J c *, * * Characterization of Local J c Distribution in Superconducting Wires and Tapes Based on Scanning Hall-probe Microscop Kohei HIGASHIKAWA *,, Masaoshi INOUE * and Taanobu KISS * Snopsis: This article reviews a method for characterizing the local critical current densit (J c ) distribution in superconducting tapes and wires based on scanning Hall-probe microscop (SHPM). This method is ver powerful for (1) finding the bottlenec that limits the global performance of a superconductor, () investigating the local inhomogeneit that ma become the origin of local quench, especiall in HTS applications, () establishing the processes for narrow and/or multifilamentar HTS conductors for reducing the magnetization in magnet applications and the AC losses in power applications. The principle and the functions of this technique are introduced b referring to characterization eamples of REBCO coated conductors and an MgB wire. Kewords: scanning Hall-probe microscop, critical current densit distribution, local homogeneit, HTS tape, MgB wire J c J c 1 μv/cm I c SQUID J c Received Jul, 14 * 819-95 744 Kushu Universit, Graduate School of ISEE 744 Motooa, Nishi-u, Fuuoa 819-95, Japan E-mail: ohei@super.ees.ushu-u.ac.jp DOI : 1.1/jcsj.49.485 J c J c J c cmμm RE-1 J c 49 9 14 485
J c SHPM J c Fig. 1 J c J c 1) J c J c Current flow Magnetic field Fig. 1 Schematic diagrams of current flows in shielding state and in remanent state. These currents flow at critical current densit according to the critical state model. z J r (,,) J B z r(,,z) Fig. Illustration of the relationship between the perpendicular component of the magnetic field, B z, measured at a position above a film, r(,, z), and the sheet current densit vector, J(J, J ), at a position in the film, r (,, ). J c Biot-Savart ( r ) ( r r ) J d B() r = μ d r (1) 4π r r r r B J d μ Fig. d (1) ) B z (,, z) (, )( ) ( ) J ( ) μ + + J, () = d d 4π ( ) + ( ) + z ) (,, z) r (,, z ) r B z B (J, J ) J=J d d () (, ) () ) μ z () b ( ) = ( ) ( ) z,, z i e j, j, b z B z j, J, i j j 1 J = (, ) i j (, ) = i j (4) ()(4)j j ) z i e (, ) = b ( ) j z, μ z i e (, ) = b ( ) j z, μ (5) (6) z b z 486 TEION KOGAKUJ. Cro. Super. Soc. Jpn.Vol. 49 No. 914
1 1 Be = 15 mt - 1 1 ࡇࡇ ὀពࡍ ࡁⅬ ࠊ㧗 㛫 Ἴ ᡂศ ቑᖜ ࡀ ࡁ ࡇ ࠊ ィ 㞧㡢 ቑᖜࡉ ࡋ ࡇ ࠋᚑࡗ ࠊ㞧㡢 ᮶ 㛫㧗ㄪἼᡂศ ษ ᤞ ᚲせࡀ ࠊලయ ḟᘧ ࡍ ࢫ ࢱ 㐺 ࡍ ࡇ ࠋ ) π 1 + cos cut -off ¹ w( ) = for cut -off ࠉ for > cut -off - Measured magnetic field Bz (T) (7) (8) Be = 9 mt Be = 75 mt - 8 1 Be = 6 mt - 6 1 Be = 45 mt 4 1- Be = mt Be = 18 mt - 1 Be = 9 mt Be = mt Be = mt - - 1 after 1 mt -8-6 -4-4 ࡓࡔࡋࠊcut-off Ἴ ࠊࡇ ᑐᛂࡍ ࠋ ᚋ ࠊ ኚ ࡇ ࠊᐇ 㛫 㠃ෆ㟁ὶᐦᗘ ࢡ J(J, J)ࡀồ ࠋ ௨ ࠊ ᙧ ヨ ホ౯ ࡀࠊཌࡉࡀ ࡁ ࡃࠊཌࡉ᪉ 㟁ὶศᕸ ホ౯ࡍ ᚲせࡀ ሙ ࠊ ᆶ ᡂศ ࡃࠊᖹ ᡂศ ィ ࡍ ᚲせ - 1 1 Be = 15 mt - Be = 9 mt Be = 75 mt - 8 1 Be = 6 mt - 6 1 Be = 45 mt 4 1- Be = mt Be = 18 mt - Be = 9 mt 1 Be = mt B = mt e - - 1 after 1 mt ࡀ ࠋ ࡓࠊホ౯ 㛫 ᛶయࡀ ሙ ࠊゎ -8 ᯒ ゎࡃࡇ ᅔ㞴 ࠊ ゎᯒ ウ ࠋ ௨ ཎ ࠊ ᰝᆺ Ꮚ㢧ᚤ㙾㸦SHPM㸧 ࡗ ሗࡀᚓ ࠊ ࡎ REBCO ࡓホ౯ ࡍ ࠋFig. ᐃ ࡓ SHPM ᴫ ᅗ ࡍࠋᾮయ ࢱ ࢡ እ㒊 ༳ຍ Bi- ࢥ ࡀタ ࡉ ࠊヨ ࡑ ෆ タ ࡉ ࠋ ࠊࢥ ヨ ᪉ ᾮయ ࢱ ࢡ ఏᑟ ࡗ ෭ ࡉ ࠊ ᆺ ヨ ᗘ 79 K ࠋ ࡓࠊ1 mt እ㒊 ༳ຍࡍ ࡇ ࡀ ࡗ ࠋヨ ࢡ ࡁࡉࡀ 5 ࢡ ゅ ࢭ ࢧࡀ ࠊ ᐦ ࢫ ࢪ ࡗ ᰝࡍ ࡇ ࡀ ࡁ ࠋ ࠊィ ヨ ࢧ ࢬ 8 mm mm ᗘ ࠊヨ 5-6 -4-4 6 8 (mm) 6 1 Sheet current densit J (A/m) ᐃ᪉ἲ 8 1 1 Ἴ㛗λcut-off ࡀᮏホ౯ᡭἲ 㛫ศゎ Ỵᐃࡍ 5( ᑻヨ ࡓホ౯ 6 (mm) In-plane magnetic field Bz (T) bz (, ) = bz (,,) = e z bz (,, z ) Be = 15 mt 4 1 Be = 9 mt Be = 75 mt 1 Be = 6 mt Be = 45 mt Be = mt Be = 18 mt - 1 Be = 9 mt Be = mt -4 1 Be = mt after 1 mt -6 1-8 -6-4 - 4 6 8 (mm) Fig. 4 Eperimental results for a 1-mm-wide short sample of IBAD-PLD GdBCO coated conductor eposed to different conditions of eternal magnetic fields: lateral distributions of measured magnetic field, magnetic field in the plane, and longitudinal component of sheet current densit, b onedimensional scanning ). A 㟁ὶ 㟁 ࡁ 㟁ὶ ഛ ࠋ እ㒊 ༳ຍ 㟁 ᮏホ౯ ࠊIBAD-PLD ἲ ࡗ స〇ࡉ ࡓ Eternal magnetic field: < 1 mt Tpical stage temperature: 79 K Active area of Hall sensor: 5 μm square Spatial step: 1 μm in,,.5 μm in z Current leads GdBCO ᑻヨ 㸦ᖜ 1 mmࠊ㛗ࡉ mm㸧 ホ౯ ࡗࡓࠋ ఏᑟ ෭ ࡋࡓヨ ᑐࡋ ࠊእ㒊 Hall sensor Observation window Bi- coil with Cu bobbin ༳ຍࡋࡓ㝿 ホ౯ ᯝ Fig. 4 ࡍࠋ ᅗ 1 ḟ Sample AlN plate z ඖ ᰝ ࡗ ᚓ ࡓヨ ᖜ᪉ ศᕸ ࡋ ࡀࠊእ㒊 ቑຍࡉࡏ ࡋࡓࡀࡗ ࠊヨ ෆ ࡀ ෆ㒊 ධࡋ ࡃᵝᏊࡀ ど ࡉ ࠋFig. 4 ヨ 17 μm ࡍ ࢭ ࢧ ᐃࡋࡓ ศ LN outlet ᕸ ࡋ ࡀࠊᘧ(7) ࡗ Fig. 4 ࡍ ヨ LN tan LN inlet 㠃ෆ ࡍ ࡇ ࠊࡑ ᵝᏊࡀ Scanning Hall-probe microscop ) Fig. Photograph and schematic diagram of a SHPM sstem. ప ᕤᏛ 49 ᕳ 9 14 ᖺ ࠋ ࡓࠊFig. 4 ࠊࡑ ศᕸ ᑐᛂࡍ ࢩ 487
J c 6 mt J c 6 mt 1 mt Fig. 4 J c J c J c Fig. 5 Fig. 51 Farada Fig. 6 J c 1-9 V/m 1-4 V/m J c Fig. 7 Fig. 7 Fig. 7 Fig. 7(d)(e) Fig. 7(e) J c Fig. 7 J c Fig. 7(e) J c Fig. 7(f) mt Fig. 7(g) J c J c J c Measured magnetic field B z (T) Measured magnetic field B z (T) 6 1-4 1-1 - - 1-5. 1-4.8 1-4.6 1-4.4 1-4. 1-1st scan nd scan -8-6 -4-4 6 8 1st scan nd scan blown up to (mm) -.6 -.4 -... (mm) Fig. 5 Magnetic relaation at the remanent state. The time interval between the 1 st and nd scans was s 4). is a blow up of a region shown in 4). Electric field E (V/m) 1-9 1-9 1 1-9 -1 1-9 - 1-9 - 1-9 -8-6 -4-4 6 8 (mm) Fig. 6 Electric field induced b the magnetic relaation 4). 488 TEION KOGAKUJ. Cro. Super. Soc. Jpn.Vol. 49 No. 914
Measured magnetic field B z (mt) Sheet current densit J (A/mm) Sheet current densit J (A/mm) Sheet current densit J (A/mm) 5 15-5 -5 5-5 5 5 B z 6 J J (d) J (e) Spatial inhomogeneit & J c (B) In-plane magnetic field B z (mt) Critical sheet current densit J c ( mt) (A/mm) 15-1 mm 4 1 B z (f) J c (g) Normalized sheet current densit J (A/m).. 1..9.8.7.6.5.4 = 4.5 mm =.5 mm =.5 mm = 1.5 mm =.5 mm = -.5 mm = -1.5 mm = -.5 mm = -.5 mm = -4.5 mm J c (B). 1-1 - 1-1 In-plane magnetic field B z (T) Spatial inhomogeneit Fig. 7 Measurement results for a piece of 1-mm-wide GdBCO coated conductor fabricated using IBAD-PLD processes and visualized b scanning Hall-probe microscop: measured magnetic field at the Hall sensor perpendicular to the sample, B z, magnetic field estimated at the superconducting laer, B z, longitudinal component of sheet current densit, J, (d) width component of sheet current densit, J, (e) absolute value of sheet current densit, J, (f) sheet current densit normalized at B z = mt, J n, as a function of B z obtained for 1 different positions on the arrow in (e), and (g) critical sheet current densit, J c, estimated at B z = mt. The arrow in indicates the location for the magnetic relaation measurement, 5). Sheet current densit J Sheet current densit J =. w =.1 w =. w =. w =.4 w =.5 w =. w =.1 w =. w =. w =.4 w =.5 w Fig. 8 Lateral distributions of sheet current densities and their absolute values in remanent states, where I c and w are the critical current and the width of a coated conductor, respectivel 6). The positions where J =.5 I c / w matches with the edges of the conductor independent of the value of cut-off wavelength,, of the distributions 6). The equivalent width, w e, can be determined b the distance between such positions. Total sheet current Cut-off wavelength Fig. 9 Relationship between the total sheet current, I, (the area of the lateral distributions shown in Fig. 8) and their cut-off wavelength,, where I c and w are the critical current and the width of a coated conductor, respectivel 6). J c Fig. 8 Fig. 8 J c I c 49 9 14 489
w Laser slit (copper coating) Mechanical slit Fig. 9 I c w I = J d < w w c for λcut-off.6λ cut-off (9) Fig. 8 I c w w e J c I c Fig. 1 5 mm 5 Fig. 1 Fig. 1 I c Critical sheet current densit J c,mt (A/mm) Critical current I (A) c,mt Equivalent width w (mm) e 8. 6. 4... 1..8.6.4.. 1 5 J c 1 mm 15 5 5 4 45 5 Longitudinal position (mm) 15 5 5 4 45 5 Longitudinal position (mm) filament_1-5 filament_1-4 filament_1- filament_1- filament_1-1 filament_1-5 filament_1-4 filament_1- filament_1- filament_1-1 Fig. 1 Eperimental results for 5-mm-wide 5-filamentar coated conductor: critical current densit distributions obtained for two pieces, local critical current for each filament and equivalent width for each filament in the upper piece 7). Side of stabilizer Side of substrate Equivalent width w e (mm). 1.5 1..5. Irradiated from substrate Irradiated from substrate Irradiated from stabilizer SC laer bent to substrate SC laer bent to stabilizer 1 (d) 4 (e) 5 (f) 6 Sample Fig. 11 Difference in the equivalent width for the samples slit b different cutting methods 6). Micrographs of the cross-section near the edge of the samples are also shown in the figure. The micrographs for samples - were taen after copper coating that was done after slitting. The error bar in the equivalent width indicates the standard deviation for each sample estimated from the longitudinal distribution 6). Fig. 11 mm J c I c RTR-SHPMFig. 1 1 m mm 1 1 6 m/h 4 1 mm 8) 49 TEION KOGAKUJ. Cro. Super. Soc. Jpn.Vol. 49 No. 914
Fig. 1 ホ౯ ࡍࠋホ౯ᑐ ࡋࡓヨ ࠊIBAD- PLD ᇶᯈ స〇ࡉ ࡓ GdBCO ࠊ MOD 㻴㼕㼓㼔㻙㼟㼜㼑㼑㼐 㼟㼠㼍㼓㼑㻌㼒㼛㼞 㼟㼏㼍㼚㼚㼕㼚㼓㻌㼍 㻴㼍㼘㼘㻌㼜㼞㼛㼎㼑 PLD ヨస PLD ᯝ ࡋ ࠋᒁᡤ Ic ศᕸ ࡘࡁ 㡰 ᑠࡉ ࡇ ࡀ ࡗ ࡀࠊࡇ せᅉ ど ࡍ ࡇ ᡂ ࡋ ࠋ ࠊ.5 m ᮇ mm ᮇ ᖜ᪉ ➃㒊 ᛶࡀప ࡋ ࠊࡇ ࡀ グ ࡘ 㻾㼑㼑㼘㻙㼠㼛㻙㼞㼑㼑㼘 㼍㼜㼜㼍㼞㼍㼠㼡㼟㻌㼒㼛㼞 㼏㼍㼞㼞㼥㼕㼚㼓 㼏㼛㼍㼠㼑㼐 㼏㼛㼚㼐㼡㼏㼠㼛㼞 ࡁ ཎᅉ ࡗ ࠋ ࠊ15 mm ᮇ ศ ᮇ ᛶప 㒊ࡀぢ ࠊࡑ ࡀせᅉ ࡇ ࡀ ࡗࡓࠋ ᪉ࠊ 㛵ࡋ ࠊ㠃ෆ 㧗ᆒ ᛶࡀᚓ ࡇ ࡗࡓࠋ ࡇ ࠊ༢ Ic ศᕸ ࡃࠊࡑ ཎᅉ 㠃ෆ ᆒ ᛶ ࡗ ㄽࡍ ࡇ ࡀ ࠋ ࡓࠊࡇ ḟඖ 㠃ෆᆒ ᛶ ሗ ࠊ ᚋ ࡅࡓ㧗ᆒ స〇ࠊ ࡑ ຍᕤᚋ ရ ࡁ ጾຊ ࡍ ࠋ Hall-probe scanning in width direction Conductor carring in longitudinal direction Fig. 1 Reel-to-reel scanning Hall-probe microscop (RTRSHPM) sstem: photograph and scanning method 9). 4 5 mm 1m 66 5 mm 1m 6 5 mm Sheet current densit 5 mm J (A/mm) 1 mm 4 1m Fig. 1 In-plane distribution of local critical current densit in 1-m-long pieces of coated conductors (CCs): MOD CC, commercial PLD CC, and R&D PLD CC 1). ప ఏᑟ ホ౯ ᒎ㛤 ௨ ウ ࠊᾮయ ᗘ ᑐ ࡋࡓ ࡀࠊ MgB 11, 1) ᚋ ᒎ㛤ࡀᮇᚅࡉ 㕲 ఏᑟయ 1) ࡉ ࡇ ㄆࡋ ࠋᮏ ሙ ࠊᒁᡤ ࡑ ಸ ᗘ ࢩ ࡋ 㒊 ࡀᏑ ᅾࡋ ࠋ ࠊ㟁ὶไ㝈ᅉᏊ ゎ ࡁ ᛶ 㣕 ࡘ ࡀ ࠋࡑࡇ ࠊᒁᡤ Ic ప 㒊 ホ౯ ࠊFig. 14 ࡍ ప ᰝᆺ ᐃࡋࡓ ࠊ㟁Ꮚ㢧ᚤ㙾ほᐹ ࡑ Ꮚ㢧ᚤ㙾 㛤 ࡋ ࠋᮏࢩࢫ ࠊ せᅉ ㄪ ࡓࠋࡑ ᯝ Fig. 16 ࡍࠋṧ ෭ ࡗ ෭ ࢫ ࢪ ᗘ 5 K ไᚚࡍ ప 㒊 ᒁᡤḞ㝗ࡀ ࡇ ࡀண ࡉ ࡀࠊࡑ ࡇ ࡀ ࡗ ࠊ ࡓ ఏᑟ ࢢ ᗘ ᯒฟ ࡀほ ࡉ ࠊඖ ศᯒ ᯝ Mg Si ࡏ ࡇ 5 T እ㒊 ༳ຍࡍ ࡇ ࡀ ಙ ࡀ ฟࡉ ࡓࠋࡇ ᯒฟ MgSi ࠊC ࡗ ࠋ ᛶ ࡗ ῧຍࡋࡓ SiC ᮶ ෆ㒊 Mg ᣑᩓἲ ࡗ స〇ࡉ ࡓከⰺ ホ౯ ᯒฟ ࡀ ᛶไ㝈ᅉᏊ ࡗ Ỵᐃ ドᣐ ࡘ Fig. 15 ࡍࠋᮏヨ ࠊࡇ ᙧ ࡇ ࡀ ࡁࡓࠋᐇ㝿 ࠊ ᅾ C ἲ ࡗ ࡃ ࡓ ཌࡉ᪉ ᆒ ᛶ ௬ᐃࡋ ࡅ ᛶ 㣕 ࡋ ࠋࡇ ࠊᵝࠎ ࡀࠊṧ ศᕸ ࡑ ࡒ ᒁ ඛ㐍 ఏᑟ ᑐࡋ ࠊࡑ ᛶไ㝈ᅉᏊ ゎ ᡤ Ic ศᕸ ホ౯ࡍ ࡇ ࡀ ࡁ ࠋ ࡓࠊᒁᡤ ᑠ ࡀ ഛࡀ ࡗ ࠋ Ic ࡗ ᅄ➃Ꮚἲ ࡗ ᚓ Ic ࡀไ㝈 ࠉప ᕤᏛ 49 ᕳ 9 14 ᖺ 491
Superconducting magnet Low-Temperature Scanning Hall-Probe Microscope (LT-SHPM) Specifications Eternal magnetic field: -5 T Stage temperature: 5-1 K Scanning range: 1 1 mm Horizontal scanning resolution:.5 μm Vertical scanning resolution:.5 μm Active area of Hall sensor: 5 5 μm Sample Sample stage Hall sensor Mirror Fig. 14 Low-temperature scanning Hall-probe microscope (LT- SHPM). 1 I c / I c_ave Measured magnetic field B z (mt) -4 5 14 B z Polished sample for the SHPM measurement Filament U Filament M Filament L Considering the proportional relationship between B z and I c sheath MgB bore Filament U Filament M Filament L.5 mm Fig. 15 Longitudinal variation of local critical current in each filament of a multifilamentar MgB wire estimated from remanent magnetic field distribution measured using scanning Hall-probe microscop (SHPM) at 1 K 1). Trapped fields come from upper (U) and lower (L) filaments as well as from the central filament (M) 1). Remanent magnetic field B z (mt) -4 5 14 B z Mg SEM.1 mm Fig. 16 Comparison between remanent magnetic field distribution measured b SHPM and SEM image. EDS elemental mappings are also shown for Mg and (d) Si 1). Si (d) U M L SHPM J c NEDO JST ALCA MgB JST S- JST SUPER- IRON: 1) E.H. Brandt and M. Indenbom: Tpe-II-superconductor strip with current in a perpendicular magnetic field, Phs. Rev. B 48 (199) 189-196 ) B.J. Roth, N.G. Sepulveda and J.P. Wiswo Jr: Using a magnetometer to image a two-dimensional current distribution, J. Appl. Phs. 65 (1989) 61-7 ) K. Higashiawa, M. Inoue, T. Kawaguchi, K. Shiohara, K. Imamura, T. Kiss, Y. Iijima, K. Kaimoto, T. Saitoh and T. Izumi: Scanning Hall-probe microscop sstem for two-dimensional imaging of critical current densit in RE-1 coated conductors, Phsica C 471 (11) 16-14 4) K. Shiohara, K. Higashiawa, M. Inoue, T. Kiss, Y. Iijima, T. Saitoh, M. Yoshizumi and T. Izumi: Measurement of in-plane magnetic relaation in RE-1 coated conductors b use of scanning Hall probe microscop, Phsica C 484 (1) 19-141 5) K. Higashiawa, K. Shiohara, M. Inoue, T. Kiss, M. Yoshizumi and T. Izumi: Estimation of local current transport properties in thin film superconductor based on scanning Hall-probe microscop, MRS Proceedings 144 (1) mrss1-144-i-4 6) K. Higashiawa, K. Katahira, K. Oumura, K. Shiohara, M. Inoue, T. Kiss, Y. Shingai, M. Konishi, K. Ohmatsu, M. Yoshizumi, T. Izumi and H. Oamoto: Lateral distribution of critical current densit in coated conductors slit b different cutting methods, IEEE Trans. Appl. Supercond. () (1) 6674 49 TEION KOGAKUJ. Cro. Super. Soc. Jpn.Vol. 49 No. 914
7) K. Higashiawa, K. Shiohara, M. Inoue, T. Kiss, T. Machi, N. Chiumoto, S. Lee, K. Tanabe, T. Izumi and H. Oamoto: Noncontact characterization of in-plane distribution of critical current densit in multifilamentar coated conductor, IEEE Trans. Appl. Supercond. () (1) 9574 8) K. Higashiawa, K. Shiohara, Y. Komai, K. Oumura, K. Imamura, M. Inoue, T. Kiss, Y. Iijima, T. Saitoh, T. Machi, M. Yoshizumi, T. Izumi and H. Oamoto: High-speed scanning Hallprobe microscop for two-dimensional characterization of local critical current densit in long-length coated conductor, Phsics Procedia 7 (1) 8-1 9) K. Higashiawa, K. Katahira, M. Inoue, T. Kiss, Y. Shingai, M. Konishi, K. Ohmatsu, T. Machi, M. Yoshizumi, T. Izumi and Y. Shiohara: Nondestructive diagnostics of narrow coated conductors for electric power applications, IEEE Trans. Appl. Supercond. 4 () (14) 6674 1) T. Kiss, et al.: Spatial frequenc analsis on local critical current distribution in REBCO coated conductors, Abstract of 75th autumn meeting of JSAP (14) in press REBCO 14 75 75 (14) in press 11) K. Higashiawa, A. Yamamoto, T. Kiss, S. Ye, A. Matsumoto and H. Kumaura: Magnetic microscop for characterization of local critical current in iron-sheathed MgB wires, Phsica C 5 (14) 6 64 1) K. Higashiawa, et al.: Local critical current and its relationship with microstructure in MgB wires fabricated b internal Mg diffusion process, Abstracts of CSSJ Conference 87 (1) 98 Mg MgB 87 1 (1) 98 1) V. Braccini, S. Kawale, E. Reich, E. Bellingeri, L. Pellegrino, A. Sala, M. Putti, K. Higashiawa, T. Kiss, B. Holzapfel and C. Ferdeghini: Highl effective and isotropic pinning in epitaial Fe(Se,Te) thin films grown on CaF substrates, Appl. Phs. Lett. 1 (1) 1761 1979 7 7 PD 1 1 1 1974 8 1997 1 1991 199 1991 1996 7 1999 Wisconsin Twente 6 IEEEMRSACerS 49 9 14 49