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

Abstract

To provide a glass for a magnetic recording medium substrate having excellent heat resistance and rigidity, a magnetic recording medium substrate made of the glass, a magnetic recording medium including the substrate, and a glass spacer for a magnetic recording device.SOLUTION: A glass for a magnetic recording medium substrate is an amorphous oxide glass having a SiO2 content of 56 to 80 mol%, a Li2O content of 1 to 10 mol%, a B2O3 content of 0 to 4 mol%, a total content of MgO and CaO (MgO+CaO) of 9 to 40 mol%, a specific gravity of 2.75 g / cm3 or less, a glass transition temperature of 650°C or more, and a Young's modulus of 90 GPa or more.SELECTED DRAWING: None

Description

[Technical Field]

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

[0006] As described above, glass for use as a magnetic recording medium substrate is desired to have excellent heat resistance and rigidity.

[0007] 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 and rigidity. [Means for solving the problem]

[0008] One aspect of the present invention is SiO2 content of 56 to 80 mol%, Li2O content is 1 to 10 mol%, B2O3 content is 0-4 mol%, The total content of MgO and CaO (MgO + CaO) is 9 to 40 mol%, and Specific gravity is 2.75g / cm 3 Hereinafter, a glass for magnetic recording medium substrates (hereinafter also referred to as "glass A") is an amorphous oxide glass having a glass transition temperature of 650°C or higher and a Young's modulus of 90 GPa or higher. Regarding.

[0009] Another aspect of the present invention is SiO2 content of 56 to 80 mol%, Li2O content is 1 to 10 mol%, B2O3 content is 0-4 mol%, The total content of MgO and CaO (MgO + CaO) is 9 to 40 mol%, The molar ratio of the total content of SiO2 and ZrO2 to the content of Al2O3 ((SiO2 + ZrO2) / Al2O3) is 2 to 13, and A glass for magnetic recording medium substrates (hereinafter also referred to as "glass B") that is an amorphous oxide glass having a glass transition temperature of 650°C or higher and a Young's modulus of 90 GPa or higher; Regarding.

[0010] Glass A and glass B have the above glass composition, and have high heat resistance with a glass transition temperature of 650° C. or higher and high rigidity with a Young's modulus of 90 GPa or higher. [Effects of the Invention]

[0011] According to one aspect of the present invention, a glass for a magnetic recording medium substrate having excellent heat resistance and rigidity can be provided. Furthermore, according to another aspect, 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 can also be provided. Furthermore, according to another aspect, a glass spacer for a magnetic recording device can be provided. DETAILED DESCRIPTION OF THE INVENTION

[0012] [Glass for magnetic recording media substrates] Glasses A and B are glasses for magnetic recording medium substrates that have the above glass composition and are amorphous oxide glasses with a glass transition temperature of 650° C. or higher and a Young's modulus of 90 GPa or higher.

[0013] Glasses A and B are amorphous and oxide glasses. Unlike glass-ceramics, amorphous glasses do not contain a crystalline phase and exhibit a glass transition phenomenon upon heating. Oxide glasses are glasses in which the main network-forming component of the glass is an oxide. Glasses A and B will be described in more detail below. Unless otherwise specified, the matters described apply to both glasses A and B.

[0014] <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%.

[0015] The glass compositions of glasses A and B will be described below.

[0016] SiO2 is a glass network-forming component that improves glass stability. SiO2 is also a component that contributes to improving chemical durability. From the viewpoint of improving rigidity, the SiO2 content in Glasses A and B is 80% or less, preferably 75% or less, more preferably 70% or less, even more preferably 66% or less, even more preferably 64% or less, even more preferably 63% or less, and even more preferably 62% or less. From the viewpoint of maintaining chemical durability, the SiO2 content in Glasses A and B is 56% or more, preferably 57% or more, more preferably 58% or more, even more preferably 59% or more, and even more preferably 60% or more.

[0017] Al2O3 is a glass network-forming component and functions to improve heat resistance. Al2O3 also functions to improve chemical durability. In glass B, Al2O3 is an essential component. From the viewpoint of improving heat resistance and chemical durability, the Al2O3 content in glasses A and B is preferably 5% or more, more preferably 8% or more, even more preferably 10% or more, still more preferably 12% or more, and even more preferably 13% or more. Furthermore, from the viewpoint of improving glass stability, the Al2O3 content in glasses A and B is preferably 20% or less, more preferably 18% or less, even more preferably 17% or less, still more preferably 16% or less, and even more preferably 15.5% or less.

[0018] In Glasses A and B, the total content of SiO2 and Al2O3 (SiO2 + Al2O3) is preferably 65% ​​or more, more preferably 67% or more, even more preferably 69% or more, still more preferably 70% or more, still more preferably 72% or more, and still more preferably 74% or more, from the viewpoint of maintaining chemical durability. Furthermore, from the viewpoint of improving rigidity, the total content of SiO2 and Al2O3 (SiO2 + Al2O3) in Glasses A and B is preferably 80% or less, more preferably 78% or less, even more preferably 77% or less, still more preferably 76% or less, and still more preferably 75% or less.

[0019] B2O3 is a glass network-forming component, a component that reduces the specific gravity of the glass, and a component that improves meltability. On the other hand, 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 Glasses A and B is set to 0 to 4%. The B2O3 content is preferably 3.00% or less, more preferably 2.00% or less, even more preferably 1.50% or less, even more preferably 1.00% or less, still more preferably 0.50% or less, and even more preferably 0.30% or less. The B2O3 content is preferably 0.00% or more, more preferably 0.05% or more, even more preferably 0.10% or more, and even more preferably 0.15% or more.

[0020] Among the alkaline earth metal oxides MgO, CaO, SrO, and BaO, MgO increases the Young's modulus of glass, increases the thermal expansion coefficient, and improves the meltability and formability of glass. The specific elastic modulus will be described in detail later. In one embodiment, MgO is an essential component. From the viewpoint of achieving the above functions well, the MgO content in Glasses A and B is preferably 3% or more, more preferably 5% or more, even more preferably 7% or more, still more preferably 8% or more, and even more preferably 9% or more. Furthermore, from the viewpoint of maintaining glass stability, the MgO content in Glasses A and B is preferably 28% or less, more preferably 25% or less, even more preferably 22% or less, still more preferably 20% or less, even more preferably 18% or less, even more preferably 16% or less, still more preferably 15% or less, and even more preferably 14% or less.

[0021] The CaO content in Glasses A and B 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 Glasses A and B is preferably 2% or more, more preferably 3% or more, even more preferably 4% or more, and still more preferably 5% or more. Furthermore, from the viewpoint of maintaining chemical durability, the CaO content in Glasses A and B is preferably 18% or less, more preferably 15% or less, even more preferably 12% or less, even more preferably 10% or less, even more preferably 8% or less, and still more preferably 7% or less.

[0022] From the viewpoint of improving rigidity, the total content of MgO and CaO (MgO+CaO) in Glasses A and B is 9% or more, preferably 11% or more, more preferably 13% or more, even more preferably 15% or more, still more preferably 16% or more, and even more preferably 17% or more. From the viewpoint of glass stability, the total content of MgO and CaO (MgO+CaO) in Glasses A and B is 40% or less, preferably 35% or less, more preferably 30% or less, even more preferably 27% or less, even more preferably 25% or less, even more preferably 23% or less, even more preferably 21% or less, and still more preferably 20% or less.

[0023] The molar ratio of the CaO content to the MgO content (CaO / MgO) in Glasses A and B is preferably 2.5 or less, more preferably 2.3 or less, even more preferably 2.1 or less, even more preferably 1.9 or less, still more preferably 1.6 or less, even more preferably 1.4 or less, even more preferably 1.1 or less, and still more preferably 1.0 or less, from the viewpoint of increasing Young's modulus and specific elastic modulus and improving glass stability. The molar ratio (CaO / MgO) is 0 or more, and from the viewpoint of improving glass stability, it is preferably 0.05 or more, more preferably 0.100 or more, even more preferably 0.120 or more, and even more preferably 0.150 or more.

[0024] SrO improves the meltability, formability, and glass stability of the glass, and functions to increase the thermal expansion coefficient. From the viewpoints of maintaining chemical durability, reducing the specific gravity, and reducing raw material costs, the SrO content in Glasses A and B is preferably 0 to 2%. The SrO content is more preferably in the range of 0 to 1.5%, even more preferably 0 to 1%, and even more preferably 0 to 0.5%. In one embodiment, it is 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 Glasses A and B is preferably 0 to 2%, more preferably 0 to 1.5%, even more preferably 0 to 1%, and even more preferably 0 to 0.5%. In one embodiment, it is preferable that neither BaO nor 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 functions to increase the thermal expansion coefficient. From the viewpoints of maintaining chemical durability, reducing the specific gravity, and reducing raw material costs, the BaO content in Glasses A and B is preferably 0 to 2%, more preferably 0 to 1.5%, even more preferably 0 to 1.0%, and even more preferably 0 to 0.5%. In one embodiment, it is preferable that no BaO is contained, i.e., the BaO content is 0%.

[0027] The molar ratio of the BaO content to the total content of MgO, CaO, SrO, and BaO (BaO / (MgO+CaO+SrO+BaO)) is preferably 0.1 or less, more preferably 0.08 or less, even more preferably 0.06 or less, still more preferably 0.04 or less, still more preferably 0.03 or less, still more preferably 0.02 or less, and still more preferably 0.01 or less, from the viewpoints of improving rigidity, lowering specific gravity, increasing specific elastic modulus, and improving glass stability. In one embodiment, the molar ratio (BaO / (MgO+CaO+SrO+BaO)) is preferably 0.

[0028] The molar ratio of the MgO content to the total content of MgO, CaO, SrO, and BaO (MgO / (MgO+CaO+SrO+BaO)) is preferably 1 or less, more preferably 0.8 or less, and even more preferably 0.75 or less, from the viewpoint of improving glass stability. Furthermore, the molar ratio (MgO / (MgO+CaO+SrO+BaO)) is preferably 0.3 or more, more preferably 0.5 or more, even more preferably 0.6 or more, and even more preferably 0.65 or more, from the viewpoint of increasing Young's modulus and specific elastic modulus and improving glass stability.

[0029] The total content of alkaline earth metal oxides (MgO+CaO+SrO+BaO) in Glasses A and B is preferably 9% or more, more preferably 11% or more, even more preferably 13% or more, even more preferably 14% or more, and still more preferably 16% or more, from the viewpoint of improving 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 Glasses A and B is preferably 40% or less, more preferably 35% or less, even more preferably 30% or less, even more preferably 28% or less, even more preferably 25% or less, even more preferably 22% or less, even more preferably 21% or less, and even more preferably 20% or less.

[0030] Among alkaline earth metal oxides, MgO increases the Young's modulus and specific modulus of 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 glass, and is particularly effective for increasing the Young's modulus and specific gravity. CaO also increases the Young's modulus and specific modulus of glass, contributes to suppressing an increase in specific gravity, and is also an effective component for increasing the thermal expansion coefficient of glass. On the other hand, SrO and BaO increase the specific gravity and raw material costs. From these perspectives, in Glasses A and B, the molar ratio of the total content of MgO and CaO to the total content of MgO, CaO, SrO, and BaO {(MgO + CaO) / (MgO + CaO + SrO + BaO)} is preferably 0.7 to 1. The lower limit of the molar ratio is more preferably 0.750 or more, even more preferably 0.800 or more, even more preferably 0.850 or more, even more preferably 0.900 or more, even more preferably 0.925 or more, even more preferably 0.950 or more, even more preferably 0.975 or more, and even more preferably 0.980 or more.

[0031] Among alkali metal oxides, Li2O has a strong effect of improving the meltability and formability of glass. It is also a suitable component for increasing the Young's modulus and imparting suitable rigidity to magnetic recording medium substrates. Furthermore, Li2O also plays a role in lowering the specific resistance. The specific resistance will be described in detail later. Li2O also plays a role in increasing the thermal expansion coefficient. Furthermore, when Glasses A and B are used as glasses for chemical strengthening, Li2O also plays a role in ion exchange during chemical strengthening. On the other hand, Li2O also plays a role in lowering the glass transition temperature. Taking these functions into consideration, the Li2O content in Glasses A and B is set to 1 to 10%. The lower limit of the Li2O content is preferably 2% or more, more preferably 3% or more, even more preferably 4% or more, and even more preferably 4.2% or more. The upper limit of the Li2O content is preferably 8% or less, more preferably 7% or less, even more preferably 6% or less, and even more preferably 5% or less.

[0032] The molar ratio of the Li2O content to the total content of MgO and CaO (Li2O / (MgO+CaO)) is preferably 0.4 or less, more preferably 0.37 or less, even more preferably 0.35 or less, still more preferably 0.32 or less, and even more preferably 0.30 or less, from the viewpoint of suppressing a decrease in the glass transition temperature. Furthermore, the molar ratio (Li2O / (MgO+CaO)) is preferably 0.03 or more, more preferably 0.05 or more, even more preferably 0.08 or more, still more preferably 0.10 or more, even more preferably 0.15 or more, still more preferably 0.20 or more, and still more preferably 0.21 or more, from the viewpoint of improving the meltability of the glass, suppressing the generation of bubbles, and reducing the specific resistance.

[0033] 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, accelerating bubble removal. Furthermore, when Glasses A and B are used as glasses for chemical strengthening, Na2O is also a component responsible for ion exchange during chemical strengthening. Taking these functions into consideration, the Na2O content in Glasses A and B is preferably 0 to 10%, more preferably 0 to 8%, even more preferably 0 to 7%, even more preferably 0 to 6%, still more preferably 0 to 5%, even more preferably 0 to 3%, and even more preferably 0 to 1%.

[0034] From the viewpoint of improving rigidity, the molar ratio of the NaO content to the LiO content (NaO / LiO) is preferably 5 or less, more preferably 3 or less, even more preferably 2 or less, still more preferably 1 or less, and even more preferably 0.5 or less. In one embodiment, the molar ratio (NaO / LiO) can be 0.

[0035] KO also functions to improve the meltability and formability of glass and is a component that increases the thermal expansion coefficient. On the other hand, if incorporated in excess, it tends to reduce chemical durability, particularly acid resistance, and when used as 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 Glasses A and B is preferably 0 to 5%, more preferably 0 to 3%, even more preferably 0 to 2%, and even more preferably 0 to 1%. In one embodiment, it is preferable that no KO is contained.

[0036] Regarding the Li2O content, Na2O content, and K2O content, from the viewpoint of improving the meltability of the glass, the sum of 4 × Li2O content, 2 × Na2O content, and K2O content (4Li2O + 2Na2O + K2O) is preferably 8% or more. Furthermore, a sum of 4Li2O + 2Na2O + K2O of 8% or more is also preferable for reducing the specific resistance. From the above viewpoints, the sum is more preferably 10% or more, even more preferably 12% or more, even more preferably 14% or more, still more preferably 16% or more, even more preferably 17% or more, even more preferably 17.3% or more, even more preferably 17.5% or more, and even more preferably 17.8% or more. Furthermore, from the viewpoint of increasing the glass transition temperature, the sum of the 4 × LiO content, the 2 × NaO content, and the KO content (4LiO + 2NaO + KO) is preferably 27% or less, more preferably 25% or less, even more preferably 23% or less, still more preferably 21% or less, even more preferably 20% or less, and still more preferably 19% or less.

[0037] The molar ratio of the SiO content to the total content of LiO, NaO, and KO (SiO / (LiO+NaO+KO)) is preferably 4 or more, more preferably 6 or more, even more preferably 8 or more, still more preferably 10 or more, and even more preferably 10.5 or more, from the viewpoint of increasing the glass transition temperature. Furthermore, from the viewpoint of improving the meltability and rigidity of the glass, the molar ratio (SiO / (LiO+NaO+KO)) is preferably 22 or less, more preferably 20 or less, even more preferably 17 or less, still more preferably 15 or less, and even more preferably 14 or less.

[0038] The molar ratio of the LiO content to the total content of LiO, NaO, and KO (LiO / (LiO+NaO+KO)) is preferably 0.10 or more, more preferably 0.30 or more, even more preferably 0.50 or more, still more preferably 0.70 or more, even more preferably 0.80 or more, still more preferably 0.90 or more, and still more preferably 0.95 or more, from the viewpoints of increasing Young's modulus and specific elastic modulus, improving solubility, and reducing specific resistance. In one embodiment, the molar ratio (LiO / (LiO+NaO+KO)) can be 1.

[0039] The molar ratio of the NaO content to the total content of LiO, NaO, and KO (NaO / (LiO+NaO+KO)) can be 0 or more, and from the viewpoint of suppressing a decrease in Young's modulus and specific elastic modulus, is preferably 5 or less, more preferably 2 or less, still more preferably 1 or less, even more preferably 0.8 or less, and still more preferably 0.5 or less.

[0040] From the viewpoint of increasing the thermal expansion coefficient, the sum of 5×Li2O content, 3×Na2O content, 3×K2O content, 2×B2O3 content, MgO content, 2×CaO content, 3×SrO content and BaO content (5Li2O+3Na2O+3K2O+2B2O3+MgO+2CaO+3SrO+BaO) is preferably 32% or more, more preferably 35% or more, even more preferably 38% or more, still more preferably 40% or more, even more preferably 42% or more, and still more preferably 44% or more. Furthermore, from the viewpoint of increasing the glass transition temperature, the above sum (5Li2O+3Na2O+3K2O+2B2O3+MgO+2CaO+3SrO+BaO) is preferably 58% or less, more preferably 56% or less, even more preferably 54% or less, still more preferably 52% or less, even more preferably 51% or less, and still more preferably 50% or less.

[0041] From the viewpoint of improving glass stability, the molar ratio of the total content of CaO and Na2O to the total content of MgO and Li2O ((CaO + Na2O) / (MgO + Li2O)) is preferably 0 or more, more preferably 0.03 or more, even more preferably 0.05 or more, even more preferably 0.10 or more, even more preferably 0.15 or more, even more preferably 0.20 or more, and even more preferably 0.25 or more. Furthermore, from the viewpoint of suppressing a decrease in Young's modulus and specific elastic modulus, the molar ratio ((CaO + Na2O) / (MgO + Li2O)) is preferably 2.5 or less, more preferably 2 or less, even more preferably 1.5 or less, even more preferably 1.0 or less, even more preferably 0.8 or less, and even more preferably 0.6 or less.

[0042] Li2O, Na2O, and K2O are components that improve the meltability and formability of the glass and increase the thermal expansion coefficient. To fully utilize the functions of these components, the total content of alkali metal oxides (Li2O + Na2O + K2O) in Glasses A and B is preferably 2.5% or more, more preferably 3% or more, even more preferably 3.5% or more, and even 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 Glasses A and B is preferably 10% or less, more preferably 8% or less, even more preferably 7% or less, even more preferably 6% or less, and even more preferably 5% 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.

[0043] TiO2 improves glass stability and chemical durability as well as rigidity, but if incorporated in excess, the liquidus temperature of the glass may rise, leading to a deterioration in devitrification resistance and an increase in specific gravity. Therefore, the TiO2 content in Glasses A and B is preferably 0 to 10%, more preferably 0 to 8%, even more preferably 0 to 6%, still more preferably 0 to 4%, even more preferably 0 to 3%, even more preferably 0 to 2%, and still more preferably 0 to 1%.

[0044] The molar ratio of the total content of TiO2 and Al2O3 to the total content of MgO and CaO ((TiO2 + Al2O3) / (MgO + CaO)) is preferably 0.35 or more, more preferably 0.400 or more, even more preferably 0.440 or more, even more preferably 0.500 or more, and still more preferably 0.640 or more, from the viewpoints of suppressing bubble formation, improving chemical durability, and increasing glass hardness. Furthermore, the molar ratio ((TiO2 + Al2O3) / (MgO + CaO)) is preferably 2 or less, more preferably 1.800 or less, even more preferably 1.600 or less, even more preferably 1.400, even more preferably 1.200 or less, and still more preferably 1.000 or less, from the viewpoints of improving glass stability, increasing specific elastic modulus, and decreasing specific gravity.

[0045] ZrO2 has the function of improving chemical durability and rigidity. However, excessive incorporation of ZrO2 may reduce the meltability of the glass, resulting in unmelted raw materials. Therefore, the ZrO2 content in Glasses A and B is preferably 0 to 5%, more preferably 0 to 3%, even more preferably 0 to 2%, even more preferably 0 to 1%, and even more preferably 0 to 0.5%. In one embodiment, the ZrO2 content may be 0.00%.

[0046] The sum of the MgO content, 0.7×CaO content, Li2O content, TiO2 content, and ZrO2 content (MgO + 0.7CaO + Li2O + TiO2 + ZrO2) is preferably 16% or more, more preferably 18% or more, even more preferably 19% or more, even more preferably 20% or more, and still more preferably 21% or more, from the viewpoint of increasing the specific elastic modulus. Furthermore, from the viewpoint of suppressing a decrease in glass stability and a decrease in glass transition temperature, the sum (MgO + 0.7CaO + Li2O + TiO2 + ZrO2) is preferably 40% or less, more preferably 35% or less, even more preferably 30% or less, even more preferably 28% or less, and still more preferably 25% or less.

[0047] In Glass B, the molar ratio of the total content of SiO2 and ZrO2 to the content of Al2O3 ((SiO2 + ZrO2) / Al2O3) is 2 or more, preferably 3 or more, more preferably 3.5 or more, and even more preferably 4 or more, from the viewpoint of improving chemical durability. Furthermore, in Glass B, the molar ratio ((SiO2 + ZrO2) / Al2O3) is 13 or less, preferably 10 or less, more preferably 9 or less, even more preferably 8 or less, even more preferably 7 or less, and even more preferably 6 or less, from the viewpoint of improving rigidity and achieving a high specific elastic modulus. In Glass A, the molar ratio of the total content of SiO2 and ZrO2 to the content of Al2O3 ((SiO2 + ZrO2) / Al2O3) is preferably 2 or more, from the viewpoint of improving chemical durability, and is preferably 13 or less, from the viewpoint of improving rigidity and achieving a high specific elastic modulus. For a more preferable range of the molar ratio of the total content of SiO2 and ZrO2 to the content of Al2O3 in glass A ((SiO2+ZrO2) / Al2O3), see the above description of glass B.

[0048] ZnO functions to improve meltability and rigidity, but excessive incorporation raises the liquidus temperature. From the above viewpoints, the ZnO content in Glasses A and B is preferably 0 to 5%, more preferably 0 to 3%, even more preferably 0 to 2%, even more preferably 0 to 1%, and still more preferably 0 to 0.5%. In one embodiment, the ZnO content can be 0.00%.

[0049] Although P2O5 can be incorporated into Glasses A and B, excessive incorporation tends to reduce chemical durability, so the P2O5 content in Glasses A and B is preferably 0 to 2%. The P2O5 content is more preferably 0 to 1%, and even more preferably 0 to 0.5%. In one embodiment, the P2O5 content can be 0.00%.

[0050] From the viewpoint of improving rigidity, the sum of the SiO2 content, Al2O3 content, B2O3 content, P2O5 content, 1.5×Na2O content, 1.5×K2O content, 2×SrO content, 3×BaO content and ZnO content (SiO2+Al2O3+B2O3+P2O5+1.5Na2O+1.5K2O+2SrO+3BaO+ZnO) is preferably 86% or less, more preferably 84% or less, even more preferably 82% or less, still more preferably 80% or less, even more preferably 78% or less, and still more preferably 77% or less.

[0051] From the viewpoint of achieving a high specific elastic modulus, the sum of the SiO2 content, Al2O3 content, B2O3 content, P2O5 content, Na2O content, K2O content, CaO content, 2×SrO content and 3×BaO content (SiO2+Al2O3+B2O3+P2O5+Na2O+K2O+CaO+2SrO+3BaO) is preferably 92% or less, more preferably 90% or less, even more preferably 88% or less, even more preferably 86% or less, and even more preferably 85% or less.

[0052] The molar ratio of the P2O5 content to the total content of B2O3, SiO2, Al2O3, and P2O5 (P2O5 / (B2O3+SiO2+Al2O3+P2O5)) is preferably 0.005 or less, more preferably 0.003 or less, even more preferably 0.002 or less, and even more preferably 0.001 or less, from the viewpoints of improving glass stability, improving rigidity, increasing specific elastic modulus, and improving chemical durability. In one embodiment, the molar ratio (P2O5 / (B2O3+SiO2+Al2O3+P2O5)) can be 0.

[0053] Glasses A and B may contain one or more elements selected from the group consisting of SnO2, CeO2, and Sb2O3 from the viewpoint of achieving a fining effect. In one embodiment, the total content of SnO2 and CeO2 may be 0%. In another embodiment, glasses A and B may 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 achieved, and residual bubbles can be reduced or suppressed. Furthermore, when the total content (SnO2 + CeO2) is 2% or less, it is possible to prevent the glass melt from blowing up during glass melting, thereby preventing a decrease in 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.

[0054] 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, it is preferable to incorporate SnO2 into Glasses A and B in order to remove bubbles from glasses with high melting temperatures. 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.

[0055] 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 Glasses A and B 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, Glasses A and B preferably contain both SnO2 and CeO2.

[0056] From the viewpoint of reducing the environmental impact, it is desirable to refrain from using Sb2O3. The content of Sb2O3 in Glasses A and B 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 even more preferably 0.02% or less, and it is particularly preferable that no Sb2O3 is contained.

[0057] When CeO2 is contained, the molar ratio of the SnO2 content to the CeO2 content (SnO2 / CeO2) can be 0 or more, preferably 0.1 or more, more preferably 0.3 or more, even more preferably 0.5 or more, even more preferably 0.7 or more, even more preferably 0.9 or more, even more preferably 1.0 or more, even more preferably 1.2 or more, and even more preferably 1.4 or more. The molar ratio (SnO2 / CeO2) is preferably 7 or less, more preferably 6 or less, even more preferably 5 or less, even more preferably 4 or less, even more preferably 3.5 or less, even more preferably 3.0 or less, even more preferably 2.5 or less, even more preferably 2.3 or less, even even more preferably 2.1 or less, 1.9 or less, 1.8 or less, or 1.7 or less. As will be described later with respect to resistivity, in embodiments in which electrical heating is performed during glass melting, glass with low resistivity is preferred. Glass with low resistivity can be melted by lowering the temperature inside the furnace during electrical heating. When the temperature inside the furnace is lowered, it is preferable to increase the ratio of the content of CeO2 to the content of SnO2, which can exhibit a better fining effect at low temperatures compared to SnO2, i.e., to decrease the molar ratio (SnO2 / CeO2).

[0058] The Fe content of Glasses A and B, calculated as Fe2O3 in the oxide-based glass composition expressed by mass, can be 1% by mass or less, 0.7% by mass or less, 0.5% by mass or less, 0.4% by mass or less, 0.3% by mass or less, 0.1% by mass or less, 0.07% by mass or less, 0.05% by mass or less, 0.04% by mass or less, or 0.03% by mass or less. In one embodiment, Glasses A and B can be free of Fe (the above content is 0% by mass). Glasses A and B can also contain one or more elements selected from the group consisting of Cu, Co, Yb, Mn, Nd, Pr, Nb, V, Cr, Ni, Mo, Ho, and Er.

[0059] Since Pb, Cd and As are substances that have a negative impact on the environment, it is preferable to avoid introducing them.

[0060] Glasses A and B 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. The mixture is then heated and melted in a melting vessel, for example, at a temperature in the range of 1400 to 1600°C, refined, stirred, and then molded into a homogenized glass melt that is sufficiently bubble-free. 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 glass melt in a refining vessel, maintain it at 1450 to 1600°C, and then cool it to 1200 to 1400°C, allowing the glass to flow and be molded. The glass may be heated by electrical heating or by a heating method other than electrical heating.

[0061] <Glass properties> Glasses A and B can have the various glass properties described below by adjusting their compositions as described above.

[0062] (glass transition temperature) As mentioned 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 to increase the recording density of 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, Glasses A and B have glass transition temperatures (hereinafter also referred to as "Tg"), an indicator of heat resistance, of 650°C or higher. Substrates made of highly heat-resistant glasses with glass transition temperatures of 650°C or higher can maintain excellent flatness even after high-temperature treatment. However, Glasses A and B are not limited to glasses for use as substrates for magnetic recording media having a magnetic recording layer containing a magnetic material requiring high-temperature treatment, and can be used to fabricate magnetic recording media containing various magnetic materials. The glass transition temperature is preferably 660° C. or higher, more preferably 670° C. or higher, even more preferably 675° C. or higher, even more preferably 680° C. or higher, even more preferably 685° C. or higher, and even more preferably 687° C. or higher. The upper limit of the glass transition temperature is, for example, about 770° C. or about 750° C., but is not particularly limited because a higher glass transition temperature is preferable from the viewpoint of heat resistance.

[0063] (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 glasses to have high rigidity. In this regard, Glasses A and B have a Young's modulus, an index of rigidity, of 90 GPa or more. Magnetic recording medium substrate glasses with high rigidity, exhibiting a Young's modulus of 90 GPa or more, can suppress substrate deformation during spindle motor rotation, thereby suppressing warpage and deflection of the magnetic recording medium due to substrate deformation. The Young's modulus of Glasses A and B is preferably 91 GPa or more, more preferably 92 GPa or more, even more preferably 93 GPa or more, even more preferably 94 GPa or more, and even more preferably 95 GPa or more. The upper limit of 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.

[0064] (specific gravity) The specific gravity of glass A is 2.75 or less. Furthermore, the specific gravity of glass B is preferably 2.75 or less. The specific gravity of glasses A and B is more preferably 2.73 or less, even more preferably 2.70 or less, even more preferably 2.68 or less, even more preferably 2.64 or less, even more preferably 2.62 or less, and even more preferably 2.60 or less. Lowering the specific gravity of the glass for magnetic recording medium substrates enables the weight of magnetic recording medium substrates to be reduced, and furthermore, the weight of magnetic recording media, which in turn enables the power consumption of magnetic recording and reproducing devices (commonly called HDDs (hard disk drives)) to be reduced. The lower limit of the specific gravity is, for example, about 2.40, but is not particularly limited, as a lower specific gravity is preferable.

[0065] (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 3This can be thought of as a value with the unit "modulus of elasticity." From the viewpoint of providing a substrate that is less susceptible to deformation, the specific elastic modulus of Glasses A and B is preferably 30 MNm / kg or more, more preferably 32 MNm / kg or more, even more preferably 33 MNm / kg or more, even more preferably 34 MNm / kg or more, and even more preferably 35 MNm / kg or more. 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 preferable.

[0066] (coefficient of thermal expansion) HDDs incorporating magnetic recording media are designed to rotate the magnetic recording media itself by holding 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, a mismatch will occur between the thermal expansion and contraction of the spindle and the magnetic recording media substrate in response to ambient temperature changes during use, resulting in deformation of the magnetic recording media. 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 for the glass used for the magnetic recording media substrate to have a moderate thermal expansion coefficient similar to that of the spindle material (e.g., stainless steel). Generally, HDD spindle materials have a thermal expansion coefficient of 70×10°C 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 40 × 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 Glasses A and B at 100 to 300°C is 40 × 10 -7 / °C or more, and -7 / °C or more, and 42 × 10 -7 / °C or more, and more preferably 43 × 10 -7 / °C or more, and more preferably 44 × 10 -7 / °C or more, and more preferably 45 × 10 -7 / °C or more. The average linear expansion coefficient (α) of glasses A and B at 100 to 300°C is 70×10 -7 / °C or less, and -7 / °C or less, and more preferably 65 × 10 -7 / °C or less, and more preferably 63 × 10 -7 / °C or less, and more preferably 60 × 10 -7 / °C or less, and more preferably 57 × 10 -7 / °C or less, and even more preferably 55 x 10 -7 / °C or less, and more preferably 53 x 10 -7 / °C or less, and even more preferably 50 × 10 -7 / °C or less. In one embodiment, the average linear expansion coefficient (α) of glasses A and B at 100 to 300°C is 48 × 10 -7 / °C can be less than 47 x 10 -7 / °C or less.

[0067] (glass stability) Glasses A and B 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 evaluation result in the 1300°C 16-hour holding test is preferably A, more preferably the evaluation result in the 1300°C 16-hour holding test is A and the evaluation result in the 1250°C 16-hour holding test is A or B, and even more preferably the evaluation result in both holding tests is A.

[0068] (foam density) Glasses A and B can also have reduced bubbles by adjusting their compositions as described above. Reducing bubbles is desirable in glasses for magnetic recording medium substrates. This is for the following reasons: With the recent trend toward higher density recording, there is a demand for a narrower distance (called the "flying height") between the head (magnetic head) used to write and read data and the surface of the magnetic recording medium. However, if irregularities due to bubbles exist on the surface of a glass substrate for magnetic recording media, these irregularities are reflected on the surface of the magnetic recording medium, reducing the surface smoothness of the magnetic recording medium. Bringing a magnetic head close to the surface of a magnetic recording medium with poor surface smoothness could result in contact with the surface of the magnetic recording medium and damage the magnetic head. Therefore, a certain level of flying height must be maintained to prevent contact. For these reasons, it is desirable to reduce bubbles in glasses for magnetic recording medium substrates in order to narrow the flying height and produce magnetic recording media with high surface smoothness. Regarding bubbles in the glass, the density of bubbles per unit mass, as measured using an optical microscope (magnification 40 to 100 times) with 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.

[0069] (specific resistance) The resistivity of glasses A and B can be reduced by adjusting the composition as described above. Glasses with low resistivity are preferred in that they are less likely to corrode a melting furnace (e.g., made of brick) when electrical heating is performed during glass melting. When electricity flows through a melting furnace during electrical heating, the furnace is corroded. However, if the resistivity of the glass in the furnace is high, electricity flows more easily through the furnace. Corrosion of the melting furnace can lead to glass leakage from the melting furnace or the incorporation of components resulting from the corrosion of the melting furnace into the glass. Therefore, it is preferable to be able to suppress the corrosion of the melting furnace. The natural logarithm lnρ of the resistivity ρ can be used as an indicator of low resistivity. In one embodiment, the natural logarithm lnρ (unitless) of the resistivity ρ (unit: Ω cm) at 1450°C of Glasses A and B is preferably 3.20 or less, more preferably 3.00 or less, even more preferably 2.90 or less, still more preferably 2.80 or less, even more preferably 2.70 or less, still more preferably 2.68 or less, and still more preferably 2.65 or less. Furthermore, the natural logarithm lnρ of the resistivity ρ at 1450°C can be, for example, 1 or more.

[0070] [Magnetic recording medium substrate] A magnetic recording medium substrate according to one embodiment of the present invention is made of Glass A or Glass B.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] [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.

[0078] 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.

[0079] 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.

[0080] 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 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.

[0081] 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. As described above, when 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. Therefore, a certain flying height must be maintained to prevent this 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. This is because reducing bubbles in the substrate enables the flying height to be narrowed. 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.

[0082] There are no particular limitations on the dimensions of the magnetic recording medium substrate (e.g., magnetic disk substrate) or 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, the nominal diameter can be 2.5 inches, of course, but it can also be made smaller (e.g., 1 inch, 1.8 inches), or even 3 inches, 3.5 inches, etc. Furthermore, the plate thickness can be, for example, 0.3 to 2 mm, but Glasses A and B are suitable for thin plates due to their excellent heat resistance and rigidity. Thinner plates allow for more magnetic recording media to be installed in HDDs, thereby increasing the storage capacity of HDDs. From this perspective, the nominal thickness is preferably 0.635 mm or less, more preferably 0.550 mm or less, even more preferably 0.500 mm or less, and even more preferably 0.400 mm or less.

[0083] [Glass spacer for magnetic recording / reproducing devices] One aspect of the present invention is SiO2 content of 56 to 80 mol%, Li2O content is 1 to 10 mol%, B2O3 content is 0-4 mol%, The total content of MgO and CaO (MgO + CaO) is 9 to 40 mol%, and Specific gravity is 2.75g / cm 3 Hereinafter, a glass spacer for a magnetic recording / reproducing device containing an amorphous oxide glass having a glass transition temperature of 650°C or higher and a Young's modulus of 90 GPa or higher, Regarding.

[0084] Another aspect of the present invention is SiO2 content of 56 to 80 mol%, Li2O content is 1 to 10 mol%, B2O3 content is 0-4 mol%, The total content of MgO and CaO (MgO + CaO) is 9 to 40 mol%, The molar ratio of the total content of SiO2 and ZrO2 to the content of Al2O3 ((SiO2 + ZrO2) / Al2O3) is 2 to 13, and a glass spacer for a magnetic recording / reproducing device, comprising an amorphous oxide glass having a glass transition temperature of 650°C or higher and a Young's modulus of 90 GPa or higher; Regarding.

[0085] 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 for magnetic recording medium substrates, these glass spacers are also desired to have excellent heat resistance and rigidity. In contrast, as detailed above for Glasses A and B, the above glasses can have excellent heat resistance and rigidity, making them suitable as glass spacers for magnetic recording and reproducing devices.

[0086] 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. The glass spacer for a magnetic recording / reproducing device can be made of the above-mentioned glass, or can be configured with one or more films, such as a conductive film, provided on the surface of the above-mentioned glass. For example, to remove static electricity generated during rotation of the magnetic recording medium, a conductive film, such as a NiP alloy, can 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 increase its surface smoothness (for example, to an average surface roughness of 1 μm or less), which strengthens the adhesion between the magnetic recording medium and the spacer and suppresses misalignment.

[0087] [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.

[0088] 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.

[0089] 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]

[0090] 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.

[0091] [Examples No. 1 to No. 76] 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 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) to a range of 1200 to 1400°C for 1 hour, and the molten glass was then formed to obtain glass (amorphous oxide glass) for the evaluations described below. The heating can be performed using various heating methods, such as electrical heating.

[0092] [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).

[0093] (2) Young's modulus The Young's modulus of each glass was measured by an ultrasonic method.

[0094] (3) Specific gravity The specific gravity of each glass was measured by Archimedes' method.

[0095] (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).

[0096] (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.

[0097] (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.

[0098] (7) Natural logarithm lnρ of resistivity ρ(1450℃) The glass to be measured for resistivity was melted to prepare a glass melt at a temperature of t°C. The electrical conductivity σ (unit: S / cm) of this glass melt was measured using Pt-Rh ring electrodes (electrode distance 15 mm) at a voltage of 50 mV and a frequency of 20 kHz. The cell constant was measured using a reference solution with a concentration of 1 mol / dm 3 The electrical conductivity σ was determined using a KCl solution (temperature: 25°C). In the temperature range from about 1200°C to about 1600°C, the electrical conductivity σ was determined for each temperature t at intervals of about 25 to 45°C, and the determined σ was plotted in an Arrhenius plot to obtain a straight line from which the σ at 1450°C was determined. From the σ at 1450°C thus determined, the resistivity ρ at 1450°C was determined using the equation resistivity ρ = 1 / σ, and the natural logarithm lnρ of the determined ρ was calculated.

[0099] The results are shown in the table below.

[0100] [Table 1]

[0101] [Table 2]

[0102] [Table 3]

[0103] [Table 4]

[0104] [Table 5]

[0105] [Table 6]

[0106] [Table 7]

[0107] [Table 8]

[0108] [Table 9]

[0109] [Table 10]

[0110] The results shown in the table above confirm that the glasses for magnetic recording medium substrates of the Examples all have excellent heat resistance and rigidity. Furthermore, it was also confirmed that the glasses for magnetic recording medium substrates of the Examples have low specific gravity and an appropriate thermal expansion coefficient. 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. Furthermore, all of the glasses of the examples shown in the above table were ranked S, A or B in bubble density, confirming that bubble generation was suppressed.

[0111] [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.

[0112] (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.

[0113] [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.

[0114] First, using a vacuum-drawn film-forming apparatus, an adhesive layer, an underlayer, and a magnetic recording layer were sequentially formed in an Ar atmosphere by DC magnetron sputtering.

[0115] 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.

[0116] 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 Heating may be performed to create an ordered structure.

[0117] 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.

[0118] Furthermore, a glass spacer obtained by the above manufacturing process using the glass of the example and having a conductive NiP alloy film formed on its surface (a glass spacer with a NiP alloy film) was mounted 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 that of one embodiment of the glass of the present invention (glass A or glass B). No collision between the magnetic head and the magnetic disk surface (crash failure) was observed.

[0119] 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, it is clear that the phenomenon that can occur due to the difference in thermal expansion coefficient described above does not occur.

[0120] According to one aspect of the present invention, it is possible to provide a magnetic recording medium suitable for high-density recording.

[0121] Finally, the above-mentioned aspects will be summarized.

[0122] According to one aspect, SiO2 content: 56-80 mol%, Li2O content: 1-10 mol%, B2O3 content: 0-7 mol%, total content of MgO and CaO (MgO+CaO): 9-40 mol% and the specific gravity is 2.75 g / cm 3 Glass for magnetic recording medium substrates, which is an amorphous oxide glass having a glass transition temperature of 650°C or higher and a Young's modulus of 90 GPa or higher; and a glass for a magnetic recording medium substrate which is an amorphous oxide glass having an SiO2 content of 56 to 80 mol%, a Li2O content of 1 to 10 mol%, a B2O3 content of 0 to 4 mol%, a total MgO and CaO content (MgO+CaO) of 9 to 40 mol%, and a molar ratio of the total SiO2 and ZrO2 content to the Al2O3 content ((SiO2+ZrO2) / Al2O3) of 2 to 13, and which has a glass transition temperature of 650°C or higher and a Young's modulus of 90 GPa or higher; is provided.

[0123] Also, according to one aspect, a glass spacer for a magnetic recording / reproducing device, comprising an amorphous oxide glass having an SiO2 content of 56 to 80 mol%, an Li2O content of 1 to 10 mol%, an B2O3 content of 0 to 4 mol%, and a total content of MgO and CaO (MgO+CaO) of 9 to 40 mol%, a specific gravity of 2.75 g / cm3 or less, a glass transition temperature of 650°C or more, and a Young's modulus of 90 GPa or more; and a glass spacer for a magnetic recording / reproducing device comprising an amorphous oxide glass having an SiO2 content of 56 to 80 mol%, an Li2O content of 1 to 10 mol%, a B2O3 content of 0 to 7 mol%, a total content of MgO and CaO (MgO+CaO) of 9 to 40 mol%, a molar ratio of the total content of SiO2 and ZrO2 to the content of Al2O3 ((SiO2+ZrO2) / Al2O3) of 2 to 13, and having a glass transition temperature of 650°C or higher and a Young's modulus of 90 GPa or higher; is provided.

[0124] The glass for a magnetic recording medium substrate has excellent heat resistance and rigidity, as does the glass spacer for a magnetic recording / reproducing device.

[0125] In one embodiment, the average linear expansion coefficient of the oxide glass at 100 to 300°C is 40 × 10 -7 ~70×10 -7 / °C range.

[0126] In one embodiment, the CaO content of the oxide glass can be in the range of 0 to 18 mol %.

[0127] In one embodiment, the total content of BaO and SrO in the oxide glass can be in the range of 0 to 2 mol %.

[0128] In one embodiment, the total content of Li2O, Na2O and K2O (Li2O+Na2O+K2O) in the oxide glass can be in the range of 2.5 to 10 mol %.

[0129] In one embodiment, the total content of SnO2 and CeO2 (SnO2+CeO2) in the oxide glass can be in the range of 0.05 to 2 mol%.

[0130] In one embodiment, the molar ratio of the SnO2 content to the CeO2 content (SnO2 / CeO2) of the oxide glass can be 1 or greater.

[0131] In one embodiment, the oxide glass has an SiO2 content of 56 to 65% in mole percent. Al2O3 content is 5-20%, B2O3 content is 0-4%, MgO content is 3-28%, Li2O content is 1-10%, the total content of SiO2 and Al2O3 (SiO2 + Al2O3) is 65-80%, the total content of MgO and CaO (MgO + CaO) is 11-30%, the total content of MgO, CaO, SrO and BaO (MgO + CaO + SrO + BaO) is 12-30%, the sum of MgO content, 0.7 × CaO content, Li2O content, TiO2 content and ZrO2 content (MgO + 0.7 × CaO + Li2O + TiO2 + Zr O2) is 16% or more, 5×Li2O content, 3×Na2O content, 3×K2O content, 2×B2O3 content, MgO content, 2×CaO content, 3×SrO content and sum of BaO content (5Li2O+3Na2O+3K2O+2B2O3+MgO+2CaO+3S rO+BaO) is 32~58%, SiO2 content, Al2O3 content, B2O3 content, P2O5 content, 1.5× Na2O content, 1.5× K2O content, 2× SrO content, 3× Sum of BaO content and ZnO content (SiO2+Al2O3+B2O3+P2O5 The sum of the SiO2 content, Al2O3 content, B2O3 content, P2O5 content, Na2O content, K2O content, CaO content, 2×SrO content, and 3×BaO content (SiO2+Al2O3+B2O3+P2O5+Na2O+K2O+CaO+2SrO+3BaO) is 92% or less; the molar ratio of the CaO content to the MgO content (CaO / MgO) is 2.5 or less; and the molar ratio of the Na2O content to the Li2O content (Na2O / Li2O) is 5 or less. The molar ratio of the Li2O content to the total content of MgO and CaO (Li2O / (MgO+CaO)) is 0.03-0.4, the molar ratio of the SiO2 content to the total content of Li2O, Na2O, and K2O (SiO2 / (Li2O+Na2O+K2O)) is 4-22, the molar ratio of the total content of SiO2 and ZrO2 to the total content of Al2O3 ((SiO2+ZrO2) / Al2O3) is 2-10, and the molar ratio of the total content of TiO2 and Al2O3 to the total content of MgO and CaO ((TiO2+Al2O3) / (MgO+CaO)) is 0.The molar ratio of the total content of MgO and CaO to the total content of MgO, CaO, SrO, and BaO ((MgO + CaO) / (MgO + CaO + SrO + BaO)) can be 0.7 to 1, the molar ratio of the content of BaO to the total content of MgO, CaO, SrO, and BaO (BaO / (MgO + CaO + SrO + BaO)) can be 0.1 or less, and the molar ratio of the content of P2O5 to the total content of B2O3, SiO2, Al2O3, and P2O5 (P2O5 / (B2O3 + SiO2 + Al2O3 + P2O5)) can be 0.005 or less.

[0132] According to one embodiment, there is provided a magnetic recording medium substrate made of the above-mentioned glass for magnetic recording medium substrates.

[0133] In one embodiment, the magnetic recording medium substrate has a homogeneous composition on the surface and in the interior.

[0134] According to one embodiment, there is provided a magnetic recording medium having a magnetic recording layer on the above magnetic recording medium substrate.

[0135] According to one aspect, there is provided a magnetic recording and reproducing device including at least one of the above magnetic recording medium and the above glass spacer for a magnetic recording and reproducing device.

[0136] 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. SiO 2 The content is 56 to 80 mol %, Li 2 an O content of 2.00 mol% or less; B 2 O 3 The content is 0 to 4 mol %, The total content of MgO and CaO (MgO + CaO) is 9 to 40 mol %, and Specific gravity is 2.75 g / cm 3 and a glass transition temperature of 650°C or higher.

2. SiO 2 The content is 56 to 80 mol %, B 2 O 3 The content is 0 to 4 mol %, Li 2 O, Na 2 O and K 2 Li relative to the total content of O 2 Molar ratio of O content (Li 2 O / (Li 2 O + Na 2 O+K 2 O)) is 0.50 or less, The total content of MgO and CaO (MgO + CaO) is 9 to 40 mol %, and Specific gravity is 2.75 g / cm 3 and a glass transition temperature of 650°C or higher.

3. SiO 2 The content is 56 to 80 mol %, B 2 O 3 The content is 0 to 4 mol %, Li 2 Na content relative to O content 2 Molar ratio of O content (Na 2 O / Li 2 O) is 1.25 or more, The total content of MgO and CaO (MgO + CaO) is 9 to 40 mol %, and Specific gravity is 2.75 g / cm 3 and a glass transition temperature of 650°C or higher.

4. Li 2 3. The glass of claim 2, wherein the O content is 2.00 mol % or less.

5. Li 2 The glass according to claim 3, wherein the O content is 2.00 mol % or less.

6. Li 2 Na content relative to O content 2 Molar ratio of O content (Na 2 O / Li 2 3. The glass of claim 2 wherein .theta..sub.O) is 1.25 or greater.

7. Li 2 Na content relative to O content 2 Molar ratio of O content (Na 2 O / Li 2 5. The glass of claim 4, wherein .O) is 1.25 or greater.

8. SiO 2 The content is 56 to 80 mol %, Li 2 an O content of 10 mol% or less; B 2 O 3 The content is 0 to 4 mol %, SiO 2 and Al 2 O 3 The total content (SiO 2 +Al 2 O 3 ) is 76.60 mol % or more, The total content of MgO and CaO (MgO + CaO) is 9 to 40 mol %, and Specific gravity is 2.75 g / cm 3 and a glass transition temperature of 650°C or higher.

9. SiO 2 and Al 2 O 3 The total content (SiO 2 +Al 2 O 3 8. The glass according to claim 1, wherein the content of C₂O₁₄ is 76.60 mol % or more.

10. Li 2 O, Na 2 O and K 2 The total content of O (Li 2 O + Na 2 O+K 2 10. The glass of claim 1, wherein O) is in the range of 2.5 to 10 mol %.

11. A magnetic recording medium substrate made of the glass according to any one of claims 1 to 10.

12. A magnetic recording medium comprising the magnetic recording medium substrate according to claim 11 and a magnetic recording layer.

13. A magnetic recording and reproducing device comprising the magnetic recording medium according to claim 12 and a magnetic head.

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