WO1994018673A1

WO1994018673A1 - Light-transmitting high recording density magnetic recording medium having an undercoating layer

Info

Publication number: WO1994018673A1
Application number: PCT/US1994/001354
Authority: WO
Inventors: Koji Etchu; Shigeto Oiri
Original assignee: Minnesota Mining And Manufacturing Company
Priority date: 1993-02-08
Filing date: 1994-02-07
Publication date: 1994-08-18
Also published as: JPH06236541A

Abstract

A light-transmitting servo tracking type high recording density magnetic recording medium provided with a nonmagnetic supporting member and a magnetic layer, which magnetic recording medium is characterized by the fact that an undercoating layer containing an electrically conductive, inorganic, nonmagnetic substance is interposed between the nonmagnetic supporting member and the magnetic layer. The magnetic layer is less than about 1 νm thick and has a light transmittance of at least 40 %.

Description

LIGHT-TRANSMITTING fflGH RECORDING DENSITY MAGNEΗC RECORDING MEDIUM HAVING AN UNDERCOATING LAYER Field of the Invention

This invention relates generally to a high recording density magnetic recording medium for use with a light-transmitting servo-tracking system, and more specifically to an undercoating layer used in such media.

Background of the Invention

In the disc type magnetic recording medium which is used for the purpose of recording data, the practice of projecting a beam of light on the surface of the magnetic recording medium and utilizing the light reflected on the surface for effecting servo tracking with a view to improving the recording density has been in done. This is because the improvement of recording density requires the recording surface to be accurately tracked. The disc type magnetic recording device of this operating principle is disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 2-14,436, for example.

The dissemination of note type and book-type portable personal computers has been urging the magnetic recording media toward more reduction in size and weight and the magnetic disc devices toward further reduction in thickness. As a measure necessary for this "reduction in thickness" of the magnetic disc devices intended to effect optical servo tracking, the system of performing the servo tracking with the light transmitting through the magnetic disc instead of the light reflected on the surface of the magnetic recording layer has been studied.

With the magnetic disc for use in the conventional optical servo tracking type magnetic disc device operating with the reflected light, accurate tracking is attained by forming regions of varying reflectance on the surface of the magnetic recording layer superposed on the nonmagnetic supporting member and producing pertinent servo signals by virtue of the differences in intensity of the reflected light.

In contrast, in the case of a magnetic disc for use in a system effecting the servo tracking by transmitting light through the magnetic disc, areas of varying light transmittance are formed on the surface of the magnetic recording layer. The differences in intensity of the transmitting light obtained as a consequence permit formation of servo signals capable of dense tracking. For the magnetic disc to be used in the light-transmitting type servo tracking of this operating principle, therefore, the overall light transmission ratio must exceed a predetermined level. To be more specific, if a laser beam of about 800 nm is used in the servo, the transmission ratio throughout the entire magnetic disc must be in the range of from 20 to 40%. If the light transmission ratio falls outside this particular range, the formation of a servo tracking signal due to the differences in light transmission ratio is difficult.

The magnetic recording layer should be decreased based on the recording wavelength to enhance the recording density of the magnetic disc. The recording medium, however, is notably degraded in strength and durability in proportion as the magnetic recording layer is reduced in thickness. The interposition of an undercoating layer between the nonmagnetic supporting member and the magnetic layer to enhance the durability of the magnetic recording medium is disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 63-317,926. Since this undercoating layer uses carbon black for antistatic effect, however, the magnetic recording medium is incapable of acquiring the light transmission ratio necessary for the light transmission servo tracking.

Japanese Unexamined Patent Publication (KOKAI) No. 61-214, 127 discloses the idea of adding tin oxide or white electroconductive titanium dioxide to the undercoating layer for enhancing the light transmission while maintaining high electric conductivity. In this case, however, the magnetic recording medium is not allowed to acquire sufficiently high durability because the amount of the inorganic powder which is to be added to the undercoating layer is small and the thickness of the undercoating layer is small.

Summary of the Invention The present invention, therefore, is aimed at providing a magnetic recording medium which attain light transmission servo tracking and effects high density recording by interposing between the nonmagnetic supporting member and the magnetic layer an undercoating layer which is electrically conductive and transparent to light.

The present inventors have discovered that interposition of an undercoating layer containing an electrically conductive, inorganic, nonmagnetic substance between the nonmagnetic supporting member and the magnetic layer permits production of a magnetic recording medium which exhibits sufficient durability and antistatic property, and provides light transmittance sufficient for the sake of light- transmitting servo tracking when the magnetic layer is not more than 1 μm thick. This invention, therefore, is directed to a high recording density magnetic recording medium using the light-transmitting servo-tracking system and comprising a nonmagnetic supporting member and a magnetic layer, which magnetic recording medium is characterized in that an undercoating layer containing an electrically conductive, inorganic, nonmagnetic substance is interposed between the nonmagnetic supporting member and the magnetic layer and the magnetic layer not more than 1 μm thick.

Detailed Description

The magnetic recording medium of this invention comprises a nonmagnetic supporting member, an undercoating layer, and a magnetic layer. The nonmagnetic supporting member of the present invention may be any of the light- transmitting supporting members which are conventionally used for supporting magnetic recording media. The light-transmitting supporting members may include organic and inorganic materials such as, for example, polyethylene terephthalate film, polyethylene naphthalate film, acetate film, polyimide film, polyamide film, and glass, which have a light transmission ratio of at least 40% . This invention is characterized by providing an undercoating layer containing an electrically conductive, inorganic, magnetic substance for a magnetic recording medium. Generally, carbon black is generally used as an conductivity- imparting material. The carbon black, however owing to the blackness thereof, notably degrades light transmittance of a layer using it. The magnetic recording medium of the light-transmitting servo-tracking system contemplated by this invention, therefore, does not permit use of the carbon black. Further, the carbon black generally disperses in a binder only with difficulty, tends to cohere and form an aggregate, and may disrupt the transparency of a magnetic recording medium. In contrast to the carbon black, the electrically conductive organic nonmagnetic substance is preferably generally white and is easily dispersed uniformly in a binder. The undercoating layer which has this substance dispersed therein, therefore, is allowed to acquire sufficient light transmittance. Preferred electrically conductive, inorganic, nonmagnetic substances include conductive tin oxide, titanium dioxide, zinc oxide, indium oxide, zinc sulfide, barium sulfate, silicon oxide, and magnetic carbonate, for example. As used herein, the term "electrically conductive" means that when the inorganic nonmagnetic substance of the quality mentioned above is compressed with a pressure of 100 kg /cm², the compressed mass exhibits a specific resistance of not more than 300 Ωcm. Though the method for imparting conductivity to the inorganic nonmagnetic substance is not specifically restricted, this is accomplished by doping or coating the surface of the substance with Ga, In, Sn, or CuS or by plating the surface therewith, for example.

In order for the undercoating layer to acquire sufficient light transmittance in this invention, the average particle diameter of the electrically conductive, inorganic, nonmagnetic substance must be suitably defined. When the substance is of a granular form, for example, the average particle diameter thereof must fall in the range of from 0.02 to 0.1 μm. If the average particle diameter is larger than 0.1 μm, the undercoating layer fails to acquire ample light transmittance and exhibits poor surface smoothness and, as a result, the produced magnetic recording medium suffers from deficiency in electromagnetic conversion property. Conversely, if the average particle diameter falls shown of the lower limit of 0.02 μm, the particles tend to cohere, disperse in a binder only with difficultly, and fail to produce an undercoating layer having good surface smoothness.

When the electrically conductive, inorganic, nonmagnetic substance happens to assume an acicular form, the average major axis diameter thereof must be in the range of from 0.05 to 0.3 μm for the same reason as described above with respect to the granular substance. The undercoating layer comprises the electrically conductive, nonmagnetic particles of the nature described above and a binder. The binder in this undercoating layer can be any known binder such as, for example, urethane resin, polyvinyl chloride resin, phenoxy resin, polyester resin, and copolymers of vinyl chloride monomer with other vinyl monomers which are popularly used in magnetic recording media.

In the present invention, the binder is preferably a binder resin which, when incorporated in the electrically conductive undercoating layer, allows the layer to acquire a light transmission ratio in the range of from 40 to 90% . The mixing ratio of the electrically conductive, inorganic, nonmagnetic substance and the binder contained in the undercoating layer is preferably in the range of from 50/50 to 85/15 by weight. The undercoating layer fails to manifest sufficient electroconductivity if the proportion of the electrically conductive, inorganic, nonmagnetic substance is smaller than the lower limit of this range. This insufficient supply of the nonmagnetic substance is also disadvantageous in that the formability of servo tracks, the durability of the formed servo tracks, and the electromagnetic conversion characteristic thereof are degraded. If the proportion of the inorganic, electrically conductive, nonmagnetic substance exceeds the upper limit of the range mentioned above, the adhesiveness of the undercoating layer to the nonmagnetic supporting member is so inferior as to jeopardize the practical utility of the magnetic recording medium.

The high recording density magnetic recording medium using the light- transmitting servo tracking system is required to form on the surface of the magnetic layer thereof a plurality of grooves i.e., servo tracks. A notable improvement in recording density may be realized by effecting servo tracking with the "light" by virtue of these "grooves" and consequently improving the track accuracy. These servo tracks (grooves or depressions) may be formed by pressing a die (protuberances) of servo tracks against the surface of the magnetic layer, thereby depressing the magnetic layer. If the magnetic layer has an unduly small thickness or unduly high hardness, however, the magnetic layer will not readily succumb to depression and will prevent efficient formation of servo tracks. The thickness of the undercoating layer, therefore, should be in the range of from 0.2 to 5 μm.

If the thickness of the undercoating layer is less than 0.2 μ , the efficiency with which the servo tracks are formed is notably impaired and the servo tracking no longer functions normally. This creates a peculiar problem for the high recording density magnetic recording medium of the servo tracking type which makes use of light. Another problem results if the thickness exceeds 5 μm: the magnetic layer is not easily superposed on the undercoating layer by spreading and the magnetic layer if produced at all, fails to acquire a sufficiently smooth surface and exhibits an inferior electromagnetic conversion characteristic. This phenomenon may be explained by theorizing that when the undercoating layer is excessively thick as compared with the magnetic layer, the undercoating layer excessively absorbs the solvent of the magnetic layer.

Further, from the standpoint of the function of the undercoating layer as a reservoir for a lubricant, if the undercoating layer is unduly thick, the undercoating layer encounters difficulty in bringing the required supply of the reserved lubricant to the magnetic layer and the magnetic tape suffers from a decline in durability. These problems are particularly conspicuous in the high recording density magnetic recording medium which comprises the magnetic layer not exceeding 1.0 μm in thickness and the undercoating layer.

There are cases where the electrical conductivity of the undercoating layer must be controlled within a certain range, although dependent on the particular kind of the magnetic recording medium which incorporates the undercoating layer therein. In the case of the floppy disc, for example, the conductivity (surface resistance) of the magnetic layer is required to be controlled within the range of from 10⁶ to 10⁹ ohms per square. For the purpose of controlling the conductivity within this range, non-electrically conductive, inorganic, magnetic substance such as titanium dioxide can be used as mixed with the aforementioned conductive nonmagnetic member. In this case, it is desirable to use the non-electrically conductive titanium dioxide identical in shape and average particle diameter with the aforementioned conductive nonmagnetic particles so that the undercoating layer is enabled to maintain the properties (transparency, surface smoothness, etc.) other than the conductivity.

The magnetic recording medium of the present invention is provided on the aforementioned undercoating layer with the magnetic layer. The present invention aims to provide a light-transmitting servo tracking type magnetic recording medium having a capacity for high recording density. For this purpose, the magnetic layer should be not more than 1.0 μm thick.

For the magnetic layer of this invention, any of magnetic paniculate substances in conventional use such as needle-shaped iron oxide type magnetic substances (such as 7-Fe*-.O₃ and F-^O.) , cobalt-containing needle-shaped iron oxide type magnetic substances, metallic ferromagnetic substances (metallic magnetic substances), hexagonal magnetic substances (such as barium ferrite), and iron carbide type magnetic substances can be used.

Since magnetic particles are varied in tinting strength with differences in kind, the magnetic layers formed of such magnetic particles having the same thickness may exhibit different light transmission ratios. For example, the magnetic layer of a metallic ferromagnetic substance tends to exhibit a higher light-shielding property than that of a needle-shaped iron oxide type magnetic substance. This problem of difference in tinting strength can be solved, however, by adjusting the thickness of the magnetic layer and the thickness of the undercoating layer within the possible ranges so as to control the light transmission ratio. When the solution is not attained by adjusting the thickness of the magnetic layer and that of the undercoating layer, the method which comprises increasing the light transmission ratio by dispersing in the binder of the magnetic layer a superfine powder of SiO₂ or Al₂O₃ having a larger refractive index than the binder (Japanese Unexamined Patent Publication KOKAI No. 4-214,217) is capable of attaining the solution. Alternatively, the method which attains the control of the light transmission ratio of the magnetic layer by combining a plurality of different magnetic layers in a superposed construction can be adapted for the purpose of the solution. The magnetic recording medium of this invention is produced as follows:

The coating material for the undercoating layer is produced by kneading and dispersing electrically conductive tin oxide conforming to this invention in combination with a binder, a lubricant, a solvent, etc. At the outset of the production of this coating material, the components mentioned above are simultaneously placed in the kneading and dispersing devices either wholly at once or as split in several fractions. In the solvent containing the binder, the electroconductive tin oxide is placed and then kneaded therein continuously. The blend is transferred into the dispersing device and thoroughly dispersed. Then, the dispersed liquid is combined with a lubricant to complete the coating material for the undercoating layer.

For the kneading and dispersing operations which are involved in the preparation of the coating material for the undercoating layer, various known devices adopted for the purpose of this operation may be used. For example, such kneading devices as kneader, planetary mixer, extruder, homogenizer, and high speed mixer can be used for the kneading operation. Such dispersing devices as sand mill, ball mill, attriter, tornado dispersing device, and high speed shock mill can be used for the dispersing operation.

The coating material for the undercoating layer, when necessary, may further incorporate therein various known additives such as curing agent, antifungal agent, and surfactant heretofore popularly used in magnetic layers and similar additives heretofore used in backcoat layers.

The devices which are effectively usable herein for the work of applying the coating material to the supporting member include various known devices such as air doctor coater, blade coater, air knife coater, squeeze coater, reverse roll coater, gravure coater, kiss coater, spray coater, and die coater, for example.

The coating material for the overcoating layer is produced by kneading and dispersing magnetic particles of a varying kind in combination with a binder, a lubricant, an abrasive, and a solvent. This coating material for the overcoating layer is superposed by spreading on the undercoating layer.

The operations of kneading, dispersing, and applying the coating material for the overcoating layer and the additives used therefor are identical to those adopted with respect to the coating material for the undercoating layer described above.

The overcoating layer is superposed on the undercoating layer after the coating material for the undercoating layer has been applied to the supporting member and then dried until the undercoating layer is formed completely. It is superposed on the undercoating layer before the latter is thoroughly dried. In the alternative, the undercoating layer and the overcoating layer may be simultaneously applied and dried.

The drying temperature for the undercoating layer and the overcoating layer is variable depending on the type of solvent and supporting member used. The drying operation is preferably carried out at a temperature in the range of from 40 to 120°C for a period in the range of from 30 seconds to 10 minutes, with the drying air being fed at a flow rate in the range of from 1 to 5 kl/m²s. The drying operation may be performed by irradiation with an infrared ray, far infrared ray, or electron beam.

Before the overcoating layer has been dried, the magnetic particles may be oriented or disoriented as occasion demands. The orientation of the magnetic particles is effected by exerting a magnetic field longitudinally, vertically, or obliquely (at an angle of 45° relative to the plane of the supporting member, for example) on the overcoating layer by the use of a permanent magnet or an electromagnet and drying the overcoating layer outside or inside the magnetic field. The disorientation is effected by randomly changing the direction of the magnetic member in a horizontal plane or three-dimensionally by the use of an alternating magnetic field or a rotary magnetic field. The orientation or disorientation may be attained by known methods.

The dried undercoating layer and overcoating layer may be subjected to an appropriate calendaring treatment. The calendaring treatment is carried out by the conventional method using a metal-plated roll or an elastic roll, for example. Although the conditions for the calendaring treatment are variable with the kinds of materials (binder, supporting member, etc.) to be used in the undercoating layer and the overcoating layer, the treatment should be performed at a temperature in the range of from 30 to 90 °C at a pressure in the range of from 500 to 4,000 pounds per lineal inch (pli).

The various materials to be used in this invention other than the electrically conductive, inorganic, nonmagnetic substance, and non-electrically conductive, inorganic, nonmagnetic substance which are specifically defined for this invention may be selected from among those materials known heretofore in the art.

The servo tracks are then formed by stamping, for example.

The light-transmitting servo tracking type high recording density magnetic recording medium which is obtained by this invention enjoys sufficient durability, resistance to electrification, and light-transmitting property.

The present invention will be further illustrated by the following non- limiting example.

Example An undercoating material was produced by kneading and dispersing the raw materials indicated in Table 1 as follows. By the use of a high speed mixer, the entire amount of a nonmagnetic substance indicated in the Table 1 was kneaded for about 10 minutes in a sclution having the entire amount of urethane resin and the entire amount of vinyl resin dissolved in the entire amount of solvent. The kneading in the high speed mixture was continued for about 50 minutes to obtain a blend. The blend was transferred into a sand mill and dispersed therein for 20 hours, to obtain a dispersion. In this dispersion, the entire amount of oleic acid, the entire amount of isocetyl stearate, and the entire amount of polyisocyanate were stirred by the use of a high speed mixer for about 30 minutes, to obtain the undercoating material in its finished form.

TABLE 1

Undercoating layer (Parts by Weight)

Varying nonmagnetic substance (see Samples listed below) 100

Urethane resin (molecular resin [Mw] about 8,000, (Mixing ratio produced by Takeda Chemical Industries, Ltd. and of urethane marketed under product code of "XE-148") resin to vinyl resin 1: 1)

Vinyl resin (molecular weight [Mw] about 34,000, produced by Sekisui Chemical Co., Ltd. and marketed under product code of "C-130")

Oleic acid (produced by Kao Soap Co. , Ltd. and marketed 1 under trademark designation of "Lunac-OA")

Isocetyl stearate (produced by Kokyu Alcohol K.K. and 2 marketed under product code of "ICS-R")

Polyisocyanate (produced by Sumitomo Bayer K.K. and 7 marketed under product code of "SBU-0856")

Methylethyl ketone 110

Cyclohexanone 35

Toluene 35

A coating material for a magnetic layer was produced by kneading and dispersing the raw materials indicated in Table 2 as follows. Barium ferrite was added piecemeal while stirring with a speed mixer to a solution having the entire amounts of Dispersant 1 and Dispersant 2 dissolved in the entire amount of a solvent. Thirty minutes after the introduction of the entire amount of barium ferrite, the entire amounts of urethane resin and vinyl resin were introduced and stirred continuously. Several minutes thereafter, the entire amount of alumina was introduced and continuously stirred for about 10 minutes to obtain a blend. This blend was transferred into a sand mill and dispersed therein for about 30 hours to obtain a dispersion. In this dispersion, the entire amount of oleic acid, the entire amount of isocetyl stearate, and the entire amount of polyisocyanate were placed and stirred with a high speed mixer for about 30 minutes to obtain the coating material for the magnetic layer in its finished form. TABLE 2

Magnetic Layer (Parts by Weight)

Barium ferrite (He = 750 Oe, BET value = 35 m²/g) 100

Alumina (BET value 8.4 m²/g, produced by Sumitomo 8 Chemical Co. , Ltd. and marketed under product code of "HIT-50")

Dispersant 1 [phosphorylated polyoxyalkyl polyol disclosed 4 in KOKAI (Japanese Unexamined Patent Publication) No. 63-14,326]

Dispersant 2 (N,N-dialkyl-N-hydroxyalkylpolyoxyalkylene 2 ammonium salt disclosed in U.S. Patent No. 3, 123,641)

Urethane resin (polyether polyurethane having a molecular 4 weight [Mw] of about 10,000)

Vinyl acetate (estimated average polymerization degree = 6 50, produced by Union Carbide K.K. and marketed under trademark designation of "VAGH")

Oleic acid (produced by Kao Soap Co., Ltd. and marketed 3 under trademark designation of "Lunac-O-A")

Isocetyl stearate (Kokyu Alcohol K.K. and marketed under 2 product code of "ICS-R")

Polyisocyanate (produced by Sumitomo Bayer Urethane 7 K.K. and marketed under product code of "SBU-0856")

Methylethyl ketone 120

Cyclohexanone 40

Toluene 40

The undercoating material mentioned above was applied with a gravure cater to a base film 2 μm thick (produced by Teijin Limited and marketed under product code of IIAXP-54/6") and the applied layer of the coating material was dried at 40 °C for 40 seconds and then at 100°C for 30 seconds. The dried layer of the coating material was calendared with a metalplated roll at a temperature of 45°C at a pressure of 1,500 pli. After the undercoating layer was amply hardened, the coating material for the magnetic layer mentioned above was applied on the undercoating layer by the use of a gravure cater and then dried at 40 °C for 30 seconds and at 80 °C for 30 seconds. The dried magnetic layer was calendared with a metal -plated roll at a temperature of 45° C at a pressure of 1 ,500 pli.

The thickness of the undercoating layer and that of the magnetic layer were measured after the respective layers had undergone the calendaring treatment. The following samples were prepared:

Sample No. 1: an undercoating layer 2.0 μm thick formed of conductive tin oxide (produced by Ishihara Sangyo Kaisha, Ltd. and marketed under product code of "SN-100") and a binder in a gravimetric mixing ratio of 85: 15.

Sample No. 2: a magnetic layer superposed directly on a supporting member without interposition of an undercoating layer.

Sample No. 3: an undercoating layer comprising carbon black, iron oxide (average particle diameter 0.1 μm), and a binder, with the mixing ratio of the total of nonmagnetic substances to the binder at 85: 15.

Sample No. 4: an undercoating layer comprising non-conductive titanium dioxide, the same tin oxide as used in Sample 1 , and a binder, with the mixing ratio of the total of nonmagnetic substances to the binder at 85: 15.

Sample No. 5: an undercoating layer comprising the same tin oxide as used in Sample 1 and a binder, with the mixing ratio of the tin oxide to the binder at 60:40.

Sample No. 6: an undercoating layer comprising the same tin oxide as used in Sample 1 and a binder, with the mixing ratio of the tin oxide to the binder at 40:60.

Sample No. 7: an undercoating layer of the same composition as used in

Sample 1, except that the thickness of the undercoating layer was changed to 0.3 μm. Sample No. 8: an undercoating layer of the same composition as used in Sample 1, except that the thickness of the undercoating layer was changed to 0.1 μm.

Sample No. 9: an undercoating layer 1.5 μm thick formed in the same composition as in Sample 1, except that granular electroconductive titanium dioxide (produced by Ishihara Sangyo Kaisha, Ltd. and marketed under product code of "ET-300W") was used in the place of tin oxide.

Sample No. 10: provided with an undercoating layer 1.5 μm thick formed in the same composition as in Sample 1, excepting granular electroconductive titanium dioxide of a large particle diameter (produced by Ishihard Sangyo Kaisha, Ltd. and marketed under product code of "ET-500W") was used in the place of tin oxide.

Sample No. 11: an undercoating layer 2.5 μm thick formed in the same composition as in Sample 1, except that needle shaped electroconductive titanium dioxide (produced by Ishihara Sangyo Kaisha, Ltd. and procured as sample) was used in the place of tin oxide.

Sample No. 12: an undercoating layer 2.5 μm thick formed in the same composition as in Sample 1, excepting needle shaped conductive titanium dioxide having a large major diameter (sample for composition) was used in the place of tin oxide.

Sample No. 13: an equivalent of Sample 1 , except that zinc oxide (produced by Mitsui Mining and Smelting Co., Ltd.) was used in the place of tin oxide.

Sample No. 14: an equivalent of Sample 1 , except that indium oxide

(produced by Fuji Titanium Industry, Ltd. and marketed under product code of "In2O3") was used in the place of tin oxide. Sample No. 15: an equivalent of Sample 1 , except that zinc sulfide (produced by Mitsui mining and Smelting Co. , Ltd.) was used in the place of tin oxide.

Sample No. 16: an equivalent of Sample 1, except that barium sulfate

(produced by Mitsui Mining and Smelting Co., Ltd.) was used in the place of the tin oxide.

Sample No. 17: an equivalent of Sample 1 , except that silicon oxide (produced by Tokuyama Soda Co. , Ltd.) was used in the place of tin oxide.

Sample No. 18: an equivalent of Sample 1, excepting magnesium carbonate (produced by Tokuyama Soda Co. , Ltd.) was used in the place of tin oxide.

The 18 samples are compared in Tables 3-6.

TABLE 3 (Tin oxide average particle diameter 0.1 μm)

Sample Thickness of Thickness of Composition of Ra of Light Formability Durability (x Surface Electric No. magnetic undercoating undercoating magnetic transmission of servo 10,000 passes) resistance characteristic layer (μm) layer (μm) (gravimetric layer ratio of tracks (Ω/O) regenerative ratio) (nm) magnetic output (dB) medium (%)

1 0.5 2.0 Tin oxide 85 5 32 OK 2300 2xl0⁶ + 1.4 Binder 15

^'2 0.4 None 6 38 NG 300 3xl0¹⁰ ± 10

3 0.5 2.0 Tin oxide 80 5 31 OK 2400 5xl0⁷ -1.2 Carbon black 5 Binder 15

4 0.5 2.0 Tin oxide 80 5 31 OK 2400 5xl0⁷ + 1 Non-elective 5 conductive

ON titanium dioxide Binder 15

5 0.5 2.0 Tin oxide 60 7 33 OK 2200 lxlO⁸ + .08 Binder 40

6 0.5 2.0 Tin oxide 40 10 34 NG 1400 5xl0⁸ -0.4 Binder 60

7 0.5 0.3 Tin oxide 85 7 36 OK 2200 lxlO⁷ + .05 Binder 15

8 0.5 0.1 Tin oxide 85 12 37 NG 1000 2xl0⁸ -1.5 Binder 15

TABLE 4

(Gravimetric ratio of granular titanium dioxide and binder = 85: 15)

Sample Thickness Thickness of Size of Ra of Light Formability Electric characteristics: No. of magnetic undercoating nonmagnetic magnetic transmission of servo regenerative output layer (μm) layer (μm) substance layer ratio of tracks (dB) (μm) (nm) magnetic medium (%)

9 0.7 1.5 0.05 6 28 OK + .09

10 0.7 1.5 0.2 11 10 OK -1.6

I

TABLE 5

(Gravimetric ratio of acicular titanium dioxide and binder = 85: 15)

Sample Thickness Thickness Size of Ra of Light Formability Electric No. of magnetic of under¬ nonmagnetic magnetic transmission of servo characteristics: layer (μm) coating substance layer ratio of tracks regenerative layer (μm) (μm) (nm) magnetic output (dB) medium (%)

11 1.0 2.5 0.2 5 23 OK + 1.7

12 1.0 2.5 0.4 9 7 OK -0.3

TABLE v

Sample Thickness Thickness Size of non¬ Ra of Light Servo-track Regeneration No. of magnetic of under¬ magnetic magnetic transmittance work-ability output (dB) layer (μm) coating substance layer of magnetic layer (μm) (μm) (nm) medium (%)

13 0.5 2.0 0.1 7 23 OK +0.6

14 0.5 2.0 0.1 7 26 OK +0.5

15 0.5 2.0 0.1 7 27 OK +0.5

00 16 0.5 2.0 0.1 7 29 OK +0.8

17 0.5 2.0 0.1 7 28 OK + 1.0

18 0.5 2.0 0.1 7 24 OK +0.7

*Ra: Average roughness along the central line of the surface of the magnetic layer, determined with a three-dimensional optical roughness tester produced by WYKO K.K. and marked under trademark designation of "TOPO-30. "

^♦Light transmission ratio: The transmission rate of light at 830 nm was measured.

^♦Formability of servo tracks: This property was determined by stamping servo tracks (grooves on a sample and measuring the difference between grooves (servo track region) and the region devoid of such grooves (recording region) with a contact type roughness tester. A sample showing a difference of not less than 0.2 μ was rated as OK.

*Durability: This property was determined by running a sample as held in contact with a head by the use of floppy disc drive and finding, by visual observation, the number of passes required for the surface of the magnetic layer to sustain a scratch. It was evaluated in terms of the number of such passes.

^♦Signal output: This property was determined by recording signals of a sample at 500 kHz by the use of a trially manufactured FD drive and thereafter reviewing the regenerative signals.

*Surface resistance: This property of the magnetic layer of a sample was determined with a high resistivity meter (Hiresta IP) produced by Mitsubishi Petrochemical Co. , Ltd.

As shown above, the tin oxide-containing undercoating layer in this invention acquired sufficient transparency (not less than 40% and not more than

90%) and sufficient conductivity when the thickness of the layer was 2 μm (on one side of the supporting member). When this undercoating layer is used as an undercoating layer in a magnetic disc for a magnetic disc device using a light- transmitting optical servo tracking, therefore, the produced magnetic disc acquires high durability without a sacrifice of the performance of servo tracking.

Claims

CLAIMS:What is claimed is:

1. A high recording density magnetic recording medium for use with a light-transmitting servo-tracking system, comprising: a nonmagnetic supporting member; a magnetic layer; and an undercoating layer interposed between the supporting member and the magnetic layer, wherein the undercoating layer has a thickness of not more than 1 μm and is comprised of an electrically conductive, inorganic, nonmagnetic substance, and wherein the undercoating layer has a light transmittance of at least

40% .

2. The medium of claim 1 , wherein the undercoating layer contains the nonmagnetic substance and a binder in a gravimetric ratio in the range of from

50/50 to 85/15.

3. The medium of claim 2, wherein the nonmagnetic substance is selected from among the group of granular tin oxide, titanium dioxide, zinc oxide, indium oxide, zinc sulfide, barium sulfate, silicon oxide, and magnesium carbonate, and wherein said inorganic nonmagnetic substance has an average particle diameter in the range of from 0.02 to 0.1 μm.

4. The medium of claim 1, wherein said nonmagnetic substance is selected from acicular titanium dioxide and acicular indium oxide, and said nonmagnetic substance has an average major axis diameter in the range of from 0.05 to 0.3 μm.

5. The medium of claim 1 , wherein the nonmagnetic substance is titanium dioxide which has undergone a surface treatment with electroconductive oxide.

6. The medium of claim 1, wherein the undercoating layer further comprises an inorganic, non-electrically conductive, non-magnetic substance.

7. The medium of claim 1 , wherein the non-electrically conductive substance is non-electrically conductive titanium dioxide.

8. The medium of claim 1 , wherein the total light transmittance of the high recording density magnetic recording medium is in the range of from 20 to 40%.