JP4193489B2 - Glass substrate for magnetic disk and magnetic disk using the same - Google Patents

Description

【００２３】
表１において、希土類元素以外の母ガラス組成は、同一組成のアルミノホウケイ酸ガラスとした。含有させる希土類酸化物の量はいずれも３重量％と一定にした。ガラス基板の特性として、マイクロビッカース硬さ，可視光の透過率、及び着色性及びガラス基板の歩留まりを評価した。マイクロビッカース硬さは荷重５００ｇ，荷重印加時間１５秒の条件で荷重を印加し、１０点の平均値として求めた。可視光の透過率は、分光光度計を用いて３００ｎｍから７００ｎｍまでの波長の分光透過率曲線より透過率スペクトルを測定し、この波長範囲の光の全透過率の積分値として求めた。着色性は目視により着色の程度を評価し、無色のものは×、着色しているものは○とした。歩留まりの評価は、ガラス基板をレーザー光照射による散乱光により異物数を検査する装置により評価し、気泡，研磨傷，かけ，表面異物等の不良がディスク片面当たり２０個以上のものを不良としてカウントし、不良でないものの割合を評価した。
【００２４】
また磁気ディスクの特性として磁化、及び磁化の標準偏差，記録再生特性及び磁気ディスクの歩留まりを評価した。また基板加工前のブロック作製から磁気ディスク作製にいたるまでの総合歩留まりを評価した。磁化及び磁化の標準偏差は、Ｂ−Ｈ曲線を振動試料型磁力計（ＶＳＭ）によって測定し、磁性膜のヒステリシスループのバックグラウンド成分を基板からの磁性とし、そのバックグラウンド成分の大きさを評価した。表には磁界として１ｋＯｅ印加したときのバックグラウンドの磁化の大きさを掲載した。
【００２５】
またこの磁気ディスクの記録再生特性を評価した。図３に、本発明で作製した記録再生特性評価用の磁気ディスクドライブを示す。図３において、９は磁気ディスク、１０はスピンドル、１１は磁気ヘッド、１２は磁気ヘッドのアーム、１３はヘッドを駆動するためのボイスコイルモーター、１４は全体を支える筐体である。なお、この図では記されていないが、磁気ディスク９の下部にはスピンドルモーターが設置されており、ディスク全体を回転させる。図３の磁気ディスクドライブに各磁気ディスクを搭載し、２０Ｇｂ／ｉｎ² に相当する磁気信号を記録し、磁気記録再生特性を評価した。この評価を１５０枚のディスクに対して行い、十分な記録再生特性が得られたものの割合を磁気ディスク歩留まりとして表記した。
【００２６】
さらに上記のガラス基板歩留まりと磁気ディスク歩留まりより総合歩留まりを評価した。総合での歩留まりが８０％未満のものを×、８０％以上９０％未満のものを○、９０％以上のものを◎とした。
【００２７】
表１の基板特性のマイクロビッカース硬さはいずれの基板でも６４０以上が得られており、良好であることが分かった。また可視光の透過率は、いずれの基板でも８０％以上であった。これらのうちＮｄ，Ｐｒ，Ｓｍ，Ｅｕ，Ｈｏ，Ｅｒは可視光域に希土類のｆ−ｆ遷移に起因するシャープな吸収が見られた。このため、他の元素に比べて透過率は若干低下しており、８５％以下であった。しかしながらこの鋭い吸収のため、ガラス基板に明確な着色が見られた。白熱灯下での目視観察による評価では、Ｐｒは黄緑、Ｎｄは紫色、Ｓｍ，Ｅｕは非常に淡いがそれぞれ黄色と桃色に着色しているのが見られた。また、Ｅｒ，Ｈｏも桃色に着色していた。
【００２８】
そのほかの希土類元素を含有させた基板は無色であり、透過率はいずれも８５％を超え、着色は見られなかった。
【００２９】
これらの基板に対するガラス基板の歩留まりを評価すると、明瞭な着色の見られたＰｒ，Ｎｄ，Ｓｍ，Ｅｕ，Ｈｏ，Ｅｒでは加工による不良、特に傷不良が着色していないものに比べて少なく、歩留まりが９５％以上となった。これは、基板加工工程，洗浄工程において基板が可視であるため、取扱いが容易なことから歩留まりが向上したと考えられる。
【００３０】
また比較例として酸化ニッケル（ＮｉＯ）を含有する着色性の高いガラス基板について評価した。この基板は透過率が４７％と低く、ガラス中に存在する気泡、あるいは熔融時のるつぼを構成する成分のガラス中への溶損を発見することが難しく、基板表面にこれらが残存することから歩留まりが低下していた。また若干ＮｉＯ含有量を低下させ、透過率を５０％以下としたガラス基板では、基板を光が透過して、基板内部を観察可能であることが分かった。このため、気泡などによる不良が減少し、歩留まりが向上した。
【００３１】
以上のことから、透過率と磁気ディスクの歩留まりとの間に明瞭な相関関係が見られた。ガラス基板の透過率が５０％以上９０％以下のとき着色性が良好で、歩留まりが良好なガラス基板が得られた。透過率が５０％未満となると基板中に残存する気泡，炉材混入が発見し難く、歩留まり低下の要因となった。また基板の透過率が９０％をこえると、基板取扱いが難しくなり、傷などの加工不良が増加していたため、好ましいといえなかった。
【００３２】
また上記の光学的な特性を達成するため、添加する元素はＰｒ，Ｎｄ，Ｓｍ，Ｅｕ，Ｅｒ又はＨｏが良好であることが分かった。このうち、Ｐｒ，Ｎｄ，Ｅｒ又はＨｏであれば着色が顕著であり、より好ましかった。
【００３３】
次に、磁気ディスクの磁気特性について評価した。Ｓｃ，Ｙ，Ｌａを含有した磁気ディスクでは、基板の磁化の大きさが１０^-4ｅｍｕ／ccのオーダーであり、きわめて小さい磁化量であった。Ｓｍを用いたときは、磁化は反磁性的な挙動を示しており、−４×１０^-4ｅｍｕ／ccとなった。またＰｒ，Ｎｄ，Ｅｕでは１〜３×１０^-3ｅｍｕ／ccのオーダーであったが、Ｇｄ，Ｔｂ，Ｄｙ，Ｈｏ，Ｅｒ，Ｔｍ，Ｙｂでは５×１０^-3〜２×１０^-2ｅｍｕ／ccと、磁化の値が大きくなっていた。
【００３４】
磁化の固体差を示す磁化の標準偏差を評価したところ、磁化の大きさの大きいものほど大きくなっており、基板によるばらつきが大きくなっていた。特に磁化の大きさが３×１０^-3を超えるＧｄ，Ｔｂ，Ｄｙ，Ｈｏ，Ｅｒ，Ｔｍ，Ｙｂでは、磁化の標準偏差が１×１０^-3ｅｍｕ／cc以上となり、基板による磁気特性のばらつきが大きくなった。
【００３５】
磁気記録再生特性による磁気ディスク歩留まりを見ると、磁化が３×１０^-3ｅｍｕ／cc以下で、磁化の標準偏差が１×１０^-3ｅｍｕ／cc未満の試料では、良好な磁気特性の得られる磁気ディスクが９０％以上と良好であったが、磁化が３×１０^-3ｅｍｕ／ccを超え、かつ磁化の標準偏差が１×１０^-3ｅｍｕ／cc以上となる試料では、歩留まりが８０％以下と急激に低下していることが分かった。これは、基板に含有される希土類元素の若干の固体差により基板の磁気特性が変化し、そのために標準偏差が大きくなるため、記録する際の磁界を一定にした場合の記録にばらつきが生じたためと考えられる。
【００３６】
以上より、基板の磁化に与える影響が小さい希土類元素としてＳｃ，Ｙ，Ｌａ，Ｐｒ，Ｎｄ，Ｓｍ，Ｅｕが良好であった。また、磁化の大きさが３×１０^-3ｅｍｕ／cc以下であれば磁気記録再生のばらつきが小さい磁気記録媒体が得られた。磁化の大きさが３×１０^-3ｅｍｕ／ccを超えると磁気記録再生特性に基板ごとのばらつきが大きくなるため、好ましいとはいえなかった。
【００３７】
上記の光学的な特性が及ぼすガラス基板の歩留まりに与える影響、及び磁気的な特性が記録再生特性に及ぼす影響を考慮して総合歩留まりを評価した。その結果、両者とも良好なＰｒ，Ｎｄ，Ｓｍ，Ｅｕを用いたガラス基板の場合、総合歩留まりが８０％以上となり、良好であった。これに対してその他の希土類を添加した場合には、総合歩留まりが８０％以下となるため、良好といえなかった。
【００３８】
また、特に希土類元素としてＰｒを用いると、総合歩留まりが９０％となり、さらに良好な結果が得られた。
【００３９】
次に、希土類酸化物の種類と添加量の関係について詳細に調べた。着色については、表１で透明であったものについては含有量を増減させても透過率に変化は見られなかった。このため、着色した元素のうち、Ｐｒ，Ｅｒ，Ｓｍについて、その含有量を変化させたガラス基板を作製し、表１と同様の検討を行った。表２に、検討した結果を示す。
【００４０】
【表２】

【００４１】
Ｐｒの含有量を変化させていったところ、試料Ｎo.１７のＰｒ₂Ｏ₃を０.７ 重量％含有するガラス基板では、マイクロビッカース硬さが低く、そのためガラスの機械的強度が低いためにガラス基板歩留まりが８２％と低かった。１％の試料Ｎo.１６、及び１.５％ 〜７％の実施例１８〜２１では、マイクロビッカース硬さも高い値を示しており、着色，磁気特性とも良好であった。この事から総合歩留まりも８０％を超えており、良好な結果となった。
【００４２】
一方、試料Ｎo.２２のようにＰｒ₂Ｏ₃含有量が７％を超えるものでは着色に関しては問題無かったものの、磁気ディスクの磁化が３×１０^-3ｅｍｕ／ccを超える値となった。このため、磁化のばらつきが大きくなり、磁気ディスク歩留まりが８０％を下回り、好ましい結果とは言えなかった。さらに実施例２３のガラス基板ではガラス中の希土類元素が均一にガラス中に溶解せず、不良品数が多く、歩留まりが１５％と低かった。このため、ガラス基板材料としては好ましくなかった。
【００４３】
さらに希土類元素をＥｒに変えた場合の実施例をみると、Ｅｒ含有量が０.５％の試料Ｎo.２５では含有量が少なく、マイクロビッカース硬さが小さいため、歩留まりが悪かった。またこのとき磁気ディスクの磁化の値が３.１ ×１０^-3ｅｍｕ／ccと高く、磁気ディスクとしての歩留まりも低下していた。またＥｒ含有量が１％からＥｒ含有量を増加させていくと、マイクロビッカース硬さも高くなり、透過率は低くなって基板歩留まりは上昇するものの、磁化が依然として３×１０^-3ｅｍｕ／ccを超えるため、基板としての歩留まりが低下していた。以上より、希土類元素としてＥｒを用いた場合では、基板の硬さ，光学特性，磁気特性の両者を同時に満たす組成範囲が存在しないことが分かった。
【００４４】
Ｓｍについてみると、光学的特性については２.５ 重量％以上であると透過率が８５％以下で適正な範囲となった。磁気特性は１０重量％含有させても適正であったが、１０重量％を超えるとＰｒの時と同様にガラス中に残存原料が残り、好ましくなかった。
【００４５】
同様にＮｄ，Ｅｕ，Ｈｏについて検討を行ったところ、Ｎｄ，Ｅｕについては、８重量％を超える場合に磁気特性が良好でなかったものの、Ｓｍと同様の結果が得られた。ＨｏについてはＥｒと同じく、光学的特性と磁気特性の両者を同時に満たす組成範囲が存在しなかったたため、好ましい結果が得られなかった。
【００４６】
以上より、光学的特性，磁気特性の双方で好ましい組成範囲をとる希土類元素としてＰｒ，Ｎｄ，Ｓｍ，Ｅｕが良好であると判断できた。その中でもＰｒ，Ｎｄを用いれば１重量％〜７重量％の組成範囲で良好な組成範囲をとることができた。またＳｍの場合では２.５ 〜９重量％、Ｅｕの場合では２.５ 〜８重量％で良好な特性を得ることができた。またＰｒを１.５重量％〜５.２重量％含有させた場合については、総合歩留まりが９０％以上となり、非常に良好な結果が得られた。
【００４７】
これらの希土類の含有量が少ないと、マイクロビッカース硬さなどの機械的強度が低かったり、透過率が高かったりし、ガラス基板の歩留まりが低下するため好ましくなかった。また希土類含有量が多いと、基板の磁化の値が大きくなるため、磁気特性が良好でなくなった。また、さらに多量に添加するとガラス中に原料が残存するため、好ましくなかった。
【００４８】
【実施例２】
次に、磁気ディスク用ガラス基板として適切なガラス組成範囲について検討した。表３に、本発明で作製したガラス基板の実施例を示す。添加する希土類としては実施例１で良好な結果が得られたＰｒを用いた。表のガラス組成の中で、Ｒ₂Ｏ とはＬｉ₂Ｏ ，Ｎａ₂Ｏ ，Ｋ₂Ｏ のトータルのアルカリ金属酸化物含有量を示す。
【００４９】
【表３】

【００５０】
各試料について、磁気ディスク用ガラス基板作製の際のガラスの安定性，ガラス材の熱膨張係数，ガラス基板表面のマイクロビッカース硬さ，円環強度を示した。
【００５１】
ここで、熱膨張係数は各ガラスのブロックを作製し、１５mm×４mm×４mmの熱膨張測定用試験片を切り出し、熱膨張測定装置を用いて測定した。測定温度範囲は、３０℃〜１００℃とした。
【００５２】
マイクロビッカース硬さは、ダイヤモンド圧子をガラス基板の表面に荷重500ｇ，荷重印加時間１５秒の条件で印加し、１０点の平均値として求めた。
【００５３】
また得られた基板より実施例１と同様に磁気ディスクを作製し、これを図３に示した磁気ディスクドライブに搭載してドライブの熱衝撃試験を実施した。熱衝撃試験によりガラスにクラックや割れ，トラックずれによる読み取りエラーなどの問題が生じなかった場合は○を、生じた場合を×とした。熱衝撃試験は−４０℃で２時間保持後、８０℃まで急速加熱させ、８０℃で２時間保持後、−４０℃まで急冷する。これを５回繰り返し、その間で上記した問題が生じるか否かを判定した。
【００５４】
ガラスの安定性では、ガラス溶解後ガラス基板中に見られる気泡，脈理，異物などが顕著に見られたものは×とし、そのような物が見られず、清澄なガラスが得られた場合は○とした。
【００５５】
また、円環強度は以下のようにして求めた。２.５″ 基板の内周部の上部に、外径２２mmφの円環を載せ、また内径６３mmφ，外径６５mmφの円環の基板の下部に設置した後、円環に荷重をかけて破壊強度を測定した。
【００５６】
まず、Ｌｉ₂Ｏ ，Ｎａ₂Ｏ ，Ｋ₂Ｏ のトータルのアルカリ酸化物量（表中のＲ₂Ｏ ）を変化させた試料を表３のＮo.４２〜４９に示す。
【００５７】
Ｎo.４３，Ｎo.４４のようにアルカリ金属酸化物含有量が１３％未満のガラスを用いた場合、熱衝撃試験でガラスにクラックが生じた。また、逆にＮo.４９のようにアルカリ金属酸化物の含有量が１７重量％を超えるような場合には、熱衝撃試験ではトラックずれによるエラーが生じ、また高速回転試験では回転歪が生じやすくなるため、トラックずれによるエラーが生じるため好ましくなかった。Ｎo.４２，４５〜４８のガラスのようにアルカリ金属酸化物の合計の含有量が１３重量％〜１７重量％の場合、熱衝撃試験で良好な結果が得られ、好ましい結果となった。
【００５８】
これらのガラスの熱膨張係数に着目すると、表３よりＮo.４３，Ｎo.４４のガラスはそれぞれ５９×１０^-7／℃，６１×１０^-7／℃とドライブ装置部材の熱膨張係数である７０×１０^-7／℃〜８０×１０^-7／℃よりもかなり小さくなっていた。これらのガラスでは、他の装置部材との熱膨張の差異により熱衝撃試験においてクラックが発生したりトラックずれが生じたりしたため、好ましいとは言えなかった。
【００５９】
Ｎo.４２，４５〜４８のガラスに示すように、熱膨張係数が６５×１０^-7／℃以上，９０×１０^-7／℃以下であれば熱衝撃試験で良好な結果が得られた。以上より、適正な熱膨張係数は６５×１０^-7／℃以上，９０×１０^-7／℃以下であった。
【００６０】
つぎに、Ｎo.５０〜５５においてＳｉＯ₂ 含有量について検討した。ＳｉＯ₂ 含有量が５４重量％のＮo.５２のガラスでは、円環強度が十分でなく、磁気ディスク用ガラス基板として適切ではなかった。またＮo.５５のようにＳｉＯ₂ 量が７０重量％を超えると、ガラス溶解時に気泡などの発生が顕著になり、好ましくなかった。以上より、ＳｉＯ₂ 含有量は５５重量％以上７０重量％以下であると磁気ディスク用ガラス基板として良好な結果が得られた。
【００６１】
次にＡｌ₂Ｏ₃含有量について検討した。Ｎ o. ５７のＡｌ ₂ Ｏ ₃ 含有量が１８重量％であるガラスでは、ガラスの熔融温度が高くなりすぎ、１６００℃の熔融ではガラスの原料が残存したため、好ましくなかった。Ａｌ₂Ｏ₃含有量が１０重量％のＮo.５８に記載のガラスでは、安定したガラスが得られ、磁気ディスク用ガラス基板としても良好な結果が得られたが、Ａｌ₂Ｏ₃含有量が９.５重量％のＮo.５９ガラスでは、ガラス中に脈理等の不均一が生じた。以上より、Ａｌ₂Ｏ₃の含有量が１０重量％以上，１７重量％以下のとき良好なガラスが得られた。
【００６２】
さらにＢ₂Ｏ₃含有量について検討した。Ｂ₂Ｏ₃含有量が増加するに伴い、円環強度は向上したが、Ｎo.６０に示すように、９重量％であるガラスでは熱衝撃試験でガラスにクラックが生じた。以上より、Ｂ₂Ｏ₃含有量が８重量％以下であれば良好なガラスが得られた。
【００６３】
ＺｎＯに関しては、Ｎo.７２に示すように、添加量が１０重量％を超えるとガラス中に結晶の析出が著しくなり、安定なガラスを得ることが難しかった。１０重量％ではこのような結晶の析出は認められなかった。従って、ＺｎＯ含有量は、１０重量％以下であることが好ましかった。
【００６４】
次に、アルカリ金属酸化物のうち、Ｌｉ₂Ｏ含有量とＮａ₂Ｏ 含有量に着目して、組成比について検討した。Ｌｉ₂Ｏ／Ｎａ₂Ｏ比が０.６０ のＮo.６５のガラスおよび６.１５ のＮo.６７のガラスでは熱衝撃試験においてガラスにクラックが発生し、また円環強度も低下しており、Ｌｉ₂Ｏ／Ｎａ₂Ｏ 比としては、0.61以上，６.００以下であることが好ましかった。
【００６５】
【発明の効果】
本発明の磁気ディスク用ガラス基板は、基板が着色しており、磁界印加時の磁化が小さいため、量産性に優れた磁気ディスク用ガラス基板、及びそれを用いた磁気ディスクが作製できる。さらに本発明の磁気ディスク用ガラス基板は熱膨張係数が６５〜９０×１０^-7／℃であるため磁気ディスクドライブ装置部材の熱膨張係数と整合性が良好なため、熱衝撃試験等によるガラスのクラック発生やトラックずれなどの問題が少ないため、高記録密度，高信頼性が要求される磁気ディスクの基板材料として最適である。
【図面の簡単な説明】
【図１】本発明の磁気ディスク用ガラス基板の平面図。
【図２】本発明の磁気ディスク用ガラス基板を用いた磁気ディスクの断面図。
【図３】本発明で作製した磁気ディスク装置の概略図。
【符号の説明】
１…磁気ディスク用ガラス基板、２…内周チャック用穴、３…チャンファー部、４…粒径制御層、５…配向制御層、６…磁性膜、７…保護膜、８…潤滑膜、９…磁気ディスク、１０…スピンドル、１１…磁気ヘッド、１２…磁気ヘッドのアーム、１３…ボイスコイルモーター、１４…筐体。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a glass substrate for a magnetic disk, and more particularly to a glass substrate for a magnetic disk suitable for high-density recording, which has a suitable thermal expansion coefficient and mechanical characteristics, and has good mass productivity, and a magnetic disk using the same.
[0002]
[Prior art]
Currently, a magnetic disk device is used as a recording medium for general-purpose large-sized computers and personal computers, and also as a home server for temporarily storing images distributed as digital signals. Conventionally, as a substrate for this magnetic disk, a 3.5 ″ size aluminum substrate is used for general purpose or desktop personal computers, and a 2.5 ″ glass substrate is mainly used for portable notebook personal computers. Has been used.
[0003]
Since this glass substrate is harder and less deformable than an aluminum substrate and has excellent surface smoothness, it is also applied to the general-purpose 3.5 "or 3" size substrate. . Further, this glass substrate is also being applied to a recording device for small portable terminals such as 1.8 ″ and 1 ″.
[0004]
In addition to such miniaturization, there is an increasing demand for a large capacity for magnetic disk devices, and in recent years its storage capacity is increasing at an annual rate of 100%. In order to cope with this, since it is necessary to further reduce the flying height of the magnetic head of the recording unit, it is necessary to develop a magnetic disk having a smoother recording surface.
[0005]
Currently, the use of a chemically tempered glass substrate or a crystallized glass substrate overcomes the inherent cracking problem of glass. However, in the chemically strengthened amorphous glass substrate, the surface is roughened by the replacement of alkali ions during the chemical strengthening process, and it is difficult to cope with the future low head flying. Furthermore, in the use environment as described above, the surface of the chemically strengthened glass is chemically unstable because the substituted alkali ions having a large ion radius are chemically unstable. There is a concern that the alkali ions may move and precipitate on the substrate surface under the environment of long-term use or high temperature and humidity, resulting in deterioration of the magnetic properties of the magnetic film, peeling of the film, and defects such as adhesion. The
[0006]
Crystallized glass substrates, on the other hand, produce crystalline fine particles in amorphous glass, but the polishing rate differs depending on the hardness difference between the amorphous part and the crystalline part. However, there is a problem that it is difficult to produce a recording surface having sufficient smoothness that can cope with further higher density.
[0007]
In order to overcome the above problems, the inventors have improved the mechanical strength by adding rare earth ions to the glass substrate as described in JP-A-10-083531, and solved this problem. Yes.
[0008]
[Patent Document 1]
Japanese Patent Laid-Open No. 10-083531
[0009]
[Problems to be solved by the invention]
In the above-mentioned Japanese Patent Application Laid-Open No. 10-083531, a glass substrate having a high mechanical strength can be obtained, but the optimization of the thermal expansion coefficient and the mass productivity, which are characteristics necessary for a glass substrate for a magnetic disk, are sufficiently taken into consideration. It was hard to say. Therefore, in the thermal environment test such as thermal shock, there is a track shift that occurs at the time of high-speed rotation of the drive device due to the generation of cracks due to mismatch of thermal expansion characteristics between the glass substrate and the magnetic disk drive device member that supports the glass substrate. Can be considered. This is likely to become a more severe issue as capacity increases in the future.
[0010]
Accordingly, an object of the present invention is to obtain a glass substrate for a magnetic disk suitable for high-density recording, which has a particularly suitable thermal expansion coefficient and mechanical characteristics, and has good mass productivity, and a magnetic disk using the same.
[0011]
[Means for Solving the Problems]
  In order to solve the above problems, the glass substrate for a magnetic disk of the present invention is in percentage by weight.
  SiO₂: 55% to 70%
  Al₂O_Three:10% to 17%,
  B₂O_Three: 0% to 8%
  R₂O: 13% to 17% (R represents an alkali metal element),
  ZnO: 0% to 10%,
In addition, the following oxide-based weight percentage of Pr is contained.₂O_ThreeOr Nd₂O_Three1% to 7%, or Sm₂O_Three2.5% to 9%, or Eu₂O_Three2.5% to 8%, R₂O (R represents an alkali metal element) but Li₂O, Na₂O, K₂O and Li₂O and Na₂The proportion of O is Li₂O / Na₂The ratio of O is 0.61 or more and 6.00 or less, and the thermal expansion coefficient in the temperature range of 30 ° C. to 100 ° C. is 65 × 10 6.^-7/ ℃, 90 × 10^-7/ ° C or less.
[0012]
Further, the glass substrate for magnetic disk of the present invention has a transmittance of 50% to 90% in visible light having a wavelength of 300 nm to 700 nm at a thickness of 0.635 mm, and has a magnetization of 3 when a magnetic field of 1 kOe is applied. × 10^-3emu / cc or less.
[0013]
  The magnetic disk of the present invention is a magnetic disk having a glass substrate for magnetic disk and a magnetic layer formed on the substrate directly or via another layer, the glass substrate being in weight percentage.
  SiO₂: 55% to 70%
  Al₂O_Three:10% to 17%,
  B₂O_Three: 0% to 8%
  R₂O: 13% to 17% (R represents an alkali metal element),
  ZnO: 0% to 10%
In addition, the following oxide-based weight percentage of Pr is contained.₂O_ThreeOr Nd₂O_Three1% to 7%, or Sm₂O_Three2.5% to 9%, or Eu₂O_Three2.5% to 8%, R₂O (R represents an alkali metal element) but Li₂O, Na₂O, K₂O and Li₂O and Na₂The proportion of O 2 is Li₂O / Na₂The ratio of O is 0.61 or more and 6.00 or less, and the thermal expansion coefficient in the temperature range of 30 ° C. to 100 ° C. is 65 × 10 6.^-7/ ℃, 90 × 10^-7/ ° C or less.
[0014]
The magnetic disk of the present invention is a magnetic disk having at least a glass substrate and a magnetic film formed on the surface thereof directly or via another layer, and a wavelength of 300 nm of a glass substrate having a thickness of 0.635 mm. The transmittance when visible light of ˜700 nm is 50% or more and 90% or less, and the magnetization when a magnetic field of 1 kOe is applied to this glass substrate is 3 × 10^-3emu / cc or less.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Next, embodiments of the present invention will be described in detail.
[0016]
[Example 1]
FIG. 1 is a plan view of a magnetic disk substrate according to the present invention. In the present invention, a 2.5 "type glass substrate having a diameter of 65 mmφ and a thickness of 0.635 mm was prepared as the glass substrate 1 for the magnetic disk. This substrate is formed with a round hole 2 having a diameter of 20 mmφ for the inner chuck. Further, the inner periphery and the outer periphery are formed with a chamfer portion 3. The chamfer is chamfered at 45 ° on both sides at the substrate edge portion.
[0017]
The magnetic disk glass substrate was produced as follows. First, the raw material powder of the quantity defined so that it might become the target glass composition was measured and mixed, it put into the crucible made from platinum, and it melt | dissolved at 1600 degreeC in the electric furnace. After the raw materials were sufficiently dissolved, a stirring blade was inserted into the glass melt and stirred for about 4 hours. Thereafter, the stirring blade was taken out and allowed to stand for 30 minutes, and then the melt was poured into a mold to obtain a glass block having a diameter of about 70 mmφ and a thickness of about 1 mm. Thereafter, the glass block was reheated to the vicinity of the glass transition point of the glass and slowly cooled to remove strain.
[0018]
Next, the distorted glass block was cut out using a core drill with the inner periphery and the outer periphery being concentric. Further, the chamfer portion was chamfered on the inner and outer circumferences using a diamond grindstone. Thereafter, both surfaces were roughly polished, followed by polishing, and the substrate was further washed with a cleaning agent and pure water to obtain a magnetic disk glass substrate. As described above, the glass substrate for magnetic disk of the present invention is not subjected to a special strengthening process such as a chemical strengthening process.
[0019]
A magnetic film was formed on this substrate to produce a magnetic disk. FIG. 2 shows a schematic diagram of a cross-sectional structure of a magnetic recording medium manufactured according to the present invention. In FIG. 2, 1 is a glass substrate produced by the present invention, 4 is a particle size control layer for controlling the particle size of the magnetic film, 5 is an orientation control layer for controlling the orientation of the magnetic film, 6 is a magnetic film, 7 is a protective film and 8 is a lubricating film. In the present invention, a NiAl alloy film of 20 nm was formed as the grain size control layer 4. Further, a CrMo alloy thin film of 10 nm was formed as the orientation control layer 5 and a CoCrPrB magnetic film 20 nm was formed as the magnetic film 6. Further, 4 nm of C was deposited on the protective film. All these thin films were formed by sputtering. The lubricating film was formed by a coating method after the completion of sputtering.
[0020]
The characteristics and mass productivity of the magnetic disk glass substrate produced as described above and the magnetic disk on which the magnetic film was formed were evaluated, and the glass composition was examined.
[0021]
First, focusing on the kind of rare earth element to be added, glasses having various compositions were produced. Table 1 shows the composition of the glass produced in the present invention and the characteristics of the glass substrate and magnetic disk.
[0022]
[Table 1]

[0023]
In Table 1, the aluminoborosilicate glass having the same composition was used as the mother glass composition other than the rare earth elements. The amount of rare earth oxide to be contained was constant at 3% by weight. As the characteristics of the glass substrate, the micro Vickers hardness, the visible light transmittance, the colorability, and the yield of the glass substrate were evaluated. The micro Vickers hardness was obtained as an average value of 10 points by applying a load under conditions of a load of 500 g and a load application time of 15 seconds. The transmittance of visible light was obtained as an integral value of the total transmittance of light in this wavelength range by measuring a transmittance spectrum from a spectral transmittance curve of wavelengths from 300 nm to 700 nm using a spectrophotometer. As for the colorability, the degree of coloration was visually evaluated. Yield evaluation is performed using a device that inspects the number of foreign objects on the glass substrate using scattered light from laser light irradiation, and counts 20 or more defects per side of the disk as defects, such as bubbles, polishing scratches, scratches, and surface foreign substances. Then, the proportion of non-defective products was evaluated.
[0024]
Further, as the characteristics of the magnetic disk, the magnetization, the standard deviation of the magnetization, the recording / reproducing characteristics, and the yield of the magnetic disk were evaluated. We also evaluated the overall yield from block fabrication before substrate processing to magnetic disk fabrication. Magnetization and standard deviation of magnetization are measured by measuring the BH curve with a vibrating sample magnetometer (VSM), and the background component of the hysteresis loop of the magnetic film is defined as the magnetism from the substrate, and the size of the background component is evaluated. did. The table shows the magnitude of the background magnetization when 1 kOe is applied as the magnetic field.
[0025]
In addition, the recording / reproducing characteristics of the magnetic disk were evaluated. FIG. 3 shows a magnetic disk drive for recording / reproduction characteristics evaluation manufactured according to the present invention. In FIG. 3, 9 is a magnetic disk, 10 is a spindle, 11 is a magnetic head, 12 is an arm of the magnetic head, 13 is a voice coil motor for driving the head, and 14 is a housing that supports the whole. Although not shown in this figure, a spindle motor is installed below the magnetic disk 9 and rotates the entire disk. Each magnetic disk is mounted on the magnetic disk drive of FIG. 3, and 20 Gb / in² Was recorded, and magnetic recording / reproducing characteristics were evaluated. This evaluation was performed on 150 disks, and the ratio of those with sufficient recording / reproduction characteristics was expressed as magnetic disk yield.
[0026]
Furthermore, the overall yield was evaluated from the glass substrate yield and the magnetic disk yield. An overall yield of less than 80% was rated as x, an yield of 80% or more and less than 90% was rated as ◯, and a yield of 90% or more was rated as ◎.
[0027]
As for the micro Vickers hardness of the board | substrate characteristic of Table 1, 640 or more was obtained with any board | substrate, and it turned out that it is favorable. Further, the visible light transmittance was 80% or more for any of the substrates. Among these, Nd, Pr, Sm, Eu, Ho, and Er showed sharp absorption due to rare earth ff transitions in the visible light region. For this reason, the transmittance was slightly lower than that of other elements, and was 85% or less. However, due to this sharp absorption, the glass substrate was clearly colored. Evaluation by visual observation under an incandescent lamp showed that Pr was yellowish green, Nd was purple, and Sm and Eu were very pale but colored yellow and pink, respectively. Er and Ho were also colored pink.
[0028]
The other substrates containing rare earth elements were colorless, the transmittance exceeded 85%, and no coloration was observed.
[0029]
When the yield of the glass substrate relative to these substrates is evaluated, Pr, Nd, Sm, Eu, Ho, and Er, in which clear coloring is seen, have fewer defects due to processing, especially scratch defects compared to those that are not colored, and the yield. Was over 95%. This is probably because the substrate is visible in the substrate processing step and the cleaning step, and the yield is improved because it is easy to handle.
[0030]
As a comparative example, a highly colored glass substrate containing nickel oxide (NiO) was evaluated. Since this substrate has a low transmittance of 47%, it is difficult to find bubbles present in the glass or melting damage of the components constituting the crucible during melting into the glass, and these remain on the substrate surface. Yield was decreasing. In addition, it was found that in a glass substrate in which the NiO content is slightly reduced and the transmittance is 50% or less, light can pass through the substrate and the inside of the substrate can be observed. For this reason, defects due to bubbles and the like are reduced, and the yield is improved.
[0031]
From the above, a clear correlation was observed between the transmittance and the yield of the magnetic disk. When the transmittance of the glass substrate was 50% or more and 90% or less, a glass substrate having good colorability and good yield was obtained. When the transmittance was less than 50%, it was difficult to find bubbles remaining in the substrate and mixing of furnace materials, which caused a decrease in yield. Further, if the transmittance of the substrate exceeded 90%, it was difficult to handle the substrate and processing defects such as scratches increased, which was not preferable.
[0032]
Moreover, in order to achieve said optical characteristic, it turned out that the element to add has favorable Pr, Nd, Sm, Eu, Er, or Ho. Of these, Pr, Nd, Er, or Ho is more preferable because it is markedly colored.
[0033]
Next, the magnetic characteristics of the magnetic disk were evaluated. In the magnetic disk containing Sc, Y, and La, the magnitude of the magnetization of the substrate is 10^-FourThe order of emu / cc was very small. When Sm is used, the magnetization exhibits a diamagnetic behavior and is −4 × 10^-Fouremu / cc. For Pr, Nd, and Eu, 1 to 3 × 10^-3emu / cc order, but 5 × 10 for Gd, Tb, Dy, Ho, Er, Tm, Yb^-3~ 2x10^-2The value of magnetization was large, emu / cc.
[0034]
When the standard deviation of the magnetization showing the solid difference of the magnetization was evaluated, the larger the magnitude of the magnetization, the larger the value, and the greater the variation among the substrates. Especially the magnitude of magnetization is 3 × 10^-3For Gd, Tb, Dy, Ho, Er, Tm, and Yb exceeding 1, the standard deviation of magnetization is 1 × 10^-3It became emu / cc or more, and the variation of the magnetic characteristic by a board | substrate became large.
[0035]
Looking at the magnetic disk yield due to the magnetic recording / reproducing characteristics, the magnetization is 3 × 10^-3The standard deviation of magnetization is 1 × 10 at emu / cc or less^-3In the sample of less than emu / cc, the magnetic disk capable of obtaining good magnetic properties was as good as 90% or more, but the magnetization was 3 × 10.^-3emu / cc and the standard deviation of magnetization is 1 × 10^-3It was found that the yield of the sample with emu / cc or more was drastically decreased to 80% or less. This is because the magnetic characteristics of the substrate change due to slight solid differences in the rare earth elements contained in the substrate, and therefore the standard deviation increases, resulting in variations in recording when the magnetic field during recording is constant. it is conceivable that.
[0036]
From the above, Sc, Y, La, Pr, Nd, Sm, and Eu were good as rare earth elements having a small effect on the magnetization of the substrate. Also, the magnitude of magnetization is 3 × 10^-3If it was below emu / cc, a magnetic recording medium with small variations in magnetic recording and reproduction was obtained. Magnetization size is 3 × 10^-3If it exceeds emu / cc, the magnetic recording / reproducing characteristics vary greatly from substrate to substrate, which is not preferable.
[0037]
The overall yield was evaluated in consideration of the influence of the above optical characteristics on the glass substrate yield and the influence of the magnetic characteristics on the recording / reproducing characteristics. As a result, in the case of glass substrates using good Pr, Nd, Sm, and Eu in both cases, the overall yield was 80% or more, which was good. On the other hand, when other rare earths were added, the overall yield was 80% or less, so that it was not good.
[0038]
In particular, when Pr was used as the rare earth element, the overall yield was 90%, and even better results were obtained.
[0039]
Next, the relationship between the type of rare earth oxide and the amount added was examined in detail. As for coloring, no change was seen in the transmittance even when the content was increased or decreased for those that were transparent in Table 1. For this reason, among the colored elements, glass substrates in which the contents of Pr, Er, and Sm were changed were prepared, and the same examination as in Table 1 was performed. Table 2 shows the results of the study.
[0040]
[Table 2]

[0041]
When the Pr content was changed, the Pr of sample No. 17 was changed.₂O_ThreeIn the glass substrate containing 0.7% by weight, the micro Vickers hardness is low, and therefore the mechanical strength of the glass is low, so that the glass substrate yield is as low as 82%. In the samples No. 16 of 1% and Examples 18 to 21 of 1.5% to 7%, the micro Vickers hardness was also high, and both coloring and magnetic properties were good. As a result, the overall yield exceeded 80%, which was a good result.
[0042]
On the other hand, Pr as in sample No. 22₂O_ThreeWhen the content exceeds 7%, there is no problem with coloring, but the magnetization of the magnetic disk is 3 × 10.^-3The value exceeded emu / cc. For this reason, the variation in magnetization becomes large, and the magnetic disk yield falls below 80%, which is not a preferable result. Furthermore, in the glass substrate of Example 23, the rare earth elements in the glass were not uniformly dissolved in the glass, the number of defective products was large, and the yield was as low as 15%. For this reason, it was not preferable as a glass substrate material.
[0043]
Further, in the example in which the rare earth element was changed to Er, the sample No. 25 having an Er content of 0.5% had a low yield because the content was small and the micro Vickers hardness was small. At this time, the magnetization value of the magnetic disk is 3.1 × 10^-3emu / cc was high, and the yield as a magnetic disk was also lowered. When the Er content is increased from 1%, the micro Vickers hardness increases and the transmittance decreases and the substrate yield increases, but the magnetization is still 3 × 10.^-3Since it exceeded emu / cc, the yield as a board | substrate fell. From the above, it was found that when Er is used as the rare earth element, there is no composition range that satisfies both the hardness, optical characteristics, and magnetic characteristics of the substrate at the same time.
[0044]
As for Sm, the optical characteristics were 2.5% by weight or more, and the transmittance was 85% or less, which was an appropriate range. The magnetic properties were appropriate even if contained in 10% by weight. However, if the content exceeded 10% by weight, the remaining raw material remained in the glass as in the case of Pr, which was not preferable.
[0045]
Similarly, when Nd, Eu, and Ho were examined, the magnetic characteristics of Nd and Eu exceeding 8% by weight were not good, but the same results as Sm were obtained. As with Er, there was no composition range that satisfies both the optical characteristics and the magnetic characteristics at the same time, and therefore, favorable results could not be obtained.
[0046]
From the above, it has been determined that Pr, Nd, Sm, and Eu are good as rare earth elements that have a preferable composition range in both optical characteristics and magnetic characteristics. Among these, when Pr and Nd were used, a favorable composition range could be obtained with a composition range of 1 wt% to 7 wt%. In the case of Sm, good characteristics could be obtained at 2.5 to 9% by weight, and in the case of Eu 2.5 to 8% by weight. When Pr was contained in an amount of 1.5% to 5.2% by weight, the overall yield was 90% or more, and a very good result was obtained.
[0047]
If the content of these rare earths is small, the mechanical strength such as micro Vickers hardness is low, the transmittance is high, and the yield of the glass substrate is lowered, which is not preferable. In addition, when the rare earth content is large, the value of magnetization of the substrate increases, so that the magnetic properties are not good. Moreover, since the raw material remains in the glass when added in a larger amount, it was not preferable.
[0048]
[Example 2]
Next, the glass composition range suitable as a glass substrate for magnetic disks was examined. Table 3 shows examples of glass substrates produced in the present invention. As the rare earth to be added, Pr obtained with good results in Example 1 was used. Among the glass compositions in the table, R₂O is Li₂O 2, Na₂O, K₂The total alkali metal oxide content of O 2 is shown.
[0049]
[Table 3]

[0050]
For each sample, the stability of the glass, the thermal expansion coefficient of the glass material, the micro Vickers hardness of the surface of the glass substrate, and the ring strength were shown.
[0051]
Here, the thermal expansion coefficient was measured using a thermal expansion measuring device by preparing a block of each glass, cutting out a 15 mm × 4 mm × 4 mm test piece for thermal expansion measurement. The measurement temperature range was 30 ° C to 100 ° C.
[0052]
The micro Vickers hardness was obtained as an average value of 10 points by applying a diamond indenter to the surface of the glass substrate under the conditions of a load of 500 g and a load application time of 15 seconds.
[0053]
Further, a magnetic disk was produced from the obtained substrate in the same manner as in Example 1, and this was mounted on the magnetic disk drive shown in FIG. 3, and a thermal shock test of the drive was performed. In the case where no problems such as cracks, cracks, or reading errors due to track deviation occurred in the glass by the thermal shock test, a circle was given. In the thermal shock test, after being held at −40 ° C. for 2 hours, rapidly heated to 80 ° C., held at 80 ° C. for 2 hours, and then rapidly cooled to −40 ° C. This was repeated 5 times, and it was determined whether or not the above-described problems occurred during that time.
[0054]
In the stability of the glass, if the bubbles, striae, foreign matter, etc. that are noticeable in the glass substrate after melting the glass are marked as x, such a thing is not seen and a clear glass is obtained. Was marked with ○.
[0055]
Further, the ring strength was determined as follows. An annular ring with an outer diameter of 22 mmφ is placed on the upper part of the inner periphery of a 2.5 ″ substrate, and is placed under the annular substrate with an inner diameter of 63 mmφ and an outer diameter of 65 mmφ. Was measured.
[0056]
First, Li₂O 2, Na₂O, K₂Total alkali oxide amount of O 2 (R in the table₂Samples with different O 2) are shown in Nos. 42 to 49 in Table 3.
[0057]
When glass having an alkali metal oxide content of less than 13% was used, such as No. 43 and No. 44, cracks occurred in the glass in the thermal shock test. On the other hand, when the content of alkali metal oxide exceeds 17% by weight as in No. 49, errors due to track deviation occur in the thermal shock test, and rotational distortion easily occurs in the high-speed rotation test. Therefore, an error due to a track deviation occurs, which is not preferable. When the total content of alkali metal oxides was 13% by weight to 17% by weight as in the case of glass No. 42, 45 to 48, good results were obtained in the thermal shock test, and favorable results were obtained.
[0058]
Focusing on the thermal expansion coefficient of these glasses, the glass of No. 43 and No. 44 is 59 × 10 0 from Table 3, respectively.^-7/ ℃, 61 × 10^-7/ ° C and the thermal expansion coefficient of the drive device member is 70 × 10^-7/ ° C to 80 × 10^-7It was considerably smaller than / ° C. These glasses were not preferred because cracks and track deviations occurred in the thermal shock test due to differences in thermal expansion from other apparatus members.
[0059]
As shown in No. 42, 45-48 glass, the coefficient of thermal expansion is 65 × 10 6.^-7/ ℃, 90 × 10^-7When the temperature was not higher than / ° C, good results were obtained in the thermal shock test. From the above, the appropriate thermal expansion coefficient is 65 × 10^-7/ ℃, 90 × 10^-7/ ° C. or less.
[0060]
Next, in No. 50-55, SiO₂ The content was examined. SiO₂ The glass having a No. 52 content of 54% by weight was not suitable as a glass substrate for a magnetic disk because of insufficient ring strength. Also, like No.55, SiO₂ When the amount exceeds 70% by weight, the generation of bubbles and the like becomes remarkable when the glass is melted, which is not preferable. From the above, SiO₂ When the content was 55% by weight or more and 70% by weight or less, good results were obtained as a glass substrate for a magnetic disk.
[0061]
  Next, Al₂O_ThreeThe content was examined.N o. 57 Al ₂ O _Three Content is 18% by weightIn the case of glass, the glass melting temperature was too high, and melting at 1600 ° C. was not preferable because the glass raw material remained. Al₂O_ThreeWith the glass described in No. 58 having a content of 10% by weight, a stable glass was obtained, and good results were obtained as a glass substrate for a magnetic disk.₂O_ThreeIn the No. 59 glass having a content of 9.5% by weight, unevenness such as striae occurred in the glass. From the above, Al₂O_ThreeContent of 10% by weight or more,17% by weight or lessIn this case, a good glass was obtained.
[0062]
B₂O_ThreeThe content was examined. B₂O_ThreeAs the content increased, the ring strength improved, but as shown in No. 60, the glass with 9% by weight cracked the glass in the thermal shock test. From the above, B₂O_ThreeIf the content was 8% by weight or less, a good glass was obtained.
[0063]
Regarding ZnO, as shown in No. 72, when the addition amount exceeds 10% by weight, precipitation of crystals becomes remarkable in the glass, and it is difficult to obtain a stable glass. At 10% by weight, no such crystal precipitation was observed. Therefore, the ZnO content was preferably 10% by weight or less.
[0064]
Next, among the alkali metal oxides, Li₂O content and Na₂Focusing on the O 2 content, the composition ratio was examined. Li₂O / Na₂In the glass of No. 65 with an O ratio of 0.60 and No. 67 of 6.15, cracks occurred in the glass in the thermal shock test, and the ring strength also decreased.₂O / Na₂The O 2 ratio was preferably 0.61 or more and 6.00 or less.
[0065]
【The invention's effect】
Since the glass substrate for magnetic disks of the present invention is colored and has a small magnetization when a magnetic field is applied, a glass substrate for magnetic disks excellent in mass productivity and a magnetic disk using the same can be produced. Further, the glass substrate for magnetic disk of the present invention has a coefficient of thermal expansion of 65 to 90 × 10.^-7Because the temperature coefficient is consistent with the thermal expansion coefficient of the magnetic disk drive device because of / ° C, there are few problems such as glass cracking and track misalignment due to thermal shock tests, etc., so high recording density and high reliability are required. It is most suitable as a substrate material for magnetic disks.
[Brief description of the drawings]
FIG. 1 is a plan view of a glass substrate for a magnetic disk according to the present invention.
FIG. 2 is a sectional view of a magnetic disk using the magnetic disk glass substrate of the present invention.
FIG. 3 is a schematic view of a magnetic disk device manufactured according to the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Glass substrate for magnetic discs, 2 ... Hole for inner periphery chuck | zipper, 3 ... Chamfer part, 4 ... Grain size control layer, 5 ... Orientation control layer, 6 ... Magnetic film, 7 ... Protective film, 8 ... Lubrication film, DESCRIPTION OF SYMBOLS 9 ... Magnetic disk, 10 ... Spindle, 11 ... Magnetic head, 12 ... Arm of magnetic head, 13 ... Voice coil motor, 14 ... Housing | casing.

Claims (4)

In terms of weight percentage, SiO ₂ : 55% to 70%,
Al ₂ O ₃ : 10 to 17%,
B ₂ O ₃ : 0% to 8%
R ₂ O: 13% to 17% (R represents an alkali metal element),
ZnO: 0% to 10%,
1 to 7% of Pr ₂ O ₃ or Nd ₂ O ₃ , or 2.5% to Sm ₂ O ₃ , in the following oxide-based weight percentage: 9%, or Eu ₂ O ₃ is contained 2.5% to 8%, and the R ₂ O (R represents an alkali metal element) is composed of Li ₂ O, Na ₂ O, K ₂ O, and Li ₂ O and Na ₂ O ratio of Li ₂ O / Na ₂ O ratio in the 0.61 or more and 6.00 or less, 30 ° C. to 100 ° C. thermal expansion coefficient of 65 × 10 ^-7 at a temperature range of A glass substrate for a magnetic disk, wherein the glass substrate is / ° C or higher and 90 × 10 ^-7 / ° C or lower. At a thickness of 0.635 mm, the transmittance for visible light having a wavelength of 300 nm to 700 nm is 50% or more and 90% or less, and the magnetization when a magnetic field of 1 kOe is applied is 3 × 10 ⁻³ emu / cc or less. A glass substrate for magnetic disks characterized by A magnetic disk having a magnetic disk glass substrate and a magnetic layer formed directly or via another layer on the substrate, wherein the glass substrate is SiO ₂ : 55% to 70% by weight,
Al ₂ O ₃ : 10% to 17%,
B ₂ O ₃ : 0% to 8%
R ₂ O: 13% to 17% (R represents an alkali metal element),
ZnO: 0% to 10%,
1 to 7% of Pr ₂ O ₃ or Nd ₂ O ₃ , or 2.5% to Sm ₂ O ₃ , in the following oxide-based weight percentage: 9%, or Eu ₂ O ₃ is contained 2.5% to 8%, and the R ₂ O (R represents an alkali metal element) is composed of Li ₂ O, Na ₂ O, K ₂ O, and Li ₂ O and Na ₂ O ratio of Li ₂ O / Na ₂ O ratio in the 0.61 or more and 6.00 or less, 30 ° C. to 100 ° C. thermal expansion coefficient of 65 × 10 ^-7 at a temperature range of A magnetic disk characterized by a temperature of not less than / ° C and not more than 90 × 10 ^-7 / ° C. A magnetic disk having at least a glass substrate and a magnetic film formed on the surface thereof directly or via another layer, the transmittance of visible light having a wavelength of 300 nm to 700 nm of a glass substrate having a thickness of 0.635 mm And a magnetization of 3 × 10 ⁻³ emu / cc or less when a magnetic field of 1 kOe is applied to the glass substrate.

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