JP3704549B2

JP3704549B2 - Method and apparatus for evaluating superconducting critical current characteristics of superconducting film

Info

Publication number: JP3704549B2
Application number: JP2002381028A
Authority: JP
Inventors: アシナラヤナン，スンダレサン; 康資田中; 重利大嶋; 正暢楠
Original assignee: Japan Science and Technology Agency; National Institute of Advanced Industrial Science and Technology AIST; National Institute of Japan Science and Technology Agency
Current assignee: Japan Science and Technology Agency; National Institute of Advanced Industrial Science and Technology AIST; National Institute of Japan Science and Technology Agency
Priority date: 2002-12-27
Filing date: 2002-12-27
Publication date: 2005-10-12
Anticipated expiration: 2022-12-27
Also published as: JP2004212168A

Description

【０００１】
【発明の属する技術分野】
この発明は、超伝導膜の超伝導特性を、高感度、簡便、且つ、低コストで評価する方法に関する。
【０００２】
【従来の技術】
近年、銅酸化物系の高温超伝導膜を用いたデバイスの実用化が進んでいる。例えば、移動体通信用基地局は、限られた周波数帯域にできるだけ多くの加入者周波数帯を取り込む必要があり、高性能のマイクロ波バンドパスフィルターを必要としている。マイクロ波バンドパスフィルターの特性はフィルター回路の抵抗成分によって左右されるので、抵抗成分の極めて小さい高温超伝導膜を使用してマイクロ波バンドパスフィルターの特性を飛躍的に向上させている（非特許文献１参照）。また、高温超伝導膜をテープ上に積層した超伝導電線が実用化されている（非特許文献２参照）。
【０００３】
しかしながら、銅酸化物系の高温超伝導膜に代表されるように、超伝導膜の生産技術が極めて精力的に研究されているにもかかわらず、再現性良く良品の超伝導膜のみを生産することが困難であり、デバイスの生産に当たっては、デバイスに使用する超伝導膜を全数検査し、材料選別することが必要である。
このため、超伝導膜を用いたデバイスを安定した品質で、且つ、低コストで生産するには、簡便、且つ低コストな超伝導膜の評価技術が必要不可欠である。
上記のデバイスに使用する超伝導膜に要求される超伝導特性は、表面抵抗、臨界電流密度であるが、一般に、超伝導特性の優れた超伝導膜は、表面抵抗が小さく、臨界電流密度が大きく、また、臨界磁場強度が大きいといった関係にある。
【０００４】
次に、超伝導膜の超伝導特性の従来の評価方法を説明する。
超伝導膜の品質検査方法として、２００２年３月に超伝導体のマイクロ波表面抵抗試験方法の国際規格：ＩＥＣ６１７８８−７（非特許文献３参照）が制定された。この方法は誘電体共振器法と呼ばれ、図１０に示すように、誘電体円柱を２枚の超伝導膜で挟むことにより、共振器を構成し、共振特性（Ｑ値）の測定から超伝導膜による損失を求め、表面抵抗を導出するものである。この方法によれば、非破壊、かつ、簡便に表面抵抗が精度良く測れ、超伝導膜フィルター等の超伝導材料の選別に適している。
【０００５】
しかしながらこの方法は、試料のサイズを２０ｍｍ×２０ｍｍにしないと正確な表面抵抗が求まらないことから、５０ｍｍ×５０ｍｍといった大きな面積を必要とするフィルター等に使用する超伝導膜の材料選別には使用できない。また、規格外の大きさで評価しようとすると、大がかりな装置となり、また、測定値から表面抵抗を導出する計算が複雑になってしまい、製造現場等で簡便に使用することができない。
【０００６】
また従来の超伝導膜の品質評価方法には、一般用途の交流帯磁率計を利用するものがある（非特許文献４参照）。この方法は図１１に示すように、超伝導膜上に励磁コイルを配置し、励磁コイルの磁場によって超伝導膜に反磁場を発生させ、反磁場をピックアップコイルによって測定する。励磁コイルに流す電流を徐々に増加し励磁コイルの磁場を強くしていくと臨界磁場強度付近で、超伝導膜の発生する反磁場に第３高調波が現れる。第三高調波が現れるときの励磁コイルに流す電流値は、臨界磁場強度に対応するので、励磁コイル電流値によって超伝導膜の評価ができる。
【０００７】
しかしながらこの方法は、交流帯磁率計の励磁コイルに流すことができる電流値に限界があるため、最近の高温超伝導膜のように臨界磁場強度が大きい超伝導膜の評価が困難である。もちろん、大がかりな電流源及び大がかりな電磁石を使用すれば、励磁コイルの発生する磁場を強くすることができるが、このような装置は、装置のコストが高く、また、生産現場で簡便に使用することができない。
【０００８】
また従来の超伝導膜の評価方法には、レーザー加熱法と呼ばれる方法がある（非特許文献５参照）。この方法は図１２に示すように、超伝導膜をテープ状に加工し、テープに一定電流を流し、テープに照射するレーザービームによって生ずるテープ両端の電圧差から臨界電流密度を算出する方法である（非特許文献４参照）。この方法によれば、レーザービーム径を小さくしてテープ上を走査することによって、超伝導膜の臨界電流密度の分布を評価することができる。
しかしながらこの方法は、テープ状に加工したり、テープ両端に電極を接続することが必要不可欠といった破壊検査であるため、生産現場で材料選別といった用途に使用することはできない。
【０００９】
【発明が解決しようとする課題】
このように、超伝導膜を用いたデバイスの実用化に伴い、超伝導膜の評価方法が重要になってきているが、上記に説明したように、従来の方法では、大きな面積の超伝導膜には適用できない、臨界磁場強度の大きな超伝導膜には適用できない、あるいは、破壊検査である、といった課題があり、生産現場で簡便に且つ低コストで使用できる超伝導膜の評価方法がなかった。
本発明は上記課題に鑑み、高感度、簡便、且つ、低コストな超伝導膜の評価方法を提供することを目的とする。
【００１０】
【非特許文献１】
ＳＵＰＥＲＣＯＮＤＵＣＴＩＶＩＴＹＣＯＭＭＵＮＩＣＡＴＩＯＮＳ，Ｖｏｌ．１１，Ｎｏ．２，Ａｐｒｉｌ．２００２
【非特許文献２】
ＳＵＰＥＲＣＯＮＤＵＣＴＩＶＩＴＹＣＯＭＭＵＮＩＣＡＴＩＯＮＳ，Ｖｏｌ．７，Ｎｏ．３，Ｊｕｎｅ．１９９８
【非特許文献３】
ＳＵＰＥＲＣＯＮＤＵＣＴＩＶＩＴＹＣＯＭＭＵＮＩＣＡＴＩＯＮＳ，Ｖｏｌ．１１，Ｎｏ．３，Ｊｕｎｅ．２００２
【非特許文献４】
ＳＵＰＥＲＣＯＮＤＵＣＴＩＶＩＴＹＣＯＭＭＵＮＩＣＡＴＩＯＮＳ，Ｖｏｌ．１１，Ｎｏ．１，Ｆｅｂ．２００２
【非特許文献５】
Ｄ．Ａｂｒａｉｍｏｖ，Ａ．Ｇ．Ｓｉｖａｋｏｖ，Ａ．Ｖ．Ｌｕｋａｓｈｅｎｋｏ，Ｍ．Ｖ．Ｆｉｓｕｔｕｌ，Ｐ．ＭｕｌｌｅｒａｎｄＡ．Ｖ．Ｕｓｔｉｎｏｖ，ＩＥＥＥＴＲＡＮＳＡＣＴＩＯＮＳＯＮＡＰＰＬＩＥＤＳＵＰＥＲＣＯＮＤＵＣＴＩＶＩＴＹ，ＶＯＬ．１１，ＭＡＲＣＨ２０（２００１），ｐ３１７０−３１７３
【非特許文献６】
ＹａｓｕｎｏｒｉＭａｗａｔａｒｉ，ＨｉｒｏｆｕｍｉＹａｍａｓａｋｉ，ａｎｄＹｏｓｉｈｉｋｏＮａｋａｇａｗａＡＰＰＬＩＥＤＰｈｙｓｉｃｓＬｅｔｔｅｒｓＶｏｌ．８１，Ｎｏ．１３，ｐｐ２４２４〜２４２５（１９９９）
【非特許文献７】
Ｊ．Ｈ．Ｃｌｕａｓｓｅｎ，Ｍ．Ｅ．Ｒｅｅｖｅｓ．ａｎｄＲ．Ｊ．Ｓｏｕｌｅｎ，Ｊｒ．，Ｒｅｖ．Ｓｃｉ．Ｉｎｓｔｒｕｍ，Ｖｏｌ．６２，ｐ９９６（１９９１）
【００１１】
【課題を解決するための手段】
上記目的を達成するため、本発明の超伝導膜の超伝導臨界電流特性の評価方法は、超伝導膜に磁場発生源により磁場を印加し、磁場発生源の磁場により超伝導膜と磁場発生源との間に生ずる斥力が増大から減少に転ずるまで磁場強度を連続して増大させ、斥力が増大から減少に転ずる磁場強度から、上記超伝導膜の超伝導臨界電流特性を評価することを特徴とする。
本発明の超伝導膜の評価方法は、超伝導膜に磁場発生源からの磁場を印加し、超伝導膜に印加される磁場強度を変化させて、超伝導膜と磁場発生源との間に働く斥力が増大から減少に転ずる磁場強度から超伝導膜の超伝導臨界電流特性を評価する。
この方法によれば、超伝導膜に印加される磁場強度が超伝導膜の超伝導特性に依存した一定値未満であれば、磁気遮蔽効果及びマイスナー効果によって互いに斥力が働くが、超伝導膜の超伝導特性に依存した特定の磁場強度で、超伝導膜に磁束が進入し、磁束ピン止め現象が生ずる。磁束が進入すると、磁場発生源と超伝導膜との間に引力が生ずる。磁束が進入し磁束ピン止め現象が生ずる磁場強度は、超伝導膜の超伝導特性の善し悪しによって異なる。すなわち、臨界磁場強度の大きい超伝導膜の場合には、大きな磁場強度で磁束が進入し始め、臨界磁場強度の小さい超伝導膜の場合には、小さな磁場強度で磁束が進入し始める。このようにして、斥力が増大から減少に転ずる磁場強度から超伝導膜の超伝導特性を評価できる。また、第２種超伝導体においては、磁束の進入が臨界磁場強度よりも小さな磁場強度で生ずるので、磁場発生源の生成する磁場強度は小さくて良い。
【００１２】
また、超伝導膜に印加される磁場は、超伝導膜の膜端における磁場強度が超伝導膜上の磁場の最大強度以下であり、超伝導膜の膜端における磁場圧縮に基づく超伝導エッジ効果（非特許文献７参照）が生じないことを特徴とする。
この方法によれば、超伝導膜端の磁場圧縮による高い磁場により超伝導膜端から磁束が進入する効果、所謂超伝導エッジ効果が生じないので、正確に被測定超伝導膜の超伝導特性を評価できる。
また、超伝導膜の膜端における磁場強度が超伝導膜上の磁場の最大強度以下にする方法は、磁場発生源の磁極断面積の超伝導膜の面積に対する比を、超伝導膜の膜端における磁場強度が超伝導膜上の磁場の最大強度以下になるように小さくすることによって達成できる。
また、超伝導膜表面の被測定個所のみに磁場が印加されるように磁場発生源を磁気シールドしても良い。また、超伝導膜を超伝導膜表面の被測定個所を除いて磁気シールドしても良い。これらの場合には、磁束の回り込みによる磁性基板に生ずる力をほぼ完全に防止できるので、超伝導膜が磁性基板上に形成される場合の精密な評価方法として最適である。
【００１３】
また、斥力が増大から減少に転ずる磁場強度は、超伝導膜、または磁場発生源を秤上に固定し、超伝導膜または磁場発生源に働く力を秤によって測定し、この測定値の磁場強度に対する変化率の符合が変化する磁場強度から求められる。この方法によれば、簡便、低コスト、且つ、高感度に斥力が増大から減少に転ずる磁場強度を求めることができる。また、超伝導薄膜表面の磁場強度は、磁場発生源の構成から電磁気学における鏡像法等によってを容易に知ることができる。
【００１４】
また、斥力が増大から減少に転ずる磁場強度から評価する超伝導特性は超伝導臨界電流密度であり、斥力が増大から減少に転ずる磁場強度をＨ_a、超伝導膜の膜厚をｄ、そして超伝導膜の超伝導臨界電流密度をＪ_cとして、超伝導臨界電流密度Ｊ_cを、既知の計算式（非特許文献６参照）Ｊ_c＝２Ｈ_a／ｄ、から求めることを特徴とする。この方法によれば、極めて簡便に超伝導臨界電流密度を評価できる。
【００１５】
また、磁場強度を変化させる方法は、磁場発生源の発生する磁場を一定に保ち、超伝導膜と磁場発生源との距離を変化させることを特徴とする。または、超伝導膜と磁場発生源との距離を一定に保ち、磁場発生源の発生する磁場を変化させてもよい。
【００１６】
また、磁場発生源は、超伝導膜の表面に垂直に配置した永久磁石、または超伝導膜の表面に平行に配置した磁石であれば好ましい。または、超伝導膜の表面に垂直に配置した電磁石、または超伝導膜の表面に平行に配置した電磁石であっても良い。
【００１７】
また、本発明では、超伝導膜の平均的超伝導特性を評価する場合に、超伝導膜に印加される印加磁場面積が大きな磁場発生源を用い、超伝導膜の超伝導特性の分布を評価する場合に、印加磁場面積が小さい磁場発生源を用いれば好ましい。この方法によれば、印加磁場面積が大きな磁場発生源を用いれば、超伝導膜の平均の超伝導特性になり、平均の超伝導特性が重要なデバイス用に適している。また、印加磁場面積が小さい磁場発生源を用いれば、超伝導膜の超伝導特性の分布を評価できる。
【００１８】
また、本発明によれば、超伝導膜の裏面に常磁性体を配置し、本発明の前記いずれかの方法を用いて超伝導膜の超伝導特性を評価方法することができる。この方法によれば、超伝導膜に磁場が進入することによって、常磁性体膜に磁場が印加され、常磁性体にも力が働くので、斥力が増大から減少に転ずる磁場強度を正確に評価することができる。
【００１９】
また本発明の超伝導膜の超伝導特性の評価方法は、前記いずれかの方法を金属シース超伝導線材、または、テープ状超伝導線材に適用することができる。
この方法によれば、金属シース超伝導線材、または、テープ状超伝導線材の品質管理に使用できる。
【００２０】
【発明の実施の形態】
以下、図面に基づいて本発明の実施の形態を詳細に説明する。なお、図において実質的に同一の部材は、同一の符号を付して説明する。
図１は、本発明の超伝導膜の評価方法を説明するための模式的装置を示す図である。以下、図１を用いて本発明の方法を説明する。図において、超伝導膜１は秤２上に固定され、秤２は容器３の底面に固定されており、容器３には超伝導膜１を冷却する液体窒素等の冷媒４が入れられている。超伝導膜１の真上に、磁場発生源である磁石５が上下方向に移動可能に配置されている。磁石５は、アーム６を介してスケール７に摺動可能に接続されており、超伝導膜１と磁石５との間の距離をスケール７で測定する。秤２は、超伝導膜１と磁石５との間に働く垂直方向の力を測定する。２ａは秤２の目盛りを模式的に示している。
【００２１】
磁石５は、種々の構成が可能である。図２は、磁石５の種々の構成を示す図であり、図２（ａ）は細長い永久磁石を使用する例を示す。図２（ｂ）は支持棒８の先端に永久磁石５をつけた例を示しており、また、図２（ｃ）は支持棒８の先端に支持棒８の軸方向に直交するように永久磁石をつけた例を示す。この磁石５は電磁石であってもよい。
【００２２】
以下、図１を用いて本発明の超伝導膜の評価方法を説明する。
初めに、超伝導膜１を秤２に固定し、容器３に冷媒４を入れ、超伝導膜１を冷却して超伝導状態にする。次に、磁石５を徐々に下降させると共に秤２で超伝導膜１と磁石５に働く力を検出し、斥力から引力に変わる超伝導膜１と磁石５との間の距離Ｚを測定する。
【００２３】
図３は、超伝導膜と磁石との間に働く力と、超伝導膜と磁石との間の距離との関係を示す図である。縦軸は秤が表示する力Ｆを表し、正領域が斥力、負領域が引力を示す。横軸は距離Ｚである。図の実線、点線及び一点鎖線は、それぞれ、超伝導特性の異なる超伝導膜ａ１，ａ２、ａ３の特性であり、超伝導特性は、ａ１＞ａ２＞ａ３の順番で低くなっている。図の矢印で示したように、距離Ｚを減少させる方向に対して、力Ｆが増大から減少に転ずる距離を、斥力から引力に変わる距離Ｚａと定義する。図に示すように、斥力から引力に変わる距離は、Ｚａ１＜Ｚａ２＜Ｚａ３となり、超伝導特性が高いほど、斥力から引力に変わる距離Ｚａが短くなる。この斥力から引力に変わる距離Ｚａにより、超伝導特性の良否を判定する。
【００２４】
図４は斥力から引力に変わる距離Ｚａと超伝導臨界電流密度Ｊｃとの相関を示す図である。図に示すように、距離Ｚａが小さいほど臨界電流密度Ｊｃは大きくなる。
【００２５】
次に、本発明の方法の原理を説明する。
図５は、第２種超伝導体の反磁化特性を示す図である。縦軸は第２種超伝導体の反磁化の大きさを示し、横軸は印加する磁場強度を示す。
Ｈ_c1は下部臨界磁場と呼ばれており、Ｈ_c2は上部臨界磁場と呼ばれている。第２種超伝導体においては、磁場強度Ｈ_c1から磁束が進入し始め、徐々に進入する磁束が増え、Ｈ_c2において超伝導状態が消失する。磁場強度Ｈ_c1とＨ_c2の間においては超伝導状態と常伝導状態が共存する。磁束ピン止め中心が導入された第２種超伝導体は、臨界磁場強度Ｈ_c2が極めて大きな超伝導体である。
【００２６】
図６は、本発明の評価方法の原理を示す図である。図６（ａ）は斥力が生じている場合を示し、図６（ｂ）は引力が生じている場合を示す。
図６（ａ）に示すように、磁石５と超伝導膜１との距離Ｚが大きく、超伝導膜１に印加される磁場強度が下部臨界磁場Ｈ_c1未満の場合には、磁気遮蔽効果及びマイスナー効果によって磁束Ｂが超伝導膜１に進入しないので、磁場エネルギーを下げるべく、超伝導膜１と磁石５との間に斥力が働く。
図６（ｂ）に示すように、磁石５と超伝導膜１との距離Ｚが小さく、超伝導膜１に印加される磁場強度が下部臨界磁場Ｈ_c1に達したときに磁束が超伝導膜１中に進入し、磁束ピン止め中心に固定される。このように、磁束Ｂが超伝導膜１中に進入できるようになると、磁場エネルギーを下げるべく、超伝導膜１と磁石５との間に引力が働く。
【００２７】
一般に第２種超伝導体においては、超伝導特性の優れた超伝導膜は、表面抵抗が小さく、臨界電流密度が大きく、且つ、臨界磁場強度Ｈ_c1及びＨ_c2が大きいという関係がある。従って、本発明の方法によって測定する距離は、直接的には下部臨界磁場強度Ｈ_c1に対応するが、表面抵抗、臨界電流密度、及び臨界磁場強度Ｈ_c2にも対応しており、本発明の方法によれば超伝導特性の良否を判定することができる。
【００２８】
また、本発明の磁場印加方法は、超伝導膜の膜端における、所謂超伝導エッジ効果（非特許文献７参照）が生じない。
図７は、本発明に使用する磁場印加方法を示す図である。
図７（ａ）は、超伝導膜の面積Ｓ₁に対して磁石の磁極断面積Ｓ₂を十分小さくして、超伝導膜の膜端１０における磁場圧縮に基づく磁場強度が超伝導膜上の磁場の最大強度以下にする例を示している。
図７（ｂ）は、磁石５に磁気シールド９を施し、超伝導膜の膜端１０における磁場強度が無視できるようにする例を示している。
図７（ｃ）は、超伝導膜及び基板１１自体を磁気シールド材料で覆って磁気シールド９を施し、超伝導膜の膜端１０における磁場強度が無視できるようにする例を示している。
【００２９】
上記に説明したように、超伝導膜に磁束が進入しない場合には、磁気遮蔽効果及びマイスナー効果によって超伝導膜は完全反磁性であり、超伝導膜下には磁場が存在しない。従って、この場合の超伝導膜表面の磁石直下の磁場は、電磁気学における鏡像法を用いて容易に求めることができる。一方、超伝導膜端における磁場は、所謂磁場圧縮効果によって強度が強くなるが、この強度は、被測定磁性膜の形状によって種々変化し、その強度を求めるのは容易ではない。
上記の図７に示した方法によれば、磁石直下の磁場強度よりも超伝導膜端の磁場強度が小さく、または、超伝導膜端の磁場が無視できるので、磁場強度が正確にわかる磁石直下で最初に磁束の進入が生じ、正確に超伝導膜に磁束が進入する磁場強度（外部臨界磁場）Ｈ_aを求めることができる。
【００３０】
次に、このようにして求めた外部臨界磁場Ｈ_aから、超伝導臨界電流密度Ｊ_cを求める方法を説明する。
外部磁界をうち消すように、超伝導膜中には超伝導電流が流れている。磁束が進入する直前の超伝導電流は、超伝導臨界電流密度Ｊ_cに超伝導膜の膜厚ｄの半分をかけた値となり、この超伝導電流の形成する磁場と外部臨界磁場Ｈ_aが釣り合うことから超伝導臨界電流密度Ｊ_cは次式で表される（非特許文献６参照）。
Ｊ_c＝２Ｈ_a／ｄ
このようにして、超伝導臨界電流密度を求めることができる。
【００３１】
また、図５に示したように、本発明の方法は上部臨界磁場強度Ｈ_c2よりも遙かに磁場強度が小さい下部臨界磁場Ｈ_c1程度の磁場強度で評価できるので、必要とされる磁石の磁場強度が小さくてよく、上部臨界磁場強度Ｈ_c2が大きい超伝導膜の評価にも対応することができる。
また、磁石の磁極断面積を小さくすれば、微少部分の超伝導特性を評価でき、超伝導膜上を走査して測定することにより、超伝導特性の分布を測定することができる。用途に応じて、磁極断面積の異なる磁石を使用すれば、簡便に、且つ、低コストで様々な用途に使用できる。
また、表面抵抗を評価する場合には、表面抵抗が超伝導特性のみならず、薄膜表面のトポロジーによっても影響を受けるので、本発明の方法による評価に加えて、薄膜表面の顕微鏡観察をおこない、粒状突起物、あるいは針状突起物の有無を確認するようにすれば、きわめて良好な材料選別ができる。
上記説明では、超伝導膜の評価について説明したが、本発明の方法は力学的力に基づいた評価方法であるから、秤上に固定できる形状の超伝導体であれば何でも測定できる。例えば、金属シース超伝導線材、または、テープ状超伝導線材に用いれば、極めて低コストに、且つ簡便に品質検査ができる。
【００３２】
次に、超伝導膜を積層する基板が磁性体である場合の、本発明の超伝導特性の評価方法を説明する。
超伝導膜を積層する基板１１が磁性体である場合には、図１で示した方法で測定すると、磁石５から超伝導膜を回り込んだ磁場が基板に力を及ぼすので正確な評価が難しくなる。このような場合の本発明の方法を説明する。
基板１１が磁性体である場合には、図７（ｂ）に示した磁気シールド９した磁石５を用いて、図１と同様に測定することによって、または、図７（ｃ）に示した、超伝導膜１及び基板１１を被測定個所を残して磁気シールド９して図１と同様に測定すれば、基板１１の磁性体に回り込む磁束が無視でき、正確な評価ができる。
【００３３】
図８は、基板が磁性体である場合の本発明の超伝導膜の超伝導特性の評価方法による、超伝導膜と磁石との間に働く力と、超伝導膜と磁石との間の距離との関係を示す図である。図３と同様に、実線、点線及び一点鎖線は、それぞれ、超伝導特性の異なる超伝導膜ａ１，ａ２、ａ３の特性であり、超伝導特性は、ａ１＞ａ２＞ａ３の順番で低くなっており、それに対応して斥力から引力に変わる距離は、Ｚａ１＜Ｚａ２＜Ｚａ３となる。図３と異なるのは、斥力から引力に変わった後に急峻な勾配で引力が増大することであるが、これは、磁束が超伝導膜を貫通し、磁性体基板を引きつけるためである。
また、上記の効果を利用して、超伝導膜に働く力の向きの変化が判別しにくい超伝導膜の裏面に常磁性体を配置することにより、超伝導膜に磁場が進入した時点で、常磁性体に磁場が印加され、常磁性体にも力が働くので、超伝導膜に働く力の向きの変化が明確に判別できるようになり、磁場強度を正確に評価できるようになる。
【００３４】
上記説明では、斥力から引力に変化する場合を例にとって説明したが、引力から斥力に変化する場合は、斥力から引力に変化する場合と等価であるから説明を省略した。また、磁場発生源が磁石の場合を例にとり、超伝導膜表面の外部磁場強度の変化を磁石と超伝導膜の相対距離の変化でおこなう場合を説明したが、磁石が電磁石であり、電磁石を固定し、電磁石のコイルに流す電流を制御して外部磁場強度を変化させても同等の効果が得られることは明らかである。
【００３５】
次に、実施例を示す。
超伝導薄膜（Ｔｌ−１２２３）の厚さが５００ｎｍ、基板面積が２０ｍｍ×２０ｍｍ、超伝導薄膜と基板の重さが合わせて６００ｍｇの試料を評価に用いた。磁石は、磁極断面積が８ｍｍ×８ｍｍの永久磁石（Ｎｄ−Ｆｅ−Ｂ）を多段に積み重ねて作製した。この磁石表面の磁場強度は０．５Ｔ（テスラ）である。液体窒素温度に冷却した試料に、この磁石を近づけて、この磁石に吸い付くか否かで材料選別をおこなった。
図９は磁石に試料の超伝導薄膜が吸い付けられた状態を示す写真である。図に示すように、磁石に引き寄せられつり上げることができる試料と、つり上げることができない試料が存在した。
他の手段で臨界電流密度を測定したところ、磁石に引き寄せられつり上げることができる試料の臨界電流密度は、１０⁶Ａ／ｃｍ²以上であったが、つり上げることができなかった試料の臨界電流密度は、１０⁵Ａ／ｃｍ²であった。また７７Ｋ、３８ＧＨｚにおける表面抵抗は、つり上げることができる試料が５ｍΩ以下であったのに対し、つり上げることができなかった試料は、５０ｍΩ程度であった。なお、表面トポロジーは同程度であった。
この実施例は、秤を使用して斥力から引力に変化する磁場を検出したものではないが、超伝導膜と磁石との間に働く力が大きいものほど超伝導臨界電流密度が高いことを示した点で、本発明の原理を実証するものである。
【００３６】
【発明の効果】
上記説明から理解されるように、本発明の方法は、超伝導膜に磁束が進入することによる、磁石と超伝導膜の相互作用力の方向の変化を検出して超伝導特性の評価を行うので、磁石の磁場強度が小さくてよく、また、極めて高感度である。また、力学的力を利用しているので、簡便、且つ、低コストである。従って、本発明の評価方法を超伝導薄膜フィルターや、超伝導電線の品質管理などに用いれば、極めて有用である。
【図面の簡単な説明】
【図１】本発明の超伝導膜の評価方法を説明するための模式的装置を示す図である。
【図２】本発明の超伝導膜の評価方法に用いる磁石の種々の構成を示す図である。
【図３】超伝導膜と磁石との間に働く力と、超伝導膜と磁石との間の距離との関係を示す図である。
【図４】斥力から引力に変わる距離と臨界電流密度との相関を示す図である。
【図５】第２種超伝導体の反磁化特性を示す図である。
【図６】本発明の原理を示す図である。
【図７】本発明に使用する磁場印加方法を示す図である。
【図８】基板が磁性体である場合の超伝導膜と磁石との間に働く力と、超伝導膜と磁石との間の距離との関係を示す図である。
【図９】磁石に試料の超伝導薄膜が吸い付けられた状態を示す写真である。
【図１０】従来の超伝導膜の表面抵抗を評価する誘電体共振器法を説明する図である。
【図１１】従来の超伝導膜の臨界磁場強度を評価する交流帯磁率計を利用する方法を説明する図である。
【図１２】従来の超伝導膜の臨界電流密度を評価するレーザー加熱法の説明図である。
【符号の説明】
１超伝導膜
２秤
２ａ指示針
３容器
４冷媒
５磁石
６アーム
７スケール
８支持棒
９磁気シールド
１０超伝導膜端
１１基板[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for evaluating the superconducting properties of a superconducting film with high sensitivity, simplicity, and low cost.
[0002]
[Prior art]
In recent years, devices using copper oxide-based high-temperature superconducting films have been put into practical use. For example, a base station for mobile communication needs to capture as many subscriber frequency bands as possible in a limited frequency band, and requires a high-performance microwave bandpass filter. Since the characteristics of the microwave bandpass filter depend on the resistance component of the filter circuit, the characteristics of the microwave bandpass filter are dramatically improved by using a high-temperature superconducting film with a very small resistance component (non-patented) Reference 1). Moreover, a superconducting electric wire in which a high-temperature superconducting film is laminated on a tape has been put into practical use (see Non-Patent Document 2).
[0003]
However, as represented by copper oxide-based high-temperature superconducting films, only superconducting films with good reproducibility are produced despite the fact that superconducting film production technology has been studied very vigorously. In the production of devices, it is necessary to inspect all the superconducting films used in the devices and to select materials.
Therefore, in order to produce a device using a superconducting film with stable quality and low cost, a simple and low-cost superconducting film evaluation technique is indispensable.
Superconducting properties required for superconducting films used in the above devices are surface resistance and critical current density. In general, superconducting films with excellent superconducting properties have low surface resistance and critical current density. The relationship is large and the critical magnetic field strength is large.
[0004]
Next, a conventional method for evaluating the superconducting properties of the superconducting film will be described.
As a quality inspection method for superconducting films, an international standard IEC 61788-7 (see Non-Patent Document 3) was established in March 2002 for a microwave surface resistance test method for superconductors. This method is called a dielectric resonator method. As shown in FIG. 10, a resonator is formed by sandwiching a dielectric cylinder between two superconducting films, and the measurement of resonance characteristics (Q value) is The loss due to the conductive film is obtained and the surface resistance is derived. According to this method, non-destructive and simple surface resistance can be measured with high accuracy, and it is suitable for selection of superconducting materials such as a superconducting film filter.
[0005]
However, in this method, since accurate surface resistance cannot be obtained unless the sample size is 20 mm × 20 mm, it is necessary to select a material for a superconducting film used for a filter or the like that requires a large area of 50 mm × 50 mm. I can not use it. Further, if an attempt is made to evaluate with a size outside the standard, it becomes a large-scale device, and the calculation for deriving the surface resistance from the measured value becomes complicated, and cannot be used easily at the production site.
[0006]
Further, there is a conventional method for evaluating the quality of a superconducting film that uses an AC susceptibility meter for general use (see Non-Patent Document 4). In this method, as shown in FIG. 11, an exciting coil is arranged on a superconducting film, a demagnetizing field is generated in the superconducting film by the magnetic field of the exciting coil, and the demagnetizing field is measured by a pickup coil. When the current flowing through the exciting coil is gradually increased to increase the magnetic field of the exciting coil, the third harmonic appears in the demagnetizing field generated by the superconducting film near the critical magnetic field strength. Since the current value that flows through the exciting coil when the third harmonic appears corresponds to the critical magnetic field strength, the superconducting film can be evaluated by the exciting coil current value.
[0007]
However, since this method has a limit on the current value that can be passed through the exciting coil of the AC susceptibility meter, it is difficult to evaluate a superconducting film having a large critical magnetic field strength like a recent high-temperature superconducting film. Of course, if a large current source and a large electromagnet are used, the magnetic field generated by the exciting coil can be strengthened. However, such a device is expensive and easy to use at the production site. I can't.
[0008]
As a conventional method for evaluating a superconducting film, there is a method called a laser heating method (see Non-Patent Document 5). In this method, as shown in FIG. 12, a superconducting film is processed into a tape shape, a constant current is applied to the tape, and a critical current density is calculated from a voltage difference between both ends of the tape generated by a laser beam applied to the tape. (Refer nonpatent literature 4). According to this method, the critical current density distribution of the superconducting film can be evaluated by reducing the laser beam diameter and scanning the tape.
However, since this method is a destructive inspection in which it is indispensable to process into a tape shape or connect electrodes to both ends of the tape, it cannot be used for material selection at the production site.
[0009]
[Problems to be solved by the invention]
As described above, with the practical application of devices using superconducting films, the superconducting film evaluation method has become important. As described above, the conventional method has a large area superconducting film. In other words, there is a problem that it cannot be applied to a superconducting film having a large critical magnetic field strength, or that it is a destructive inspection. .
In view of the above problems, an object of the present invention is to provide a method for evaluating a superconducting film with high sensitivity, simplicity, and low cost.
[0010]
[Non-Patent Document 1]
SUPERCONDUCTIVITY COMMUNICATIONS, Vol. 11, no. 2, April. 2002
[Non-Patent Document 2]
SUPERCONDUCTIVITY COMMUNICATIONS, Vol. 7, no. 3, June. 1998
[Non-Patent Document 3]
SUPERCONDUCTIVITY COMMUNICATIONS, Vol. 11, no. 3, June. 2002
[Non-Patent Document 4]
SUPERCONDUCTIVITY COMMUNICATIONS, Vol. 11, no. 1, Feb. 2002
[Non-Patent Document 5]
D. Abramov, A .; G. Sivakov, A .; V. Lukashenko, M .; V. Fistur, P.M. Muller and A.M. V. Ustinov, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 11, MARCH 20 (2001), p3170-3173.
[Non-Patent Document 6]
Yasuni Mawatari, Hirofumi Yamasaki, and Yoshihiko Nakagawa APPLIED Physics Letters Vol. 81, no. 13, pp 2424-2425 (1999)
[Non-Patent Document 7]
J. et al. H. Classen, M.M. E. Reeves. and R.R. J. et al. Soulen, Jr. Rev. Sci. Instrum, Vol. 62, p996 (1991)
[0011]
[Means for Solving the Problems]
In order to achieve the above object, the method for evaluating the superconducting critical current characteristics of a superconducting film according to the present invention applies a magnetic field to a superconducting film by a magnetic field source, and the superconducting film and the magnetic field source by the magnetic field of the magnetic field source. The magnetic field strength is continuously increased until the repulsive force generated between the increase and decrease decreases, and the superconducting critical current characteristic of the superconducting film is evaluated from the magnetic field strength at which the repulsive force changes from increase to decrease. To do.
The superconducting film evaluation method of the present invention applies a magnetic field from a magnetic field generation source to the superconducting film, changes the magnetic field strength applied to the superconducting film, and between the superconducting film and the magnetic field generation source. The superconducting critical current characteristics of the superconducting film are evaluated from the magnetic field strength at which the repulsive force changes from increasing to decreasing .
According to this method, if the strength of the magnetic field applied to the superconducting film is less than a certain value depending on the superconducting characteristics of the superconducting film, the repulsive force works due to the magnetic shielding effect and the Meissner effect. Magnetic flux enters the superconducting film with a specific magnetic field strength depending on the superconducting characteristics, and a magnetic flux pinning phenomenon occurs. When the magnetic flux enters, an attractive force is generated between the magnetic field generation source and the superconducting film. The magnetic field strength at which magnetic flux enters and magnetic flux pinning phenomenon occurs varies depending on whether the superconducting properties of the superconducting film are good or bad. That is, in the case of a superconducting film having a large critical magnetic field strength, the magnetic flux begins to enter with a large magnetic field strength, and in the case of a superconducting film having a small critical magnetic field strength, the magnetic flux begins to enter with a small magnetic field strength. In this way, the superconducting properties of the superconducting film can be evaluated from the magnetic field strength at which the repulsive force changes from increasing to decreasing . In the type 2 superconductor, since the magnetic flux enters with a magnetic field intensity smaller than the critical magnetic field intensity, the magnetic field intensity generated by the magnetic field generation source may be small.
[0012]
In addition, the magnetic field applied to the superconducting film is such that the magnetic field strength at the film end of the superconducting film is less than the maximum intensity of the magnetic field on the superconducting film, and the superconducting edge effect based on magnetic field compression at the film end of the superconducting film. (See Non-Patent Document 7) does not occur.
According to this method, since the effect of magnetic flux entering from the edge of the superconducting film due to the high magnetic field due to the magnetic field compression at the edge of the superconducting film, the so-called superconducting edge effect does not occur, the superconducting characteristics of the measured superconducting film can be accurately determined. Can be evaluated.
In addition, the method in which the magnetic field intensity at the film edge of the superconducting film is less than or equal to the maximum intensity of the magnetic field on the superconducting film is obtained by setting the ratio of the magnetic pole cross-sectional area of the magnetic field source to the area of the superconducting film. This can be achieved by reducing the magnetic field strength of the magnetic field to be less than the maximum strength of the magnetic field on the superconducting film.
In addition, the magnetic field generation source may be magnetically shielded so that the magnetic field is applied only to the portion to be measured on the surface of the superconducting film. Further, the superconducting film may be magnetically shielded except for the portion to be measured on the surface of the superconducting film. In these cases, since the force generated on the magnetic substrate due to the wraparound of the magnetic flux can be almost completely prevented, it is optimal as a precise evaluation method when the superconducting film is formed on the magnetic substrate.
[0013]
The magnetic field strength at which repulsive force changes from increasing to decreasing is determined by fixing the superconducting film or magnetic field source on the scale and measuring the force acting on the superconducting film or magnetic field source with a scale. It is obtained from the magnetic field intensity at which the sign of the rate of change with respect to changes. According to this method, it is possible to obtain a magnetic field strength at which the repulsive force is changed from increasing to decreasing with high sensitivity in a simple, low-cost manner. Further, the magnetic field intensity on the surface of the superconducting thin film can be easily known from the configuration of the magnetic field generation source by the mirror image method in electromagnetics.
[0014]
Furthermore, the superconducting properties of voted magnetic field strength starts to decrease repulsive force from the increase is superconducting critical current density, magnetic field strength starts to decrease repulsive force from increasing H _a, the film thickness of the superconducting film d and super The superconducting critical current density of the conductive film is _defined as J _c , and the superconducting critical current density J _c is obtained from a known calculation formula (see Non-Patent Document 6) J _c = 2H _a / d. According to this method, the superconducting critical current density can be evaluated very simply.
[0015]
The method of changing the magnetic field strength is characterized in that the magnetic field generated by the magnetic field generation source is kept constant and the distance between the superconducting film and the magnetic field generation source is changed. Alternatively, the magnetic field generated by the magnetic field generation source may be changed while keeping the distance between the superconducting film and the magnetic field generation source constant.
[0016]
The magnetic field generating source is preferably a permanent magnet disposed perpendicular to the surface of the superconducting film or a magnet disposed in parallel to the surface of the superconducting film. Alternatively, an electromagnet arranged perpendicular to the surface of the superconducting film or an electromagnet arranged parallel to the surface of the superconducting film may be used.
[0017]
Further, in the present invention, when evaluating the average superconducting property of the superconducting film, the distribution of the superconducting property of the superconducting film is evaluated by using a magnetic field source having a large applied magnetic field area applied to the superconducting film. when, preferable if being used a small magnetic source applied magnetic field area. According to this method, if a magnetic field generation source having a large applied magnetic field area is used, the superconducting film has an average superconducting characteristic, which is suitable for devices in which the average superconducting characteristic is important. Moreover, if a magnetic field generating source with a small applied magnetic field area is used, the distribution of superconducting characteristics of the superconducting film can be evaluated.
[0018]
In addition, according to the present invention, a paramagnetic material can be disposed on the back surface of the superconducting film, and the superconducting characteristics of the superconducting film can be evaluated using any one of the methods of the present invention. According to this method, when a magnetic field enters the superconducting film, a magnetic field is applied to the paramagnetic film, and a force is also applied to the paramagnetic film. Therefore, the magnetic field strength at which the repulsive force turns from increasing to decreasing is accurately evaluated. can do.
[0019]
In addition, in the method for evaluating the superconducting property of the superconducting film of the present invention, any of the above methods can be applied to a metal sheath superconducting wire or a tape-shaped superconducting wire.
According to this method, the metal sheath superconducting wire or the tape-shaped superconducting wire can be used for quality control.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, substantially the same members will be described with the same reference numerals.
FIG. 1 is a diagram showing a schematic apparatus for explaining a method for evaluating a superconducting film of the present invention. Hereinafter, the method of the present invention will be described with reference to FIG. In the figure, the superconducting film 1 is fixed on a scale 2, and the scale 2 is fixed to the bottom surface of a container 3, and the container 3 contains a refrigerant 4 such as liquid nitrogen that cools the superconducting film 1. . A magnet 5 which is a magnetic field generation source is arranged directly above the superconducting film 1 so as to be movable in the vertical direction. The magnet 5 is slidably connected to the scale 7 via the arm 6, and the distance between the superconducting film 1 and the magnet 5 is measured by the scale 7. The scale 2 measures a vertical force acting between the superconducting film 1 and the magnet 5. 2a schematically shows the scale of the scale 2.
[0021]
The magnet 5 can have various configurations. FIG. 2 is a diagram showing various configurations of the magnet 5, and FIG. 2 (a) shows an example in which an elongated permanent magnet is used. FIG. 2B shows an example in which the permanent magnet 5 is attached to the tip of the support bar 8, and FIG. 2C is permanent so that the tip of the support bar 8 is orthogonal to the axial direction of the support bar 8. An example with a magnet attached is shown. The magnet 5 may be an electromagnet.
[0022]
Hereinafter, the evaluation method of the superconducting film of the present invention will be described with reference to FIG.
First, the superconducting film 1 is fixed to the balance 2, the refrigerant 4 is put into the container 3, and the superconducting film 1 is cooled to be in a superconducting state. Next, the magnet 5 is gradually lowered and the force acting on the superconducting film 1 and the magnet 5 is detected by the balance 2, and the distance Z between the superconducting film 1 and the magnet 5 that changes from repulsive force to attractive force is measured.
[0023]
FIG. 3 is a diagram showing the relationship between the force acting between the superconducting film and the magnet and the distance between the superconducting film and the magnet. The vertical axis represents the force F displayed by the scale, with the positive region indicating repulsive force and the negative region indicating attractive force. The horizontal axis is the distance Z. The solid line, dotted line, and alternate long and short dash line in the figure are the characteristics of the superconducting films a1, a2, and a3 having different superconducting characteristics, and the superconducting characteristics are lower in the order of a1>a2> a3. As indicated by the arrows in the figure, the distance at which the force F changes from increasing to decreasing with respect to the direction in which the distance Z decreases is defined as a distance Za that changes from repulsive force to attractive force. As shown in the figure, the distance from repulsive force to attractive force is Za1 <Za2 <Za3, and the higher the superconducting property, the shorter the distance Za from repulsive force to attractive force. The quality of the superconducting characteristics is determined by the distance Za that changes from repulsive force to attractive force.
[0024]
FIG. 4 is a diagram showing the correlation between the distance Za that changes from repulsive force to attractive force and the superconducting critical current density Jc. As shown in the figure, the critical current density Jc increases as the distance Za decreases.
[0025]
Next, the principle of the method of the present invention will be described.
FIG. 5 is a diagram showing the demagnetization characteristics of the type 2 superconductor. The vertical axis represents the magnitude of the demagnetization of the type 2 superconductor, and the horizontal axis represents the applied magnetic field strength.
H _c1 is called the lower critical magnetic field, and H _c2 is called the upper critical magnetic field. In the type 2 superconductor, the magnetic flux starts to enter from the magnetic field strength H _c1 , the magnetic flux entering gradually increases, and the superconducting state disappears at H _c2 . A superconducting state and a normal state coexist between the magnetic field strengths H _c1 and H _c2 . The type 2 superconductor in which the magnetic flux pinning center is introduced is a superconductor having an extremely large critical magnetic field strength H _c2 .
[0026]
FIG. 6 is a diagram showing the principle of the evaluation method of the present invention. FIG. 6A shows a case where repulsive force is generated, and FIG. 6B shows a case where attractive force is generated.
As shown in FIG. 6A, when the distance Z between the magnet 5 and the superconducting film 1 is large and the magnetic field strength applied to the superconducting film 1 is less than the lower critical magnetic field H _c1 , the magnetic shielding effect and Since the magnetic flux B does not enter the superconducting film 1 due to the Meissner effect, a repulsive force acts between the superconducting film 1 and the magnet 5 in order to reduce the magnetic field energy.
As shown in FIG. 6B, when the distance Z between the magnet 5 and the superconducting film 1 is small and the magnetic field strength applied to the superconducting film 1 reaches the lower critical magnetic field H _c1 , the magnetic flux is superconducting. 1 enters and is fixed to the magnetic flux pinning center. Thus, when the magnetic flux B can enter the superconducting film 1, an attractive force acts between the superconducting film 1 and the magnet 5 in order to lower the magnetic field energy.
[0027]
In general, in the type 2 superconductor, a superconducting film having excellent superconducting properties has a relationship that the surface resistance is small, the critical current density is large, and the critical magnetic field strengths H _c1 and H _c2 are large. Therefore, the distance measured by the method of the present invention directly corresponds to the lower critical magnetic field strength H _c1, but also corresponds to the surface resistance, critical current density, and critical magnetic field strength H _c2 . According to the method, the quality of superconducting characteristics can be determined.
[0028]
In addition, the magnetic field application method of the present invention does not cause a so-called superconducting edge effect (see Non-Patent Document 7) at the film edge of the superconducting film.
FIG. 7 is a diagram showing a magnetic field application method used in the present invention.
FIG. 7A shows that the magnetic pole cross-sectional area S ₂ of the magnet is sufficiently smaller than the area S ₁ of the superconducting film, and the magnetic field strength based on the magnetic field compression at the film end 10 of the superconducting film is on the superconducting film. An example is shown in which the maximum intensity of the magnetic field is made lower than the maximum.
FIG. 7B shows an example in which the magnetic shield 9 is applied to the magnet 5 so that the magnetic field strength at the film end 10 of the superconducting film can be ignored.
FIG. 7C shows an example in which the superconducting film and the substrate 11 itself are covered with a magnetic shielding material and the magnetic shield 9 is applied so that the magnetic field strength at the film end 10 of the superconducting film can be ignored.
[0029]
As described above, when the magnetic flux does not enter the superconducting film, the superconducting film is completely diamagnetic due to the magnetic shielding effect and the Meissner effect, and no magnetic field exists under the superconducting film. Therefore, the magnetic field directly under the magnet on the surface of the superconducting film in this case can be easily obtained by using a mirror image method in electromagnetism. On the other hand, the strength of the magnetic field at the edge of the superconducting film is increased by the so-called magnetic field compression effect, but this strength varies depending on the shape of the magnetic film to be measured, and it is not easy to obtain the strength.
According to the method shown in FIG. 7, the magnetic field strength at the end of the superconducting film is smaller than the magnetic field strength immediately below the magnet, or the magnetic field at the end of the superconducting film can be ignored. occurs first entry flux can be determined accurately field strength magnetic flux in the superconducting film enters (external critical magnetic field) H _a in.
[0030]
Then, the external critical magnetic field H _a thus determined, a method of obtaining a superconducting critical current density J _c.
A superconducting current flows in the superconducting film so as to extinguish the external magnetic field. Superconducting current immediately before the flux enters becomes a value obtained by multiplying the half of the thickness d of the superconducting film to the superconducting critical current density J _c, the magnetic field and the external critical field H _a of forming the superconducting current matches Therefore, the superconducting critical current density _Jc is expressed by the following equation (see Non-Patent Document 6).
J _c = 2H _a / d
In this way, the superconducting critical current density can be determined.
[0031]
Also, as shown in FIG. 5, the method of the present invention can be evaluated with a magnetic field strength of the lower critical magnetic field H _c1, which is much smaller than the upper critical magnetic field strength H _c2 . The magnetic field strength may be small, and it can correspond to the evaluation of a superconducting film having a large upper critical magnetic field strength _Hc2 .
Further, if the magnetic pole cross-sectional area of the magnet is reduced, the superconducting characteristics of a minute portion can be evaluated, and the distribution of superconducting characteristics can be measured by scanning the superconducting film and measuring it. If a magnet having a different magnetic pole cross-sectional area is used according to the application, it can be used for various applications simply and at low cost.
In addition, when evaluating the surface resistance, the surface resistance is affected not only by the superconducting properties, but also by the topology of the thin film surface, so in addition to the evaluation by the method of the present invention, perform the microscopic observation of the thin film surface, If the presence or absence of granular protrusions or needle-like protrusions is confirmed, extremely good material selection can be performed.
In the above description, evaluation of a superconducting film has been described. However, since the method of the present invention is an evaluation method based on mechanical force, any superconductor having a shape that can be fixed on a scale can be measured. For example, if it is used for a metal sheath superconducting wire or a tape-like superconducting wire, quality inspection can be performed easily at a very low cost.
[0032]
Next, the superconducting property evaluation method of the present invention when the substrate on which the superconducting film is laminated is a magnetic material will be described.
When the substrate 11 on which the superconducting film is laminated is a magnetic material, when measured by the method shown in FIG. 1, the magnetic field that has passed around the superconducting film from the magnet 5 exerts a force on the substrate, making accurate evaluation difficult. Become. The method of the present invention in such a case will be described.
In the case where the substrate 11 is a magnetic body, by using the magnet 5 with the magnetic shield 9 shown in FIG. 7B, the measurement is performed in the same manner as in FIG. 1, or as shown in FIG. If the superconducting film 1 and the substrate 11 are measured in the same manner as in FIG. 1 with the magnetic shield 9 leaving the portion to be measured, the magnetic flux that wraps around the magnetic body of the substrate 11 can be ignored and accurate evaluation can be performed.
[0033]
FIG. 8 shows the force acting between the superconducting film and the magnet and the distance between the superconducting film and the magnet according to the method for evaluating the superconducting property of the superconducting film of the present invention when the substrate is a magnetic material. It is a figure which shows the relationship. As in FIG. 3, the solid line, the dotted line, and the alternate long and short dash line are the characteristics of the superconducting films a1, a2, and a3 having different superconducting characteristics, and the superconducting characteristics become lower in the order of a1>a2> a3. Accordingly, the distance corresponding to the change from repulsive force to attractive force is Za1 <Za2 <Za3. The difference from FIG. 3 is that the attractive force increases with a steep gradient after changing from repulsive force to attractive force, because the magnetic flux penetrates the superconducting film and attracts the magnetic substrate.
In addition, by utilizing the above effect, by placing a paramagnetic material on the back surface of the superconducting film where it is difficult to determine the change in the direction of the force acting on the superconducting film, when the magnetic field enters the superconducting film, Since a magnetic field is applied to the paramagnetic material and a force is also applied to the paramagnetic material, a change in the direction of the force acting on the superconducting film can be clearly discriminated, and the magnetic field strength can be accurately evaluated.
[0034]
In the above description, the case where the repulsive force is changed to the attractive force has been described as an example, but the case where the repulsive force is changed to the repulsive force is equivalent to the case where the repulsive force is changed to the attractive force. In addition, taking the case where the magnetic field generation source is a magnet as an example, the case where the change in the external magnetic field intensity on the surface of the superconducting film is performed by the change in the relative distance between the magnet and the superconducting film has been described. It is clear that the same effect can be obtained even if the external magnetic field strength is changed by controlling the current flowing through the coil of the electromagnet.
[0035]
Next, an example is shown.
A sample having a thickness of the superconducting thin film (Tl-1223) of 500 nm, a substrate area of 20 mm × 20 mm, and a weight of the superconducting thin film and the substrate of 600 mg was used for evaluation. The magnet was produced by stacking permanent magnets (Nd—Fe—B) having a magnetic pole cross-sectional area of 8 mm × 8 mm in multiple stages. The magnetic field strength of the magnet surface is 0.5T (Tesla). The material was selected based on whether or not the magnet was brought close to the sample cooled to the liquid nitrogen temperature and attracted to the magnet.
FIG. 9 is a photograph showing a state in which the superconducting thin film of the sample is attracted to the magnet. As shown in the figure, there were samples that could be lifted by the magnet and samples that could not be lifted.
When the critical current density was measured by other means, the critical current density of the sample that could be attracted and lifted by the magnet was 10 ⁶ A / cm ² or more, but the critical current density of the sample that could not be lifted. Was 10 ⁵ A / cm ² . The surface resistance at 77K and 38 GHz was 5 mΩ or less for samples that could be lifted, whereas the sample that could not be lifted was about 50 mΩ. The surface topology was similar.
This example does not detect a magnetic field that changes from repulsive force to attractive force using a scale, but shows that the higher the force acting between the superconducting film and the magnet, the higher the superconducting critical current density. This demonstrates the principle of the present invention.
[0036]
【The invention's effect】
As understood from the above description, the method of the present invention evaluates the superconducting characteristics by detecting the change in the direction of the interaction force between the magnet and the superconducting film due to the magnetic flux entering the superconducting film. Therefore, the magnetic field strength of the magnet may be small and the sensitivity is extremely high. Moreover, since mechanical force is utilized, it is simple and low-cost. Therefore, if the evaluation method of the present invention is used for superconducting thin film filters and quality control of superconducting wires, it is extremely useful.
[Brief description of the drawings]
FIG. 1 is a diagram showing a schematic apparatus for explaining a method for evaluating a superconducting film of the present invention.
FIG. 2 is a diagram showing various configurations of a magnet used in the superconducting film evaluation method of the present invention.
FIG. 3 is a diagram showing a relationship between a force acting between a superconducting film and a magnet and a distance between the superconducting film and the magnet.
FIG. 4 is a diagram showing a correlation between a distance from a repulsive force to an attractive force and a critical current density.
FIG. 5 is a diagram showing the demagnetization characteristics of the type 2 superconductor.
FIG. 6 is a diagram illustrating the principle of the present invention.
FIG. 7 is a diagram showing a magnetic field application method used in the present invention.
FIG. 8 is a diagram showing a relationship between a force acting between a superconducting film and a magnet and a distance between the superconducting film and the magnet when the substrate is a magnetic body.
FIG. 9 is a photograph showing a state in which a sample superconducting thin film is attracted to a magnet.
FIG. 10 is a diagram for explaining a dielectric resonator method for evaluating the surface resistance of a conventional superconducting film.
FIG. 11 is a diagram for explaining a method using an AC susceptibility meter for evaluating the critical magnetic field strength of a conventional superconducting film.
FIG. 12 is an explanatory diagram of a laser heating method for evaluating the critical current density of a conventional superconducting film.
[Explanation of symbols]
1 Superconducting film 2 Scale 2a Indicator needle 3 Container 4 Refrigerant 5 Magnet 6 Arm 7 Scale 8 Support bar 9 Magnetic shield 10 Superconducting film end 11 Substrate

Claims

The magnetic field is applied by the magnetic field generation source to the superconducting layer increases continuously magnetic field strength to repulsive force generated is starts to decrease from the increase between the superconducting film and the magnetic field generation source by the magnetic field of the magnetic source , from the magnetic field intensity starts to decrease repulsive force from increasing, and evaluating the superconducting critical current characteristics of the superconducting film, method for evaluating the superconducting critical current characteristics of the superconducting film.

The superconducting critical current characteristic of the superconducting film is evaluated by determining the superconducting critical current density of the superconducting film from the magnetic field intensity Ha where the repulsive force changes from increasing to decreasing and the film thickness d of the superconducting film. 2. The method for evaluating the superconducting critical current characteristics of a superconducting film according to claim 1, wherein the superconducting critical current density Jc is determined from the relationship of Jc = 2Ha / d.

The increase in the magnetic field strength is performed by decreasing the distance between the magnetic field generation source that generates a constant magnetic field and the superconductivity, and the superconductivity of the superconducting film is increased by the distance at which the repulsive force changes from increasing to decreasing. The method for evaluating a superconducting critical current characteristic of a superconducting film according to claim 1, wherein the critical current characteristic is evaluated.

The supermagnetic film according to any one of claims 1 to 3 , wherein a paramagnetic material is disposed on the back surface of the superconducting film to detect the magnetic field strength at which the repulsive force changes from increasing to decreasing with high sensitivity. Evaluation method of superconducting critical current characteristics of conductive films.

The magnetic field generation source is magnetically shielded except for an opening provided in a part of a surface facing the superconducting film, and the opening is formed at a film end of the superconducting film for each shape of the superconducting film. The method for evaluating a superconducting critical current characteristic of a superconducting film according to any one of claims 1 to 3 , wherein the magnetic field strength in the region is smaller than the magnetic field strength immediately below the opening .

The superconducting film is magnetically shielded except for an opening provided in a part of a surface facing the magnetic field generation source, and the opening is formed at each end of the superconducting film for each shape of the superconducting film. The method for evaluating a superconducting critical current characteristic of a superconducting film according to any one of claims 1 to 3 , characterized in that the magnetic field strength in the region is smaller than the magnetic field strength of the opening .

A scale equipped with a superconducting film for evaluating the superconducting critical current characteristics, a refrigerant for cooling the superconducting film mounted on the scale, a container for holding the refrigerant, and a surface of the superconducting film mounted on the scale A magnetic field source held on the straight line perpendicular to the superconducting film so as to be close and separable.
By bringing the magnetic field generation source close to the superconducting film mounted on the scale, the magnetic field strength applied from the magnetic field generation source to the superconducting film is increased, and the superconducting film, the magnetic field generation source, The repulsive force generated between the superconducting film and the magnetic field source is measured according to the distance between the superconducting film where the repulsive force changes from increasing to decreasing. A superconducting critical current characteristic measuring device for a superconducting film, characterized by evaluating the superconducting critical current characteristic of the film .