Glass for magnetic recording medium substrate, magnetic recording medium substrate, glass spacer for magnetic recording medium and magnetic recording / reproducing device

Abstract

To provide a glass for a magnetic recording medium substrate excellent in heat resistance, rigidity, and glass stability, having a low specific gravity, and having an appropriate thermal expansion coefficient.SOLUTION: There is provided a glass for a magnetic recording medium substrate which is an amorphous oxide glass having, in terms of mol%, a SiO2 content of 45-68%, an Al2O3 content of 5-20%, a total content of SiO2 and Al2O3 of 60-80%, a B2O3 content of 0-5%, a MgO content of 3-28%, a CaO content of 0-18%, a total content of BaO and SrO of 0-2%, a total content of alkali earth metal oxides of 12-30%, a total content of alkali metal oxides of 3.5-15%, containing at least one selected from a Sn oxide and Ce oxide, and having a total content of these of 0.05-2.00%, having a glass transition temperature of 625°C or higher, a Young's modulus of 83 GPa or higher, a specific gravity of 2.85 or higher, and a mean coefficient of linear expansion at 100-300°C of 48×10-7 / °C or higher.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to glass for a magnetic recording medium substrate, a magnetic recording medium substrate, a magnetic recording medium, and a glass spacer for a magnetic recording / reproducing device. [Background technology]

[0002] 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 and do not have sufficient smoothness on the substrate surface after polishing. For this reason, glass magnetic recording medium substrates are now widely used (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-64921 Summary of the Invention [Problem to be solved by the invention]

[0004] In the process of forming a magnetic recording layer on a magnetic recording medium substrate, film formation is usually carried out at high temperatures, or a heat treatment is carried out at high temperatures after film formation. Therefore, glass for magnetic recording medium substrates is required to have high heat resistance that can withstand high-temperature treatment, specifically, a high glass transition temperature.

[0005] On the other hand, magnetic recording and playback devices (commonly referred to as hard disk drives (HDDs)) incorporating magnetic recording media are designed to rotate the magnetic recording media itself by clamping its central portion 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 media substrate and the spindle material that makes up the spindle, changes in ambient temperature during use can cause a mismatch between the thermal expansion and contraction of the spindle and the magnetic recording media substrate, resulting in deformation of the magnetic recording media. This can prevent the head from reading the written information, thereby reducing the reliability of recording and playback. Therefore, glass for magnetic recording media substrates is required to have an appropriate thermal expansion coefficient similar to that of the spindle material (e.g., stainless steel).

[0006] Furthermore, as magnetic recording media become thinner and their recording densities increase, there are growing demands for further reduction in warping and deflection of magnetic recording media during spindle motor rotation, as well as for magnetic recording media with higher practical strength. To meet these demands, it is desirable for glass for magnetic recording medium substrates to have high rigidity, specifically a high Young's modulus.

[0007] In addition, it is desirable to reduce the specific gravity of glass for magnetic recording medium substrates, because this reduces the weight of the magnetic recording medium substrate. Reducing the substrate weight reduces the weight of the magnetic recording medium, reducing the power required to rotate the magnetic recording medium and thus the power consumption of HDDs.

[0008] Furthermore, it is desirable for glass for magnetic recording medium substrates to have excellent glass stability. Here, glass stability refers to the resistance to crystal precipitation in molten glass. For example, it can be evaluated by the presence or absence and degree of crystal precipitation after holding molten glass at a holding temperature near the liquidus temperature for a long period of time. Furthermore, the less crystal precipitation occurs even when the holding temperature is lowered, the more excellent the glass stability. Glass with excellent glass stability can be molded at a lower molding temperature, thereby extending the life of components of the molding device, such as heating elements, furnace bodies, and pipes. Furthermore, in order to suppress the occurrence of volatilization, striae, and molding bubbles, it is desirable to mold by increasing the glass viscosity by lowering the molding temperature. However, glass with poor glass stability will precipitate crystals when the molding temperature is lowered. In contrast, glass with excellent glass stability can be molded at a lower molding temperature while suppressing crystal precipitation.

[0009] As described above, it is desirable for glass for use in magnetic recording medium substrates to have excellent heat resistance, rigidity, and glass stability, a low specific gravity, and an appropriate coefficient of thermal expansion.

[0010] Therefore, an object of one aspect of the present invention is to provide a glass for a magnetic recording medium substrate that is excellent in heat resistance, rigidity, and glass stability, has a low specific gravity, and has an appropriate thermal expansion coefficient. [Means for solving the problem]

[0011] One aspect of the present invention is a compound represented by the formula: SiO2 content 45-68%, Al2O3 content is 5-20%, The total content of SiO2 and Al2O3 (SiO2+Al2O3) is 60-80%, B2O3 content is 0-5% MgO content is 3-28%; CaO content is 0-18%; The total content of BaO and SrO (BaO+SrO) is 0-2%. The total content of alkaline earth metal oxides (MgO + CaO + SrO + BaO) is 12 to 30%, The total content of alkali metal oxides (Li2O + Na2O + K2O) is 3.5 to 15%. and containing at least one selected from the group consisting of Sn oxide and Ce oxide, and the total content of Sn oxide and Ce oxide is 0.05 to 2.00%, Glass transition temperature is 625°C or higher, Young's modulus is 83GPa or more, Specific gravity is 2.85 or less, and Average linear expansion coefficient at 100-300°C is 48 x 10 -7 / °C or higher, Regarding.

[0012] By having the above glass composition, the glass for magnetic recording medium substrates can combine high heat resistance with a glass transition temperature of 625°C or higher, high rigidity with a Young's modulus of 83 G or higher, a specific gravity of 2.85 or lower, and the above-mentioned appropriate thermal expansion coefficient, and can also exhibit excellent glass stability. [Effects of the Invention]

[0013] According to one aspect of the present invention, it is possible to provide a glass for a magnetic recording medium substrate that has excellent heat resistance and rigidity, a low specific gravity, a moderate thermal expansion coefficient, and excellent glass stability. Furthermore, according to another aspect, it is also possible to provide a magnetic recording medium substrate made of the above-mentioned glass for a magnetic recording medium substrate, and a magnetic recording medium including this substrate. Furthermore, according to another aspect, it is possible to provide a glass spacer for a magnetic recording device. DETAILED DESCRIPTION OF THE INVENTION

[0014] [Glass for magnetic recording media substrates] One aspect of the present invention is a glass composition having the above glass composition, a glass transition temperature of 625°C or higher, a Young's modulus of 83 GPa or higher, a specific gravity of 2.85 or lower, and an average linear expansion coefficient of 48 × 10 at 100 to 300°C. -7The present invention relates to a glass for a magnetic recording medium substrate (hereinafter also simply referred to as "glass"), which is an amorphous oxide glass having a temperature of 1000 K / °C or higher.

[0015] The glass is an amorphous glass and an oxide glass. Unlike glass-ceramics, amorphous glass does not contain a crystalline phase and exhibits a glass transition phenomenon upon heating. Furthermore, oxide glass is glass in which the main network-forming component of the glass is an oxide. The above glass will be described in more detail below.

[0016] <Glass composition> 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 being 0%, not containing, or not incorporating a component means that the component is substantially not contained, and the content of the component is at or below the impurity level, for example, less than 0.01%.

[0017] The glass composition of the above glass will be described below.

[0018] SiO2 is a glass network-forming component and has the function of improving glass stability. SiO2 is also a component that contributes to improving chemical durability. From the viewpoints of improving the thermal expansion coefficient and maintaining meltability and formability, the SiO2 content in the above glass is 68% or less, preferably 65% ​​or less, more preferably 64% or less, even more preferably 63% or less, and still more preferably 62% or less. From the viewpoints of maintaining heat resistance and chemical durability, the SiO2 content in the above glass is 45% or more, preferably 48% or more, more preferably 50% or more, even more preferably 52% or more, even more preferably 53% or more, and still more preferably 54% or more.

[0019] Al2O3 is also a network-forming component of glass and has the function of improving heat resistance. Al2O3 also has the function of improving chemical durability. From the viewpoint of improving heat resistance and chemical durability, the content of Al2O3 in the above glass is 5% or more, preferably 8% or more, more preferably 10% or more, even more preferably 12% or more, and still more preferably 13% or more. From the viewpoint of improving glass stability, the content of Al2O3 in the above glass is 20% or less, preferably 18% or less, more preferably 17% or less, and even more preferably 16% or less.

[0020] The respective contents of SiO2 and Al2O3 are as described above. Furthermore, in the above glass, the total content of SiO2 and Al2O3 (SiO2 + Al2O3) is 60% or more, more preferably 65% ​​or more, even more preferably 67% or more, still more preferably 69% or more, and even more preferably 70% or more, from the viewpoint of stabilizing the glass structure. Furthermore, in view of the viscosity characteristics of the glass during melting, the total content of SiO2 and Al2O3 (SiO2 + Al2O3) in the above glass is 80% or less, preferably 78% or less, more preferably 77% or less, even more preferably 76% or less, and even more preferably 75% or less.

[0021] B2O3 is also a component that forms a glass network, reduces the specific gravity of the glass, and improves meltability. However, B2O3 is prone to volatilization during melting, making the glass component ratio unstable. Furthermore, excessive incorporation of B2O3 tends to reduce chemical durability. For these reasons, the B2O3 content in the above glass is set to 0 to 5%. The B2O3 content is preferably 3% or less, more preferably 2% or less, even more preferably 1% or less, even more preferably 0.5% or less, and even more preferably 0.3% or less.

[0022] Among the alkaline earth metal oxides MgO, CaO, SrO, and BaO, MgO increases the Young's modulus and specific modulus of the glass, increases the thermal expansion coefficient, and improves the meltability and formability of the glass. The specific modulus will be described in detail later. To achieve these functions effectively, the MgO content in the glass is 3% or more, preferably 5% or more, more preferably 6% or more, even more preferably 7% or more, and still more preferably 8% or more. Furthermore, to maintain chemical durability, the MgO content in the glass is 28% or less, preferably 26% or less, more preferably 23% or less, even more preferably 20% or less, even more preferably 17% or less, and still more preferably 15% or less.

[0023] The CaO content in the above glass is 0% or more. CaO also functions to increase the Young's modulus and specific elastic modulus of the glass, to increase the thermal expansion coefficient, and to improve the meltability and formability of the glass. From the viewpoint of obtaining these functions well, the CaO content in the above glass is preferably 1% or more, more preferably 2% or more, even more preferably 3% or more, still more preferably 4% or more, and even more preferably 5% or more. Furthermore, from the viewpoint of maintaining chemical durability, the CaO content in the above glass is 18% or less, preferably 15% or less, more preferably 13% or less, and even more preferably 12% or less.

[0024] SrO improves the meltability, formability, and glass stability of the glass, and increases the thermal expansion coefficient. From the viewpoints of maintaining chemical durability, reducing the specific gravity, and reducing raw material costs, the SrO content in the above glass is preferably 0 to 2%. The SrO content is more preferably in the range of 0 to 1%, and even more preferably in the range of 0 to 0.5%. It is most preferable that no SrO is contained, i.e., the SrO content is 0%.

[0025] Both BaO and SrO are components that increase the specific gravity of the glass. From the viewpoint of lowering the specific gravity of the glass, the total content of BaO and SrO (BaO+SrO) in the glass is 0 to 2%, preferably 0 to 1%, more preferably 0 to 0.5%, and most preferably no BaO or SrO is contained, i.e., the total content of BaO and SrO (BaO+SrO) is 0%.

[0026] BaO also improves the meltability, formability, and glass stability of the glass and increases the thermal expansion coefficient. From the viewpoints of maintaining chemical durability, reducing the specific gravity, and reducing raw material costs, the BaO content in the above glass is preferably 0 to 2%, more preferably 0 to 1%, and even more preferably 0 to 0.5%, and it is most preferable that the glass does not contain BaO, i.e., the BaO content is 0%.

[0027] The total content of alkaline earth metal oxides (MgO+CaO+SrO+BaO) in the glass is 12% or more, preferably 14% or more, more preferably 15% or more, and even more preferably 16% or more, from the viewpoint of the meltability and stability of the glass. Furthermore, from the viewpoint of the chemical durability of the glass, the total content of alkaline earth metal oxides (MgO+CaO+SrO+BaO) in the glass is 30% or less, preferably 28% or less, more preferably 26% or less, and even more preferably 25% or less.

[0028] Among alkaline earth metal oxides, MgO has the function of increasing the Young's modulus and specific modulus of the glass and also contributes to suppressing an increase in specific gravity. Therefore, MgO is a very useful component for increasing the Young's modulus, specific modulus, and specific gravity of the glass, and is particularly effective for increasing the Young's modulus and specific gravity. CaO also has the function of increasing the Young's modulus and specific modulus of the glass, contributes to suppressing an increase in specific gravity, and is also an effective component for increasing the thermal expansion coefficient of the glass. On the other hand, SrO and BaO increase the specific gravity and raw material costs. From these perspectives, in the above glass, the molar ratio of the total content of MgO and CaO to the total content of alkaline earth metal oxides {(MgO + CaO) / (MgO + CaO + SrO + BaO)} is preferably 0.75 to 1.00. The lower limit of the molar ratio is more preferably 0.80 or more, even more preferably 0.85 or more, even more preferably 0.90 or more, and even more preferably 0.95 or more.

[0029] Among alkali metal oxides, Li2O has a strong effect of improving the meltability and formability of glass, and is also a suitable component for increasing the Young's modulus and imparting suitable rigidity to a magnetic recording medium substrate. Li2O is also a component that increases the thermal expansion coefficient. When the above glass is used as a glass for chemical strengthening, it is also a component that is responsible for ion exchange during chemical strengthening. On the other hand, Li2O is also a component that lowers the glass transition temperature. In consideration of the above effects, the Li2O content in the above glass is preferably 0 to 10%. The lower limit of the Li2O content is more preferably 0.5% or more, even more preferably 1.0% or more, and even more preferably 2.0% or more. The upper limit of the Li2O content is more preferably 8% or less, even more preferably 7% or less, and even more preferably 6% or less.

[0030] Na2O is a component that improves the meltability and formability of glass, increases the thermal expansion coefficient, and reduces the viscosity of the glass during fining, facilitating bubble removal. Furthermore, when the above-mentioned glass is used as a glass for chemical strengthening, Na2O is also a component responsible for ion exchange during chemical strengthening. In consideration of the above-mentioned functions, the Na2O content in the above-mentioned glass is preferably 0 to 10%. The lower limit of the Na2O content is more preferably 0.5% or more, even more preferably 1.0% or more, and even more preferably 2.0% or more. The upper limit of the Na2O content is more preferably 8% or less, even more preferably 7% or less, and even more preferably 6% or less.

[0031] KO also functions to improve the meltability and formability of glass, and is a component that increases the thermal expansion coefficient. However, if incorporated in excess, chemical durability, particularly acid resistance, decreases, and when formed into a glass substrate, alkali elution from the substrate surface increases, and the precipitated alkali may affect the film properties of the magnetic recording layer, etc. In consideration of the above, the KO content in the above glass is preferably 0 to 5%, more preferably 0 to 3%, even more preferably 0 to 2%, even more preferably 0 to 1%, and most preferably no KO is contained.

[0032] As described above, Li2O, Na2O, and K2O are components that improve the meltability and formability of glass and increase the thermal expansion coefficient. To effectively utilize these components, the total content of alkali metal oxides (Li2O + Na2O + K2O) in the glass is 3.5% or more, and more preferably 4% or more. To maintain the heat resistance and chemical durability of the glass, the total content of alkali metal oxides (Li2O + Na2O + K2O) in the glass is 15% or less, preferably 13% or less, more preferably 12% or less, and even more preferably 11% or less. By including two or more alkali metal oxides in the glass, a mixed alkali effect can be achieved, reducing or preventing alkali elution from the glass surface.

[0033] Among alkali metal oxides, Li2O is a component that increases the Young's modulus of the glass and contributes more effectively to increasing the thermal expansion coefficient. Therefore, in one embodiment, from the viewpoint of increasing the Young's modulus and improving the thermal expansion coefficient, the molar ratio of the Li2O content to the total content of alkali metal oxides {Li2O / (Li2O + Na2O + K2O)} is preferably 0.4 to 1. In one embodiment of the glass in which the molar ratio {Li2O / (Li2O + Na2O + K2O)} is within the above range, a Young's modulus of 90 GPa or more can be obtained. In one embodiment, the lower limit of the molar ratio {Li2O / (Li2O + Na2O + K2O)} is more preferably 0.5 or more, even more preferably 0.6 or more, even more preferably 0.7 or more, and even more preferably 0.8 or more.

[0034] Among alkali metal oxides, Na2O is a component that more effectively contributes to improving the meltability and formability of glass and increasing the thermal expansion coefficient. Furthermore, compared with Li2O, Na2O has a smaller effect of lowering the glass transition temperature. Therefore, in one embodiment, from the viewpoint of improving heat resistance and the thermal expansion coefficient, the molar ratio of the Na2O content to the total content of alkali metal oxides {Na2O / (Li2O + Na2O + K2O)} is preferably 0.4 to 1. In one embodiment of the above glass in which the molar ratio {Na2O / (Li2O + Na2O + K2O)} is in the above range, the glass transition temperature can be set to 690°C or higher. In one embodiment, the lower limit of the molar ratio {Na2O / (Li2O + Na2O + K2O)} is more preferably 0.5 or higher, even more preferably 0.6 or higher, even more preferably 0.7 or higher, and even more preferably 0.8 or higher.

[0035] TiO2 improves glass stability and chemical durability, as well as increasing Young's modulus, but if incorporated in excess, the liquidus temperature of the glass may rise, leading to deterioration in devitrification resistance and an increase in specific gravity. Therefore, the TiO2 content in the above glass is preferably 0 to 8%, more preferably 0 to 6%, even more preferably 0 to 5%, still more preferably 0 to 4%, even more preferably 0 to 3%, and even more preferably 0 to 2%.

[0036] HfO2, Nb2O5, Ta2O5, La2O3, and Y2O3 function to improve chemical durability, especially alkali resistance. However, excessive incorporation of these elements may deteriorate meltability and increase specific gravity. Therefore, from the viewpoint of improving chemical durability, especially alkali resistance, while maintaining meltability, the total content of HfO2, Nb2O5, Ta2O5, La2O3, and Y2O3 (HfO2 + Nb2O5 + Ta2O5 + La2O3 + Y2O3) is preferably 0 to 5%. The lower limit is preferably 0.3% or more, more preferably 0.5% or more, and even more preferably 1.0% or more. The upper limit is preferably 5% or less, more preferably 3% or less, and even more preferably 2% or less.

[0037] ZrO2 has the function of improving chemical durability and also increasing Young's modulus. However, excessive incorporation of ZrO2 may reduce the meltability of the glass and cause residual melting of the raw materials. Therefore, the ZrO2 content in the above glass is preferably 0 to 5%, more preferably 0 to 3%, even more preferably 0 to 2%, and even more preferably 0 to 1%.

[0038] ZnO has the functions of improving meltability and increasing Young's modulus, but excessive incorporation raises the liquidus temperature, so the ZnO content in the above glass is preferably 0 to 3%, more preferably 0 to 2%, even more preferably 0 to 1%, and may even be 0%.

[0039] Although a small amount of P2O5 can be incorporated, excessive incorporation tends to reduce chemical durability, so the P2O5 content in the above glass is preferably 0 to 2%, more preferably 0 to 1%, and even more preferably no P2O5 is contained, i.e., the P2O5 content is 0%.

[0040] The glass contains at least one oxide selected from the group consisting of Sn oxide and Ce oxide to obtain a fining effect, and the total content of Sn oxide and Ce oxide is 0.05% to 2.00%. When the total content of Sn oxide and Ce oxide is 0.05% or more, a sufficient fining effect can be obtained, and residual bubbles can be reduced or suppressed. Furthermore, when the total content is 2.00% or less, it is possible to prevent the molten glass from blowing up during glass melting, thereby preventing a decrease in productivity. The lower limit of the total content of Sn oxide and Ce oxide is preferably 0.10% or more, more preferably 0.15% or more, and even more preferably 0.20% or more. The upper limit of the total content of Sn oxide and Ce oxide is preferably 1.50% or less, more preferably 1.20% or less, and even more preferably 1.00% or less.

[0041] Sn oxides have the function of promoting fining when the melting temperature of glass is relatively high (temperature range of approximately 1400 to 1600°C). Under the current restrictions on the use of environmentally harmful fining agents such as Sb2O3 and arsenous acid, it is preferable to incorporate Sn oxides (e.g., SnO2) into the glass to remove bubbles in the glass with a high glass transition temperature. From the viewpoint of obtaining a fining effect, the Sn oxide 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 Sn oxide content is preferably 2.00% or less, more preferably 1.50% or less, even more preferably 1.00% or less, and even more preferably 0.80% or less.

[0042] Ce oxide, like Sn oxide, is a component that exhibits a fining effect on glass. Ce oxide functions to capture oxygen and fix it as a glass component when the glass melting temperature is relatively low (in the temperature range of about 1200 to 1400°C). Therefore, it is preferable to introduce Ce oxide (e.g., CeO2) into the glass as a fining agent. From the viewpoint of obtaining a fining effect, the content of Ce oxide is preferably 0.01% or more, more preferably 0.05% or more, even more preferably 0.10% or more, and even more preferably 0.20% or more. Furthermore, the content of Ce oxide is preferably 1.00% or less, more preferably 0.80% or less, even more preferably 0.50% or less, and even more preferably 0.40% or less.

[0043] The glass contains at least one oxide selected from the group consisting of Sn oxide and Ce oxide, and in one embodiment contains only Sn oxide, in another embodiment contains only Ce oxide, or in another embodiment contains both Sn oxide and Ce oxide. In particular, the coexistence of Sn oxide and Ce oxide in the glass can provide a fining effect over a wide temperature range, so it is preferable that the glass contain both Sn oxide and Ce oxide.

[0044] Sb2O3, which has been widely used as a fining agent in the past, is desirably used sparingly from the viewpoint of reducing environmental impact. Therefore, the Sb2O3 content in the above glass is preferably in the range of 0 to 0.5%. The Sb2O3 content is more preferably 0.2% or less, even more preferably 0.1% or less, even more preferably 0.05% or less, and even more preferably 0.02% or less, and it is particularly preferable that no Sb2O3 is contained.

[0045] The glass may contain about 0.5% or less Fe, calculated as Fe2O3. The Fe content, calculated as Fe2O3, is preferably 0.2% or less, more preferably 0.1% or less, and even more preferably 0.05% or less, and it is even more preferable that the glass contains no Fe.

[0046] Since Pb, Cd, As, etc. are substances that have a negative impact on the environment, it is preferable to avoid introducing them.

[0047] The above-mentioned glasses 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, for example, at a temperature in the range of 1400 to 1600°C, refining, stirring, and molding the homogenized glass melt 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 and hold the resulting glass melt in a refining vessel at 1450 to 1600°C, and then cool it to 1200 to 1400°C, at which point the glass is poured out and molded.

[0048] <Glass characteristics> By adjusting the composition as described above, the above glass can have the various glass properties described below.

[0049] (glass transition temperature) As described above, 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 for higher-density recording in magnetic recording media, film formation is typically performed at high temperatures, or a high-temperature heat treatment is performed after film formation. If the magnetic recording medium substrate does not have the heat resistance to withstand such high-temperature treatment, the substrate's flatness will be impaired by exposure to high temperatures during high-temperature treatment. In contrast, the above-mentioned glass has a glass transition temperature (hereinafter also referred to as "Tg"), which is an indicator of heat resistance, of 625°C or higher. A substrate made of a highly heat-resistant glass with a glass transition temperature of 625°C or higher can maintain excellent flatness even after high-temperature treatment. However, the above-mentioned glass is not limited to glass for use as a substrate for a magnetic recording medium having a magnetic recording layer containing a magnetic material that requires high-temperature treatment, and can be used to fabricate magnetic recording media containing various magnetic materials. The glass transition temperature is preferably 630° C. or higher, more preferably 640° C. or higher, even more preferably 650° C. or higher, even more preferably 660° C. or higher, even more preferably 670° C. or higher, even more preferably 675° C. or higher, and even more preferably 680° C. or higher. The upper limit of the glass transition temperature is, for example, about 770° C., but is not particularly limited because a higher glass transition temperature is preferable from the viewpoint of heat resistance.

[0050] (Young's modulus) In order to meet the aforementioned demand for improved rigidity of magnetic recording media, it is desirable for magnetic recording medium substrate glass to have high rigidity. In this regard, the glass has a Young's modulus, an index of rigidity, of 83 G or more. Magnetic recording medium substrate glass with a high rigidity, showing a Young's modulus of 83 G or more, can suppress substrate deformation during rotation of a spindle motor, thereby suppressing warpage and deflection of the magnetic recording medium due to substrate deformation. The Young's modulus of the glass is preferably 85 GPa or more, more preferably 86 GPa or more, even more preferably 88 GPa or more, and even more preferably 90 GPa or more. The upper limit of the Young's modulus is, for example, about 120 GPa, but is not particularly limited, as a higher Young's modulus is preferable as a higher rigidity is higher.

[0051] (specific gravity) The specific gravity of the glass is 2.85 or less, preferably 2.80 or less, more preferably 2.75 or less, even more preferably 2.70 or less, and even more preferably 2.65 or less. By lowering the specific gravity of the glass for magnetic recording medium substrates, the weight of the magnetic recording medium substrate can be reduced, and further the weight of the magnetic recording medium can be reduced, thereby enabling the power consumption of HDDs to be reduced. The lower limit of the specific gravity is, for example, about 2.40, but is not particularly limited, as the lower the specific gravity, the better.

[0052] (specific elastic modulus) The specific elastic modulus is the Young's modulus of the glass divided by its density, where density is the specific gravity of the glass multiplied by the ratio of g / cm 3 The specific elastic modulus of the glass is preferably 31 MNm / kg or more, more preferably 33 MNm / kg or more, and even more preferably 34 MNm / kg or more, from the viewpoint of providing a substrate that is less susceptible to deformation. The upper limit of the specific elastic modulus is, for example, about 40 MNm / kg, but is not particularly limited, as a higher specific elastic modulus is more preferable.

[0053] (coefficient of thermal expansion) From the viewpoint of improving the reliability of magnetic recording media, it is desirable for the glass for magnetic recording medium substrates to have an appropriate thermal expansion coefficient, as mentioned above. Generally, HDD spindle materials have a thermal expansion coefficient of 70×10 in the temperature range of 100 to 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 48 × 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. The average linear expansion coefficient (hereinafter also referred to as "α") of the above glass at 100 to 300°C is 48×10 -7 / ℃ or more, 50 × 10 -7 / °C or more, and more preferably 51 × 10 -7 / °C or more. The average linear expansion coefficient (α) of the glass at 100 to 300°C is 90×10 -7 / °C or less is preferable.

[0054] (glass stability) The glass can exhibit high glass stability. Methods for evaluating glass stability include a 1300°C 16-hour holding test and a 1250°C 16-hour holding test, which will be described in detail below. The glass preferably achieves an evaluation result of A in the 1300°C 16-hour holding test, more preferably achieves an evaluation result of A in the 1300°C 16-hour holding test and an evaluation result of A or B in the 1250°C 16-hour holding test, and even more preferably achieves an evaluation result of A in both holding tests.

[0055] (foam density) The glass can also have reduced bubbles by adjusting the composition as described above. The bubble density per unit mass of the glass, as measured by an optical microscope (magnification 40 to 100 times) and having a diameter of more than 0.03 mm, is preferably less than 50 bubbles / kg, more preferably less than 20 bubbles / kg, even more preferably less than 10 bubbles / kg, still more preferably 2 bubbles / kg or less, and most preferably 0 bubbles / kg.

[0056] [Magnetic recording medium substrate] A magnetic recording medium substrate according to one aspect of the present invention is made of the above-mentioned glass for magnetic recording medium substrates.

[0057] 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 any of the press molding, down-draw, and float processes, and then processing the resulting glass melt. For example, in the press molding process, 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. Next, a center hole is formed in the resulting substrate blank, and the inner and outer peripheries are processed, and both main surfaces are lapped and polished. Next, a cleaning process including acid washing and alkali washing is performed to obtain a disk-shaped substrate.

[0058] 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., no ion exchange layer is present). For example, a magnetic recording medium substrate without an ion exchange layer can be used when an HDD (hard disk drive) incorporating the magnetic recording medium is used in an environment that is unlikely to be subjected to external shocks. Furthermore, since a magnetic recording medium substrate without an ion exchange layer has not been subjected to an ion exchange process, the manufacturing cost can be significantly reduced.

[0059] 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 the magnetic recording layer is provided or 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.

[0060] 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.

[0061] 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 mm or less, with the lower limit of the thickness being preferably 0.3 mm. The magnetic recording medium substrate is preferably in the shape of a disk having a center hole.

[0062] 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.

[0063] [Magnetic recording media] One aspect of the present invention relates to a magnetic recording medium having a magnetic recording layer on the above magnetic recording medium substrate.

[0064] Magnetic recording media are called magnetic disks, hard disks, etc., and are suitable for use as internal storage devices (such as fixed disks) in various magnetic recording and playback devices, such as desktop personal computers, server computers, notebook personal computers, and mobile personal computers, internal storage devices in portable recording and playback devices that record and play back images and / or audio, and recording and playback devices for in-car audio. 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.

[0065] 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. For example, CrTi can be used as the adhesive layer, and CrRu can be used as the underlayer. After the above film formation, a protective layer is formed using C2H4, for example, by a CVD (Chemical Vapor Deposition) method, and a nitriding process is performed in the same chamber to introduce nitrogen into the surface, thereby forming a magnetic recording medium. Then, for example, a lubricating layer can be formed by applying PFPE (polyfluoropolyether) to the protective layer using a dip coating method.

[0066] 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 preferably has 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 laser 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.

[0067] In recent years, magnetic heads have been equipped with a dynamic flying height (DFH) mechanism, which significantly reduces the gap between the read / write element of the magnetic head and the surface of the magnetic recording medium (reducing the flying height), thereby achieving even higher recording densities. The DFH mechanism involves installing a heating element, such as a tiny heater, near the read / write element of the magnetic head, protruding only the area around the element toward the surface of the medium. This reduces the distance (flying height) between the magnetic head and the magnetic recording layer of the medium, enabling signals from smaller magnetic particles to be picked up, thereby achieving even higher recording densities. However, this also results in an extremely small gap (flying height) between the magnetic head element and the medium surface. If unevenness due to bubbles exists on the surface of the magnetic recording medium substrate, these unevenness is reflected on the surface of the magnetic recording medium, reducing the surface smoothness of the magnetic recording medium. If a magnetic head is brought close to the surface of a magnetic recording medium with poor surface smoothness, the magnetic head may come into contact with the surface of the magnetic recording medium and be damaged, so a certain flying height must be maintained to prevent contact. For these reasons, it is desirable to reduce bubbles in the magnetic recording medium substrate in order to produce a magnetic recording medium with high surface smoothness. Reducing bubbles in the substrate allows for a narrower flying height. Since the magnetic recording medium substrate preferably has reduced bubbles, the magnetic recording medium equipped with such a substrate is also suitable for a magnetic recording device equipped with a DFH mechanism that achieves an extremely narrow flying height.

[0068] There are no particular limitations on the dimensions of the magnetic recording medium substrate (e.g., magnetic disk substrate) and the magnetic recording medium (e.g., magnetic disk), but since high recording density is possible, it is also possible to miniaturize the medium and substrate. For example, they can have a nominal diameter of 2.5 inches, as well as smaller diameters (e.g., 1 inch or 1.8 inches), or even 3 inches or 3.5 inches.

[0069] [Glass spacer for magnetic recording / reproducing devices] One aspect of the present invention is In mole percent, SiO2 content 45-68%, Al2O3 content is 5-20%, The total content of SiO2 and Al2O3 (SiO2+Al2O3) is 60-80%, B2O3 content is 0-5% MgO content: 3-28% CaO content is 0-18%; The total content of BaO and SrO (BaO+SrO) is 0-2%. The total content of alkaline earth metal oxides (MgO + CaO + SrO + BaO) is 12 to 30%, The total content of alkali metal oxides (Li2O + Na2O + K2O) is 3.5 to 15%. and containing at least one selected from the group consisting of Sn oxide and Ce oxide, and the total content of Sn oxide and Ce oxide is 0.05 to 2.00%, Glass transition temperature is 625°C or higher, Young's modulus is 83GPa or more, Specific gravity is 2.85 or less, and Average linear expansion coefficient at 100-300°C is 48 x 10 -7 a glass spacer for a magnetic recording / reproducing device, which is made of amorphous oxide glass having a temperature of 1000 K / °C or higher; Regarding.

[0070] 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 securing 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 glasses for magnetic recording medium substrates, these glass spacers are also desired to have excellent heat resistance, rigidity, and glass stability, a low specific gravity, and a moderate coefficient of thermal expansion. In contrast, as previously described in detail for the glass for magnetic recording medium substrates according to one embodiment of the present invention, the above-mentioned glasses are suitable as glass spacers for magnetic recording and reproducing devices because they can have excellent heat resistance, rigidity, and glass stability, a low specific gravity, and a moderate coefficient of thermal expansion.

[0071] 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 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 glass properties of the glass spacer for a magnetic recording / reproducing device according to one aspect of the present invention, reference can be made to the above descriptions regarding the glass for a magnetic recording medium substrate, the magnetic recording medium substrate, and the magnetic recording medium according to one aspect of the present invention. To remove static electricity generated when the magnetic recording medium rotates, a conductive film can be formed on the surface of the glass spacer by immersion, vapor deposition, sputtering, etc. Furthermore, the surface smoothness of the glass spacer can be increased by polishing (for example, an average surface roughness of 1 μm or less), which strengthens the adhesion between the magnetic recording medium and the spacer and prevents misalignment.

[0072] [Magnetic recording and playback device] One aspect of the present invention is A magnetic recording medium according to one embodiment of the present invention; and A glass spacer according to one aspect of the present invention; a magnetic recording and reproducing device including at least one of the above; Regarding.

[0073] 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.

[0074] A magnetic recording / reproducing device according to an embodiment of the present invention may include at least one of a magnetic recording medium according to an embodiment of the present invention and a glass spacer according to an embodiment of the present invention. Other aspects of the magnetic recording / reproducing device may be achieved by applying known techniques related to magnetic recording / reproducing devices. 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, a film is typically deposited at a high temperature, or a heat treatment is performed at a high temperature after deposition. A magnetic recording medium substrate according to an embodiment of the present invention is preferable as a magnetic recording medium substrate having 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]

[0075] 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.

[0076] [Examples No. 1 to No. 47, Comparative Examples 1 to 5, Reference Example 1] Raw materials such as oxides, carbonates, nitrates, and hydroxides were weighed and mixed to prepare a raw material blend so as to obtain glass having the composition shown in Table 1. This raw material blend was charged into a melting vessel and heated and melted in the range of 1400 to 1600°C. The resulting molten glass 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.

[0077] [Evaluation method] (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).

[0078] (2) Young's modulus The Young's modulus of each glass was measured by an ultrasonic method.

[0079] (3) Specific gravity The specific gravity of each glass was measured by Archimedes' method.

[0080] (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).

[0081] (5) Glass stability 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 1250°C or 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. The presence or absence of crystals in each glass was observed under an optical microscope and evaluated according to the following criteria. A: No crystals were observed when observed under an optical microscope at a magnification of 40 to 100 times. B: Crystals are observed under magnification with an optical microscope (40 to 100 times magnification), but not by visual observation. C: Crystals are confirmed by visual observation.

[0082] (6) Foam density ranking A glass plate (substrate blank) approximately 1.2 mm thick was prepared from the molten glass obtained above. The surface of this glass plate was polished to a flat and smooth surface, and the interior of the glass was observed from the polished surface under an optical microscope at a magnification of 40 to 100 times, and the number of bubbles with a diameter of more than 0.03 mm (hereinafter simply referred to as "bubbles") was counted. The bubble density was determined by dividing the number of bubbles counted by the mass of glass corresponding to the area observed under magnification. The bubble density rank was evaluated from rank S to rank F according to the bubble density obtained by the above method. Specifically, a bubble density of 0 bubbles / kg was ranked S, a bubble with bubbles present and a bubble density of 2 bubbles / kg or less was ranked A, a bubble density of more than 2 bubbles / kg but less than 10 bubbles / kg was ranked B, a bubble density of 10 bubbles / kg or more but less than 20 bubbles / kg was ranked C, a bubble density of 20 bubbles / kg or more but less than 50 bubbles / kg was ranked D, a bubble density of 50 bubbles / kg or more but less than 80 bubbles was ranked E, and a bubble density of 80 bubbles / kg or more was ranked F.

[0083] The above results are shown in Table 1 (Table 1-1 to Table 1-6).

[0084] [Table 1-1]

[0085] [Table 1-2]

[0086] [Table 1-3]

[0087] [Table 1-4]

[0088] [Table 1-5]

[0089] [Table 1-6]

[0090] The results shown in Table 1 confirm that the glasses for magnetic recording medium substrates of the examples all have excellent heat resistance and rigidity, low specific gravity, an appropriate thermal expansion coefficient, and excellent glass stability. Regarding glass stability, in the retention test performed by the above method, the better the evaluation result at a lower temperature, the less likely the glass is to precipitate crystals in the molten state, and the lower the molding temperature can be used for molding. The lower the molding temperature, the longer the life of components of the molding device, such as heating elements, furnace bodies, and pipes. In particular, when producing substrate blanks by press molding, the lower the molding temperature, the more preferable. Furthermore, if the molding temperature can be lowered, the glass viscosity can be increased for molding, thereby suppressing volatilization, striae, and molding bubbles. On the other hand, the glass for a magnetic recording medium substrate of Comparative Example 1 has the composition of Example 9 in JP 2010-64921 A (Patent Document 1), the glass for a magnetic recording medium substrate of Comparative Example 2 has the composition of Example 18 in the same publication, the glass for a magnetic recording medium substrate of Comparative Example 3 has the composition of Example 27 in the same publication, the glass for a magnetic recording medium substrate of Comparative Example 4 has the composition of Example 29 in the same publication, and the glass for a magnetic recording medium substrate of Comparative Example 5 has the composition of Example 30 in the same publication. The glass for a magnetic recording medium substrate of Comparative Example 1 has an excessive total content of SiO2 and Al2O3, while the glasses for a magnetic recording medium substrate of Comparative Examples 4 and 5 have excessive SiO2 contents and excessive total contents of SiO2 and Al2O3. Furthermore, the glasses for a magnetic recording medium substrate of Comparative Examples 4 and 5 have a low total content of alkaline earth oxides. Furthermore, the glasses for a magnetic recording medium substrate of Comparative Examples 1 to 3 contain no alkali metal oxides or have a low content of alkali metal oxides. All of these glasses for a magnetic recording medium substrate of Comparative Examples 1 to 5 had low thermal expansion coefficients and poor glass stability. Furthermore, all of the glasses of the examples shown in Table 1 were ranked S, A or B in bubble density, confirming that bubble generation was suppressed. In contrast, the glass for a magnetic recording medium substrate of Reference Example 1, in which the total content of Sn oxide and Ce oxide was less than 0.05 mass %, showed significant bubble formation (bubble density rank E).

[0091] [Magnetic recording medium substrate fabrication] (1) Preparation of substrate blank Next, a disk-shaped substrate blank was produced by the following method A or B. Furthermore, a glass blank for producing a glass spacer for a magnetic recording / reproducing device can be obtained by the same method. (Method A) The clarified and homogenized glass melt of the above-described example was allowed to flow out of the flow pipe at a constant rate and was received by a press-molding lower die. The flowing glass melt was cut with a cutting blade so as to obtain a predetermined amount of glass melt gob on the lower die. The lower die bearing the glass melt gob was then immediately removed from the bottom of the pipe, and press-molded into a thin disk shape with a diameter of 66 mm and a thickness of 1.2 mm using an upper die and a barrel die facing the lower die. The press-molded product was cooled to a temperature at which it would not deform, then removed from the die and annealed to obtain a substrate blank. Note that in the above-described molding, multiple lower dies were used to successively form the flowing glass melt into disk-shaped substrate blanks. (Method B) The clarified and homogenized glass melt of the above-described example was continuously poured from above into a heat-resistant mold having a cylindrical through-hole, formed into a cylindrical shape, and removed from the bottom of the through-hole. The removed glass was annealed, and then sliced ​​at regular intervals in a direction perpendicular to the cylindrical axis using a multi-wire saw to produce disk-shaped substrate blanks. 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) The molten glass of the above-described examples can be poured onto a float bath and formed into a sheet of glass (formed by the float method), and then annealed, after which a disk of glass can be bored out of the sheet of glass to obtain a substrate blank. (Method D) The molten glass of the above-described embodiment can be formed into a sheet of glass by the overflow downdraw method (fusion method), annealed, and then a disk of glass can be bored from the sheet of glass to obtain a substrate blank.

[0092] (2) Preparation of glass substrate A through hole was drilled in the center of the substrate blank obtained by each of the above methods, the outer and inner circumferences were ground, and the main surfaces of the disk were lapped and polished (mirror polished) to finish it into a glass substrate for a magnetic disk with a diameter of 65 mm and a thickness of 0.8 mm. In addition, 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 by the same method. The glass substrate obtained above was washed with a 1.7% by mass aqueous solution of hydrofluorosilicic acid (HSiF) and then with a 1% by mass aqueous solution of potassium hydroxide, and then rinsed with pure water and dried. When the surface of the substrate made from the glass of the example was observed under magnification, no surface roughness was observed and the surface was smooth.

[0093] [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 a glass substrate obtained from the glass of the example by the following method, to obtain a magnetic disk.

[0094] 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.

[0095] The adhesion layer was formed using a CrTi target to form an amorphous CrTi layer with a thickness of 20 nm. Subsequently, a 10 nm thick underlayer made of CrRu was formed by DC magnetron sputtering in an Ar atmosphere using a single-wafer, stationary facing deposition system. The magnetic recording layer was formed using an FePt or CoPt target at a deposition temperature of 400°C to form a 10 nm thick FePt or CoPt layer.

[0096] The magnetic disk, after the film formation up to the magnetic recording layer, was transferred from the film formation apparatus into a heating furnace and annealed at a temperature in the range of 650 to 700°C.

[0097] 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 (flying height: 8 nm) equipped with a DFH mechanism, and magnetic signals were recorded in the recording area on the main surface of the magnetic disk at a recording density of 20 gigabits per square inch. No collision between the magnetic head and the magnetic disk surface (crash failure) was observed.

[0098] According to one aspect of the present invention, it is possible to provide a magnetic recording medium that is optimal for high-density recording.

[0099] Finally, the above-mentioned aspects will be summarized.

[0100] According to one embodiment, the SiO2 content is 45 to 68%, the Al2O3 content is 5 to 20%, the total content of SiO2 and Al2O3 (SiO2 + Al2O3) is 60 to 80%, the B2O3 content is 0 to 5%, the MgO content is 3 to 28%, the CaO content is 0 to 18%, the total content of BaO and SrO (BaO + SrO) is 0 to 2%, the total content of alkaline earth metal oxides (MgO + CaO + SrO + B aO) is 12 to 30%, the total content of alkali metal oxides (Li2O + Na2O + KO) is 3.5 to 15%, it contains at least one oxide selected from the group consisting of Sn oxide and Ce oxide, the total content of Sn oxide and Ce oxide is 0.05 to 2.00%, it has a glass transition temperature of 625°C or higher, a Young's modulus of 83 GPa or higher, a specific gravity of 2.85 or lower, and an average linear expansion coefficient at 100 to 300°C of 48 × 10 -7 / °C or higher.

[0101] According to one embodiment, the SiO2 content is 45 to 68%, the Al2O3 content is 5 to 20%, the total content of SiO2 and Al2O3 (SiO2 + Al2O3) is 60 to 80%, the B2O3 content is 0 to 5%, the MgO content is 3 to 28%, the CaO content is 0 to 18%, the total content of BaO and SrO (BaO + SrO) is 0 to 2%, and the total content of alkaline earth metal oxides (MgO + CaO + SrO and BaO) is 12 to 30%, the total content of alkali metal oxides (Li2O+Na2O+K2O) is 3.5 to 15%, and the composite oxide contains at least one oxide selected from the group consisting of Sn oxide and Ce oxide, with the total content of Sn oxide and Ce oxide being 0.05 to 2.00%, the composite oxide has a glass transition temperature of 625°C or higher, a Young's modulus of 83 GPa or higher, a specific gravity of 2.85 or lower, and an average linear expansion coefficient at 100 to 300°C of 48 × 10 -7 The glass spacer for a magnetic recording / reproducing device is made of amorphous oxide glass having a temperature of 1000K / °C or higher.

[0102] The glass for a magnetic recording medium substrate has excellent heat resistance and rigidity, a low specific gravity, an appropriate thermal expansion coefficient, and excellent glass stability, as does the glass spacer for a magnetic recording / reproducing device.

[0103] In one embodiment, the oxide glass contains 0 to 8 mol % of Li2O, 0 to 10 mol % of Na2O, and 0 to 5 mol % of K2O.

[0104] In one embodiment, the oxide glass contains Sn oxide and Ce oxide.

[0105] In one embodiment, the molar ratio of the total content of MgO and CaO to the total content of alkaline earth metal oxides in the oxide glass {(MgO+CaO) / (MgO+CaO+SrO+BaO)} is 0.75 to 1.00.

[0106] In one embodiment, the molar ratio of the Li2O content to the total content of alkali metal oxides in the oxide glass {Li2O / (Li2O+Na2O+K2O)} is 0.4-1.

[0107] In one embodiment, the molar ratio of the Na2O content to the total content of alkali metal oxides in the oxide glass {Na2O / (Li2O+Na2O+K2O)} is 0.4-1.

[0108] In one embodiment, the CaO content of the oxide glass is 2 to 15%.

[0109] According to one aspect, there is provided a magnetic recording medium substrate comprising the above magnetic recording medium.

[0110] In one embodiment, the magnetic recording medium substrate has a homogeneous composition on the surface and in the interior.

[0111] In one embodiment, the magnetic recording medium substrate has an ion-exchange layer on a part or all of the surface.

[0112] According to one embodiment, there is provided a magnetic recording medium having a magnetic recording layer on the above magnetic recording medium substrate.

[0113] 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. In mol %, SiO ２ The content is 52-68%, Al ２ O ３ The content is 12-18%, SiO ２ and Al ２ O ３ The total content (SiO ２ +Al ２ O ３ ) is 80% or less, B ２ O ３ The content is 0-5%, MgO content is 3-28%, The content of CaO is 0 to 4.00%, TiO ２ The content is 0-5%, ZrO ２ The content is 0-3%, The total content of BaO and SrO (BaO + SrO) is 0 to 2%; The total content of alkaline earth metal oxides (MgO + CaO + SrO + BaO) is 12 to 30%, the molar ratio of the total content of MgO and CaO to the total content of alkaline earth metal oxides {(MgO + CaO) / (MgO + CaO + SrO + BaO)} is 0.75 to 1.00; The total content of alkali metal oxides (Li ２ O + Na ２ O+K ２ O) is 3.5 to 13%, Li ２ O content is 8% or less, Na ２ O content is 6% or less, K ２ O content is 5% or less, HfO ２ , Nb ２ O ５ , Ta ２ O ５ , La ２ O ３ and Y ２ O ３ The total content of HfO ２ +Nb ２ O ５ +Ta ２ O ５ +La ２ O ３ +Y ２ O ３ ) is 0 to 3%, ZnO content is 0-3%, P ２ O ５ The content is 0-2%, The glass for magnetic recording medium substrates is an amorphous oxide glass.

2. 2. The glass for a magnetic recording medium substrate according to claim 1, which has a specific elastic modulus of 31 MNm / kg or more.

3. 3. The glass for magnetic recording medium substrates according to claim 1, wherein when the glass is left to stand for 16 hours while maintaining the temperature at 1250°C, and then cooled to room temperature, and then the presence or absence of crystals in the glass is observed with an optical microscope at a magnification of 40 to 100 times, no crystals are confirmed.

4. 4. The glass for a magnetic recording medium substrate according to claim 1, wherein the density of bubbles per unit mass of the glass is less than 50 bubbles / kg, as determined by the density of bubbles having a diameter of more than 0.03 mm as observed with an optical microscope at a magnification of 40 to 100 times.

5. A magnetic recording medium substrate comprising the glass for magnetic recording medium substrates according to any one of claims 1 to 4.

6. 6. A magnetic recording medium comprising a magnetic recording layer on the magnetic recording medium substrate according to claim 5.

7. A magnetic recording and reproducing device comprising the magnetic recording medium according to claim 6.

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