr alloy layer, a carbon layer, a hydrogenated carbon layer, a zirconia layer, and a silica layer. These protective layers can be formed continuously with an in-line type sputtering apparatus, such as an underlayer and a magnetic film. In addition, these protective layers may be a single layer, or may have a multilayer structure including the same or different layers. Note that another protective layer may be formed on the protective layer or instead of the protective layer. For example, in place of the protective layer, colloidal silica fine particles are dispersed and applied to a Cr layer diluted with an alcohol solvent, and then fired to form a silicon dioxide (SiO2) layer. May be.

  The magnetic disk has been described above as one embodiment of the recording medium. However, the recording medium is not limited to this, and the glass substrate of the present invention can be used for a magneto-optical disk, an optical disk, and the like.

It is a schematic diagram which shows the holding member used for a chemical strengthening process. It is a schematic diagram which shows the jig | tool for the transfer of the conventional glass substrate. It is a mimetic diagram showing one embodiment of a transfer jig concerning the present invention. It is a schematic diagram which shows the process cassette used at the manufacturing process of a glass substrate. It is a schematic diagram which shows another embodiment of the jig | tool for transfer which concerns on this invention. It is a flowchart which shows the example of the manufacturing process of the glass substrate for recording media. It is a flowchart which shows the content of the chemical strengthening process process in FIG. It is a perspective view containing the partial cross section of a magnetic disc.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Glass substrate for recording media 2 Magnetic film 30 Holding member 31 Glass substrate 32 Shaft upper body 36 Side plate 4, 5 Transfer jig 41, 51 Grip part 40 Shaft 501 Plate material 502 Core material 503 Outer peripheral member 6 Process cassette P1, P2 support Part

Claims (6)

In a transfer jig for transferring a glass substrate having an opening in the center,
The transfer jig has a rod-shaped transfer part having a recess, and a grip part,
A transfer jig having support portions for supporting openings of the glass substrate at two locations of the recess. The said transfer part has a rod-shaped core material and a pair of board | plate material which has a recessed part fixedly hold | maintained to the side surface of this core material, The transfer jig of Claim 1 characterized by the above-mentioned. The transfer jig according to claim 2, wherein the rod-shaped core member is a metal, and the pair of plate members is made of a resin. 4. A method for manufacturing a glass substrate for a recording medium comprising a step of immersing a glass substrate in a chemical strengthening solution and performing chemical strengthening, wherein the transfer jig according to any one of claims 1 to 3 is used. A method for producing a glass substrate for a medium. A glass substrate for a recording medium, which is produced using the method for producing a glass substrate for a recording medium according to claim 4. A recording medium comprising a magnetic film on a surface of the glass substrate for a recording medium according to claim 5.

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