JP6015876B1 - Glass substrate for magnetic recording medium and method for manufacturing magnetic recording medium - Google Patents

Abstract

To provide a glass substrate for a magnetic recording medium, in which deformation due to heating at a temperature equal to or higher than a strain point is suppressed. A glass substrate for a magnetic recording medium having a compressive stress layer formed by chemical strengthening on a surface layer and having a disk shape with a through hole in the center, wherein the glass has a strain point (Ts) of 585 ° C. or higher. Retardation measured by irradiating light having a wavelength of 543 nm perpendicular to the main surface at a position 0.2 mm radially outward from the inner peripheral edge and 0.2 mm radially inner from the outer peripheral edge. The maximum value of the retardation is 4 nm or less, and the difference between the maximum value and the minimum value of the retardation is 1.2 nm or less. A glass substrate for a magnetic recording medium in which the depth (λ) of the introduced element introduced by chemical strengthening is 3 to 9 μm in all the portions that are separated by 5 mm or more and separated from the outer peripheral edge by 0.5 mm or more radially inward . [Selection] Figure 1

Description

  The present invention relates to a glass substrate for a magnetic recording medium and a method for manufacturing the magnetic recording medium.

In recent years, with the increase in recording capacity of magnetic recording media, higher recording density has been progressing at a high pace. However, with increasing recording density, miniaturization of magnetic particles impairs thermal stability, and crosstalk and a decrease in the S / N ratio of a reproduction signal have become problems. That is, when the high recording density, because the area for recording the 1-bit reduced, need to further reduce scratching or unevenness of the main surface of the glass substrate occurs.

  Furthermore, in order to increase the recording density, a heat-assisted magnetic recording technique is attracting attention as a fusion technique of light and magnetism (see, for example, Patent Document 1). This is a technique in which a magnetic recording layer is irradiated with a laser beam or near-field light and recorded by applying an external magnetic field in a state where the coercive force is lowered in a locally heated portion, and the recorded magnetization is read by a GMR element or the like. Since recording can be performed on a high coercive force medium, the magnetic particles can be miniaturized while maintaining thermal stability. However, in order to form a high coercive force medium on a glass substrate, it is necessary to heat the glass substrate to a high temperature (see, for example, Patent Document 2).

  In addition, the glass substrate is required to be thinned and rotated at high speed, and the fluttering characteristics of the glass substrate are required to be improved. There is a close relationship between the fluttering characteristics and the flatness of the glass substrate, and it is required to keep the flatness of the glass substrate low.

  It is also required that the glass substrate is difficult to break during high-speed rotation and handling. Therefore, the glass substrate is also required to have improved mechanical strength. In order to improve mechanical strength, it has been proposed to apply a chemical strengthening treatment to a glass substrate.

  Patent Document 3 proposes a method of selecting non-defective products by evaluating the variation of the chemically strengthened layer by retardation.

International Publication No. 2013/140469 International Publication No. 2014/129633 JP2013-12260A

  Conventionally, a glass substrate having a chemically strengthened layer may be greatly deformed when heated to a temperature equal to or higher than the strain point in the manufacturing process of the magnetic recording medium.

  The present invention has been made in view of the above problems, and a main object of the present invention is to provide a glass substrate for a magnetic recording medium in which deformation due to heating at a temperature equal to or higher than the strain point is suppressed.

In order to solve the above problems, according to one aspect of the present invention,
A glass substrate for a magnetic recording medium having a compressive stress layer by chemical strengthening on a surface layer and having a disk shape with a through hole in the center,
The strain point (Ts) of the glass is 585 ° C. or higher,
Maximum retardation measured by irradiating light with a wavelength of 543 nm perpendicular to the main surface at a position 0.2 mm radially outward from the inner peripheral edge and a position 0.2 mm radially inner from the outer peripheral edge The value is 4 nm or less, and the magnitude of the difference between the maximum value and the minimum value of the retardation is 1.2 nm or less,
The depth of the introduced element introduced by chemical strengthening in all portions of each main surface that are separated by 0.5 mm or more radially outward from the inner peripheral end and 0.5 mm or more radially inward from the outer peripheral end. There is provided a glass substrate for a magnetic recording medium, wherein (λ) is 3 to 9 μm.

  According to one embodiment of the present invention, a glass substrate for a magnetic recording medium is provided in which deformation due to heating at a temperature equal to or higher than the strain point is suppressed.

1 is a cross-sectional view of a magnetic recording medium according to an embodiment. It is explanatory drawing of the measuring method of the depth of the introductory element of the glass substrate by an example. It is explanatory drawing of the heat processing test of the glass substrate by an example. It is sectional drawing of the glass substrate along the IV-IV line of FIG. It is a figure which shows the shape change of one main surface of the glass substrate before and behind the heat processing test by an example. It is a figure which shows the variation | change_quantity of each measurement point before and after the heat processing test by an example.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. In the drawings, the same or corresponding components are denoted by the same or corresponding reference numerals, and description thereof is omitted. In this specification, “to” representing a numerical range means a range including numerical values before and after the numerical range.

  FIG. 1 is a cross-sectional view of a magnetic recording medium according to an embodiment.

  The magnetic recording medium 10 is an energy assisted magnetic recording type recording medium. The energy-assisted magnetic recording system is a system in which the coercive force of the magnetic recording layer 14 is reduced by applying energy (heat), and recording is performed by applying an external magnetic field in this state. The magnetic particles are recorded while maintaining thermal stability. Can be miniaturized.

  The magnetic recording medium 10 can be used not only in an air atmosphere but also in an inert atmosphere. As the inert atmosphere, in addition to a nitrogen atmosphere and an argon atmosphere, a helium atmosphere having a small atomic weight is particularly preferable because the influence of an air flow accompanying rotation can be reduced. The rotational speed of at least one of the magnetic recording media 10 at the time of recording and reproduction may be 7200 to 20000 rpm.

  The magnetic recording medium 10 includes a glass substrate 11, a heat sink layer 12, a seed layer 13, a magnetic recording layer 14, and a protective layer 15 as shown in FIG. The magnetic recording medium 10 is not limited to the configuration shown in FIG. The magnetic recording medium 10 only needs to have the glass substrate 11 and the magnetic recording layer 14. For example, the magnetic recording medium 10 may not have the heat sink layer 12, the seed layer 13, and the protective layer 15. The magnetic recording medium 10 may further include an adhesion layer, a soft magnetic backing layer, an intermediate layer, and the like between the glass substrate 11 and the magnetic recording layer 14. Further, the magnetic recording medium 10 may have a magnetic recording layer 14 on both sides of the glass substrate 11.

  First, the glass substrate 11 will be described. The glass substrate 11 has a compression stress layer by chemical strengthening on the surface layer, and has a disk shape with a through hole in the center. The glass substrate 11 typically has an inner diameter of 20 mm, an outer diameter of 65 mm or 95 mm, and a thickness of 0.635 mm or 0.8 mm.

  Examples of the chemical strengthening method include an ion exchange method. In the ion exchange method, the glass substrate 11 is immersed in a treatment liquid (for example, a sodium nitrate molten salt, a potassium nitrate molten salt, or a mixed salt thereof). Thereby, ions with a small ion radius contained in the glass are exchanged for ions with a large ion radius. For example, Li ions contained in the glass are exchanged for Na ions. Alternatively, Na ions contained in the glass are exchanged for K ions. Or both. By ion exchange, elements of Na, K, or both are introduced, and a compressive stress layer is formed. When an element is introduced by the chemical strengthening method, compressive stress is generated at that site. Since the generated stress may be weakened due to the relaxation of the glass structure, the depth of the compressive stress layer is shallower than or equal to the depth of the introduced element.

The element introduced by chemical strengthening (hereinafter also simply referred to as “introduced element”) is preferably only K from the viewpoint of reducing the depth λ. Further, from the viewpoint of lowering the LiO ₂ content in the glass and increasing the strain point Ts of the glass, the introduced element is preferably only K.

  The chemical strengthening treatment conditions are appropriately selected according to the thickness of the glass substrate 11 and the like, but it is typical to immerse the glass substrate 11 in a treatment liquid at 350 to 550 ° C. for 0.1 to 20 hours. From an economical viewpoint, the temperature of the treatment liquid is preferably 350 to 500 ° C., and the immersion time is 0.2 to 16 hours. A more preferable immersion time is 0.5 to 10 hours.

  The compressive stress layer may be formed on the entire surface of the glass substrate 11. The glass substrate 11 has both main surfaces 11a and 11b, an inner peripheral connection surface 11c, and an outer peripheral connection surface 11d as surfaces. Both main surfaces 11a and 11b are parallel to each other.

  The inner peripheral connection surface 11c connects the inner peripheral edges of both the main surfaces 11a and 11b. For example, as shown in FIG. 1, the inner peripheral connection surface 11c has a vertical surface perpendicular to both the main surfaces 11a and 11b, and each main surface 11a is between the vertical surface and each main surface 11a and 11b. , 11b. The inner peripheral connection surface 11c may have curved surfaces between the inclined surface and the main surfaces 11a and 11b and between the inclined surface and the vertical surface. Moreover, the inner peripheral connection surface 11c does not need to have a vertical surface or an inclined surface, and may have a curved surface having an arcuate cross section as a whole. The width W1 represents the width of the inner peripheral connection surface 11c in the radial direction of the glass substrate 11 (left and right direction in FIG. 1).

  11 d of outer periphery connection surfaces connect the outer periphery of both the main surfaces 11a and 11b. For example, as shown in FIG. 1, the outer peripheral connection surface 11d has a vertical surface perpendicular to both the main surfaces 11a and 11b, and the main surfaces 11a and 11b are arranged between the vertical surface and the main surfaces 11a and 11b. It further has an inclined surface inclined with respect to 11b. The outer peripheral connection surface 11d may have curved surfaces between the inclined surface and the main surfaces 11a and 11b and between the inclined surface and the vertical surface. Moreover, the outer peripheral connection surface 11d does not need to have a vertical surface or an inclined surface, and may have a curved surface having an arcuate cross section as a whole. The width W2 represents the width of the outer peripheral connection surface 11d in the radial direction of the glass substrate 11 (left-right direction in FIG. 1).

  The strain point Ts of the glass of the glass substrate 11 is 585 ° C. or higher. If it is lower than 585 ° C., it is necessary to keep the film forming temperature low in the film forming process of the magnetic recording layer 14, which may make it difficult to form the magnetic recording layer 14 having a high coercive force.

  The strain point Ts is preferably 600 ° C. or higher, more preferably 635 ° C. or higher, further preferably 650 ° C. or higher, particularly preferably 655 ° C. or higher, more preferably 660 ° C. or higher. When the strain point Ts is 635 ° C. or higher, the magnetic recording layer 14 can be formed at 650 ° C. or higher, which contributes to improving the quality of the magnetic recording layer 14. The strain point Ts is preferably 750 ° C. or lower, more preferably 720 ° C. or lower, and still more preferably 700 ° C. or lower, from the viewpoint of moldability during glass production.

  The inventor appropriately adjusts the retardation due to the stress distribution in the vicinity of the inner and outer peripheral edges after the chemical strengthening of the glass substrate 11 and the depth λ of the introduced element due to the chemical strengthening of the glass substrate 11, respectively. It has been found that deformation due to heating at the above temperature can be suppressed. Focusing on the stress distribution in the vicinity of the inner and outer peripheral edges of the glass substrate 11 after chemical strengthening is because the surface roughness is rougher in the vicinity of the inner and outer peripheral edges of the glass substrate 11 than in the portion far from the inner and outer peripheral ends. This is because the later stress anisotropy increases.

  The anisotropy of the stress of the glass substrate 11 can be represented by the retardation of the glass substrate 11. The retardation of the glass substrate 11 is measured by irradiating light with a wavelength of 543 nm perpendicularly to the main surfaces 11a and 11b and detecting the phase difference between two orthogonal linearly polarized waves.

First, retardation of the glass substrate 11 will be described. Hereinafter, the retardation at a position 0.2 mm radially outward from the inner peripheral end 11e of the glass substrate 11 is denoted as RI _0.2 . RI _0.2 is measured over the entire circumference. The maximum value of RI _0.2 is expressed as RI _0.2MAX , the minimum value of RI _0.2 is expressed as RI _0.2MIN , and the difference between the maximum value and the minimum value of RI _0.2 is expressed as RI _0.2DEF . . Further, the retardation at a position of 0.2 mm radially inward from the outer peripheral end 11f of the glass substrate 11 is expressed as RO _0.2 . RO _0.2 is measured over the entire circumference. The maximum value of RO _0.2 is expressed as RO _0.2MAX , the minimum value of RO _0.2 is expressed as RO _0.2MIN , and the difference between the maximum value and the minimum value of RO _0.2 is expressed as RO _0.2DEF . .

In the glass substrate 11, RI _0.2MAX and RO _0.2MAX are each 4 nm or less, and RI _0.2DEF and RO _0.2DEF are 1.2 nm or less, respectively. Therefore, the anisotropy of stress is small in the vicinity of the inner and outer peripheral edges of the glass substrate 11 and the variation thereof is small, and deformation of the glass substrate 11 during heat treatment due to these influences can be suppressed. That is, it is possible to suppress deformation when the chemically strengthened glass substrate is heated at a temperature equal to or higher than the strain point Ts.

RI _0.2MAX and RO _0.2MAX are preferably 3.5 nm or less, more preferably 3 nm or less, still more preferably 2.5 nm, particularly preferably 2 nm, and still more preferably 1.5 nm.

Further, RI _0.2DEF and RO _0.2DEF are preferably 1.1 nm or less, more preferably 1.0 nm or less, still more preferably 0.9 nm or less, particularly preferably 0.8 nm or less, and more preferably 0.7 nm. It is as follows.

RI _0.2 is chemically strengthened prior to and depends on the difference in surface roughness between the both main surfaces 11a, 11b and the inner circumferential connecting surface 11c. Similarly, RO _0.2 depends on a difference in surface roughness between the main surfaces 11a and 11b and the outer peripheral connection surface 11d before chemical strengthening. As the surface is rougher, ion exchange is more likely to proceed, and as the difference in surface roughness is greater, the anisotropy of stress after chemical strengthening is greater. Therefore, in order to reduce RI _0.2 and RO _0.2 , it is effective to mirror-polish the inner peripheral connection surface 11c and the outer peripheral connection surface 11d before chemical strengthening in the same manner as the two main surfaces 11a and 11b. is there.

In order to reduce RI _0.2 and RO _0.2 , the inner peripheral connection surface 11c and its vicinity, and the outer peripheral connection surface 11d and its vicinity may be protected with a protective layer such as clay during chemical strengthening. It is valid. It is also effective to etch the inner peripheral connection surface 11c and its vicinity and the outer peripheral connection surface 11d and its vicinity after chemical strengthening.

RI _0.2 is affected by the width W1. The smaller the width W1, the smaller the RI _0.2 . Similarly, RO _0.2 is affected by the width W2. The smaller the width W2, the smaller the RO _0.2 . Each of the width W1 and the width W2 is preferably 0.15 mm or less, more preferably 0.12 mm or less, further preferably 0.10 mm or less, particularly preferably 0.08 mm or less, and further preferably 0.06 mm or less. The width W1 and the width W2 are each 0.03 mm or more from the viewpoint of preventing the glass substrate 11 from being chipped.

When the width W1 or the width W2 is 0.08 mm or less, the retardation RI is _0.1 at a position 0.1 mm radially outward from the inner peripheral end 11e and the position is 0.1 mm radially inward from the outer peripheral end 11f. A retardation RO of _0.1 is important. RI _0.1 and RO _0.1 are measured over the entire circumference. _Hereinafter, the maximum value of the _{RI 0.1 RI 0.1MAX,} minimum value _RI 0.1 min of _{RI 0.1,} the magnitude of the difference between the maximum value and the minimum value of the _{RI 0.1} and _{RI 0.1DEF} write. _The maximum value _RO 0.1MAX of _{RO 0.1,} the minimum value _RO 0.1 min of _{RO 0.1,} the magnitude of the difference between the maximum value and the minimum value of _{RO 0.1} and _{RO 0.1DEF} write.

RI _0.1MAX and RO _0.1MAX are each preferably 5 nm or less, more preferably 4.5 nm or less, further preferably 4 nm or less, particularly preferably 3.5 nm, and more preferably 3 nm. The lower the RI _0.1MAX and the RO _0.1MAX , the better, but typically 0.1 nm or more.

RI _0.1DEF and RO _0.1DEF are each preferably 2.5 nm or less, more preferably 2.0 nm or less, still more preferably 1.5 nm or less, particularly preferably 1.0 nm or less, and still more preferably 0. 8 nm or less.

  Next, the depth λ of the introduced element due to the chemical strengthening of the glass substrate 11 will be described. Of all the main surfaces 11a and 11b, the depth λ of the introduced element is 3 in all the portions that are 0.5 mm or more away from the inner peripheral end 11e radially outward and 0.5 mm or more away from the outer peripheral end 11f radially inward. ~ 9 μm. Thereby, a deformation | transformation when the glass substrate after chemical strengthening is heated at the temperature more than the strain point Ts can be suppressed. The reason is that the introduced amount of the introduced element is an appropriate amount, and the introduced element (more specifically, ions of the introduced element) moves moderately so as to absorb the stress of the glass substrate 11 during the heat treatment after chemical strengthening. It is done. The stress applied to the glass substrate 11 during the heat treatment is caused by uneven heating, uneven support, gravity, and the like.

  The depth λ of the introduced element is preferably 3.5 μm or more, more preferably 4 μm or more, still more preferably 4.5 μm or more, and particularly preferably 5 μm or more.

The depth λ of the introduced element is measured by analyzing the chemical composition of the cut surface of the glass substrate 11 in the plate thickness direction or by analyzing the chemical composition while etching the main surfaces 11a and 11b of the glass substrate 11 in the depth direction. Is done. For the chemical composition analysis of the cut surface, electron microanalyzer analysis (EPMA), scanning electron microscope / energy dispersive X-ray spectroscopy (SEM-EDX), or the like is used. For chemical composition analysis involving etching, X-ray photoelectron spectroscopy (XPS) or the like is used. In order to suppress movement of elements introduced by ion exchange during etching, etching at C ₆₀ is preferable.

  FIG. 2 is an explanatory diagram of a method for measuring the depth of an introduced element in a glass substrate according to an example. 2A is an overall view, and FIG. 2B is a partially enlarged view.

  First, as shown in FIG. 2A, the concentration of the introduced element is measured every 0.1 μm in the depth direction from the main surfaces 11a and 11b. The concentration is measured from the depth at which the measured value decreases to a substantially constant level to a depth of 21 μm. However, when the substantially constant depth is less than 9 μm, the concentration up to 30 μm is measured. In FIGS. 2A and 2B, if all the concentrations of the introduced elements are plotted every 0.1 μm, the number of display points increases and it becomes difficult to see the figure.

Next, L (μm) shown in FIG. 2A is obtained from the equation “L = (ΔC / 10) + C _MIN ”. Here, ΔC represents the magnitude of the difference between the maximum density C _MAX and the minimum density C _MIN up to the deepest point measured from the surface (in this example, the depth is 30 μm). L is rounded down to two decimal places.

Next, as shown in FIG. 2A, the average value C _AVE of the density from the position where the depth is 10 μm deeper than L to the position where the depth is 20 μm deeper than L is obtained.

Finally, as shown in FIG. 2B, the density distribution from the position where the depth is 0.75 × L to the position where the depth is 1.05 × L is approximated by a least square line LS, and the least square is obtained. concentration and C _AVE and introduce a depth becomes deep λ on the straight line LS.

  In addition, when there are a plurality of introduced elements, the depth of the introduced element that has penetrated deeper is the depth λ of the introduced element.

By the way, it is considered that the introduced element moves while exchanging ions in the glass during the heat treatment after chemical strengthening. When the introduced element is Na, the element to be ion exchanged is Li. When the introduced element is K, the element to be ion exchanged is Na. When the introduced elements are both Na and K, the elements to be ion exchanged are both Li and Na. The amount of movement of the introduced element is represented by X calculated by the following formula (1).
X = CA × CB × (CC / CD) × λ / d (1)
In the above formula (1), when the total number of moles of all oxides when the glass composition is expressed in terms of oxide is 100, the number of moles of Al ₂ O _{3 in} the glass is CA, The number of moles in terms of oxide of the element to be ion-exchanged is represented by CB, the number of moles of MgO in the glass by CC, and the total number of moles of MgO and CaO in the glass by CD. These values are measured at a position deeper than the depth at which the introduced element is introduced. Since Al ₂ O ₃ promotes ion exchange, the larger the CA, the easier the introduced element moves. CB is the total content of the plurality of elements when there are a plurality of elements to be ion-exchanged (for example, both Li and Na). The greater the CB, the greater the amount of elements that are ion-exchanged, and the easier it is for the introduced element to move. Of the oxides of alkaline earth elements, MgO has a stronger function to facilitate ion exchange than CaO. Therefore, the larger the CC / CD, the easier the introduced element moves. On the other hand, λ is the depth (μm) of the introduced element by ion exchange, d is the thickness (μm) of the glass substrate 11, and λ / d relatively represents the introduced amount of the introduced element. The larger the λ / d is, the more the introduced element is introduced and the more the introduced element is moved.

  X is the product of the ease of movement of the introduced element and the amount of introduced element introduced. X is preferably 0.1 to 1.3 so that the deformation of the glass substrate can be suppressed by appropriately moving the introduced element during the heat treatment after chemical strengthening. X is more preferably 0.15 or more, further preferably 0.2 or more, and particularly preferably 0.3 or more. X is more preferably 1.1 or less, still more preferably 0.9 or less, particularly preferably 0.8 or less, and still more preferably 0.7 or less. When CA is 8 or more, the introduced element is very easy to move. Therefore, X is preferably 0.4 or less, more preferably 0.35 or less, particularly preferably 0.3 or less, and further preferably 0.25 or less. . When CA is 7 or less, since the introduced element is difficult to move, X is more preferably 0.35 or more, further preferably 0.45 or more, and particularly preferably 0.55 or more so that the deformation of the glass substrate can be suppressed. Preferably, 0.6 or more is more preferable.

As the glass of the glass substrate 11, for example, aluminosilicate glass containing SiO ₂ and Al ₂ O ₃ as main components is used. Aluminosilicate glass before the chemical strengthening may be one which contains 1% to 25% of Na ₂ O and Li ₂ O in mol% based on oxides.

The glass substrate 11 is expressed in mol% on the basis of oxide at a position deeper than the depth at which the introduced element is introduced, and SiO ₂ is 60 to 75%, Al ₂ O ₃ is 5 to 15%, and B ₂ O ₃ is 0-12%, 0-20% of MgO, the CaO 0-12%, 0% of SrO, 0% of BaO, Na ₂ O 20% more than 0% or less, the _{K 2} O 0 10%, Li ₂ O 0-15%, TiO ₂ 0-5%, ZrO ₂ 0-5%, and a total of 95% or more of these, Na ₂ O and Li ₂ O The total content of is preferably 1 to 25%.

The glass substrate 11 has a SiO ₂ content of 65 to 70%, an Al ₂ O ₃ content of 6 to 12%, and a B ₂ O ₃ content of not less than the depth at which the introduced element is introduced and expressed in terms of oxide-based mol%. 0 to 3%, MgO 1 to 18%, CaO 0 to 10%, SrO 0 to 6%, BaO 0 to 2%, Na ₂ O 0.5 to 12%, K ₂ O 0 to 0% 3%, Li ₂ O 0-5%, TiO ₂ 0-3%, ZrO ₂ 0-3%, and a total of 95% or more of these, Na ₂ O and Li ₂ O The total content is more preferably 2 to 15%.

  Next, each component of the glass substrate 11 will be described. In the description of each component,% means mol%.

SiO ₂ is an essential component for forming a glass skeleton. When the content of SiO ₂ is less than 60%, the acid resistance is remarkably lowered, the strain point Ts is easily lowered, and scratches are liable to occur. In this case, the liquidus temperature rises, and the temperature T ₂ (the temperature at which the viscosity becomes 10 ² dPa · s) and the temperature T ₄ (the temperature at which the viscosity becomes 10 ⁴ dPa · s) are likely to decrease, resulting in a loss. Permeability tends to occur. Therefore, the content of SiO ₂ is 60% or more, preferably 63% or more, more preferably 66% or more, still more preferably 67% or more, and particularly preferably 68% or more.

On the other hand, when the content of SiO ₂ is more than 75%, the temperature T ₂ and the temperature T ₄ are likely to rise, and it becomes difficult to melt the glass, and the defoaming property at the time of refining is lowered and defects are likely to occur. In this case, the specific elastic modulus E / ρ tends to decrease. Furthermore, in this case, the average linear expansion coefficient α (hereinafter also simply referred to as “average linear expansion coefficient α”) of the glass at 50 to 350 ° C. tends to be small. Therefore, the content of SiO ₂ is 75% or less, preferably 70% or less.

Al ₂ O ₃ is an essential component that enhances Young's modulus and alkali resistance. When the content of Al ₂ O ₃ is less than 5%, the Young's modulus E tends to decrease, and the strain point Ts tends to decrease. Therefore, the content of Al ₂ O ₃ is 5% or more, preferably 6% or more, more preferably 7% or more, still more preferably 8% or more, particularly preferably 9% or more, more preferably 10% or more. is there.

On the other hand, when the content of Al ₂ O ₃ is more than 15%, the density ρ is large and the specific modulus E / ρ is likely to be small, and the fluttering characteristics are likely to be deteriorated. In this case, the acid resistance tends to decrease. Furthermore, in this case, since the liquidus temperature tends to be too high, molding tends to be difficult. Therefore, the content of Al ₂ O ₃ is 15% or less, preferably 12% or less, more preferably 11.5% or less, still more preferably 11% or less, and particularly preferably 10.5% or less.

B ₂ O ₃ may be contained in order to improve chemical resistance, scratch resistance, and glass solubility. On the other hand, when the content of B ₂ O ₃ is more than 12%, the Young's modulus E and thus the specific elastic modulus E / ρ are likely to be lowered, and the strain point Ts is also likely to be lowered. Therefore, the content of B ₂ O ₃ is 12% or less, preferably 3% or less, more preferably 2% or less, still more preferably 1% or less, and still more preferably substantially free.

  MgO is not essential, but may be contained in a range of 20% or less in order to improve the solubility, improve the Young's modulus E and consequently the specific elastic modulus E / ρ, and increase the strain point Ts. When the content of MgO exceeds 20%, the liquidus temperature tends to rise, and the production tends to be difficult. The content of MgO is preferably 18% or less, more preferably 17% or less, still more preferably 15% or less, particularly preferably 12% or less, and even more preferably 10% or less. Further, the content of MgO is preferably 1% or more, more preferably 2% or more, further preferably 3% or more, particularly preferably 5% or more, and further preferably 8% or more.

CaO may be contained in order to lower the melting temperature and lower the devitrification temperature while suppressing a decrease in strain point. In addition, there is a function of slowing the ion exchange rate at the time of chemical strengthening, which is effective for keeping the depth λ of the introduced element shallow. The content of CaO is preferably 1% or more, more preferably 2% or more, further preferably 3% or more, particularly preferably 4% or more, and further preferably 5% or more.

  On the other hand, when the content of CaO is more than 12%, Young's modulus E, specific elastic modulus E / ρ, and glass transition point Tg tend to decrease. In addition, the ion exchange rate during chemical strengthening may become extremely slow, and it may take too much time to form the compressive stress layer. Therefore, the CaO content is preferably 11% or less, more preferably 10% or less, further preferably 9% or less, particularly preferably 8% or less, and even more preferably 7% or less.

  SrO may be contained as necessary, but may be contained in a range of 10% or less in order to improve solubility. If it exceeds 10%, the ion exchange rate during chemical strengthening becomes extremely slow, and it may take too much time to form the compressive stress layer. Preferably it is 6%, more preferably 2% or less, still more preferably 1% or less, particularly preferably 0.5% or less, and still more preferably substantially free.

  BaO may be contained as necessary, but may be contained in a range of 10% or less in order to improve solubility. If it exceeds 10%, the ion exchange rate during chemical strengthening becomes extremely slow, and it may take too much time to form the compressive stress layer. In addition, the glass becomes brittle and may be easily scratched. Preferably it is 2% or less, More preferably, it is 1% or less, More preferably, it is 0.5% or less, Most preferably, it is 0.2% or less, More preferably, it does not contain substantially.

  The total content of MgO, CaO, SrO, and BaO (hereinafter collectively referred to as “RO”) is included in order to increase the specific elastic modulus E / ρ and to improve the solubility of the glass. May be. Preferably it is 4% or more, More preferably, it is 7% or more, More preferably, it is 10% or more, Especially preferably, it is 13% or more, More preferably, it is 15% or more. When the total content of RO exceeds 22%, the liquidus temperature rises, the viscosity tends to decrease, and the acid resistance tends to decrease. The total RO content is 22% or less, preferably 20% or less, more preferably 19% or less, still more preferably 18% or less, particularly preferably 17% or less, and even more preferably 16% or less.

Na ₂ O is a component that forms a compressive stress layer by ion exchange using K molten salt and improves the meltability of glass, and is essential. Preferably it is 0.5% or more, More preferably, it is 2% or more, More preferably, it is 3% or more, Especially preferably, it is 4%, More preferably, it is 5% or more. On the other hand, if Na ₂ O exceeds 20%, the strain point may decrease, or the weather resistance of the glass may decrease, and the Young's modulus may decrease. It is preferably 20% or less, more preferably 12% or less, further preferably 10% or less, particularly preferably 8% or less, and still more preferably 7% or less.

K ₂ O is not essential, but may be contained in order to prevent the depth λ of the introduced element from becoming too deep during chemical strengthening. However, if it exceeds 10%, the ion exchange rate during chemical strengthening becomes extremely slow, and it may take too much time to form the compressive stress layer. Alternatively, the strain point may be lowered, or the weather resistance of the glass may be lowered. Preferably it is 10% or less, More preferably, it is 3% or less, More preferably, it is 1% or less, Most preferably, it is 0.5% or less, More preferably, it does not contain substantially.

Li ₂ O forms a compressive stress layer by ion exchange using Na molten salt, and may be included because it is a component that increases the Young's modulus. The ion exchange rate is high, and the surface compression layer may become too deep. Moreover, there exists a possibility that a strain point may fall and heat resistance may deteriorate. Preferably it is 15% or less, more preferably 5% or less, even more preferably 2% or less, particularly preferably 0.8%, even more preferably 0.5% or less, even more preferably 0.1% or less, and even more preferably. Is 0.05% or less, particularly preferably not substantially contained.

Na ₂ O and Li ₂ O are elements that cause ion exchange by chemical strengthening, and are components that improve the meltability of glass, so that they are contained in a total amount of 1% or more. The total content of Na ₂ O and Li ₂ O is preferably at least 2%, more preferably at least 3%, even more preferably at least 4%, particularly preferably at least 5%, more preferably at least 5.5%. is there.

On the other hand, when the total content of Na ₂ O and Li ₂ O exceeds 25%, the Young's modulus E and thus the specific modulus E / ρ tends to decrease, and fluttering characteristics may be deteriorated. In addition, the strain point Ts is likely to be lowered, and thermal deformation is likely to occur when the substrate is heated during the formation of the magnetic recording layer. Therefore, the total content of Na ₂ O and Li ₂ O is 25% or less, preferably 20% or less, more preferably 15% or less, still more preferably 12% or less, and particularly preferably 10% or less.

Na ₂ O, Li ₂ O, and K ₂ O are components that improve the meltability of the glass, and are contained in a total of 1% or more. The total content of Na ₂ O, Li ₂ O and K ₂ O is preferably 2% or more, more preferably 3% or more, still more preferably 4% or more, particularly preferably 5% or more, and still more preferably 5. 5% or more.

On the other hand, when the total content of Na ₂ O, Li ₂ O, and K ₂ O exceeds 25%, the strain point tends to decrease, and thermal deformation tends to occur when the substrate is heated during the formation of the magnetic recording layer. . Further, since the linear expansion coefficient α increases and the thermal shock resistance decreases, the cooling rate of the glass substrate 11 cannot be increased during the production of the magnetic recording medium as described later, and the productivity may be reduced. The total content of Na ₂ O and Li ₂ O is 25% or less, preferably 18% or less, more preferably 13% or less, still more preferably 11% or less, particularly preferably 9.5% or less, more preferably 8.5% or less, most preferably 8% or less.

TiO ₂ can be contained in a range of 5% or less for improving solubility or improving chemical durability against acids and alkalis. When the content of TiO ₂ exceeds 5%, the devitrification temperature of the glass tends to increase, the moldability tends to deteriorate, and the surface tends to be damaged. The content of TiO ₂ is preferably 3% or less, more preferably 2% or less, further preferably 1% or less, particularly preferably 0.5% or less, and still more preferably substantially free.

ZrO ₂ may be contained for improving the Young's modulus E and specific modulus E / ρ of the glass, or for improving the chemical durability against acids and alkalis. However, when there is too much ZrO ₂ , the devitrification temperature increases. Therefore, the ZrO ₂ content is preferably 5% or less, more preferably 3% or less, further preferably 1% or less, particularly preferably 0.5% or less, and still more preferably substantially free.

The glass substrate 11 may contain other components at 1% or less and 5% or less in total. For example, the glass substrate 11 is made of ZnO, WO ₃ , Nb ₂ O ₅ , V ₂ O ₅ , Bi ₂ O ₃ , MoO ₃ , P ₂ for the purpose of improving rigidity, weather resistance, solubility, devitrification and the like. O ₅ or the like may be contained. Further, in order to improve solubility of the glass, the clarity, the glass substrate 11, as a component resulting from the refining _{agent, SO 3, F, Cl,} SnO 2, PbO, As 2 O 3, CeO 2, Sb 2 O ₃ may be contained. The defoaming property is improved when the SnO ₂ content is 0.01% or more. Moreover, when the content of SnO ₂ is more than 0.5%, the material characteristics are easily affected. _{Incidentally, PbO, As 2 O 3,} Sb 2 O 3 is may contain, for ease of recycling glass, it is preferred not substantially contained.

Β-OH representing the moisture content of the glass substrate 11 is preferably 0.05 to 0.35 mm ⁻¹ . If β-OH exceeds 0.35 mm ⁻¹ , the strain point Ts may be lowered. β-OH is more preferably 0.3 mm ⁻¹ , further preferably 0.25 mm ⁻¹ , particularly preferably 0.2 mm ⁻¹ , more preferably 0.15 mm ⁻¹ .

  The Young's modulus E of the glass substrate 11 is preferably 70 GPa or more. If it is less than 70 GPa, the specific elastic modulus described later becomes small, and not only fluttering is likely to occur during rotation of the magnetic recording medium 10, but the crack resistance and breaking strength of the glass may be insufficient. More preferably, it is 75 GPa or more, More preferably, it is 80 GPa or more, Especially preferably, it is 81 GPa or more, More preferably, it is 82 GPa or more. Typically, Young's modulus is 88 GPa or less due to restrictions of the glass material.

The density ρ of the glass substrate 11 is preferably 2.55 g / cm ³ or less. When the density ρ exceeds 2.55, the specific modulus E / ρ obtained by dividing the Young's modulus E by the density ρ becomes small, and fluttering is likely to occur. The density [rho, more preferably 2.53 g / cm ³ or less, more preferably 2.51 g / cm ³ or less, particularly preferably 2.49 g / cm ³ or less, more preferably 2.47 g / cm ³ or less. The density ρ is typically 2.35 g / cm ³ or more.

  The specific elastic modulus E / ρ of the glass substrate 11 is preferably 28 MNm / kg or more. When the specific elastic modulus E / ρ is smaller than 28 MNm / kg, not only the fluttering described above is likely to occur, but also it is bent by its own weight during roller conveyance or partial support, and flows normally in the manufacturing process. There is a risk that it will not be allowed. The specific modulus E / ρ is more preferably 30 MNm / kg or more, further preferably 31 MNm / kg or more, particularly preferably 32 MNm / kg or more, and further preferably 33 MNm / kg or more.

In order to set the specific modulus E / ρ to 28 MNm / kg or more, for example, if the Young's modulus E is 70 GPa or more, the density ρ is 2.50 g / cm ³ or less and the Young's modulus E is 80 GPa or more. The density ρ may be 2.85 g / cm ³ or less. The specific modulus E / ρ is typically 35 MNm / kg or less.

The average linear expansion coefficient α of the glass substrate 11 is preferably 30 × 10 ⁻⁷ to 80 × 10 ⁻⁷ / ° C. If the average linear expansion coefficient α is less than 30 × 10 ⁻⁷ / ° C. or more than 80 × 10 ⁻⁷ / ° C., the difference in thermal expansion from the magnetic recording layer 14 becomes too large, and defects such as peeling tend to occur.

If the average linear expansion coefficient α is 35 × 10 ⁻⁷ / ° C. or more, the difference between the average linear expansion coefficient and the metal spindle chuck holding the glass substrate 11 is small, and the glass substrate 11 is cracked. Is more preferable. The average linear expansion coefficient α is more preferably 40 × 10 ⁻⁷ / ° C. or more, particularly preferably 45 × 10 ⁻⁷ / ° C. or more, and further preferably 50 × 10 ⁻⁷ / ° C. or more. On the other hand, when the average linear expansion coefficient α is 70 × 10 ⁻⁷ / ° C. or more, the thermal shock resistance is lowered, and thus the cooling rate of the glass substrate 11 cannot be increased in the manufacturing process of the magnetic recording medium. Decreases. The average linear expansion coefficient α is preferably 65 × 10 ⁻⁷ / ° C. or less, more preferably 60 × 10 ⁻⁷ / ° C. or less, and still more preferably 55 × 10 ⁻⁷ / ° C. or less.

  The glass substrate 11 is formed by processing glass formed into a plate shape by, for example, a float method, a slot down draw method, or a fusion method (so-called overflow down draw method). Or you may process the glass shape | molded by the press method. In addition, plate-shaped glass may be produced by cutting glass formed into a cylindrical shape with a wire saw.

  The glass substrate 11 is used for manufacturing the magnetic recording medium 10. On the glass substrate 11, for example, as described above, the heat sink layer 12, the seed layer 13, the magnetic recording layer 14, the protective layer 15 and the like are formed. In this process, the glass substrate 11 is heated to a temperature equal to or higher than the strain point Ts.

  The heat sink layer 12 effectively absorbs excess heat of the magnetic recording layer 14 generated during energy-assisted magnetic recording. The heat sink layer 12 can be formed of a metal having high thermal conductivity and specific heat capacity. As the material of the heat sink layer 12, a general material is used.

  The seed layer 13 ensures adhesion between the heat sink layer 12 and the magnetic recording layer 14. The seed layer 13 controls the grain size and crystal orientation of the magnetic crystal grains of the magnetic recording layer 14. Further, the seed layer 13 controls the temperature rise and temperature distribution of the magnetic recording layer 14 as a thermal barrier. As the material of the seed layer 13, a general material is used.

  The magnetic recording layer 14 is made of a material for energy-assisted magnetic recording, and a material for heat-assisted magnetic recording is particularly preferable. The material is preferably a material containing at least one of Co and Fe, and further contains at least one of Pt, Pd, Ni, Mn, Cr, Cu, Ag, and Au. It is preferable. For example, a CoCr alloy, a CoCrPt alloy, a FePt alloy, a FePd alloy, or the like can be used. From the viewpoint of improving magnetic properties, an FePt alloy is particularly preferable.

  The magnetic recording layer 14 is a layer for writing signals. The magnetic recording layer 14 may have a multi-layer structure, and each layer has a granular structure composed of magnetic crystal grains and nonmagnetic portions. As the material of the magnetic recording layer 14, a general material is used, but it is a material for energy-assisted magnetic recording, and a material for heat-assisted magnetic recording is particularly preferable. The magnetic recording layer 14 may have a single layer structure.

The surface recording density of the magnetic recording layer 14 may be 800 Gbits / in ² or more.

  The protective layer 15 protects the magnetic recording layer 14. The protective layer 15 may have either a single layer structure or a laminated structure. As the material of the protective layer 15, a general material is used.

  By the way, the manufacturing method of the magnetic recording medium 10 has a heating process of heating the glass substrate 11 at a temperature equal to or higher than the strain point Ts.

  According to the present embodiment, as described above, the stress distribution in the vicinity of the inner and outer peripheral edges after the chemical strengthening of the glass substrate 11 and the depth λ of the introduced element due to the chemical strengthening of the glass substrate 11 are adjusted appropriately. Therefore, deformation due to heating at a temperature equal to or higher than the strain point Ts in the heating process can be suppressed.

In the heating step, a time average temperature during heating the glass substrate 11 to a temperature T1 (T1 = Ts−10) or higher at a temperature 10 ° C. lower than the strain point Ts is T _ave (° C.) or higher than the temperature T1. If the time for heating the glass substrate 11 to the temperature is t _ave (min), Y calculated by the following formula (2) is 140 or less.
Y = (T _ave −Ts) × t _ave (2)
The reason why the temperature equal to or higher than the temperature T1 is considered in the above formula (2) is that the glass easily flows at the temperature equal to or higher than the temperature T1. T _ave is obtained by integrating the heating temperature while the heating temperature is equal to or higher than temperature T1, and dividing the integrated value by time t _ave . The difference between T _ave and Ts (T _ave −Ts) represents the ease of glass flow.

  Y is the product of the ease of glass flow and the time during which the glass can flow, and represents the ease of deformation of the glass substrate 11 due to glass flow. If Y is 140 or less, the deformation of the glass substrate 11 due to glass flow can be suppressed. Y is more preferably 130 or less, still more preferably 120 or less, particularly preferably 110 or less, and still more preferably 100 or less.

Further, in the above heating step, _{T 500} (° C.) the temperature of the time average during heating the glass substrate 11 to a temperature above 500 ° C., the time to heat the glass substrate 11 to a temperature above 500 ° C. _{t 500} (min ), Z calculated by the following formula (3) is 5 to 32.
Z = (T ₅₀₀ / Ts) × (t ₅₀₀ ) ^1/2 × λ (3)
The reason why the temperature of 500 ° C. (° C.) or higher is considered in the above formula (3) is that the introduced element easily moves while ion exchange with other elements at a temperature of 500 ° C. or higher. In typical chemical strengthening, ion exchange is performed at 400 to 500 ° C. T ₅₀₀ is obtained by integrating the temperature of the glass substrate 11 in the heating step while the temperature of the glass substrate 11 is 500 ° C. or higher and dividing the integrated value by the time t ₅₀₀ . T ₅₀₀ / Ts is an index of the ease of movement of the introduced element in the heating process. The square root of t ₅₀₀ relatively represents the moving distance of the introduced element in the heating process. λ represents the depth of the introduced element due to chemical strengthening, and relatively represents the introduced amount. Therefore, Z is the product of the ease of movement of the introduced element in the heating step, the relative value of the distance of movement of the introduced element in the heating step, and the relative value of the amount of introduced element introduced by chemical strengthening. This is an amount that relatively indicates how far the introduced element has moved.

  If Z is 5 to 32, the introduced element moves to an appropriate range, and the deformation of the glass substrate 11 can be suppressed. Z is more preferably 7 or more, further preferably 10 or more, particularly preferably 15 or more, and still more preferably 20 or more. Z is more preferably 30 or less, still more preferably 28 or less, particularly preferably 26 or less, and still more preferably 24 or less.

The maximum temperature T _MAX of the glass substrate 11 in the heating step is preferably 600 ° C. or higher. Thereby, the characteristics of the magnetic recording layer 14 can be improved by forming the magnetic recording layer 14 at a temperature of 600 ° C. or higher. T _MAX is more preferably 620 ° C. or higher, further preferably 630 ° C. or higher, particularly preferably 640 ° C. or higher, more preferably 650 ° C. or higher.

Further, T _MAX is preferably not less than the strain point Ts and not more than a temperature T2 (T2 = Ts + 15) which is 15 ° C. higher than the strain point Ts. When T _MAX is Ts or more and T2 or less, the effect of suppressing deformation during heating can be enjoyed. When T _MAX is higher than T2, deformation due to the flow of glass becomes remarkable. Since it is difficult to produce the glass substrate 11 having a strain point Ts of 735 ° C. or higher, T _MAX is preferably 750 ° C. or lower. T _MAX is more preferably 730 ° C. or less, further preferably 720 ° C. or less, particularly preferably 710 ° C. or less, and further preferably 700 ° C. or less.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention further in detail, this invention is not limited to these Examples. Examples 1 to 5, 11 to 12, and 17 to 19 are examples, and examples 6 to 10, 13 to 16, and 20 to 22 are comparative examples.

(Production of glass base plate)
A raw material was prepared so as to have the glass composition (mole% based on oxide) displayed in Table 1, and the prepared raw material was put in a platinum crucible and heated for 3 hours at a temperature of 1650 ° C. to prepare a molten glass. Molten glass was formed into a plate shape to produce a glass base plate. A test piece was cut out from the produced glass base plate, average linear expansion coefficient α (× 10 ⁻⁷ / ° C.), strain point Ts (° C.), density ρ (g / cm ³ ), Young's modulus E (GPa), specific elasticity. The rate E / ρ was measured. The measuring method of each physical property is shown below.

  The average linear expansion coefficient α was measured using a differential thermal dilatometer (TMA), and was measured in accordance with JIS R3102 (1995).

  The strain point Ts was measured according to JIS R3103-2 (2001) using a strain point measuring apparatus.

  The density ρ was measured by the Archimedes method.

  Young's modulus E was measured at 25 ° C. by an ultrasonic pulse method (Olympus, DL35).

  The results are shown in Table 1.

表１から明らかなように、ガラスＡ、ガラスＢ、ガラスＣのいずれも、酸化物基準のモル％表示で、ＳｉＯ_２を６０〜７５％、Ａｌ_２Ｏ_３を５〜１５％、Ｂ_２Ｏ_３を０〜１２％、ＭｇＯを０〜２０％、ＣａＯを０〜１２％、ＳｒＯを０〜１０％、ＢａＯを０〜１０％、Ｎａ_２Ｏを０〜２０％、Ｋ_２Ｏを０〜１０％、Ｌｉ_２Ｏを０〜１５％、ＴｉＯ_２を０〜５％、ＺｒＯ_２を０〜５％の範囲内で含有し、Ｎａ_２ＯおよびＬｉ_２Ｏの含有量の合計が１〜２５％の範囲内であった。

As is clear from Table 1, all of glass A, glass B, and glass C are expressed in terms of mol% on the basis of oxide, and SiO ₂ is 60 to 75%, Al ₂ O ₃ is 5 to 15%, and B ₂ O. ₃ 0 to 12% of MgO 0 to 20% 0 to 12% of CaO, 0% to SrO, 0% to BaO, 0 to 20% of Na ₂ O, 0 to a _{K 2} O 10%, Li ₂ O 0-15%, TiO ₂ 0-5%, ZrO ₂ in the range of 0-5%, the total content of Na ₂ O and Li ₂ O is 1-25 %.

In particular, the glass B and glass C is a SiO ₂ 65 to 70% of _{Al 2 O 3 6~12%, B} 2 O 3 and 0-3% to 18% of MgO, the CaO 0% SrO 0-6%, BaO 0-2%, Na ₂ O 0.5-12%, K ₂ O 0-3%, Li ₂ O 0-5%, TiO ₂ 0-3 %, ZrO ₂ was contained in the range of 0 to 3%, and the total content of Na ₂ O, Li ₂ O, and K ₂ O was in the range of 2 to 15%.

(Production of glass substrate)
The obtained glass base plate was processed according to the following procedure.

  In the first step, a disk-shaped glass substrate having a circular hole in the center was obtained by processing the glass base plate.

  In the second step, the inner and outer circumferences of the glass substrate were ground with a grindstone. When the glass of the glass substrate is glass A, a vertical surface perpendicular to both main surfaces and an inclined surface inclined at 45 ° with respect to each main surface are formed between the main surfaces and the vertical surface on the inner and outer circumferences. did. On the other hand, when the glass of the glass substrate was glass B or glass C, only vertical surfaces perpendicular to both main surfaces were formed on the inner and outer circumferences.

  In the third step, both main surfaces of the glass substrate were lapped using alumina abrasive grains, and the abrasive grains were washed and removed.

  In the fourth step, the outer periphery of the glass substrate was brush-polished, the processing-affected layer (scratches, etc.) in the second step was removed, mirror-finished, and then washed. In brush polishing, a polishing liquid containing cerium oxide abrasive grains was used, and the work-affected layer was reliably removed, and polishing was performed for a long time in order to sufficiently reduce the surface roughness. Thereby, when the glass of the glass substrate was glass A, the corners between the main surface and the inclined surface of the glass substrate and between the vertical surface and the inclined surface were chamfered to form a curved surface. On the other hand, when the glass of the glass substrate was glass B or glass C, the corner between the main surface and the vertical surface of the glass substrate was chamfered to form a curved surface. Further, when the glass of the glass substrate was glass B, a polishing time was shortened compared to other examples, and a glass substrate having a high surface roughness on the outer periphery of the glass substrate was prepared.

  In the fifth step, the inner periphery of the glass substrate was brush-polished, and the work-affected layer (scratches, etc.) in the second step was removed, mirror-finished, and then washed. In brush polishing, a polishing liquid containing cerium oxide abrasive grains was used, and the work-affected layer was reliably removed, and polishing was performed for a long time in order to sufficiently reduce the surface roughness. Thereby, when the glass of the glass substrate was glass A, the corners between the main surface and the inclined surface of the glass substrate and between the vertical surface and the inclined surface were chamfered to form a curved surface. On the other hand, when the glass of the glass substrate was glass B or glass C, the corner between the main surface and the vertical surface of the glass substrate was chamfered to form a curved surface. In addition, when the glass of the glass substrate was glass B, the polishing time was shortened compared with other examples, and a glass substrate having a high surface roughness on the inner periphery of the glass substrate was prepared.

  In the sixth step, both main surfaces of the glass substrate were lapped using a fixed grain tool containing diamond abrasive grains and a grinding liquid, and washed.

  In the seventh step, both main surfaces of the glass substrate were primarily polished by a double-side polishing apparatus and cleaned. In the primary polishing, a hard urethane polishing pad and a polishing liquid containing cerium oxide abrasive grains were used.

  In the eighth step, both main surfaces of the glass substrate were secondarily polished by the double-side polishing apparatus and washed. In the secondary polishing, a soft urethane polishing pad and a polishing liquid containing cerium oxide abrasive grains having an average particle size smaller than that of the primary polishing were used.

  In the ninth step, both main surfaces of the glass substrate were subjected to third polishing by the double-side polishing apparatus and washed. In the tertiary polishing, a soft urethane polishing pad and a polishing liquid containing colloidal silica were used.

  In the tenth step, scrub cleaning, ultrasonic cleaning in a state of being immersed in a detergent solution, and ultrasonic cleaning in a state of being immersed in pure water are sequentially performed on the glass substrate after the third polishing, followed by drying with isopropyl alcohol vapor. Went.

  By the above procedure, a disk-shaped glass substrate having a diameter of 65 mm, a plate thickness of 0.8 mm, and a 20 mm circular hole at the center was obtained. The width W1 of the inner peripheral connection surface and the width W2 of the outer peripheral connection surface were as shown in Tables 2 to 4.

Then, in Examples 1-6, 8, 10-13, 15-19, 21-22, the glass substrate was chemically strengthened. In the chemical strengthening treatment, these glass substrates were dipped in a molten salt mixed with 95 wt% KNO ₃ and 5 wt% NaNO ₃ at 425 ° C. for dipping times shown in Tables 2 to 4, respectively.

(Evaluation of glass substrate)
The depth λ of the introduced element by chemical strengthening was calculated by the method described above using electron beam microanalyzer analysis (EPMA). The depth λ of the introduced element is 0.5 mm radially outward from the inner peripheral edge of the glass substrate, two locations 11 mm radially outward from the inner peripheral edge of the glass substrate, and radial from the outer peripheral edge of the glass substrate. Measurements were taken at two locations of 0.5 mm inside. Tables 2 to 4 show the maximum value λ _MAX and the minimum value λ _{MIN of} λ measured at a total of six locations.

Retardation was measured with a WPA-micro manufactured by Photonic Lattice before the thermal deformation test described later. For comparison with Patent Document 3, in Example 6 and Example 10 in which the depth λ of the introduced element by chemical strengthening is 10 μm or more, the glass substrate is positioned from a position 0.5 mm radially outward from the inner peripheral edge of the glass substrate. Retardation R _0.5 was measured from the outer peripheral edge to the position 0.5 mm radially inward. The measurement results indicate the maximum value of retardation R _0.5 as R _0.5MAX and the difference between the maximum value and minimum value of retardation R _0.5 as R _0.5DEF . The glass substrate of Example 6 had R _0.5MAX of 0.8 nm and R _{0.5DEF of 0.6} nm. The glass substrate of Example 10 had R _{0.5MAX of 0.9 nm} and R _0.5DEF of 0.7 _nm . In Example 6 and Example 10, R _0.5MAX was less than 1 nm, but the deformation amount ΔD before and after the heat treatment test described later was more than 3.5 μm.

In the heat treatment test, after heating the electric furnace to a furnace temperature of 400 ° C., a glass substrate was put into the electric furnace, and as shown in FIGS. 3 and 4, the lower part of the glass substrate with both main surfaces vertical was made of iron. The pedestals 20 and 21 were clamped. After 0.5 minutes from the introduction of the glass substrate, the set temperature of the electric furnace is raised to the maximum temperature T _MAX in Tables 2 to 4 at a rate of 200 ° C./min, and then the holding time t at the maximum temperature T _MAX is reached. Hold by _MAX . Thereafter, the temperature of the electric furnace was lowered at a rate of 60 ° C./min. After the furnace temperature in the electric furnace reached 400 ° C. or lower, the glass substrate was taken out of the electric furnace, placed on silica wool at room temperature with the main surface substantially horizontal, and allowed to cool as it was. The iron pedestals 20 and 21 were rectangular parallelepipeds, and their external dimensions were about 70 mm × 90 mm × height 12 mm. The holding times t _MAX , t _AVE , t ₅₀₀ , T _AVE , T ₅₀₀ at the maximum temperature T _MAX were as shown in Tables 2 to 4.

  FIG. 5 is a diagram illustrating a shape change of one main surface of the glass substrate before and after the heat treatment test according to an example. In FIG. 5, the broken line 31 exaggerates the shape of the main surface before the heat treatment test, and the solid line 32 exaggerates the shape of the main surface after the heat treatment test.

  The shape of the main surface before the heat treatment test was measured with a flatness tester FT-17 manufactured by NIDEK. The measurement range was a range of a radius of 13.30 to 32.06 mm from the center of the glass substrate. A plurality of vertical lines were set at a pitch of 0.5 mm and a plurality of horizontal lines were set at a pitch of 0.5 mm over the entire measurement range, and an intersection of the vertical lines and the horizontal lines was used as a measurement point.

  The shape of the main surface before the heat treatment test was represented by xyz orthogonal coordinates at each measurement point. Here, the xy plane including the x-axis and the y-axis is a least square plane of a plurality of measurement points. The x axis was parallel to the horizontal line, and the y axis was parallel to the vertical line.

  The shape of the main surface after the heat treatment test was represented by xyz orthogonal coordinates at each measurement point, similarly to the shape of the main surface before the heat treatment test.

  Before and after the heat treatment test, the coordinate surfaces were aligned, the shapes of the main surfaces were compared, and the amount of change Δz in the z-axis direction at each measurement point was measured.

  FIG. 6 is a diagram illustrating the amount of change at each measurement point before and after the heat treatment test according to an example. Here, the sign of the change amount Δz represents the change direction. The magnitude of the difference between the maximum value (> 0) of the change amount Δz and the minimum value (<0) of the change amount Δz was defined as the deformation amount ΔD.

  A result is shown to Tables 2-4 with the kind of glass, chemical strengthening conditions, heat processing conditions, etc. The conditions other than those shown in Tables 2 to 4 were the same in Examples 1 to 21.

表２〜４から明らかなように、例１〜５、１１〜１２、１７〜１９では、ＲＩ_{０．２ＭＡＸ}およびＲＯ_{０．２ＭＡＸ}が４ｎｍ以下、ＲＩ_{０．２ＤＥＦ}およびＲＯ_{０．２ＤＥＦ}が１．２ｎｍ以下、且つ導入元素の深さλが３〜９μｍであった。また、これらの例では、上記式（１）で算出されるＸが０．１〜１．３の範囲内であり、上記式（２）で算出されるＹが１４０以下であり、上記式（３）で算出されるＺが５〜３２の範囲内であった。そのため、これらの例では、熱処理試験の前後での変形量ΔＤが３．５μｍ以下であった。

As is clear from Tables 2 to 4, in Examples 1 to 5, 11 to _12, and ₁₇ to ₁₉ , RI _0.2MAX and RO _0.2MAX are 4 nm or less, and RI _0.2DEF and RO _0.2DEF are 1.2 nm. Hereinafter, the depth λ of the introduced element was 3 to 9 μm. In these examples, X calculated by the above formula (1) is in the range of 0.1 to 1.3, Y calculated by the above formula (2) is 140 or less, and the above formula ( Z calculated in 3) was in the range of 5 to 32. Therefore, in these examples, the deformation amount ΔD before and after the heat treatment test was 3.5 μm or less.

In particular, in Examples 11 to 12 and 17 to 19, the Young's modulus E is 80 GPa or more, the specific modulus E / ρ is 30 MNm / kg or more, the strain point Ts is 635 ° C. or more, and the average linear expansion coefficient α is 40 × 10. It was in the range of ⁻⁷ to 70 × 10 ⁻⁷ / ° C. In Examples 11 to 12 and 17 to 19, since glass having a strain point Ts of 635 ° C. or higher was used, heat treatment at 650 ° C. or higher was possible.

On the other hand, in Examples 6 to 10, 14 to 16, and 20 to 22, the depth λ of the introduced element was outside the range of 3 to 9 μm. In Examples 6, 8, 10, ₁₃ , 15, ₁₆ , and ₂₂ , RI _0.2MAX was larger than 4 nm. In Examples 6, 8, 10, ₁₃ , 15, ₁₆ , 21, and ₂₂ , RI _0.2DEF was larger than 1.2 nm. Therefore, in these examples, the deformation amount ΔD before and after the heat treatment test was larger than 3.5 μm, or the deformation amount ΔD before and after the heat treatment test was too large, and the glass substrate was cracked. In Example 13, the depth of the introduced element was in the range of 3 to 9 μm, but the surface roughness of the inner and outer peripheral edges was high, so RI _0.2MAX was larger than 4 nm, and RI _0.2DEF was also 1.2 nm. There was a problem that became larger.

  In particular, Examples 6 and 10 satisfy the requirements described in Patent Document 3, but have problems.

  The embodiment of the glass substrate for the magnetic recording medium has been described above, but the present invention is not limited to the above embodiment, and various modifications are possible within the scope of the gist of the present invention described in the claims. Improvements are possible.

  For example, the recording method of the magnetic recording medium of the above embodiment is a heat-assisted magnetic recording method, but the present invention is not limited to this. The recording system of the magnetic recording medium may be another energy assisted magnetic recording system such as a microwave assisted magnetic recording system, or a normal magnetic recording system.

DESCRIPTION OF SYMBOLS 10 Magnetic recording medium 11 Glass substrate 11a, 11b Main surface 11c Inner peripheral connection surface 11d Outer peripheral connection surface 11e Inner peripheral end 11f Outer peripheral end 12 Heat sink layer 13 Seed layer 14 Magnetic recording layer 15 Protective layer

Claims (12)

A glass substrate for a magnetic recording medium having a compressive stress layer by chemical strengthening on a surface layer and having a disk shape with a through hole in the center,
The strain point (Ts) of the glass is 585 ° C. or higher,
Maximum retardation measured by irradiating light with a wavelength of 543 nm perpendicular to the main surface at a position 0.2 mm radially outward from the inner peripheral edge and a position 0.2 mm radially inner from the outer peripheral edge The value is 4 nm or less, and the magnitude of the difference between the maximum value and the minimum value of the retardation is 1.2 nm or less,
The depth of the introduced element introduced by chemical strengthening in all portions of each main surface that are separated by 0.5 mm or more radially outward from the inner peripheral end and 0.5 mm or more radially inward from the outer peripheral end. A glass substrate for a magnetic recording medium, wherein (λ) is 3 to 9 μm. The glass substrate for a magnetic recording medium according to claim 1, wherein the strain point (Ts) is 600 ° C. or higher. At a position deeper than the depth at which the introduction element is introduced, the total number of moles of all oxides when the glass composition is expressed in terms of oxide is 100, and the Al ₂ O ₃ content in the glass is 100%. The number of moles is CA, the number of moles in terms of oxide of the element ion-exchanged with the introduced element in the glass is CB, the number of moles of MgO in the glass is CC, and the total number of moles of MgO and CaO in the glass When the number is CD, the depth of the introduced element is λ (μm), and the thickness of the glass substrate is d (μm), X calculated by the following formula (1) is 0.1 or more and 1.3 or less. The glass substrate for magnetic recording media according to claim 1 or 2.
X = CA × CB × (CC / CD) × λ / d (1) At a position deeper than the depth at which the introduced element is introduced, SiO ₂ is 60 to 75%, Al ₂ O ₃ is 5 to 15%, and B ₂ O ₃ is 0 to 12% in terms of mol% based on oxide. MgO 0-20%, CaO 0-12%, SrO 0-10%, BaO 0-10%, Na ₂ O more than 0% but not more than 20%, K ₂ O 0-10%, Li ₂ O 0 to 15% of TiO ₂ 0-5%, a ZrO ₂ containing 0-5%, and, they contain more than 95% in total, the content of Na ₂ O and Li ₂ O The glass substrate for magnetic recording media according to any one of claims 1 to 3, wherein the total is 1 to 25%. In a position deeper than the depth at which the introduced element is introduced, the SiO ₂ is 65 to 70%, the Al ₂ O ₃ is 6 to 12%, and the B ₂ O ₃ is 0 to 3% in terms of mol% based on the oxide. the MgO 1 to 18%, 0% and CaO, the SrO 0~6%, 0~2% of BaO, 0.5 to 12% of Na ₂ O, 0 to 3% of _{K 2} O, Li the ₂ O 0 to 5%, the TiO ₂ 0 to 3%, containing ZrO ₂ 0 to 3%, and the total content of these contained more than 95% in total, Na ₂ O and Li ₂ O The glass substrate for magnetic recording media according to claim 4, wherein is 2 to 15%. As surfaces, there are two main surfaces parallel to each other, an inner peripheral connection surface that connects the inner peripheral edges of both main surfaces, and an outer peripheral connection surface that connects the outer peripheral edges of both main surfaces,
The width of the inner peripheral connection surface in the radial direction of the glass substrate and the width of the outer peripheral connection surface in the radial direction of the glass substrate are 0.03 to 0.15 mm, respectively. The glass substrate for magnetic recording media as described in 2. The width of the inner peripheral connection surface in the radial direction of the glass substrate and the width of the outer peripheral connection surface in the radial direction of the glass substrate are 0.03 to 0.08 mm, respectively.
The maximum value of the retardation is 5 nm or less at each of the position 0.1 mm radially outward from the inner peripheral edge and the position 0.1 mm radially inner from the outer peripheral edge, and the maximum value of the retardation The glass substrate for magnetic recording media according to claim 6, wherein the difference between the minimum value and the minimum value is 2.5 nm or less.   The glass substrate for a magnetic recording medium according to any one of claims 1 to 7, which is used for a magnetic recording medium for energy-assisted magnetic recording. A method for producing a magnetic recording medium, comprising the glass substrate for a magnetic recording medium according to any one of claims 1 to 8 and a magnetic recording layer,
A method for producing a magnetic recording medium, comprising a heating step of heating the glass substrate to a temperature equal to or higher than a strain point (Ts). In the heating step, the time average temperature during heating the glass substrate to a temperature of 10 ° C. or lower with respect to the strain point (Ts) is T _ave (° C.), and the strain point (Ts) is the reference. 10. The magnetic recording medium according to claim 9, wherein Y calculated by the following formula (2) is 140 or less, where t _ave (min) is a time during which the glass substrate is heated to a temperature of 10 ° C. or lower. Production method.
Y = (T _ave −Ts) × t _ave (2) In the heating step, T ₅₀₀ (° C.) is a time average temperature during heating the glass substrate to a temperature of 500 ° C. or higher, and t ₅₀₀ (min) is a time for heating the glass substrate to a temperature of 500 ° C. or higher. 11. The manufacturing of a magnetic recording medium according to claim 9, wherein Z calculated by the following formula (3) is 5 to 32, where λ (μm) is the depth of the introduced element before the heating step. Method.
Z = (T ₅₀₀ / Ts) × (t ₅₀₀ ) ^1/2 × λ (3)   The method for manufacturing a magnetic recording medium according to claim 9, wherein the maximum temperature of the glass substrate in the heating step is 650 ° C. or higher.

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