Magnetic recording medium

Abstract: A magnetic recording medium suitable for high density recording comprising a non-magnetic substrate and a mag­ netic layer which contains a non-magnetic binder and ferro­ magnetic powder dispersed therein and is formed on said substrate, coercive force of said magnetic layer being 1,400 to 2,000 oersteds, residual magnetic flux density being at least 1,100 gausses and a thickness of said layer being at least 0.5 µm, which effectively works when it is used togeth­ er with a magnetic head a part of which near a megnetic gap is made of a magnetic material having saturation magnetic flux density of at least 7,000 gausses and which has a mag­ netic gap length of 0.15 to 0.4 µm.

Description: The present invention relates to a magnetic recording disc. More particularly, it relates to a magnetic recording disc suitable for high density recording. For a conventional magnetic recording and reproducing apparatus with a magnetic disc by which high density recording is possible, there is used, as a magnetic material of the magnetic recording medium, cobalt-containing γ-Fe₂O₃ with coercive force of 47,7 · 10³-63,7 · 10³ A · m⁻¹ (600-800 oersteds) and, as a magnetic head, a ferrite head with a gap length of 1-2 µm. With such conventional apparatus, however, recording and reproducing are carried out with D₅₀ of only 20 KBPI. (k bits per inch). One KBPI corresponds to 0,039 k bit per mm (kbpmm) "D₅₀" is a recording density at which output decreases to 50% of recording and reproducing output as a long wavelength and indicates a maximum recording density achieved by a recording and reproducing apparatus. To significantly increase the recording capacity of the magnetic recording medium such as the magnetic disc, high density recording with D₅₀ of 40 KBPI or larger is required. EP-A-0 107 985 discloses a magnetic recording medium which comprises a magnetic layer containing a ferromagnetic powder having a coercitive force (Hc) of from 107,4 · 10³ to 127 · 10³ Am⁻¹. DE-PS-24 46 487 discloses the improvement of the surface smoothness of the magnetic layer to increase the output and the S/N-ratio of a tape. In US-4,455,345 a magnetic recording medium is disclosed. The medium is comprised of a support having a magnetic layer coated thereon. The magnetic layer is comprised of ferro magnetic metal powder and a binder. The ferro magnetic metal powder has a specific surface area of 35 to 70 m²/g, a saturation magnetization (σ s ) of 120 to 160 lmu/g and a metal content of 75 to 98 wt%. The magnetic recording medium has a high S/N ratio, particular a high video S/N ratio and has stable magnetic properties. Although the claims and specification generally refers to a magnetic medium, the Examples and Comparative Examples refer to a magnetic tape. Nevertheless, the specification states that the coated materials are slit into any desired width to provide magnetic recording tapes or discs. Although indicating specific surface areas of the magnetic particles used in this document, no specific values are given for the particle size thereof. Moreover, the specification states that the particle size of the ferro magnetic metal powder particles does not necessarily correspond to a specific particle size. Thus, it was the object of the invention to provide a magnetic recording disc, having a high output level, an improved S/N in combination with a good corrosion resistance and good reliability. The magnetic recording disc should be suitable for high density recording. The object of the invention is met by the magnetic recording disc as defined in claim 1. Further embodiments of the invention can be found in the dependent claims. Fig. 1 is a cross section of a magnetic head used according to the present invention, Fig. 2 is a graph showing relationships between the thickness of the magnetic layer, and S/N ratio, surface roughness and durability, and Fig. 3 is an enlarged cross section of a magnetic recording medium according to the present invention. For producing a magnetic recording disc excellent in high density recording, one comprising metal magnetic powder is proposed. However, it is believed that as a thickness of a magnetic layer of the medium is changed to achieve the high density recording, loss due to thickness varies. Accordingly, the thickness of the magnetic layer is made as thin as possible (Record of Electrical Communication Meeting in Tohoku University, Vol 31, No. 3, page 95 (September 1962)). Our study has revealed that when the magnetic layer has coercive force of larger than 111,4 · 10³ Am⁻¹ (1,400 oersteds) and residual magnetic flux density more of than 0,13 T (1,300 gausses) in case of the thickness of less than 0.5 µm, the loss due to thickness varies, while in case of the thickness of 0.5 µm or larger, the loss due to thickness does not substantially change and D₅₀ characteristics does not decrease at a constant coercive force and residual magnetic flux density. Based on the fact that when the thickness of the magnetic layer is at least 0.5 µm, the D₅₀ characteristics do not vary with the thickness, it is possible to form a thick magnetic layer with thickness of 0.5 µm or more instead of a thin one which tends to form an ununiform layer. According to the above findings, the above objects are achieved by a magnetic disc in accordance with claim 1. Our further study to improve S/N ratio of a magnetic disc has revealed that S/N ratio is influenced by the thickness of the magnetic layer through its surface roughness, which finding is contrary to the conventional observation on the relationship between the thickness of the magnetic layer and the S/N ratio. In other words, as the thickness of the magnetic layer is increased, leveling treatment is effectively carried out so that the surface roughness is minimized and, in turn, the S/N ratio is improved. Such leveling effect is significant when the thickness is at least 0.5 µm, particularly at least 1 µm, and the surface roughness can be made smaller than 0.025 µm so that S/N ratio is improved. In addition, it has been found that when the residual magnetic flux density is within a certain specific range, the S/N ratio is further improved. Therefore, in the embodiments of the magnetic recording medium of the invention, the residual magnetic flux density of the magnetic layer is at least 0,13 T (1,300 gausses) and the roughness of the surface layer of the magnetic layer is 0.025 µm or less. To improve corrosion resistance of the metal magnetic powder, an oxide protective layer is formed on the surface of each powder particle. Since the oxide protective layer is formed by oxidizing the metal powder, the saturation magnetization of the powder is significantly decreased by oxidization so that suitable residual magnetic flux density is not maintained when contained in the magnetic layer of the magnetic recording medium. According to our investigation, corrosion resistance of the magnetic powder in the magnetic layer is improved by selecting the amount of the metal powder based on the total amount of the powder and the binder in the magnetic layer instead of providing the oxide protective layer for the powder particle. By this manner, the magnetic recording characteristics are also improved. Therefore, the amount of the magnetic powder is limited so that the residual magnetic flux density is not larger than 0,20 T (2,000 gausses). In the present invention, the metal magnetic powder may be conventional one, for example, magnetic powder of iron, nickel, cobalt and chromium and magnetic alloys thereof. A particle size of the metal magnetic powder is from 0.1 to 1.0 µm, and its specific surface area measured by a nitrogen adsorption method is not larger than 60 m²/g. When the specific surface area of the magnetic powder is larger than 60 m²/g, the particle size becomes so small that the corrosion resistance of the magnetic layer tends to be deteriorated. The saturation magnetization of the metal magnetic powder is usually from 100 to 130 emu/g, preferably from 110 to 125 emu/g. When it is less than 100 emu/g, the residual magnetic flux density of the magnetic layer is not made higher than a specific value and the S/N ratio is not improved. When it is larger than 130 emu/g, the residual magnetic flux density is not made lower than a specific value and the corrosion resistance of the magnetic layer is deteriorated. The amount of the magnetic powder in the magnetic layer is 60 to 85% by weight, preferably 75 to 82% by weight based on the total amount of the magnetic powder and the binder. The amount of the magnetic powder is important in view of not only corrosion resistance but also magnetic characteristics. That is, when it is less than 65% by weight, the residual magnetic flux density of the magnetic layer is not made higher than a specific value. When it is larger than 85% by weight, a friction coefficient of the surface of the magnetic layer becomes so large that traveling property of the magnetic recording medium, particularly the magnetic disc in a disc drive is deteriorated. In either case, the S/N ratio of the medium is not improved. The thickness of the magnetic layer should be at least 0.5 µm, preferably from 1 to 5 µm to effectively improve the S/N ratio. When the thickness of the magnetic layer is less than 0.5 µm, not only the S/N ratio is not improved due to surface roughness but also the magnetic layer does not have enough durability due to weakness of the layer. However, when the thickness of the magnetic layer is larger than 5 µm, the strength of the layer tends to be weakened by calendering. Thus, the magnetic layer preferably has a thickness of 1 to 5 µm, particularly 2.0 to 5 µm. To achieve the satisfactorily high S/N ratio, the surface roughness of the magnetic layer is preferably less than 0.025 µm. Such the surface roughness is easily achieved by adequately selecting the calendering conditions such as temperature and pressure. In case of the magnetic disc, the magnetic powder should be in a substantially non-oriented condition. This is achieved by eliminating an orientating step, which is carried out after coating and drying the magnetic coating composition in the production of the magnetic recording tape. If necessary, the magnetic disc may be subjected to a non-orientation treatment. In the present specification, the "substantially non-oriented condition" of the magnetic powder means that a ratio of output in one arbitrary direction of the magnetic disc to that in a direction perpendicular to said arbitrary direction is about 0.9 to 1.1. According to the present invention, the residual magnetic flux density is at least 0,13 T (1,300 gausses). As it increases, the S/N ratio is more improved. However, if it exceeds 0,2 T (2,000 gausses), the corrosion resistance is deteriorated. Thus, in view of the corrosion resistance, the residual magnetic flux density of the magnetic layer is preferably from 0,13 T to 0,2 T (1,300 to 2,000 gausses), more preferably from 0,13 T to 0,18 T (1,300 to 1,800 gausses). The binder in which the magnetic powder is dispersed is any of conventionally used ones such as vinyl chloride copolymers, cellulose resins, polyurethane resins, polyacetal resins, polyester resins, isocyanate compounds and the like. The magnetic coating composition may further contain conventionally used additives such as a lubricant, an abrasive, a surfactant, an antistatic agent and the like. The magnetic recording disc of the invention may be produced by a per se conventional method. For example, the metal magnetic powder, the resinous binder and optionally other additives are mixed in a suitable solvent to prepare a magnetic coating composition, coating said composition on one or both surfaces of the non-magnetic substrate and drying it to form the magnetic layer(s) on the substrate, calendering it and finally cutting the substrate having the magnetic layer to produce a magnetic disc. Specific examples of the solvent used in the preparation of the coating composition are ketones, alcohols, and toluene. The substrate can be made of a resin film such as a polyester or polyimide resin film or of an aluminum foil. Our further study has revealed that, to make full use of the characteristics of the magnetic recording disc, its coercive force, residual magnetic flux density and surface roughness should be correlated with a saturation magnetic flux density of a magnetic material placed near a magnetic gap and a gap length of the magnetic head (i.e., length of a magnetic gap along the traveling direction of the magnetic recording disc, and therefore, these factors should be determined in relation to each other. Accordingly, in addition to make the magnetic recording disc having the above described characteristics, the magnetic head should be so designed that a part of the magnetic head near the magnetic gap is made of a magnetic material having saturation magnetic flux density of at least 7,000 gausses and the magnetic gap length is 0.15 to 0.4 µm. Fig. 1 shows a cross section of the magnetic head according to the present invention. The magnetic head comprises the first core half 1, the second core half 2 and an exciting coil 3 wound around either of the core halves 1 and 2. The first core half 1 comprises the first core base 2 and the first laminated magnetic film 6 attached on one surface of the base 2 facing the magnetic gap 5. The laminated magnetic film 6 comprises two magnetic layers 8,8 and a non-magnetic layer 7 interposed between the magnetic layers 8,8. The second core half has substantially the same structure as that of the first one as shown in Fig. 1. Namely, the second core half 2 comprises the second core base 9 and the second laminated magnetic film 10 attached on one surface of the base 9 facing the magnetic gap 5. The laminated magnetic film 10 comprises two magnetic layers 12,12 and a non-magnetic layer 11 interposed between the magnetic layers 12,12. The core bases 4 and 9 are made of a magnetic material with good magnetic permeability such as manganese-zinc ferrite and nickel-zinc ferrite, or a non-magnetic material such as ceramic or zinc ferrite. The non-magnetic layers 7 and 11 are made of a non-magnetic material such as silicon dioxide and glass. The magnetic layers 8 and 12 are made of an amorphous metal alloy having high saturation magnetic flux density and large magnetic permittivity. Specific examples of the amorphous metal alloy are those comprising at least one element selected from the group consisting of iron, nickel and cobalt and at least one element selected from the group consisting of phosphorus, carbon, boron and silicon; an alloy comprising one of these element as a primary component and at least one additional element selected from the group consisting of aluminum, germanium, beryllium, tin, molybdenum, indium, tungsten, titanium, manganese, chromium, zirconium, hafnium and niobium; and an alloy comprising cobalt or zirconium as a primary component and at least one of the above described additional elements. The present invention will be hereinafter explained further in detail by following examples, wherein parts and % are by weight unless otherwise indicated, specific surface area is measured by the nitrogen adsorption method and "σ s ", "Hc", "Br", "T" ("Oe" and "G") stand for "saturation magnetization", "coercive force", "residual magnetic flux density", "Tesla" ("oersted" and "gauss", respectively). Example 1 The following components were mixed by a ball mill for 72 hours to homogeneously disperse them. Then, an isocyanate compound (COLONATA, trifunctional low molecular weight isocyanate compound manufactured by Nippon Polyurethane Kogyo Co.) (10 parts) was added and mixed for additional one hour to prepare a magnetic coating composition. The composition was coated on both surfaces of a film of polyethylene terephthalate with a thickness of 75 µm as a substrate and dried to form magnetic layers. Then, the substrate coated with the magnetic layers was calendered and punched to shape into a magnetic disc with a diameter of 20.02 cm. The percentage of the magnetic powder in the magnetic layer based on the total amount of the magnetic powder and the binder consisting of the vinyl chloride/vinyl acetate/vinyl alcohol copolymer, the polyurethane resin and the isocyanate compound was 75%. The used magnetic powder had a coercive force of 99,5 · 10³-150,4 · 10³ Am⁻¹ (1,250-1,890 Oe) and the saturation magnetization of 78-140 emu/g. By changing and calendering temperature and pressure. magnetic recording media with various surface roughness were produced. The coercive force, residual magnetic flux density and surface roughness of the sample recording media and D₅₀ and head output at various thickness of the magnetic layer are shown in Table 1. The magnetic recording medium with D₅₀ larger than 50 KBPI is satisfactory as a recording medium for high density recording. Head output is preferably at least 70 µV in view of amplifier noise. The coercive force and residual magnetic flux density were measured by a sample vibration type flux meter, the thickness of the magnetic layer was measured by a tracer type surface roughness meter and the surface roughness was measured by the same surface roughness meter with a stylass diameter of 2 µm, a tracer load of 23 mg, cut off of 0.08 mm at a scanning rate of 0.03 mm/sec. Electromagnetic conversion characteristics were measured by a magnetic head comprising magnetic cores made of iron-nickel alloy with a gap length of 0.3 µm, a track width of 30 µm and a relative rate against the magnetic recording medium of 3.1 m/sec. As understood from the results of Table 1, although the conventional magnetic recording medium produced in Comparative Example 1 has large residual magnetic flux density of 0,173 T (1,730 gausses) and small surface roughness of 0.019 µm, it has small D₅₀ and head output since the coercive force is only 99,5 · 10³ Am⁻¹ (1,250 oersteds). In contrast, the magnetic layers of the magnetic recording mediums of Examples 1-6 and 8 according to the present invention have the coercive force of not less than 111,4 · 10³ Am⁻¹ (1.400 oersteds) and the residual magnetic flux density of 0.13 T (1,300 gausses), and therefore have D₅₀ of 56 KBPI or larger. When the coercive force of the magnetic layer exceeds 159,1 · 10³ Am⁻¹ (2,000 oersteds), the core of the magnetic head may be saturated. Therefore. the coercive force should be in a range between 111,4 · 10³ and 159,1 · 10³ Am⁻¹ (1,400 and 2,000 oersteds). When the residual magnetic flux density is less than 0,11 T (1,100 gausses), sufficient head output is not obtained in connection with amplifier noise. Example 9 In the same manner as in Example 1 but coating the magnetic layer in a thickness of 0.4, 0.5, 4.0 or 5.5 µm, a magnetic recording medium was produced. D₅₀ and application ununiformity in terms of modulation of the magnetic layer were measured. The results are shown in Table 2. As understood from the above results, the magnetic layers with a thickness of 0.5 µm or larger have substantially the same D₅₀ and small modulation which corresponds to small application ununiformity of the layer. Examples 10-18 Ten magnetic discs were produced in the same manner as in Example 1 but using metal powder having specific surface area and magnetic characteristics as shown in Table 3 in a predetermined amount and calendering the magnetic layer to obtain surface roughness as shown in Table 3. Hc and Br of the magnetic discs are shown in Table 3. Comparative Example 2 In the same manner as in Example 1 but using Co-containing Fe₂O₃ powder (specific surface area 24 m², Hc 53,3 · 10³ Am⁻¹ (670 Oe), σ s , 78 emu/g) in place of iron powder, a comparative magnetic disc comprising a magnetic layer having surface roughness of 0.018 µm, Hc of 51,7 · 10³ Am⁻¹ (650 Oe) and Br of 0,08 T (800 G). The magnetic discs produced in Examples 10-18 and Comparative Example 2 were tested on the S/N ratio, D₅₀, modulation and corrosion resistance. The results are shown in Table 3. The corrosion resistance was tested by measuring the decrease of Br after keeping the magnetic disc at 60°C, 90% RH for one week. When the decrease is less than 10%, the corrosion resistance of the magnetic disc is practically sufficient. Other characteristics were measured by means of an iron-nickel alloy made magnetic head with a head gap of 0.3 µm, and S/N ratio was represented by a relative value against that of Comparative Example 2 being 0 (zero) dB. In Table 3, an orientation ratio of the magnetic disc, specific surface area, magnetic characteristics of the magnetic powder used and surface roughness and magnetic characteristics of the magnetic layer are also shown. As understood from the above results, the magnetic layers with Br of smaller than 0,13 T (1,300 gausses) in Examples 16 and 17 show comparatively small S/N ratio of +1.3 and +1.0 dB. Although the magnetic layer of Example 18 has Br of larger than 0,135 T (1,350 gausses), its S/N ratio is only +4.0 dB since its surface roughness is larger than 0.025 µm. In contrast, the magnetic discs of Examples 10-15 have S/N ratio of +4.5 dB or larger. Particularly, when Br of the magnetic layer is larger than 0,15 T (1,500 gausses) and the surface roughness is smaller than 0.020 µm as in Examples 10-14, the S/N ratio is +5.2 dB or larger. The magnetic layers of Examples 10-13 contain the metal magnetic powder in an amount of 65 to 85% so that its corrosion resistance is less than 10%. Example 19 In the same manner as in Example 1 but changing a thickness of a magnetic layer, a magnetic disc was produced, and its S/N ratio, surface roughness and durability were measured. Durability of the magnetic disc is measured and evaluated by loading the magnetic disc in a magnetic recording and reproducing apparatus, traveling the disc with contacting it with a magnetic head while observing decrease of reproducing output level due to abrasion of the magnetic disc and measuring traveling time till the reproducing output decreased to 70% of the initial output. The results are shown in Fig. 2. As understood from Fig. 2, when the thickness of the magnetic layer is 1 µm or larger, the calendering of the layer is effectively carried out so that the surface roughness is decreased and S/N ratio and durability are improved. However, the durability tends to decrease when the thickness of the magnetic layer exceeds 5 µm.

Claims: A magnetic recording disc comprising a non-magnetic substrate and a magnetic layer comprising a non-magnetic binder and ferromagnetic powder dispersed therein formed on said substrate wherein said magnetic layer has a coercive force of 111.4 x 10³ to 159.2 x 10³ Am⁻¹ (1400 to 2000 Oersteds), a residual magnetic flux density from 0.13 to 0.2 T ( 1300 to 2000 gausses) and a thickness of at least 0.5 micrometre, wherein the ferromagnetic powder is in a substantially non-orientated condition, has a particle size of 0.1 to 1.0 micrometre and is contained in the magnetic layer in an amount of 60 to 85 % by weight based on the total weight of the magnetic powder and the binder, wherein the magnetic recording disc has a D₅₀-value of larger than 1.97 kbpmm (50 KBPI), when measured by a magnetic head comprising magnetic cores made of ironnickel alloy with a gap length of 0.3 µm, a track width of 30 µm and a relative rate against the magnetic recording disc of 3.1 m/s. A magnetic recording disc according to claim 1, wherein the magnetic layer has surface roughness of 0.025 µm or less. A magnetic recording disc according to claim 1, wherein the thickness of the magnetic layer is 1 to 5 µm. A magnetic recording disc according to claim 3, wherein the thickness of the magnetic layer is 2.0 to 5 µm. A magnetic recording disc according to claim 1, wherein the ferromagnetic powder has a specific surface area of not larger than 60 m²/g measured by a nitrogen adsorption method. A magnetic recording disc according to claim 1, wherein the amount of the ferromagnetic powder is 65 to 85 % by weight based on the total weight of the ferromagnetic powder and the binder. A magnetic recording disc according to claim 1, wherein the D₅₀-value is at least 2,20 kbpmm (56 KBPI).

Inventors: FUJIWARA HIDEO, MIYAKE AKIRA C O HITACHI MAXEL, MIZUSHIMA KUNIO, SAKEMOTO AKITO, FUJIWARA, HIDEO, MIYAKE, AKIRA C/O HITACHI MAXELL LTD., MIZUSHIMA, KUNIO, SAKEMOTO, AKITO
Applicants: HITACHI MAXELL, HITACHI MAXELL LTD.
Application year: 1985
Main classification: G11B5
IPC codes: G11B5