Sputtering target for magnetic recording medium

Abstract

The present invention relates to a sputtering target for a magnetic recording medium, the sputtering target including Co and Pt as metal components and also including at least one oxide. In the present invention, a tungsten oxide and / or a molybdenum oxide is included as an essential oxide for improving magnetic separability, and the essential oxide content is 15 vol% or less. As the oxide in the sputtering target, other oxides such as B2O3 can be included as optional oxides, and in such case, the total oxide content is preferably 20 vol% to 50 vol% inclusive. Through use of the essential oxide, the present invention makes it possible to appropriately set the total oxide content while ensuring magnetic separability. With the present invention, it is possible to form a granular thin film that is composed of a CoPt-based alloy and has good magnetic separability.

Description

Sputtering targets for magnetic recording media

[0001] The present invention relates to a sputtering target for forming a thin film made of a CoPt-based alloy for magnetic recording media, and more particularly to a sputtering target for forming a thin film made of a CoPt-based alloy having a granular structure and exhibiting good magnetic separation between magnetic particles.

[0002] The recording density of magnetic recording media such as hard disk drives (HDDs) has been increasing rapidly, and the demand and trend for higher recording densities is expected to continue in the future. To achieve higher recording densities in magnetic recording media, perpendicular magnetic recording methods have been proposed. Furthermore, to improve recording densities while optimizing such recording methods, it is considered preferable to reduce the size of magnetic grains in the magnetic thin film that constitutes the magnetic recording layer while reducing inter-particle interactions.

[0003] As a magnetic thin film that reduces the inter-particle interaction of magnetic particles, a magnetic thin film having a structure called a granular structure has attracted attention (hereinafter, a magnetic thin film having a granular structure will be referred to as a granular thin film). A granular thin film has a structure in which magnetic particles are dispersed in a non-magnetic material matrix such as an oxide, and the inter-particle interaction is reduced by the non-magnetic grain boundaries of the oxide or the like. In a granular thin film, multiple magnetic particles constitute a recording bit. Examples of oxides that constitute the non-magnetic grain boundaries of a magnetic recording layer to which such a granular thin film is applied include SiO ２ , TiO ２ , B ２ O ３ and the like are known (Patent Documents 1 and 2).

[0004] Typically, magnetic thin films of magnetic recording media are often produced by sputtering. Granular thin films can also be produced by sputtering, and a sputtering target containing an oxide along with an alloy or metal such as Co or Pt that constitutes the magnetic material is used (e.g., Patent Documents 3 and 4). Furthermore, in the formation of granular thin films, an underlayer made of Ru or a Ru-containing alloy, which has the effect of promoting the epitaxial growth of magnetic particles, is used. Then, by sputtering the surface of the underlayer with a sputtering target containing the oxide, a two-phase precipitation of magnetic particles and an oxide (non-magnetic grain boundaries) occurs, forming a granular thin film.

[0005] Japanese Patent Publication No. 2002-83411 Japanese Patent Publication No. 2001-43526 Japanese Patent No. 5032706 Japanese Patent No. 6317636

[0006] As mentioned at the beginning, miniaturizing magnetic particles is an effective way to increase the recording density of magnetic recording media. Furthermore, miniaturizing magnetic particles in granular thin films is possible by increasing the oxide content in the sputtering target. Increasing the oxide content increases the thickness of the nonmagnetic grain boundaries, making them clearer, and enabling the formation of granular thin films in which magnetic particles are dispersed and separated.

[0007] However, increased recording density is achieved by reducing the size of magnetic particles and improving the separation between recording bits formed by those magnetic particle groups. Here, separation refers to magnetic separation. Increasing the oxide content reduces the size of magnetic particles and thickens nonmagnetic grain boundaries, but this does not necessarily lead to improved magnetic separation in the granular thin film. Considering that recording bits are composed of multiple magnetic particles, it is necessary to consider not only the amount of oxide (nonmagnetic grain boundaries) but also the effect of their composition in order to ensure magnetic separation throughout the thin film.

[0008] Furthermore, increasing the amount of oxide in the granular thin film can reduce the size of the magnetic particles, but this inevitably leads to a decrease in the volume ratio of the magnetic particles. A decrease in the volume ratio of the magnetic particles reduces the amount of magnetization of the entire granular thin film, which can be a factor in a decrease in signal strength when reproducing from a recording medium. While oxide is an essential component of the granular thin film, its content cannot be unlimited.

[0009] The present invention was made in light of the above background and relates to a sputtering target for magnetic recording media for producing a magnetic thin film made of a CoPt-based alloy, which is one of the magnetic thin films most commonly used in magnetic recording media. The present invention also provides a sputtering target capable of depositing a granular thin film with good magnetic isolation. To achieve this goal, the inventors have sought to find a means for optimizing the oxide content while maintaining magnetic properties, including magnetic isolation.

[0010] According to previous studies by the inventors, the magnetic properties of granular thin films vary depending on the type of oxide that constitutes the nonmagnetic grain boundaries. Since these magnetic properties are thought to include magnetic isolation, it can be said that magnetic isolation varies depending on the type of oxide, even when the oxide content is the same. Furthermore, the various magnetic properties of magnetic thin films can sometimes be in a contradictory relationship, and improving one property can result in a deterioration of the other.

[0011] The most basic evaluation method for magnetic recording media is the MH hysteresis loop. By measuring the MH hysteresis loop, the saturation magnetization (M ｓ ), perpendicular coercivity (H ｃ ) can be grasped. The magnetic isolation of a granular thin film can also be determined from the M-H hysteresis loop, and can be determined by the α value, which indicates the slope (4π × (dM / dH)) of the magnetization curve near the coercive force (near M = 0). The α value increases as the magnetic isolation between recording bits decreases, and the closer the α value is to 1, the better the magnetic isolation.

[0012] In order to solve the above problems, the present inventors have improved the magnetic isolation of granular thin films and also improved the saturation magnetization (M ｓ As a result, it was discovered that oxides of W (tungsten) and Mo (molybdenum) can form magnetic thin films with low α values ​​and good magnetic isolation even when the oxides are contained in relatively small amounts, and this led to the present invention.

[0013] That is, the present invention provides a sputtering target for a magnetic recording medium that contains Co and Pt as metal components and at least one oxide, characterized in that the sputtering target contains tungsten oxide and / or molybdenum oxide as essential oxides, and the content of the essential oxides is 15 vol % or less.

[0014] The structure and manufacturing method of the sputtering target for magnetic recording media according to the present invention will be described below.

[0015] A. Structure of the sputtering target for magnetic recording media according to the present invention (i) Metallic components As described above, the sputtering target for magnetic recording media according to the present invention contains Co and Pt as essential metallic components. Co is a ferromagnetic metal and is the main component of CoPt-based alloy particles that become magnetic particles in the granular thin film that constitutes the magnetic recording layer. Pt is also a paramagnetic metal, but has a large spin-orbit interaction, and improves magnetic anisotropy when alloyed with Co, a ferromagnetic material. Pt is also a metal that is an essential component of CoPt-based alloy particles that become magnetic particles.

[0016] The preferred composition of the CoPt-based alloy grains that make up the magnetic recording layer is a uniaxial magnetic anisotropy constant K ｕFor the purpose of optimizing the above, the Co content is 60 atomic % or more and 95 atomic % or less. Therefore, in the sputtering target for magnetic recording media according to the present invention, the Co content relative to the total metal components is preferably 60 atomic % or more and 95 atomic % or less. Note that the metal components refer to Co, Pt, and, in the case where other metal components are included, the other metals described below. Metal components in the form of oxides are not included. Note that the Co content relative to the total metal components is more preferably 70 atomic % or more and 90 atomic % or less.

[0017] The magnetic grains that make up the magnetic recording layer are made of a CoPt-based alloy, and Co and Pt are essential metals, but other metals may be included. Examples of other metals include Cr, Ru, and B. These metal elements have a saturation magnetization M ｓ These metals can be added because they have the effect of adjusting the composition ratio and improving the magnetic separation of the magnetic particles. The CoPt-based alloy, which is the magnetic particles of the magnetic recording layer, can contain 20 atomic % or less of these metals. Therefore, the sputtering target according to the present invention can contain 20 atomic % or less of other metals relative to the total metal components. The content of Pt in the metal components is the remainder of the contents of Co and the other metals.

[0018] In the sputtering target for magnetic recording media according to the present invention, the metal components may be metallic Co and metallic Pt present as single metals or may be present in the form of an alloy (CoPt-based alloy), as well as other metal components such as Cr.

[0019] (ii) Oxides (essential oxides and optional oxides) The sputtering target for magnetic recording media according to the present invention contains at least one oxide to form non-magnetic grain boundaries in the granular thin film. In the present invention, tungsten oxide and / or molybdenum oxide are contained as essential oxides. A granular thin film formed by a sputtering target containing tungsten oxide and / or molybdenum oxide has a low α value and good magnetic isolation even with a relatively small oxide content. In this case, the saturation magnetization M ｓThe reason why the magnetic isolation (α value) of the granular thin film containing tungsten oxide and molybdenum oxide is good is unknown, and is based on the results of screening by the present inventors, which will be described later.

[0020] In a magnetic thin film made of a CoPt-based alloy, the essential oxides tungsten oxide and molybdenum oxide can significantly reduce the α value even when added in extremely small amounts (more than 0 vol%). Furthermore, as the content of the essential oxides increases, the α value decreases and approaches 1. However, when the content of the essential oxides exceeds 15 vol%, the effect of reducing the α value decreases. For this reason, in the present invention, the content of the essential oxides is set to 15 vol% or less with respect to the total volume of the target. The upper limit of the content of the essential oxides may be 13 vol% or less, 10 vol% or less, or 8 vol% or less. The lower limit of the content of the essential oxides is preferably 0.5 vol% or more, more preferably 1.0 vol% or more, and even more preferably 2.0 vol% or more.

[0021] The sputtering target of the present invention may contain either tungsten oxide or molybdenum oxide, which are essential oxides. Alternatively, the sputtering target may contain both tungsten oxide and molybdenum oxide. When both tungsten oxide and molybdenum oxide are contained, the total amount thereof may be 15 vol % or less.

[0022] In the present invention, the tungsten oxide contained in the sputtering target is WO ３ and / or WO ２ Molybdenum oxide is MoO ３ and / or MoO ２ For example, WO ３ Even if a sputtering target is manufactured using this as a raw material, a portion of it is reduced to WO ２ In such a case, WO may be present in the sputtering target. ３ and W.O. ２The tungsten oxide and molybdenum oxide included in the present invention also include a case where oxides of tungsten and molybdenum having different valences are mixed.

[0023] The sputtering target for a magnetic recording medium according to the present invention essentially contains tungsten oxide and / or molybdenum oxide, and may contain other oxides different from these as optional oxides. ２ O ３ , SiO ２ , TiO ２ , Cr ２ O ３ , CoO, Co ３ O ４ These optional oxides combine with the essential oxides to form non-magnetic grain boundaries.

[0024] Among the above-mentioned optional oxides, the oxide that is preferably added together with the essential oxide is B. ２ O ３ B at the non-magnetic grain boundary ２ O ３ The granular thin film containing ｕ This tends to lead to an increase in the uniaxial anisotropy constant K ｕ is related to the resistance to thermal fluctuations (thermal stability) of the magnetic thin film. Such thermal stability depends on the uniaxial anisotropy constant K ｕ and activation volume V ａ (or the volume V of the magnetization reversal unit). ｕ The increase in B increases the value of the above product, and therefore the increase in B ２ O ３ The addition of leads to an improvement in thermal fluctuation resistance.

[0025] Also, B ２ O ３ For any oxide other than SiO ２ has the effect of amorphizing the non-magnetic grain boundaries. ２ Since Ti has a large valence, it has the effect of preventing segregation to magnetic particles due to oxygen deficiency. ２ O ３ , CoO and Co３ O ４ has the effect of filling oxygen deficiency at grain boundaries. Which of these optional oxides the sputtering target according to the present invention contains can be appropriately selected depending on the properties required for the magnetic thin film to be formed.

[0026] The sputtering target for magnetic recording media according to the present invention comprises essential oxides (tungsten oxide and / or molybdenum oxide) and optional oxides (B ２ O ３ , SiO ２ , TiO ２ , Cr ２ O ３、 CoO, Co ３ O ４ When the magnetic recording medium contains optional oxides, the total content of the essential oxides and optional oxides (total oxide content) is preferably 20 vol% or more and 50 vol% or less. This is to form a granular structure that ensures magnetic separation and to ensure signal strength during playback from the recording medium. When the magnetic recording medium contains optional oxides, the content of the essential oxides is preferably 15 vol% or less.

[0027] Furthermore, the α-value-reducing effect of the essential oxides is not inhibited by the optional oxides. In the present invention, when the optional oxides are included, the content of the essential oxides can be 10 vol% or less, or even 8 vol% or less, or 5 vol% or less, within the above-mentioned range of the total oxide content. This indicates that the essential oxides, tungsten oxide and / or molybdenum oxide, are useful for reducing the total oxide content and ensuring the proportion of magnetic components. In this case, the lower limit of the content of the essential oxides is preferably 0.5 vol% or more, more preferably 1.0 vol% or more, and even more preferably 2.0 vol% or more.

[0028] B. Method for manufacturing a sputtering target for magnetic recording media according to the present invention The sputtering target according to the present invention comprises a metal phase (CoPt-based alloy phase, or metallic Co phase and metallic Pt phase) made of the above-mentioned metal components, and an oxide phase (essential oxide phase and optional oxide phase). Powder metallurgy is preferably used as a method for manufacturing such a sputtering target in order to uniformly disperse the oxide phase in the metal phase. In the powder metallurgy method, a metal powder and an oxide powder that will form the metal phase are mixed, appropriately molded, and then pressure-sintered to produce a sputtering target.

[0029] The metal powder can be an alloy powder made of a CoPt-based alloy, or a metal Co powder and a metal Pt powder. The alloy powder can be produced by an atomization method such as gas atomization from a molten CoPt-based alloy having the above-described composition. When using metal Co powder and metal Pt powder, commercially available powders of the respective metals can be used, and it is preferable to use high-purity products. Then, in the mixing step described below, the metal Co powder and metal Pt powder are mixed to obtain the above-described composition. In producing the sputtering target according to the present invention, the particle size of the metal powder is preferably 100 μm or less.

[0030] The above metal powder and oxide powder are thoroughly mixed before sintering to form a mixed powder. Mixing at this time can be performed using a mixing and dispersing means such as a ball mill, a bead mill, or a rocking mill. The mixed powder obtained by these mixing means may be preformed before the next step, pressure sintering.

[0031] The mixed powder is then appropriately molded and then pressure-sintered to obtain a sputtering target according to the present invention. The sintering method may be hot pressing (HP), hot isostatic sintering (HIP), spark plasma sintering (SPS), or the like. The sintering temperature during pressure sintering is preferably 600°C or higher and 1500°C or lower, and the applied pressure is preferably 10 MPa or higher and 200 MPa or lower. The heating atmosphere is preferably a vacuum or a non-oxidizing atmosphere.

[0032] As explained above, the sputtering target according to the present invention can form a magnetic thin film having a granular structure that is excellent in magnetic isolation while maintaining saturation magnetization. In the present invention, from the viewpoint of magnetic isolation, tungsten oxide and / or molybdenum oxide are used as the essential oxides that form the non-magnetic grain boundaries of the granular thin film. In addition, in the sputtering target according to the present invention, B other than the essential oxides is used. ２ O ３ The uniaxial anisotropy constant K ｕ It is possible to set an appropriate oxide content while balancing the improvement of magnetic properties such as the above with magnetic separation. Optimizing the oxide content leads to optimizing the proportion of magnetic components in the magnetic thin film and ensuring the amount of magnetization.

[0033] The α value and M due to the oxide content of the magnetic film made from the sputtering target (75 atomic % Co-25 atomic % Pt+oxide) manufactured in the first embodiment ｓ Graph showing the measurement results of the sputtering target (80 atomic % Co-20 atomic % Pt / 20 vol % B) manufactured in the second embodiment. ２ O ３ +Oxide) and M ｓ Graph showing the measurement results of the sputtering target (80 atomic % Co-20 atomic % Pt / 30 vol % B) manufactured in the second embodiment. ２ O ３ +Oxide) and M ｓ Graph showing the measurement results.

[0034] First embodiment: Hereinafter, an embodiment of the present invention will be described. In this embodiment, various oxides (WO ３ , MoO ３ , B ２ O ３ , SiO ２ , TiO ２ , Cr ２ O ３Then, magnetic thin films were formed using these sputtering targets, and the MH hysteresis loops were measured to determine the saturation magnetization M ｓ and the magnetic separation ability α value were evaluated.

[0035] In manufacturing the sputtering target, first, metallic Co and metallic Pt were weighed and melted at 1500°C or higher to prepare a molten alloy of CoPt alloy (75 atomic % Co-25 atomic % Pt). Then, alloy powder was produced from this molten alloy by gas atomization. After that, classification treatment was performed to obtain CoPt alloy powder (75 atomic % Co-25 atomic % Pt alloy powder) with an average particle size of 100 μm or less.

[0036] The CoPt alloy powder obtained above and various oxide powders (WO ３ , MoO ３ , B ２ O ３ , SiO ２ , TiO ２ , Cr ２ O ３ The CoPt alloy powder and the oxide powder were mixed together in a ball mill (grinding medium: ceramic balls).

[0037] The mixed powder obtained above was pressure-sintered to form a sputtering target. The pressure-sintering was carried out by a hot press method under conditions of a sintering temperature of 920°C, a pressure of 24.5 MPa, and a pressure-sintering time of 30 minutes in a 5×10 atmosphere. －２ The vacuum conditions were set to 100 Pa or less. The manufactured sputtering target had a diameter of 180 mm and a thickness of 5 mm, and the relative density was set to be in the range of 95% to 100%.

[0038] In this embodiment, the oxide is WO ３ , MoO ３ , B ２ O ３ , SiO ２ , TiO ２ , Cr ２ O ３ The inventors have decided to manufacture sputtering targets containing the above and having oxide contents of 2.5 vol % to 40 vol %, and to evaluate the characteristics of the magnetic thin film in relation to the type and content of the oxide.

[0039] Next, magnetic thin films (granular thin films) were formed using the various sputtering targets produced. The magnetic thin film substrate was a glass substrate with Ni-Ta (60 nm) / 90Ni-10W alloy (6 nm) / Ru (25 nm) / CoCrRu-TiO ２ A substrate with a layer structure of 2 nm was used. The magnetic thin film was deposited using a sputtering device (Ar gas pressure 1-5 Pa, room temperature to 250°C), with the target film thickness set to 4-16 nm. After the magnetic thin film was deposited, a C thin film (7 nm) was sputtered as a protective layer for the magnetic thin film, and a sample for evaluating magnetic properties was prepared.

[0040] The magnetic properties of each magnetic thin film were evaluated by measuring the MH hysteresis loop using a vibrating magnetometer (VSM). The saturation magnetization M ｓ was calculated, and the slope α value near M=0 was calculated.

[0041] Various oxides (WO ３ , MoO ３ , B ２ O ３ , SiO ２ , TiO ２ , Cr ２ O ３ The α value and saturation magnetization M of a magnetic thin film formed using a 75 atomic % Co-25 atomic % Pt alloy target containing ｓ The measurement results are shown in Figure 1. In Figure 1, the value for a magnetic thin film (75 atomic % Co-25 atomic % Pt) that does not contain any oxide (X=0 vol %) is indicated by a dotted line.

[0042] As can be seen from FIG. 1, the α value basically decreases and approaches 1 with increasing content of any oxide. ３ and MoO ３The magnetic thin film containing tungsten oxide and molybdenum oxide exhibits a remarkable effect of reducing the α value even at the minimum content (2.5 vol%) investigated in this embodiment. It can be said that even small amounts of tungsten oxide and molybdenum oxide are effective as essential oxides for reducing the α value of granular thin films. Furthermore, the saturation magnetization M ｓ Taking this into consideration, it is confirmed that the content of these essential oxides is preferably 15 vol % or less.

[0043] Second embodiment: In this embodiment, a CoPt alloy (80 atomic % Co-20 atomic % Pt) and B ２ O ３ Based on a magnetic thin film consisting of B, sputtering targets were manufactured for depositing magnetic thin films to which various oxides were added. ２ O ３ The magnetic isolation properties of a granular thin film formed by adding an oxide are investigated.

[0044] The manufacturing method of the sputtering target was basically the same as that of the first embodiment. CoPt alloy powder was produced from a molten CoPt alloy (80 atomic % Co-20 atomic % Pt alloy) by gas atomization, and B was added to this CoPt alloy powder. ２ O ３ Powder and various oxides (WO ３ , MoO ３ , B ２ O ３ , SiO ２ , TiO ２ , Cr ２ O ３ The mixed powder was hot pressed and pressure sintered to produce a sputtering target having a relative density in the range of 95% to 100%.

[0045] In this embodiment, two types of reference structures for the sputtering target were set. The first reference structure was a CoPt alloy (80 atomic % Co-20 atomic % Pt) and 20 vol% B ２ O ３The second standard is a target consisting of a CoPt alloy (80 atomic % Co-20 atomic % Pt) and 30 vol% B. ２ O ３ In this embodiment, the target is made up of these two basic structures and WO ３ , MoO ３ , B ２ O ３ , SiO ２ , TiO ２ , Cr ２ O ３ A number of sputtering targets were prepared using the same additive.

[0046] Thereafter, magnetic thin films were formed using various sputtering targets obtained in the same manner as in Embodiment 1. The substrate configuration and film formation conditions were the same as in Embodiment 1. Furthermore, the measurement of the M-H hysteresis loop, which is an evaluation method for the prepared magnetic thin film samples, was also the same as in Embodiment 1.

[0047] The evaluation results are shown in FIG. 2. ２ O ３ The α value and M when various oxides are added to ｓ 3 shows the second reference structure of 80 atomic % Co-20 atomic % Pt / 30 vol % B. ２ O ３ The α value and M when various oxides are added to ｓ These figures show the magnetic thin film (80 atomic % Co-20 atomic % Pt / 20 vol% and 30 vol% B) before adding various oxides (X=0 vol%). ２ O ３ ) values ​​are also shown.

[0048] 2 and 3, the optional oxide (B ２ O ３ The CoPt alloy magnetic film containing WO ３ or MoO ３ It can be seen that the α value is significantly reduced by including B as an optional oxide. ２ O ３ A sputtering target containing WO ３or MoO ３ Or, these are the evaluation results when these different oxides are added. ３ and MoO ３ The effect of improving magnetic isolation is superior to that of other oxides. This tendency is similar to that shown in the first embodiment.

[0049] And B ２ O ３ It has been confirmed that even in a sputtering target containing optional oxides such as tungsten oxide and molybdenum oxide, a small amount of the oxide can exhibit the effect of reducing the α value. Furthermore, from this embodiment, it has also been confirmed that the sputtering target according to the present invention can have a total content of essential oxides and optional oxides (total oxide content) of 20 vol% or more and 50 vol% or less.

[0050] As mentioned above, B ２ O ３ forms the nonmagnetic grain boundary of the granular thin film and has the uniaxial anisotropy constant K ｕ Therefore, in the sputtering target, B ２ O ３ It is expected that it will be necessary to mix optional oxides such as tungsten oxide and molybdenum oxide, which may result in a higher oxide content in the magnetic thin film (sputtering target). For such magnetic thin films (sputtering targets), it is believed that tungsten oxide and molybdenum oxide are both useful as additives that improve magnetic separation properties when mixed in small amounts.

[0051] As explained above, the sputtering target according to the present invention is effective for forming a magnetic thin film having excellent magnetic isolation properties when forming a magnetic thin film having a granular structure. The sputtering target according to the present invention contains tungsten oxide and / or molybdenum oxide as essential oxides, as well as B ２ O ３ In this case, the essential oxides of tungsten oxide and molybdenum oxide can be optionally contained. ２ O ３This can function as a small amount of additive to any oxide such as those mentioned above. This makes it possible to optimize the oxide content in the sputtering target and, ultimately, in the magnetic thin film, thereby ensuring the magnetization of the magnetic thin film. The sputtering target according to the present invention is particularly suitable for forming magnetic thin films for perpendicular recording, which require significantly higher densities.

Claims

1. A sputtering target for a magnetic recording medium, comprising Co and Pt as metal components and at least one oxide, characterized in that the sputtering target contains tungsten oxide and / or molybdenum oxide as essential oxides, and the content of the essential oxides is 15 vol % or less.

2. A sputtering target for a magnetic recording medium according to claim 1, which contains, together with an essential oxide, an oxide different from said essential oxide as an optional oxide.

3. Optional oxide is B ２ O ３ , SiO ２ , TiO ２ , Cr ２ O ３ , CoO, Co ３ O ４ 3. The sputtering target for a magnetic recording medium according to claim 2, wherein 4. A sputtering target for magnetic recording media according to claim 2 or 3, wherein the total oxide content, which is the sum of the content of essential oxides and the content of optional oxides, is 20 vol.% or more and 50 vol.% or less, and the content of essential oxides is 15 vol.% or less.

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