JP2004212168A

JP2004212168A - Method for evaluating superconductivity property of superconductive film

Info

Publication number: JP2004212168A
Application number: JP2002381028A
Authority: JP
Inventors: Sundaresan Athinarayanan; アシナラヤナン，スンダレサン; Kosuke Tanaka; 康資田中; Shigetoshi Oshima; 重利大嶋; Masanobu Kusunoki; 正暢楠
Original assignee: Japan Science and Technology Agency; National Institute of Advanced Industrial Science and Technology AIST
Current assignee: Japan Science and Technology Agency; National Institute of Advanced Industrial Science and Technology AIST
Priority date: 2002-12-27
Filing date: 2002-12-27
Publication date: 2004-07-29
Anticipated expiration: 2022-12-27
Also published as: JP3704549B2

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for sensitively, conveniently and inexpensively evaluating a superconductive film. <P>SOLUTION: The superconductive film 1 is fixed to a scale 2. A cooling medium 4 is placed in a container 3. The superconductive film 1 is cooled and comes into a superconductive state. While a magnet 5 gradually falls, the scale 2 measures a force applied to the superconductive film 1. A distance Z between the superconductive film 1 and the magnet 5 is measured when a repulsive force changes to an attractive force. When magnetic field strength applied to the superconductive film 1 reaches a lower critical magnetic field H<SB>c1</SB>, a magnetic flux enters into the superconductive film 1. When the magnetic flux enters the superconductive film 1, the attractive force acts on the superconductive film 1 and the magnet 5. The superconductive film with an excellent superconductive property has the small distance Z. <P>COPYRIGHT: (C)2004,JPO&NCIPI

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力が、斥力から引力、または引力から斥力に変化する磁場強度から超伝導膜の超伝導特性を評価する。
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、斥力から引力に変化す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、斥力から引力、または引力から斥力に変化する磁場強度は、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ることを特徴とする。
この方法によれば、簡便、低コスト且つ高感度に斥力から引力、または引力から斥力に変化す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、斥力から引力、または引力から斥力に変化す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、斥力から引力、または引力から斥力に変化する磁場強度をＨ_ａ、超伝導膜の膜圧を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ることを特徴とする。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ることを特徴とする。
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、または磁場発生源に働く力の向きの変化が判別しにくい超伝導膜であっても、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、超伝導膜、または磁場発生源に働く力の向きが変化す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Ｓ_１に対して磁石の磁極断面積Ｓ_２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、１０^６Ａ／ｃｍ^２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、１０^５Ａ／ｃｍ^２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]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for evaluating the superconducting properties of a superconducting film with high sensitivity, simply, and at low cost.
[0002]
[Prior art]
In recent years, devices using a copper oxide-based high-temperature superconducting film have been put into practical use. For example, a base station for mobile communication needs to take in as many subscriber frequency bands as possible in a limited frequency band, and requires a high-performance microwave bandpass filter. Since the characteristics of a 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 having an extremely small resistance component (Non-patented) Reference 1). Also, a superconducting electric wire in which a high-temperature superconducting film is laminated on a tape has been put to practical use (see Non-Patent Document 2).
[0003]
However, as represented by copper oxide-based high-temperature superconducting films, despite the fact that production techniques for superconducting films are being studied very energetically, only good quality superconducting films are produced with good reproducibility. Therefore, it is necessary to inspect all the superconducting films used in the device and to sort the materials in the production of the device.
Therefore, in order to produce a device using a superconducting film with stable quality and at low cost, a simple and low-cost superconducting film evaluation technique is indispensable.
The superconducting properties required for the superconducting film used in the above devices are surface resistance and critical current density.In general, a superconducting film having excellent superconducting properties has low surface resistance and critical current density. There is such a relationship that it 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 method of inspecting the quality of a superconducting film, in March 2002, the international standard for a method for testing the surface resistance of microwaves of superconductors: IEC 61788-7 (see Non-Patent Document 3) was established. This method is called a dielectric resonator method, and as shown in FIG. 10, a dielectric cylinder is sandwiched between two superconducting films to form a resonator, and the resonance characteristics (Q value) are measured. This is to determine the loss due to the conductive film and derive the surface resistance. According to this method, the surface resistance can be measured nondestructively and easily with high accuracy, and is suitable for selecting a superconducting material such as a superconducting membrane filter.
[0005]
However, in this method, since accurate surface resistance cannot be obtained unless the sample size is set to 20 mm × 20 mm, it is necessary to select 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 evaluation is made with a size outside the standard, a large-scale apparatus is required, and the calculation for deriving the surface resistance from the measured values becomes complicated, so that it cannot be easily used at a manufacturing site or the like.
[0006]
A conventional superconducting film quality evaluation method uses an AC susceptometer 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 a 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, a third harmonic appears in the demagnetizing field generated by the superconducting film near the critical magnetic field strength. Since the value of the current flowing through the exciting coil when the third harmonic appears corresponds to the critical magnetic field strength, the superconducting film can be evaluated based on the current value of the exciting coil.
[0007]
However, this method has a limitation in the current value that can be passed through the excitation coil of the AC susceptometer, and thus it is difficult to evaluate a superconducting film having a large critical magnetic field strength, such as 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 increased. However, such a device has a high device cost and is easily used at a production site. I can't.
[0008]
A conventional method for evaluating a superconducting film includes 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 caused by a laser beam applied to the tape. (See Non-Patent Document 4). According to this method, the critical current density distribution of the superconducting film can be evaluated by scanning the tape with the laser beam diameter reduced.
However, this method is a destructive inspection in which it is indispensable to process into a tape shape or to connect electrodes to both ends of the tape, and therefore cannot be used for applications such as material selection at a production site.
[0009]
[Problems to be solved by the invention]
As described above, with the practical use of a device using a superconducting film, the evaluation method of the superconducting film has become important. However, as described above, in the conventional method, the superconducting film having a large area has a large area. There is a problem that it cannot be applied to superconducting films with large critical magnetic field strength, or it is a destructive inspection, and there is no method for evaluating superconducting films that can be used easily and at low cost at production sites. .
In view of the above problems, an object of the present invention is to provide a highly sensitive, simple, and low-cost method for evaluating a superconducting film.
[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. Abraimov, A .; G. FIG. Sivakov, A .; V. Lukasenko, M .; V. Fisutul, P .; Muller and A. V. Usteinov, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 11, MARCH 20 (2001), p3170-3173
[Non-Patent Document 6]
Yasunori Mawarari, Hirofumi Yamasaki, and Yoshishiko Nakagawa APPLIED Physics Letters Vol. 81, No. 13, pp2424-2425 (1999)
[Non-Patent Document 7]
J. H. Cluassen, M .; E. FIG. Reeves. and R. J. Soulen, Jr. Rev., Rev .. Sci. Instrum, Vol. 62, p996 (1991)
[0011]
[Means for Solving the Problems]
In order to achieve the above object, a method for evaluating a superconducting film of the present invention comprises applying a magnetic field from a magnetic field source to the superconducting film, changing a magnetic field intensity applied to the superconducting film, and The superconducting property of the superconducting film is evaluated based on the magnetic field strength at which the force acting between the superconducting film and the magnetic field is changed from repulsive force to attractive force or from attractive force to repulsive force.
According to this method, if the magnetic field intensity applied to the superconducting film is less than a certain value depending on the superconducting characteristics of the superconducting film, repulsive forces act due to the magnetic shielding effect and the Meissner effect. At a specific magnetic field strength depending on the superconducting characteristics, magnetic flux enters the superconducting film, and a magnetic flux pinning phenomenon occurs. When the magnetic flux enters, an attractive force is generated between the magnetic field source and the superconducting film. The magnetic field strength at which the magnetic flux enters and the magnetic flux pinning phenomenon occurs differs depending on the quality of the superconducting characteristics of the superconducting film. That is, in the case of a superconducting film having a large critical magnetic field intensity, the magnetic flux starts to enter at a large magnetic field intensity, and in the case of a superconducting film having a small critical magnetic field intensity, the magnetic flux starts to enter at a small magnetic field intensity. In this way, the superconducting properties of the superconducting film can be evaluated from the magnetic field strength that changes from the repulsive force to the attractive force. Further, in the second-class superconductor, since the entry of the magnetic flux occurs at 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]
Also, 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 strength of the magnetic field on the superconducting film, and the superconducting edge effect based on the magnetic field compression at the film end of the superconducting film. (See Non-Patent Document 7) does not occur.
According to this method, the effect that magnetic flux enters from the superconducting film edge due to the high magnetic field due to the magnetic field compression of the superconducting film edge, the so-called superconducting edge effect does not occur, so that the superconducting characteristics of the measured superconducting film can be accurately determined Can be evaluated.
In addition, the method of setting the magnetic field intensity at the film edge of the superconducting film to be equal to or less than the maximum intensity of the magnetic field on the superconducting film is based on the ratio of the magnetic pole cross-sectional area to the area of the superconducting film. Can be achieved by reducing the magnetic field strength at the time to be not more than the maximum strength of the magnetic field on the superconducting film.
Further, the magnetic field generation source may be magnetically shielded so that the magnetic field is applied only to the measured portion on the surface of the superconducting film. Further, the superconducting film may be magnetically shielded except for a 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]
In addition, the magnetic field intensity that changes from repulsive force to attractive force or from attractive force to repulsive force is measured by measuring the force acting on the superconducting film or the magnetic field source by fixing the superconducting film or the magnetic field source on a balance, The measurement value is obtained from the magnetic field strength at which the sign of the rate of change with respect to the magnetic field strength changes.
According to this method, it is possible to easily, at low cost and with high sensitivity, obtain the magnetic field strength that changes from repulsive force to attractive force or from attractive force to repulsive force. Further, the strength of the magnetic field on the surface of the superconducting thin film can be easily known from the configuration of the magnetic field source by a mirror image method or the like in electromagnetics.
[0014]
Furthermore, the superconducting properties of voted magnetic field strength that varies attraction from repulsive or from attraction to repulsion is superconducting critical current density, magnetic field strength that varies attraction from repulsive or from attraction to repulsion H _{a superconducting} film the film thickness d, and the superconducting critical current density of the superconducting film as _{J c,} the superconducting critical current density _{J c,} known equation (see non-Patent Document _{_{6) J c = 2H a /}} d, from It is characterized by seeking.
According to this method, the superconducting critical current density can be evaluated extremely easily.
[0015]
Further, the method of changing the magnetic field strength is characterized in that the magnetic field generated by the magnetic field source is kept constant and the distance between the superconducting film and the magnetic field source is changed. Alternatively, the distance between the superconducting film and the magnetic field generation source may be kept constant, and the magnetic field generated by the magnetic field generation source may be changed.
[0016]
Further, the magnetic field source is a permanent magnet arranged perpendicular to the surface of the superconducting film or a magnet arranged 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]
Also, in the present invention, when evaluating the average superconducting properties of the superconducting film, the distribution of the superconducting properties of the superconducting film is evaluated using a magnetic field source having a large applied magnetic field applied to the superconducting film. In this case, a magnetic field source having a small applied magnetic field area is used.
According to this method, when a magnetic field source having a large applied magnetic field area is used, the superconducting film has an average superconducting characteristic, and is suitable for a device in which the average superconducting characteristic is important. Further, if a magnetic field source having a small applied magnetic field area is used, the distribution of superconducting properties of the superconducting film can be evaluated.
[0018]
Further, according to the present invention, a paramagnetic substance can be disposed on the back surface of the superconducting film, and the superconducting film can be evaluated for superconducting properties using any one of the above-described methods of the present invention.
According to this method, even if a superconducting film or a superconducting film in which a change in the direction of a force acting on a magnetic field source is difficult to determine, a magnetic field penetrates into the superconducting film, and the magnetic field is applied to the paramagnetic film. Is applied and a force also acts on the paramagnetic material, so that it is possible to accurately evaluate the magnetic field strength at which the direction of the force acting on the superconducting film or the magnetic field generation source changes.
[0019]
In the method for evaluating the superconducting properties of a superconducting film according to 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, it can be used for quality control of a metal sheath superconducting wire or a tape-shaped superconducting wire.
[0020]
BEST MODE FOR CARRYING OUT 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 device 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, a superconducting film 1 is fixed on a scale 2, and the scale 2 is fixed on a bottom surface of a container 3, and the container 3 contains a refrigerant 4 such as liquid nitrogen for cooling the superconducting film 1. . Immediately above the superconducting film 1, a magnet 5, which is a magnetic field source, is arranged so as to be movable in the vertical direction. The magnet 5 is slidably connected to a scale 7 via an arm 6, and measures the distance between the superconducting film 1 and the magnet 5 with the scale 7. The balance 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. 2A shows an example in which an elongated permanent magnet is used. FIG. 2B shows an example in which a permanent magnet 5 is attached to the tip of the support rod 8, and FIG. 2C shows a permanent magnet that is perpendicular to the tip of the support rod 8 in the axial direction of the support rod 8. An example with a magnet is shown. This magnet 5 may be an electromagnet.
[0022]
Hereinafter, a method for evaluating a superconducting film of the present invention will be described with reference to FIG.
First, the superconducting film 1 is fixed to the scale 2, the refrigerant 4 is put in the container 3, and the superconducting film 1 is cooled to 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 scale 2, and the distance Z between the superconducting film 1 and the magnet 5, which changes from repulsive force to attractive force, is measured.
[0023]
FIG. 3 is a diagram illustrating 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 balance, with the positive area indicating repulsion and the negative area indicating attractive force. The horizontal axis is the distance Z. The solid line, the dotted line, and the one-dot chain line in the figure are the characteristics of the superconducting films a1, a2, and a3 having different superconducting characteristics, respectively, and the superconducting characteristics decrease in the order of a1>a2> a3. As shown by the arrow in the figure, the distance at which the force F changes from increasing to decreasing in the direction in which the distance Z decreases is defined as the distance Za at which repulsive force changes to attractive force. As shown in the figure, the distance from the repulsive force to the attractive force is Za1 <Za2 <Za3, and the higher the superconductivity, the shorter the distance Za from the repulsive force to the attractive force. Whether the superconductivity is good or not is determined based on the distance Za at which the repulsive force changes to the attractive force.
[0024]
FIG. 4 is a diagram showing the correlation between the distance Za at which the repulsive force changes to the attractive force and the superconducting critical current density Jc. As shown in the drawing, the smaller the distance Za, the larger the critical current density Jc.
[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 indicates the magnitude of the demagnetization of the second type superconductor, and the horizontal axis indicates the intensity of the applied magnetic field.
H _c1 is called the lower critical magnetic field, and H _c2 is called the upper critical magnetic field. In the second type superconductor begins to flux enters the magnetic field strength H _c1, it increased flux gradually enters, the superconducting state is lost at the H _c2. A superconducting state and a normal conducting state coexist between the magnetic field strengths _Hc1 and _Hc2 . The type 2 superconductor in which the magnetic flux pinning center is introduced is a superconductor having an extremely large critical magnetic field strength _Hc2 .
[0026]
FIG. 6 is a diagram showing the principle of the evaluation method of the present invention. FIG. 6A shows a case where a repulsive force is generated, and FIG. 6B shows a case where an 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 intensity applied to the superconducting film 1 is less than the lower critical magnetic field _Hc1 , 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 to reduce the magnetic field energy.
As shown in FIG. 6 (b), when the distance Z between the magnet 5 and the superconducting film 1 is small, and the magnetic field intensity applied to the superconducting film 1 reaches the lower critical magnetic field _Hc1 , the magnetic flux is increased. 1 and fixed at the flux pinning center. As described above, when the magnetic flux B can enter the superconducting film 1, an attractive force acts between the superconducting film 1 and the magnet 5 to reduce the magnetic field energy.
[0027]
In general, a superconducting film having excellent superconducting properties in a type 2 superconductor has a relationship that the surface resistance is small, the critical current density is large, and the critical magnetic field strengths _Hc1 and _Hc2 are large. Therefore, the distance measured by the method of the present invention directly corresponds to the lower critical magnetic field strength _Hc1, but also corresponds to the surface resistance, the critical current density, and the critical magnetic field strength _Hc2 . According to the method, the quality of the superconductivity can be determined.
[0028]
Further, according to the magnetic field applying method of the present invention, the so-called superconducting edge effect (see Non-Patent Document 7) does not occur at the end of the superconducting film.
FIG. 7 is a diagram showing a magnetic field application method used in the present invention.
7 (a) is sufficiently small pole sectional area S ₂ of the magnet relative to the area S ₁ of superconducting films, the magnetic field intensity based on the magnetic field compression in film end 10 of the superconducting film on the superconductor film An example in which the intensity is equal to or less than the maximum intensity of the magnetic field is shown.
FIG. 7B shows an example in which a 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 magnetic shield 9 is provided by covering the superconducting film and the substrate 11 with a magnetic shielding material so that the magnetic field strength at the film end 10 of the superconducting film can be ignored.
[0029]
As described above, when no magnetic flux enters the superconducting film, the superconducting film is completely diamagnetic due to the magnetic shielding effect and the Meissner effect, and no magnetic field exists below the superconducting film. Therefore, the magnetic field immediately below the magnet on the surface of the superconducting film in this case can be easily obtained by using the mirror image method in electromagnetism. On the other hand, the strength of the magnetic field at the end of the superconducting film increases due to the so-called magnetic field compression effect, but the strength varies depending on the shape of the magnetic film to be measured, and it is not easy to find 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 neglected. 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 cancel 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]
Further, as shown in FIG. 5, the present invention method since it evaluated at a field strength of about the lower critical field H _c1 field strength much smaller than the upper critical magnetic field strength H _c2, the magnets are required The magnetic field strength may be small, and it is possible to cope with 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 the superconducting characteristics can be measured by scanning and measuring the superconducting film. If magnets having different magnetic pole cross-sectional areas are used depending on the application, it can be used simply and at low cost for various applications.
Further, when evaluating the surface resistance, since the surface resistance is affected not only by the superconducting properties but also by the topology of the thin film surface, in addition to the evaluation by the method of the present invention, a microscopic observation of the thin film surface is performed. If the presence or absence of granular projections or needle-like projections is confirmed, very good material sorting can be performed.
In the above description, evaluation of the 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 balance can be measured. For example, when used for a metal-sheathed superconducting wire or a tape-shaped superconducting wire, quality inspection can be performed at a very low cost and easily.
[0032]
Next, a method for evaluating the superconducting characteristics 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, it is difficult to accurately evaluate the measurement by the method shown in FIG. 1 because the magnetic field wrapping around the superconducting film from the magnet 5 exerts a force on the substrate. Become. The method of the present invention in such a case will be described.
When the substrate 11 is a magnetic material, the measurement is performed in the same manner as in FIG. 1 using the magnet 5 with the magnetic shield 9 illustrated in FIG. 7B, or as illustrated in FIG. If the superconducting film 1 and the substrate 11 are magnetically shielded 9 except for the portion to be measured and measured in the same manner as in FIG. 1, the magnetic flux flowing around the magnetic material 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 characteristics of the superconducting film when the substrate is a magnetic material. FIG. Similar to FIG. 3, the solid line, the dotted line, and the dashed line are the characteristics of the superconducting films a1, a2, and a3 having different superconducting characteristics, respectively, and the superconducting characteristics decrease in the order of a1>a2> a3. Accordingly, the distance from the repulsive force to the attractive force corresponding thereto is Za1 <Za2 <Za3. The difference from FIG. 3 is that the attractive force increases with a steep gradient after changing from the repulsive force to the attractive force, because the magnetic flux penetrates the superconducting film and attracts the magnetic substrate.
In addition, by utilizing the above effect, by arranging 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 a magnetic field enters the superconducting film, Since a magnetic field is applied to the paramagnetic material and a force also acts on the paramagnetic material, a change in the direction of the force acting on the superconducting film can be clearly determined, and the magnetic field strength can be accurately evaluated.
[0034]
In the above description, the case where the repulsive force changes to the attractive force has been described as an example. However, the case where the attractive force changes to the repulsive force is equivalent to the case where the repulsive force changes to the attractive force, so the description is omitted. Also, taking the case where the magnetic field source is a magnet as an example, the case where the change in the external magnetic field strength on the surface of the superconducting film is performed by changing the relative distance between the magnet and the superconducting film has been described, but the magnet is an electromagnet, and the electromagnet is used. It is apparent that the same effect can be obtained even when the external magnetic field strength is changed by controlling the current flowing through the coil of the electromagnet while fixing the coil.
[0035]
Next, examples will be described.
A sample in which the thickness of the superconducting thin film (Tl-1223) was 500 nm, the substrate area was 20 mm x 20 mm, and the total weight of the superconducting thin film and the substrate was 600 mg was used for evaluation. The magnet was produced by stacking permanent magnets (Nd-Fe-B) having a magnetic pole sectional area of 8 mm x 8 mm in multiple stages. The magnetic field strength on the surface of this magnet is 0.5 T (tesla). The magnet was brought close to the sample cooled to the temperature of liquid nitrogen, and material selection was performed based on whether or not the magnet attracted the magnet.
FIG. 9 is a photograph showing a state where the superconducting thin film of the sample is attracted to the magnet. As shown in the figure, there were a sample that was attracted to the magnet and could be lifted, and a sample 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 to 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 Was 10 ⁵ A / cm ² . Further, the surface resistance at 77 K and 38 GHz was 5 mΩ or less for the sample that could be lifted, but was about 50 mΩ for the sample that could not be lifted. Note that the surface topologies were comparable.
This example does not use a balance to detect a magnetic field that changes from repulsive to attractive, 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 can be understood from the above description, the method of the present invention detects 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, and evaluates the superconducting characteristics. Therefore, the magnetic field strength of the magnet may be small, and the sensitivity is extremely high. Further, since the mechanical force is used, the cost is simple and the cost is low. Therefore, if the evaluation method of the present invention is used for quality control of a superconducting thin film filter or a superconducting electric wire, it is extremely useful.
[Brief description of the drawings]
FIG. 1 is a view showing a schematic apparatus for explaining a method for evaluating a superconducting film of the present invention.
FIG. 2 is a view showing various configurations of a magnet used in the method for evaluating a superconducting film of the present invention.
FIG. 3 is a diagram illustrating a relationship between a force acting between a superconducting film and a magnet and a distance between the superconducting film and a magnet.
FIG. 4 is a diagram illustrating a correlation between a critical current density and a distance from a repulsive force to an attractive force.
FIG. 5 is a diagram showing the demagnetization characteristics of a type 2 superconductor.
FIG. 6 is a diagram showing 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 the relationship between the force acting between the superconducting film and the magnet when the substrate is a magnetic material and the distance between the superconducting film and the magnet.
FIG. 9 is a photograph showing a state in which a superconducting thin film of a sample is attracted to a magnet.
FIG. 10 is a diagram illustrating a conventional dielectric resonator method for evaluating the surface resistance of a superconducting film.
FIG. 11 is a diagram illustrating a conventional method using an AC susceptometer for evaluating the critical magnetic field strength of a superconducting film.
FIG. 12 is an explanatory view of a conventional laser heating method for evaluating a critical current density of a superconducting film.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Superconducting film 2 Scale 2a Pointing needle 3 Container 4 Refrigerant 5 Magnet 6 Arm 7 Scale 8 Support rod 9 Magnetic shield 10 Superconducting film end 11 Substrate

Claims

Applying a magnetic field from a magnetic field source to the superconducting film, changing the strength of the magnetic field applied to the superconducting film, the force acting between the superconducting film and the magnetic field source is attractive from repulsion, or A method for evaluating superconducting properties of a superconducting film, wherein the superconducting properties of the superconducting film are evaluated from the magnetic field intensity that changes from attractive force to repulsive force.

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 or equal to the maximum strength of the magnetic field on the superconducting film, and the superconducting edge based on the magnetic field compression at the film end of the superconducting film. 2. The method according to claim 1, wherein no effect is produced.

The method of reducing the magnetic field intensity at the film edge of the superconducting film to the maximum intensity of the magnetic field on the superconducting film is as follows: the ratio of the magnetic pole cross-sectional area of the magnetic field source to the area of the superconducting film, 3. The method for evaluating superconducting properties of a superconducting film according to claim 2, wherein the magnetic field intensity at the film edge is reduced so as to be equal to or less than the maximum intensity of the magnetic field on the superconducting film.

The method of setting the magnetic field strength at the film edge of the superconducting film to be not more than the maximum strength of the magnetic field on the superconducting film is such that the magnetic field generating source is magnetized so that the magnetic field is applied only to the measured portion on the superconducting film surface. The method for evaluating superconductivity of a superconducting film according to claim 2, wherein the superconducting film is shielded.

The method of reducing the magnetic field strength at the end of the superconducting film to the maximum intensity of the magnetic field on the superconducting film is characterized in that the superconducting film is magnetically shielded except for a portion to be measured on the superconducting film surface. A method for evaluating superconducting properties of a superconducting film according to claim 2.

The magnetic field strength changing from the repulsive force to the attractive force or from the attractive force to the repulsive force, the superconducting film, or the magnetic field source is fixed on a balance, the force acting on the superconducting film, or the magnetic field source by the balance. 2. The method for evaluating superconducting properties of a superconducting film according to claim 1, wherein the superconducting properties of the superconducting film are measured from the magnetic field strength at which the sign of the rate of change of the measured value with respect to the magnetic field strength changes.

Superconductivity which voted magnetic field strength changes repulsive from attraction or attraction from the repulsive force is superconducting critical current density, magnetic field strength of the changing H _a, the film thickness of the superconducting film d, and the the superconducting critical current density of the superconducting film as J _c, the superconducting critical current density J _c, and obtaining from the relationship J c ₌ 2H a / _d, of the superconducting film according to claim 1 Superconductivity evaluation method.

The magnetic field intensity is changed by keeping the magnetic field generated by the magnetic field source constant and changing the distance between the superconducting film and the magnetic field source, wherein the magnetic field intensity is changed. A method for evaluating superconducting properties of a superconducting film according to any one of the above.

The magnetic field intensity is changed by maintaining a constant distance between the superconducting film and the magnetic field source and changing a magnetic field generated by the magnetic field source. A method for evaluating superconducting properties of a superconducting film according to any one of the above.

The superconducting film according to claim 8, wherein the magnetic field generating source is a permanent magnet arranged perpendicular to the surface of the superconducting film, or a magnet arranged parallel to the surface of the superconducting film. Of superconducting properties of

The superconducting film according to claim 9, wherein the magnetic field source is an electromagnet arranged perpendicular to the surface of the superconducting film, or an electromagnet arranged parallel to the surface of the superconducting film. Evaluation method of superconductivity.

When evaluating the average superconducting properties of the superconducting film, when the applied magnetic field area applied to the superconducting film is used to evaluate the distribution of superconducting properties of the superconducting film using the large magnetic field source. The method for evaluating superconducting properties of a superconducting film according to any one of claims 1 to 11, wherein the magnetic field generating source having a small applied magnetic field area is used.

Placing a paramagnetic material on the back surface of the superconducting film, characterized by evaluating the superconducting properties of the superconducting film using any one of the methods according to claim 1 to 12, Evaluation method of superconductivity.

The method for evaluating superconducting properties of a superconducting film according to any one of claims 1 to 13 is applied to the evaluation of superconducting properties of a metal-sheathed superconducting wire or a tape-shaped superconducting wire. Evaluation method of superconductivity of superconducting film.