Glass Disk for Magnetic Recording Medium and Magnetic Recording Apparatus Using the Same

Abstract

A glass substrate for magnetic recording media according to the present invention is characterized by being shaped like a disk, exhibiting a strain point of 695-780°C, exhibiting a temperature of not more than 1300°C at 104.5dPa•s, and having a Young's modulus of not less than 78 GPa.

Description

Technical Field

[0001] The present invention relates to a glass disk for a magnetic recording medium and a magnetic recording apparatus using the same.

Background Art

[0002] A magnetic recording apparatus includes a magnetic recording medium in which a magnetic layer is formed on a substrate for a magnetic recording medium, and information can be recorded using the magnetic layer. Conventionally, an aluminum alloy substrate has been used as the substrate for a magnetic recording medium used in a magnetic recording apparatus. Currently, with the demand for higher recording density, thinning of the substrate for a magnetic medium is being studied. However, when the aluminum alloy substrate is thinned, its rigidity is lost, so attention has been focused on a glass disk (glass substrate) having excellent rigidity, flatness, smoothness, etc.

[0003] In recent years, in order to meet the need for further higher recording density, a magnetic recording medium using an energy-assisted magnetic recording method, that is, an energy-assisted magnetic recording medium, has been studied. Also for an energy-assisted magnetic recording medium, a glass disk is used, and a magnetic layer or the like is formed on the surface of the glass disk. In an energy-assisted magnetic recording medium, an ordered alloy having a large magnetic anisotropy coefficient Ku (hereinafter referred to as "high Ku") is used as the magnetic material of the magnetic layer.

[0004] In order to increase the degree of ordering (orderliness) of the magnetic layer and achieve high Ku, the base material including the glass disk may be heat-treated at a high temperature of about 800°C during or before and after the formation of the magnetic layer. Since this heat treatment temperature needs to be higher as the recording density increases, higher heat resistance than that of a conventional glass disk for a magnetic recording medium is required. Also, after the formation of the magnetic layer, laser irradiation may be performed on the base material including the glass disk. Such heat treatment and laser irradiation also aim to increase the annealing temperature and coercive force of the magnetic layer containing an FePt-based alloy or the like.

Summary of the Invention

Problems to be Solved by the Invention

[0005] By the way, in order not to cause large deformation during high-speed rotation, a glass disk for a magnetic recording medium is required to have high rigidity (Young's modulus). More specifically, in a disk-shaped magnetic recording medium, while the medium is rotated at high speed around the central axis, the magnetic head is moved in the radial direction, and information is written and read along the rotation direction. In recent years, the rotational speed for increasing the writing speed and reading speed has been advancing in the direction of higher speed, from 5400 rpm to 7200 rpm, and further to 10000 rpm. In a disk-shaped magnetic recording medium, positions for recording information are assigned in advance according to the distance from the central axis. Therefore, if the glass disk is deformed during rotation, misalignment of the magnetic head occurs, making accurate reading difficult.

[0006] In recent years, by mounting a DFH (Dynamic Flying Height) mechanism on the magnetic head, a significant narrowing (reduction in flying height) of the gap between the recording and reproducing element part of the magnetic head and the surface of the magnetic recording medium has been achieved, aiming for further high recording density. The DFH mechanism is a mechanism in which a heating part such as an extremely small heater is provided near the recording and reproducing element part of the magnetic head, and only the periphery of the element part is thermally expanded in the direction of the medium surface. By providing such a mechanism, the distance between the magnetic head and the magnetic layer of the medium approaches, so that signals of smaller magnetic particles can be picked up, making it possible to achieve high recording density. On the other hand, since the gap between the recording and reproducing element part of the magnetic head and the surface of the magnetic recording medium becomes extremely small, for example, 2 nm or less, there is a risk that the magnetic head may collide with the surface of the magnetic recording medium even by a slight impact. This tendency becomes more prominent as the rotation speed increases. Therefore, at high rotation speeds, it is important to prevent the occurrence of deflection and flutter of the glass disk that cause such collisions.

[0007] Furthermore, with the worldwide increase in the use of data centers and servers in recent years, there has been a demand for cost reduction of these glass disks. In order to reduce the cost of glass disks, formability is important. Furthermore, it is also effective to adopt the overflow down-draw method or the float method to form a large glass substrate with a plate thickness close to the product thickness and process it into a disk shape.

[0008] Therefore, the present invention has been made in view of the above circumstances, and its object is to create a glass disk for a magnetic recording medium that is less likely to bend or flutter during high-speed rotation, has sufficient heat resistance to achieve a significantly high recording density, and contributes to cost reduction.

Means for Solving the Problems

[0009] As a result of repeating various experiments, the present inventor has found that the above technical problems can be solved by strictly regulating the glass properties of the glass disk, and proposes the present invention. That is, the glass disk for a magnetic recording medium of the present invention has a disk shape, the strain point is 695 to 780 ° C, the temperature at 10 4.5 dPa·s is 1300 ° C or lower, and the Young's modulus is 78 GPa or more. Further, it is preferable that the glass disk for a magnetic recording medium of the present invention has a circular opening formed in the center. Here, the "strain point" refers to a value measured based on the method of ASTM C336. "The temperature at 10 4.5 dPa·s" refers to a value measured by the platinum ball pulling-up method. The "Young's modulus" can be measured by a well-known resonance method.

[0010] FIG. 1 is an upper perspective view for showing the disk shape. The disk shape preferably refers to a disk shape, and more preferably a shape in which a circular opening is formed in the center (see FIG. 1).

[0011] In the glass disk for a magnetic recording medium of the present invention, the strain point is regulated to be 695 °C or higher. By doing so, even when heat treatment at a high temperature such as heat assist or laser irradiation is performed, deformation of the glass disk is less likely to occur. As a result, when aiming for a higher Ku value, a higher heat treatment temperature can be adopted, making it easier to fabricate a magnetic recording device with a high recording density.

[0012] Also, in the glass disk for a magnetic recording medium of the present invention, the temperature at a high-temperature viscosity of 10 4.5 dPa·s is regulated to be 1300 °C or lower. By doing so, the formability is improved, which can contribute to cost reduction of the glass disk.

[0013] Furthermore, in the glass disk for a magnetic recording medium of the present invention, the Young's modulus is regulated to be 78 GPa or higher. By doing so, during high-speed rotation, bending and fluttering of the glass disk are less likely to occur, so that collision between the information recording medium and the magnetic head can be prevented.

[0014] The glass disk for a magnetic recording medium of the present invention preferably contains, in mol%, 60 to 71% of SiO2, 10 to 16% of Al2O3, 0 to 5% of B2O3, 0 to 0.1% of Na2O, 0 to 1% of K2O, 0 to 12% of MgO, 0 to 12% of CaO, 0 to 10% of SrO, 0 to 10% of BaO, 0 to 1% of ZrO2, and 0 to 1% of SnO2 as the glass composition.

[0015] Also, the glass disk for a magnetic recording medium of the present invention preferably has an average surface roughness Ra of the surface of 1.0 nm or less. By doing so, even when the bit size is miniaturized for high recording density, improvement of magnetic characteristics becomes possible. Here, the "average surface roughness Ra of the surface" refers to the average surface roughness Ra of the main surfaces (both surfaces) excluding the end faces, and can be measured by, for example, an atomic force microscope (AFM).

[0016] Also, the glass disk for a magnetic recording medium of the present invention preferably has an average linear transmittance of 70% or higher in an optical path length of 1 mm and a wavelength range of 350 to 1500 nm.

[0017] In addition, the glass disk for a magnetic recording medium of the present invention preferably has a magnetic layer on its surface. This makes it easier to apply to an energy-assisted magnetic recording medium.

[0018] The glass substrate for a magnetic recording medium of the present invention is characterized in that the strain point is 695 to 740 °C, the temperature at 10 4.5 dPa·s is 1300 °C or lower, and the Young's modulus is 78 GPa or higher.

[0019] In addition, the glass substrate for a magnetic recording medium of the present invention preferably contains, in mol%, 60 to 71% of SiO2, 10 to 16% of Al2O3, 0 to 5% of B2O3, 0 to 0.1% of Na2O, 0 to 1% of K2O, 0 to 12% of MgO, 0 to 12% of CaO, 0 to 10% of SrO, 0 to 10% of BaO, 0 to 1% of ZrO2, and 0 to 1% of SnO2 as the glass composition.

[0020] In addition, the magnetic recording device of the present invention preferably includes the above glass disk for a magnetic recording medium.

Brief Description of the Drawings

[0021]

Figure 1

Embodiments for Carrying Out the Invention

[0022] In the glass disk for a magnetic recording medium of the present invention, the strain point is 695 °C or higher, preferably 697 °C or higher, 700 °C or higher, 702 °C or higher, 705 °C or higher, 710 °C or higher, 711 °C or higher, 712 °C or higher, 713 °C or higher, 714 °C or higher, particularly 715 °C or higher. If the strain point is too low, it becomes difficult to perform heat treatment or laser irradiation at high temperatures, and it becomes difficult to manufacture a magnetic recording medium with a high recording density. On the other hand, if the strain point is too high, the melting temperature and the forming temperature become high, so the production efficiency of the glass substrate tends to decrease. Therefore, the strain point is 780 °C or lower, preferably 775 °C or lower, 770 °C or lower, 768 °C or lower, 765 °C or lower, 763 °C or lower, 760 °C or lower, 758 °C or lower, 755 °C or lower, 753 °C or lower, 750 °C or lower, 748 °C or lower, 745 °C or lower, 743 °C or lower, 740 °C or lower, 738 °C or lower, 735 °C or lower, 733 °C or lower, 730 °C or lower, 725 °C or lower, 720 °C or lower, particularly 715 °C or lower. The most preferable range of the strain point is 715 to 770 °C.

[0023] In the glass disk for a magnetic recording medium of the present invention, the lower the temperature at 10 4.5 dPa·s of the high-temperature viscosity, the lower the load on the forming equipment can be reduced. The temperature at 10 4.5 dPa·s is 1300 °C or lower, preferably 1290 °C or lower, 1280 °C or lower, 1275 °C or lower, 1270 °C or lower, 1265 °C or lower, 1260 °C or lower, 1255 °C or lower, particularly 1250 °C or lower. On the other hand, if the temperature at 10 4.5 dPa·s is too low, the strain point cannot be designed to be high. Therefore, the temperature at 10 4.5 dPa·s is preferably 1150 °C or higher, 1170 °C or higher, 1180 °C or higher, 1185 °C or higher, 1190 °C or higher, 1195 °C or higher, particularly 1200 °C or higher.

[0024] In the glass disk for a magnetic recording medium of the present invention, the Young's modulus is 78 GPa or higher, preferably 80 GPa or higher, 81 GPa or higher, 82 GPa or higher, particularly preferably 83 to 100 GPa. If the Young's modulus is too low, when rotating at high speed, the glass disk is likely to bend or flutter, so the information recording medium and the magnetic head are likely to collide.

[0025] The glass disk for a magnetic recording medium of the present invention preferably contains, in mol%, 60 to 71% of SiO2, 10 to 16% of Al2O3, 0 to 5% of B2O3, 0 to 0.1% of Na2O, 0 to 1% of K2O, 0 to 12% of MgO, 0 to 12% of CaO, 0 to 10% of SrO, 0 to 10% of BaO, 0 to 1% of ZrO2, and 0 to 1% of SnO2 as a glass composition. The reasons for limiting the content ranges of the respective components as described above are shown below. In the description of the content ranges of the respective components, the % notation means mol%.

[0026] If the content of SiO2 is too small, the chemical resistance, particularly the acid resistance, tends to decrease, and the strain point tends to decrease. On the other hand, if the content of SiO2 is too large, the etching rate with hydrofluoric acid or a mixed solution of hydrofluoric acid tends to be slow, the high-temperature viscosity becomes high, the meltability tends to decrease, and further, SiO2-based crystals, particularly cristobalite, precipitate, and the liquid-phase viscosity tends to decrease. Therefore, the preferable upper limit content of SiO2 is 71%, 70.5%, 70%, 69.5%, 69%, 68.5%, 68%, particularly 67.5%, and the preferable lower limit content is 60%, 61%, 62%, 62.5%, 63%, 63.5%, 64%, 64.5%, particularly 65%. The most preferable content range is 66 to 70.5%.

[0027] If the content of Al2O3 is too small, the strain point decreases, the thermal shrinkage amount increases, the Young's modulus decreases, and the glass disk tends to bend. On the other hand, if the content of Al2O3 is too large, the resistance to BHF (buffered hydrofluoric acid) decreases, cloudiness easily occurs on the glass surface, and the crack resistance tends to decrease. Further, SiO2-Al2O3-based crystals, particularly mullite, precipitate in the glass, and the liquid-phase viscosity tends to decrease. The preferable upper limit content of Al2O3 is 16%, 15.5%, 15%, 14.5%, particularly 14%, and the preferable lower limit content is 10%, 10.5%, 11%, 11.5%, particularly 12%. The most preferable content range is 12 to 14%.

[0028] B2O3 acts as a flux, a component that reduces viscosity and improves meltability. If the content of B2O3 is too low, it does not sufficiently act as a flux, and the BHF resistance and crack resistance are liable to decrease. Furthermore, the liquidus temperature is liable to increase. On the other hand, if the content of B2O3 is too high, the strain point, heat resistance, and acid resistance are liable to decrease, and particularly the strain point is liable to decrease. Also, the glass is liable to phase-separate. The preferable upper limit content of B2O3 is 5%, particularly 4.5%, and the preferable lower limit content is 0%, 1%, 1.5%, 2%, particularly 2.5%. The most preferable content range is 2.5 - 4.5%.

[0029] Alkali metal oxides (Li2O, Na2O, K2O) deteriorate the properties of the magnetic film formed on the glass disk, so it is preferable to reduce their contents to 0.1% (desirably 0.06%, 0.05%, 0.02%, particularly 0.01%) respectively.

[0030] MgO is a component that reduces the high-temperature viscosity without lowering the strain point and improves meltability. Also, among RO, MgO has the most effect of reducing density, but if introduced in excess, SiO2-based crystals, particularly cristobalite, precipitate, and the liquid-phase viscosity is liable to decrease. Furthermore, MgO is a component that easily reacts with BHF to form a product. This reaction product may adhere or attach to the glass surface, causing the glass to become cloudy. Furthermore, impurities such as Fe2O3 may be mixed into the glass from the raw material for introducing MgO such as dolomite, and the transmittance of the glass disk may decrease. Therefore, the preferable upper limit content of MgO is 12%, 11.5%, 11%, 10.5%, 10%, 9.5%, 9.3%, 9%, 8.5%, 8%, 7.5%, 7%, 6.5%, particularly 6%, and the preferable lower limit content is 0%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, particularly 4.5%. The most preferable content range is 4.5 - 6%.

[0031] Similar to MgO, CaO is a component that reduces the high-temperature viscosity without lowering the strain point, significantly improving the fusibility. However, if the CaO content is too high, SiO2-Al2O3-RO system crystals, especially anorthite, will precipitate, making it easier for the liquid-phase viscosity to decrease and the BHF resistance to decline. There is a risk that the reaction products will adhere or stick to the glass surface, clouding the glass. Therefore, the preferred upper limit content of CaO is 12%, 11.5%, 11%, 10.5%, 10%, 9.5%, 9%, especially 8.5%, and the preferred lower limit content is 0%, 1%, 2%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 5.6%, 6%, especially 6.5%. The most preferred content range is 6.5 - 8.5%.

[0032] SrO is a component that enhances chemical resistance and devitrification resistance. However, if its proportion is too high in the overall RO, the fusibility is likely to decrease, and the density and thermal expansion coefficient are likely to increase. Therefore, the content of SrO is preferably 0 - 10%, 0 - 9%, 0 - 8%, 0 - 7%, 0 - 6%, especially 0 - 5%.

[0033] BaO is a component that enhances chemical resistance and devitrification resistance. However, if its content is too high, the density is likely to increase. Also, since SiO2-Al2O3-B2O3-RO system glass is generally difficult to melt, from the perspective of supplying high-quality glass disks inexpensively and in large quantities, it is very important to increase the fusibility and reduce the defect rate due to bubbles, foreign substances, etc. However, among RO, BaO has a poor effect of increasing fusibility. Therefore, the preferred upper limit content of BaO is 10%, 9%, 8%, 7%, 6%, especially 5%, and the preferred lower limit content is 0%, 0.1%, 0.3%, especially 0.2%.

[0034] SnO2 functions as a fining agent that reduces bubbles in the glass. On the other hand, if the SnO2 content is too high, devitrification crystals of SnO2 are likely to form in the glass. The preferred upper limit content of SnO2 is 1%, 0.5%, 0.4%, especially 0.3%, and the preferred lower limit content is 0%, 0.01%, 0.03%, especially 0.05%. The most preferred content range is 0.05 - 0.3%.

[0035] ZrO₂ is a component that enhances chemical durability. However, when the introduction amount increases, crystallization of ZrSiO₄ is likely to occur. The preferable upper limit content of ZrO₂ is 1%, 0.5%, 0.3%, 0.2%, especially 0.1%. From the perspective of chemical durability, it is preferable to introduce 0.001% or more. The most preferable content range is 0.001% - 0.1%. Note that ZrO₂ may be introduced from raw materials or by elution from refractories.

[0036] In addition to the above components, other components may be introduced. The introduction amount is preferably 5% or less, 3% or less, especially 1% or less.

[0037] ZnO is a component that improves fusibility and resistance to BHF. However, if the content is too high, the glass is likely to devitrify or the strain point decreases, making it difficult to ensure heat resistance. Therefore, the content of ZnO is preferably 0 - 10%, 0 - 5%, 0 - 3%, 0 - 2%, especially 0 - 1%.

[0038] P₂O₅ is a component that lowers the liquidus temperature of SiO₂ - Al₂O₃ - CaO - based crystals (especially anorthite) and SiO₂ - Al₂O₃ - based crystals (especially mullite). However, when a large amount of P₂O₅ is introduced, the glass is likely to phase - separate. Therefore, the content of P₂O₅ is preferably 0 - 10%, 0 - 5%, 0 - 3%, 0 - 2%, 0 - 1%, especially 0 - 0.1%.

[0039] TiO₂ is a component that lowers the high - temperature viscosity and enhances fusibility, and also enhances chemical durability. However, when introduced in excess, the ultraviolet transmittance is likely to decrease. The content of TiO₂ is preferably 3% or less, 1% or less, 0.5% or less, 0.1% or less, 0.05% or less, 0.03%, especially 0.01% or less. Note that when a very small amount of TiO₂ is introduced (for example, 0.0001% or more), an effect of suppressing coloring by ultraviolet rays can be obtained. The most preferable content range is 0.0001 - 0.01%.

[0040] As2O3 and Sb2O3 are components that act as clarifying agents. However, since they are environmentally burdensome chemical substances, it is desirable to avoid using them as much as possible. The content of As2O3 and Sb2O3 is preferably less than 0.3%, less than 0.1%, less than 0.09%, less than 0.05%, less than 0.03%, less than 0.01%, less than 0.005%, and particularly preferably less than 0.003%.

[0041] Iron is a component that is mixed in from raw materials as an impurity. However, if the iron content is too high, there is a risk that the ultraviolet transmittance will decrease. Therefore, the preferred lower limit content of iron, in terms of Fe2O3 conversion, is 0.0001%, 0.0005%, 0.001%, and particularly 0.0015%. The preferred upper limit content, in terms of Fe2O3 conversion, is 0.01%, 0.009%, 0.008%, 0.007%, and particularly 0.006%. The most preferred content range is 0.0015% to 0.006%.

[0042] Cr2O3 is a component that is mixed in from raw materials as an impurity. However, if the Cr2O3 content is too high, when inspecting for foreign substances inside the glass disk due to scattered light, it becomes difficult for light to pass through, and there is a risk that problems will occur in the foreign substance inspection. In particular, when the substrate size is 730 mm × 920 mm or more, this problem is likely to occur. Also, when the plate thickness of the glass disk is small (for example, 0.5 mm or less, 0.4 mm or less, and particularly 0.3 mm or less), the amount of scattered light decreases, so the significance of regulating the Cr2O3 content increases. The preferred upper limit content of Cr2O3 is 0.001%, 0.0008%, 0.0006%, 0.0005%, and particularly 0.0003%. The preferred lower limit content is 0.00001%. The most preferred content range is 0.00001 to 0.0003%.

[0043] SO3 is a component that is mixed in from raw materials as an impurity. However, if the SO3 content is too high, there is a risk of generating bubbles called reboil during melting and forming, which can cause defects in the glass. The preferred upper limit content of SO3 is 0.005%, 0.003%, 0.002%, and particularly 0.001%. The preferred lower limit content is 0.0001%. The most preferred content range is 0.0001% to 0.001%.

[0044] The glass disk for a magnetic recording medium of the present invention preferably has the following characteristics.

[0045] The glass disk for a magnetic recording medium is required to have an appropriate coefficient of thermal expansion in order to enhance the reliability of recording and reproduction of the magnetic recording medium. Specifically, a hard disk drive (HDD) incorporating the magnetic recording medium has a structure in which the central portion is pressed by the spindle of a spindle motor to rotate the magnetic recording medium itself. For this reason, if the difference in the coefficient of thermal expansion between the glass disk and the spindle material is too large, the thermal expansion and contraction of both are different with respect to the ambient temperature change, and thus a phenomenon occurs in which the magnetic recording medium is deformed. When such a phenomenon occurs, the written information cannot be read by the magnetic head, and there is a risk of impairing the reliability of recording and reproduction. Therefore, it is desirable that the glass disk for a magnetic recording medium has a coefficient of thermal expansion that matches the coefficient of thermal expansion of the spindle material (for example, stainless steel, etc.). From such a viewpoint, the average linear coefficient of thermal expansion in the temperature range of 30 to 380 °C is preferably 25×10 -7 ~60×10 -7 / °C, 28×10 -7 ~55×10 -7 / °C, particularly 30×10 -7 ~50×10 -7 / °C.

[0046] The liquidus temperature is preferably 1350 °C or lower, 1330 °C or lower, 1300 °C or lower, 1280 °C or lower, 1260 °C or lower, 1250 °C or lower, 1240 °C or lower, particularly 1230 °C or lower. The liquidus viscosity is preferably 10 3.8 dPa·s or more, 10 4.4 dPa·s or more, 10 4.6 dPa·s or more, 10 4.8 dPa·s or more, particularly 10 5.0It is 0.1 dPa·s or more. By doing so, devitrified crystals are less likely to precipitate during molding, and it becomes easier to mold into a plate shape by the overflow down-draw method or the like. Therefore, without polishing the surface or with a small amount of polishing, the average surface roughness Ra of the surface can be made 1.0 nm or less, particularly 0.2 nm or less. As a result, it becomes possible to enhance the magnetic properties by miniaturizing the bit size. Also, by reducing devitrified crystals and the amount of polishing, the cost of the glass disk can be reduced. Here, the "liquidus temperature" can be calculated by putting glass powder that passes through a standard sieve of 30 mesh (500 μm) and remains on a 50 mesh (300 μm) into a platinum boat and then holding it in a temperature gradient furnace for 24 hours to measure the temperature at which crystals precipitate. The "liquidus viscosity" refers to the viscosity of the glass at the liquidus temperature and can be measured by the platinum ball pulling-up method.

[0047] The average linear transmittance in an optical path length of 1 mm and a wavelength range of 350 to 1500 nm is preferably 70% or more, 80% or more, particularly 90% or more. If the average linear transmittance in an optical path length of 1 mm and a wavelength range of 350 to 1500 nm is too low, when laser irradiation is performed, the laser light is not sufficiently irradiated to the magnetic layer, and it becomes difficult to increase the Ku value of the magnetic layer.

[0048] β-OH is preferably 0.30 / mm or less, 0.25 / mm or less, 0.20 / mm or less, 0.15 / mm or less, particularly 0.10 / mm or less. If β-OH is too large, the annealing point is likely to decrease. In addition, if β-OH is too small, there is a high need to introduce dry components such as chlorine. In that case, chlorine etc. in the glass may remain at a high level, increasing the environmental load. Therefore, β-OH is preferably 0.01 / mm or more, particularly 0.02 / mm or more.

[0049] As methods for reducing β-OH, the following methods can be mentioned. (1) Select raw materials with a low water content. (2) Add components (Cl, SO3, etc.) that reduce β-OH to the glass. (3) Reduce the moisture content in the furnace atmosphere. (4) Perform N2 bubbling in the molten glass. (5) Adopt a small melting furnace. (6) Increase the flow rate of the molten glass. (7) Adopt an electric melting method.

[0050] Here, "β-OH" refers to the value obtained by measuring the transmittance of the glass disk using FT-IR and calculating it with the following formula.

[0051] [Equation 1] β-OH=(1 / X)log(T1 / T2) X: Plate thickness (mm) T1: Transmittance (%) at a reference wavelength of 3846 cm -1 T2: Minimum transmittance (%) near the hydroxyl absorption wavelength of 3600 cm -1

[0052] The average surface roughness Ra of the surface is preferably 1.0 nm or less, 0.7 nm or less, 0.4 nm or less, and particularly 0.2 nm or less. If the average surface roughness Ra of the surface is too large, even if the bit size is miniaturized for high recording density, no improvement in magnetic properties can be expected.

[0053] The plate thickness is preferably 1.5 mm or less, 1.2 mm or less, 0.2 to 1.0 mm, and particularly 0.3 to 0.9 mm. If the plate thickness is too thick, it may be necessary to polish it to the desired plate thickness, which may increase the processing cost.

[0054] The total thickness variation (TTV) of the entire plate is preferably less than 2.0 μm, 1.5 μm or less, 1.0 μm or less, and particularly less than 0.1 to 1.0 μm. If the total thickness variation (TTV) of the entire plate is too large, even if the bit size is miniaturized for high recording density, no improvement in magnetic properties can be expected. Here, the "total thickness variation (TTV)" is the difference between the maximum plate thickness and the minimum plate thickness of the entire plate, and can be measured, for example, by SBW-331ML / d manufactured by Kobe Steel, Ltd.

[0055] The glass disk for a magnetic recording medium of the present invention preferably has a polished surface over the entire surface. By doing so, it becomes easier to regulate the total thickness variation (TTV) to less than 2.0 μm, 1.5 μm or less, 1.0 μm or less, and particularly less than 1.0 μm. As a method of the polishing treatment, various methods can be adopted, but a method of sandwiching both surfaces of the glass disk between a pair of polishing pads and polishing the glass disk while rotating both the glass disk and the pair of polishing pads together is preferable. Further, it is preferable that the pair of polishing pads have different outer diameters, and it is preferable to perform the polishing treatment so that a part of the glass disk intermittently protrudes from the polishing pads during polishing. Thereby, it becomes easier to reduce the total thickness variation (TTV), and also easier to reduce the amount of warpage. In the polishing treatment, the polishing depth is not particularly limited, but the polishing depth is preferably 50 μm or less, 30 μm or less, 20 μm or less, and particularly 10 μm or less. The smaller the polishing depth, the higher the productivity of the glass disk.

[0056] The glass disk for a magnetic recording medium of the present invention can be produced, for example, by the following method. First, a glass raw material prepared to have a desired glass composition is charged into a continuous melting furnace, heated and melted at 1500 to 1700 ° C., clarified, and then the molten glass is supplied to a forming device and preferably formed into a plate shape and cooled. As a method of cutting the plate-shaped body into a disk shape after forming, a well-known method can be adopted. As a method of forming the glass substrate, various methods can be adopted, but in order to improve the surface smoothness, it is preferable to adopt an overflow down-draw method, a slot down method, or the like. Further, polishing of the disk surface for adjusting the plate thickness and TTV, drilling of a circular opening in the center of the disk, polishing of the inner and outer peripheral end faces, formation of a magnetic layer on the disk surface, etc. can be appropriately adopted.

Examples

[0057] Hereinafter, the present invention will be described based on examples. The following examples are merely illustrative. The present invention is not limited to the following examples at all.

[0058] Tables 1 to 5 show the examples (Sample Nos. 1 to 131) of the present invention.

[0059] [Table 1]

[0060] [Table 2]

[0061] [Table 3]

[0062] [Table 4]

[0063] [Table 5]

[0064] [Table 6]

[0065] [Table 7]

[0066] [Table 8]

[0067] [Table 9]

[0068] [Table 10]

[0069]

Table 11

[0070]

Table 12

[0071]

Table 13

[0072] Samples were prepared as follows. First, a glass batch prepared by blending glass raw materials to have the glass compositions shown in the table was placed in a platinum crucible and melted at 1600 °C for 24 hours. When melting the glass batch, stirring was performed using a platinum stirrer to achieve homogenization. Next, the molten glass was poured onto a carbon plate, formed into a flat plate shape, and then cut into a disk shape. For each of the obtained samples, the β-OH value, density, coefficient of thermal expansion, Young's modulus, strain point, temperature at 10 4.5 dPa·s, liquidus temperature, liquidus viscosity, and thermal shrinkage rate were evaluated.

[0073] The β-OH value is the value calculated by the above formula.

[0074] The density is the value measured by the well-known Archimedes method.

[0075] The coefficient of thermal expansion is the average coefficient of thermal expansion measured with a dilatometer in the temperature range of 30 to 380 °C.

[0076] Young's modulus is the value measured by the dynamic elastic modulus measurement method (resonance method) based on JIS R1602.

[0077] The strain point is the value measured based on the method of ASTM C336.

[0078] High-temperature viscosity 10 4.5The temperature in dPa·s is the value measured by the platinum sphere pulling-up method.

[0079] The liquidus temperature is the temperature at which devitrified crystals (crystalline foreign substances) are observed in the glass after each sample is pulverized, passed through a standard sieve of 30 mesh (500 μm), the glass powder remaining on a 50 mesh (300 μm) sieve is placed in a platinum boat, and held in a temperature gradient furnace set at 1100 °C to 1350 °C for 24 hours, then the platinum boat is taken out. The liquid-phase viscosity is the value measured by the platinum sphere pulling-up method for the viscosity of the glass at the liquidus temperature.

[0080] As is clear from the table, Samples No. 1 to 131 are suitable as glass disks for magnetic recording media because their strain points are 715 °C or higher, the temperature in 4.5 dPa·s is 1290 °C or lower, and their Young's moduli are 81.7 GPa or higher.

[0081] A glass batch prepared by mixing glass raw materials to have the glass compositions of Samples No. 1 to 131 in the table was charged into a melting furnace, melted, clarified, and homogenized at 1500 to 1700 °C for 24 hours, then formed into a plate shape by the overflow down-draw method so as to have a plate thickness of 0.675 mm, and then processed into a disk shape. When the surface roughness Ra of the surface of the obtained glass disk was measured with an atomic force microscope (AFM), it was 0.10 to 0.20 nm. Also, the overall thickness variation (TTV) was 1.0 μm. Furthermore, for the obtained glass disks, when the average linear transmittance in an optical path length of 1 mm and a wavelength range of 350 to 1500 nm was measured with a spectrophotometer UV-3100 manufactured by Shimadzu Corporation, all were 85% or higher.

Claims

1. It has a disk shape and, as a glass composition, contains, in mol%, 60 to 71% of SiO₂, 10 to 16% of Al₂O₃, 0 to 5% of B₂O₃, 0 to 0.1% of Na₂O, 0 to 1% of K₂O, 0 to 7.42% of MgO, 0 to 12% of CaO, 0 to 10% of SrO, 0 to 10% of BaO, 0 to 1% of ZrO₂, 0 to 1% of SnO₂, has a strain point of 695 to 780 °C, and ４．５ a temperature at 10 dPa·s is 1300 °C or lower and a Young's modulus is 78 to 87.5 GPa, and is a glass disk for a magnetic recording medium.

2. The glass disk for a magnetic recording medium according to claim 1, wherein a circular opening is formed in the central portion.

3. As a glass composition, in mol%, SiO ２ 60 to 71%, Al ２ O ３ 12 to 16%, B ２ O ３ 0 to 5%, Na ２ O 0 to 0.1%, K ２ O 0 to 1%, MgO 0 to 7%, CaO 0 to 12%, SrO 0 to 10%, BaO 0 to 10%, ZrO ２ 0 to 1%, SnO ２ The glass disk for a magnetic recording medium according to claim 1 or 2, characterized by containing 0 to 1%.

4. The glass disk for a magnetic recording medium according to any one of claims 1 to 3, wherein the average surface roughness Ra of the surface is 1.0 nm or less.

5. The glass disk for a magnetic recording medium according to any one of claims 1 to 4, wherein the average linear transmittance in an optical path length of 1 mm and a wavelength range of 350 to 1500 nm is 70% or more.

6. It has a disk shape, and as a glass composition, in mol%, SiO ２ 60 to 71%, Al ２ O ３ 10 to 16%, B ２ O ３ 0 to 5%, Na ２ O 0 to 0.1%, K ２ O 0 to 1%, MgO 0 to 7.42%, CaO 0 to 12%, SrO 0 to 10%, BaO 0 to 10%, ZrO ２ 0 to 1%, SnO ２ 0 to 1%, and is characterized in that the Young's modulus is 78 to 87.5 GPa. A glass disk for a magnetic recording medium.

7. The glass disk for a magnetic recording medium according to any one of claims 1 to 6, wherein a magnetic layer is provided on the surface.

8. A magnetic recording apparatus comprising the glass disk for a magnetic recording medium according to any one of claims 1 to 7. **Claim 9**: A glass substrate for a magnetic recording medium, wherein the content of Na₂O in the glass composition is 0.1 mol% or less, the content of MgO is 7.42 mol% or less, the strain point is 695 to 740 °C, and the temperature at 10 ４．５ dPa·s is 1300 °C or less, and the Young's modulus is 78 to 87.5 GPa.

10. As a glass composition, in mol%, SiO ２ 60 to 71%, Al ２ O ３ 12 to 16%, B ２ O ３ 0 to 5%, Na ２ O 0 to 0.1%, K ２ O 0 to 1%, MgO 0 to 7%, CaO 0 to 12%, SrO 0 to 10%, BaO 0 to 10%, ZrO ２ 0 to 1%, SnO ２ The glass substrate for a magnetic recording medium according to claim 9, characterized by containing 0 to 1%.

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