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High pinning potential in YBCO thin films deposited from a target prepared from YBCO nanopowder
Physica C 408–410 (2004) 647–648 www.elsevier.com/locate/physc
High pinning potential in YBCO thin films deposited from a target prepared from YBCO nanopowder J. Raittila a
a,b,*
, M. Peurla a, H. Huhtinen a, P. Paturi a, R. Laiho
a
Wihuri Physical Laboratory, Department of Physics, University of Turku, FIN-20014 Turku, Finland b Graduate School of Materials Research, Turku, Finland
Abstract Thin YBa2 Cu3 O6þx (YBCO) films were prepared on SrTiO3 substrates by pulsed laser deposition using a target made from nanosized powder. These films show high critical current densities Jc 1 107 A/cm2 at 77 K in zero field as obtained by SQUID magnetization measurements. The pinning potentials, U ðT Þ, were calculated from the magnetic relaxation curves measured at several temperatures after cooling the sample in 1 T. From these data the U (0 K) was extrapolated to be 900 K, which is higher than previously reported for as-prepared films. Ó 2004 Elsevier B.V. All rights reserved. PACS: 74.60.Ge Keywords: Superconducting films; YBCO; Flux pinning
1. Introduction YBCO thin films prepared by laser deposition from a target prepared from YBCO nanopowder were previously shown to exhibit very high critical current density, Jc , and a very smooth surface [1]. Since no artificial defects are induced and no evidence from defects in the surface structure was found, a pinning mechanism is important matter to be resolved. In this work we have investigated the hysteresis loops and magnetic relaxation in the films. Jc is calculated using the Bean model and the pinning potential U (0 K) is obtained by fitting the obtained data to a vortex-glass model. 2. Experimental and results Superconducting YBCO thin films were prepared by pulsed laser deposition from a target [1] sintered from * Corresponding author. Address: Wihuri Physical Laboratory, Department of Physics, University of Turku, FIN-20014 Turku, Finland. Tel.: +358-2-333-5676; fax: +358-2-231-9836. E-mail address: jussi.raittila@utu.fi (J. Raittila).
nanophase powder prepared by the citrate gel method [2]. The films were ablated with a XeCl (308 nm) excimer laser on SrTiO3 (1 0 0) substrates. During the deposition oxygen pressure was 0.3 Torr and the temperature of the substrate was 750 °C. Magnetic properties of the films were investigated with a SQUID magnetometer. The films were attached to the sample holder so that their plane was perpendicular to the magnetic field during the measurements. The critical temperature, Tc ¼ 91 K, was determined from the ZFC magnetization curve. Hysteresis loops were measured at temperatures between 5 and 80 K and the Jc (in SI units) was calculated from the Bean model [3] Jc ¼ 3DM=R, where DM (A/m) is half of the opening of the loop and R (m) is the radius of the disk like film. Very high values of Jc 9 107 A/cm2 and Jc 1 107 A/cm2 at 5 and 77 K, respectively, in zero field were obtained. To measure the magnetic relaxation the sample was cooled in a field of 1 T to the measurement temperature and then the field was switched off and the magnetization was recorded with defined time intervals. Magnetic relaxation at 5 K is shown in Fig. 1(a). The relaxation was very fast during the first 5 min after which it slowed down. The line fitted to the latter part of the relaxation
0921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.03.094
648
J. Raittila et al. / Physica C 408–410 (2004) 647–648
t (min) 10
100
0.025
1000
(b)
-12.12 0.02
S
2 ln(M(A m ))
(a) -12.14
0.015
-12.16 0.01
-12.18
0.005
-12. 2 5
6
7
8
9
10
11
0
20
ln(t(s))
40
60
80
T (K )
T/S (K)
Fig. 1. (a) Magnetic time relaxation of the film at 5 K. (b) The temperature dependence of the normalized relaxation rate, S.
12000
3. Conclusions
10000
We have prepared YBCO thin films by PLD using a target prepared from YBCO nanopowder. These films have very high critical currents Jc 1 107 A/cm2 at 77 K in zero field which is previously concluded to be caused by the nanostructure of the film [7]. From the T =S curve we have extrapolated the pinning potential to be U ð0 KÞ ¼ 900 K, which is higher than previously reported 342 K in 1 T for as-prepared films [8].
8000 6000 4000 2000 0 0
20
40
60
80
T (K) Fig. 2. Temperature dependence the T =S curve and extrapolation of value of U (0 K) from the 10, 20 and 30 K points.
curve was used for determination of the normalized relaxation rate [4] d lnM ; ð1Þ S¼ d lnt where M is the magnetization and t is time. The temperature dependence of S is shown in Fig. 1(b). The vortex-glass models [5,6] give the following relation between S and U (0 K), T U ð0 KÞ ¼ þ CkT ; ð2Þ S k where k is the Boltzmann constant and C is a constant. U (0 K) was obtained by plotting T =S curve in Fig. 2 and extrapolating the curve to 0 K. In the extrapolation only the low temperature region (<40 K) was used and the 5 K point was not used since the curve bends down due to quantum creep [5,6]. The obtained value is U ð0 KÞ ¼ 900 K.
Acknowledgements Academy of Finland and the Wihuri Foundation are acknowledged for financial support.
References [1] H. Huhtinen, P. Paturi, E. L€ahderanta, R. Laiho, Supercond. Sci. Technol. 12 (1999) 81. [2] J. Raittila, H. Huhtinen, P. Paturi, Y.P. Stepanov, Physica C 371 (2002) 90. [3] C.P. Bean, Phys. Rev. Lett. 8 (1962) 250. [4] Y. Yeshurun, A.P. Malozemoff, A. Shaulov, Rev. Mod. Phys. 68 (1996) 911. [5] H.-H. Wen, H.G. Schnack, R. Griessen, B. Dam, J. Rector, Physica C 241 (1995) 353. [6] F.C. Klaassen, G. Doornbos, J.M. Huijbregtse, R.C.F. van der Geest, B. Dam, R. Griessen, Phys. Rev. B 64 (2001) 184523. [7] P. Paturi, H. Huhtinen, K. Laajalehto, R. Laiho, Supercond. Sci. Technol. 13 (2000) 622. [8] E. Moraitakis, M. Pissas, G. Kallias, D. Niarchos, Supercond. Sci. Technol. 12 (1999) 682.
High pinning potential in YBCO thin films deposited from a target prepared from YBCO nanopowder J. Raittila a
a,b,*
, M. Peurla a, H. Huhtinen a, P. Paturi a, R. Laiho
a
Wihuri Physical Laboratory, Department of Physics, University of Turku, FIN-20014 Turku, Finland b Graduate School of Materials Research, Turku, Finland
Abstract Thin YBa2 Cu3 O6þx (YBCO) films were prepared on SrTiO3 substrates by pulsed laser deposition using a target made from nanosized powder. These films show high critical current densities Jc 1 107 A/cm2 at 77 K in zero field as obtained by SQUID magnetization measurements. The pinning potentials, U ðT Þ, were calculated from the magnetic relaxation curves measured at several temperatures after cooling the sample in 1 T. From these data the U (0 K) was extrapolated to be 900 K, which is higher than previously reported for as-prepared films. Ó 2004 Elsevier B.V. All rights reserved. PACS: 74.60.Ge Keywords: Superconducting films; YBCO; Flux pinning
1. Introduction YBCO thin films prepared by laser deposition from a target prepared from YBCO nanopowder were previously shown to exhibit very high critical current density, Jc , and a very smooth surface [1]. Since no artificial defects are induced and no evidence from defects in the surface structure was found, a pinning mechanism is important matter to be resolved. In this work we have investigated the hysteresis loops and magnetic relaxation in the films. Jc is calculated using the Bean model and the pinning potential U (0 K) is obtained by fitting the obtained data to a vortex-glass model. 2. Experimental and results Superconducting YBCO thin films were prepared by pulsed laser deposition from a target [1] sintered from * Corresponding author. Address: Wihuri Physical Laboratory, Department of Physics, University of Turku, FIN-20014 Turku, Finland. Tel.: +358-2-333-5676; fax: +358-2-231-9836. E-mail address: jussi.raittila@utu.fi (J. Raittila).
nanophase powder prepared by the citrate gel method [2]. The films were ablated with a XeCl (308 nm) excimer laser on SrTiO3 (1 0 0) substrates. During the deposition oxygen pressure was 0.3 Torr and the temperature of the substrate was 750 °C. Magnetic properties of the films were investigated with a SQUID magnetometer. The films were attached to the sample holder so that their plane was perpendicular to the magnetic field during the measurements. The critical temperature, Tc ¼ 91 K, was determined from the ZFC magnetization curve. Hysteresis loops were measured at temperatures between 5 and 80 K and the Jc (in SI units) was calculated from the Bean model [3] Jc ¼ 3DM=R, where DM (A/m) is half of the opening of the loop and R (m) is the radius of the disk like film. Very high values of Jc 9 107 A/cm2 and Jc 1 107 A/cm2 at 5 and 77 K, respectively, in zero field were obtained. To measure the magnetic relaxation the sample was cooled in a field of 1 T to the measurement temperature and then the field was switched off and the magnetization was recorded with defined time intervals. Magnetic relaxation at 5 K is shown in Fig. 1(a). The relaxation was very fast during the first 5 min after which it slowed down. The line fitted to the latter part of the relaxation
0921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.03.094
648
J. Raittila et al. / Physica C 408–410 (2004) 647–648
t (min) 10
100
0.025
1000
(b)
-12.12 0.02
S
2 ln(M(A m ))
(a) -12.14
0.015
-12.16 0.01
-12.18
0.005
-12. 2 5
6
7
8
9
10
11
0
20
ln(t(s))
40
60
80
T (K )
T/S (K)
Fig. 1. (a) Magnetic time relaxation of the film at 5 K. (b) The temperature dependence of the normalized relaxation rate, S.
12000
3. Conclusions
10000
We have prepared YBCO thin films by PLD using a target prepared from YBCO nanopowder. These films have very high critical currents Jc 1 107 A/cm2 at 77 K in zero field which is previously concluded to be caused by the nanostructure of the film [7]. From the T =S curve we have extrapolated the pinning potential to be U ð0 KÞ ¼ 900 K, which is higher than previously reported 342 K in 1 T for as-prepared films [8].
8000 6000 4000 2000 0 0
20
40
60
80
T (K) Fig. 2. Temperature dependence the T =S curve and extrapolation of value of U (0 K) from the 10, 20 and 30 K points.
curve was used for determination of the normalized relaxation rate [4] d lnM ; ð1Þ S¼ d lnt where M is the magnetization and t is time. The temperature dependence of S is shown in Fig. 1(b). The vortex-glass models [5,6] give the following relation between S and U (0 K), T U ð0 KÞ ¼ þ CkT ; ð2Þ S k where k is the Boltzmann constant and C is a constant. U (0 K) was obtained by plotting T =S curve in Fig. 2 and extrapolating the curve to 0 K. In the extrapolation only the low temperature region (<40 K) was used and the 5 K point was not used since the curve bends down due to quantum creep [5,6]. The obtained value is U ð0 KÞ ¼ 900 K.
Acknowledgements Academy of Finland and the Wihuri Foundation are acknowledged for financial support.
References [1] H. Huhtinen, P. Paturi, E. L€ahderanta, R. Laiho, Supercond. Sci. Technol. 12 (1999) 81. [2] J. Raittila, H. Huhtinen, P. Paturi, Y.P. Stepanov, Physica C 371 (2002) 90. [3] C.P. Bean, Phys. Rev. Lett. 8 (1962) 250. [4] Y. Yeshurun, A.P. Malozemoff, A. Shaulov, Rev. Mod. Phys. 68 (1996) 911. [5] H.-H. Wen, H.G. Schnack, R. Griessen, B. Dam, J. Rector, Physica C 241 (1995) 353. [6] F.C. Klaassen, G. Doornbos, J.M. Huijbregtse, R.C.F. van der Geest, B. Dam, R. Griessen, Phys. Rev. B 64 (2001) 184523. [7] P. Paturi, H. Huhtinen, K. Laajalehto, R. Laiho, Supercond. Sci. Technol. 13 (2000) 622. [8] E. Moraitakis, M. Pissas, G. Kallias, D. Niarchos, Supercond. Sci. Technol. 12 (1999) 682.