Magnetic disk substrate and magnetic disk

Abstract

To provide a magnetic disk substrate and a magnetic disk capable of suppressing particle generation due to external impact, even when the diameter of the magnetic disk substrate is increased and the plate thickness is thinned.SOLUTION: A disk-shaped magnetic disk substrate 1 has a diameter D of 85 mm or more, a plate thickness T of 0.6 mm or less, and is made of a material with a Young's modulus E of 90 GPa or more. It is preferable that the diameter D be 90 mm or more, and that when a 70[G] impact is applied to the substrate in the direction normal to the main surface for 2 [m second] with the inner peripheral edge of the substrate fixed, the maximum amplitude of vibration in the thickness direction of the outer peripheral edge of the substrate is 0.25 mm or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a magnetic disk substrate and a magnetic disk. [Background technology]

[0002] Glass substrates and aluminum alloy substrates have traditionally been used as magnetic disk substrates. Magnetic disks are formed on the main surfaces of these substrates, with magnetic films formed on them. It is desirable for magnetic disks to have few surface defects, to have no problems with reading and writing information, and to be able to read and write large amounts of information. Furthermore, in response to demands for increased storage capacity in hard disk drive devices (hereinafter referred to as HDDs), efforts are being made to increase the density of magnetic recording.

[0003] For example, when increasing the density of magnetic recording, a method for manufacturing a glass substrate for a magnetic disk is known in which the surface roughness (arithmetic mean roughness Ra) of the glass substrate for a magnetic disk is reduced so that reading and writing can be performed without difficulty using a magnetic head (DFH head) equipped with a DFH (Dynamic Flying Height) mechanism (Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2014 / 051153 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, the hard disk drive industry has seen a slowdown in the pace of improvements in recording density, as the miniaturization of magnetic particles in magnetic disks approaches its limits. Meanwhile, demand for increased storage capacity in HDDs is becoming increasingly intense due to big data analysis and other factors. As a result, increasing the number of magnetic disks installed in a single HDD is being considered. To store such large amounts of data, 3.5-inch HDDs are typically used. In order to increase the storage capacity by increasing the number of magnetic disks installed in an HDD, it is necessary to reduce the thickness of the magnetic disk substrate, which accounts for the majority of the thickness of the magnetic disk within the limited space inside the HDD. Furthermore, in order to increase the storage capacity, it is also desirable to increase the size of the magnetic disk.

[0006] However, it has been found that increasing the diameter and thinning the thickness of magnetic disk substrates reduces the rigidity of the substrate, making it more susceptible to large vibrations and making it difficult to contain those vibrations. For example, cloud data centers use an extremely large number of HDDs, and HDDs are frequently replaced due to failure. It has been found that the impact of installing a new HDD in the rack can cause failure or shorten the time until failure. Further detailed investigation revealed that when an HDD is subjected to external impact, it can be damaged even though the magnetic disk is not yet rotating because power is not yet being supplied to the HDD.

[0007] Unlike steady-state flutter vibrations caused by the rotating magnetic disk and the surrounding airflow, vibrations caused by external impacts decay over time. However, if the amplitude of these vibrations is large, they are more likely to come into contact with the ramp inside the HDD and with adjacent magnetic disks. Furthermore, the topmost magnetic disk of a group of magnetic disks spaced at regular intervals may come into contact with the ceiling of the HDD's magnetic disk storage container. Such contact can cause chipping of the magnetic disk at the contacted portion, generating particles. Particles can also be generated by friction and abrasion. These particles often scatter within the storage container and adhere to the large read / write area (main surface) of the magnetic disk. As the diameter of magnetic disk substrates increases and the thickness decreases, vibrations due to external shocks and the resulting generation of particles, which previously did not pose a problem, become non-negligible. In particular, for large magnetic disk substrates with magnetic disks nominally 3.5 inches (e.g., 95 mm in diameter) or larger, the problem of particles generated by the contact caused by substrate vibrations cannot be ignored.

[0008] Therefore, an object of the present invention is to provide a magnetic disk substrate and a magnetic disk that can suppress the generation of particles caused by external impacts even when the diameter of the magnetic disk substrate is increased and the plate thickness is reduced. [Means for solving the problem]

[0009] One aspect of the present invention is a disk-shaped magnetic disk substrate. The diameter D of the substrate is 85 mm or more, and the thickness T of the substrate is 0.6 mm or less, The substrate is made of a material having a Young's modulus E of 90 GPa or more.

[0010] The diameter D is preferably 90 mm or more. It is preferable that when an impact of 70 [G] is applied to the substrate in the normal direction of the main surface of the substrate for 2 [ms] with the inner peripheral edge of the substrate fixed, the maximum amplitude of vibration in the thickness direction of the outer peripheral edge of the substrate is 0.25 mm or less.

[0011] Another aspect of the present invention is a disk-shaped magnetic disk substrate. The diameter D of the substrate is 85 mm or more, and the thickness T of the substrate is 0.6 mm or less, When an impact of 70 [G] is applied to the substrate in the normal direction of the main surface of the substrate for 2 [ms] with the inner peripheral edge of the substrate fixed, the maximum amplitude of vibration in the thickness direction of the outer peripheral edge of the substrate is 0.25 mm or less.

[0012] The diameter D is preferably 90 mm or more. The substrate is preferably a glass substrate made of glass having a glass transition point of 650° C. or higher.

[0013] It is preferable that the change in flatness of the substrate after heating at 730° C. and the flatness of the substrate before heating is 4 μm or less.

[0014] The substrate has a linear expansion coefficient of 70×10 -7 It is preferable that the material be [1 / K] or less.

[0015] The Vickers hardness Hv of the substrate is 650 [kgf / mm 2 ] or more.

[0016] The Knoop hardness Hk of the substrate is 600 [kgf / mm 2 ] or more.

[0017] a chamfered surface is provided on at least the outer peripheral edge surface of the substrate; It is preferable that the ratio W1 / R of the width W1 of the chamfered surface along the radial direction of the substrate to the radius R of the substrate is 0.0025 or less.

[0018] It is preferable that the ratio (2·W2) / T of twice the width W2 of the chamfered surface along the thickness direction of the substrate to the thickness T is 0.4 or less.

[0019] With respect to the Young's modulus E of the substrate and the thickness T, E·T 3 The value of 3 to 18 [GPa mm 3 ] is preferable.

[0020] The Q value of the material at 3000 Hz at room temperature is preferably 1500 or less.

[0021] ρ is the density of the material at room temperature [g / cm 3 ], Q is the Q value of the material at room temperature at 3000 Hz, E is the Young's modulus [GPa] of the material at room temperature, and ν is the Poisson's ratio of the material at room temperature, and ρ (1-ν) of the material 2 · Q / E value is 25 [g / cm 3 / GPa] or less.

[0022] Another aspect of the present invention is a magnetic disk having at least a magnetic film on the surface of the magnetic disk substrate. [Effects of the Invention]

[0023] According to the magnetic disk substrate and magnetic disk described above, even if the diameter of the magnetic disk substrate is increased and the plate thickness is reduced, the generation of particles caused by external impact can be suppressed. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a diagram showing an example of the external shape of a magnetic disk substrate according to an embodiment; [Figure 2] 1A and 1B are diagrams illustrating an example of vibration of a magnetic disk substrate according to an embodiment. [Figure 3] 2 is a diagram illustrating an example of an outer peripheral end portion of a magnetic disk according to an embodiment. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0025] The magnetic disk substrate of the present invention will be described in detail below. In the following description, a magnetic disk glass substrate will be used, but the magnetic disk substrate may be a non-magnetic metal substrate in addition to a glass substrate. In the case of a glass substrate, aluminosilicate glass, soda-lime glass, borosilicate glass, etc. can be used as the glass. In particular, amorphous aluminosilicate glass can be preferably used because it can be chemically strengthened as necessary and can produce a magnetic disk glass substrate excellent in flatness of the main surface and substrate strength. Examples of materials that can be used for the metal substrate include aluminum alloys, titanium alloys, and single crystal Si. A magnetic disk is produced by forming at least a magnetic film on the surface of the magnetic disk substrate.

[0026] Fig. 1 is a diagram showing the external shape of a magnetic disk substrate according to this embodiment. As shown in Fig. 1, a magnetic disk substrate 1 (hereinafter simply referred to as substrate 1) according to this embodiment is a thin, disk-shaped substrate having an inner hole 2. A magnetic disk is produced by forming films such as a magnetic film, an underlayer, and a soft magnetic layer on this substrate 1. There are no restrictions on the size of the substrate 1 as long as the diameter D is 85 mm or more, preferably 90 mm or more, but the substrate 1 is suitable for use as a magnetic disk substrate with a nominal diameter of 3.5 inches, for example. In the case of a magnetic disk substrate with a nominal diameter of 3.5 inches, the diameter D (outer diameter) of the disk shape is 85 mm or more, preferably 90 mm or more. Specifically, the nominal value of the outer diameter of the disk shape is 95 mm or 97 mm. As described above, the amplitude of vibration of the magnetic disk caused by an external impact different from flutter vibration increases and becomes more difficult to attenuate as the outer diameter of the substrate 1 increases. Therefore, the substrate 1 of this embodiment is preferable when used with magnetic disks with a nominal size of 3.5 inches or larger.

[0027] In this embodiment, the diameter D of the substrate 1 is 85 mm or more, preferably 90 mm or more, and the thickness T of the substrate 1 is 0.6 mm or less. Furthermore, the substrate 1 is made of a material with a Young's modulus E of 90 GPa or more. The substrate 1 has a larger diameter D and a thinner thickness T than conventional substrates. This allows for a larger number of magnetic disks to be incorporated into the HDD, thereby increasing storage capacity. Because the diameter D of the substrate 1 is large and the thickness T is thin, as described above, vibrations of the substrate 1 caused by impacts during installation of the HDD can cause the magnetic disk to come into contact with the ramp inside the HDD and with adjacent magnetic disks inside the HDD. Furthermore, the magnetic disk located at the top of the magnetic disks may come into contact with the ceiling surface of the magnetic disk storage container of the HDD. The upper limit of the diameter D of the substrate 1 is, for example, 100 mm, in consideration of the standard size of a nominal 3.5-inch HDD. The lower limit of the thickness T of the substrate 1 is, for example, 0.30 mm, in consideration of suppressing arcing when a bias voltage is applied during the film formation process. There is no particular need to set an upper limit for the Young's modulus E, but from the viewpoint of ease of processing, it is, for example, 120 GPa.

[0028] FIG. 2 illustrates an example of vibration of the substrate 1 caused by the impact. This vibration differs from steady-state flutter vibration, which occurs during steady rotation due to the rotating magnetic disk and the surrounding airflow. Impact-induced vibration is vibration in which the main surface of the substrate 1 is displaced out of plane relative to the main surface. In particular, in a HDD, the inner peripheral edge is fixed to the spindle, while the outer peripheral edge is free and displaces out of plane relative to the main surface. Such vibration displaced out of plane relative to the main surface (vibration in the thickness direction) can cause contact with the ramp inside the HDD, adjacent magnetic disks, and even the ceiling of the magnetic disk storage container. Such contact can chip the magnetic disk at the contacted portion, generating particles. The generated particles often scatter within the storage container and adhere to the read / write area of ​​the magnetic disk. For this reason, in this embodiment, a material with a Young's modulus of 90 GPa or higher is used for the substrate 1.

[0029] When the substrate 1 is a glass substrate, for example, the following glass composition can provide an amorphous oxide glass having a Young's modulus of 90 GPa or more.

[0030] (Glass 1) SiO2 56-80 mol%, 1 to 10 mol% of Li2O, 0 to 4 mol% of B2O3, The total content of MgO and CaO (MgO + CaO) is 9 to 40 mol%, is. The specific gravity of glass 1 is 2.75 g / cm 3 Hereinafter, the glass transition temperature Tg is 650°C or higher.

[0031] (Glass 2) SiO2 56-80 mol%, 1 to 10 mol% of Li2O, 0 to 4 mol% of B2O3, The total content of MgO and CaO (MgO + CaO) is 9 to 40 mol%, and The molar ratio of the total content of SiO2 and ZrO2 to the content of Al2O3 ((SiO2 + ZrO2) / Al2O3) is 2 to 13, is. The glass transition temperature Tg of glass 2 is 650° C. or higher.

[0032] (Glass 3) In mole percent, SiO2 56-65%, Al2O3 5-20%, B2O3 0~4%, MgO 3-28%, Li2O 1-10%, and The total content of SiO2 and Al2O3 (SiO2+Al2O3) is 65-80%. Total content of MgO and CaO (MgO+CaO) 11~30% The total content of MgO, CaO, SrO and BaO (MgO+CaO+SrO+BaO) is 12-30%; Sum of MgO content, 0.7×CaO content, Li2O content, TiO2 content and ZrO2 content (MgO+0.7CaO+Li2O+TiO2+ZrO2) 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+3SrO+BaO) 32~58%, Sum of 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) 86% or less, and The sum of 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) 92% or less, and The molar ratio of CaO to MgO (CaO / MgO) is 2.5 or less; The molar ratio of the NaO content to the LiO content (NaO / LiO) 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 to 0.4, The molar ratio of the SiO2 content to the total content of Li2O, Na2O, and K2O (SiO2 / (Li2O+Na2O+K2O)) is 4 to 22, The molar ratio of the total content of SiO2 and ZrO2 to Al2O3 ((SiO2+ZrO2) / Al2O3) is 2 to 10, The molar ratio of the total content of TiO2 and Al2O3 to the total content of MgO and CaO ((TiO2+Al2O3) / (MgO+CaO)) is 0.35 to 2. 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 0.7 to 1, the molar ratio of the BaO content to the total content of MgO, CaO, SrO, and BaO (BaO / (MgO+CaO+SrO+BaO)) is 0.1 or less; The molar ratio of the P2O5 content to the total content of B2O3, SiO2, Al2O3 and P2O5 (P2O5 / (B2O3+SiO2+Al2O3+P2O5)) is 0.005 or less; and Glass transition temperature is 670°C or higher and Young's modulus is 90GPa or higher. Specific gravity is 2.75 or less, Average linear expansion coefficient at 100-300°C is 40 x 10 -7 ~70×10 -7 / °C range.

[0033] In this case, when an impact (acceleration) of 70 G is applied to the substrate 1 in the normal direction to the main surface of the substrate 1 for 2 ms with the inner peripheral edge of the substrate 1 fixed, it is preferable that the maximum amplitude of vibration in the thickness direction of the outer peripheral edge of the substrate 1 is 0.25 mm or less. By keeping the maximum amplitude at 0.25 mm or less, the above-mentioned contact can be prevented. The above impact test was conducted using an AVEX-SM-110-MP testing machine manufactured by Air Brown Co., Ltd. Therefore, in one embodiment, the diameter D of the substrate 1 is 85 mm or more, preferably 90 mm or more, the thickness T of the substrate 1 is 0.6 mm or less, and when an impact of 70 [G] is applied to the substrate 1 for 2 [ms] in the normal direction to the main surface of the substrate 1 with the inner peripheral edge of the substrate 1 fixed, the maximum amplitude due to vibration in the thickness direction of the outer peripheral edge of the substrate 1 is 0.25 mm or less.

[0034] According to one embodiment, the substrate 1 is preferably made of glass having a glass transition temperature Tg of 650°C or higher, and more preferably a glass transition temperature Tg of 680°C or higher. The higher the glass transition temperature Tg, the higher the thermal durability, and the more effectively the substrate 1 can be prevented from deforming, for example reducing flatness, when it is heat-treated. In consideration of the heat treatment that is performed when forming a magnetic film or the like of a magnetic disk on the substrate 1, the glass transition temperature Tg is preferably set to 650°C or higher in order to prevent thermal deformation. Specifically, when a magnetic disk is fabricated by forming a metal film containing a magnetic film or the like on the substrate 1 to a thickness of about 30 nm, the substrate 1 is heated. During this heating process, the substrate 1 is susceptible to deformation due to thermal history. For this reason, according to one embodiment, it is preferable that the change in flatness of the substrate 1 after heating at 730°C and the flatness of the substrate 1 before heating (flatness after heating - flatness before heating) be 4 μm or less. By limiting the change in flatness in this way, a flat magnetic disk can be obtained, and micro-vibrations during rotation of the magnetic disk can also be reduced.

[0035] According to one embodiment, the substrate 1 has a linear expansion coefficient of 70×10 -7 It is preferable that the material has a linear expansion coefficient of 60×10 -7 The lower limit of the linear expansion coefficient of the substrate 1 is, for example, 40×10 -7 [1 / K]. The linear expansion coefficient here is the linear expansion coefficient calculated from the difference in thermal expansion between 100°C and 300°C. By using such a linear expansion coefficient, it is possible to suppress thermal expansion during the heat treatment when forming a magnetic film, etc., and when the gripping member of the film forming apparatus fixes and grips the substrate 1 at the outer peripheral end surface (hereinafter referred to as the outer peripheral end surface), it is possible to suppress thermal distortion of the substrate 1 around the gripped portion. For example, the linear expansion coefficient of a conventional aluminum alloy substrate is 242×10 -7 [1 / K], and for conventional glass substrates, it is 95 × 10 -7 [1 / K] or more, whereas the linear expansion coefficient of the glass substrate 1 of one embodiment is 51×10 -7[1 / K]. On the other hand, there is no need to set a lower limit for the linear expansion coefficient of the substrate 1. However, if the linear expansion coefficient of the substrate 1 becomes too small, when the temperature inside the HDD rises, the spindle may expand and come into contact with and press against the circular hole in the substrate, causing deformation of the substrate. Therefore, for example, if the lower limit for the linear expansion coefficient is set to 20×10 -7 It is even more preferable to set it to [1 / K].

[0036] According to one embodiment, the Vickers hardness Hv of the substrate 1 is 650 [kgf / mm 2 According to one embodiment, the Knoop hardness Hk of the substrate 1 is preferably 600 [kgf / mm 2 ] or more. By increasing the Vickers hardness Hv or Knoop hardness Hk, it is possible to prevent the substrate 1 from chipping and generating particles even when the substrate 1 comes into contact with another substrate 1 or with another member due to vibration caused by an external impact. Furthermore, when the outer peripheral end face is gripped by a gripping member of a film-forming device during film formation, for example, a part of the outer peripheral end face is chipped due to gripping, generating particles, and it is possible to prevent these particles from adhering to the main surface of the substrate 1. For example, the Vickers hardness Hv of an aluminum alloy substrate is 128 [kgf / mm 2 ], and for conventional glass substrates, it is 620 [kgf / mm 2 ], whereas the glass substrate 1 of one embodiment has a resistance of 741 [kgf / mm 2 ].

[0037] Figure 3 is an enlarged view of an example of the edge of a magnetic disk. Figure 3 shows two adjacent magnetic disks in a HDD. In Figure 3, the film thickness of the magnetic film, etc. is much smaller than the thickness of the substrate 1, on the order of several tens of nanometers, so the magnetic film, etc. are not shown. The substrate 1 has a pair of main surfaces 3, side wall surfaces 4 arranged in a direction perpendicular to the pair of main surfaces 3, and a pair of chamfered surfaces 5 arranged between the pair of main surfaces 3 and the side wall surfaces 4. The side wall surfaces 4 and the chamfered surfaces 5 are formed on the outer peripheral edge and the inner peripheral edge of the substrate 1, respectively.

[0038] In an HDD, magnetic disks 10, each having a magnetic film or the like (not shown) formed on the surface of a substrate 1, are arranged side by side as shown in Figure 3. When the magnetic disks 10 vibrate in the normal direction of the main surface 3 and come into contact with the adjacent magnetic disks 10, corners 6, which are the connection points between the main surface 3 at the outer periphery and the chamfered surface 5, are likely to come into contact. This contact causes a large impact on the chamfered surface 5 near the corners 6. The chamfered surface 5 is formed by grinding with a shaped grinding wheel and then polishing the edge with a brush or other tool. It has been found that substrates made of glass materials with a Young's modulus E of 90 GPa or higher have more latent cracks or microcracks on the surface of the chamfered surface 5 than substrates made of conventional glass with a lower Young's modulus. While the cause is not entirely clear, it is presumed that, for example, high-Young's modulus glass generally has high hardness, which increases the load during grinding, resulting in deeper cracks than conventional glass, some of which remain as latent or microcracks after polishing. In this state, if the corner 6 is subjected to impact due to contact with a ramp or other component, the latent or microcracks may propagate, resulting in chipping of the corner 6 or part of the chamfered surface 5 of the substrate 1 in the magnetic disk 10, potentially generating particles along with the magnetic film thereon. In such cases, the greater the width W1 (described below), the more likely the corner 6 or part of the chamfered surface 5 is to chip. Particles generated at the corners 6 or the chamfered surfaces 5 are scattered within the HDD container and often adhere to the read / write area of ​​the magnetic disk 10 .

[0039] For this reason, according to one embodiment, it is preferable to reduce the machining allowance when forming the chamfered surface 5 in the chamfering process. When the magnetic disk 10 has a diameter of 85 mm or more, preferably 90 mm or more, and a thickness of 0.600 mm or less, the corner 6 is prone to contact due to vibration. Therefore, it is preferable to reduce the machining allowance when forming the chamfered surface 5 to reduce the number of latent cracks and microcracks. This can prevent particle generation, even if the corner 6 of the magnetic disk 10 comes into contact with a ramp or other component due to strong vibration. Here, the machining allowance when forming the chamfered surface 5 refers to the width W1, which will be described later. Alternatively, the outer diameter may be adjusted by removing a certain amount of the outer periphery at the beginning of grinding. However, research by the present inventors has shown that such initial machining allowance does not affect latent cracks. This is presumably because latent cracks and other defects caused by the initial machining allowance are eliminated by the final machining allowance. Specifically, the chamfered surface 5 is provided at least on the outer peripheral edge of the substrate 1. In this case, the width W1 (see FIG. 3 ) of the chamfered surface 5 along the radial direction of the substrate 1 is preferably 120 μm or less. More preferably, the width W1 is 90 μm or less. By adjusting the machining allowance when forming the chamfered surface 5 so that the width W1 is 120 μm or less, the number of latent cracks or microcracks on the chamfered surface 5 can be reduced, even when the magnetic disk 10 using a high Young's modulus glass substrate has a large diameter and a thin plate thickness, making the corners 6 more likely to come into contact due to vibration. Therefore, even if the corners 6 come into contact, chipping of the corners 6 or the chamfered surface 5 can be suppressed, preventing particle generation. The lower limit of the width W1 is, for example, 20 μm. If the width W1 is less than 20 μm, the chamfered surface 5 is too small, which may result in chipping during the substrate manufacturing process or film formation process after shaping. 3, the chamfered surface 5 may have a linear shape or an outwardly convex arc or curved shape in a cross section taken along the radial direction passing through the center of the magnetic disk 10. In this case, the chamfered surface 5 refers to a portion where the inclination angle of the tangent at each position relative to the side wall surface 4 and the main surface 3 is 5 to 85 degrees. FIG. 3 shows the chamfered surface 5 where the inclination angle θ1 relative to the main surface 3 is constant.

[0040] According to one embodiment, the ratio (2·W2) / T of twice the width W2 (see FIG. 3) of chamfered surface 5 at the outer periphery of substrate 1 along the thickness direction of substrate 1 to the thickness T is preferably 0.4 or less. The ratio (2·W2) / T is more preferably 0.3 or less. If the ratio (2·W2) / T exceeds 0.4, sidewall surface 4 becomes too small, which may cause chipping or cracking of the outer peripheral edge due to gripping or application of a bias voltage when forming a magnetic film or the like.

[0041] Furthermore, according to one embodiment, the Young's modulus E [GPa] and the plate thickness T [mm] are expressed as follows: E·T 3 The value is 3 to 18 GPa / mm 3 ], and 3 to 16 [GPa / mm 3 ], and more preferably 5 to 15 [GPa / mm 3 ] is particularly preferred. 3 If the value of E·T is in this range, contact of the magnetic disk 10 due to vibration can be suppressed. 3 The value of 3 [GPa / mm 3 If the magnetic disk 10 is less than 1 / 2, the magnetic disk 10 is likely to come into contact with the magnetic disk 10 due to vibration. 3 The value is 18 [GPa / mm 3 ], the plate thickness cannot be increased in order to ensure the number of magnetic disks 10 that can be built into the HDD, and so Young's modulus E is increased. In this case, as Young's modulus E increases, substrate 1 tends to become harder than necessary, and the polishing time for main surface 3 becomes longer, which is undesirable in terms of the production efficiency of magnetic disk substrate 1. According to one embodiment, Young's modulus E [GPa] and density ρ [g / cm 3 ], the specific elastic modulus calculated by E / ρ is 36 [GPa cm 3 / g] or more. Even if the Young's modulus is high, if the density is high, the weight of the substrate itself may cause large vibrations. There is no need to set an upper limit for the specific modulus, but from the viewpoint of productivity, it is recommended to set it to, for example, 41 [GPa cm]. 3 / g] may also be used.

[0042] When such substrates 1 are mounted on an HDD, the number of substrates 1 mounted on the HDD depends on the thickness T of the substrates 1. The film thickness of the magnetic film formed on the substrate 1 is several tens of nanometers, which is thin enough to be ignored compared to the thickness T of the substrate 1. For example, if the thickness T of the substrate 1 is 0.635 mm, nine or more substrates 1 can be mounted; if the thickness T of the substrate 1 is 0.5 mm, ten or more substrates 1 can be mounted; and if the thickness T of the substrate 1 is 0.38 mm, twelve or more substrates 1 can be mounted. In this way, the number of substrates 1 mounted varies depending on the thickness T of the substrate 1. Therefore, the storage capacity can be increased by reducing the thickness T of the substrate 1 and increasing the number of substrates mounted. In this case, as described above, the substrate 1 of this embodiment is less likely to come into contact with other substrates 1 or components due to vibrations, even if the thickness T is reduced. Furthermore, according to one embodiment, it is preferable that the substrate 1 does not break in an impact test in which the inner peripheral edge of the substrate 1 is fixed to reproduce the clamping of a magnetic disk and an impact of 600 G is applied to the substrate 1 for 2 ms. Such a substrate 1 is preferable from the viewpoint of improving durability, since the substrate 1 does not break even when a large impact is applied to the HDD. This substrate 1 can be achieved, for example, by using the above-mentioned glasses 1 to 3.

[0043] According to one embodiment, the material of the substrate 1 preferably has a Q value (quality factor) of 1500 or less at room temperature (25°C) and 3000 Hz. The Q value is the vibration energy stored in the vibrating substrate 1 during one cycle divided by the energy dissipated from the vibrating substrate 1; the smaller this value, the greater the vibration damping. Therefore, the smaller the Q value of a material used, the faster the vibration damping can be. This reduces the number of contacts with adjacent substrates 1 or lamps in the HDD and also mitigates the impact of contact, making it possible to suppress adverse effects such as particle generation due to contact. The Q value at room temperature and 3000 Hz is more preferably 1300 or less. The Q value at 3000 Hz was obtained as follows. First, a laser Doppler vibrometer (LDV) was used to generate vibrations on the substrate 1 rotated on a spin stand. The laser was then applied to the approximate outer periphery of the substrate 1 to measure the vibrations. The obtained data was then appropriately Fourier-transformed to obtain a frequency response function (horizontal axis: frequency (unit: Hz), vertical axis: NRRO (Non-Repeatable Runout) Amplitude (unit: nm)). Next, for each peak observed in the frequency response function, the Q value (= f0 / (f2-f1)) was calculated using the half-width method (a calculation method using the frequencies f1 and f2 (>f1) corresponding to values ​​3 dB lower than the NRRO peak value and the frequency f0 (resonance frequency) corresponding to the peak value). The obtained measurement results were plotted on an XY plane with frequency on the horizontal axis and Q value on the vertical axis. Linear approximation by the least-squares method was performed to obtain an approximate line. The Q value at 3000 Hz was obtained on the obtained approximate line or, if necessary, by extrapolating the approximate line. The substrate 1 used for evaluation using the laser Doppler vibrometer had an outer diameter of 95 mm, an inner diameter of 25 mm, and a thickness of 0.635 mm. The substrate rotation speed was 6900 rpm, and the measurement position was a radius of 46.5 mm from the center of the substrate (1 mm inside from the outer edge), and the measurement was performed at room temperature.

[0044] According to one embodiment, ρ is defined as the density at room temperature of the material of the substrate 1 [g / cm 3 ], Q is the Q value of the material of substrate 1 at 3000 Hz at room temperature, E is the Young's modulus [GPa] of the material of substrate 1 at room temperature, and ν is the Poisson's ratio of the material at room temperature. 2 · Q / E value is 25 [g / cm 3 / Gpa]. The amplitude at the outer peripheral edge of the substrate 1 is preferably less than ρ·(1−ν) 2 The amplitude is proportional to / E / ξ (ξ is the damping ratio of the material of the substrate 1), and the damping ratio ξ is expressed as 1 / (2·Q) (Q is the Q value), so the amplitude is 2·ρ·(1-ν) 2 ·Proportional to Q / E, where 2·ρ(1-ν) 2 Q / E is 25 [g / cm 3 / Gpa], it was found that the amplitude of vibration in a specific frequency band can be efficiently reduced, that is, the RSS (root of sum of squares) of flutter vibration in the 1000-4000 Hz range can be reduced to less than 80 nm. Specifically, the RSS in the 1000-4000 Hz range is the square root of the integrated value of the squares of the amplitude of flutter vibration in the 1000-4000 Hz range. In other words, 2·ρ(1-ν) 2 Q / E is 25 [g / cm 3 / GPa], the RSS in the range from 1000Hz to 4000Hz can be reduced. 2 ·Q / E is 20 [g / cm 3 / Gpa] or less, the RSS in the range of 1000 Hz to 4000 Hz can be made 68 nm or less, which is more preferable. The impact of flutter vibrations in the frequency band below 1000 Hz has been reduced due to recent advances in head servo technology, while flutter vibrations in the band above 4000 Hz are still small to begin with. Therefore, it has become important to reduce flutter vibrations in the 1000 to 4000 Hz band. The substrate 1 of this embodiment has such characteristics.

[0045] Such a substrate 1 is produced, for example, as follows: Here, as an example, a case where a glass substrate is used as the substrate 1 will be described. First, a glass blank, which will be the material for a plate-shaped magnetic disk substrate having a pair of main surfaces, is formed. Next, the glass blank is roughly ground. After this, the glass blank is shaped and its edges are polished. After this, the main surfaces of the substrate obtained from the glass blank are finely ground using fixed abrasive grains. After this, the substrate is subjected to first polishing of the main surfaces, chemical strengthening, and second polishing of the main surfaces. Note that in this embodiment, the substrate is fabricated in the above-described sequence, but the above processes do not always need to be performed, and the order of these processes may be changed or omitted as appropriate. For example, among the above processes, the fine grinding, first polishing, and chemical strengthening may not be performed. Each process will be described below.

[0046] (a) Forming of glass blanks For example, a press molding method can be used to form a glass blank. A circular glass blank can be obtained by the press molding method. Furthermore, it can be manufactured using known manufacturing methods such as a downdraw method, a redraw method, and a fusion method. A disk-shaped substrate that serves as the base for a magnetic disk substrate can be obtained by appropriately shaping the plate-shaped glass blank manufactured by these known manufacturing methods.

[0047] (b) Rough grinding In rough grinding, grinding is performed on both main surfaces of the glass blank. For example, loose abrasive grains are used as the abrasive. In rough grinding, the glass blank is ground so as to approximate the target thickness dimension and flatness of the main surfaces. Note that rough grinding is performed depending on the dimensional accuracy or surface roughness of the formed glass blank, and may not be performed in some cases.

[0048] (c) Shape processing Next, shaping is performed. In shaping, first, after molding the glass blank, a circular hole and an outer periphery are formed using a known processing method to obtain a disk-shaped substrate with a circular hole (circular hole forming process). Then, the edge surface of the substrate is chamfered (chamfering process). As a result, on the edge surface of the substrate, side wall surfaces 4 perpendicular to the main surface 3 and chamfered surfaces 5 inclined with respect to the main surface 3 are formed between the side wall surfaces 4 and both main surfaces 3. In the chamfering process, the side wall surfaces 4 and two chamfered surfaces 5 may be formed simultaneously by grinding the edge surface of the substrate using a forming grindstone.

[0049] (d) Edge polishing Next, the edge of the substrate is polished. Edge polishing is a process in which a polishing solution containing free abrasive grains is supplied between the polishing brush and the outer edge (side wall surface 4 and chamfered surface 5) and inner edge (side wall surface 4 and chamfered surface 5) of the substrate, and the polishing brush and the substrate are moved relative to each other. In edge polishing, the inner and outer edge of the substrate are polished to a mirror finish.

[0050] (e) Fine grinding Next, the main surface of the substrate is subjected to precision grinding. For example, a double-sided grinding device with a planetary gear mechanism is used to grind the main surface 3 of the substrate. In this case, for example, a fixed abrasive is provided on a surface plate for grinding. Alternatively, grinding can be performed using loose abrasive. Note that precision grinding may not be performed in some cases.

[0051] (f) First polishing Next, the main surface 3 of the substrate is subjected to a first polishing. The first polishing uses a polishing pad attached to a surface plate with loose abrasive grains. The first polishing removes cracks and distortions remaining on the main surface 3 after precision grinding with a fixed abrasive grain, for example. The first polishing can reduce the surface roughness of the main surface 3, such as the arithmetic mean roughness Ra, while preventing the edge shape of the main surface 3 from being excessively depressed or protruded. The free abrasive grains used in the first polishing are not particularly limited, but may be, for example, cerium oxide abrasive grains, zirconia abrasive grains, etc. Note that the first polishing may not be performed in some cases.

[0052] (g)Chemical strengthening Depending on the embodiment, the substrate 1 may be chemically strengthened as appropriate. When chemically strengthening is performed, a molten liquid obtained by heating, for example, potassium nitrate, sodium nitrate, or a mixture thereof, can be used as the chemical strengthening liquid. By immersing the substrate in the chemical strengthening liquid, lithium ions and sodium ions in the glass composition on the surface layer of the substrate are replaced by sodium ions and potassium ions, which have relatively large ionic radii, respectively, in the chemical strengthening liquid, forming a compressive stress layer in the surface layer and strengthening the substrate. The timing of chemical strengthening can be determined as appropriate, but polishing after chemical strengthening is particularly preferred, as this not only smoothes the surface but also removes any foreign matter that has adhered to the surface of the substrate due to chemical strengthening.

[0053] (h) Second polishing (mirror polishing) Next, the chemically strengthened substrate is subjected to a second polishing. The second polishing aims to mirror-polish the main surface 3. The second polishing is also performed using a polishing device with the same configuration as the first polishing. In the second polishing, the type and particle size of the free abrasive grains are changed from those in the first polishing, and a softer resin polisher is used as the polishing pad to perform the mirror polishing. This reduces the roughness of the main surface 3 while preventing excessive depression or protrusion of the edge shape of the main surface 3. The roughness of the main surface 3 is preferably 0.2 nm or less in terms of arithmetic mean roughness Ra (JIS B 0601 2001). Thereafter, the substrate is washed, and the substrate 1 can be obtained.

[0054] (Examples, Comparative Examples, Conventional Examples) In order to examine the effect of the substrate 1, various substrates were prepared (Conventional Examples 1 and 2, Comparative Examples 1 and 2, Examples 1 to 12, Examples 61 to 67, Examples 81 to 84, Examples 111 to 114, Examples 121 to 124). The substrates used were glass substrates or aluminum alloy substrates. Glass 4 having the following composition was used for the substrates of Conventional Example 1 and Comparative Example 1. The substrate 1 of Examples 1 and 2 was made of Glass 1 described above. The substrate 1 of Examples 3 to 5 was made of Glass 2 described above. The substrate 1 of Examples 6 to 10 was made of Glass 3 described above. The substrate 1 of Examples 11 and 12 was made of Glasses 5 and 6, which were amorphous aluminosilicate glasses with a Young's modulus E of 100 GPa or more and different compositions from Glasses 1 to 4. The substrate 1 of Examples 61 to 67 and Examples 81 to 84 was made of Glass 3 described above. Furthermore, Glass 5 was used for the substrate 1 of Examples 111 to 114, and Glass 6 was used for the substrate 1 of Examples 121 to 124. None of the substrates were chemically strengthened. In addition, the specific elastic modulus of glasses 1 to 3, 5, and 6 is 36 [GPa cm 3 / g] or more. Glass 4 is less than 36.

[0055] (Glass 4) SiO2, Al2O3, one or more alkali metal oxides selected from the group consisting of Li2O, Na2O, and K2O, one or more alkaline earth metal oxides selected from the group consisting of MgO, CaO, SrO, and BaO, ZrO2, HfO2, Nb2O5, Ta2O5, La2O3, and YO 3、 and one or more oxides selected from the group consisting of TiO2, SiO2 50 mol% or more, Al2O3 3 mol% or more, and the total content of SiO2 and Al2O3 is 70 to 85 mol%; Contains Li2O and Na2O, with Li2O at 4.3 mol% or more, Na2O 5 mol% or more, and the total content of Li2O and Na2O is 24 mol% or less, a total content of the alkali metal oxide and the alkaline earth metal oxide of 8 mol% or more; Contains the molar ratio of the total content of the oxides to the total content of the alkali metal oxides and the alkaline earth metal oxides ((ZrO2+HfO2+Nb2O5+Ta2O5+La2O3+YO3+TiO2) / (Li2O+Na2O+K2O+MgO+CaO+SrO+BaO)) is 0.035 or more; Contains MgO and CaO, less than 3 mol% MgO, CaO 4 mol% or less, and the molar ratio of the MgO content to the CaO content (MgO / CaO) is 0.130 to 0.700; Amorphous glass.

[0056] The fabricated substrates had outer diameters (diameters) of 85 mm to 97 mm and inner diameters (circular hole diameters) of 25 mm. The specifications of the chamfered surfaces were a width W1 along the radial direction of 60 μm to 150 μm and a width W2 along the plate thickness direction of 60 μm to 150 μm. Specifically, for Conventional Examples 1 and 2, which had plate thicknesses exceeding 0.6 mm, the width W1 along the radial direction of the chamfered surface was 150 μm, and the width W2 along the plate thickness direction was 150 μm. For Comparative Examples 1 and 2 and Examples 1 to 12, which had plate thicknesses of 0.6 mm or less, the width W1 was 100 μm and the width W2 was 100 μm. The widths W1 and W2 of Examples 61 to 67 and 81 to 84 are shown in Tables 3A to 3D below. Therefore, when widths W1 and W2 are equal, the inclination angle θ1 (see FIG. 3) is 45 degrees. In the shaping process, when forming the outer periphery of the substrate, a diamond scriber was used to make a vertical cut, which was then extended to the opposite surface to break it. In the subsequent chamfering process, a chamfered surface was formed using a grinding wheel.

[0057] (Experiment 1) The fabricated substrates of Conventional Examples 1 and 2, Comparative Examples 1 and 2, and Examples 1 to 12 were attached to an evaluation device equipped with a high-speed camera to measure the maximum amplitude. This evaluation device can apply an external impact (acceleration) of any magnitude and capture the resulting movement (vibration) of the outer peripheral edge of the substrate as a video. The video can then be analyzed to measure the displacement of the outer peripheral edge in the normal direction to the main surface. Using this evaluation device, an impact test was conducted in which a 70G impact was applied to the substrate for 2 ms in the normal direction to the main surface of the substrate, and the vibration of the outer edge in the normal direction to the main surface was measured. The measurement results are shown as waveform data as shown in Figure 2. From this waveform data, the maximum displacement in either direction in the normal direction relative to the center of the displacement amount of 0 at the outer edge of the substrate was calculated as the maximum amplitude.

[0058] Actual HDDs incorporate ramps for the magnetic head's ramp loading mechanism, which, when installed, leave a 0.25 mm gap between each main surface. In other words, the gap between the ramps for the magnetic disk is the thickness of the magnetic disk plus 0.5 mm. Actual HDDs are designed to maintain this gap constant regardless of changes in substrate thickness. The evaluation device, on the other hand, does not have such ramps. Therefore, determining whether the substrate's vibration will cause contact with other components, such as the ramp (adjacent substrates, ramps, or HDD housings), in an actual HDD is based on the maximum amplitude of the substrate's vibration. If the maximum amplitude is 0.25 mm or less, it can be determined that contact with the ramp will not occur. If the maximum amplitude exceeds 0.25 mm, there is a high possibility of contact with other components. The maximum amplitude was measured for three substrates, and the average of the maximum amplitudes was used. In this evaluation, the substrates were stationary, not rotating. The thickness of the magnetic film formed in the media process is approximately 100 nm or less on the main surface, and can therefore be substantially ignored.

[0059] The evaluation results of the maximum amplitude are shown in Tables 1, 2A, and 2B below. The aluminum alloys ("Al alloys") of Conventional Example 2 and Comparative Example 2 are Al-Mg alloys having the following composition: Mg: 3.5-5%, Si: 0-0.05%, Fe: 0-0.1%, Cu: 0-0.12%, Mn: 0-0.3%, Cr: 0-0.1%, Zn: 0-0.5%, Ti: 0-0.1%, and the balance being Al. Furthermore, a film of a Ni-P alloy (P: 10% by mass, balance Ni) was formed on the surface of an Al-Mg alloy substrate by electroless plating so as to cover the entire surface of the substrate. The thickness of the substrate on which the plating film is formed refers to the thickness including the film.

[0060] [Table 1]

[0061] According to Table 1, in Conventional Examples 1 and 2, where the plate thickness T exceeds 0.6 mm, even if the Young's modulus is less than 90 GPa, the plate thickness T is large, so the maximum amplitude is small and the substrate does not come into contact with other members. However, as shown in Comparative Examples 1 and 2, when the plate thickness T is 0.6 mm or less, the maximum amplitude exceeds 0.25 mm, making it highly likely that the substrate will come into contact with other members. In contrast, in Examples 1 and 2, even if the plate thickness T is 0.6 mm or less, the Young's modulus is 90 GPa or more, so the maximum amplitude is 0.25 mm or less.

[0062] [Table 2A]

[0063] [Table 2B]

[0064] In Examples 3 to 12, similarly to Examples 1 and 2, even if the plate thickness T is 0.6 mm or less, the Young's modulus is 90 [GPa] or more, so the maximum amplitude is 0.25 mm or less.

[0065] From the above, according to Table 1 and Tables 2A and 2B, even if the diameter D of substrate 1 is 85 mm or more and the thickness T of substrate 1 is 0.6 mm or less, if the Young's modulus E of the material of substrate 1 is 90 GPa or more, the maximum amplitude will be 0.25 mm or less, so that vibrations caused by external impacts will not cause substrate 1 to come into contact with other components. This makes it possible to suppress the generation of particles inside the HDD.

[0066] (Experiment 2) Furthermore, using the substrates 1 of Examples 6, 8, 11, and 12 as references, the quality of the substrates 1 after impact testing was evaluated using substrates 1 (Examples 61 to 67, Examples 81 to 84, Examples 111 to 114, and Examples 121 to 124) in which the widths W1 and W2 of the chamfered surface 5 were variously changed. The widths W1 and W2 were varied by varying the grindstone shape and processing conditions when performing chamfering using a shaped grindstone. Examples 61 to 67 are the same as in Example 6, except that the widths W1 and W2 of the substrate 1 were varied; Examples 81 to 84 are the same as in Example 8, except that the widths W1 and W2 of the substrate 1 were varied; Examples 111 to 114 are the same as in Example 11, except that the widths W1 and W2 of the substrate 1 were varied; and Examples 121 to 124 are the same as in Example 12, except that the widths W1 and W2 of the substrate 1 were varied. Therefore, the materials, Young's modulus E, plate thickness, and outer diameter of Examples 61 to 67 are the same as those of Example 6, the materials, Young's modulus E, plate thickness, and outer diameter of Examples 81 to 84 are the same as those of Example 8, the materials, Young's modulus E, plate thickness, and outer diameter of Examples 111 to 114 are the same as those of Example 11, and the materials, Young's modulus E, plate thickness, and outer diameter of Examples 121 to 124 are the same as those of Example 12.

[0067] To evaluate the quality of the fabricated substrate 1, a commercially available HDD was disassembled. The substrate 1 and spacer of each example were attached to a spindle, and a simulated ramp member made of engineering plastic was attached so that it protruded from the substrate surface. A gap of 0.25 mm was maintained between the ramp member and the substrate. Then, while the substrate 1 was still attached, an impact test was performed in which an impact of 200 G was applied for 2 ms in the normal direction to the main surface of the substrate 1. This test was an accelerated test in which the outer edge of the substrate 1 was intentionally collided with the ramp member several times or more. The particle distribution around the area where the ramp member came into contact on the surface of the substrate 1 was then observed. Due to the difficulty of quantifying the results, a relative evaluation was performed. Rank 1: Almost no particles Rank 2: Medium number of particles Rank 3: High number of particles The evaluation results are shown in Tables 3A to 3D below. The lower the rank value, the better the quality evaluation, with rank 1 representing the highest evaluation.

[0068] [Table 3A]

[0069] [Table 3B]

[0070] [Table 3C]

[0071] [Table 3D]

[0072] Tables 3A to 3D show that the number of particles can be reduced by setting width W1 to 120 μm or less. Furthermore, the number of particles can be further reduced by setting width W1 to 90 μm or less. Therefore, when the diameter D of substrate 1 is 85 mm or more, the thickness T of substrate 1 is 0.6 mm or less, and a material with a Young's modulus E of 90 GPa or more is used for substrate 1, the number of particles adhering to the main surface of substrate 1 in an HDD can be further reduced by setting width W1 to 120 μm or less.

[0073] The magnetic disk substrate and magnetic disk of the present invention have been described in detail above, but the present invention is not limited to the above-mentioned embodiments and examples, and various improvements and modifications may be made without departing from the spirit and scope of the present invention. [Explanation of symbols]

[0074] 1. Magnetic disk substrate 2 inner hole 3 Main surface 4 Side wall 5 Chamfered surface 6 angles 10 Magnetic Disk

Claims

1. A disk-shaped magnetic disk substrate, The diameter D of the substrate is 85 mm or more, and the thickness T of the substrate is 0.6 mm or less, A magnetic disk substrate, characterized in that the substrate is made of a material having a Young's modulus E of 90 GPa or more.

2. 2. The magnetic disk substrate according to claim 1, wherein the diameter D is 90 mm or more.

3. 3. The magnetic disk substrate according to claim 1, wherein when an impact of 70 G is applied to the substrate for 2 ms in the normal direction to the main surface of the substrate while the inner peripheral edge of the substrate is fixed, the maximum amplitude of vibration in the thickness direction of the outer peripheral edge of the substrate is 0.25 mm or less.

4. A disk-shaped magnetic disk substrate, The diameter D of the substrate is 85 mm or more, and the thickness T of the substrate is 0.6 mm or less, A magnetic disk substrate characterized in that, when an impact of 70 [G] is applied to the substrate for 2 [ms] in the normal direction of the main surface of the substrate while the inner peripheral edge of the substrate is fixed, the maximum amplitude of vibration in the thickness direction of the outer peripheral edge of the substrate is 0.25 mm or less.

5. 5. The magnetic disk substrate according to claim 4, wherein the diameter D is 90 mm or more.

6. 6. The magnetic disk substrate according to claim 1, wherein the substrate is a glass substrate made of glass having a glass transition point of 650° C. or higher.

7. 7. The magnetic disk substrate according to claim 1, wherein the change in flatness of the substrate after heating at 730° C. and the flatness of the substrate before heating is 4 μm or less.

8. The substrate has a linear expansion coefficient of 70×10 －７ 8. The magnetic disk substrate according to claim 1, which is made of a material having a viscosity of not more than [1 / K].

9. The Vickers hardness Hv of the substrate is 650 [kgf / mm ２ 9. The magnetic disk substrate according to claim 1, wherein the thickness of the magnetic disk substrate is 100 nm or more.

10. The Knoop hardness Hk of the substrate is 600 [kgf / mm ２ 10. The magnetic disk substrate according to claim 1, wherein the thickness of the magnetic disk substrate is 100 nm or more.

11. a chamfered surface is provided on at least the outer peripheral edge surface of the substrate; The width W of the chamfered surface along the radial direction of the substrate １ The magnetic disk substrate according to any one of claims 1 to 10, wherein the thickness is 120 µm or less.

12. The width W of the chamfered surface along the thickness direction of the substrate ２ The ratio of the thickness T to the thickness T is twice (2.W ２ 12. The magnetic disk substrate according to claim 1, wherein ) / T is 0.4 or less.

13. With respect to the Young's modulus E and the thickness T of the substrate, E·T ３ The value is 3 to 18 [GPa mm ３ 13. The magnetic disk substrate according to claim 1, wherein:

14. 14. The magnetic disk substrate according to claim 1, wherein the material has a Q value of 1500 or less at 3000 Hz at room temperature.

15. ρ is the density of the material at room temperature [g / cm ３ ], Q is the Q value of the material at room temperature at 3000 Hz, E is the Young's modulus [GPa] of the material at room temperature, and ν is the Poisson's ratio of the material at room temperature, and ρ (1-ν) of the material ２ ・Q / E value is 25 [g / cm ３ 15. The magnetic disk substrate according to claim 1, wherein the surface tension is less than [0.1 GPa / GPa].

16. A magnetic disk having at least a magnetic film on the surface of the magnetic disk substrate according to any one of claims 1 to 15.

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