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Evaluation of crystallographic and electromagnetic defects in the superconducting crystals and films by analysis of magneto-microwave response
Physica C 337 Ž2000. 49–56 www.elsevier.nlrlocaterphysc
Evaluation of crystallographic and electromagnetic defects in the superconducting crystals and films by analysis of magneto-microwave response Hachizo Muto ) , Takeshi Kusumori, Shozi Kawakami National Industrial Research, Institute of Nagoya Hirate-cho 1, Kita-ku, Nagoya, 462-8510 Japan
Abstract Magnetic-flux trapping in crystals and films of high-Tc superconductors is investigated from the microwave ŽMW. response detected by electron spin resonance ŽESR. spectrometer. For Bi2212 single crystals, the periodic MW spectra were observed at external magnetic fluxes F X s Ž k " 1r2.F 0 ; where k is an integer, along with the surface impedance signal. The periodic spectrum is a clear evidence for the stepwise transitions Ž k k q 1. between the energy states of Josephson-junction ŽJJ. links by MW absorption, which follows the stepwise penetration of quantized flux F 0 . Their analysis provided information on the phase shift D w s 0 or p of the vortex current along the link in addition to the link size, link-surface orientation, and the number of the penetrated quanta. The YBCO epitaxial films, which were fabricated by pulsed laser ablation–deposition ŽPLD., showed the three kinds of MW responses or their superimposed spectra: Ž1. surface impedance, Ž2. periodic spectra due to intra-crystal JJ, and Ž3. a complex spectrum characteristic of the flux dynamics at inter-granular JJ defects. They are discussed in comparison with the film-crystallinity studied by X-ray diffraction and scanning electron microscope ŽSEM. and have made it possible to evaluate the degree of epitaxy and the defects that work as flux-trapping sites. q 2000 Elsevier Science B.V. All rights reserved.
™
Keywords: Microwave absorption; Flux pinning; Thin films; Josephson effect; Grain boundaries
1. Introduction Magnetic-flux trapping governs the superconductivity in magnetic fields. Therefore, the study of the trapping modes in the single crystals and epitaxial films is important in the understanding and application of high-Tc superconductivity. The study of microwave ŽMW. magneto-absorption spectra measured by electron spin resonance ŽESR. spectrome-
) Corresponding author. Tel.: q81-52-911-2111; fax: q81-52916-2802. E-mail address: [email protected] ŽH. Muto..
ters is useful and can be applied to a magnetic field as high as H0 s 5000–20 000 Oe Žlimit field of the spectrometers and larger than the field of ; 200 Oe in bitter decoration method. w1,2x. The method directly probes the quantized flux F 0 trapped in Josephson-junction ŽJJ. links via the detection of MW periodic spectra w3–9x. Previously, we studied Bi2212 single crystals and observed a few periodic spectra due to F 0 penetration into JJ links w10x. The MW responses were also studied for a homogeneous sample of high purity Pb solid and inhomogeneous samples made of Pb metal and PbO w11x. The former sample gives a simple MW response of surface impedance. The latter samples, having PbrPbOrPb
0921-4534r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 0 0 . 0 0 0 5 4 - X
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H. Muto et al.r Physica C 337 (2000) 49–56
inter-granular JJ, give a dynamic MW response due to the trapping and detrapping behavior of the vortexes by field modulation at the junctions. Thus, the MW responses are classified to three categories: Ž1. a weak and broad spectrum arising only from the surface impedance, Ž2. periodic spectra due to intracrystal JJ, and Ž3. a complex spectrum characteristic of the flux dynamics at inter-granular JJ defects. These results have been applied in investigating the YBCO crystalline films, which are fabricated by pulsed laser ablation–deposition ŽPLD. method w12x. It was found that the above characteristics ŽŽ1. – Ž3.. were useful in evaluating the crystallographic and electromagnetic quality of the films. 2. Experimental A crystal of Bi 2 Sr2 Ca 1Cu 2 O y superconductor was grown by the melting flux flow method w10x. A few small single crystals were prepared by cleaving and cutting it. Thin films of YBCO were fabricated by PLD method using the fourth harmonic of the Nd:YAG laser w12x. They are deposited on MgOŽ001. single crystal substrates. The PLD conditions are as follows: substrate temperature 730–7608C, oxygen pressure 30–50 Pa, laser frequency 1 Hz, and irradiation laser energy 10–40 mJrpulse. The set-up of the apparatus and the other conditions are described elsewhere w12x. The films were investigated by X-ray diffraction ŽRigaku Geigerflex. to examine the epitaxy, by scanning electron microscope ŽSEM; JEOL JIS-T200. to know the surface morphology, and by measuring the temperature dependence of resistance using the four-probe method to estimate the critical temperature. The magneto-MW response was measured using an ESR spectrometer ŽJEOL-FGX3.. The magnetic power supply was improved, so as to sweep a wide range of the magnetic field across zero: from y1000 to q10 000 Oe w13x.
and Zimmmerman w3x and other groups w4,5x. Here, it should be briefly described so the results may be applied to the epitaxial films w10x. The current I and energy E are given by, I s Ic sin Ž D w .
Ž 1.
E s LI 2r2 y Ž F 0 Icr2p . cos Ž D w . ,
Ž 2.
where Ic is the critical current, L is the inductance of the loop, and D w is the phase shift of I across JJ. The energy of JJ links is multi-valued and has a potential minimum at a phase shift D w s 0 q 2 kp Žcalled 0 junctions., where k is an integer, or shift D w s p q 2 kp Žp junction. for cases Ic ) 0 or Ic - 0, ŽFig. 1.. By increasing the field H0 and applying MW, the flux state absorbs the MW energy and transits to the next state Ž k k q 1. followed by the F 0 penetration into JJ. Current I flows simultaneously across the JJ by changing the phase by 0 or p . JJ link can accept many quanta, since it has multi-eigen states. It differs from the Abrikosov vortex by following only one F 0 . For links having a number M of JJ, D w can be given by Eq. Ž3. for 0
™
3. Results and discussions 3.1. MW responses from single crystals Previously, we discussed the magnetic-flux states of JJ link, which is a supercurrent loop across a JJ boundary, based on the theory developed by Silver
Fig. 1. Energy Ž E . states of a superconducting link having Ža. a 0-type and Žc. a p-type JJ. Žb. and Žd. are MW spectra due to the transitions Žvertical arrows. between the states for Žb. 0-type and Žd. p-type JJ links, which occur at a flux shifted from the E minima by F 0 r2 Žsee text..
H. Muto et al.r Physica C 337 (2000) 49–56
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Fig. 2. Ža. Periodic spectra due to MW response from JJ, measured at 77 K for a thin Bi2212 single by ESR spectrometer Ž H0 makes 758 with c axis in the ca crystal plane. and Žb. their angular dependence in the ca plane. The diversion of spectra comes from that of 1rcos u in Eq. Ž5. at u s 908, where H0 ´ is the vortex surface SJ .
junctions and even number Ž2 m q 1. of p junctions, and by Eq. Ž4. for odd number Ž2 m q 1. of p junctions w10x.
w named 0-type linkx D w s Ž 2prM . Ž k q 1r2 y FrF 0 . Ž 4 . w named p-type linkx
external flux F X . Transitions between flux states by MW absorption will be observed stepwise at a flux shifted by Ž1r2.F 0 from the energy minimum position Ž D w 0., i.e. at F X s Ž k q 1r2.F 0 for the 0-type, and at F X s kF 0 for the p-type links ŽFig. 1b and d.. Since only one F 0 penetrates into the JJ links Žhaving a surface SJ . in each step, the line separation D B of periodic spectrum is given by
The energy of the JJ links is illustrated in Fig. 1a and c for 0- and p-type links, as a function of the
D B s F 0rSeff s F 0rSJ cos u .
D w s Ž 2prM . Ž k y FrF 0 .
Ž 3.
X
™
Ž 5.
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H. Muto et al.r Physica C 337 (2000) 49–56
Seff is the effective surface that is given by the cross-section of SJ projected along the external field H0 . u is the angle between H0 and normal of SJ . Fig. 2a shows the periodic spectra observed at a
crystal orientation for a thin Bi2212 single crystal. The JJ concerned was identified to be a 0-type link from the observed periodicity at flux F X s Ž k q 1r2.F 0 . Fig. 2b shows their angular dependence.
Fig. 3. MW response measured at 4.2 K for Ža. homogeneous Pb solid, Žb. inhomogeneous sample with PbrPbOrPb inter-granular JJ, and Žc. an enclosed sample of Pb solid and DPPH. Žd. is the absorption form of Žc. obtained by the integration of Žc..
H. Muto et al.r Physica C 337 (2000) 49–56
The diversion of the spectrum by the spreading of the line separation D B comes from the divergence of 1rcos u in Eq. Ž5. at u s 908, where the external field H0 is applied parallel to the vortex surface SJ in the specimen. The vortex size and orientation were estimated from the analysis w10x. Thus, the periodic spectra and angular dependence provide clear evidence for the existence of JJ and F 0 penetration ŽFig. 2a.. 3.2. Pb solids with and without JJ and polycrystalline high-Tc superconductors Fig. 3 shows the MW responses observed at 4.2 K for Ža. homogeneous lead ŽPb. solid with high purity Ž99.99%. and Žb. Pb solid with PbrPbOrPb intergranular JJ. The respective samples were prepared by slow cooling to RT in vacuo and by rapid freezing to 77 K in air after melting w11x. The sharp MW spectrum Ža., named B, indicates a sharp transition from the superconductor to the normal state at Hc s 60 mT, which is characteristic of type I superconductor. Its shape showed no dependence on the H0 sweep reversal Žno hysteresis. and the modulation field Hm . It is assigned to the MW response that is associated to Meissner effect Žinset I in Fig. 3.. That may be an exclusion of MW Žsurface impedance. from the homogeneous sample surface due to flux penetration. No hysteresis can be understood by the smooth penetration and flow-out. Here, the MW exclusion has been confirmed as follows. We observed simultaneously the signal B of Pb solid and
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the ESR absorption of DPPH by enclosing them into a sample cell. B is detected with opposite phase Žsign. to the ESR absorption signal, as shown in Fig. 3c and d, which is an absorption form obtained by the integration of Fig. 3c. Sample Žb., having a PbrPbOrPb inter-granular JJ, gives rise to a response containing another component named J in addition to B ŽFig. 3b.. These results are clear evidence for J being the MW responses from the inter-granular JJ. J changes its phase for H0 sweep reversal Žd H0rdt s q and y. and depends on Hm . By increasing the field modulation Hm , its intensity increases first, however, it saturates and finally decreases. Previously, we explained phenomenologically these behaviors as due to the MW responses from magnetization, which is caused by shielding the current and vortexes w11x. However, the magnetization due to the vortexes differs from spin systems and does not precess in the MW magnetic field. Furthermore, our previous study on Bi2212 crystals clearly showed that the response came from MW absorption and emission following the flux trapping and detrapping at JJ w10x. Therefore, J component and its behavior should now be understood by the weaker inter-granular JJ as follows. J is the difference of the MW absorption Žwith phase q. and emission Žwith phase y. associating to the trapping and detrapping of fluxes at JJ by increasing and decreasing the magnetic field. The specimen in ESR cavity feels two magnetic fields: the static field H0 and the modulation field Hm cosŽ2pn t . for phase-sensitive detection. The modu-
Fig. 4. SEM pictures of YBCO thin films fabricated on MgOŽ001. single crystal substrates by PLD method using the laser energy Ža. 40, Žb. 30 and Žc. 10 mJrpulse.
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lation changes the sweep direction rapidly Ž n s 100 kHz in the ESR spectrometer. and causes the trapping and detrapping of fluxes in one cycle. When a large Hm is applied, the detrapping in the Hm -decreasing cycle will also easily occur at weak JJ, such as the inter-granular defects and valances with the trapping in the H0-increasing cycle. As a result, the accompanying MW responses with opposite phase Žq, y. are cancelled in time-average and the J component of the MW response vanish. Thus, the field modulation mechanism of J in ESR detection scheme is alternated, however, the previous assignment that J comes from the inter-granular JJ and the value in evaluation of the flux-trapping strength are still correct and valid. The J and B components were popularly observed for sintered samples of YBCO, Bi and Tl oxides superconductors w11,13,14x. 3.3. MW responses of YBCO thin films Figs. 4–6 respectively show the SEM pictures of the film surface, the X-ray diffraction spectra, and the MW magneto-absorption spectra observed for the YBCO films fabricated on MgOŽ001. substrates by PLD method. The component figures a, b, and c, respectively, are for the films deposited by using laser energy Ža. 40, Žb. 30 and Žc. 10 mJrpulse. Other PLD conditions are as follows: substrate temperature 7408C, oxygen pressure 50 Pa, laser pulse frequency 1 Hz. The critical temperatures of the films do not differ largely Ža. 83, Žb. 82 and Žc. 84 " 2 K. The SEM pictures in Fig. 4 indicate that the number of surface particles is nearly equivalent between Ža. and Žb. films, though some particles on film are Ža. larger than on Žb.. The number and size of the impurity particles on film Žc. are much smaller than those of Ža. and Žb.. X-ray spectra of Ža. and Žb. are only composed of Ž 00l . diffraction peaks except for the MgOŽ 00lX . peaks ŽFig. 5.. Their peak heights are strong and nearly identical. These results suggest equivalent epitaxial formation for both films, though the particles are slightly large on Ža.. Relatively weak diffraction pattern of film Žc. indicates a low epitaxy in spite of smaller number and smaller sizes of particles. The MW response of Ža. in Fig. 6a shows a simple spectrum of only B due to surface impedance, indicating a high quality of epitaxial film texture
Fig. 5. X-ray diffraction spectra of YBCO films fabricated on MgOŽ001. substrates by PLD method using the laser energy Ža. 40, Žb. 30 and Žc.10 mJrpulse.
without intra- and inter-granular JJ, in spite of having surface particles. The spectrum Žb. has many peaks, which resemble the periodic spectra observed for Bi2212 single crystals, in addition to the B component. The multi-line spectrum is assigned to the MW response from the intra-crystal JJ ŽJJintra .. These results indicate that there are intra-crystal defects such as the dislocations providing JJintra in film Žb.. Fig. 6c is composed of three components: B, JJintra and JJinter . The last one changes the phase for H0 sweep reversal and the signal height depends on Hm , as observed in the inhomogeneous Pb solid
H. Muto et al.r Physica C 337 (2000) 49–56
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Fig. 6. Magneto-MW spectra observed for the YBCO thin films fabricated on MgOŽ001. substrates by PLD method using the laser energy Ža. 40, Žb. 30 and Žc. 10 mJrpulse. Ža. is only surface impedance ŽB., indicating the good quality of the film, Žb. shows multi-line spectra due to the intra-crystal JJ in addition to B, and Žc. is composed of the three component responses from the intra-crystal and inter-granular JJ and B.
with PbrPbOrPb inter-granular JJ and polycrystalline samples of high-Tc superconductors of Bi, Tl and Y metal oxides w11,13,14x. It is assigned to the MW response characteristic of the trapping and detrapping at inter-granular JJ defects. These results show that film Žc. has grain-boundaries in addition to intra-crystal JJ indicating that the epitaxy of this film is bad. The cause may be that the energy of particles Žions, atoms, clusters. in the plume ablated is not enough to build good crystalline lattices, since film Žc. was deposited with a laser energy of 10 mJrpulse smaller than the others: Ža. 40 and Žb. 30 mJrpulse. The difference of the MW responses Fig. 6a and b indicates is that film Ža. has a higher quality in epitaxy than Žb. and that the surface particles, along with the number and sizes, do not directly produce intra- and inter-granular JJ nor affect the flux-trapping strength. Thus, the present study has clearly indicated that the MW response and its analysis are
very useful in the evaluation of the quality of the epitaxial films and single crystals of superconductors and in searching the optimum fabrication conditions.
4. Conclusion Magneto-MW response spectra were systematically studied for polycrystals, single crystals and epitaxial films of a various grade of superconductors. They are classified to three categories: Ž1. surface impedance, Ž2. periodic spectra due to intra-crystal JJ, and Ž3. a complex spectrum characteristic of the flux dynamics at inter-granular JJ defects. Their natures and behaviors give important information in understanding the flux trapping and dynamics in high-Tc superconductors and are useful in the evaluation of crystallographic and electromagnetic quality and defects of the films.
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References w1x R. Cubitt, E.M. Forgan, G. Yang, S.L. Lee, D.McK. Paul, H.A. Mook, M. Yethiraj, P.H. Kes, T.W. Li, A.A. Menovsky, Z. Tarnawski, K. Mortensen, Nature 365 Ž1993. 407. w2x S. Yoon, H. Dai, J. Liu, C.M. Lieber, Science 265 Ž1994. 215. w3x A.H. Silver, J.E. Zimmerman, Phys. Rev. 157 Ž1967. 317. w4x H. Vichery, F. Beuneu, P. Lejay, Physica C 159 Ž1989. 823. w5x J. Martinek, J. Stankowski, Phys. Rev. B 50 Ž1994. 399. w6x Wosik, L.M. Xie, J.C. Wolfe, Y. Yen, C.W. Chu, Phys. Rev. B 51 Ž1995. 16289.
w7x K.W. Blazy, A.M. Portis, F.H. Holtzberg, Physica C 157 Ž1989. 16. w8x K. Kish, S. Tyagi, C. Krafft, Phys. Rev. B 44 Ž1991. 225. w9x A. Poppl, L. Kevan, H. Kimura, R.N. Schwartz, Phys. Rev. B 46 Ž1992. 8559. w10x T. Kusumori, N. Murayama, H. Muto, Phys. Rev. B 55 Ž1997. 14479. w11x H. Muto, K. Matsuura, Y. Tai, Physica C 219 Ž1994. 259. w12x T. Kusumori, H. Muto, Physica C 282–287 Ž1997. 577. w13x H. Muto, K. Matsuura, T. Kusumori, Physica C 235–240 Ž1994. 2029. w14x H. Muto, K. Matsuura, K. Yasuda, I. Nakagawa, Physica C 211 Ž1993. 147.
Evaluation of crystallographic and electromagnetic defects in the superconducting crystals and films by analysis of magneto-microwave response Hachizo Muto ) , Takeshi Kusumori, Shozi Kawakami National Industrial Research, Institute of Nagoya Hirate-cho 1, Kita-ku, Nagoya, 462-8510 Japan
Abstract Magnetic-flux trapping in crystals and films of high-Tc superconductors is investigated from the microwave ŽMW. response detected by electron spin resonance ŽESR. spectrometer. For Bi2212 single crystals, the periodic MW spectra were observed at external magnetic fluxes F X s Ž k " 1r2.F 0 ; where k is an integer, along with the surface impedance signal. The periodic spectrum is a clear evidence for the stepwise transitions Ž k k q 1. between the energy states of Josephson-junction ŽJJ. links by MW absorption, which follows the stepwise penetration of quantized flux F 0 . Their analysis provided information on the phase shift D w s 0 or p of the vortex current along the link in addition to the link size, link-surface orientation, and the number of the penetrated quanta. The YBCO epitaxial films, which were fabricated by pulsed laser ablation–deposition ŽPLD., showed the three kinds of MW responses or their superimposed spectra: Ž1. surface impedance, Ž2. periodic spectra due to intra-crystal JJ, and Ž3. a complex spectrum characteristic of the flux dynamics at inter-granular JJ defects. They are discussed in comparison with the film-crystallinity studied by X-ray diffraction and scanning electron microscope ŽSEM. and have made it possible to evaluate the degree of epitaxy and the defects that work as flux-trapping sites. q 2000 Elsevier Science B.V. All rights reserved.
™
Keywords: Microwave absorption; Flux pinning; Thin films; Josephson effect; Grain boundaries
1. Introduction Magnetic-flux trapping governs the superconductivity in magnetic fields. Therefore, the study of the trapping modes in the single crystals and epitaxial films is important in the understanding and application of high-Tc superconductivity. The study of microwave ŽMW. magneto-absorption spectra measured by electron spin resonance ŽESR. spectrome-
) Corresponding author. Tel.: q81-52-911-2111; fax: q81-52916-2802. E-mail address: [email protected] ŽH. Muto..
ters is useful and can be applied to a magnetic field as high as H0 s 5000–20 000 Oe Žlimit field of the spectrometers and larger than the field of ; 200 Oe in bitter decoration method. w1,2x. The method directly probes the quantized flux F 0 trapped in Josephson-junction ŽJJ. links via the detection of MW periodic spectra w3–9x. Previously, we studied Bi2212 single crystals and observed a few periodic spectra due to F 0 penetration into JJ links w10x. The MW responses were also studied for a homogeneous sample of high purity Pb solid and inhomogeneous samples made of Pb metal and PbO w11x. The former sample gives a simple MW response of surface impedance. The latter samples, having PbrPbOrPb
0921-4534r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 0 0 . 0 0 0 5 4 - X
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H. Muto et al.r Physica C 337 (2000) 49–56
inter-granular JJ, give a dynamic MW response due to the trapping and detrapping behavior of the vortexes by field modulation at the junctions. Thus, the MW responses are classified to three categories: Ž1. a weak and broad spectrum arising only from the surface impedance, Ž2. periodic spectra due to intracrystal JJ, and Ž3. a complex spectrum characteristic of the flux dynamics at inter-granular JJ defects. These results have been applied in investigating the YBCO crystalline films, which are fabricated by pulsed laser ablation–deposition ŽPLD. method w12x. It was found that the above characteristics ŽŽ1. – Ž3.. were useful in evaluating the crystallographic and electromagnetic quality of the films. 2. Experimental A crystal of Bi 2 Sr2 Ca 1Cu 2 O y superconductor was grown by the melting flux flow method w10x. A few small single crystals were prepared by cleaving and cutting it. Thin films of YBCO were fabricated by PLD method using the fourth harmonic of the Nd:YAG laser w12x. They are deposited on MgOŽ001. single crystal substrates. The PLD conditions are as follows: substrate temperature 730–7608C, oxygen pressure 30–50 Pa, laser frequency 1 Hz, and irradiation laser energy 10–40 mJrpulse. The set-up of the apparatus and the other conditions are described elsewhere w12x. The films were investigated by X-ray diffraction ŽRigaku Geigerflex. to examine the epitaxy, by scanning electron microscope ŽSEM; JEOL JIS-T200. to know the surface morphology, and by measuring the temperature dependence of resistance using the four-probe method to estimate the critical temperature. The magneto-MW response was measured using an ESR spectrometer ŽJEOL-FGX3.. The magnetic power supply was improved, so as to sweep a wide range of the magnetic field across zero: from y1000 to q10 000 Oe w13x.
and Zimmmerman w3x and other groups w4,5x. Here, it should be briefly described so the results may be applied to the epitaxial films w10x. The current I and energy E are given by, I s Ic sin Ž D w .
Ž 1.
E s LI 2r2 y Ž F 0 Icr2p . cos Ž D w . ,
Ž 2.
where Ic is the critical current, L is the inductance of the loop, and D w is the phase shift of I across JJ. The energy of JJ links is multi-valued and has a potential minimum at a phase shift D w s 0 q 2 kp Žcalled 0 junctions., where k is an integer, or shift D w s p q 2 kp Žp junction. for cases Ic ) 0 or Ic - 0, ŽFig. 1.. By increasing the field H0 and applying MW, the flux state absorbs the MW energy and transits to the next state Ž k k q 1. followed by the F 0 penetration into JJ. Current I flows simultaneously across the JJ by changing the phase by 0 or p . JJ link can accept many quanta, since it has multi-eigen states. It differs from the Abrikosov vortex by following only one F 0 . For links having a number M of JJ, D w can be given by Eq. Ž3. for 0
™
3. Results and discussions 3.1. MW responses from single crystals Previously, we discussed the magnetic-flux states of JJ link, which is a supercurrent loop across a JJ boundary, based on the theory developed by Silver
Fig. 1. Energy Ž E . states of a superconducting link having Ža. a 0-type and Žc. a p-type JJ. Žb. and Žd. are MW spectra due to the transitions Žvertical arrows. between the states for Žb. 0-type and Žd. p-type JJ links, which occur at a flux shifted from the E minima by F 0 r2 Žsee text..
H. Muto et al.r Physica C 337 (2000) 49–56
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Fig. 2. Ža. Periodic spectra due to MW response from JJ, measured at 77 K for a thin Bi2212 single by ESR spectrometer Ž H0 makes 758 with c axis in the ca crystal plane. and Žb. their angular dependence in the ca plane. The diversion of spectra comes from that of 1rcos u in Eq. Ž5. at u s 908, where H0 ´ is the vortex surface SJ .
junctions and even number Ž2 m q 1. of p junctions, and by Eq. Ž4. for odd number Ž2 m q 1. of p junctions w10x.
w named 0-type linkx D w s Ž 2prM . Ž k q 1r2 y FrF 0 . Ž 4 . w named p-type linkx
external flux F X . Transitions between flux states by MW absorption will be observed stepwise at a flux shifted by Ž1r2.F 0 from the energy minimum position Ž D w 0., i.e. at F X s Ž k q 1r2.F 0 for the 0-type, and at F X s kF 0 for the p-type links ŽFig. 1b and d.. Since only one F 0 penetrates into the JJ links Žhaving a surface SJ . in each step, the line separation D B of periodic spectrum is given by
The energy of the JJ links is illustrated in Fig. 1a and c for 0- and p-type links, as a function of the
D B s F 0rSeff s F 0rSJ cos u .
D w s Ž 2prM . Ž k y FrF 0 .
Ž 3.
X
™
Ž 5.
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H. Muto et al.r Physica C 337 (2000) 49–56
Seff is the effective surface that is given by the cross-section of SJ projected along the external field H0 . u is the angle between H0 and normal of SJ . Fig. 2a shows the periodic spectra observed at a
crystal orientation for a thin Bi2212 single crystal. The JJ concerned was identified to be a 0-type link from the observed periodicity at flux F X s Ž k q 1r2.F 0 . Fig. 2b shows their angular dependence.
Fig. 3. MW response measured at 4.2 K for Ža. homogeneous Pb solid, Žb. inhomogeneous sample with PbrPbOrPb inter-granular JJ, and Žc. an enclosed sample of Pb solid and DPPH. Žd. is the absorption form of Žc. obtained by the integration of Žc..
H. Muto et al.r Physica C 337 (2000) 49–56
The diversion of the spectrum by the spreading of the line separation D B comes from the divergence of 1rcos u in Eq. Ž5. at u s 908, where the external field H0 is applied parallel to the vortex surface SJ in the specimen. The vortex size and orientation were estimated from the analysis w10x. Thus, the periodic spectra and angular dependence provide clear evidence for the existence of JJ and F 0 penetration ŽFig. 2a.. 3.2. Pb solids with and without JJ and polycrystalline high-Tc superconductors Fig. 3 shows the MW responses observed at 4.2 K for Ža. homogeneous lead ŽPb. solid with high purity Ž99.99%. and Žb. Pb solid with PbrPbOrPb intergranular JJ. The respective samples were prepared by slow cooling to RT in vacuo and by rapid freezing to 77 K in air after melting w11x. The sharp MW spectrum Ža., named B, indicates a sharp transition from the superconductor to the normal state at Hc s 60 mT, which is characteristic of type I superconductor. Its shape showed no dependence on the H0 sweep reversal Žno hysteresis. and the modulation field Hm . It is assigned to the MW response that is associated to Meissner effect Žinset I in Fig. 3.. That may be an exclusion of MW Žsurface impedance. from the homogeneous sample surface due to flux penetration. No hysteresis can be understood by the smooth penetration and flow-out. Here, the MW exclusion has been confirmed as follows. We observed simultaneously the signal B of Pb solid and
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the ESR absorption of DPPH by enclosing them into a sample cell. B is detected with opposite phase Žsign. to the ESR absorption signal, as shown in Fig. 3c and d, which is an absorption form obtained by the integration of Fig. 3c. Sample Žb., having a PbrPbOrPb inter-granular JJ, gives rise to a response containing another component named J in addition to B ŽFig. 3b.. These results are clear evidence for J being the MW responses from the inter-granular JJ. J changes its phase for H0 sweep reversal Žd H0rdt s q and y. and depends on Hm . By increasing the field modulation Hm , its intensity increases first, however, it saturates and finally decreases. Previously, we explained phenomenologically these behaviors as due to the MW responses from magnetization, which is caused by shielding the current and vortexes w11x. However, the magnetization due to the vortexes differs from spin systems and does not precess in the MW magnetic field. Furthermore, our previous study on Bi2212 crystals clearly showed that the response came from MW absorption and emission following the flux trapping and detrapping at JJ w10x. Therefore, J component and its behavior should now be understood by the weaker inter-granular JJ as follows. J is the difference of the MW absorption Žwith phase q. and emission Žwith phase y. associating to the trapping and detrapping of fluxes at JJ by increasing and decreasing the magnetic field. The specimen in ESR cavity feels two magnetic fields: the static field H0 and the modulation field Hm cosŽ2pn t . for phase-sensitive detection. The modu-
Fig. 4. SEM pictures of YBCO thin films fabricated on MgOŽ001. single crystal substrates by PLD method using the laser energy Ža. 40, Žb. 30 and Žc. 10 mJrpulse.
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lation changes the sweep direction rapidly Ž n s 100 kHz in the ESR spectrometer. and causes the trapping and detrapping of fluxes in one cycle. When a large Hm is applied, the detrapping in the Hm -decreasing cycle will also easily occur at weak JJ, such as the inter-granular defects and valances with the trapping in the H0-increasing cycle. As a result, the accompanying MW responses with opposite phase Žq, y. are cancelled in time-average and the J component of the MW response vanish. Thus, the field modulation mechanism of J in ESR detection scheme is alternated, however, the previous assignment that J comes from the inter-granular JJ and the value in evaluation of the flux-trapping strength are still correct and valid. The J and B components were popularly observed for sintered samples of YBCO, Bi and Tl oxides superconductors w11,13,14x. 3.3. MW responses of YBCO thin films Figs. 4–6 respectively show the SEM pictures of the film surface, the X-ray diffraction spectra, and the MW magneto-absorption spectra observed for the YBCO films fabricated on MgOŽ001. substrates by PLD method. The component figures a, b, and c, respectively, are for the films deposited by using laser energy Ža. 40, Žb. 30 and Žc. 10 mJrpulse. Other PLD conditions are as follows: substrate temperature 7408C, oxygen pressure 50 Pa, laser pulse frequency 1 Hz. The critical temperatures of the films do not differ largely Ža. 83, Žb. 82 and Žc. 84 " 2 K. The SEM pictures in Fig. 4 indicate that the number of surface particles is nearly equivalent between Ža. and Žb. films, though some particles on film are Ža. larger than on Žb.. The number and size of the impurity particles on film Žc. are much smaller than those of Ža. and Žb.. X-ray spectra of Ža. and Žb. are only composed of Ž 00l . diffraction peaks except for the MgOŽ 00lX . peaks ŽFig. 5.. Their peak heights are strong and nearly identical. These results suggest equivalent epitaxial formation for both films, though the particles are slightly large on Ža.. Relatively weak diffraction pattern of film Žc. indicates a low epitaxy in spite of smaller number and smaller sizes of particles. The MW response of Ža. in Fig. 6a shows a simple spectrum of only B due to surface impedance, indicating a high quality of epitaxial film texture
Fig. 5. X-ray diffraction spectra of YBCO films fabricated on MgOŽ001. substrates by PLD method using the laser energy Ža. 40, Žb. 30 and Žc.10 mJrpulse.
without intra- and inter-granular JJ, in spite of having surface particles. The spectrum Žb. has many peaks, which resemble the periodic spectra observed for Bi2212 single crystals, in addition to the B component. The multi-line spectrum is assigned to the MW response from the intra-crystal JJ ŽJJintra .. These results indicate that there are intra-crystal defects such as the dislocations providing JJintra in film Žb.. Fig. 6c is composed of three components: B, JJintra and JJinter . The last one changes the phase for H0 sweep reversal and the signal height depends on Hm , as observed in the inhomogeneous Pb solid
H. Muto et al.r Physica C 337 (2000) 49–56
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Fig. 6. Magneto-MW spectra observed for the YBCO thin films fabricated on MgOŽ001. substrates by PLD method using the laser energy Ža. 40, Žb. 30 and Žc. 10 mJrpulse. Ža. is only surface impedance ŽB., indicating the good quality of the film, Žb. shows multi-line spectra due to the intra-crystal JJ in addition to B, and Žc. is composed of the three component responses from the intra-crystal and inter-granular JJ and B.
with PbrPbOrPb inter-granular JJ and polycrystalline samples of high-Tc superconductors of Bi, Tl and Y metal oxides w11,13,14x. It is assigned to the MW response characteristic of the trapping and detrapping at inter-granular JJ defects. These results show that film Žc. has grain-boundaries in addition to intra-crystal JJ indicating that the epitaxy of this film is bad. The cause may be that the energy of particles Žions, atoms, clusters. in the plume ablated is not enough to build good crystalline lattices, since film Žc. was deposited with a laser energy of 10 mJrpulse smaller than the others: Ža. 40 and Žb. 30 mJrpulse. The difference of the MW responses Fig. 6a and b indicates is that film Ža. has a higher quality in epitaxy than Žb. and that the surface particles, along with the number and sizes, do not directly produce intra- and inter-granular JJ nor affect the flux-trapping strength. Thus, the present study has clearly indicated that the MW response and its analysis are
very useful in the evaluation of the quality of the epitaxial films and single crystals of superconductors and in searching the optimum fabrication conditions.
4. Conclusion Magneto-MW response spectra were systematically studied for polycrystals, single crystals and epitaxial films of a various grade of superconductors. They are classified to three categories: Ž1. surface impedance, Ž2. periodic spectra due to intra-crystal JJ, and Ž3. a complex spectrum characteristic of the flux dynamics at inter-granular JJ defects. Their natures and behaviors give important information in understanding the flux trapping and dynamics in high-Tc superconductors and are useful in the evaluation of crystallographic and electromagnetic quality and defects of the films.
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