JP7614571B2 - Glass substrate for magnetic recording medium and magnetic recording device using same - Google Patents

Description

The present invention relates to a glass substrate for magnetic recording media and a magnetic recording device using the same.

A magnetic recording device is equipped with a magnetic recording medium in which a magnetic layer is formed on a magnetic recording medium substrate, and information can be recorded using the magnetic layer. Until now, aluminum alloy substrates have been used as the magnetic recording medium substrates used in magnetic recording devices. Currently, in response to the demand for higher recording density, thinner magnetic medium substrates are being considered. However, when aluminum alloy substrates are made thinner, they lose their rigidity, so attention is being paid to glass substrates, which have excellent rigidity, flatness, smoothness, etc.

In recent years, to meet the need for even higher recording density, magnetic recording media using an energy-assisted magnetic recording method, that is, energy-assisted magnetic recording media, have been considered. In energy-assisted magnetic recording media, a glass substrate is also used, and a magnetic layer, etc. is formed on the surface of the glass substrate. In energy-assisted magnetic recording media, an ordered alloy with a large magnetic anisotropy coefficient Ku (hereinafter referred to as "high Ku") is used as the magnetic material for the magnetic layer.

In order to increase the degree of ordering (ordering) of the magnetic layer and achieve a high Ku, the substrate including the glass substrate may be heat-treated at a high temperature of about 800°C during, before, or after the deposition of the magnetic layer. The higher the recording density, the higher the heat treatment temperature required, so a higher heat resistance is required than for conventional glass substrates for magnetic recording media. In addition, after the magnetic layer is deposited, the substrate including the glass substrate may be irradiated with a laser. Such heat treatment and laser irradiation are also intended to increase the annealing temperature and coercivity of the magnetic layer including the FePt alloy, etc.

Incidentally, glass substrates for magnetic recording media are required to have high rigidity (Young's modulus) to prevent significant deformation during high-speed rotation. More specifically, in disk-shaped magnetic recording media, the medium is rotated at high speed around the central axis while the magnetic head is moved in the radial direction to write and read information along the direction of rotation. In recent years, the rotation speed to increase the writing and reading speed has been increasing from 5,400 rpm to 7,200 rpm and even 10,000 rpm, but in disk-shaped magnetic recording media, the position for recording information is assigned in advance according to the distance from the central axis. For this reason, if the glass substrate deforms during rotation, the magnetic head will shift position, making accurate reading difficult.

In recent years, a DFH (Dynamic Flying Height) mechanism has been installed in the magnetic head to significantly narrow the gap between the recording and reproducing element of the magnetic head and the surface of the magnetic recording medium (reducing the flying height), thereby achieving even higher recording density. The DFH mechanism is a mechanism in which a heating unit such as a very small heater is provided near the recording and reproducing element of the magnetic head, and only the periphery of the element is thermally expanded toward the surface of the medium. By providing such a mechanism, the distance between the magnetic head and the magnetic layer of the medium is reduced, making it possible to pick up signals from smaller magnetic particles and achieve higher recording density. On the other hand, since the gap between the recording and reproducing element of the magnetic head and the surface of the magnetic recording medium is extremely small, for example, 2 nm or less, there is a risk that the magnetic head will collide with the surface of the magnetic recording medium even with a slight impact. This tendency becomes more pronounced as the rotation speed increases. Therefore, during high-speed rotation, it is important to prevent the glass substrate from bending or fluttering, which can cause this collision.

Furthermore, with the recent increase in the global use of data centers and servers, there is a demand for reducing the cost of these magnetic disk substrates. Low-temperature melting is generally an effective way to reduce the cost of glass substrates. Furthermore, it is also effective to adopt the overflow down-draw method or the float method to form large glass substrates with a thickness close to that of the product.

The present invention was made in consideration of the above circumstances, and its purpose is to create a glass substrate for magnetic recording media that is less prone to bending or fluttering during high-speed rotation, has sufficient heat resistance to achieve significantly higher recording density, and also contributes to reducing costs.

The inventors have found that the above technical problems can be solved by strictly controlling the glass properties through various repeated experiments, and propose this as the present invention. That is, the glass substrate for magnetic recording media of the present invention is characterized in that it has an annealing point of 780° C. or higher, a temperature at a high-temperature viscosity of 10 ^2.5 dPa·s of 1640° C. or lower, and a Young's modulus of 80 GPa or higher. Here, the "annealing point" can be measured by the well-known fiber elongation method, and can be measured, for example, based on the method of ASTM C336. The "Young's modulus" can be measured by the well-known resonance method. The "temperature at a high-temperature viscosity of 10 ^2.5 dPa·s" can be measured by the well-known platinum ball pulling method.

In the glass substrate for magnetic recording media of the present invention, the annealing point is regulated to 780°C or higher. This makes it difficult for the glass substrate to deform even when heat treatment at high temperatures such as thermal assistance or laser irradiation is performed. As a result, a higher heat treatment temperature can be used when attempting to increase Ku, making it easier to manufacture a magnetic recording device with a high recording density.

In addition, in the glass substrate for magnetic recording media of the present invention, the temperature at a high-temperature viscosity of 10 ^2.5 dPa·s is restricted to 1640° C. or less. This allows low-temperature melting, which contributes to lowering the cost of the glass substrate. It also improves moldability.

Furthermore, the Young's modulus of the glass substrate for magnetic recording media of the present invention is restricted to 80 GPa or more. This makes it difficult for the glass substrate to bend or flutter during high-speed rotation, thereby preventing collisions between the information recording medium and the magnetic head.

The glass substrate for a magnetic recording medium of the present invention preferably contains, in mass %, 55 to 70% SiO ₂ , 17 to 25% Al ₂ O ₃ , 0 to 2% B ₂ O ₃ , 0 to 6% MgO, 0 to 7% CaO, 0.1 to 7% SrO, and 0.1 to 15% BaO as a glass composition.

The glass substrate for magnetic recording media of the present invention preferably has an average surface roughness Ra of 1.0 nm or less. This makes it possible to improve the magnetic properties even if the bit size is reduced to achieve high recording density. Here, the "average surface roughness Ra" refers to the average surface roughness Ra of the main surfaces (both surfaces) excluding the end faces, and can be measured, for example, with an atomic force microscope (AFM).

In addition, the glass substrate for magnetic recording media of the present invention preferably has an average linear transmittance of 70% or more in a wavelength range of 350 to 1500 nm with an optical path length of 1 mm.

The glass substrate for magnetic recording media of the present invention is preferably disk-shaped, i.e., disc-shaped, with a circular opening formed in the center (see Figure 1).

The magnetic recording device of the present invention preferably includes the above-mentioned glass substrate for magnetic recording media.

FIG. 1 is a top perspective view showing the disk shape.

In the glass substrate for magnetic recording media of the present invention, the annealing point is preferably 780°C or higher, 785°C or higher, 790°C or higher, 795°C or higher, or 800°C or higher, and particularly preferably 805 to 900°C. If the annealing point is too low, it becomes difficult to perform heat treatment or laser irradiation at high temperatures, making it difficult to produce a magnetic recording medium with a high recording density.

In the glass substrate for magnetic recording media of the present invention, the temperature at a high-temperature viscosity of 10 ^2.5 dPa·s is preferably 1640° C. or lower, 1635° C. or lower, or 1630° C. or lower, particularly preferably 1450 to 1625° C. If the temperature at a high-temperature viscosity of 10 ^2.5 dPa·s is too high, the melting property and formability decrease, and the manufacturing cost of the glass substrate increases.

In the glass substrate for magnetic recording media of the present invention, the Young's modulus is preferably 80 GPa or more, 81 GPa or more, or 82 GPa or more, and particularly preferably 83 to 100 GPa. If the Young's modulus is too low, the glass substrate is more likely to bend or flutter during high-speed rotation, making it easier for the information recording medium and the magnetic head to collide.

The glass substrate for magnetic recording media of the present invention preferably contains, in mass %, _SiO2 55-70%, _{Al2O3 17-25} %, _{B2O3 0-2} %, MgO 0-6%, CaO 0-7%, SrO 0.1-7%, and BaO 0.1-15% as a glass composition. The reasons for limiting the content of each component as above are as follows. In the explanation of the content of each component, % denotes mass % unless otherwise specified.

_SiO2 is a component that forms a glass network. The content of _SiO2 is preferably 55 to 70%, 56 to 67%, 57 to 65%, and particularly 58 to 63%. If the content of _SiO2 is too low, vitrification becomes difficult and heat resistance tends to decrease. On the other hand, if the content of _SiO2 is too high, meltability, moldability, and Young's modulus tend to decrease.

Al ₂ O ₃ is a component that increases Young's modulus and annealing point. The content of Al ₂ O ₃ is preferably 17-25%, 18-23%, 18.5-21%, particularly 19-20%. If the content of Al ₂ O ₃ is too low, Young's modulus and heat resistance tend to decrease. On the other hand, if the content of Al ₂ O ₃ is too high, meltability, moldability, and devitrification resistance tend to decrease.

B ₂ O ₃ is a component that forms a glass network, but also reduces the Young's modulus and heat resistance. Therefore, the content of B ₂ O ₃ is preferably 0 to 2%, 0.1 to 1.8%, 0.3 to 1.5%, and particularly preferably 0.4 to 1%.

MgO is a component that significantly increases Young's modulus and also reduces high-temperature viscosity, improving meltability and moldability. The MgO content is preferably 0-6%, 1-6%, 1.5-5%, 2-5%, 2-4.5%, and particularly 2.5-4%. If the MgO content is too low, it becomes difficult to obtain the above effects. On the other hand, if the MgO content is too high, heat resistance and devitrification resistance are likely to decrease.

CaO is a component that reduces high-temperature viscosity and improves meltability and moldability. The CaO content is preferably 0-7%, 1-6%, 2-5%, and particularly 2.5-4.5%. If the CaO content is too low, it becomes difficult to obtain the above effects. On the other hand, if the CaO content is too high, devitrification resistance is easily reduced.

SrO has the effect of balancing with other components and improving devitrification resistance, and is an essential component for stabilizing glass, especially in alkali-free glass systems. On the other hand, if the SrO content is too high, it promotes the precipitation of CaO-based crystals, which not only reduces devitrification resistance but also reduces Young's modulus. Therefore, the SrO content is preferably 0.1-7%, 0.1-6%, 0.5-5%, 1-4.5%, and especially 1-4%.

BaO has the effect of reducing the precipitation of CaO-based crystals, and is an essential component for stabilizing glass, especially in alkali-free glass systems. On the other hand, if the BaO content is too high, the Young's modulus is likely to decrease. Therefore, the BaO content is preferably 0.1-15%, 3-13%, 4-12%, 5-12%, and especially 6-11%.

In addition to the above ingredients, the following ingredients may also be added:

Li ₂ O, Na ₂ O and K ₂ O are components that reduce high-temperature viscosity and improve meltability and moldability, but they are components that reduce water resistance and weather resistance. The total content of Li ₂ O, Na ₂ O and K ₂ O is preferably 0 to 15%, 0 to 10%, 0 to 5%, 0 to 1%, and particularly 0 to less than 0.1%. The respective contents of Li ₂ O, Na ₂ O and K ₂ O are preferably 0 to 10%, 0 to 5%, 0 to less than 1%, and particularly 0 to less than 0.1%.

ZnO is a component that reduces high-temperature viscosity and significantly increases melting properties. The ZnO content is preferably 0-7%, 0.1-5%, and particularly 0.5-3%. If the ZnO content is too low, it becomes difficult to obtain the above effects. Furthermore, if the ZnO content is too high, the glass becomes more susceptible to devitrification.

_TiO2 is a component that enhances water resistance and weather resistance, but also colors the glass. However, if the _TiO2 content is too high, the average linear transmittance at an optical path length of 1 mm and in the wavelength range of 350 to 1500 nm tends to decrease. Therefore, the _TiO2 content is preferably 0 to 0.5%, and particularly 0 to less than 0.1%.

As a fining agent, one or more selected from the group consisting of SnO ₂ , Cl, SO ₃ and CeO ₂ (preferably SnO ₂ and SO ₃ ) may be added in an amount of 0.05 to 0.5%.

Fe ₂ O ₃ is a component that is inevitably mixed as an impurity in glass raw materials and is a coloring component. Therefore, the content of Fe _{2 O 3 is preferably 0.5% or less, 0.001 to 0.1%, 0.005 to 0.07%, 0.008 to 0.025%, and particularly 0.01 to 0.03%. If the content of Fe 2 O 3 is too} high, the average linear transmittance in the wavelength range of 350 to 1500 nm with an optical path length of 1 mm is likely to decrease.

_{V2O5 , Cr2O3} , _CoO3 and NiO are coloring components. Therefore, the content of each of _V2O5 , _Cr2O3 , _CoO3 and NiO is preferably 0.1% or less _, particularly less than 0.01%. If the content of each of _V2O5 , _Cr2O3 , _CoO3 and _NiO is too high, the average linear transmittance at an optical path length of 1 mm _and in _a wavelength range of 350 to 1500 _nm tends to decrease.

From an environmental perspective, it is preferable that the glass composition does not substantially contain As ₂ O ₃ , Sb ₂ O ₃ , PbO, Bi ₂ O ₃ and F. Here, "substantially does not contain ..." means that the specified components are not actively added as glass components, but are allowed to be mixed in as impurities, and specifically means that the content of the specified components is less than 0.05%.

The glass substrate for magnetic recording media of the present invention preferably has the following characteristics:

A glass substrate for a magnetic recording medium is required to have an appropriate thermal expansion coefficient in order to improve the reliability of recording and reproducing the magnetic recording medium. In detail, a HDD (hard disk drive) incorporating a magnetic recording medium has a structure in which the central part is pressed by the spindle of a spindle motor to rotate the magnetic recording medium itself. Therefore, if the difference in thermal expansion coefficient between the glass substrate and the spindle material is too large, the magnetic recording medium will deform due to the difference in thermal expansion and thermal contraction of the two in response to changes in the surrounding temperature. If such a phenomenon occurs, the written information cannot be read by the magnetic head, and there is a risk of impairing the reliability of recording and reproducing. Therefore, it is desirable for a glass substrate for a magnetic recording medium to have a thermal expansion coefficient that matches the thermal expansion coefficient of the spindle material (e.g., stainless steel, etc.). From this viewpoint, the average linear thermal expansion coefficient in the temperature range of 30 to 380°C is preferably 25x10 ^-7 to 60-10 ^-7 /°C, 28x10 ^-7 to 55-10 ^-7 /°C, particularly 30x10 ^-7 to 50-10 ^-7 /°C.

The liquidus temperature is preferably 1300°C or less, 1280°C or less, 1260°C or less, 1250°C or less, 1240°C or less, particularly 1230°C or less. The liquidus viscosity is preferably 10 ^3.8 dPa·s or more, 10 ^4.4 dPa·s or more, 10 ^4.6 dPa·s or more, 10 ^4.8 dPa·s or more, particularly 10 ^5.0 dPa·s or more. In this way, devitrified crystals are less likely to precipitate during molding, and it is easy to mold into a plate shape by the overflow downdraw method or the like, so that the average surface roughness Ra of the surface can be made 1.0 nm or less, particularly 0.2 nm or less, without polishing the surface or by a small amount of polishing. As a result, it is possible to improve the magnetic properties by miniaturizing the bit size. In addition, the cost of the glass substrate can be reduced by reducing the devitrified crystals and the amount of polishing. Here, the "liquidus temperature" can be calculated by placing glass powder that has passed through a standard sieve of 30 mesh (500 μm) and remains on a 50 mesh (300 μm) in a platinum boat, holding it in a temperature gradient furnace for 24 hours, and measuring the temperature at which crystals precipitate. The "liquidus viscosity" refers to the viscosity of glass at the liquidus temperature, and can be measured by a platinum ball pull-up method.

The average linear transmittance at an optical path length of 1 mm and in the wavelength range of 350 to 1500 nm is preferably 70% or more, 80% or more, and particularly 90% or more. If the average linear transmittance at an optical path length of 1 mm and in the wavelength range of 350 to 1500 nm is too low, the laser light will not be sufficiently irradiated onto the magnetic layer during laser irradiation, making it difficult to achieve a high Ku for the magnetic layer.

β-OH is preferably 0.30/mm or less, 0.25/mm or less, 0.20/mm or less, 0.15/mm or less, and particularly 0.10/mm or less. If β-OH is too large, the annealing point is likely to decrease. If β-OH is too small, there is a risk that the chlorine in the glass will remain at a high level. Therefore, β-OH is preferably 0.01/mm or more, and particularly 0.02/mm or more.

The following methods can be used to reduce β-OH: (1) Select raw materials with low water content. (2) Add components (Cl, _SO3, etc.) that reduce β-OH to the glass. (3) Reduce the amount of water in the furnace atmosphere. (4) Bubble _N2 in the molten glass. (5) Use a small melting furnace. (6) Increase the flow rate of the molten glass. (7) Use an electric melting method.

Here, "β-OH" refers to the value obtained by measuring the transmittance of a glass substrate using FT-IR and using the following formula:

[Number 1]
β-OH=(1/X)log(T ₁ /T ₂ )
X: plate thickness (mm)
T ₁ : Transmittance (%) at a reference wavelength of 3846 cm ⁻¹
T ₂ : Minimum transmittance (%) at a hydroxyl group absorption wavelength of about 3600 cm ⁻¹

The average surface roughness Ra of the surface is preferably 1.0 nm or less, 0.7 nm or less, 0.4 nm or less, and particularly 0.2 nm or less. If the average surface roughness Ra of the surface is too large, even if the bit size is reduced to achieve high recording density, improvement in magnetic properties cannot be expected.

The plate thickness is preferably 1.5 mm or less, 1.2 mm or less, 0.2 to 1.0 mm, and particularly 0.3 to 0.9 mm. If the plate thickness is too thick, it will have to be polished to the desired thickness, which may increase the processing cost.

The glass substrate for magnetic recording media of the present invention can be produced, for example, by the following method. First, glass raw materials prepared to obtain the desired glass composition are charged into a continuous melting furnace, heated and melted at 1500 to 1700°C, and then clarified. The molten glass is then preferably fed into a molding device, formed into a plate, and cooled. After forming into a plate, a well-known method can be used to cut the glass into a predetermined size. Various methods can be used to form the glass substrate, but it is preferable to use the overflow downdraw method, slot down method, etc. to increase surface smoothness.

The present invention will be described below based on examples. Note that the following examples are merely illustrative. The present invention is not limited to the following examples in any way.

Tables 1 to 3 show examples of the present invention (samples No. 1 to 50).

First, a glass batch prepared by mixing glass raw materials to obtain the glass composition shown in the table was placed in a platinum crucible, and then melted, refined, and homogenized for 24 hours at 1500 to 1700°C. When melting the glass batch, the mixture was stirred and homogenized using a platinum stirrer. Next, the molten glass was poured onto a carbon plate and formed into a plate shape, and then slowly cooled for 30 minutes at a temperature near the annealing point. For each of the obtained glass substrates, the average linear thermal expansion coefficient α in the temperature range of 30 to 380°C _{(30 to 380°C} ), density, β-OH, strain point Ps, annealing point Ta, softening point Ts, temperature at high temperature viscosity of 10 ^4.5 dPa·s, temperature at high temperature viscosity of 10 ^4.0 dPa·s, temperature at high temperature viscosity of 10 ^3.0 dPa·s, temperature at high temperature viscosity of 10 ^2.5 dPa·s, liquidus temperature TL, liquidus viscosity logη at TL, Young's modulus, and specific modulus calculated as Young's modulus/density were evaluated.

The average linear thermal expansion coefficient in the temperature range of 30 to 380°C is a value measured using a dilatometer.

Density is measured using Archimedes' method.

β-OH is the value measured using the above method.

The strain point, annealing point, and softening point were measured according to the methods of ASTM C336 and C338.

The temperatures at high temperature viscosities of 10 ^4.5 dPa·s, 10 ^4.0 dPa·s, 10 ^3.0 dPa·s, and 10 ^2.5 dPa·s are values measured by the platinum sphere pull-up method.

The liquidus temperature is the temperature at which crystals (primary phase) precipitate when glass powder that passes through a standard 30 mesh (sieve opening 500 μm) sieve and remains on a 50 mesh (sieve opening 300 μm) is placed in a platinum boat and held in a temperature gradient furnace for 24 hours. The liquidus viscosity is the viscosity of the glass at the liquidus temperature, measured using the platinum ball pull-up method.

Young's modulus refers to the value measured using the resonance method.

As is clear from the table, Samples No. 1 to 50 have an annealing point of 796° C. or higher, a Young's modulus of 81.2 GPa or higher, and a temperature at 10 ^2.5 dPa·s of 1640° C. or lower, and are therefore suitable as glass substrates for magnetic recording media.

Glass batches prepared by mixing glass raw materials to obtain the glass compositions of samples No. 1 to 50 in the table were put into a melting furnace, and then melted, clarified, and homogenized at 1500 to 1700°C for 24 hours, and formed into plates by the overflow downdraw method to a plate thickness of 0.675 mm. The surface roughness Ra of the surface of the obtained glass substrate was measured with an atomic force microscope (AFM) and found to be 0.10 to 0.20 nm. Furthermore, the average linear transmittance of the obtained glass substrate in the wavelength range of 350 to 1500 nm with an optical path length of 1 mm was measured with a Shimadzu UV-3100 spectrophotometer, and all were 90% or more.

Claims (7)

A glass substrate for a magnetic recording medium, characterized in that the glass composition contains, in mass %, 55-70% _SiO2 , _17-25 % _Al2O3 , 0-1% _Li2O + _Na2O + _K2O , 0-6% MgO, and 2-7% CaO, has an annealing point of 780°C or higher, an average linear thermal expansion coefficient in the temperature range of 30-380°C of ^25x10-7 to ^60-10-7 /°C, and a Young's modulus of 80 GPa or higher. The glass substrate for a magnetic recording medium according to claim 1, characterized in that the glass composition contains, in mass %, 55-70% SiO ₂ , 17-25% Al ₂ O ₃ , 0-2% B ₂ O ₃ , 0-0.1% Li ₂ O + Na ₂ O + K ₂ O , 0-6% MgO, 2-7% CaO, 0.1-7% SrO, and 0.1-15% BaO. The glass substrate for magnetic recording media according to claim 1 or 2, characterized in that the average surface roughness Ra of the surface is 1.0 nm or less. The glass substrate for magnetic recording media according to claim 1 or 2, characterized in that the average linear transmittance in the wavelength range of 350 to 1500 nm and the optical path length is 1 mm is 70% or more. The glass substrate for magnetic recording media according to claim 1 or 2, characterized in that it is disk-shaped. A glass substrate for a magnetic recording medium, characterized in that the glass composition contains, in _mass %, 55-70% _{SiO2 ,} 17-25% _Al2O3 , _0-2 % _B2O3 , 0-1% Li2O+ _Na2O + _K2O , 0-6% MgO, 2-7% CaO, 0.1-7% SrO, and 0.1-15% BaO. A magnetic recording device comprising the glass substrate for magnetic recording media according to claim 1 or 2.