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Ag tapes
Physica C 408–410 (2004) 643–644 www.elsevier.com/locate/physc
Irreversibility fields and pinning potentials in U/n treated Bi2223/Ag tapes I. Kusevic b
a,*
, E. Babic a, D. Marinaro b, S.X. Dou b, R. Weinstein
c
a Department of Physics, Faculty of Science, University of Zagreb, P.O. Box 331, HR-10000 Zagreb, Croatia Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia c Texas Center for Superconductivity, University of Houston, Houston, TX 77204-5506, USA
Abstract The magnetoresistance RðT ; BÞ of virgin and four 235 U doped Bi2223/Ag tapes irradiated with thermal neutrons (U/n treated) has been measured in the temperature range T P 50 K and magnetic field B 6 1 T. For B P 20 mT the pinning potentials U0 of U/n treated tapes were enhanced compared to that of virgin tape. The relative enhancement U0 ðB; U=nÞ=U0 (B, virg.) increased with matching field B/ and persisted to higher B at higher B/ . For U/n treated samples, U0 showed universal variation with B=B/ : U0 ðB K B/ Þ / ðB/ =BÞa with a 0:5 and faster variation at higher B. The resistive irreversibility fields Birr ðT Þ of all U/n treated samples for T K 100 K were also enhanced: Birr (T ; U =nÞ=Birr (T , virg.) increased with B/ and the enhancement persisted to lower T for higher B/ . The maximum enhancements of Birr scale well with the corresponding enhancements of U0 ðBÞ. Ó 2004 Elsevier B.V. All rights reserved. PACS: 74.60.Ge; 74.62.Dh; 74.72.Hs Keywords: Flux pinning; Uranium doping; Neutron irradiation
Weak flux pinning at elevated temperatures is the main obstacle for the large scale applications of Agsheathed Bi–Sr–Ca–Cu–O tapes. The efficient way for increasing flux pinning in these and other high-Tc superconductors (HTS) is the introduction of columnar defects [1]. Such defects can be produced either by irradiation with high energy ions or with nuclear fission of suitable atoms within the material. In particular, the fission of Bi-atoms in Bi2223/Ag tapes (induced by highenergy protons [1]) causes large enhancement of both the irreversibility field Birr ðJc ! 0Þ and the field Bmax (pinning force density Fp ¼ Jc B reaches maximum at Bmax ) at elevated temperatures. Similar effects are produced by the fission of 235 U atoms (induced by thermal neutrons, U/n treatment) incorporated in HTS [1]. Analysis [2] of Jc ðBÞ and Fp ðBÞ at 77 K for nine Bi2223/Ag tapes doped with different
*
Corresponding author. E-mail address: [email protected] (I. Kusevic).
amounts of 235 UO2 Æ2H2 O and irradiated to different neutron fluences revealed that the pinning capacity at 77 K (represented by either Bmax or Birr ) increases linearly with the associated matching field B/ , and that contributions to pinning due to intrinsic defects and fission tracks are roughly additive. Since these results referred to vortex solid (B 6 Birr ) in high-J limit (J Jc ), it is interesting to compare them with those for vortex liquid phase (B P Birr ) in low-J limit (J ! 0), which provide a more direct insight into the strength of pinning (represented by the pinning potential U0 and Birr ðT Þ). The details concerning 235 U doping, tape fabrication and neutron irradiation were previously reported [2,3]. For the magnetoresistance measurements we selected samples with B/ 6 0:54 T. RðT ; BÞ was measured with low frequency (18.4 Hz) ac method with IRMS ¼ 1 mA (corresponding to J 1 A/cm2 ). The applied field B 6 1 T was perpendicular to broad face of the tape (thus roughly parallel to c-axis of Bi2223 grains). For all samples, low-resistance parts of resistive transitions showed [3] the Arrhenius-like behaviour
0921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.03.092
I. Kusevic et al. / Physica C 408–410 (2004) 643–644
RðT ; BÞ ¼ R0 expðU ðB; T Þ=kB T Þ, with U ðT ; BÞ an effective activation energy and kB the Boltzmann’s constant. Pinning potentials U0 ðBÞ ¼ U ðT ¼ 0; BÞ for all samples below 0.02 T show the same U0 / Ba variation with a 0:5 (Fig. 1). Above 0.02 T, U0 ðBÞ for U/n treated samples shows strong enhancement in respect to that of virgin tape, reflecting stronger flux pinning in the vortex liquid. This enhancement is larger for larger B/ and persists to higher B on increasing B/ . As a measure of the enhancement, we defined ratio rU ¼ U0 ðB; U=nÞ=U0 (B, virg.), which increases as Bb with b ’ 0:4 for B B/ , saturates around B B/ and decreases with B for B > B/ (inset to Fig. 1). However, rU > 1 remains fulfilled even for B J 3B/ . Initial Bb variation of rU with b a is consistent with the field independent U0 for B B/ deduced from the analysis of reversible magnetization (flux liquid) in Bi2223 tapes [4] and Bi2212 single crystals [1] containing columnar defects. rU for all U/n treated samples reaches a maximum around B 0:8B/ . The maxima of rU increase with B/ and reach rU max ’ 2:6 for B/ ¼ 0:54 T. These results show that B/ sets a natural field scale for flux pinning in U/n treated tapes. Accordingly, U0 for all U/n treated samples follow the same curve when plotted against B=B/ . In particular, for B=B/ 6 0:7, log U0 vs. log B=B/ has a slope 0:5, whereas at higher fields this slope is higher (in accord with the reduced pinning efficacy for B J B/ [5]). By using low resistivity criterion qc ¼ 1 n X cm (which corresponds to criterion J < 1 A/cm2 ), we determined the irreversibility lines Birr ðTirr Þ for all samples. As expected [5] for t ¼ T =Tc J 0:95, Birr of U/n treated samples is roughly the same as that for virgin tape. At lower temperatures (i.e. for B > 0:02 T), Birr for
5
4
10
0.01
2.5 2.0 1.5 1.0 0.5 0.6
0.7
0.8
0.9
1
t
Fig. 2. Ratio rB of the irreversibility fields Birr (t, U/n treated) and Birr (t, virgin tape) vs. reduced temperature t. The symbols correspond to those in Fig. 1.
U/n treated samples is sizably enhanced compared to that for virgin tape. The relative enhancement rB ðtÞ ¼ Birr ðt; U=nÞ=Birr (t, virg.) increases with decreasing t (Fig. 2) and reaches a maximum for Birr ðU=nÞ 0:8B/ . We note remarkable similarity between rU (Fig. 1) and rB . With increasing B/ the maxima of both rU and rB are shifted to higher B (note that lower t corresponds to higher B ¼ Birr ) and the value of rB;max increases. Moreover, the values for rU;max and rB;max are almost the same for the same B/ (for example, equal to 2.6 for B/ ¼ 0:54 T) and rB > 1 is fulfilled for Birr =B/ J 3. As expected [5], actual Birr ðTirr Þ variation depends on Birr 7B/ , the decrease of Birr with Tirr being faster for Birr < B/ . The results for U0 ðBÞ and Birr ðTirr Þ of U/n treated Bi2223/Ag tapes give consistent picture of a strong pinning of vortices by fission tracks in the vortex liquid. Moreover, they complete and confirm earlier findings [2] deduced from the study of vortex solid phase in the same system.
B (T) 0.11
References 2
rU
U0 / k B (K)
10
3.0
rB
644
1 3
10
0.01
0.1 B (T)
1
Fig. 1. Field dependence of pinning potentials U0 for virgin sample () and U/n treated samples with B/ ¼ 0:21 ( ), 0.37 (M), 0.39 (O) and 0.54 T (}). Inset: ratio of pinning potentials of U/n treated and virgin tape, rU ¼ U0 ðB; U=nÞ=U0 (B, virg.).
[1] L. Civale, Supercond. Sci. Technol. 10 (1997) A52, and references therein. [2] E. Babic, I. Kusevic, D. Marinaro, S.X. Dou, J. Boldeman, R. Weinstein, Solid State Commun. 118 (2001) 607. [3] D.G. Marinaro, J. Horvat, S.X. Dou, I. Kusevic, E. Babic, R. Weinstein, A. Gandini, Supercond. Sci. Technol. 15 (2002) 1596. [4] Qiang Li et al., Phys. Rev. B 54 (1996) R788. [5] G. Blatter, M.V. Feigel’man, V.B. Geshkenbein, A.I. Larkin, V.M. Vinokur, Rev. Mod. Phys. 66 (1994) 1125.
Irreversibility fields and pinning potentials in U/n treated Bi2223/Ag tapes I. Kusevic b
a,*
, E. Babic a, D. Marinaro b, S.X. Dou b, R. Weinstein
c
a Department of Physics, Faculty of Science, University of Zagreb, P.O. Box 331, HR-10000 Zagreb, Croatia Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia c Texas Center for Superconductivity, University of Houston, Houston, TX 77204-5506, USA
Abstract The magnetoresistance RðT ; BÞ of virgin and four 235 U doped Bi2223/Ag tapes irradiated with thermal neutrons (U/n treated) has been measured in the temperature range T P 50 K and magnetic field B 6 1 T. For B P 20 mT the pinning potentials U0 of U/n treated tapes were enhanced compared to that of virgin tape. The relative enhancement U0 ðB; U=nÞ=U0 (B, virg.) increased with matching field B/ and persisted to higher B at higher B/ . For U/n treated samples, U0 showed universal variation with B=B/ : U0 ðB K B/ Þ / ðB/ =BÞa with a 0:5 and faster variation at higher B. The resistive irreversibility fields Birr ðT Þ of all U/n treated samples for T K 100 K were also enhanced: Birr (T ; U =nÞ=Birr (T , virg.) increased with B/ and the enhancement persisted to lower T for higher B/ . The maximum enhancements of Birr scale well with the corresponding enhancements of U0 ðBÞ. Ó 2004 Elsevier B.V. All rights reserved. PACS: 74.60.Ge; 74.62.Dh; 74.72.Hs Keywords: Flux pinning; Uranium doping; Neutron irradiation
Weak flux pinning at elevated temperatures is the main obstacle for the large scale applications of Agsheathed Bi–Sr–Ca–Cu–O tapes. The efficient way for increasing flux pinning in these and other high-Tc superconductors (HTS) is the introduction of columnar defects [1]. Such defects can be produced either by irradiation with high energy ions or with nuclear fission of suitable atoms within the material. In particular, the fission of Bi-atoms in Bi2223/Ag tapes (induced by highenergy protons [1]) causes large enhancement of both the irreversibility field Birr ðJc ! 0Þ and the field Bmax (pinning force density Fp ¼ Jc B reaches maximum at Bmax ) at elevated temperatures. Similar effects are produced by the fission of 235 U atoms (induced by thermal neutrons, U/n treatment) incorporated in HTS [1]. Analysis [2] of Jc ðBÞ and Fp ðBÞ at 77 K for nine Bi2223/Ag tapes doped with different
*
Corresponding author. E-mail address: [email protected] (I. Kusevic).
amounts of 235 UO2 Æ2H2 O and irradiated to different neutron fluences revealed that the pinning capacity at 77 K (represented by either Bmax or Birr ) increases linearly with the associated matching field B/ , and that contributions to pinning due to intrinsic defects and fission tracks are roughly additive. Since these results referred to vortex solid (B 6 Birr ) in high-J limit (J Jc ), it is interesting to compare them with those for vortex liquid phase (B P Birr ) in low-J limit (J ! 0), which provide a more direct insight into the strength of pinning (represented by the pinning potential U0 and Birr ðT Þ). The details concerning 235 U doping, tape fabrication and neutron irradiation were previously reported [2,3]. For the magnetoresistance measurements we selected samples with B/ 6 0:54 T. RðT ; BÞ was measured with low frequency (18.4 Hz) ac method with IRMS ¼ 1 mA (corresponding to J 1 A/cm2 ). The applied field B 6 1 T was perpendicular to broad face of the tape (thus roughly parallel to c-axis of Bi2223 grains). For all samples, low-resistance parts of resistive transitions showed [3] the Arrhenius-like behaviour
0921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.03.092
I. Kusevic et al. / Physica C 408–410 (2004) 643–644
RðT ; BÞ ¼ R0 expðU ðB; T Þ=kB T Þ, with U ðT ; BÞ an effective activation energy and kB the Boltzmann’s constant. Pinning potentials U0 ðBÞ ¼ U ðT ¼ 0; BÞ for all samples below 0.02 T show the same U0 / Ba variation with a 0:5 (Fig. 1). Above 0.02 T, U0 ðBÞ for U/n treated samples shows strong enhancement in respect to that of virgin tape, reflecting stronger flux pinning in the vortex liquid. This enhancement is larger for larger B/ and persists to higher B on increasing B/ . As a measure of the enhancement, we defined ratio rU ¼ U0 ðB; U=nÞ=U0 (B, virg.), which increases as Bb with b ’ 0:4 for B B/ , saturates around B B/ and decreases with B for B > B/ (inset to Fig. 1). However, rU > 1 remains fulfilled even for B J 3B/ . Initial Bb variation of rU with b a is consistent with the field independent U0 for B B/ deduced from the analysis of reversible magnetization (flux liquid) in Bi2223 tapes [4] and Bi2212 single crystals [1] containing columnar defects. rU for all U/n treated samples reaches a maximum around B 0:8B/ . The maxima of rU increase with B/ and reach rU max ’ 2:6 for B/ ¼ 0:54 T. These results show that B/ sets a natural field scale for flux pinning in U/n treated tapes. Accordingly, U0 for all U/n treated samples follow the same curve when plotted against B=B/ . In particular, for B=B/ 6 0:7, log U0 vs. log B=B/ has a slope 0:5, whereas at higher fields this slope is higher (in accord with the reduced pinning efficacy for B J B/ [5]). By using low resistivity criterion qc ¼ 1 n X cm (which corresponds to criterion J < 1 A/cm2 ), we determined the irreversibility lines Birr ðTirr Þ for all samples. As expected [5] for t ¼ T =Tc J 0:95, Birr of U/n treated samples is roughly the same as that for virgin tape. At lower temperatures (i.e. for B > 0:02 T), Birr for
5
4
10
0.01
2.5 2.0 1.5 1.0 0.5 0.6
0.7
0.8
0.9
1
t
Fig. 2. Ratio rB of the irreversibility fields Birr (t, U/n treated) and Birr (t, virgin tape) vs. reduced temperature t. The symbols correspond to those in Fig. 1.
U/n treated samples is sizably enhanced compared to that for virgin tape. The relative enhancement rB ðtÞ ¼ Birr ðt; U=nÞ=Birr (t, virg.) increases with decreasing t (Fig. 2) and reaches a maximum for Birr ðU=nÞ 0:8B/ . We note remarkable similarity between rU (Fig. 1) and rB . With increasing B/ the maxima of both rU and rB are shifted to higher B (note that lower t corresponds to higher B ¼ Birr ) and the value of rB;max increases. Moreover, the values for rU;max and rB;max are almost the same for the same B/ (for example, equal to 2.6 for B/ ¼ 0:54 T) and rB > 1 is fulfilled for Birr =B/ J 3. As expected [5], actual Birr ðTirr Þ variation depends on Birr 7B/ , the decrease of Birr with Tirr being faster for Birr < B/ . The results for U0 ðBÞ and Birr ðTirr Þ of U/n treated Bi2223/Ag tapes give consistent picture of a strong pinning of vortices by fission tracks in the vortex liquid. Moreover, they complete and confirm earlier findings [2] deduced from the study of vortex solid phase in the same system.
B (T) 0.11
References 2
rU
U0 / k B (K)
10
3.0
rB
644
1 3
10
0.01
0.1 B (T)
1
Fig. 1. Field dependence of pinning potentials U0 for virgin sample () and U/n treated samples with B/ ¼ 0:21 ( ), 0.37 (M), 0.39 (O) and 0.54 T (}). Inset: ratio of pinning potentials of U/n treated and virgin tape, rU ¼ U0 ðB; U=nÞ=U0 (B, virg.).
[1] L. Civale, Supercond. Sci. Technol. 10 (1997) A52, and references therein. [2] E. Babic, I. Kusevic, D. Marinaro, S.X. Dou, J. Boldeman, R. Weinstein, Solid State Commun. 118 (2001) 607. [3] D.G. Marinaro, J. Horvat, S.X. Dou, I. Kusevic, E. Babic, R. Weinstein, A. Gandini, Supercond. Sci. Technol. 15 (2002) 1596. [4] Qiang Li et al., Phys. Rev. B 54 (1996) R788. [5] G. Blatter, M.V. Feigel’man, V.B. Geshkenbein, A.I. Larkin, V.M. Vinokur, Rev. Mod. Phys. 66 (1994) 1125.