Substrates and magnetic disks for magnetic disks

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] This invention relates to a substrate for a magnetic disk and a magnetic disk. [Background technology]

[0002] Conventionally, glass substrates and aluminum alloy substrates have been used as substrates for magnetic disks. A magnetic film is formed on the main surface of these substrates to create a magnetic disk. Magnetic disks are desired to have few surface defects, allow for smooth reading and writing of information, and enable the reading and writing of large amounts of information. Furthermore, in response to the demand for increased storage capacity in hard disk drives (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 that reduces the surface roughness (arithmetic mean roughness Ra) of the glass substrate for the magnetic disk so that reading and writing can be performed without problems using a magnetic head equipped with a DFH (Dynamic Flying Height) mechanism (DFH head) (Patent Document 1). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] International Publication No. 2014 / 051153 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] Incidentally, in recent years, the hard disk drive industry has been approaching the limits of miniaturization of magnetic particles in magnetic disks, and the rate of improvement in recording density has slowed down. On the other hand, the demand for increased storage capacity in HDDs is becoming increasingly intense due to big data analysis and other applications. Therefore, increasing the number of magnetic disks installed in a single HDD is being considered. For storing such large amounts of data, a nominal 3.5-inch HDD is typically used. When increasing storage capacity by increasing the number of magnetic disks incorporated into 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 disks within the limited space inside the HDD. Furthermore, to achieve even greater storage capacity, there is a demand for larger magnetic disk sizes.

[0006] Here, it has been found that increasing the diameter of the magnetic disk substrate and decreasing its thickness reduces the rigidity of the substrate, making it more susceptible to large vibrations, and that these vibrations may be difficult to subside. For example, in cloud data centers, a very large number of HDDs are used, so HDD replacements due to failures are frequent. At this time, it has been found that new HDDs may fail due to the shock when they are installed in the rack, or the time until failure may be shortened. Further investigation revealed that when an HDD is subjected to an external shock, it can be damaged even though the magnetic disk is not yet rotating because power has not yet been supplied to the HDD.

[0007] The vibrations caused by external impacts, unlike the steady-state flutter vibrations that occur in the steady rotation state caused by the rotating magnetic disk and the air flow around it, decay over time. However, if the amplitude of these vibrations is large, it becomes easy to contact the lamp inside the HDD and also to contact the magnetic disks arranged adjacent to each other. Furthermore, the magnetic disk located at the top of a plurality of magnetic disks arranged at regular intervals may contact the ceiling surface of the magnetic disk storage container of the HDD. Such contact may cause the contacted portion of the magnetic disk to chip and generate particles. Also, particles may be generated due to rubbing or scraping. The generated particles scatter inside the storage container and often adhere to the read / write area (main surface) of the magnetic disk with a large area. Thus, by increasing the diameter of the magnetic disk substrate and reducing the plate thickness, vibrations caused by external impacts that did not cause problems conventionally and the particles generated accordingly have become impossible to ignore. In particular, in the case of a magnetic disk substrate for a large magnetic disk with a nominal size of 3.5 inches (for example, a diameter of 95 mm or more), the problem of particles generated by the above-mentioned contact due to the vibration of the substrate 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 plate thickness T of the substrate is 0.6 mm or less. A material with a Young's modulus E of 90 GPa or more is used for the substrate.

[0010] The diameter D is preferably 90 mm or more. When an impact of 70 [G] over 2 [msec] is applied to the substrate in the direction normal to the main surface of the substrate while the inner peripheral end of the substrate is fixed, it is preferable that the maximum amplitude due to vibration in the thickness direction of the outer peripheral end of the substrate is 0.25 mm or less.

[0011] Another aspect of the present invention is a substrate for a disc-shaped magnetic disk. 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 the substrate is subjected to an impact of 70 G over 2 m seconds in the direction normal to the main surface of the substrate while its inner peripheral end is fixed, the maximum amplitude due to vibration in the thickness direction of the outer peripheral end of the substrate is 0.25 mm or less.

[0012] The diameter D is preferably 90 mm or more. Preferably, the substrate is a glass substrate made of glass with a glass transition temperature of 650°C or higher.

[0013] Preferably, 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 aforementioned substrate has a coefficient of thermal expansion of 70 × 10 -7 It is preferable that the material be composed of materials with a [1 / K] or lower ratio.

[0015] The Vickers hardness Hv of the aforementioned substrate is 650 [kgf / mm²]. 2 It is preferable that it is ] or greater.

[0016] The Knoop hardness Hk of the aforementioned substrate is 600 [kgf / mm²]. 2 It is preferable that it is ] or greater.

[0017] At least the outer peripheral edge surface of the substrate is provided with a chamfered surface. Preferably, 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] Preferably, 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] Regarding the Young's modulus E and the plate thickness T in the aforementioned substrate, E·T 3 The value is 3-18 [GPa·mm 3 It is preferable that it be ].

[0020] Preferably, the Q value of the aforementioned material at room temperature and 3000 Hz is 1500 or less.

[0021] Let ρ be the density of the material at room temperature [g / cm³]. 3 Let Q be the Q value of the material at room temperature at 3000 Hz, E be the Young's modulus [GPa] of the material at room temperature, and ν be the Poisson's ratio of the material at room temperature. 2 • Q / E value is 25 [g / cm³] 3 It is preferable that the value is less than [GPa].

[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] With the above-described magnetic disk substrate and magnetic disk, even if the diameter of the magnetic disk substrate is increased and the thickness of the substrate is reduced, the generation of particles caused by external impacts can be suppressed. [Brief explanation of the drawing]

[0024] [Figure 1] This figure shows an example of the external shape of a substrate for a magnetic disk according to one embodiment. [Figure 2] This figure shows an example of vibration of a magnetic disk substrate according to one embodiment. [Figure 3] This figure illustrates an example of the outer edge of a magnetic disk according to one embodiment. [Modes for carrying out the invention]

[0025] The magnetic disk substrate of the present invention will be described in detail below. In the following description, a glass substrate for magnetic disks will be used, but the magnetic disk substrate may also be a non-magnetic metal substrate. In the case of glass substrates, aluminosilicate glass, soda-lime glass, borosilicate glass, etc., can be used as the glass. In particular, amorphous aluminosilicate glass can be suitably used because it can be chemically strengthened as needed, and a glass substrate for magnetic disks with excellent flatness of the main surface and substrate strength can be produced. For example, aluminum alloys, titanium alloys, and Si single crystals can be used as materials for the metal substrate. A magnetic disk is manufactured by forming at least one magnetic film on the surface of a substrate for this magnetic disk.

[0026] Figure 1 shows the external shape of the magnetic disk substrate according to this embodiment. As shown in Figure 1, the magnetic disk substrate 1 (hereinafter simply referred to as substrate 1) in this embodiment is a thin, disc-shaped substrate with an internal hole 2 formed therein. The magnetic disk is manufactured by forming films such as a magnetic film, an undercoat, and a soft magnetic layer on this substrate 1. The size of the substrate 1 is not limited as long as the diameter D is 85 mm or more, preferably 90 mm or more. However, the substrate 1 can be suitably applied to, for example, a magnetic disk substrate with a nominal diameter of 3.5 inches. 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 vibrations of a magnetic disk caused by external shocks, which are different from flutter vibrations, increases as the outer diameter of the substrate 1 increases, and attenuation becomes more difficult. Therefore, the substrate 1 of this embodiment is preferable when used with magnetic disks of nominal size 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 compared to conventional substrates. This makes it possible to increase the number of magnetic disks incorporated into the HDD, thereby increasing the storage capacity. However, 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 the impact when the HDD is installed can cause the magnetic disks to come into contact with the lamps inside the HDD, and also make them more likely to come into contact with adjacent magnetic disks inside the HDD. Furthermore, the magnetic disk located at the top of the magnetic disk housing may come into contact with the ceiling surface of the magnetic disk housing of the HDD. The upper limit of the diameter D of substrate 1 is, for example, 100 mm, from the perspective of the standard size of a nominal 3.5-inch HDD. The lower limit of the board thickness T of substrate 1 is, for example, 0.30 mm, from the perspective of suppressing arcing when a bias voltage is applied during the film deposition process. There is no particular upper limit for Young's modulus E, but from the perspective of ease of processing, it is, for example, 120 GPa.

[0028] Figure 2 shows an example of vibration of the substrate 1 caused by the above-mentioned impact. Such vibration is different from steady-state flutter vibration that occurs in a steady rotation state caused by a rotating magnetic disk and the airflow around it. The vibration caused by the impact is a vibration in which the main surface of the substrate 1 is displaced out of plane relative to the main surface. In particular, in an HDD, the inner circumference end is fixed to the spindle, but the outer circumference end is a free end and is displaced out of plane relative to the main surface. This out-of-plane displacement vibration of the main surface (vibration in the thickness direction) causes contact with the lamp inside the HDD, contact with adjacent magnetic disks, and even contact with the ceiling surface of the magnetic disk housing. Such contact may cause the contacted portion of the magnetic disk to chip and generate particles. The generated particles scatter inside the housing and often 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, an amorphous oxide glass having a Young's modulus of 90 GPa or more can be obtained by the following glass composition.

[0030] (Glass 1) SiO2 56 to 80 mol%, Li2O 1 to 10 mol%, B2O3 0 to 4 mol%, Total content of MgO and CaO (MgO + CaO) 9 to 40 mol%, and. 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 to 80 mol%, Li2O 1 to 10 mol%, B2O3 0 to 4 mol%, Total content of MgO and CaO (MgO + CaO) 9 to 40 mol%, and The molar ratio of the total content of SiO2 and ZrO2 to the Al2O3 content ((SiO2 + ZrO2) / Al2O3) is 2 to 13, and. The glass transition temperature Tg of Glass 2 is 650 °C or higher.

[0032] (Glass 3) In mol%, SiO2 56 to 65%, Al2O3 5 to 20%, B2O3 0 to 4%, MgO 3 to 28%, Li2O 1 to 10%, and Total content of SiO2 and Al2O3 (SiO2 + Al2O3) 65 to 80%, Total content of MgO and CaO (MgO + CaO) 11 to 30%, Total content of MgO, CaO, SrO and BaO (MgO + CaO + SrO + BaO) 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 content to MgO content (CaO / MgO) is 2.5 or less. The molar ratio of Na2O content to Li2O content (Na2O / Li2O) is 5 or less. The molar ratio of Li2O content to the total content of MgO and CaO (Li2O / (MgO+CaO)) is 0.03-0.4. The molar ratio of SiO2 content to the total content of Li2O, Na2O, and K2O (SiO2 / (Li2O+Na2O+K2O)) is between 4 and 22. The molar ratio of the total content of SiO2 and ZrO2 relative 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, and the molar ratio of the content of BaO to the total content of MgO, CaO, SrO, and BaO (BaO / (MgO+CaO+SrO+BaO)) is 0.1 or less. The molar ratio of 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 of 670°C or higher and Young's modulus of 90 GPa or higher, Specific gravity of 2.75 or less, The average coefficient of linear thermal expansion at 100-300°C is 40 × 10⁻⁶. -7 ~70×10 -7 [1 / K] Amorphous oxide glass within this range.

[0033] In this case, when the inner edge of the substrate 1 is fixed and an impact (acceleration) of 70 [G] is applied to the substrate 1 in the direction normal to the main surface of the substrate 1 in 2 [m-s], it is preferable that the maximum amplitude due to vibration in the thickness direction of the outer edge of the substrate 1 is 0.25 mm or less. By making the maximum amplitude 0.25 mm or less in this way, the above-mentioned contact can be prevented. The above impact test was performed using an AVEX-SM-110-MP type testing machine from Air Brown Co., Ltd. Therefore, in one embodiment, the substrate 1 has a diameter D of 85 mm or more, preferably 90 mm or more, a thickness T of 0.6 mm or less, and when an impact of 70 [G] is applied to the substrate 1 in the direction normal to the main surface of the substrate 1 in 2 [msec] while the inner circumference end of the substrate 1 is fixed, the maximum amplitude due to vibration in the thickness direction of the outer circumference end of the substrate 1 is 0.25 mm or less.

[0034] According to one embodiment, the substrate 1 is preferably made of glass with a glass transition temperature (Tg) of 650°C or higher, and more preferably, the glass transition temperature (Tg) is 680°C or higher. The higher the glass transition temperature (Tg), the higher the thermal durability, and the more it is possible to suppress deformation of the substrate, such as flatness, that occurs when the substrate 1 is heat-treated. Considering the heat treatment when forming a magnetic film for a magnetic disk or the like on the substrate 1, it is preferable to set the glass transition temperature (Tg) to 650°C or higher in order to suppress thermal deformation. Specifically, when manufacturing a magnetic disk by forming a metal film containing a magnetic film or the like on a substrate 1 to a thickness of approximately 30 nm, the substrate 1 is heated. During this heating process, the substrate 1 is susceptible to deformation due to thermal history. Therefore, according to one embodiment, it is preferable that the change in flatness of the substrate 1 after heating at 730°C and the change in flatness of the substrate 1 before heating (flatness after heating - flatness before heating) is 4 μm or less. By limiting the change in flatness in this way, a flat magnetic disk can be obtained, and furthermore, minute vibrations during the rotation of the magnetic disk can be reduced.

[0035] According to one embodiment, the substrate 1 has a coefficient of thermal expansion of 70 × 10 -7 It is preferable that the material be composed of a material with a coefficient of thermal expansion of [1 / K] or less, and more preferably, a coefficient of thermal expansion of 60 × 10⁻⁶. -7 It is less than or equal to [1 / K]. The lower limit of the coefficient of thermal expansion of substrate 1 is, for example, 40 × 10 -7 The coefficient of linear expansion is [1 / K]. Here, the coefficient of linear expansion is determined by the difference in thermal expansion between 100°C and 300°C. By using such a coefficient of linear expansion, thermal expansion can be suppressed during the heat treatment when forming magnetic films, and when the gripping member of the film deposition apparatus fixes and grips the substrate 1 at the outer edge surface (hereinafter referred to as the outer edge surface), thermal distortion of the substrate 1 around the gripping portion can be suppressed. For example, the coefficient of linear expansion for a conventional aluminum alloy substrate is 242 × 10 -7 [1 / K], and for conventional glass substrates, 95 × 10 -7 While [1 / K] or higher is observed, the coefficient of linear expansion of the glass substrate 1 in one embodiment is 51 × 10 -7[1 / K]. On the other hand, there is no particular need to set a lower limit for the coefficient of thermal expansion of substrate 1, but if the coefficient of thermal expansion of substrate 1 becomes too small, when the temperature inside the HDD rises, the spindle may expand and come into contact with and compress the circular hole in the substrate, deforming the substrate. Therefore, for example, the lower limit for the coefficient of thermal expansion should be 20 × 10 -7 [1 / K] is even more preferable.

[0036] According to one embodiment, the Vickers hardness Hv of the substrate 1 is 650 [kgf / mm²]. 2 It is preferable that it be ] or more. Furthermore, according to one embodiment, the Knoop hardness Hk of the substrate 1 is 600 [kgf / mm 2 It is preferable that the Vickers hardness Hv or Knoop hardness Hk is higher. By increasing the Vickers hardness Hv or Knoop hardness Hk, even if the substrate 1 comes into contact with another substrate 1 or other component due to vibrations caused by external impact, it is possible to suppress the chipping of the substrate 1 and the generation of particles. In addition, when forming a magnetic film or the like, when the gripping member of the film-forming apparatus grips the outer peripheral edge, it is possible to prevent a part of the outer peripheral edge from chipping off due to the gripping and generating particles, and to prevent these particles from adhering to the main surface of the substrate 1. For example, the Vickers hardness Hv for an aluminum alloy substrate is 128 [kgf / mm²]. 2 ] and conventional glass substrates have a load of 620 [kgf / mm²]. 2 In contrast, the glass substrate 1 of one embodiment has a strength of 741 [kgf / mm²]. 2 ]

[0037] Figure 3 is a magnified view illustrating an example of the edge of a magnetic disk. Figure 3 shows two adjacent magnetic disks within an HDD. In Figure 3, the thickness of the magnetic film, etc., is overwhelmingly smaller than the thickness of the substrate 1, being only a few tens of nanometers, so the illustration of the magnetic film, etc., is omitted. The substrate 1 has a pair of main surfaces 3, side wall surfaces 4 arranged along a direction perpendicular to the pair of main surfaces 3, and a pair of chamfered surfaces 5 positioned 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 at the outer and inner periphery ends of the substrate 1, respectively.

[0038] Inside the HDD, magnetic disks 10, each having a magnetic film (not shown) or the like formed on the surface of a substrate 1, are arranged adjacent to each other as shown in Figure 3. When a magnetic disk 10 vibrates in the direction normal to the main surface 3 and comes into contact with an adjacent magnetic disk 10, the corner 6, which is the connection point between the main surface 3 and the chamfered surface 5 at the outer edge, is prone to contact. This contact causes the chamfered surface 5 near the corner 6 to experience a significant impact. The chamfered surface 5 is formed by grinding with a full-size grinding wheel and then polishing the end face with a brush or the like. However, it has been found that in the case of substrates made of glass material with a Young's modulus E of 90 GPa or higher, there are more latent cracks or microcracks on the surface of the chamfered surface 5 than in conventional substrates made of glass with a lower Young's modulus. The cause of this is not entirely clear, but for example, since high Young's modulus glass is generally hard, the load during grinding is greater, causing deeper cracks than before, and it is presumed that some of these remain after polishing as latent cracks or microcracks. In this state, if the corner 6 is subjected to impact by contact with a lamp member or the like, the latent cracks or microcracks propagate, and as a result, a part of the corner 6 or chamfered surface 5 of the substrate 1 may chip off in the magnetic disk 10, generating particles together with the magnetic film on top. In such cases, the larger the width W1 described later, the more likely a part of the corner 6 or chamfered surface 5 is to chip. Particles generated at corners 6 or chamfered surfaces 5 often scatter within the HDD housing and adhere to the read / write area of ​​the magnetic disk 10.

[0039] Therefore, according to one embodiment, it is preferable to reduce the amount of material removed when forming the chamfered surface 5 in the chamfering process. When the diameter of the magnetic disk 10 is large, preferably 90 mm or more, and the thickness is thin, of 0.600 mm or less, the corners 6 are more likely to come into contact due to vibration, so it is preferable to reduce the amount of material removed when forming the chamfered surface 5 to reduce the number of latent cracks and fine cracks. By doing so, even if the corners 6 of the magnetic disk 10 come into contact with a lamp member or the like due to large vibrations, the generation of particles and the like can be suppressed. Here, the amount of material removed when forming the chamfered surface 5 is the value of the width W1 described later. In addition, in some cases, the outer diameter is adjusted by removing a certain amount of material from the outer circumference at the beginning of grinding, but according to the inventor's research, such initial material removal does not affect latent cracks, etc. This is presumed to be because latent cracks, etc. that are generated by the initial material removal are eliminated by the effect of the final material removal. Specifically, a chamfered surface 5 is provided at least on the outer edge surface of the substrate 1. In this case, the width W1 (see Figure 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 material removal allowance when forming the chamfered surface 5 so that the width W1 is 120 μm or less, even when the diameter of the magnetic disk 10 using a high Young's modulus glass substrate is large and the plate thickness is thin, making it easier for the corners 6 to come into contact due to vibration, the number of potential cracks or fine cracks in the chamfered surface 5 can be reduced. Therefore, even if the corners 6 come into contact, it is possible to suppress the generation of particles by chipping of the corners 6 or a part of the chamfered surface 5. 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, and chipping may occur in the substrate manufacturing process after shaping or in the film deposition process. As shown in Figure 3, the chamfered surface 5 may have a straight shape in a cross-section obtained by cutting through the center of the magnetic disk 10 along the radial direction, or it may have a convex arc or curved shape that is directed outward. In this case, the chamfered surface 5 refers to the portion where the angle of inclination of the tangent line at each position is 5 to 85 degrees with respect to the side wall surface 4 and the main surface 3. Figure 3 shows a chamfered surface 5 where the angle of inclination θ1 with respect to the main surface 3 is constant.

[0040] According to one embodiment, the ratio (2·W2) / T of the width W2 (see Figure 3) of the chamfered surface 5 on the outer periphery of the substrate 1 along the thickness direction of the substrate 1 to the thickness T is preferably 0.4 or less. It is more preferable that the ratio (2·W2) / T be 0.3 or less. If the ratio (2·W2) / T exceeds 0.4, the side wall surface 4 becomes too small, which may cause chipping or cracking of the outer edge when gripping or applying a bias voltage during the deposition of a magnetic film or the like.

[0041] Furthermore, according to one embodiment, with respect to Young's modulus E [GPa] and plate thickness T [mm], E·T 3 The value is 3-18 [GPa · mm 3 It is preferable that the temperature be 3 to 16 [GPa · mm 3 It is more preferable that the temperature be 5-15 [GPa · mm 3 It is especially preferable that ]. E.T. 3 By keeping the value within this range, contact of the magnetic disk 10 due to vibration can be suppressed. 3 The value is 3 [GPa · mm 3 If it is less than ], vibration is likely to induce contact of the magnetic disk 10. E·T 3 The value is 18 [GPa · mm 3 In the case of exceeding ], the plate thickness cannot be increased in order to secure the number of magnetic disks 10 to be built into the HDD, so the Young's modulus E is increased. In this case, as the Young's modulus E increases, the substrate 1 tends to become harder than necessary, the polishing time of the main surface 3 increases, which is undesirable in terms of the production efficiency of the magnetic disk substrate 1. Furthermore, according to one embodiment, Young's modulus E [GPa] and density ρ [g / cm³] 3 Regarding this, the specific modulus calculated by E / ρ is 36 [GPa·cm 3It is preferable that the specific modulus is 41 [GPa·cm²] or higher. Even with a high Young's modulus, if the density is high, vibration may increase due to the weight of the substrate itself. There is no particular upper limit to the value of the specific modulus, but from the viewpoint of productivity, for example, 41 [GPa·cm²] is preferable. 3 You can also use / g].

[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 thickness of the magnetic film etc. formed on the substrate 1 is several tens of nanometers, which is negligibly thin compared to the thickness T of the substrate 1. For example, if the thickness T of the substrate 1 is 0.635 mm, 9 or more substrates 1 can be mounted; if the thickness T of the substrate 1 is 0.5 mm, 10 or more substrates 1 can be mounted; and if the thickness T of the substrate 1 is 0.38 mm, 12 or more substrates 1 can be mounted. Thus, the number of substrates 1 that can be mounted changes depending on the thickness T of the substrate 1. Therefore, by reducing the thickness T of the substrate 1 and increasing the number of substrates mounted, the memory capacity can be increased. In this case, as described above, even if the thickness T of the substrate 1 of this embodiment is reduced, contact with other substrates 1 or components due to vibration is less likely to occur. Furthermore, according to one embodiment, it is preferable that the substrate 1 does not break in an impact test in which the inner circumference of the substrate 1 is fixed to replicate the clamping of a magnetic disk and a 600G impact is applied to the substrate 1 over 2 milliseconds. Such a substrate 1 is preferable from the viewpoint of improved durability because it does not break even when a large impact is applied to the HDD. This substrate 1 can be achieved, for example, using the aforementioned glass 1 to 3.

[0043] According to one embodiment, the Q value (Quality Factor) of the substrate 1 material at room temperature (25°C) and 3000 Hz is preferably 1500 or less. The Q value is calculated by dividing the vibration energy stored in the vibrating substrate 1 during one cycle 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 the material used, the faster the vibration can be dampened. Consequently, the number of contacts with adjacent substrates 1 and lamps in the HDD can be reduced, and the impact during contact can be mitigated, thereby suppressing adverse effects such as particle generation due to contact. More preferably, the above Q value at room temperature and 3000 Hz is 1300 or less. The Q value at 3000 Hz was obtained as follows. First, vibration was generated on a substrate 1 rotated on a spin stand using a laser Doppler vibrometer (LDV). The vibration was measured by shining a laser on the approximate outer edge of the substrate 1, and the obtained data was appropriately Fourier transformed to obtain the 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 determined using the half-width method (a method that uses frequencies f1 and f2 (>f1) corresponding to values ​​3 dB lower than the NRRO peak value, and the frequency f0 (resonant 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, and a linear approximation was performed using the least squares method to obtain an approximate line. The Q value at 3000 Hz was obtained on the obtained approximate line, or by extrapolating the approximate line as necessary. The substrate 1 used for evaluation with the laser Doppler vibrometer was standardized to have an outer diameter of 95 mm, an inner diameter of 25 mm, and a thickness of 0.635 mm. The substrate was rotated at 6900 rpm, and the measurement position was 46.5 mm radially from the center of the substrate (1 mm inward from the outer edge), and measurements were taken at room temperature.

[0044] According to one embodiment, ρ is the density of the substrate 1 material at room temperature [g / cm³]. 3Let Q be the Q value of the substrate 1 material at room temperature at 3000 Hz, E be the Young's modulus [GPa] of the substrate 1 material at room temperature, and ν be the Poisson's ratio of the said material at room temperature. Then, the ρ·(1-ν) of the substrate 1 material is... 2 • Q / E value is 25 [g / cm³] 3 It is preferable that the amplitude is less than [ / Gpa]. The amplitude at the outer edge of the substrate 1 is ρ·(1-ν) 2 It is proportional to / E / ξ (where ξ is the damping ratio of the substrate 1 material), and the damping ratio ξ is expressed as 1 / (2·Q) (where Q is the Q value), so the above amplitude is 2·ρ·(1-ν) 2 It is proportional to Q / E. Here, 2·ρ(1-ν) 2 • Q / E is 25 [g / cm³] 3 We found that by setting the RSS (Root of Sum of Squares) of flutter vibrations in a specific frequency band to less than 2·ρ(1-ν)[ / Gpa], the amplitude of vibrations in a specific frequency band can be efficiently reduced, meaning that the RSS of flutter vibrations in the 1000-4000Hz range can be reduced to less than 80nm. Specifically, the RSS in the 1000-4000Hz range is the square root of the cumulative sum of the squares of the amplitudes of flutter vibrations from 1000Hz to 4000Hz. That is, 2·ρ(1-ν) 2 • Q / E is 25 [g / cm³] 3 By setting it below [ / GPa], the RSS in the 1000Hz to 4000Hz range can be reduced. 2·ρ(1-ν) 2 • Q / E is 20 [g / cm³] 3 It is even more preferable to set it to less than / Gpa so that the RSS in the 1000Hz to 4000Hz region can be reduced to 68nm or less. Furthermore, the impact of flutter vibrations in the frequency band below 1000Hz has decreased due to recent advances in head servo technology, while flutter vibrations in the frequency band above 4000Hz are inherently small. Therefore, reducing flutter vibrations in the 1000-4000Hz band is becoming increasingly important. The substrate 1 of this embodiment has the following features.

[0045] Such a substrate 1 can be manufactured, for example, as follows. Here, we will describe the case where a glass substrate is used as substrate 1 as an example. 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 surface of the substrate obtained from the glass blank is finely ground using fixed abrasive grains. After this, the substrate is subjected to first polishing, chemical strengthening, and second polishing of the main surface. In this embodiment, the substrate is manufactured in the above order, but it is not always necessary to perform the above processes, and the order of these processes may be changed or omitted as appropriate. For example, fine grinding, first polishing, and chemical strengthening may not be performed. Each process will be described below.

[0046] (a) Forming of glass blanks For forming glass blanks, for example, a press forming method can be used. A circular glass blank can be obtained using the press forming method. Furthermore, it can be manufactured using known manufacturing methods such as the down-draw method, the redraw method, and the fusion method. By appropriately processing the shape of the plate-shaped glass blank produced by these known manufacturing methods, a disc-shaped substrate that will serve as the basis for a magnetic disk substrate can be obtained.

[0047] (b)Rough grinding In rough grinding, the main surfaces on both sides of the glass blank are ground. For example, free abrasive grains are used as the abrasive material. In rough grinding, the glass blank is ground to approximate the target plate thickness and the flatness of the main surface. Note that rough grinding is performed depending on the dimensional accuracy or surface roughness of the formed glass blank, and may be omitted in some cases.

[0048] (c) Shape processing Next, shaping is performed. In shaping, first, after forming the glass blank, a circular hole and outer circumference are formed using a known processing method to obtain a disc-shaped substrate with a circular hole (circular hole formation step). After that, the edge faces of the substrate are chamfered (chamfering step). As a result, a side wall surface 4 perpendicular to the main surface 3 is formed on the edge face of the substrate, and chamfered surfaces 5 inclined with respect to the main surface 3 are formed between the side wall surface 4 and the main surfaces 3 on both sides. In the chamfering step, the side wall surface 4 and the two chamfered surfaces 5 may be formed simultaneously by grinding the edge face of the substrate using a full-size grinding wheel.

[0049] (d) Edge polishing Next, the edges of the substrate are polished. Edge polishing is a process in which polishing is performed by supplying a polishing fluid containing free abrasive particles between a polishing brush and the outer edge surface (side wall surface 4 and chamfered surface 5) and inner edge surface (side wall surface 4 and chamfered surface 5) of the substrate, and moving the polishing brush and the substrate relative to each other. In edge polishing, the inner and outer edge surfaces of the substrate are polished, and the inner and outer edge surfaces are made to a mirror finish.

[0050] (e) Fine grinding Next, the main surface of the substrate is precision ground. For example, grinding is performed on the main surface 3 of the substrate using a double-sided grinding device for planetary gear mechanisms. In this case, for example, fixed abrasive grains are placed on a grinding plate and grinding is performed. Alternatively, grinding can be performed using free abrasive grains. Note that precision grinding may be omitted in some cases.

[0051] (f) First polishing Next, the main surface 3 of the substrate is subjected to a first polishing. The first polishing is performed using free abrasive grains and a polishing pad attached to a surface plate. The first polishing removes cracks and distortions that remain on the main surface 3 when, for example, precision grinding is performed with fixed abrasive grains. The first polishing can reduce the surface roughness of the main surface 3, such as the arithmetic mean roughness Ra, while preventing the edges of the main surface 3 from becoming excessively concave or protruding. The free abrasive grains used in the first polishing are not particularly limited, but for example, cerium oxide abrasive grains or zirconia abrasive grains can be used. Note that the first polishing may be omitted in some cases.

[0052] (g)Chemical strengthening Depending on the substrate 1 of one embodiment, chemical strengthening may be performed as appropriate. When chemical strengthening is performed, for example, potassium nitrate, sodium nitrate, or a mixture thereof can be heated to obtain a molten solution. By immersing the substrate in the chemical strengthening solution, lithium ions and sodium ions in the glass composition on the surface of the substrate are replaced by sodium ions and potassium ions in the chemical strengthening solution, respectively, which have relatively larger ionic radii, thereby forming a compressive stress layer on the surface and strengthening the substrate. The timing of chemical strengthening can be determined as appropriate, but performing polishing after chemical strengthening is particularly preferable because it smooths the surface and removes foreign matter that has adhered to the substrate surface due to chemical strengthening.

[0053] (h) Second polishing (mirror polishing) Next, the chemically strengthened substrate undergoes a second polishing process. The purpose of the second polishing is to achieve a mirror finish on the main surface 3. In the second polishing, the same polishing apparatus as in the first polishing is used. In the second polishing, the type and particle size of the free abrasive grains are changed compared to the first polishing, and a resin polisher with a softer hardness is used as the polishing pad to achieve a mirror finish. This prevents the edges of the main surface 3 from becoming excessively concave or protruding while reducing the roughness of the main surface 3. The arithmetic mean roughness Ra (JIS B 0601 2001) of the main surface 3 is preferably 0.2 nm or less. After this, the substrate 1 can be obtained by cleaning the substrate.

[0054] (Examples, comparative examples, conventional examples) To investigate the effects of substrate 1, various substrates were fabricated (Conventional Examples 1 and 2, Comparative Examples 1 and 2, and Examples 1 to 12, Examples 61 to 67, Examples 81 to 84, Examples 111 to 114, and Examples 121 to 124). The substrates used were either glass substrates or aluminum alloy substrates. For Conventional Example 1 and Comparative Example 1, glass 4 with the following composition was used. For substrate 1 in Examples 1 and 2, the above-mentioned glass 1 was used; for substrate 1 in Examples 3 to 5, the above-mentioned glass 2 was used; for substrate 1 in Examples 6 to 10, the above-mentioned glass 3 was used; for substrate 1 in Examples 11 and 12, glass 5 and 6, which are amorphous aluminosilicate glasses with a different composition from glasses 1 to 4 and a Young's modulus E of 100 [GPa] or higher, were used; and for substrate 1 in Examples 61 to 67 and Examples 81 to 84, the above-mentioned glass 3 was used. Furthermore, for substrate 1 in Examples 111 to 114, glass 5 was used, and for substrate 1 in Examples 121 to 124, glass 6 was used. Chemical strengthening was not performed on any of the substrates. Furthermore, the specific modulus of elasticity of glasses 1-3, 5, and 6 is 36 [GPa·cm]. 3 It is 1 / g or more. Glass 4 is less than 36.

[0055] (Glass 4) One or more alkali metal oxides selected from the group consisting of SiO2, Al2O3, Li2O, Na2O, and K2O, and one or more alkaline earth metal oxides selected from the group consisting of MgO, CaO, SrO, and BaO, and ZrO2, HfO2, Nb2O5, Ta2O5, La2O3, Y2O 3、 and one or more oxides selected from the group consisting of TiO2, SiO2 50 mol% or more, Al2O33 mol% or more, Furthermore, the total content of SiO2 and Al2O3 is 70-85 mol%, It contains Li2O and Na2O, with Li2O at 4.3 mol% or more. Na2O 5 mol% or more, Furthermore, the total content of Li2O and Na2O is 24 mol% or less. The total content of the alkali metal oxide and alkaline earth metal oxide is 8 mol% or more. Contains, The molar ratio of the total content of the alkali metal oxides to the total content of the alkaline earth metal oxides ((ZrO2+HfO2+Nb2O5+Ta2O5+La2O3+Y2O3+TiO2) / (Li2O+Na2O+K2O+MgO+CaO+SrO+BaO)) is 0.035 or greater. Contains MgO and CaO, MgO less than 3 mol%, CaO 4 mol% or less, Furthermore, the molar ratio of MgO content to CaO content (MgO / CaO) is 0.130 to 0.700. Amorphous glass.

[0056] The fabricated substrates have an outer diameter (diameter) of 85 mm to 97 mm and an inner diameter (circular hole diameter) of 25 mm. The specifications for the chamfered surface are that the width W1 along the radial direction is 60 μm to 150 μm, and the width W2 along the thickness direction is 60 μm to 150 μm. Specifically, for conventional examples 1 and 2 with a thickness exceeding 0.6 mm, the width W1 along the radial direction of the chamfered surface was 150 μm, and the width W2 along the thickness direction was 150 μm. For comparative examples 1 and 2 and examples 1 to 12 with a thickness of 0.6 mm or less, the width W1 and width W2 were 100 μm. The widths W1 and W2 for examples 61 to 67 and 81 to 84 are shown in Tables 3A to 3D below. Therefore, when the widths W1 and W2 are equal, the inclination angle θ1 (see Figure 3) is 45 degrees. In the shaping process, when forming the outer perimeter of the substrate, a diamond scriber was used to make vertical cuts, which were then extended to the opposite side to perform the splitting. In the subsequent chamfering process, a chamfered surface was formed using a full-form grinding wheel.

[0057] (Experiment 1) The 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 determine the maximum amplitude. This evaluation device can apply external impacts (acceleration) of any magnitude, and the resulting movement (vibration) of the outer edge of the substrate can be captured as a video. By analyzing this video, the displacement of the outer edge in the direction normal to the main surface can be measured. Using this evaluation device, an impact test was performed on the substrate, applying a 70G impact over 2 msec in the direction normal to the main surface of the substrate. The vibration of the outer edge of the main surface in the direction normal to the main surface was measured. The measurement results are represented as waveform data as shown in Figure 2. From this waveform data, the maximum displacement in one direction normal to the center of zero displacement at the outer edge of the substrate was determined as the maximum amplitude.

[0058] In actual HDDs, a ramp is incorporated for the magnetic head ramp loading mechanism, and when each magnetic disk is installed, a gap of 0.25 mm is created between the two main surfaces. That is, the gap between the ramps where the magnetic disks fit is the thickness of the magnetic disk + 0.5 mm. In actual HDDs, this gap is designed to remain constant even if the thickness of the circuit board changes. On the other hand, this ramp is not provided in the evaluation device. Therefore, whether or not the circuit board vibration will cause contact with other components such as the ramps (adjacent circuit boards, ramps, or the HDD housing) in an actual HDD is determined by the maximum amplitude of the circuit board vibration. If the maximum amplitude is 0.25 mm or less, it can be determined that contact with the ramps will not occur. If the maximum amplitude exceeds 0.25 mm, there is a very high possibility of contact with another component. The maximum amplitude was investigated for three circuit boards, and the average value of the maximum amplitudes was used. In this evaluation, the circuit board was not rotating and was evaluated in a stationary state. Furthermore, the thickness of the magnetic film and other materials deposited in the media process is approximately 100 nm or less on the main surface, so it can be practically ignored.

[0059] Tables 1, 2A, and 2B below show the evaluation results for the maximum amplitude. The aluminum alloys ("Al alloys") in Conventional Example 2 and Comparative Example 2 are Al-Mg alloys having the following composition: In mass%, 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%, with the remainder being Al. Furthermore, a Ni-P alloy film (P: 10 mass%, remainder Ni) was formed on the surface of the Al-Mg alloy substrate by electroless plating, covering the entire surface of the substrate. The thickness of the substrate with the plated film is 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 maximum amplitude is small due to the thickness T, and the substrate does not come into contact with other components. 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, and there is a very high possibility that the substrate will come into contact with other components. 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 higher, so the maximum amplitude is 0.25 mm or less.

[0062] [Table 2A]

[0063] [Table 2B]

[0064] In Examples 3 to 12, as in Examples 1 and 2, even when 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] Based on the above, according to Tables 1 and 2A,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 substrate 1 material is 90 [GPa] or more, the maximum amplitude will be 0.25 mm or less. Therefore, the substrate 1 will not come into contact with other components due to vibrations caused by external shocks. For this reason, the generation of particles inside the HDD can be suppressed.

[0066] (Experiment 2) Furthermore, using the substrates 1 of Examples 6, 8, 11, and 12 as a reference, the quality of the substrates 1 after impact testing was evaluated using substrates 1 with various changes in the widths W1 and W2 of the chamfered surfaces 5 (Examples 61-67, 81-84, 111-114, and 121-124). The widths W1 and W2 were varied by changing the shape of the grinding wheel and the processing conditions when performing chamfering using a full-form grinding wheel. Examples 61 to 67 are variations of Example 6 with modified widths W1 and W2 of the substrate 1; Examples 81 to 84 are variations of Example 8 with modified widths W1 and W2 of the substrate 1; Examples 111 to 114 are variations of Example 11 with modified widths W1 and W2 of the substrate 1; and Examples 121 to 124 are variations of Example 12 with modified widths W1 and W2 of the substrate 1. Therefore, the material, Young's modulus E, plate thickness, and outer diameter of Examples 61-67 are the same as those of Example 6, the material, Young's modulus E, plate thickness, and outer diameter of Examples 81-84 are the same as those of Example 8, the material, Young's modulus E, plate thickness, and outer diameter of Examples 111-114 are the same as those of Example 11, and the material, Young's modulus E, plate thickness, and outer diameter of Examples 121-124 are the same as those of Example 12.

[0067] The quality evaluation of the fabricated substrate 1 involved disassembling a commercially available HDD, attaching the substrate 1 and spacers from each embodiment to the spindle, and then attaching a simulated lamp component made of engineering plastic so that it protruded from the substrate surface. A gap of 0.25 mm was maintained between the lamp component and the substrate. With the substrate 1 still in place, an impact test was performed in which a 200 G impact was applied in the direction normal to the main surface of the substrate 1 over 2 m seconds. This test was an accelerated test in which the outer edge of the substrate 1 and the lamp component were deliberately collided several times. Afterwards, the particle distribution around the area of ​​the substrate 1 surface that came into contact with the lamp component was observed. Due to the difficulty of quantifying the results, a relative evaluation was used. 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. A lower rank value indicates better quality, with rank 1 representing the highest rating.

[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 making the width W1 120 μm or less. Furthermore, it can be seen that the number of particles can be further reduced by making the width W1 90 μm or less. Therefore, when the diameter D of the substrate 1 is 85 mm or more, the thickness T of the substrate 1 is 0.6 mm or less, and the material used for the substrate 1 has a Young's modulus E of 90 [GPa] or more, the number of particles adhering to the main surface of the substrate 1 inside the HDD can be further suppressed by making the width W1 120 μm or less.

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

[0074] 1. Circuit board for magnetic disks 2 Internal bore 3 Main surface 4 Side wall surface 5 Chamfered surface 6 angles 10 Magnetic disks

Claims

1. A substrate for a magnetic disk, The diameter D of the substrate is 90 mm or more, and the thickness T of the substrate is 0.6 mm or less. Regarding the Young's modulus E of the substrate material and the plate thickness T, the value of E・T3 is 3 to 18 [GPa・mm3]. The Q value of the aforementioned material at room temperature and 3000 Hz is 1500 or less. The substrate for a magnetic disk is characterized in that the substrate is a glass substrate composed of glass having a glass transition temperature of 680°C or higher and a B2O3 content of 0 to 4 mol%.

2. A substrate for a magnetic disk, The diameter D of the substrate is 90 mm or more, and the thickness T of the substrate is 0.6 mm or less. Regarding the Young's modulus E of the substrate material and the plate thickness T, the value of E・T3 is 3 to 18 [GPa・mm3]. Let ρ be the density of the material at room temperature [g / cm³], Q be the Q value of the material at room temperature at 3000 Hz, E be the Young's modulus of the material at room temperature [GPa], and ν be the Poisson's ratio of the material at room temperature. The value of ρ・(1-ν)²・Q / E of the material is less than 25 [g / cm³ / GPa]. The substrate for a magnetic disk is characterized in that the substrate is a glass substrate composed of glass having a glass transition temperature of 680°C or higher and a B2O3 content of 0 to 4 mol%.

3. The magnetic disk substrate according to claim 2, wherein the Q value of the material at 3000 Hz at room temperature is 1500 or less.

4. At least the outer peripheral edge surface of the substrate is provided with a chamfered surface. The magnetic disk substrate according to any one of claims 1 to 3, wherein the width W1 of the chamfered surface along the radial direction of the substrate is 90 μm or less.

5. The aforementioned substrate has a coefficient of thermal expansion of 70 × 10 －７ A magnetic disk substrate according to any one of claims 1 to 4, comprising a material with a ratio of [1 / K] or less.

6. The substrate for a magnetic disk according to any one of claims 1 to 5, wherein the thickness T of the substrate is 0.5 mm or less.

7. The magnetic disk substrate according to any one of claims 1 to 6, wherein the Young's modulus E of the substrate material is 90 GPa or more.

8. A magnetic disk substrate according to any one of claims 1 to 7, 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.

9. The Vickers hardness Hv of the aforementioned substrate is 650 [kgf / mm²]. ２ The magnetic disk substrate according to any one of claims 1 to 8.

10. The Knoop hardness Hk of the substrate is 600 [kgf / mm²]. ２ The magnetic disk substrate according to any one of claims 1 to 9.

11. A magnetic disk having at least a magnetic film on the surface of a substrate for a magnetic disk according to any one of claims 1 to 10.

12. A hard disk drive comprising the magnetic disk described in Claim 11 and a magnetic head.

13. The hard disk drive according to claim 12, comprising 10 or more of the magnetic disks.

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