JP6793119B2 - Glass for magnetic recording medium substrate, magnetic recording medium substrate and magnetic recording medium - Google Patents

Description

Cross-reference of related applications

This application claims the priority of Japanese Patent Application No. 2015-132038 filed on June 30, 2015, the entire description of which is incorporated herein by reference in particular.

The present invention relates to glass for a magnetic recording medium substrate, a magnetic recording medium substrate, and a magnetic recording medium.

Conventionally, an aluminum substrate has been widely used as a substrate for a magnetic recording medium such as a hard disk (magnetic recording medium substrate). However, in recent years, in hard disk drives, in addition to impact resistance, the demand for improving the surface smoothness and thinning of the magnetic recording medium substrate is becoming more and more severe as the magnetic recording medium becomes denser and thinner. It has become. Therefore, there is a limit to the aluminum substrate having inferior surface hardness and rigidity. Therefore, the development of a glass magnetic recording medium substrate is currently the mainstream (see, for example, Patent Documents 1 to 4). The entire description of Patent Documents 1 to 4 is incorporated herein by reference in particular.

Patent No. 5658030 WO2011 / 019010A1 Patent No. 55425953 Patent No. 4691135

In recent years, a glass substrate having a magnetic recording layer containing a magnetic material having a high magnetic anisotropy energy has been developed for the purpose of achieving higher density recording of a magnetic recording medium. In order to form a magnetic recording layer made of such a magnetic material, a film formation is usually performed at a high temperature, or a heat treatment is performed at a high temperature after the film formation (see, for example, Patent Documents 2 and 3). Therefore, such a glass substrate is required to have high heat resistance that can withstand high temperature treatment, that is, a high glass transition temperature (hereinafter, also referred to as “Tg”).

On the other hand, glass for a magnetic recording medium substrate is also required to suppress the generation of bubbles. This is due to the following reasons. With the progress of high-density recording in recent years, it is desired to reduce the distance (called "flying height") between the head (magnetic head) for writing / reading data and the surface of the magnetic recording medium. There is. However, if the surface of the glass substrate for the magnetic recording medium has irregularities due to bubbles, the irregularities are reflected on the surface of the magnetic recording medium, and the surface smoothness of the magnetic recording medium is lowered. If the magnetic head is brought close to the surface of the magnetic recording medium having poor surface smoothness, the magnetic head may come into contact with the surface of the magnetic recording medium and damage the magnetic head. Therefore, it is necessary to secure a certain flying height to prevent the contact. I don't get it. From the above points, the magnetic recording medium substrate is required to reduce bubbles in the glass substrate in order to produce a magnetic recording medium having high surface smoothness in order to narrow the flying height. The bubbles in the glass can be reduced by using a component (called a "fining agent") that removes the bubbles during glass melting. Conventionally, Sb oxide (for example, Sb ₂ O ₃ ) has been widely used as a fining agent, but since Sb oxide is a component that gives a load to the environment, its use should be refrained from. Patent Documents 1 to 4 also propose the use of Sn oxide and Ce oxide as fining agents, but in order to achieve both improvement in heat resistance (higher Tg) and reduction of bubbles, further There was a need for improvement.

One aspect of the present invention is to provide a glass for a magnetic recording medium substrate having high heat resistance and reduced bubbles.

One aspect of the present invention is displayed in mol%.
SiO ₂ content is 56-75%,
Al ₂ O ₃ content 0.1-10%,
Li ₂ O content 0-2%,
Total content of Na ₂ O and K ₂ O is 3-15%,
The total content of MgO, CaO and SrO is 14-35%,
Ti oxide content is 0.25 to 2.50%,
The total content of Sn oxide and Ce oxide is 0.10 to 1.55%,
Sb oxide content 0-0.02%,
And
The molar ratio of Li ₂ O content to the total content of SiO ₂ and Al ₂ O ₃ {Li ₂ O / (SiO ₂ + Al ₂ O ₃ )} is 0.02 or less, and the glass transition temperature is 600 ° C or more. Glass for magnetic recording medium substrate,
Regarding.

By having the above-mentioned composition, the glass for a magnetic recording medium substrate can achieve both high heat resistance with a glass transition temperature of 600 ° C. or higher and reduction of bubbles. The present inventor considers this point as follows. However, the following is speculation and does not limit the present invention in any way.
Since Li ₂ O is a component that lowers the heat resistance (lowers the glass transition temperature) even when introduced in a small amount, the amount of Li ₂ O introduced should be suppressed, specifically, the content of Li ₂ O should be adjusted as described above. The present inventor believes that setting the molar ratio {Li ₂ O / (SiO ₂ + Al ₂ O ₃ )} to 0.02 or less contributes to the improvement of the heat resistance of the glass. However, on the other hand, glass having such a composition tends to have low meltability, which is considered to make it difficult to remove bubbles during glass melting. This is because Li ₂ O is also a component that works to improve the meltability of glass. In a glass having such a composition, the Ti oxide contained in the above amount and the Sn oxide and Ce oxide contained in the above total content exert a clarifying action as a fining agent, whereby the glass is being melted. The inventor speculates that the removal of bubbles can be promoted. Based on the above, the present inventor believes that it is possible to achieve both improvement in heat resistance and reduction of bubbles.

According to the above-described aspect, it is possible to provide a glass for a magnetic recording medium substrate having excellent heat resistance and reduced bubbles. Further, according to one aspect, it is also possible to provide a magnetic recording medium substrate made of the glass for the magnetic recording medium substrate, and a magnetic recording medium including the substrate.

[Glass for magnetic recording medium substrate]
One aspect of the present invention relates to glass for a magnetic recording medium substrate having the above-mentioned glass composition and having a glass transition temperature of 600 ° C. or higher (hereinafter, also simply referred to as “glass”). Hereinafter, the above-mentioned glass will be described in more detail.

<Glass composition>
In the present invention, the glass composition of glass is indicated on an oxide basis. Here, the "oxide-based glass composition" refers to a glass composition obtained by converting all glass raw materials into those that are decomposed at the time of melting and exist as oxides in glass. Unless otherwise specified, the glass composition shall be indicated on a molar basis (molar%, molar ratio).
The glass composition in the present invention can be determined by a method such as ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Quantitative analysis is performed for each element using ICP-AES. The analytical values are then converted to oxide notation. The analysis value by ICP-AES may include, for example, a measurement error of about ± 5% of the analysis value. Therefore, the oxide notation value converted from the analysis value may also contain an error of about ± 5%.
Further, in the present specification and the present invention, when the content of the constituent component is 0% or not contained or introduced, it means that the constituent component is substantially not contained, and the content of the constituent component is the impurity level. It means that it is below the degree. The impurity level or less means, for example, less than 0.01%.

The glass composition of the above-mentioned glass will be described in more detail below. The above-mentioned glass is preferably an amorphous glass.

Ti is a transition metal, and the Ti oxide, which is an oxide thereof, exerts a clarifying effect in the glass having the above-mentioned glass composition together with the Sn oxide and / or the Ce oxide contained in the above-mentioned total content. The present inventor considers it to be an ingredient (an ingredient that functions as a clarifying agent). The content thereof is in the range of 0.20 to 2.50% in order to provide a glass for a magnetic recording medium substrate having high heat resistance and reduced bubbles. From the viewpoint of further reducing bubbles in the glass, the Ti oxide content is preferably 0.30% or more, more preferably 0.40% or more, and more preferably 0.50% or more. Is more preferable, 0.60% or more is more preferable, and 0.70% or more is even more preferable. On the other hand, the Ti oxide content is preferably 2.40% or less from the viewpoint of further reducing bubbles in the glass, improving the meltability of the glass, and suppressing the coloring of the glass. It is more preferably 30% or less, further preferably 2.25% or less, further preferably 1.75% or less, and even more preferably 1.50% or less.

The above-mentioned glass contains, together with the above-mentioned content of Ti oxide, one or more oxides selected from the group consisting of Sn oxide and Ce oxide. In order to provide a glass for a magnetic recording medium substrate having high heat resistance and reduced bubbles, the total content of Sn oxide and Ce oxide is in the range of 0.10 to 1.55%. The total content of Sn oxide and Ce oxide is preferably 0.15% or more, and more preferably 0.20% or more, from the viewpoint of further reducing bubbles in the glass. From the same viewpoint, the total content of Sn oxide and Ce oxide is preferably 1.50% or less, more preferably 1.25% or less, and 1.00% or less. Is even more preferable. Among the Sn oxide and the Ce oxide, the above-mentioned glass may contain only the Sn oxide, may contain only the Ce oxide, or may contain the Sn oxide and the Ce oxide. From the viewpoint of further reducing bubbles, the above-mentioned glass preferably contains Sn oxide and Ce oxide as essential components.

From the viewpoint of further reduction of bubbles, the Sn oxide content is preferably 0.10% or more, more preferably 0.15% or more, still more preferably 0.20% or more. From the same viewpoint, the Sn oxide content is preferably 1.50% or less, more preferably 1.20% or less, still more preferably 1.00% or less, still more preferably. It is 0.80% or less.
The content of Ce oxide is preferably 0.05% or more, more preferably 0.10% or more, still more preferably 0.15% or more, from the viewpoint of further reducing bubbles. From the same viewpoint, the content of Ce oxide is preferably 0.70% or less, more preferably 0.50% or less, and further preferably 0.40% or less.

Since the metal atoms constituting the oxides of Ti oxides, Sn oxides and Ce oxides can have multiple valences, the valences change at high temperatures in the clarification process to release oxygen (O ₂ ). It is considered that this causes bubbles to float from the molten glass and promotes the removal of bubbles. Furthermore, regarding Ti oxide, the present inventor may exhibit a strong oxidizing action in the low temperature region in the latter half of the clarification process, deprive the molten glass of oxygen present, and eliminate bubbles in the glass. I'm guessing it might be. This makes it possible to remove bubbles from glass that is difficult to remove by adjusting the composition for improving heat resistance by containing Ti oxide in the above amount together with Sn oxide and Ce oxide. The present inventor speculates that this is the reason for this. Above all, the fact that Ti oxide can function as a fining agent is a new finding that has not been known in the past. However, the above is a speculation and does not limit the present invention in any way.

Ti oxides, Sn oxides, and Ce oxides are preferably TiO ₂ , SnO ₂ , and CeO ₂ , but may be contained as oxides of other forms.

As described above, Sb oxide, which has been widely used as a fining agent, should be refrained from being used from the viewpoint of reducing the environmental load. Therefore, the Sb oxide content in the above-mentioned glass is in the range of 0 to 0.02%. The Sb oxide content is preferably 0.01% or less. It is particularly preferable that the above-mentioned glass is substantially free of Sb oxide.

In the above-mentioned glass, the total content of Ti oxide, Sn oxide and Ce oxide (Ti oxide + Sn oxide + Ce oxide) is 0.5% (specifically, 0) from the viewpoint of further reduction of bubbles. It is preferably .50% or more, more preferably 0.8% (specifically 0.80%) or more, and even more preferably 1.1% (specifically 1.10%) or more. .. From the same viewpoint, the total content (Ti oxide + Sn oxide + Ce oxide) is preferably 4.0% (specifically 4.00%) or less, and 3.5% (specifically, detailed). It is more preferably 3.50% or less, further preferably 3.0% (specifically 3.00%) or less, and more preferably 2.5% (specifically 2.50%) or less. More preferred. Further, from the same viewpoint, the molar ratio of Ti oxide content to the total content of Sn oxide and Ce oxide {Ti oxide / (Sn oxide + Ce oxide)} is 0.4 to 10.0. It is preferably in the range of. From the viewpoint of further reduction of bubbles, the molar ratio {Ti oxide / (Sn oxide + Ce oxide)} is more preferably 0.6 or more, further preferably 0.7 or more. It is more preferably 0.8 or more, and even more preferably 1.0 or more. From the same viewpoint, the molar ratio {Ti oxide / (Sn oxide + Ce oxide)} is more preferably 8.0 or less, further preferably 6.0 or less, and 5.0 or less. It is more preferably present, and even more preferably 4.0 or less.

SiO ₂ is a network-forming component of glass and has an effect of improving glass stability and chemical durability (for example, acid resistance). In order to obtain these effects, the above-mentioned glass contains 56 to 75% of SiO ₂ . The SiO ₂ content is preferably 57% or more, more preferably 58% or more, still more preferably 59% or more, still more preferably 60% or more. The SiO ₂ content is preferably 73% or less, more preferably 70% or less, and further preferably 68% or less.

Al ₂ O _{3 is} also a network-forming component of glass and has the effect of improving chemical durability and heat resistance. In order to obtain these effects, the above-mentioned glass contains 0.1 to 10% of Al ₂ O ₃ . The Al ₂ O ₃ content is preferably 9% or less, more preferably 8% or less, still more preferably 7% or less, still more preferably 6% or less. The Al ₂ O ₃ content is preferably 0.2% or more, more preferably 0.3% or more.

From the viewpoint of further reduction of bubbles, the molar ratio of the total content of Ti oxide, Sn oxide and Ce oxide to the total content of SiO ₂ and Al ₂ O _{3 in} the above glass {(Ti oxide + Sn) Oxide + Ce oxide) / (SiO ₂ + Al ₂ O ₃ )} is preferably in the range of 0.010 to 0.070, more preferably in the range of 0.010 to 0.050, and is 0. It is more preferably in the range of .015 to 0.040.
Further, from the viewpoint of further reducing bubbles in the glass and improving the meltability of the glass, the molar ratio of the Ti oxide content to the total content of SiO ₂ and Al ₂ O ₃ {Ti oxide / (SiO _2). + Al ₂ O ₃ )} is preferably 0.030 or less. Further, the molar ratio {Ti oxide / (SiO ₂ + Al ₂ O ₃ )} is more preferably in the range of 0.003 to 0.030, and further preferably in the range of 0.005 to 0.030. It is preferably in the range of 0.005 to 0.025, more preferably.

Li ₂ O is a component that works to improve the meltability and moldability of glass and increases the coefficient of thermal expansion, but lowers the glass transition temperature. Therefore, in order to improve the heat resistance (higher Tg), the Li ₂ O content of the above-mentioned glass is 0 to 2%. From the viewpoint of further improving the heat resistance, the Li ₂ O content is preferably in the range of 0 to 1.5%, further preferably in the range of 0 to 1.2%, still more preferably in the range of 0 to 1.0%, and even more preferably. The range is 0 to 0.8%, the still more preferable range is 0 to 0.5%, the further preferable range is 0 to 0.2%, and it is particularly preferable that Li ₂ O is substantially not contained. ..
Furthermore, in order to improve heat resistance (higher Tg), the molar ratio of Li ₂ O content to the total content of SiO ₂ and Al ₂ O ₃ contained as network components of the glass in the above glass {Li ₂ O / (SiO ₂ + Al ₂ O ₃ )} is 0.02 or less, preferably 0.01 or less, and more preferably 0.
It is considered that it is difficult to remove bubbles in the glass whose composition has been adjusted to improve heat resistance as described above, but the content of Ti oxide, the total content of Sn oxide and Ce oxide are contained. By setting the amount within the above range, it is possible to achieve both improvement in heat resistance and reduction of bubbles.

Na ₂ O and K ₂ O are components that improve the meltability and moldability of glass, reduce the viscosity of glass during clarification, promote foam breakage, and increase the coefficient of thermal expansion. Is. Further, among the alkaline components, the action of lowering the glass transition temperature is smaller than that of Li ₂ O. In the above-mentioned glass, the total content of Na ₂ O and K ₂ O is 3% in order to impart the desired homogeneity (no unmelted matter or residual bubbles) and thermal expansion characteristics to the magnetic recording medium substrate. That is all. Further, from the viewpoint of improving heat resistance and chemical durability (for example, acid resistance), the total content of Na ₂ O and K ₂ O shall be 15% or less. The preferred range for the total content of Na ₂ O and K ₂ O is 5 to 13%, the more preferred range is 6 to 13%, and the more preferred range is 8 to 11%. In one aspect, the Na ₂ O content is preferably in the range of 2-11%, more preferably in the range of 3-10%. In one aspect, the K ₂ O content is preferably in the range of 0 to 13%, more preferably in the range of 2 to 10%, and even more preferably in the range of 2.5 to 8%.

The above-mentioned glass may be used as a magnetic recording medium substrate without ion exchange, or may be used as a magnetic recording medium substrate after ion exchange. When performing ion exchange, Na ₂ O is a suitable component as a component responsible for ion exchange. Further, Na ₂ O and K ₂ O can coexist as a glass component, and an alkali elution suppressing effect can be obtained by a mixed alkali effect. From the above points and the improvement of heat resistance and chemical durability, the total content of Na ₂ O and K ₂ O is set to the above range, and the range of Na ₂ O content is set to 0 to 5%. It is preferably 0.1 to 5%, more preferably 1 to 5%, and even more preferably 2 to 5%. The range of K ₂ O content is preferably 1 to 10%, more preferably 1 to 9%, further preferably 1 to 8%, further preferably 3 to 8%. It is preferably 5 to 8%, and even more preferably 5 to 8%.

The alkaline earth metal components MgO, CaO, and SrO all work to improve the meltability, moldability, and glass stability of the glass, and to increase the coefficient of thermal expansion. In order to obtain such a function, the total content of MgO, CaO and SrO in the above glass is set to 14 to 35%. The total content is more preferably in the range of 15 to 30%, still more preferably in the range of 15 to 25%.

By the way, the substrate of a magnetic recording medium used for mobile applications is required to have high rigidity and hardness to withstand an impact during carrying, and to be lightweight. Therefore, it is desirable that the glass for producing such a substrate has a high Young's modulus, a high specific elastic modulus, and a low specific gravity. Among the alkaline earth metal components, MgO and CaO have a function of increasing rigidity and hardness and suppressing an increase in specific gravity. Therefore, it is a very useful component for obtaining glass having a high Young's modulus, a high specific elastic modulus, and a low specific gravity. In particular, MgO is effective for increasing Young's modulus and lowering the specific gravity, and CaO is an effective component for increasing thermal expansion. Therefore, in the above-mentioned glass, the molar ratio of the total content of MgO and CaO to the total content of MgO, CaO and SrO is {(MgO + CaO) in order to increase the Young's modulus, the specific elastic modulus and the low specific gravity of the glass. / (MgO + CaO + SrO)} is preferably in the range of 0.85 to 1.0, and more preferably in the range of 0.9 to 1.0.

From the viewpoints of high Young's modulus, high specific elastic modulus, low specific gravity and maintenance of chemical durability, the preferable range of MgO content is 1 to 23%, and the preferable lower limit of MgO content is 2%. The more preferable lower limit is 5%, the preferable upper limit of the MgO content is 20%, the more preferable upper limit is 18%, the further preferable upper limit is 15%, and the further preferable upper limit is 12%.
From the viewpoints of high Young's modulus, high specific elastic modulus, low specific gravity, high thermal expansion and maintenance of chemical durability, the preferable range of CaO content is 6 to 21%, and the more preferable range is 10 to 10. 20%, a more preferred range is 10-18%, and a more preferred range is 10-15%.
From the above viewpoint, the range of the total content of MgO and CaO is preferably 15 to 35%, more preferably 15 to 32%, further preferably 15 to 30%, and 15 to 25%. It is more preferably set to%, and even more preferably 15 to 20%.

SrO has the above-mentioned effect, but if it is contained in an excessive amount, the specific gravity increases. In addition, the raw material cost increases as compared with MgO and CaO. Therefore, the content of SrO is preferably in the range of 0 to 5%, more preferably in the range of 0 to 2%, further preferably in the range of 0 to 1%, and 0 to 0.5. It is even more preferable to set it in the range of%. SrO does not have to be introduced as a glass component, that is, the glass described above may be a glass that is substantially free of SrO.

BaO also has the above-mentioned effect like the other alkaline earth metal components described above, but when it is contained in excess, the specific gravity increases, the Young's modulus decreases, the chemical durability decreases, and the specific gravity increases. There is a tendency for raw material costs to increase. Further, a glass substrate containing a large amount of BaO tends to have a tendency to deteriorate in quality during long-term use. It is considered that this is because Ba in the glass reacts with carbon dioxide in the atmosphere, and BaCO ₃ is deposited on the surface of the substrate and becomes an deposit. In order to reduce or prevent the occurrence of such deposits, it is desirable not to excessively contain BaO. From the above viewpoint, the content of BaO in the above-mentioned glass is preferably 0 to 5%. A more preferable range of the BaO content is 0 to 3%, a further preferable range is 0 to 2%, a more preferable range is 0 to 1%, and a further preferable range is 0 to 0.5%. BaO does not have to be introduced as a glass component, that is, the glass described above may be a glass that does not substantially contain BaO.
From the above viewpoint, the total content of SrO and BaO is preferably 0 to 5%, more preferably 0 to 3%, further preferably 0 to 2%, and 0 to 1%. Is more preferable, and 0 to 0.5% is even more preferable.

As described above, MgO and CaO have the effect of increasing Young's modulus and coefficient of thermal expansion. On the other hand, Al ₂ O ₃ has a small function of increasing Young's modulus and a function of reducing the coefficient of thermal expansion. Therefore, in order to obtain a glass having a high Young's modulus and a high thermal expansion, in the above-mentioned glass, the molar ratio of the Al ₂ O ₃ content to the total content of MgO and CaO {Al ₂ O ₃ / (MgO + CaO)} is 0 to 0. It is preferably in the range of .4, more preferably in the range of 0.01 to 0.2, and even more preferably in the range of 0.01 to 0.1.

ZrO ₂ has a large function of increasing the glass transition temperature to improve heat resistance and a function of improving chemical durability (for example, alkali resistance), and also has an effect of increasing Young's modulus and increasing rigidity. The preferred range of ZrO ₂ content is 2-9%, the more preferred range is 2-8%, the more preferred range is 2-7%, the more preferred range is 2-6%, and the more preferred range is 2-5%. An even more preferable range is 3 to 5%.

ZnO is a component that improves the meltability, moldability and glass stability of glass, increases rigidity, and increases the coefficient of thermal expansion. The ZnO content in the above-mentioned glass is preferably in the range of 0 to 5%. From the viewpoint of maintaining good heat resistance and chemical durability, the more preferable range of ZnO content is 0 to 4%, the more preferable range is 0 to 3%, and the more preferable range is 0 to 2%. A more preferable range is 0 to 1%, a further preferable range is 0 to 0.5%, and it is not necessary to substantially contain ZnO.

La ₂ O ₃ , Y ₂ O ₃ , Yb ₂ O ₃ , Ta ₂ O ₅ , Nb ₂ O ₅ , and HfO ₂ have a large ability to increase the specific gravity, so the content of each component is to suppress the increase in specific gravity. Is preferably in the range of 0 to 4%, more preferably in the range of 0 to 3%, further preferably in the range of 0 to 2%, and further in the range of 0 to 1%. It is preferably in the range of 0 to 0.5%, and even more preferably. La ₂ O ₃ , Y ₂ O ₃ , Yb ₂ O ₃ , Ta ₂ O ₅ , Nb ₂ O ₅ , and HfO ₂ need not be introduced as glass components.

Other glass components that can be introduced include B ₂ O ₃ and P ₂ O ₅ .
B ₂ O ₃ has a function of reducing brittleness and improving meltability, but since chemical durability is lowered due to excessive introduction, the preferable range of the content is 0 to 3%, which is a more preferable range. Is 0 to 1%, more preferably 0 to 0.5%, and it is more preferable not to introduce.
P ₂ O ₅ can be introduced in a small amount within a range that does not impair the object of the present invention, but since the chemical durability is lowered due to excessive introduction, the content thereof is preferably 0 to 1%, and is 0. It is more preferably ~ 0.5%, further preferably 0 to 0.3%, and even more preferably not introduced.

Since Pb, Cd, As and the like are substances that adversely affect the environment, it is preferable to avoid their introduction.

In the above-mentioned glass, glass raw materials such as oxides, carbonates, nitrates, sulfates, and hydroxides are weighed, mixed, and sufficiently mixed so that a predetermined glass composition can be obtained, for example, in a melting vessel. It can be produced by heating, melting, clarifying, and stirring in the range of 1400 to 1600 ° C. to form a homogenized molten glass in which bubbles are sufficiently broken. For example, the glass raw material is melted by heating in a melting tank at 1400 to 1550 ° C., and the obtained molten glass is heated in a clarification tank and held at 1450 to 1600 ° C., and then lowered to 1200 to 1400 ° C. It is preferable to clarify. Assuming that the time for holding at 1450 to 1600 ° C. is TH and the time for holding at 1200 to 1400 ° C. is TL, TL / TH is preferably 0.5 or less, and more preferably 0.2 or less. Further, TL / TH is preferably larger than 0.01, more preferably larger than 0.02, further preferably larger than 0.03, and even more preferably larger than 0.04. .. The temperature difference when lowering the temperature from the range of 1450 to 1600 ° C. to the range of 1200 to 1400 ° C. is preferably 30 ° C. or higher, more preferably 50 ° C. or higher, and even more preferably 80 ° C. or higher. , 100 ° C. or higher is more preferable, and 150 ° C. or higher is even more preferable. The upper limit of the temperature difference is, for example, 400 ° C.

<Glass characteristics>
The above-mentioned glass having the composition described above has high heat resistance, and preferably has high rigidity and a high coefficient of thermal expansion. Hereinafter, preferable physical properties of the above-mentioned glass will be sequentially described.

1. 1. Glass transition temperature As described above, when the recording density of a magnetic recording medium is increased by introducing a magnetic material having high magnetic anisotropy energy, the magnetic recording medium substrate is exposed to a high temperature in high temperature treatment of the magnetic material. It will be. At that time, in order to prevent the flatness of the substrate from being impaired, the glass for the magnetic recording medium substrate is required to have excellent heat resistance. The glass transition temperature can be used as an index of heat resistance, and the above-mentioned glass for a magnetic recording medium substrate has a glass transition temperature of 600 ° C. or higher due to the composition adjustment described above. As a result, excellent flatness can be maintained even after high temperature treatment. Therefore, according to the above-mentioned glass, it is possible to provide a substrate suitable for producing a magnetic recording medium having a magnetic recording layer containing a magnetic material having a high magnetic anisotropy energy. However, the above-mentioned glass is not limited to the substrate glass of a magnetic recording medium having a magnetic recording layer containing a magnetic material having a high magnetic anisotropy energy, and is used for producing a magnetic recording medium provided with various magnetic materials. Can be used.
The preferred range of the glass transition temperature is 620 ° C. or higher, the more preferred range is 630 ° C. or higher, the more preferred range is 650 ° C. or higher, the more preferred range is 660 ° C. or higher, the further preferred range is 670 ° C. or higher, and the further preferred range is 675 ° C. ° C. or higher, and even more preferably 680 ° C. or higher. The upper limit of the glass transition temperature is, for example, about 750 ° C., but it is not particularly limited because the higher the glass transition temperature is, the more preferable it is from the viewpoint of heat resistance.

2. 2. Foam Density The above-mentioned glass can achieve both high heat resistance and reduction of bubbles due to the composition described above. The glass transition temperature, which is an index of heat resistance, is as described above. Regarding bubbles in glass, the density of bubbles per unit mass is preferably less than 50 cells / kg as the density of bubbles having a diameter of more than 0.03 mm observed by an optical microscope (magnification 40 to 100 times). It is more preferably less than 20 pieces / kg, further preferably less than 10 pieces / kg, still more preferably 2 pieces / kg or less, and most preferably 0 pieces / kg.

3. 3. Thermal expansion coefficient By the way, an HDD (hard disk drive) incorporating a magnetic recording medium usually has a structure in which the central portion is pressed by the spindle of a spindle motor to rotate the magnetic recording medium itself. Therefore, if there is a large difference in the coefficient of thermal expansion between the magnetic recording medium substrate and the spindle material constituting the spindle portion, the thermal expansion / contraction of the spindle and the heat of the magnetic recording medium substrate with respect to the ambient temperature change during use The expansion and thermal contraction may shift, and as a result, the magnetic recording medium may be deformed. If such a phenomenon occurs, the written information cannot be read by the head, which causes a decrease in the reliability of recording / playback. Therefore, in order to improve the reliability of the magnetic recording medium, the glass substrate is required to have a coefficient of thermal expansion as high as that of the spindle material (for example, stainless steel). Generally, the spindle material of an HDD has an average linear expansion coefficient (coefficient of thermal expansion) of 70 × 10 ^-7 / ° C or higher in a temperature range of 100 to 300 ° C. However, the above-mentioned glass for a magnetic recording medium substrate is used. For example, the coefficient of linear expansion in the temperature range of 100 to 300 ° C. can be set to 60 × 10 ^-7 / ° C. or higher, and the above reliability can be improved. The preferable range of the average coefficient of linear expansion is 64 × 10 ^-7 / ° C or higher, the more preferable range is 67 × 10 ^-7 / ° C or higher, the more preferable range is 70 × 10 ^-7 / ° C or higher, and the more preferable range is 73 ×. It is 10 ^-7 / ° C or higher. Considering the thermal expansion characteristics of the spindle material, the upper limit of the coefficient of linear expansion is preferably, for example, about 120 × 10 ^-7 / ° C, more preferably 100 × 10 ^-7 / ° C, and 88 × It is more preferably 10 ^-7 / ° C.

4. Young's modulus Deformation of the magnetic recording medium includes deformation due to temperature change of the HDD and deformation due to high-speed rotation. From the viewpoint of suppressing deformation during high-speed rotation, it is desired to increase the Young's modulus of the glass for the magnetic recording medium substrate. According to the above-mentioned glass for a magnetic recording medium substrate, Young's modulus can be set to 75 GPa or more, substrate deformation at high speed rotation is suppressed, and a high recording density provided with a magnetic material having a relatively high magnetic anisotropy energy. Data can be read and written accurately even on a magnetic recording medium.
The preferred range of Young's modulus is 75 GPa or more, the more preferable range is 78 GPa or more, and the more preferable range is 80 GPa or more. The upper limit of Young's modulus is, for example, about 95 GPa, but is not particularly limited.

5. Specific elastic modulus / specific gravity In order to provide a substrate that is not easily deformed when the magnetic recording medium is rotated at high speed, the specific elastic modulus of the glass for the magnetic recording medium substrate is preferably 28 MNm / kg or more, preferably 30 MNm / kg or more. Is more preferable. The upper limit is, for example, about 35 MN m / kg, but is not particularly limited. The specific elastic modulus is the Young's modulus of glass divided by the density. Here, the density can be thought of as an amount obtained by adding a unit of g / cm ³ to the specific gravity of glass. By lowering the specific gravity of glass, the specific elastic modulus can be increased and the weight of the substrate can be reduced. By reducing the weight of the substrate, the weight of the magnetic recording medium can be reduced, the power required for rotation of the magnetic recording medium can be reduced, and the power consumption of the HDD can be suppressed. The preferable range of the specific gravity of the glass for the magnetic recording medium substrate is less than 3.0, the more preferable range is 2.9 or less, the more preferable range is 2.85 or less, and the more preferable range is 2.80 or less.

[Magnetic recording medium board]
The magnetic recording medium substrate according to one aspect of the present invention is made of the above-mentioned glass for a magnetic recording medium substrate. The above-mentioned magnetic recording medium substrate can be a magnetic recording medium substrate having excellent heat resistance (that is, a glass transition temperature of 600 ° C. or higher) and reduced bubbles.

The above-mentioned magnetic recording medium substrate was obtained by preparing molten glass by heating a glass raw material and molding the molten glass into a plate shape by any method of a press molding method, a downdraw method or a float method. It can be manufactured through a process of processing plate-shaped glass. For example, in the press molding method, the molten glass flowing out from the glass outflow pipe is cut into a predetermined volume to obtain a required molten glass ingot, which is press-molded by a press molding die to produce a thin-walled disk-shaped substrate blank. .. Next, a center hole is provided in the obtained substrate blank, inner and outer circumferences are processed, and both main surfaces are wrapped and polished. Then, a disk-shaped substrate can be obtained through a cleaning step including acid cleaning and alkaline cleaning.

In one aspect, the magnetic recording medium substrate described above has a homogeneous surface and internal composition. Here, the homogeneous surface and internal composition means that ion exchange has not been performed (that is, there is no ion exchange layer). For example, when an HDD (hard disk drive) incorporating a magnetic recording medium is used in an environment where it is unlikely to receive an external impact, a magnetic recording medium substrate having no ion exchange layer can be used. However, since the above-mentioned magnetic recording medium substrate has high heat resistance and reduced bubbles, it is suitable for application to various HDDs even if it does not have an ion exchange layer. Since the magnetic recording medium substrate having no ion exchange layer is not subjected to the ion exchange treatment, the manufacturing cost can be significantly reduced.

Further, in one aspect, the above-mentioned magnetic recording medium substrate has an ion exchange layer on a part or all of the surface. Since the ion exchange layer exhibits compressive stress, the presence or absence of the ion exchange layer can be confirmed by breaking the substrate perpendicular to the main surface and obtaining a stress profile by the Babine method on the fracture surface. The "main surface" is a surface on which the magnetic recording layer of the substrate is provided or is provided. Since such a surface is the surface having the largest area among the surfaces of the magnetic recording medium substrate, it is called a main surface. In the case of a disk-shaped magnetic recording medium, the circular surface of the disk (when there is a center hole). Is equivalent to (excluding the central hole). The presence or absence of the ion exchange layer can also be confirmed by a method of measuring the concentration distribution of alkali metal ions in the depth direction from the surface of the substrate.

The ion exchange layer can be formed by bringing an alkali salt into contact with the surface of the substrate at a high temperature and exchanging the alkali metal ions in the alkali salt with the alkali metal ions in the substrate. Known techniques can be applied to ion exchange (also referred to as "strengthening treatment" or "chemical strengthening"), and paragraphs 0068 to 0069 of WO2011 / 019010A1 can be referred to as an example.

The above-mentioned magnetic recording medium substrate has, for example, a thickness of 1.5 mm or less, preferably 1.2 mm or less, more preferably 1 mm or less, and a lower limit of the thickness is preferably 0.3 mm. Further, the above-mentioned magnetic recording medium substrate preferably has a disk shape having a center hole.

[Magnetic recording medium]
One aspect of the present invention relates to a magnetic recording medium having a magnetic recording layer on the above-mentioned magnetic recording medium substrate.
Magnetic recording media are called magnetic disks and hard disks, and are used for internal storage devices (fixed disks, etc.) such as desktop computers, server computers, notebook computers, and mobile computers, and portable recording for recording and reproducing images and / or audio. It is suitable for an internal storage device of a playback device, a recording / playback device for in-vehicle audio, and the like.
The magnetic recording medium has, for example, a configuration in which at least an adhesive layer, a base layer, a magnetic layer (magnetic recording layer), a protective layer, and a lubricating layer are laminated on the main surface of a magnetic recording medium substrate in order from the one closest to the main surface. It has become.
For example, a magnetic recording medium substrate is introduced into a film forming apparatus that has been evacuated, and a magnetic layer is formed on the main surface of the magnetic recording medium substrate in an Ar atmosphere by a DC (Direct Current) magnetron sputtering method. The film is formed sequentially up to. For example, CrTi can be used as the adhesion layer, and CrRu can be used as the base layer. After the above film formation, for example, a protective layer is formed using C ₂ H ₄ by a CVD (Chemical Vapor Deposition) method, and a nitriding process for introducing nitrogen to the surface is performed in the same chamber to form a magnetic recording medium. Can be formed. Then, for example, a lubricating layer can be formed by applying PFPE (polyfluoropolyether) on the protective layer by a dip coating method.

As described above, in order to achieve higher density recording of the magnetic recording medium, the magnetic recording layer preferably contains a magnetic material having a high magnetic anisotropy energy. From this point of view, preferred magnetic materials include Fe-Pt-based magnetic materials and Co-Pt-based magnetic materials. Here, the term "system" means that it is contained. That is, the above-mentioned magnetic recording medium preferably has a magnetic recording layer containing Fe and Pt, or Co and Pt as the magnetic recording layer. For the magnetic recording layer containing such a magnetic material and the film forming method thereof, reference can be made to paragraph 0074 of WO2011 / 019010A1 and the description of Examples of the same publication. Further, the magnetic recording medium having such a magnetic recording layer is preferably applied to a magnetic recording apparatus by a recording method called an energy assist recording method. Among the energy-assisted recording methods, a recording method that assists magnetization reversal by irradiating laser light is called a heat-assisted recording method, and a recording method that assists with microwaves is called a microwave-assisted recording method. For more information on them, see paragraph 0075 of WO2011 / 019010A1.

By the way, in recent years, by mounting a DFH (Dynamic Flying Height) mechanism on a magnetic head, the gap between the recording / reproducing element portion of the magnetic head and the surface of the magnetic recording medium has been significantly narrowed (reduced levitation amount). Higher recording densities are being achieved. The DFH mechanism is a function of providing a heating portion such as a very small heater in the vicinity of the recording / reproducing element portion of the magnetic head and projecting only the periphery of the element portion toward the surface of the medium. By doing so, the distance (flying height) between the magnetic head and the magnetic recording layer of the medium becomes shorter, so that signals of smaller magnetic particles can be picked up, and further high recording density can be achieved. It will be possible. However, on the other hand, the gap (flying height) between the element portion of the magnetic head and the surface of the medium becomes extremely small. Since the above-mentioned magnetic recording medium has reduced bubbles on the substrate, it can have high surface smoothness. Therefore, it is also suitable for a magnetic recording device equipped with a DFH mechanism in which the flying height is extremely narrowed.

The dimensions of both the above-mentioned magnetic recording medium substrate (for example, magnetic disk substrate) and magnetic recording medium (for example, magnetic disk) are not particularly limited, but for example, the medium and the substrate can be miniaturized because high recording density can be achieved. It is also possible. For example, the nominal diameter may be 2.5 inches, of course, and smaller diameters (eg, 1 inch, 1.8 inches), or dimensions such as 3 inches, 3.5 inches, and the like.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the present invention is not limited to the embodiments shown in the examples.

[Example No. 1-No. 15]
Raw materials such as oxides, carbonates, nitrates, and hydroxides were weighed and mixed to prepare a blending raw material so that a glass having the composition shown in Table 1 could be obtained. The molten glass obtained by putting this compounding raw material into a melting tank, heating and melting in the range of 1400 to 1600 ° C. is held in a clarification tank at 1400 to 1550 ° C. for 6 hours, and then the temperature is lowered (lowered). The mixture was kept in the range of 1200 to 1400 ° C. for 1 hour for clarification to obtain molten glass.

(1) Foam Density Rank A glass plate (board blank) having a thickness of about 1.2 mm was prepared from the molten glass obtained above. The surface of this glass plate is polished flat and smooth, and the inside of the glass is magnified and observed with an optical microscope (magnification 40 to 100 times) from the polished surface, and bubbles having a diameter of more than 0.03 mm (hereinafter, simply referred to as "foam"). The number of) was counted. The density of the bubbles was defined as the mass of the glass corresponding to the magnified observation area divided by the number of bubbles counted.
The bubble density rank was evaluated from S rank to F rank according to the density of bubbles obtained by the above method. Specifically, those with a bubble density of 0 / kg are S rank, those with bubbles present and a bubble density of 2 / kg or less are A rank, and those with a bubble density of more than 2 / kg and less than 10 / kg. B rank, foam density of 10 / kg or more and less than 20 / kg C rank, foam density of 20 / kg or more and less than 50 / kg D rank, foam density of 50 / kg Those having a weight of kg or more and less than 80 were given an E rank, and those having a bubble density of 80 / kg or more were given a F rank.

(2) Glass transition temperature Tg, coefficient of thermal expansion The glass transition temperature Tg of each glass and the average linear expansion coefficient α at 100 to 300 ° C. were measured using a thermomechanical analysis (TMA).

(3) Young's modulus The Young's modulus of each glass was measured by an ultrasonic method.

(4) Specific gravity The specific gravity of each glass was measured by the Archimedes method.

(5) Specific elastic modulus The specific elastic modulus was calculated from the Young's modulus obtained in (3) above and the specific gravity obtained in (4).

The above results are shown in Table 1.

As shown in Table 1, all of the glasses for the magnetic recording medium substrate of the examples have a glass transition temperature (Tg) of 600 ° C. or higher and a bubble density rank of B or higher (foam density is less than 10 cells / kg). there were. That is, it was confirmed that these glasses for magnetic recording medium substrates have high heat resistance and very few bubbles.

[Example No. A1 to A11, Comparative Examples 1 to 5]
Raw materials such as oxides, carbonates, nitrates, and hydroxides were weighed and mixed to prepare a blending raw material so that a glass having the composition shown in Table 2 could be obtained. The molten glass obtained by putting this compounding raw material into a melting tank, heating and melting in the range of 1400 to 1600 ° C. is held in a clarification tank at 1400 to 1550 ° C. for 6 hours, and then the temperature is lowered (lowered). The mixture was kept in the range of 1200 to 1400 ° C. for 1 hour for clarification to obtain molten glass. The compositions shown in Table 2 fix the amounts of components other than Ti oxide (TiO ₂ ), Sn oxide (SnO ₂ ) and Ce oxide (CeO ₂ ), and are composed of Ti oxide, Sn oxide and Ce oxide. It is a composition in which the amount is changed. A glass plate (substrate blank) having a thickness of about 1.2 mm is produced from the molten glass thus obtained by press molding, and these glass plates are ground and polished to obtain a plurality of flat, smooth and transparent glass substrates. It was.
For each glass, the bubble density rank was evaluated and the glass transition temperature was measured by the method described above. The results are shown in Table 2.

As shown in Table 2, Example No. The glass for the magnetic recording medium substrate of A1 to A11 had a glass transition temperature (Tg) of 600 ° C. or higher and a bubble density rank of B or higher (foam density of less than 10 cells / kg). That is, it was confirmed that these glasses for magnetic recording medium substrates have high heat resistance and very few bubbles.

On the other hand, in Comparative Examples 1 to 5, the amounts of components other than Ti oxide (TiO ₂ ), Sn oxide (SnO ₂ ) and Ce oxide (CeO ₂ ) were found in Example No. A1 to No. Since it is equivalent to A11, the glass transition temperature (Tg) is as high as 600 ° C. or higher, but the bubble density rank is E rank or lower (foam density is 50 bubbles / kg or more), and the number of bubbles is not reduced, which is practical. It was confirmed that it was not suitable for. In particular, it was confirmed that Comparative Example 1 containing no Ti oxide (TIO ₂ ) had a higher foam density than the other Comparative Examples 2 to 5, and the foam density rank was F rank.

[Example No. B1 to B11, Comparative Examples 6 to 10]
Raw materials such as oxides, carbonates, nitrates, and hydroxides were weighed and mixed to prepare a blending raw material so that a glass having the composition shown in Table 3 could be obtained. The molten glass obtained by putting this compounding raw material into a melting tank, heating and melting in the range of 1400 to 1600 ° C. is held in a clarification tank at 1400 to 1550 ° C. for 6 hours, and then the temperature is lowered (lowered). The mixture was kept in the range of 1200 to 1400 ° C. for 1 hour for clarification to obtain molten glass. The composition shown in Table 3 fixes the amounts of components other than Ti oxide (TiO ₂ ), Sn oxide (SnO ₂ ) and Ce oxide (CeO ₂ ), and is composed of Ti oxide, Sn oxide and Ce oxide. It is a composition in which the amount is changed. A glass plate (substrate blank) having a thickness of about 1.2 mm is produced from the molten glass thus obtained by press molding, and these glass plates are ground and polished to obtain a plurality of flat, smooth and transparent glass substrates. It was.
For each glass, the bubble density rank was evaluated and the glass transition temperature was measured by the method described above. The results are shown in Table 3.

As shown in Table 3, Example No. The glass for the magnetic recording medium substrate of B1 to B11 had a glass transition temperature (Tg) of 600 ° C. or higher and a bubble density rank of B or higher (foam density was less than 10 cells / kg). That is, it was confirmed that these glasses for magnetic recording medium substrates have high heat resistance and very few bubbles.

On the other hand, in Comparative Examples 6 to 10, the amounts of components other than Ti oxide (TiO ₂ ), Sn oxide (SnO ₂ ) and Ce oxide (CeO ₂ ) were found in Example No. B1 to No. Since it is equivalent to B11, the glass transition temperature (Tg) is as high as 600 ° C. or higher, but the bubble density rank is E rank or lower (foam density is 50 bubbles / kg or more), and the number of bubbles is not reduced, which is practical. It was confirmed that it was not suitable for. In particular, it was confirmed that Comparative Example 6 containing no Ti oxide (TIO ₂ ) had a higher foam density than the other Comparative Examples 7 to 10, and the foam density rank was F rank.

[Manufacturing of magnetic recording medium substrate]
(1) Preparation of Substrate Blank Next, a disk-shaped substrate blank was prepared by the following method A or B.
(Method A)
The clarified and homogenized molten glass of the above-mentioned example was discharged from the outflow pipe at a constant flow rate and received by a lower mold for press molding, and the molten glass flowed out so that a predetermined amount of molten glass ingot was obtained on the lower mold. It was cut with a cutting blade. Then, the lower mold on which the molten glass ingot was placed was immediately carried out from below the pipe, and was press-molded into a thin disk shape having a diameter of 66 mm and a thickness of 1.2 mm using the upper mold and the body mold facing the lower mold. After cooling the press-molded product to a temperature at which it was not deformed, it was taken out from the mold and annealed to obtain a substrate blank. In the above-mentioned molding, the molten glass flowing out was formed one after another into a disk-shaped substrate blank using a plurality of lower dies.
(Method B)
The clarified and homogenized molten glass of the above-mentioned example was continuously cast from the upper part into the through hole of the heat-resistant mold provided with the cylindrical through hole, formed into a columnar shape, and taken out from the lower side of the through hole. .. After annealing the taken-out glass, the glass was sliced at regular intervals in the direction perpendicular to the cylindrical axis using a multi-wire saw to prepare a disk-shaped substrate blank.
In this embodiment, the above-mentioned methods A and B are adopted, but the following methods C and D are also suitable as a method for manufacturing a disk-shaped substrate blank.
(Method C)
The molten glass of the above-described embodiment may be poured onto a float bath, molded into a sheet-shaped glass (molded by the float method), and then annealed, and then a disk-shaped glass is hollowed out from the sheet glass to obtain a substrate blank. it can.
(Method D)
The molten glass of the above-described embodiment can be formed into a sheet-shaped glass by an overflow downdraw method (fusion method), annealed, and then a disk-shaped glass is hollowed out from the sheet glass to obtain a substrate blank.

(2) Preparation of glass substrate A through hole is made in the center of the substrate blank obtained by each of the above methods, and the outer and inner circumferences are ground, and the main surface of the disk is wrapped and polished (mirror polishing). The glass substrate for a magnetic disk having a diameter of 65 mm and a thickness of 0.8 mm was finished. The obtained glass substrate was washed with a 1.7% by mass aqueous solution of silicic acid (H ₂ SiF) and then with a 1% by mass aqueous solution of potassium hydroxide, rinsed with pure water and then dried. When the surface of the substrate made from the glass of the example was magnified and observed, no surface roughness was observed, and the surface was smooth.

[Making a magnetic recording medium (magnetic disk)]
An adhesive layer, a base layer, a magnetic recording layer, a protective layer, and a lubricating layer were formed in this order on the main surface of the glass substrate obtained from the glass of the example by the following method to obtain a magnetic disk.

First, the adhesion layer, the base layer, and the magnetic recording layer were sequentially formed in an Ar atmosphere by a DC magnetron sputtering method using a vacuum-drawn film forming apparatus.

At this time, the adhesive layer was formed with a CrTi target so as to be an amorphous CrTi layer having a thickness of 20 nm. Subsequently, a 10 nm-thick layer made of CrRu was formed as a base layer by a DC magnetron sputtering method in an Ar atmosphere using a single-wafer / statically opposed film forming apparatus. Further, the magnetic recording layer was formed at a film forming temperature of 400 ° C. using a FePt or CoPt target so as to be a FePt or CoPt layer having a thickness of 10 nm.

The magnetic disk after the film formation up to the magnetic recording layer was transferred from the film forming apparatus into the heating furnace and annealed. The temperature in the heating furnace at the time of annealing was in the range of 650 to 700 ° C.

Subsequently, a protective layer made of carbon hydride was formed at 3 nm by a CVD method using ethylene as a material gas. After that, a lubricating layer made of PFPE (perfluoropolyester) was formed by a dip coating method. The film thickness of the lubricating layer was 1 nm.
A magnetic disk was obtained by the above manufacturing process. The obtained magnetic disk is mounted on a hard disk drive (flying height: 8 nm) equipped with a DFH mechanism, and a magnetic signal is recorded in a recording area on the main surface of the magnetic disk at a recording density of 20 gigabits per square inch. As a result, the phenomenon of collision between the magnetic head and the surface of the magnetic disk (crash failure) was not confirmed.

According to one aspect of the present invention, it is possible to provide an optimal magnetic recording medium for high-density recording.

Finally, each of the above aspects will be summarized.

According to one embodiment, the SiO ₂ content is 56 to 75%, the Al ₂ O ₃ content is 0.1 to 10%, the Li ₂ O content is 0 to 2%, and Na ₂ O is expressed in mol%. And K ₂ O total content 3-15%, MgO, CaO and SrO total content 14-35%, Ti oxide content 0.25-2.50%, Sn oxide and Ce oxidation The total content of the substance is 0.10 to 1.55%, the Sb oxide content is 0 to 0.02%, and the molar ratio of the Li ₂ O content to the total content of SiO ₂ and Al ₂ O ₃ Provided is a glass for a magnetic recording medium substrate having {Li ₂ O / (SiO ₂ + Al ₂ O ₃ )} of 0.02 or less and a glass transition temperature of 600 ° C. or more.

In one aspect, the above-mentioned glass for a magnetic recording medium substrate preferably has a bubble density per unit mass of more than 0.03 mm in diameter observed by an optical microscope (magnification 40 to 100 times). It is less than 2 pieces / kg, more preferably less than 20 pieces / kg, further preferably less than 10 pieces / kg, still more preferably 2 pieces / kg or less, and most preferably 0 pieces / kg.

In one aspect, the above-mentioned glass for a magnetic recording medium substrate has a Sn oxide content of 0.10 to 1.50 mol%.

In one aspect, the above-mentioned glass for a magnetic recording medium substrate has a Ce oxide content of 0.05 to 0.70 mol%.

In one aspect, the above-mentioned glass for a magnetic recording medium substrate has a total content of Ti oxide, Sn oxide and Ce oxide of 0.50 to 4.00%.

In one aspect, the above-mentioned glass for a magnetic recording medium substrate has a molar ratio of the content of Ti oxide to the total content of Sn oxide and Ce oxide {Ti oxide / (Sn oxide + Ce oxide)}. , 0.4 to 10.0.

In one aspect, the above-mentioned glass for a magnetic recording medium substrate contains Sn oxide and Ce oxide as essential components.

In one aspect, the above-mentioned glass for a magnetic recording medium substrate has a molar ratio of TiO ₂ content to the total content of SiO ₂ and Al ₂ O ₃ {TiO ₂ / (SiO ₂ + Al ₂ O ₃ )}. It is 030 or less.

According to one aspect of the present invention, a magnetic recording medium substrate made of the above-mentioned magnetic recording medium is provided.

In one aspect, the magnetic recording medium substrate described above has a homogeneous surface and internal composition.

In one aspect, the magnetic recording medium substrate described above has an ion exchange layer on a part or all of the surface.

According to one aspect of the present invention, a magnetic recording medium having a magnetic recording layer on the above-mentioned magnetic recording medium substrate is provided.

It should be considered that the embodiments disclosed this time are exemplary in all respects and not restrictive. The scope of the present invention is shown by the scope of claims rather than the above description, and it is intended to include all modifications within the meaning and scope equivalent to the scope of claims.
For example, the glass for a magnetic recording medium substrate according to one aspect of the present invention can be produced by adjusting the composition described in the specification with respect to the above-exemplified glass composition.
In addition, it is of course possible to arbitrarily combine two or more of the items described in the specification as an example or a preferable range.

Claims (8)

In mole% display
SiO ₂ content is 56-75%,
Al ₂ O ₃ content 0.1-10%,
Li ₂ O content 0-2%,
Total content of Na ₂ O and K ₂ O is 3-15%,
The total content of MgO, CaO and SrO is 14-35%,
Ti oxide content is 0.25 to 2.50%,
The total content of Sn oxide and Ce oxide is 0.10 to 1.55%,
Sb oxide content 0-0.02%,
And
The molar ratio of Li ₂ O content to the total content of SiO ₂ and Al ₂ O ₃ {Li ₂ O / (SiO ₂ + Al ₂ O ₃ )} is 0.02 or less.
Contains Sn oxide and Ce oxide as essential components
The molar ratio of the content of Ti oxide to the total content of Sn oxide and Ce oxide {Ti oxide / (Sn oxide + Ce oxide)} is 0.6 or more.
Magnetic in which the molar ratio of TiO ₂ content to the total content of SiO ₂ and Al ₂ O ₃ {TiO ₂ / (SiO ₂ + Al ₂ O ₃ )} is 0.034 or less and the glass transition temperature is 600 ° C or more. Glass for recording medium substrate. The glass for a magnetic recording medium substrate according to claim 1, wherein the Sn oxide content is 0.10 to 1.50 mol%. The glass for a magnetic recording medium substrate according to claim 1 or 2, wherein the content of Ce oxide is 0.05 to 0.70 mol%. The glass for a magnetic recording medium substrate according to any one of claims 1 to 3, wherein the total content of Ti oxide, Sn oxide and Ce oxide is 0.50 to 4.00%. The molar ratio of the content of Ti oxide to the total content of Sn oxide and Ce oxide {Ti oxide / (Sn oxide + Ce oxide)} is 0. The glass for a magnetic recording medium substrate according to any one of claims 1 to 4, which is 6 to 10.0. Any 1 of claims 1 to 5, wherein the molar ratio of the TiO ₂ content to the total content of SiO ₂ and Al ₂ O ₃ {TiO ₂ / (SiO ₂ + Al ₂ O ₃ )} is 0.030 or less. The glass for a magnetic recording medium substrate according to the section. The magnetic recording medium substrate made of the glass for the magnetic recording medium substrate according to any one of claims 1 to 6 . The magnetic recording medium having a magnetic recording layer on the magnetic recording medium substrate according to claim 7 .