Glass for magnetic recording medium substrate, magnetic recording medium substrate, and magnetic recording and reproducing apparatus

Abstract

To provide a magnetic recording medium substrate made of glass suitable for molding by various molding methods.SOLUTION: A CaO content of 1.00 mol% or less, a MgO content of 14.00 mol% or more and a Li2O content of 3.00 mol% or less, a B2O3 content of 2.00 mol% or less, a SiO2 content of 50.00 mol% or more and 75.00 mol% or less, an Al2O3 content of 10.00 mol% or more and 20.00 mol% or less, a molar ratio of the MgO content to the total content of MgO, CaO, SrO, and BaO (MgO / (MgO+CaO+SrO+BaO)) of 0.900 or more, a glass-transition temperature Tg of 720°C. or less, and a temperature T1 at which the viscosity is 102.0 dPa·s of 1600°C. or less; The glass is an amorphous glass having a specific modulus of 34. 0MNm / kg or more.SELECTED DRAWING: None

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from Japanese Patent Application No. 2022-143093, filed September 8, 2022, the entire disclosure of which is expressly incorporated herein by reference. [Technical Field]

[0002] The present invention relates to a glass for a magnetic recording medium substrate, a magnetic recording medium substrate, and a magnetic recording / reproducing device. [Background technology]

[0003] Aluminum alloy substrates have traditionally been used as substrates for magnetic recording media such as hard disks (magnetic recording medium substrates). However, it has been pointed out that aluminum alloy substrates are prone to deformation. For this reason, glass magnetic recording medium substrates are now widely used (see, for example, Patent Document 1, the entire disclosure of which is expressly incorporated herein by reference). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-86643 Summary of the Invention

[0005] A glass magnetic recording medium substrate is produced through a process of molding molten glass.

[0006] An object of one aspect of the present invention is to provide a glass magnetic recording medium substrate suitable for molding by various molding methods.

[0007] In Patent Document 1 (JP 2021-86643 A), specifically, viscosity characteristics suitable for molding by the overflow downdraw method (see paragraph 0063 of Patent Document 1) include a high-temperature viscosity of 10 2.5A glass substrate for a magnetic recording medium having a temperature of 1590°C or more and 1640°C or less in dPa·s has been disclosed (see Tables 1 to 3 of Patent Document 1). However, the inventors' investigations have revealed that for molding by various molding methods such as press molding, a glass substrate with a viscosity of 10 2.0 Glasses with viscosity characteristics below 1600°C in dPa·s and / or viscosity of 10 2.5 It has been newly discovered that glasses with a temperature of 1500°C or less in dPa·s are suitable.

[0008] One aspect of the present invention is as follows. [1] Viscosity 10 2.0 The temperature T1 at dPa·s is 1600°C or less, and A glass for magnetic recording medium substrates (hereinafter also referred to as "glass 1-A"), which is amorphous glass having a specific modulus of elasticity of 34.0 MNm / kg or more. [2] The glass for a magnetic recording medium substrate according to [1], wherein the total content of Li2O and Na2O (Li2O + Na2O) is 0.50 mol % or more. [3] Viscosity 10 2.5 The temperature T2 at dPa·s is 1500°C or less, and A glass for magnetic recording medium substrates (hereinafter referred to as "glass 1-B"), which is amorphous glass having a specific modulus of elasticity of 34.0 MNm / kg or more. [4] The glass for a magnetic recording medium substrate according to [3], wherein the total content of Li2O and Na2O (Li2O + Na2O) is 0.50 mol % or more. [5] Viscosity 10 2.0 The temperature T1 at dPa·s is 1600°C or less, and A glass for magnetic recording medium substrates (hereinafter referred to as "glass 1-C") that is amorphous glass with a total content of Li2O and Na2O (Li2O + Na2O) of 0.50 mol % or more. [6] A magnetic recording medium substrate made of the glass for magnetic recording medium substrates according to any one of [1] to [5]. [7] A magnetic recording medium having the magnetic recording medium substrate according to [6] and a magnetic recording layer. [8] A magnetic recording and reproducing device including the magnetic recording medium described in [7].

[0009] Another aspect of the present invention is as follows. [1] Viscosity 10 2.0 The temperature T1 at dPa·s is 1700°C or less, and A glass for magnetic recording medium substrates (hereinafter also referred to as "glass 4-A"), which is amorphous glass having a specific modulus of elasticity of 34.0 MNm / kg or more. [2] The glass for a magnetic recording medium substrate according to [1], wherein the total content of Li2O and Na2O (Li2O + Na2O) is 0.50 mol % or more. [3] Viscosity 10 2.5 The temperature T2 at dPa·s is 1560°C or less, and A glass for magnetic recording medium substrates (hereinafter referred to as "glass 4-B"), which is amorphous glass having a specific modulus of elasticity of 34.0 MNm / kg or more. [4] The glass for a magnetic recording medium substrate according to [3], wherein the total content of Li2O and Na2O (Li2O + Na2O) is 0.50 mol % or more. [5] Viscosity 10 2.0 The temperature T1 at dPa·s is 1700°C or less, and A glass for magnetic recording medium substrates (hereinafter referred to as "glass 4-C") that is an amorphous glass having a total content of Li2O and Na2O (Li2O + Na2O) of 0.50 mol % or more. [6] A magnetic recording medium substrate made of the glass for magnetic recording medium substrates according to any one of [1] to [5]. [7] A magnetic recording medium having the magnetic recording medium substrate according to [6] and a magnetic recording layer. [8] A magnetic recording and reproducing device including the magnetic recording medium described in [7].

[0010] Furthermore, it is desirable for the glass for a magnetic recording medium substrate to have high rigidity for the following reasons. Magnetic recording media are typically installed inside hard disk drives (HDDs) built into devices such as personal computers. Inside an HDD, multiple magnetic recording media (magnetic disks) are attached to the rotating shaft of a spindle motor, and an actuator built into the HDD writes and reads data to and from the magnetic recording layer of the magnetic recording media, which are rotating at high speed inside the HDD. The large vibrations of the magnetic recording media during high-speed rotation due to such data writing and reading can lead to collisions between the magnetic head and the magnetic recording media surface, resulting in head crashes. HDDs can increase their storage capacity by reducing the thickness of each magnetic recording media and incorporating more magnetic recording media. However, generally, the thinner the substrate used to make the magnetic recording media thinner, the more susceptible the magnetic recording media is to vibration during high-speed rotation. To suppress such vibrations, it is desirable to use a glass substrate for magnetic recording media that is highly rigid and less likely to vibrate during high-speed rotation, even when thinned. An example of such rigidity is the specific elastic modulus.

[0011] In view of the above, another aspect of the present invention aims to provide a glass for a magnetic recording medium substrate having high rigidity, and in one embodiment, an object of the present invention is to provide a glass for a magnetic recording medium substrate having a high specific elastic modulus.

[0012] Another aspect of the present invention is as follows. [1] Contains at least Li2O, The total content of Na2O and K2O is 0 mol% or more and 6.00 mol% or less, The B2O3 content is 0 mol% or more and 10.00 mol% or less, and CaO content is 0 mol% or more and 1.00 mol% or less, The glass for a magnetic recording medium substrate or a glass spacer for a magnetic recording / reproducing device is an amorphous glass (hereinafter referred to as "glass 2"). [2] The glass according to [1], wherein the CaO content is 0 mol % or more and 0.50 mol % or less. [3] The glass according to [1] or [2], wherein the CaO content is 0 mol % or more and 0.10 mol % or less. [4] The glass according to any one of [1] to [3], which does not contain CaO. [5] The glass according to any one of [1] to [4], wherein the B2O3 content is 0.1 mol % or more and 10.00 mol % or less. [6] The glass according to any one of [1] to [5], having an SiO2 content of 50.00 mol % or more and 70.00 mol % or less. [7] The glass according to any one of [1] to [6], having an SiO2 content of 55.00 mol % or more and 68.00 mol % or less. [8] The glass according to any one of [1] to [7], wherein the Li2O content is 1.00 mol % or more and 10.00 mol % or less. [9] The glass according to any one of [1] to [8], wherein the Li2O content is 3.00 mol % or more and 6.00 mol % or less.

[10] The glass according to any one of [1] to [9], which does not contain any alkali metal oxide other than Li2O.

[11] The glass according to any one of [1] to

[10] , having an Al2O3 content of 10.00 mol % or more and 20.00 mol % or less.

[12] The glass according to any one of [1] to

[11] , wherein the Al2O3 content is 10.00 mol % or more and 17.00 mol % or less.

[13] The glass according to any one of [1] to

[12] , wherein the MgO content is 10.00 mol % or more and 25.00 mol % or less.

[14] The glass according to any one of [1] to

[13] , further containing an oxide of one or more metals selected from the group consisting of Zr, Y, La, Ti, Hf, Cu, Co, Mn, Nd, Pr, Nb, V, Cr, Ni, Mo, Ho, and Er.

[15] The glass according to any one of [1] to

[14] , wherein the molar ratio of the Li2O content to the MgO content (Li2O / MgO) exceeds 0.080.

[16] The glass according to any one of [1] to

[15] , wherein the molar ratio of the Li2O content to the total content of Li2O and MgO (Li2O / (Li2O+MgO)) exceeds 0.080.

[17] The glass according to any one of [1] to

[16] , wherein the molar ratio of the Al2O3 content to the SiO2 content (Al2O3 / SiO2) is less than 0.270.

[18] The glass according to any one of [1] to

[17] , wherein the molar ratio of the B2O3 content to the SiO2 content (B2O3 / SiO2) is greater than 0 and less than 0.020.

[19] The glass according to any one of [1] to

[18] , wherein the molar ratio of the B2O3 content to the total content of B2O3 and SiO2 (B2O3 / (B2O3+SiO2)) is greater than 0 and less than 0.020.

[20] The glass according to any one of [1] to

[19] , wherein the molar ratio of the B2O3 content to the total content of B2O3, SiO2 and Al2O3 (B2O3 / (B2O3+SiO2+Al2O3)) is greater than 0 and less than 0.010.

[21] The glass according to any one of [1] to

[20] , wherein the molar ratio of the MgO content to the total content of MgO, CaO, SrO, and BaO (MgO / (MgO+CaO+SrO+BaO)) is 0.970 or more and 1.000 or less.

[22] The glass according to any one of [1] to

[21] , wherein the molar ratio of the Li2O content to the total content of Li2O, Na2O and K2O (Li2O / (Li2O+Na2O+K2O)) is 0.760 or more and 1.000 or less.

[23] The glass according to any one of [1] to

[22] , wherein the molar ratio of the B2O3 content to the Li2O content (B2O3 / Li2O) is greater than 0 and less than 0.250.

[24] The glass according to any one of [1] to

[23] , having a specific modulus of elasticity of 35.0 MNm / kg or more.

[25] The glass according to any one of [1] to

[24] , which has a glass transition temperature of 700°C or higher.

[26] A magnetic recording medium substrate made of the glass according to any one of [1] to

[25] .

[27] A magnetic recording medium having the magnetic recording medium substrate according to

[26] and a magnetic recording layer.

[28] A glass spacer for a magnetic recording / reproducing device, comprising the glass according to any one of [1] to

[25] .

[29]

[27] The magnetic recording medium according to

[29] ,

[28] A glass spacer for a magnetic recording / reproducing device according to

[28] . A magnetic recording and reproducing device including one or more selected from the group consisting of:

[0013] Another aspect of the present invention is as follows. [1] B2O3 content is 0.10 mol% or more and 2.00 mol% or less; Na2O content is 1.00 mol% or more and 6.00 mol% or less, CaO content is 0 mol% or more and 1.00 mol% or less, MgO content is 14.00 mol% or more, The amorphous glass for a magnetic recording medium substrate or a glass spacer for a magnetic recording / reproducing device (hereinafter referred to as "glass 3-A"). [2] B2O3 content is 0.10 mol% or more and 2.00 mol% or less; Na2O content is 1.00 mol% or more and 6.00 mol% or less, CaO content is 0 mol% or more and 1.00 mol% or less, The total content of MgO, SrO, CaO and BaO (MgO + SrO + CaO + BaO) is 14.00 mol% or more, The amorphous glass for a magnetic recording medium substrate or a glass spacer for a magnetic recording / reproducing device (hereinafter referred to as "glass 3-B"). [3] The glass according to [1] or [2], wherein the molar ratio of the Na2O content to the total content of Li2O, Na2O and K2O (Na2O / (Li2O+Na2O+K2O)) is 0.950 or more. [4] The glass according to any one of [1] to [3], which is free of Li2O and K2O. [5] The glass according to any one of [1] to [4], wherein the total content of SiO2, B2O3 and Al2O3 (SiO2+B2O3+Al2O3) is 70 mol% or more. [6] The glass according to any one of [1] to [5], wherein the total content of SiO2 and B2O3 (SiO2 + B2O3) is 58 mol% or more. [7] The glass according to any one of [1] to [6], wherein the total content of SiO2 and Al2O3 (SiO2 + Al2O3) is 70 mol% or more. [8] The glass according to [1] or [2], wherein the molar ratio of the Na2O content to the total content of Li2O, Na2O and K2O (Na2O / (Li2O+Na2O+K2O)) is 0.350 or more. [9] The glass according to [1], [2] or [8], which contains Li2O.

[10] The glass according to [1], [2], [8] or [9], wherein the molar ratio of the Li2O content to the total content of Li2O, Na2O and K2O (Li2O / (Li2O+Na2O+K2O)) is 0.600 or less.

[11] The glass according to any one of [1], [2] and [8] to

[10] , wherein the molar ratio of the SiO2 content to the Na2O content (SiO2 / Na2O) is 50.0 or less.

[12] The glass according to any one of [1] to [7], wherein the molar ratio of the Na2O content to the MgO content (Na2O / MgO) exceeds 0.080.

[13] The glass according to any one of [1] to

[12] , having a TiO2 content of 4.00 mol % or less.

[14] The glass according to any one of [1] to

[13] , wherein the ZnO content is 10.00 mol % or less.

[15] The glass according to any one of [1] to

[14] , which has a specific modulus of elasticity of 35.0 MNm / kg or more.

[16] The glass according to any one of [1] to

[15] , which has a glass transition temperature of 700°C or higher.

[17] A magnetic recording medium substrate made of the glass according to any one of [1] to

[16] .

[18] A magnetic recording medium comprising the magnetic recording medium substrate according to

[17] and a magnetic recording layer.

[19] A glass spacer for a magnetic recording / reproducing device, which is made of the glass according to any one of [1] to

[16] .

[20]

[18] The magnetic recording medium according to

[18] ,

[19] A glass spacer for a magnetic recording / reproducing device according to

[19] . A magnetic recording and reproducing device including one or more selected from the group consisting of:

[0014] According to one aspect of the present invention, it is possible to provide a glass magnetic recording medium substrate that is suitable for molding by various molding methods. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, Glass 1-A, Glass 1-B, and Glass 1-C will be collectively referred to as "Glass 1." Glass 3-A and Glass 3-B will be collectively referred to as "Glass 3." Glass 4-A, Glass 4-B, and Glass 4-C will be collectively referred to as "Glass 4." In the following description, unless otherwise specified, a description of glass 1 can also be applied to glass 2, glass 3, and glass 4, a description of glass 2 can also be applied to glass 1, glass 3, and glass 4, and a description of glass 3 can also be applied to glass 1, glass 2, and glass 4. Glasses 1, 2, 3, and 4 are also collectively referred to simply as "glass." The glass corresponding to glass 1 may also be a glass that corresponds to any one, two, or all of glass 2, glass 3, and glass 4.

[0016] Glasses 1, 2, and 3 are amorphous glasses. Unlike glass-ceramics, amorphous glasses are glasses that do not substantially contain a crystalline phase and exhibit a glass transition phenomenon upon heating. On the other hand, the manufacturing process for glass-ceramics is complicated. In addition, it is not easy to achieve the high smoothness required for magnetic recording medium substrates using glass-ceramics. Glass 1, Glass 2, and Glass 3 can also be amorphous oxide glasses. Oxide glasses are glasses in which the main network-forming component of the glass is an oxide.

[0017] Glasses 1 to 4 will be explained in more detail below.

[0018] In the present invention and this specification, the glass composition is expressed as a glass composition based on oxides. Here, "glass composition based on oxides" refers to a glass composition obtained by converting the glass raw materials into oxides that are present in the glass after they are all decomposed during melting. Furthermore, unless otherwise specified, the glass composition is expressed on a molar basis (mol %, molar ratio). The glass composition of the present invention and the present specification can be determined by a method such as ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Quantitative analysis is performed for each element using ICP-AES. The analytical values ​​are then converted into oxide notation. The analytical values ​​obtained by ICP-AES may contain a measurement error of, for example, about ±5% of the analytical value. Therefore, the oxide notation values ​​converted from the analytical values ​​may also contain an error of about ±5%. In the present invention and this specification, the content of a component is 0%, 0.0%, 0.00%, or not contained or not incorporated means that the component is substantially not contained, and the content of the component is at or below the impurity level. At or below the impurity level means, for example, less than 0.01%.

[0019] [Glass 1, 4] In the following description, unless otherwise specified, the description regarding Glass 1-A can also be applied to Glass 1-B, Glass 1-C, and Glass 4, the description regarding Glass 1-B can also be applied to Glass 1-A, Glass 1-C, and Glass 4, and the description regarding Glass 1-C can also be applied to Glass 1-A, Glass 1-B, and Glass 4.

[0020] Viscosity of Glass 1-A and Glass 1-B: 10 2.0 The temperature T1 at dPa·s is 1600°C or less. The viscosity of glass 1-C is 10 2.5 The temperature T2 at dPa·s is 1500°C or less. 2.0 Temperature T1 and viscosity 10 in dPa·s2.5 The temperature T2 in dPa·s can be measured by the platinum ball pulling method or the rotation method. Glasses having a temperature T1 of 1600°C or less and glasses having a temperature T2 of 1500°C or less are suitable for forming by various forming methods and are therefore preferred. Examples of methods for forming glass melt include methods A to D described in the Examples section below. Glasses having a temperature T1 of 1600°C or less and glasses having a temperature T2 of 1500°C or less can be formed by any of methods A to D, and it is particularly preferred to apply method A or method B to forming.

[0021] The temperature T1 of Glass 1-A and Glass 1-B is 1600°C or less, preferably 1550°C or less, and more preferably 1500°C or less. The lower limit of the temperature T1 is not particularly limited and can be, for example, 1300°C or more, 1350°C or more, 1400°C or more, or 1450°C or more, but may be lower than the temperatures exemplified here. In one embodiment, the temperature T1 of Glass 1-C can be within the above range.

[0022] The temperature T2 of Glass 1-C is 1500°C or less, preferably 1450°C or less, and more preferably 1400°C or less. The lower limit of the temperature T2 is not particularly limited and can be, for example, 1200°C or more, 1250°C or more, 1300°C or more, or 1350°C or more, but may be lower than the temperatures exemplified here. In one embodiment, the temperature T2 of Glass 1-A and the temperature T2 of Glass 1-B can be within the above range.

[0023] Viscosity of Glass 4-A and Glass 4-B: 10 2.0 The temperature T1 at dPa·s is 1700°C or less. The viscosity of glass 4-C is 10 2.5The temperature T2 in dPa·s is 1560°C or lower. Glasses having a temperature T1 of 17000°C or lower and glasses having a temperature T2 of 1560°C or lower are preferred because they are suitable for forming by various forming methods. Examples of methods for forming the glass melt include methods A to D described in the Examples section below. Glasses having a temperature T1 of 1700°C or lower and glasses having a temperature T2 of 1560°C or lower can be formed by any of methods A to D, and it is particularly preferred to apply method A or method B to forming.

[0024] The temperature T1 of glass 4-A and glass 4-B is 1700°C or less, preferably 1690°C or less, and more preferably 1680°C or less, 1670°C or less, 1660°C or less, 1650°C or less, and 1640°C or less in that order. The lower limit of temperature T1 is not particularly limited and can be, for example, 1300°C or more, 1350°C or more, 1400°C or more, or 1450°C or more, but may be lower than the temperatures exemplified here. In one embodiment, the temperature T1 of glass 4-C can be within the above range.

[0025] The temperature T2 of glass 4-C is 1560°C or less, preferably 1550°C or less, and more preferably 1540°C or less, 1530°C or less, 1520°C or less, and 1510°C or less in that order. The lower limit of temperature T2 is not particularly limited and can be, for example, 1200°C or more, 1250°C or more, 1300°C or more, or 1350°C or more, but may be lower than the temperatures exemplified here. In one embodiment, the temperature T2 of glass 4-A and the temperature T2 of glass 4-B can be within the above-mentioned ranges.

[0026] In Glass 1-C and Glass 4-C, the total content of Li2O and Na2O (Li2O + Na2O) is 0.50% or more. A total content (Li2O + Na2O) of 0.50% or more is preferable for reducing the melt viscosity of the glass. Reducing the melt viscosity can contribute to maintaining the glass temperature T1 at 1670°C or less or 1600°C or less. From the above perspective, the total content (Li2O + Na2O) of Glass 1-C and Glass 4-C is preferably 1.00% or more, with 1.50% or more, 2.00% or more, 3.00% or more, and 3.50% or more being more preferable in this order. Furthermore, reducing the melt viscosity of the glass can contribute to maintaining the glass temperature T2 at 1560°C or less or 1500°C or less. Therefore, a total content (Li2O + Na2O) within the above range is also preferable for maintaining the glass temperature T2 at 1500°C or less. The total content (LiO + NaO) of Glass 1-C and Glass 4-C can be, for example, 8.00% or less, 7.00% or less, or 6.00% or less. In one embodiment, the total content (LiO + NaO) of Glass 1-A, Glass 1-B, Glass 4-A, and Glass 4-B can be within the above range.

[0027] In one embodiment, in glasses 1-A, 1-B, 1-C, 4-A, 4-B, and 4-C, the combined content of LiO, NaO, and KO (LiO + NaO + KO) can be greater than 0.50%, and can be 1.00% or greater, 1.50% or greater, 2.00% or greater, 3.00% or greater, or 3.50% or greater. In glasses 1-A, 1-B, 1-C, 4-A, 4-B, and 4-C, the combined content (LiO + NaO + KO) can be, for example, 10.00% or less, 9.00% or less, 8.00% or less, 7.00% or less, or 6.00% or less.

[0028] In one embodiment, in glasses 1-A, 1-B, 1-C, 4-A, 4-B, and 4-C, the molar ratio of the Li2O content to the MgO content (Li2O / MgO) preferably falls within the range described below for glass 2. In another embodiment, in glasses 1-A, 1-B, 1-C, 4-A, 4-B, and 4-C, the molar ratio of the Na2O content to the MgO content (Na2O / MgO) preferably falls within the range described below for glass 3.

[0029] The specific elastic modulus of Glasses 1-A, 1-B, 4-A, and 4-B is 34.0 MNm / kg or more. Glasses with a specific elastic modulus of 34.0 MNm / kg or more have high rigidity and are preferred as glasses for magnetic recording medium substrates for the reasons described above. The specific elastic modulus of Glasses 1-A, 1-B, 4-A, and 4-B is preferably 34.5 MNm / kg or more, and is even more preferably 35.0 MNm / kg or more, 35.5 MNm / kg or more, 36.0 MNm / kg or more, 36.5 MNm / kg or more, 37.0 MNm / kg or more, 37.5 MNm / kg or more, and 38.0 MNm / kg or more in this order. The specific elastic moduli of Glasses 1-A, 1-B, 4-A, and 4-B can be, for example, 45.0 MNm / kg or less, 44.0 MNm / kg or less, 43.0 MNm / kg or less, 42.0 MNm / kg or less, 41.0 MNm / kg or less, or 40.0 MNm / kg or less. However, since a higher specific elastic modulus is preferable because it increases rigidity, the specific elastic moduli are not limited to the above-mentioned exemplary values. In one embodiment, the specific elastic moduli of Glasses 1-C and 4-C can be within the above-mentioned range. For glass compositions for increasing rigidity, please refer to the descriptions of Glass 2 and Glass 3 described below.

[0030] [Glass 2] <Glass composition> Glass 2 contains a small amount of CaO of 0% or more and 1.00% or less, and contains Li2O as an essential component, with the total content of other alkali metal oxides Na2O and K2O being 0% or more and 6.00% or less, and the B2O3 content being 0% or more and 10.00% or less. Having such a composition can contribute to Glass 2 being able to exhibit high rigidity.

[0031] The CaO content of glass 2 is 1.00% or less. A CaO content of 1.00% or less contributes to improving the rigidity of the glass and can also contribute to improving the thermal stability of the glass. From these viewpoints, the CaO content is preferably 0.9% or less, and more preferably 0.8% or less, 0.7% or less, 0.6% or less, 0.5% or less, 0.4% or less, 0.3% or less, 0.2% or less, and 0.1% or less in that order, and can even be 0%. It is more preferable that glass 2 does not contain CaO.

[0032] Glass 2 contains Li2O as an essential component, and the total content of other alkali metal oxides, Na2O and K2O (Na2O + K2O) is 0% or more and 6.00% or less. This can also contribute to improving the rigidity of the glass. From the viewpoint of further increasing the rigidity of the glass, the total content of Na2O and K2O (Na2O + K2O) is preferably 5.00% or less, and more preferably 4.00% or less, 3.00% or less, 2.00% or less, 1.00% or less, 0.5% or less, and 0.1% or less, in that order. It can also be 0%. In one embodiment, Glass 2 more preferably does not contain Na2O or K2O, and even more preferably does not contain alkali metal oxides other than Li2O. In another embodiment, Glass 2 can contain one or both of Na2O and K2O, and the total content (Na2O + K2O) can be 1.00% or more, 2.00% or more, or 3.00% or more.

[0033] The Li2O content of Glass 2 is greater than 0%, and from the viewpoint of improving thermal stability, it is preferably 0.50% or more, and more preferably 1.00% or more, 2.00% or more, and 3.00% or more in that order. Furthermore, from the viewpoint of maintaining the temperature required for melting the glass within an appropriate range, it is preferable that the Li2O content be within the above range. Maintaining the temperature required for melting the glass within an appropriate range is also preferable from the viewpoints of extending the life of the melting furnace and reducing costs by reducing the input energy. On the other hand, from the viewpoints of further improving the rigidity of the glass, further improving devitrification resistance, and / or increasing the glass transition temperature, the Li2O content is preferably 10.00% or less, and more preferably 9.00% or less, 8.00% or less, 7.00% or less, 6.00% or less, 5.00% or less, and 4.00% or less in that order.

[0034] From the viewpoint of improving chemical durability and rigidity, the B2O3 content of glass 2 is 10.00% or less, preferably 9.00% or less, with 8.00% or less, 7.00% or less, 6.00% or less, 5.00% or less, 4.00% or less, 3.00% or less, 2.00% or less, 1.00% or less, 0.70% or less, and 0.50% or less being more preferred in this order. Glass with high chemical durability is preferred because it has high acid and / or alkali resistance, making it less likely to cause surface roughening during polishing, cleaning, etc., and making it easier to achieve low roughness on the glass surface. The ability to easily achieve low roughness through polishing is hereinafter referred to as low-roughness polishing performance. The B2O3 content of glass 2 can be 0% or more, and is preferably greater than 0%. A B2O3 content of greater than 0% can contribute to improved impact resistance. Glass with high impact resistance is preferred because it is less likely to crack and / or chip during processing such as cutting and grinding. From the viewpoints of improving impact resistance, meltability, and thermal stability, the B2O3 content of glass 2 is more preferably 0.10% or more, and even more preferably 0.20% or more. Glass with high thermal stability is preferred because it is less likely to devitrify.

[0035] In one form, glass 2 can include one or both of SiO2 and Al2O3.

[0036] From the viewpoint of improving chemical durability, the SiO content of glass 2 is preferably 50.00% or more, and more preferably 51.00% or more, 52.00% or more, 53.00% or more, 54.00% or more, 55.00% or more, 56.00% or more, 57.00% or more, 58.00% or more, and 59.00% or more in that order. On the other hand, from the viewpoint of further improving rigidity, the SiO content is preferably 70.00% or less, and more preferably 69.00% or less, 68.00% or less, 67.00% or less, 66.00% or less, 65.00% or less, 64.00% or less, and 63.00% or less in that order.

[0037] From the viewpoint of improving thermal stability, the Al2O3 content of Glass 2 is preferably 20.00% or less, and more preferably 19.00% or less, 18.00% or less, 17.00% or less, 16.00% or less, 15.00% or less, and 14.00% or less in that order. Furthermore, from the viewpoint of maintaining the temperature required for melting the glass within an appropriate range, the Al2O3 content within the above range is also preferred. On the other hand, from the viewpoint of further improving rigidity, the Al2O3 content is preferably 10.00% or more, and more preferably 11.00% or more.

[0038] Regarding alkaline earth metal oxides, the CaO content of glass 2 is as described above. In one embodiment, glass 2 can contain one or more alkaline earth metal oxides. Specific examples of alkaline earth metal oxides that can be contained in glass 2 include MgO, SrO, and BaO. In one embodiment, glass 2 can contain MgO. From the viewpoint of further improving rigidity, the MgO content of glass 2 is preferably 10.00% or more, and more preferably 11.00% or more, 12.00% or more, 13.00% or more, 14.00% or more, 15.00% or more, 16.00% or more, 17.00% or more, 18.00% or more, 19.00% or more, and 20.00% or more in that order. Furthermore, from the viewpoint of improving the devitrification resistance of the glass, the MgO content is preferably 28.00% or less, and more preferably 27.00% or less, 26.00% or less, 25.00% or less, 24.00% or less, 23.00% or less, 22.00% or less, 21.00% or less, 20.00% or less, 19.00% or less, and 18.00% or less, in that order.

[0039] From the viewpoints of reducing melt viscosity, lowering liquidus temperature, and improving glass roughness reduction and polishing performance, the molar ratio of Li2O content to MgO content (Li2O / MgO) is preferably 0.020 or more, with 0.030 or more, 0.040 or more, 0.050 or more, 0.060 or more, 0.070 or more, 0.080 or more, and more preferably greater than 0.080 in that order. Glass with a low liquidus temperature is preferred because of its high thermal stability. Furthermore, a molar ratio (Li2O / MgO) greater than 0.080 can contribute to reducing the resistivity of the glass melt. When electrical heating is performed during glass melting, glass with a low resistivity is preferred because it can be melted by lowering the temperature in the furnace during electrical heating. From the above viewpoints, the molar ratio (LiO / MgO) is more preferably 0.090 or more, and more preferably 0.100 or more, 0.110 or more, 0.120 or more, 0.130 or more, 0.140 or more, 0.150 or more, and 0.160 or more in that order. On the other hand, from the viewpoint of increasing the glass transition temperature and the specific elastic modulus, the molar ratio (LiO / MgO) is preferably less than 0.240, and more preferably 0.230 or less, 0.220 or less, 0.210 or less, 0.200 or less, 0.190 or less, or 0.180 or less in that order.

[0040] From the viewpoints of reducing melt viscosity, lowering liquidus temperature, improving glass roughness reduction polishing performance, and reducing the specific resistance of the glass melt, the molar ratio of the LiO content to the total content of LiO and MgO (LiO / (LiO+MgO)) is preferably 0.020 or more, and more preferably 0.030 or more, 0.040 or more, 0.050 or more, 0.060 or more, 0.070 or more, 0.080 or more, more preferably more than 0.080, 0.090 or more, 0.100 or more, 0.110 or more, 0.120 or more, 0.130 or more, and 0.140 or more in this order. On the other hand, from the viewpoint of increasing the glass transition temperature and specific elastic modulus, the molar ratio (LiO / (LiO+MgO)) is preferably less than 0.190, and more preferably 0.180 or less, 0.170 or less, or 0.160 or less in this order.

[0041] From the viewpoint of improving thermal stability, the molar ratio of the Al2O3 content to the SiO2 content (Al2O3 / SiO2) is preferably less than 0.270, and more preferably 0.260 or less, 0.250 or less, 0.240 or less, 0.230 or less, and 0.220 or less, in that order. On the other hand, from the viewpoint of increasing the specific elastic modulus, the molar ratio (Al2O3 / SiO2) is preferably 0.100 or more, and more preferably 0.120 or more, 0.130 or more, 0.140 or more, 0.150 or more, 0.160 or more, 0.170 or more, or 0.190 or more, in that order.

[0042] From the viewpoint of improving impact resistance, the molar ratio of the B2O3 content to the SiO2 content (B2O3 / SiO2) is preferably greater than 0, and more preferably 0.001 or greater, 0.002 or greater, 0.003 or greater, and 0.004 or greater in that order. On the other hand, from the viewpoint of improving chemical durability and further suppressing volatilization of glass components during glass melting, the molar ratio (B2O3 / SiO2) is preferably less than 0.020, and more preferably 0.015 or less, 0.010 or less, 0.009 or less, 0.008 or less, and 0.007 or less in that order.

[0043] From the viewpoint of improving impact resistance, the molar ratio of the B2O3 content to the total content of B2O3 and SiO2 (B2O3 / (B2O3+SiO2)) is preferably more than 0, and more preferably 0.001 or more, 0.002 or more, 0.003 or more, and 0.004 or more in that order. On the other hand, from the viewpoint of improving chemical durability and further suppressing volatilization of glass components during glass melting, the molar ratio (B2O3 / (B2O3+SiO2)) is preferably less than 0.020, and more preferably 0.015 or less, 0.010 or less, 0.009 or less, 0.008 or less, and 0.007 or less in that order.

[0044] From the viewpoint of improving impact resistance, the molar ratio of the B2O3 content to the total content of B2O3, SiO2, and Al2O3 (B2O3 / (B2O3+SiO2+Al2O3)) is preferably greater than 0, and more preferably 0.001 or greater, 0.002 or greater, and 0.003 or greater in that order. On the other hand, from the viewpoint of improving chemical durability and further suppressing volatilization of glass components during glass melting, the molar ratio (B2O3 / (B2O3+SiO2+Al2O3)) is preferably less than 0.010, and more preferably 0.009 or less, 0.008 or less, 0.007 or less, and 0.006 or less in that order.

[0045] The molar ratio of the MgO content to the total content of MgO, CaO, SrO, and BaO (MgO / (MgO+CaO+SrO+BaO)) can be 1.000, 1.000 or less, or less than 1.000. MgO has a lower specific gravity than other alkaline earth metal oxides, and therefore can contribute to increasing the specific modulus of elasticity. Furthermore, in general glasses, the glass transition temperature tends to increase when alkaline earth metal oxides with a higher specific gravity are added. In contrast, according to the inventors' investigations, the glass transition temperature of Glass 2 tends to decrease when alkaline earth metal oxides with a higher specific gravity than MgO are added. From the viewpoint of increasing the specific modulus and glass transition temperature of the above-mentioned glasses, the molar ratio (MgO / (MgO+CaO+SrO+BaO)) is preferably 0.970 or more, more preferably 0.980 or more, 0.990 or more, and 1.000 in that order.

[0046] The molar ratio of the LiO content to the total content of LiO, NaO, and KO (LiO / (LiO+NaO+KO)) can be 1.000, 1.000 or less, or less than 1.000. In one embodiment, from the viewpoint of increasing the specific elastic modulus, the molar ratio (LiO / (LiO+NaO+KO)) is preferably 0.760 or more, and is more preferably 0.800 or more, 0.850 or more, 0.900 or more, 0.950 or more, and 1.000 in that order. In another embodiment, the molar ratio (LiO / (LiO+NaO+KO)) can be less than 0.760, 0.700 or less, 0.600 or less, 0.500 or less, or 0.300 or less, or can be 0.050 or more, or 0.100 or more.

[0047] From the viewpoint of improving impact resistance, the molar ratio of the B2O3 content to the Li2O content (B2O3 / Li2O) is preferably greater than 0, and is more preferably 0.010 or more, 0.015 or more, 0.020 or more, 0.025 or more, 0.030 or more, 0.035 or more, 0.040 or more, 0.045 or more, 0.050 or more, 0.055 or more, 0.060 or more, 0.065 or more, and 0.070 or more, in that order. On the other hand, from the viewpoints of improving chemical durability, suppressing devitrification, and suppressing a decrease in homogeneity due to composition fluctuations, the molar ratio (B2O3 / Li2O) is preferably 0.700 or less, and more preferably 0.600 or less, 0.500 or less, 0.400 or less, 0.300 or less, 0.250 or less, less than 0.250, 0.245 or less, 0.240 or less, 0.235 or less, 0.230 or less, 0.225 or less, 0.220 or less, 0.215 or less, 0.210 or less, 0.205 or less, and 0.200 or less, in that order.

[0048] The Fe2O3 content of glass 2, expressed as an exclusive percentage, can be 1 mol% or less, 0.7 mol% or less, 0.5 mol% or less, 0.4 mol% or less, 0.3 mol% or less, 0.1 mol% or less, 0.07 mol% or less, 0.05 mol% or less, 0.04 mol% or less, 0.03 mol% or less, or 0.02 mol% or less. The Fe2O3 content is expressed as an exclusive percentage. That is, when the total content of components other than Fe2O3 contained in the glass (the total content of the glass components and additives when additives are included in addition to the glass components) is taken as 100%, the Fe2O3 content is expressed as a molar percentage of the Fe2O3 content relative to 100%. In one embodiment, glass 2 can be free of Fe (the Fe2O3 content expressed as an exclusive percentage is 0 mol%).

[0049] Glass 2 may or may not contain one or more metals or oxides thereof selected from the group consisting of Zr, Y, La, Ti, Hf, Cu, Co, Mn, Nd, Pr, Nb, V, Cr, Ni, Mo, Ho, and Er. Inclusion of an oxide of one or more metals selected from the group consisting of Zr, Y, La, Ti, Hf, Cu, Co, Mn, Nd, Pr, Nb, V, Cr, Ni, Mo, Ho, and Er in a glass composition based on oxides tends to enhance the thermal stability of the glass. In one embodiment, the content of one or more metal oxides selected from the group consisting of Zr, Y, La, Ti, Hf, Cu, Co, Mn, Nd, Pr, Nb, V, Cr, Ni, Mo, Ho, and Er (the total content when multiple metal oxides are included) can be 0%, 0% or more, or more than 0%, and can be 5.00% or less, 4.00% or less, 3.00% or less, 2.00% or less, 1.00% or less, 0.5% or less, or 0.1% or less. For example, the YO content can be 0% or more or more than 0%, and can be 3.00% or less, 2.00% or less, or 1.00% or less. The TiO content can be 0% or more or more than 0%, and can be 3.00% or less, 2.00% or less, or 1.00% or less. In one embodiment, glass 2 can include one or both of YO and TiO.

[0050] F is a component that easily volatilizes during melting and is also a component that causes striae, so it is preferable that Glass 2 does not contain F. Not containing F is also preferable from the viewpoints of suppressing erosion of the melting furnace, suppressing a decrease in Young's modulus, and suppressing a decrease in specific elastic modulus.

[0051] Since Pb, Cd and As are substances that have a negative impact on the environment, it is preferable to avoid introducing them.

[0052] From the viewpoint of obtaining a fining effect, glass 2 can contain one or more selected from the group consisting of SnO2, CeO2, and Sb2O3. In one embodiment, the total content of SnO2 and CeO2 can be 0%. In another embodiment, glass 2 can contain SnO2 and / or CeO2, and the total content of SnO2 and CeO2 (SnO2 + CeO2) is preferably 0.05 to 2%. When the total content of SnO2 and CeO2 is 0.05% or more, a sufficient fining effect can be obtained and residual bubbles can be reduced. Furthermore, when the total content (SnO2 + CeO2) is 2% or less, it is possible to prevent the molten glass from blowing up during glass melting, which can reduce productivity. The lower limit of the total content (SnO2 + CeO2) is preferably 0.10% or more, more preferably 0.20% or more, even more preferably 0.25% or more, still more preferably 0.30% or more, even more preferably 0.35% or more, and even more preferably 0.40% or more. The upper limit of the total content (SnO2 + CeO2) is preferably 1.5% or less, more preferably 1.2% or less, even more preferably 1.0% or less, even more preferably 0.70% or less, even more preferably 0.65% or less, even more preferably 0.60% or less, even more preferably 0.55% or less, and even more preferably 0.50% or less.

[0053] SnO2 has the function of promoting fining when the melting temperature of the glass is relatively high (in the temperature range of approximately 1400 to 1600°C). With the use of environmentally harmful fining agents such as Sb2O3 and arsenous acid being restricted, in one embodiment, SnO2 is preferably incorporated into glass 2 to remove bubbles in glass with a high melting temperature. From the viewpoint of obtaining a fining effect, the SnO2 content is preferably 0.01% or more, more preferably 0.05% or more, even more preferably 0.10% or more, even more preferably 0.15% or more, and even more preferably 0.20% or more. Furthermore, the SnO2 content is preferably 2% or less, more preferably 1.5% or less, even more preferably 1.0% or less, even more preferably 0.8% or less, and even more preferably 0.5% or less.

[0054] Like SnO2, CeO2 is a component that exhibits a glass fining effect. CeO2 functions to capture oxygen and fix it as a glass component when the glass melting temperature is relatively low (a temperature range of approximately 1200 to 1400°C). Therefore, in one embodiment, it is preferable to incorporate CeO2 into glass 2 as a fining agent. From the viewpoint of obtaining a fining effect, the CeO2 content is preferably 0.01% or more, more preferably 0.05% or more, even more preferably 0.08% or more, and even more preferably 0.10% or more. Furthermore, the CeO2 content is preferably 2% or less, more preferably 1.5% or less, even more preferably 1.0% or less, even more preferably 0.8% or less, even more preferably 0.5% or less, and even more preferably 0.3% or less. The coexistence of SnO2 and CeO2 can obtain a fining effect over a wide temperature range. Therefore, in one embodiment, glass 2 preferably contains both SnO2 and CeO2.

[0055] From the viewpoint of reducing the environmental impact, it is desirable to refrain from using Sb2O3. The content of Sb2O3 in glass 2 is preferably in the range of 0 to 0.5%. The content of Sb2O3 is more preferably 0.3% or less, further preferably 0.1% or less, even more preferably 0.05% or less, and still further preferably 0.02% or less, and it is particularly preferable that no Sb2O3 is contained.

[0056] Glass 2 can be produced by weighing, blending, and thoroughly mixing glass raw materials such as oxides, carbonates, nitrates, sulfates, and hydroxides so as to obtain a predetermined glass composition, heating and melting the mixture in a melting vessel at, for example, a temperature range of 1400 to 1600°C, refining, stirring, and molding the homogenized molten glass that has been thoroughly defoamed. For example, it is preferable to heat and melt the glass raw materials in a melting vessel at 1400 to 1550°C, and then heat the resulting molten glass in a refining vessel and hold it at 1450 to 1600°C, then lower the temperature to 1200 to 1400°C, at which point the glass is allowed to flow and be molded.

[0057] <Glass properties> By adjusting the composition as described above, the glass 2 can have the various glass properties described below.

[0058] (specific elastic modulus) Glass 2 can have high rigidity due to the composition described above. The specific modulus of elasticity can be used as an index of the rigidity of glass. The specific modulus of elasticity is calculated by dividing the Young's modulus of the glass by its density. Here, density refers to the specific gravity of the glass in g / cm. 3The specific elastic modulus of glass 2 is preferably 35.0 MNm / kg or more, and is more preferably 35.5 MNm / kg or more, 36.0 MNm / kg or more, 36.5 MNm / kg or more, 37.0 MNm / kg or more, 37.5 MNm / kg or more, and 38.0 MNm / kg or more in that order. The specific elastic modulus can be, for example, 45.0 MNm / kg or less, 44.0 MNm / kg or less, 43.0 MNm / kg or less, 42.0 MNm / kg or less, 41.0 MNm / kg or less, or 40.0 MNm / kg or less, but is not limited to the above-exemplified values ​​because a higher specific elastic modulus is preferable because it results in higher rigidity.

[0059] (Young's modulus E) Young's modulus can also be used as an indicator of the rigidity of glass. The Young's modulus of glass 2 is preferably 86.0 GPa or more, more preferably 87.0 GPa or more, and more preferably 88.0 GPa or more, 89.0 GPa or more, 90.0 GPa or more, 91.0 GPa or more, 92.0 GPa or more, 93.0 GPa or more, 94.0 GPa or more, and 95.0 GPa or more in this order. The Young's modulus of glass 2 can be, for example, 120.0 GPa or less or 110.0 GPa or less, but since a higher Young's modulus indicates higher rigidity and is preferable, it is not limited to the above-mentioned values.

[0060] (specific gravity d) By reducing the specific gravity of the glass for magnetic recording medium substrates, it is possible to reduce the weight of the magnetic recording medium substrate, and further reduce the weight of the magnetic recording medium, thereby enabling the reduction of power consumption of the HDD. The specific gravity of glass 2 is preferably 2.80 or less, more preferably 2.75 or less, even more preferably 2.70 or less, still more preferably 2.65 or less, and even more preferably 2.60 or less. The specific gravity of glass 2 can be, for example, 2.40 or more, but since a lower specific gravity is preferable, it is not limited to the values ​​exemplified above.

[0061] (glass transition temperature Tg) Magnetic recording medium substrates are typically subjected to high-temperature treatment during the process of forming a magnetic recording layer on the substrate. For example, to form a magnetic recording layer containing a magnetic material with high magnetic anisotropy energy, which has been developed in recent years to increase the recording density of magnetic recording media, film formation is typically performed at high temperatures, or a heat treatment is performed at high temperatures after film formation. It is preferable for magnetic recording medium substrates to have heat resistance that can withstand such high-temperature treatments in order to prevent the substrate's flatness from decreasing due to exposure to high temperatures. In this regard, glass 2 can also exhibit high heat resistance by having the above-mentioned glass composition. Regarding the glass transition temperature Tg, which is an index of heat resistance, the Tg of glass 2 can be, for example, 550°C or higher, preferably 600°C or higher, and more preferably 610°C or higher, 620°C or higher, 630°C or higher, 640°C or higher, 650°C or higher, 660°C or higher, 670°C or higher, 680°C or higher, 690°C or higher, 700°C or higher, 710°C or higher, 720°C or higher, 730°C or higher, 740°C or higher, and 750°C or higher. The Tg of glass 2 can be, for example, 850°C or lower, 830°C or lower, 810°C or lower, 790°C or lower, 770°C or lower, or 750°C or lower, but is not limited to the above-mentioned exemplary values ​​because a higher Tg is preferable from the viewpoint of heat resistance.

[0062] (coefficient of thermal expansion) HDDs incorporating magnetic recording media are typically designed to rotate the magnetic recording medium itself by holding its center with the spindle and clamp of a spindle motor. Therefore, if there is a large difference in the thermal expansion coefficients of the magnetic recording medium substrate and the spindle material that makes up the spindle, a mismatch will occur between the thermal expansion and contraction of the spindle and the magnetic recording medium substrate in response to ambient temperature changes during use, resulting in deformation of the magnetic recording medium. This phenomenon can cause the head to be unable to read written information, resulting in reduced reliability of recording and playback. Therefore, it is desirable to avoid too large a difference between the thermal expansion coefficient of the glass used for the magnetic recording medium substrate and that of the spindle material (e.g., stainless steel). Generally, HDD spindle materials have a thermal expansion coefficient of 70×10 in the temperature range of 100–300°C. -7 / °C or more, and the average linear expansion coefficient (thermal expansion coefficient) of the glass for magnetic recording medium substrates at 100 to 300°C is 30.0 × 10 -7 / °C or more, the difference in thermal expansion coefficient with the spindle material is small, which can contribute to improving the reliability of the magnetic recording medium. On the other hand, if the thermal expansion coefficient of the glass substrate is too large, the substrate may crack due to heating during film formation. The average linear expansion coefficient α of Glass 2 at 100 to 300°C is 34.0 × 10 -7 / °C or more, and 35.0 × 10 -7 / ℃ or more, 36.0×10 -7 / ℃ or more, 37.0×10 -7 / ℃ or more, 38.0×10 -7 / ℃ or more, 39.0×10 -7 / ℃ or more, 40.0×10 -7 / °C or more. The average linear expansion coefficient α of glass 2 at 100 to 300°C is 70.0 × 10 -7 / °C or less, and -7 / ℃ or less, 65.0×10 -7 / ℃ or less, 63.0×10 -7 / ℃ or less, 60.0×10 -7 / ℃ or less, 57.0×10 -7 / ℃ or less, 55.0×10 -7 / ℃ or less, 50.0×10 -7 / °C or less is more preferable.

[0063] (Liquidus temperature LT) An index of the thermal stability of glass can be the liquidus temperature (LT). The LT of glass 2 is preferably 1330°C or lower, more preferably 1320°C or lower, and further preferably 1310°C or lower, 1300°C or lower, 1290°C or lower, 1280°C or lower, 1270°C or lower, 1260°C or lower, 1250°C or lower, and 1240°C or lower in that order. Glasses with low liquidus temperatures LT are preferred because they are less likely to devitrify. The lower limit of LT can be, for example, 800°C or higher, but is not particularly limited.

[0064] The various physical properties described above and the various physical properties described below can be determined by the methods described later in the examples.

[0065] [Glass 3] <Glass composition> From the viewpoint of improving chemical durability and rigidity, the B2O3 content of glass 3 is 2.00% or less, preferably 1.80% or less, and more preferably 1.50% or less, 1.20% or less, 1.00% or less, 0.80% or less, 0.70% or less, 0.60% or less, 0.50% or less, and 0.40% or less, in that order. Glass with high chemical durability is preferred for the reasons described above. From the viewpoint of improving impact resistance, meltability, and thermal stability, the B2O3 content of glass 3 is 0.10% or more, preferably 0.20% or more. As described above, glass with high impact resistance is preferred because it is less likely to crack and / or chip during processing such as cutting and grinding. Furthermore, glass with high thermal stability is preferred because it is less likely to devitrify.

[0066] In one form, the glass 3 can include one or both of SiO2 and Al2O3.

[0067] From the viewpoint of improving chemical durability, the SiO content of glass 3 is preferably 50.00% or more, and more preferably in this order: 51.00% or more, 52.00% or more, 53.00% or more, 54.00% or more, 55.00% or more, 56.00% or more, 57.00% or more, 58.00% or more, 59.00% or more, 60.00% or more, 61.00% or more, 62.00% or more, 63.00% or more, and 64.00% or more. Glass with high chemical durability is preferred because it is less likely to cause surface roughening during polishing or cleaning. On the other hand, from the viewpoint of further improving rigidity, the SiO content is preferably 75.00% or less, and more preferably 74.00% or less, 73.0% or less, 72.0% or less, 71.0% or less, 70.0% or less, 69.0% or less, 68.0% or less, 67.0% or less, 66.0% or less, 65.00% or less, 64.00% or less, 63.00% or less, 62.00% or less, and 61.00% or less in that order.

[0068] From the viewpoint of improving thermal stability, the Al2O3 content of Glass 3 is preferably 20.00% or less, and more preferably 19.00% or less, 18.00% or less, 17.00% or less, and 16.00% or less in that order. Furthermore, from the viewpoint of maintaining the temperature required for melting the glass within an appropriate range, the Al2O3 content within the above range is also preferred. Meanwhile, from the viewpoint of further improving rigidity, the Al2O3 content is preferably 10.00% or more, and more preferably 11.00% or more, 12.00% or more, and 13.00% or more in that order.

[0069] In Glass 3, the total content of SiO2, B2O3, and Al2O3 (SiO2+B2O3+Al2O3) is preferably 70.00% or more from the viewpoint of enhancing the thermal stability of the glass, and is more preferably 71.00% or more, 72.00% or more, 73.00% or more, 74.00% or more, 75.00% or more, 76.00% or more, 77.00% or more, 78.00% or more, and 79.00% or more in that order. From the viewpoint of further improving the meltability, the total content of SiO2, B2O3, and Al2O3 (SiO2 + B2O3 + Al2O3) in glass 3 is preferably 85.00% or less, and more preferably 84.00% or less, 83.00% or less, 82.00% or less, 81.00% or less, 80.00% or less, 79.00% or less, 78.00% or less, 77.00% or less, 76.00% or less, 75.00% or less, and 74.00% or less in that order.

[0070] From the viewpoint of enhancing the thermal stability of the glass, the total content of SiO2 and B2O3 (SiO2 + B2O3) is preferably 58.00% or more, and more preferably 59.00% or more, 60.00% or more, 61.00% or more, 62.00% or more, 63.00% or more, and 64.00% or more in that order. From the viewpoint of further improving rigidity, the total content of SiO2 and B2O3 (SiO2 + B2O3) in glass 3 is preferably 77.00% or less, and more preferably in order of 76.00% or less, 75.00% or less, 74.00% or less, 73.00% or less, 72.00% or less, 71.00% or less, 70.00% or less, 69.00% or less, 68.00% or less, 67.00% or less, 66.00% or less, 65.00% or less, 64.00% or less, 63.00% or less, and 62.00% or less.

[0071] In glass 3, from the viewpoint of chemical durability such as acid resistance and alkali resistance, the total content of SiO2 and Al2O3 (SiO2+Al2O3) is preferably 70.00% or more, and more preferably in this order: 71.00% or more, 72.00% or more, 73.00% or more, 74.00% or more, 75.00% or more, 76.00% or more, 77.00% or more, 78.00% or more, and 79.00% or more. Furthermore, from the viewpoint of the meltability of the glass, the total content of SiO2 and Al2O3 (SiO2 + Al2O3) in glass 3 is preferably 85.00% or less, and is more preferably 84.00% or less, 83.00% or less, 82.00% or less, 81.00% or less, 80.00% or less, 79.00% or less, 78.00% or less, 77.00% or less, 76.00% or less, 75.00% or less, and 74.00% or less in that order.

[0072] Glass 3 contains Na2O as an essential component. Na2O is a component that has various functions, such as increasing the specific elastic modulus of glass, improving meltability, increasing the thermal expansion coefficient, and reducing the viscosity of glass during fining to promote bubble removal. Furthermore, among alkali metal oxides, the inclusion of Na2O in glass can contribute to increasing the glass transition temperature compared to the addition of the same amount of Li2O. The Na2O content of Glass 3 is 1.00% or more, preferably 1.50% or more, with 2.00% or more, 2.50% or more, and 3.00% or more being more preferred in that order. Furthermore, from the viewpoint of increasing the glass transition temperature, the Na2O content of Glass 3 is 6.00% or less, preferably 5.50% or less, with 5.00% or less, 4.50% or less, and 4.00% or less being more preferred in that order.

[0073] From the viewpoint of improving the water resistance of the glass, the molar ratio of the SiO content to the NaO content (SiO / NaO) in Glass 3 is preferably 12.0 or more, and more preferably 15.0 or more, 16.0 or more, 17.0 or more, and 18.0 or more in that order. Excellent water resistance of glass is preferable for preventing the glass surface from becoming rough and losing its smoothness (i.e., increasing surface roughness) when an ultra-smoothly polished glass surface comes into contact with water during cleaning or the like. The lower the surface roughness of the glass, the better the orientation of the magnetic particles and the easier it is to reduce the gap between the magnetic head and the magnetic recording medium (the flying height of the magnetic head), which is preferable. On the other hand, from the viewpoint of improving the meltability of the glass, the molar ratio (SiO / NaO) is preferably 50.0 or less, more preferably 40.0 or less, and even more preferably 30.0 or less.

[0074] The glass 3 may also contain alkali metal oxides other than Na2O. Li2O is a component that functions similarly to Na2O, but as described above, it is more likely to lower the glass transition temperature compared to adding the same amount of Na2O. The Li2O content of Glass 3 can be, for example, 6.00% or less, 5.00% or less, 4.00% or less, 3.00% or less, 2.00% or less, 1.50% or less, 1.00% or less, 0.50% or less, or 0.10% or less. The Li2O content of Glass 3 can be 0%, 0% or more, or more than 0%. In one embodiment, Glass 3 can be a glass that does not contain Li2O.

[0075] K2O is a component that functions similarly to Na2O, but because it has a larger specific gravity than Na2O, it is easier to lower the specific elastic modulus compared to adding the same amount of Na2O. The K2O content of Glass 3 can be, for example, 6.00% or less, 5.00% or less, 4.00% or less, 3.00% or less, 2.00% or less, 1.50% or less, 1.00% or less, 0.50% or less, or 0.10% or less. The K2O content of Glass 3 can be 0%, 0% or more, or more than 0%. In one embodiment, Glass 3 can be a glass that does not contain K2O. In another embodiment, Glass 3 can be a glass that contains K2O, for example, a glass that contains 0.10% or more of K2O.

[0076] From the viewpoint of increasing the glass transition temperature and specific elastic modulus, the molar ratio of the NaO content to the total content of LiO, NaO, and KO (NaO / (LiO+NaO+KO)) in Glass 3 is preferably 0.350 or more, and is more preferably 0.400 or more, 0.450 or more, 0.500 or more, 0.550 or more, 0.600 or more, 0.650 or more, 0.700 or more, 0.750 or more, 0.800 or more, 0.850 or more, 0.900 or more, 0.950 or more, 0.960 or more, 0.970 or more, 0.980 or more, and 0.990 or more in this order. The molar ratio (NaO / (LiO+NaO+KO)) in Glass 3 can be 1.000, or can be 1.000 or less or less than 1.000. In one embodiment, glass 3 can be glass that does not contain LiO or KO. From the viewpoint of improving the water resistance and reducing the roughness of the glass, the molar ratio (NaO / (LiO+NaO+KO)) is preferably less than 1.000, and more preferably 0.950 or less, and more preferably 0.900 or less, in that order.

[0077] From the viewpoint of improving the water resistance of the glass, the molar ratio of the LiO content to the total content of LiO, NaO, and KO in the above glass (LiO / (LiO+NaO+KO)) is preferably 0.050 or more, and more preferably 0.100 or more. On the other hand, from the viewpoint of suppressing a decrease in the glass transition temperature, the molar ratio (LiO / (LiO+NaO+KO)) is preferably 0.600 or less, and more preferably 0.500 or less, 0.400 or less, and 0.300 or less, in that order.

[0078] The CaO content of Glass 3 is 1.00% or less. A CaO content of 1.00% or less contributes to increasing the specific elastic modulus and glass transition temperature of the glass, and can also contribute to improving the thermal stability of the glass. From these viewpoints, the CaO content is preferably 0.90% or less, and more preferably 0.80% or less, 0.70% or less, 0.60% or less, 0.50% or less, 0.40% or less, 0.30% or less, 0.20% or less, and 0.10% or less in this order. The CaO content of Glass 3 can be 0%, 0% or more, or more than 0%. In one embodiment, Glass 3 preferably does not contain CaO.

[0079] Regarding alkaline earth metal oxides, the CaO content of Glass 3 is as described above. In one embodiment, Glass 3 may contain one or more alkaline earth metal oxides. Specific examples of alkaline earth metal oxides that may be contained in Glass 3 include MgO, SrO, and BaO. Glass 3-A contains MgO. From the viewpoint of improving rigidity, the MgO content of Glass 3-A is 14.00% or more, preferably 14.50% or more, with 15.00% or more, 15.50% or more, and 16.00% or more being more preferred in that order. Furthermore, from the viewpoint of improving the devitrification resistance of the glass, the MgO content of Glass 3-A is preferably 28.00% or less, and more preferably 27.00% or less, 26.00% or less, 25.00% or less, 24.00% or less, 23.00% or less, 22.00% or less, 21.00% or less, 20.00% or less, 19.00% or less, and 18.00% or less in that order. Glass 3-B may contain MgO. For the MgO content of Glass 3-B, please refer to the description of the MgO content of Glass 3-A.

[0080] In Glass 3-B, the total content of MgO, SrO, CaO, and BaO (MgO + SrO + CaO + BaO) is, from the viewpoint of improving rigidity, 14.00 mol% or more, preferably 14.50% or more, and more preferably 15.00% or more, 15.50% or more, and 16.00% or more in that order. Furthermore, from the viewpoint of improving devitrification resistance of the glass, the total content of MgO, SrO, CaO, and BaO (MgO + SrO + CaO + BaO) in Glass 3-B is preferably 28.00% or less, and more preferably 27.00% or less, 26.00% or less, 25.00% or less, 24.00% or less, 23.00% or less, 22.00% or less, 21.00% or less, 20.00% or less, 19.00% or less, and 18.00% or less in that order. For the total content of MgO, SrO, CaO, and BaO (MgO+SrO+CaO+BaO) in Glass 3-A, reference can be made to the description of the total content of MgO, SrO, CaO, and BaO (MgO+SrO+CaO+BaO) in Glass 3-B.

[0081] From the viewpoint of reducing the specific gravity and raw material costs, the SrO content of glass 3 is preferably 4.00% or less, and is more preferably 3.00% or less, 2.00% or less, 1.00% or less, and 0.50% or less in that order. In one embodiment, the SrO content of glass 3 can be 0%, or can be 0% or more or exceed 0%.

[0082] From the viewpoints of decreasing the specific gravity of the glass, improving Young's modulus, and improving the specific elastic modulus, the BaO content of glass 3 is preferably 3.00% or less, and more preferably 2.00% or less, 1.00% or less, and 0.50% or less in that order. In one embodiment, the BaO content of glass 3 can be 0%, 0% or more, or more than 0%.

[0083] The molar ratio of the MgO content to the total content of MgO, CaO, SrO, and BaO (MgO / (MgO+CaO+SrO+BaO)) can be 1.000, 1.000 or less, or less than 1.000. MgO has a lower specific gravity than other alkaline earth metal oxides, and therefore can contribute to increasing the specific modulus of elasticity. Furthermore, in general glasses, the addition of alkaline earth metal oxides with a higher specific gravity tends to increase the glass transition temperature. In contrast, according to the inventors' investigations, the addition of alkaline earth metal oxides with a higher specific gravity than MgO in Glass 3 tends to decrease the glass transition temperature. From the viewpoint of increasing the specific modulus and glass transition temperature of Glass 3, the molar ratio (MgO / (MgO+CaO+SrO+BaO)) is preferably 0.800 or more, and more preferably 0.850 or more, 0.900 or more, 0.950 or more, and 1.000, in that order.

[0084] In glass 3, from the viewpoints of reducing melt viscosity, lowering liquidus temperature, and improving glass roughness reduction polishing performance, the molar ratio of Na2O content to MgO content (Na2O / MgO) is preferably greater than 0.080. Furthermore, a molar ratio (Na2O / MgO) of glass 3 greater than 0.080 can contribute to reducing the specific resistance of the glass melt. From the above viewpoints, the molar ratio (Na2O / MgO) of glass 3 is more preferably 0.090 or greater, and further preferably 0.100 or greater, 0.110 or greater, 0.120 or greater, 0.130 or greater, 0.140 or greater, 0.150 or greater, and 0.160 or greater, in that order. On the other hand, from the viewpoint of increasing the glass transition temperature and specific elastic modulus, the molar ratio (Na2O / MgO) of glass 3 is preferably less than 0.240, and more preferably 0.230 or less, 0.220 or less, 0.210 or less, 0.200 or less, 0.190 or less, or 0.180 or less in that order.

[0085] Regarding TiO2, from the viewpoint of lowering the specific gravity and improving devitrification resistance, the TiO2 content of glass 3 is preferably 4.00% or less, and more preferably 3.00% or less, 2.00% or less, and 1.00% or less in that order. The TiO2 content of glass 3 can be 0%, or can be 0% or more or exceed 0%.

[0086] Regarding ZnO, from the viewpoints of reducing the specific gravity, improving Young's modulus, and improving the specific elastic modulus, the ZnO content is preferably 10.00% or less, and more preferably 8.00% or less, 6.00% or less, 4.00% or less, 2.00% or less, and 1.00% or less in that order. The ZnO content of glass 3 can be 0%, 0% or more, or more than 0%.

[0087] The Fe2O3 content of glass 3, expressed as an exclusive percentage, can be 1 mol% or less, 0.7 mol% or less, 0.5 mol% or less, 0.4 mol% or less, 0.3 mol% or less, 0.1 mol% or less, 0.07 mol% or less, 0.05 mol% or less, 0.04 mol% or less, 0.03 mol% or less, or 0.02 mol% or less. The Fe2O3 content is expressed as an exclusive percentage. That is, when the total content of components other than Fe2O3 contained in the glass (the total content of the glass components and additives when additives are included in addition to the glass components) is taken as 100%, the Fe2O3 content is expressed as a molar percentage of the Fe2O3 content relative to 100%. In one embodiment, glass 3 can be free of Fe (the Fe2O3 content expressed as an exclusive percentage is 0 mol%).

[0088] Glass 3 may or may not contain one or more metals or oxides thereof selected from the group consisting of Zr, Y, La, Ti, Hf, Cu, Co, Mn, Nd, Pr, Nb, V, Cr, Ni, Mo, Ho, and Er. Inclusion of an oxide of one or more metals selected from the group consisting of Zr, Y, La, Ti, Hf, Cu, Co, Mn, Nd, Pr, Nb, V, Cr, Ni, Mo, Ho, and Er in a glass composition based on oxides tends to enhance the thermal stability of the glass. In one embodiment, the content of one or more metal oxides selected from the group consisting of Zr, Y, La, Ti, Hf, Cu, Co, Mn, Nd, Pr, Nb, V, Cr, Ni, Mo, Ho, and Er (the total content when multiple metals are included) can be 0%, 0% or more, or more than 0%, and can be 5.00% or less, 4.00% or less, 3.00% or less, 2.00% or less, 1.00% or less, 0.50% or less, or 0.10% or less. For example, the YO content can be 0% or more, or more than 0%, and can be 3.00% or less, 2.00% or less, or 1.00% or less.

[0089] F is a component that easily volatilizes during melting and is also a component that causes striae, so it is preferable that glass 3 does not contain F. Not containing F is also preferable from the viewpoints of suppressing erosion of the melting furnace, suppressing a decrease in Young's modulus, and suppressing a decrease in specific elastic modulus.

[0090] Since Pb, Cd and As are substances that have a negative impact on the environment, it is preferable to avoid introducing them.

[0091] From the viewpoint of obtaining a fining effect, glass 3 can contain one or more selected from the group consisting of SnO2, CeO2, and Sb2O3. In one embodiment, the total content of SnO2 and CeO2 can be 0%. In another embodiment, glass 3 can contain SnO2 and / or CeO2, and the total content of SnO2 and CeO2 (SnO2 + CeO2) is preferably 0.05 to 2%. When the total content of SnO2 and CeO2 is 0.05% or more, a sufficient fining effect can be obtained and residual bubbles can be reduced. Furthermore, when the total content (SnO2 + CeO2) is 2% or less, it is possible to prevent the molten glass from blowing up during glass melting, which can reduce productivity. The lower limit of the total content (SnO2 + CeO2) is preferably 0.10% or more, more preferably 0.20% or more, even more preferably 0.25% or more, still more preferably 0.30% or more, even more preferably 0.35% or more, and even more preferably 0.40% or more. The upper limit of the total content (SnO2 + CeO2) is preferably 1.5% or less, more preferably 1.2% or less, even more preferably 1.0% or less, even more preferably 0.70% or less, even more preferably 0.65% or less, even more preferably 0.60% or less, even more preferably 0.55% or less, and even more preferably 0.50% or less.

[0092] SnO2 promotes fining when the glass melting temperature is relatively high (approximately 1400 to 1600°C). With the use of environmentally harmful fining agents such as Sb2O3 and arsenous acid being restricted, in one embodiment, SnO2 is preferably incorporated into glass 3 to remove bubbles in glass with a high melting temperature. From the viewpoint of achieving a fining effect, the SnO2 content is preferably 0.01% or more, more preferably 0.05% or more, even more preferably 0.10% or more, even more preferably 0.15% or more, and even more preferably 0.20% or more. Furthermore, the SnO2 content is preferably 2% or less, more preferably 1.5% or less, even more preferably 1.0% or less, even more preferably 0.8% or less, and even more preferably 0.5% or less.

[0093] Like SnO2, CeO2 is a component that exhibits a fining effect on glass. CeO2 functions to capture oxygen and fix it as a glass component when the glass melting temperature is relatively low (a temperature range of approximately 1200 to 1400°C). Therefore, in one embodiment, it is preferable to incorporate CeO2 into glass 3 as a fining agent. From the viewpoint of obtaining a fining effect, the content of CeO2 is preferably 0.01% or more, more preferably 0.05% or more, even more preferably 0.08% or more, and even more preferably 0.10% or more. Furthermore, the content of CeO2 is preferably 2% or less, more preferably 1.5% or less, even more preferably 1.0% or less, even more preferably 0.8% or less, even more preferably 0.5% or less, and even more preferably 0.3% or less. The coexistence of SnO2 and CeO2 can obtain a fining effect over a wide temperature range. Therefore, in one embodiment, glass 3 preferably contains both SnO2 and CeO2.

[0094] From the viewpoint of reducing the environmental impact, it is desirable to refrain from using Sb2O3. The content of Sb2O3 in glass 3 is preferably in the range of 0 to 0.5%. The content of Sb2O3 is more preferably 0.3% or less, further preferably 0.1% or less, even more preferably 0.05% or less, and still further preferably 0.02% or less, and it is particularly preferable that no Sb2O3 is contained.

[0095] Glass 3 can be produced by weighing, blending, and thoroughly mixing glass raw materials such as oxides, carbonates, nitrates, sulfates, and hydroxides so as to obtain a predetermined glass composition, heating and melting the mixture in a melting vessel at, for example, a temperature range of 1400 to 1600°C, refining, stirring, and molding the homogenized molten glass that has been thoroughly defoamed. For example, it is preferable to heat and melt the glass raw materials in a melting vessel at 1400 to 1550°C, and then heat the resulting molten glass in a refining vessel, hold it at 1450 to 1600°C, and then cool it to 1200 to 1400°C, at which point the glass flows out and is molded.

[0096] According to another aspect of the present invention, B2O3 content is 0.1 mol% or more and 2.0 mol% or less, The Na2O content is 1.0 mol% or more and 6.0 mol% or less, and CaO content is 0 mol% or more and 1.0 mol% or less, and a glass for a magnetic recording medium substrate or a glass spacer for a magnetic recording / reproducing device, which is an amorphous glass and further satisfies any one or more of the following (1) to (10): can be provided.

[0097] (1) The molar ratio (Na2O / (Li2O+Na2O+K2O)) is 0.950 or more, and the total content (SiO2+B2O3+Al2O3) is 70.00% or more.

[0098] (2) Does not contain Li2O and K2O, and the total content (SiO2 + B2O3 + Al2O3) is 70.00% or more.

[0099] (3) The molar ratio (Na2O / (Li2O+Na2O+K2O)) is 0.950 or more, and the total content (SiO2+B2O3) is 58.00% or more.

[0100] (4) Does not contain Li2O and K2O, and the total content (SiO2 + B2O3) is 58.00% or more.

[0101] (5) The molar ratio (Na2O / (Li2O+Na2O+K2O)) is 0.950 or more, and the total content (SiO2+Al2O3) is 70.00% or more.

[0102] (6) Does not contain Li2O and K2O, and the total content (SiO2 + Al2O3) is 70.00% or more.

[0103] (7) The molar ratio (Na2O / (Li2O+Na2O+K2O)) is 0.950 or more, and the TiO2 content is 4.00 mol% or less.

[0104] (8) Contains no Li2O and no K2O, and has a TiO2 content of 4.00 mol% or less.

[0105] (9) The molar ratio (Na2O / (Li2O+Na2O+K2O)) is 0.950 or more, and the ZnO content is 10.00% or less.

[0106] (10) Contains no Li2O and no K2O, and has a ZnO content of 10.00% or less.

[0107] For each of the above glasses, reference can be made to the above description regarding the compositions of Glass 3-A and Glass 3-B and the below description regarding the physical properties of Glass 3-A and Glass 3-B.

[0108] According to another aspect of the present invention, B2O3 content is 0.10 mol% or more and 2.00 mol% or less, a total content of Na2O and K2O of 1.00 mol% or more and 6.00 mol% or less; CaO content is 0 mol% or more and 1.00 mol% or less, MgO content is 14.00 mol% or more, a glass for a magnetic recording medium substrate or a glass spacer for a magnetic recording / reproducing device (hereinafter referred to as "glass 3-a"), which is an amorphous glass of the above formula: B2O3 content is 0.10 mol% or more and 2.00 mol% or less, a total content of Na2O and K2O of 1.00 mol% or more and 6.00 mol% or less; CaO content is 0 mol% or more and 1.00 mol% or less, The total content of MgO, SrO, CaO and BaO (MgO + SrO + CaO + BaO) is 14.00 mol% or more, a glass for a magnetic recording medium substrate or a glass spacer for a magnetic recording / reproducing device (hereinafter referred to as "glass 3-b"), which is an amorphous glass of the above formula: can be provided.

[0109] For Glass 3-a and Glass 3-b, reference can be made to the above description regarding the compositions of Glass 3-A and Glass 3-B and the below description regarding the physical properties of Glass 3-A and Glass 3-B.

[0110] <Glass properties> By adjusting the composition as described above, the glass 3 can have the various glass properties described below.

[0111] (specific elastic modulus) Glass 3 can have high rigidity due to its composition as described above. The specific elastic modulus of glass 3 is preferably 35.0 MNm / kg or more, with 35.5 MNm / kg or more, 36.0 MNm / kg or more, 36.5 MNm / kg or more, and 37.0 MNm / kg or more being even more preferred in this order. The specific elastic modulus can be, for example, 45.0 MNm / kg or less, 44.0 MNm / kg or less, 43.0 MNm / kg or less, 42.0 MNm / kg or less, 41.0 MNm / kg or less, 40.0 MNm / kg or less, 39.0 MNm / kg or less, or 38.0 MNm / kg or less. However, since a higher specific elastic modulus is preferable because it increases rigidity, the specific elastic modulus is not limited to the above-mentioned exemplary values. Note that the specific elastic modulus of glass 3 may be 37.0 MNm / kg or less, taking into account the balance with the glass transition temperature.

[0112] (Young's modulus E) The Young's modulus of glass 3 is preferably 86.0 GPa or more, more preferably 87.0 GPa or more, and even more preferably 88.0 GPa or more, 89.0 GPa or more, 90.0 GPa or more, 91.0 GPa or more, 92.0 GPa or more, and 93.0 GPa or more in this order. The Young's modulus of glass 3 can be, for example, 120.0 GPa or less, 110.0 GPa or less, 100 GPa or less, or 95.0 GPa or less, but since a higher Young's modulus is preferable because it increases rigidity, the Young's modulus is not limited to the above-mentioned values. Note that the Young's modulus of glass 3 may be 94.0 GPa or less, 93.0 GPa or less, or 92.0 GPa or less, taking into account the balance with the glass transition temperature.

[0113] (specific gravity d) The specific gravity of glass 3 is preferably 2.80 or less, more preferably 2.75 or less, even more preferably 2.70 or less, still more preferably 2.65 or less, and even more preferably 2.60 or less. The specific gravity of glass 3 can be, for example, 2.40 or more, but since a lower specific gravity is more preferable, it is not limited to the above-mentioned values.

[0114] (glass transition temperature Tg) By having the above glass composition, Glass 3 can also exhibit high heat resistance. Regarding the glass transition temperature Tg, which is an index of heat resistance, Glass 3's Tg can be, for example, 550°C or higher, preferably 600°C or higher, and is more preferably 610°C or higher, 620°C or higher, 630°C or higher, 640°C or higher, 650°C or higher, 660°C or higher, 670°C or higher, 680°C or higher, 690°C or higher, 700°C or higher, 710°C or higher, 720°C or higher, 730°C or higher, 740°C or higher, and 750°C or higher. Glass 3's Tg can be, for example, 850°C or lower, 830°C or lower, 810°C or lower, 790°C or lower, 770°C or lower, or 750°C or lower. However, since a higher Tg is preferable from the viewpoint of heat resistance, the Tg is not limited to the above-exemplified values.

[0115] (thermal stability) Glass 3 preferably exhibits high thermal stability. Methods for evaluating thermal stability include a 1300°C 16-hour holding test and a 1320°C 16-hour holding test, which will be described in detail below. In the 1300°C 16-hour holding test, the evaluation result is preferably A or B, and more preferably A. In the 1320°C 16-hour holding test, the evaluation result is preferably A or B, and more preferably A.

[0116] [Magnetic recording medium substrate] A magnetic recording medium substrate according to one aspect of the present invention is made of any of the glasses described above.

[0117] A magnetic recording medium substrate can be manufactured by heating glass raw materials to prepare a glass melt, forming the glass melt into a plate by, for example, a press molding method, a down-draw method, or a float method, and then processing the resulting glass melt. For example, in a press molding method, the glass melt flowing out of a glass flow pipe is cut to a predetermined volume to obtain a desired glass melt lump, which is then press-molded in a press mold to produce a thin, disk-shaped substrate blank. The resulting substrate blank is then provided with a central hole, and the inner and outer peripheries are processed, and both main surfaces are polished by lapping, polishing, or the like. A disk-shaped substrate can then be obtained by a cleaning process that includes acid cleaning and alkali cleaning. Known techniques related to the manufacture of magnetic recording medium substrates can be applied to the various steps performed to obtain the magnetic recording medium substrate.

[0118] In one embodiment, the magnetic recording medium substrate has a homogeneous surface and internal composition. Here, "homogeneous surface and internal composition" means that ion exchange has not been performed (i.e., the substrate does not have an ion exchange layer). Magnetic recording medium substrates without an ion exchange layer can be manufactured without ion exchange treatment, thereby significantly reducing manufacturing costs.

[0119] In one embodiment, the magnetic recording medium substrate has an ion-exchange layer on part or all of its surface. Because the ion-exchange layer exhibits compressive stress, the presence or absence of the ion-exchange layer can be confirmed by breaking the substrate perpendicular to the main surface and obtaining a stress profile on the fracture surface using the Babinet method. The "main surface" refers to the surface of the substrate on which or on which the magnetic recording layer is provided. This surface is called the main surface because it has the largest area among the surfaces of the magnetic recording medium substrate. In the case of a disk-shaped magnetic recording medium, this corresponds to the circular surface of the disk (excluding the center hole, if any). The presence or absence of the ion-exchange layer can also be confirmed by measuring the concentration distribution of alkali metal ions in the depth direction from the substrate surface.

[0120] The ion exchange layer can be formed by contacting an alkali salt with the substrate surface at high temperature and exchanging the alkali metal ions in the alkali salt with the alkali metal ions in the substrate. Ion exchange (also called "strengthening treatment" or "chemical strengthening") can be performed using known techniques; see, for example, paragraphs 0068-0069 of WO2011 / 019010A1.

[0121] The magnetic recording medium substrate has a thickness of, for example, 1.5 mm or less, preferably 1.2 mm or less, more preferably 1.0 mm or less, even more preferably 0.8 mm or less, even more preferably less than 0.8 mm, even more preferably 0.7 mm or less, and even more preferably 0.6 mm or less. The thickness of the magnetic recording medium substrate is, for example, 0.2 mm or more. Reducing the thickness of the magnetic recording medium substrate is preferable from the viewpoint of improving the recording capacity of HDDs. The magnetic recording medium substrate is preferably in the shape of a disk with a center hole.

[0122] The magnetic recording medium substrate is made of amorphous glass, which can achieve superior surface smoothness when processed into a substrate compared to glass-ceramics.

[0123] The magnetic recording medium substrate is made of any of the above glasses, and therefore can have the glass properties described above for the above glasses.

[0124] [Magnetic recording media] One aspect of the present invention relates to a magnetic recording medium having the above magnetic recording medium substrate and a magnetic recording layer.

[0125] Magnetic recording media are called magnetic disks, hard disks, etc., and are suitable for various magnetic recording and playback devices, such as internal storage devices (fixed disks, etc.) of desktop personal computers, server computers, notebook personal computers, mobile personal computers, etc., internal storage devices of portable recording and playback devices that record and play back images and / or audio, recording and playback devices for in-car audio, etc. In this invention and this specification, the term "magnetic recording and playback device" refers to a device that is capable of either or both of magnetically recording information and magnetically playing back information.

[0126] A magnetic recording medium is configured, for example, by stacking at least an adhesive layer, an underlayer, a magnetic layer (magnetic recording layer), a protective layer, and a lubricating layer on the main surface of a magnetic recording medium substrate, in that order from the side closest to the main surface. For example, a magnetic recording medium substrate is placed in a vacuum-evacuated film-forming apparatus, and films from an adhesive layer to a magnetic layer are sequentially formed on the main surface of the magnetic recording medium substrate using a DC (Direct Current) magnetron sputtering method in an Ar atmosphere. The adhesive layer can be made of, for example, CrTi, and the underlayer can be made of, for example, a material containing Ru or MgO. A soft magnetic layer and a heat sink layer may also be added as appropriate. After the above film formation, a protective layer can be formed using, for example, C2H4 by a CVD (Chemical Vapor Deposition) method, and a nitriding process to introduce nitrogen into the surface can be performed in the same chamber to form the magnetic recording medium. A lubricating layer can then be formed by applying, for example, PFPE (polyfluoropolyether) to the protective layer by a dip coating method.

[0127] To achieve even higher recording densities in magnetic recording media, the magnetic recording layer preferably contains a magnetic material with high magnetic anisotropy energy. From this perspective, preferred magnetic materials include Fe-Pt-based magnetic materials or Co-Pt-based magnetic materials. Here, "based" means "contained." That is, the magnetic recording medium can have a magnetic recording layer containing Fe and Pt, or Co and Pt. For magnetic recording layers containing such magnetic materials and methods for forming them, see paragraph 0074 of WO2011 / 019010A1 and the examples therein. Furthermore, magnetic recording media having such magnetic recording layers are preferably applied to magnetic recording devices using a recording method known as energy-assisted recording. Among energy-assisted recording methods, a recording method that assisted magnetization reversal by irradiation with near-field light or the like is called thermally assisted recording, and a recording method that assisted magnetization reversal by microwaves is called microwave-assisted recording. For details, see paragraph 0075 of WO2011 / 019010A1. As the magnetic material for forming the magnetic recording layer, a conventional CoPtCr-based material may be used.

[0128] Recently, by incorporating a DFH (Dynamic Flying Height) mechanism into a magnetic head, the gap between the read / write element of the magnetic head and the surface of the magnetic recording medium can be significantly narrowed (reduced flying height), thereby achieving even higher recording densities. The DFH mechanism is a function in which a heating element such as a tiny heater is provided near the read / write element of the magnetic head, causing only the periphery of the element to protrude toward the surface of the medium. This reduces the distance (flying height) between the magnetic head and the magnetic recording layer of the medium, making it possible to pick up signals from smaller magnetic particles and achieving even higher recording densities. In one embodiment, the magnetic recording medium substrate can be used as a substrate for a magnetic recording medium applied to a magnetic recording / reproducing device equipped with a magnetic head equipped with a DFH mechanism.

[0129] There are no particular limitations on the dimensions of the magnetic recording medium substrate (e.g., glass substrate for magnetic disk) or the magnetic recording medium (e.g., magnetic disk). For example, since high recording density is possible, it is possible to miniaturize the medium and substrate. It is also possible to enlarge the medium and substrate in order to increase the recording capacity per magnetic recording medium. For example, the nominal diameter can be 2.5 inches, of course, but they can also be made smaller (e.g., 1 inch, 1.8 inches), or 3 inches, 3.5 inches, or even larger.

[0130] [Glass spacer for magnetic recording / reproducing devices] A glass spacer for a magnetic recording / reproducing device according to one aspect of the present invention (hereinafter also simply referred to as a "glass spacer") is made of the above glass.

[0131] Magnetic recording media can be used in magnetic recording and / or reproducing devices to magnetically record and / or reproduce information. Magnetic recording and reproducing devices typically include spacers for fixing the magnetic recording media to the spindle of a spindle motor and / or for maintaining a distance between multiple magnetic recording media. In recent years, the use of glass spacers as such spacers has been proposed. For reasons similar to those detailed above for the glass used for magnetic recording medium substrates, it is desirable for these glass spacers to have high rigidity. In contrast, glass having the above composition can have high rigidity, making it suitable as a glass spacer for magnetic recording and reproducing devices.

[0132] The spacer for a magnetic recording / reproducing device is a ring-shaped member, and details such as the configuration and manufacturing method of the glass spacer are publicly known. For the manufacturing method of the glass spacer, reference can be made to the above-mentioned descriptions regarding the manufacturing method of glass for a magnetic recording medium substrate and the manufacturing method of a magnetic recording medium substrate. For other details such as the glass composition and physical properties of the glass spacer, reference can be made to the above-mentioned descriptions regarding the glass, the magnetic recording medium substrate made of the glass, and the magnetic recording medium having such a magnetic recording medium substrate. The spacer for the magnetic recording / reproducing device may be made of the glass spacer described above, or may have one or more films, such as a conductive film, formed on the surface of the glass spacer. For example, to remove static electricity generated during rotation of the magnetic recording medium, a conductive film, such as a NiP alloy, may be formed on the surface of the glass spacer by plating, immersion, vapor deposition, sputtering, or other methods. Furthermore, the glass spacer can be polished to enhance its surface smoothness (for example, to an average surface roughness Ra of 1 μm or less), thereby strengthening the adhesion between the magnetic recording medium and the spacer and suppressing misalignment.

[0133] [Magnetic recording and playback device] One aspect of the present invention relates to a magnetic recording and reproducing device including one or more members selected from the group consisting of the magnetic recording medium and the glass spacer.

[0134] A magnetic recording and reproducing device includes at least one magnetic recording medium and at least one spacer, and typically further includes a spindle motor for rotating the magnetic recording medium and at least one magnetic head for recording and / or reproducing information on the magnetic recording medium. The magnetic recording and reproducing device according to an embodiment of the present invention may include a magnetic recording medium according to an embodiment of the present invention as at least one magnetic recording medium, or may include multiple magnetic recording media according to an embodiment of the present invention. The magnetic recording and reproducing device according to an embodiment of the present invention may include a glass spacer according to an embodiment of the present invention as at least one spacer, or may include multiple glass spacers according to an embodiment of the present invention. A small difference between the thermal expansion coefficient of the magnetic recording medium and the thermal expansion coefficient of the spacer is preferable from the viewpoint of suppressing the occurrence of phenomena that may arise due to the difference in the thermal expansion coefficients, such as distortion of the magnetic recording medium and reduced stability during rotation due to misalignment of the magnetic recording medium. From this viewpoint, the magnetic recording and reproducing device according to an embodiment of the present invention preferably includes a magnetic recording medium according to an embodiment of the present invention as at least one magnetic recording medium, or as more magnetic recording media if multiple magnetic recording media are included, and preferably includes a glass spacer according to an embodiment of the present invention as at least one spacer, or as more spacers if multiple spacers are included. Furthermore, for example, in the magnetic recording and reproducing device according to an embodiment of the present invention, the glass constituting the magnetic recording medium substrate included in the magnetic recording medium and the glass constituting the glass spacer may have the same glass composition.

[0135] A magnetic recording / reproducing device according to one embodiment of the present invention may include at least one of a magnetic recording medium according to one embodiment of the present invention and a glass spacer according to one embodiment of the present invention; otherwise, known techniques related to magnetic recording / reproducing devices can be applied. In one embodiment, the magnetic head may be an energy-assisted magnetic recording head having an energy source (e.g., a heat source such as a laser light source, microwaves, etc.) for assisting magnetization reversal (assisting the writing of magnetic signals), a recording element, and a reproducing element. Such an energy-assisted magnetic recording head-equipped magnetic recording / reproducing device is useful as a magnetic recording / reproducing device with high recording density and high reliability. Furthermore, during the manufacture of magnetic recording media used in energy-assisted recording magnetic recording / reproducing devices, such as thermally assisted recording magnetic recording devices equipped with a thermally assisted magnetic recording head having a laser light source, a magnetic recording layer containing a magnetic material with high magnetic anisotropy energy may be formed on a magnetic recording medium substrate. To form such a magnetic recording layer, typically, film deposition is performed at high temperature, or a heat treatment is performed at high temperature after film deposition. In one embodiment, the magnetic recording medium substrate according to one embodiment of the present invention may be a magnetic recording medium substrate with high heat resistance capable of withstanding such high-temperature treatment. However, the magnetic recording and reproducing device according to one aspect of the present invention is not limited to an energy-assisted magnetic recording and reproducing device. [Example]

[0136] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to the embodiments shown in the examples. [Examples 1 to 81] Raw materials such as oxides, carbonates, nitrates, sulfates, and hydroxides were weighed and mixed to prepare a raw material blend so as to obtain glass having the composition shown in the table below. This raw material blend was charged into a melting vessel and heated and melted in the range of 1400 to 1600°C. The resulting glass melt was then held in a refining vessel at 1400 to 1550°C for 6 hours, after which the temperature was lowered (cooled down) and held in the range of 1200 to 1400°C for 1 hour, after which the glass melt was formed to obtain glasses (amorphous oxide glasses) for the following evaluations. The glasses of Examples 1 to 62 correspond to at least Glass 2. The glasses of Examples 63 to 81 correspond to at least Glass 3. Among the glasses of Examples 1 to 81, some also fall under Glass 1 and / or Glass 4, as will be described later in the table and with respect to T1 and T2.

[0137] <Evaluation of glass properties> (1) Glass transition temperature Tg, average linear expansion coefficient α The glass transition temperature Tg and the average linear expansion coefficient α at 100 to 300° C. of each glass were measured using a thermomechanical analyzer (TMA).

[0138] (2) Young's modulus E The Young's modulus of each glass was measured by an ultrasonic method.

[0139] (3) Specific gravity d The specific gravity of each glass was measured by Archimedes' method.

[0140] (4) Specific elastic modulus The specific elastic modulus was calculated from the Young's modulus obtained in (2) above and the specific gravity obtained in (3).

[0141] (5) 1300℃ 16h holding test 100 g of each glass was placed in a platinum crucible, which was then placed in a heating furnace with the furnace temperature set to 1300°C, and left for 16 hours (holding test) while maintaining the furnace temperature. After 16 hours, the crucible was removed from the heating furnace, and the glass in the crucible was transferred onto a refractory material and cooled to room temperature (20°C to 25°C). The presence or absence of crystals in each glass was observed under an optical microscope (magnification 40 to 100 times) and evaluated according to the following criteria. A: No crystals on the glass surface, inside, or at the interface with the platinum crucible bottom B: There are less than 10 crystals with a diameter of several tens of micrometers on the glass surface and at the interface with the bottom of the platinum crucible per 100g. C: More than 10 crystals with a diameter of several tens of micrometers on the glass surface and at the interface with the bottom of the platinum crucible / 100g D: Crystals present inside the glass E: Crystals are present on the glass surface, inside, and at the interface with the platinum crucible bottom. F: The glass is cloudy with many crystals G: The glass is cloudy

[0142] (6) 1320℃ 16h retention test Except for the fact that the temperature inside the furnace was set to 1320°C, the retention test and the evaluation of the glass after the retention test were carried out in accordance with the method described in (5) above.

[0143] (7) Liquidus temperature LT The liquidus temperatures of the glasses shown in the table below were determined by the following method. 50 g of glass was weighed into a platinum crucible. This platinum crucible, with a platinum lid attached, was placed in a heating furnace with an internal atmosphere temperature of 1350°C. After the glass in the crucible was completely molten, the internal atmosphere temperature was lowered to a predetermined temperature and maintained at that temperature for 16 hours. After maintaining the temperature for 16 hours, the platinum crucible was removed from the furnace and left to cool the glass to room temperature (approximately 20°C to 25°C). After cooling, the glass was visually inspected for the presence or absence of crystal precipitation. The above operation was carried out from 1330°C to 1200°C in 10°C increments, and the lowest temperature at which no crystal precipitation was observed was taken as the liquidus temperature LT.

[0144] The glasses of Examples 17, 18, 22, 29, 32, and 33 were confirmed to have a liquidus temperature LT of 1300° C. or lower by the following method. 50 g of glass was weighed into a platinum crucible. This platinum crucible, with a platinum lid attached, was placed in a heating furnace with an internal atmosphere temperature of 1350°C. After the glass in the crucible was completely molten, the internal atmosphere temperature was lowered to 1300°C and maintained at that temperature for 16 hours. After maintaining the temperature for 16 hours, the platinum crucible was removed from the furnace and left to cool the glass to room temperature (approximately 20°C to 25°C). The cooled glass was visually inspected for the presence or absence of crystal precipitation, and no crystal precipitation was observed.

[0145] (8) Roughness evaluation Using the method described below, magnetic disk glass substrates having the respective glass compositions shown in the table below were fabricated. Here, the basic processing conditions, such as polishing conditions and cleaning conditions, were the same. The arithmetic mean roughness Ra (JIS B0601:2001) of the main surfaces of the magnetic disk glass substrates after final cleaning was measured using an AFM (atomic force microscope). The Ra measured for each glass is shown as a relative value (ratio) when the Ra measured for Example 66 is set to 1.00. The degree of roughness was ranked according to the following criteria. Note that even rank C does not pose a practical problem. Rank A: Less than 0.80 Rank B: 0.80 or more and less than 0.90 Rank C: 0.90 or higher and 1.00 or lower

[0146] The results are shown in the table below.

[0147] [Table 1-1]

[0148] [Table 1-2]

[0149] [Table 1-3]

[0150] Table 2-1

[0151] Table 2-2

[0152] Table 2-3

[0153] Table 3-1

[0154] Table 3-2

[0155] Table 3-3

[0156] Table 4

[0157] Table 5

[0158] Table 6

[0159] Table 7

[0160] [Table 8]

[0161] The results shown in the above table confirm that the glasses of Examples 1 to 81 have high rigidity. For example, the glass of Example 27 shown in Table 5 of JP 2002-358626 A (hereinafter referred to as "Comparative Glass") has a glass composition, expressed in mole percent, of SiO2: 65.0%, Al2O3: 7.0%, MgO: 1.0%, CaO: 1.0%, Li2O: 10.0%, Na2O: 10.5%, and KO: 2.5%, which differs from the glass according to one embodiment of the present invention described above. The value calculated by dividing the Young's modulus value shown in Table 5 of JP 2002-358626 A by the density value for this Comparative Glass was approximately 32.45, which was lower than the specific modulus values ​​of Examples 1 to 81 shown in the above table.

[0162] (7) Viscosity 10 2.0 Temperature T1, viscosity 10 in dPa·s 2.5 Temperature T2 in dPa·s For the glasses of Examples 1 to 81, the temperatures T1 and T2 were measured by the platinum sphere pulling method. The temperature T1 of the glasses in Examples 1 to 62 was 1600° C. or less. For example, the temperature T1 was 1468° C. in Example 4 and 1593° C. in Example 21. The temperature T1 of the glasses in Examples 63 to 79 was 1670° C. or lower. For example, the temperature T1 was 1635° C. in Example 71 and 1662° C. in Example 72. The temperature T1 of the glasses of Examples 80 and 81 was 1700°C or less. The temperature T2 of the glasses in Examples 1 to 62 was 1500° C. or less. For example, the temperature T2 was 1364° C. in Example 4 and 1481° C. in Example 21. The temperature T2 of the glasses in Examples 63 to 79 was 1530° C. or lower. For example, the temperature T1 was 1510° C. in Example 71 and 1520° C. in Example 72. The temperature T2 of the glasses of Examples 80 and 81 was 1560°C or lower.

[0163] <Preparation of magnetic recording medium substrate> (1) Preparation of substrate blank A disk-shaped substrate blank was prepared by the following method A or B. Also, a glass blank for preparing a glass spacer for a magnetic recording / reproducing device can be obtained by the same method. (Method A) For glass having the composition shown in the above table, a clarified and homogenized glass melt was flowed out of an outflow pipe at a constant flow rate and received by a press-molding lower mold. The flowing glass melt was cut with a cutting blade so that a predetermined amount of glass melt gob was obtained on the lower mold. The lower mold with the glass melt gob placed thereon was then immediately removed from the bottom of the pipe, and using an upper mold and a barrel mold opposing the lower mold, the glass melt was press-molded into a thin disk having a diameter of 99 mm and a thickness of 0.7 mm. The press-molded product was cooled to a temperature at which it would not deform, then removed from the mold and annealed to obtain a substrate blank (amorphous oxide glass). Note that in the above-mentioned molding, multiple lower molds were used to successively form the flowing glass melt into disk-shaped substrate blanks. (Method B) For glass having the composition shown in the above table, a refined and homogenized molten glass was continuously poured from above into a cylindrical through-hole of a heat-resistant mold, formed into a cylindrical shape, and removed from the bottom of the through-hole. After annealing the removed glass, the glass was sliced ​​at regular intervals in a direction perpendicular to the cylindrical axis using a multi-wire saw to produce disk-shaped substrate blanks (amorphous oxide glass). Although the present example employs the above-described methods A and B, the following methods C and D are also suitable for manufacturing a disk-shaped substrate blank. Furthermore, the following methods C and D are also suitable for manufacturing a glass blank for producing a glass spacer for a magnetic recording / reproducing device. (Method C) Molten glass can be poured onto a float bath and formed into a sheet of glass (forming by the float method), and then annealed, after which a disk of glass can be cut from the sheet of glass to obtain a substrate blank (amorphous oxide glass). (Method D) Molten glass can also be formed into a glass sheet by the overflow downdraw method (fusion method), annealed, and then a disk-shaped glass can be cut from the glass sheet to obtain a substrate blank (amorphous oxide glass).

[0164] (2) Preparation of glass substrate A through hole was drilled in the center of the substrate blank (amorphous oxide glass) obtained by each of the above methods, and the outer and inner peripheries were ground (to simultaneously form chamfered surfaces), the edges were polished, and the main surfaces of the disk were lapped and polished (mirror polished) to produce a glass substrate for a magnetic disk with an outer diameter of 97 mm, an inner circular hole diameter of 25 mm, and a thickness of 0.5 mm. Furthermore, by using a similar method, a glass blank for producing a glass spacer for a magnetic recording / reproducing device can be finished into a glass spacer for a magnetic recording / reproducing device. The glass substrate obtained above was scrubbed, washed with an aqueous potassium hydroxide solution, rinsed with pure water, and then dried. When the surface of the substrate made from the glass of the example was observed with an atomic force microscope (AFM), no surface roughness was observed, and the surface was smooth (low roughness).

[0165] <Magnetic recording medium (magnetic disk) production> An adhesive layer, an underlayer, a magnetic recording layer, a protective layer, and a lubricating layer were formed in this order on the main surface of the magnetic disk glass substrate prepared above by the following method, to obtain a magnetic disk.

[0166] First, using a vacuum-drawn film-forming apparatus, an adhesive layer, an underlayer, and a magnetic recording layer were successively formed in an Ar atmosphere by DC magnetron sputtering.

[0167] The adhesive layer was formed using a CrTi target to be an amorphous CrTi layer with a thickness of 20 nm. A 10 nm thick MgO layer was then formed as an underlayer. The magnetic recording layer was formed using an FePtC or CoPtC target at a deposition temperature of 200 to 400°C to be a 10 nm thick FePt or CoPt granular layer.

[0168] After the magnetic recording layer was formed, the magnetic disk was transferred from the film forming apparatus to a heating furnace and annealed. The temperature in the heating furnace during annealing was set to a range of 500 to 700°C. 10 Magnetic particles of CoPt-based alloy or FePt-based alloy with an ordered structure are formed. 10 The magnetic recording layer may be heated to generate an ordered structure. For example, the temperature of the substrate during the deposition of the FePt or CoPt magnetic recording layer may be set to 500 to 700°C. 10 An ordered structure may be formed.

[0169] Next, a protective layer made of hydrogenated carbon was formed to a thickness of 3 nm by CVD using ethylene as the source gas. After this, a lubricating layer made of PFPE (perfluoropolyether) was formed by dip coating. The lubricating layer had a thickness of 1 nm. A magnetic disk was obtained through the above manufacturing process. The obtained magnetic disk was mounted in a hard disk drive equipped with a DFH mechanism, and magnetic signals were recorded and reproduced in the recording area on the main surface of the magnetic disk at a recording density of 1000 gigabits per square inch. No collision between the magnetic head and the magnetic disk surface (crash failure) was observed.

[0170] Furthermore, a glass spacer obtained by the above manufacturing process using glass of the composition shown in the above table and having a NiP alloy conductive film formed on its surface (glass spacer with NiP alloy film) was installed in a hard disk drive equipped with a DFH mechanism, and magnetic signals were recorded and reproduced at a recording density of 1000 gigabits per square inch in a recording area on the main surface of a magnetic disk separately prepared using a substrate made of a material different from the glass of one embodiment of the present invention.No collision between the magnetic head and the magnetic disk surface (crash failure) was observed.

[0171] Furthermore, when the magnetic disk and the glass spacer with NiP alloy film manufactured as described above using the same glass material according to one embodiment of the present invention were mounted in a hard disk drive equipped with a DFH mechanism, and magnetic signals were recorded and reproduced in the recording area on the main surface of the magnetic disk at a recording density of 1000 gigabits per square inch, no collision between the magnetic head and the magnetic disk surface (crash failure) was observed. Here, since the glass substrate and the glass spacer included in the magnetic disk are made of the same glass material, the phenomenon that can occur due to the difference in thermal expansion coefficient described above does not occur.

[0172] According to one aspect of the present invention, it is possible to provide a magnetic recording medium suitable for high-density recording.

[0173] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims. For example, by adjusting the composition as described in the specification to the glass compositions exemplified above, it is possible to produce a glass for a magnetic recording medium substrate and a glass spacer for a magnetic recording / reproducing device according to one embodiment of the present invention. Furthermore, it is of course possible to arbitrarily combine two or more of the items described in the specification as examples or preferred ranges.

Claims

1. CaO content is 1.00 mol% or less, MgO content is 14.00 mol% or more, Li ２ an O content of 3.00 mol% or less; B ２ O ３ a content of 2.00 mol% or less, SiO ２ The content is 50.00 mol% or more and 75.00 mol% or less, Al ２ O ３ The content is 10.00 mol% or more and 20.00 mol% or less, a molar ratio of the MgO content to the total content of MgO, CaO, SrO, and BaO (MgO / (MgO+CaO+SrO+BaO)) of 0.900 or more; A glass transition temperature Tg of 720°C or higher, Viscosity 10 ２．０ The temperature T1 at dPa s is 1600°C or less, and A glass for a magnetic recording medium substrate, which is an amorphous glass having a specific elastic modulus of 34.0 MNm / kg or more.

2. CaO content is 1.00 mol% or less, MgO content is 14.00 mol% or more, Li ２ an O content of 3.00 mol% or less; B ２ O ３ a content of 2.00 mol% or less, SiO ２ The content is 50.00 mol% or more and 75.00 mol% or less, Al ２ O ３ The content is 10.00 mol% or more and 20.00 mol% or less, a molar ratio of the MgO content to the total content of MgO, CaO, SrO, and BaO (MgO / (MgO+CaO+SrO+BaO)) of 0.900 or more; A glass transition temperature Tg of 720°C or higher, Viscosity 10 ２．５ The temperature T2 at dPa s is 1500°C or less, and A glass for a magnetic recording medium substrate, which is an amorphous glass having a specific elastic modulus of 34.0 MNm / kg or more.

3. CaO content is 1.00 mol% or less, MgO content is 14.00 mol% or more, Li ２ an O content of 3.00 mol% or less; B ２ O ３ a content of 2.00 mol% or less, SiO ２ The content is 50.00 mol% or more and 75.00 mol% or less, Al ２ O ３ The content is 10.00 mol% or more and 20.00 mol% or less, a molar ratio of the MgO content to the total content of MgO, CaO, SrO, and BaO (MgO / (MgO+CaO+SrO+BaO)) of 0.900 or more; A glass transition temperature Tg of 720°C or higher, and Viscosity 10 ２．０ A glass for a magnetic recording medium substrate, which is an amorphous glass having a temperature T1 at dPa·s of 1600° C. or less.

4. Li ２ 2. The glass for a magnetic recording medium substrate according to claim 1, wherein the O content is 0.50 mol % or less.

5. Li ２ 3. The glass for a magnetic recording medium substrate according to claim 2, wherein the O content is 0.50 mol % or less.

6. Li ２ 4. The glass for a magnetic recording medium substrate according to claim 3, wherein the O content is 0.50 mol % or less.

7. Na ２ 2. The glass for a magnetic recording medium substrate according to claim 1, wherein the O content is 5.00 mol % or less.

8. Na ２ 3. The glass for a magnetic recording medium substrate according to claim 2, wherein the O content is 5.00 mol % or less.

9. Na ２ 4. The glass for a magnetic recording medium substrate according to claim 3, wherein the O content is 5.00 mol % or less.

10. SiO ２ 2. The glass for a magnetic recording medium substrate according to claim 1, wherein the content is 55.00 mol % or more and 75.00 mol % or less.

11. SiO ２ 3. The glass for a magnetic recording medium substrate according to claim 2, wherein the content is 55.00 mol % or more and 75.00 mol % or less.

12. SiO ２ 4. The glass for a magnetic recording medium substrate according to claim 3, wherein the content is 55.00 mol % or more and 75.00 mol % or less.

13. A magnetic recording medium substrate comprising the glass for magnetic recording medium substrates according to any one of claims 1 to 12.

14. A magnetic recording medium comprising the magnetic recording medium substrate according to claim 13 and a magnetic recording layer.

15. A magnetic recording and reproducing device comprising the magnetic recording medium according to claim 14 and a magnetic head.

Images