Pinning in Al doped YBCO bulk superconductors V. Antal 1,2, M. Kaňuchová 1, M. Šefčiková 1, P. Diko 1,3 M. Eisterer 2, N. Hörhager 2, M. Zehetmayer 2, H. W. Weber 2 X. Chaud 3 1 Material Physics Laboratory, Institute of Experimental Physics, SAS, Košice, Slovakia 2 Atomic Institute of the Austrian Universities, Vienna, Austria 3 CNRS/CRETA, 25, Avenue des Martyrs, Grenoble Cedex 9, France

Motivation : Cu substitutions in TSMG YBCO bulk superconductors Zn Krabbes et al., Physica C, 2000 Li Shlyk et al., Appl. Phys. Lett., 2002 Pd, Ni Shlyk et al., Physica C, 2002 Fe Shlyk et al., Journal of Physics, 2006 Zn, Ni, Co, Ga Zhou, Scruggs, Salama SUST, 2006 Ag T. Nakashima, Journal of Physics, 2008 Al substitution was not studied in YBCO bulks so far Al Al substitutes in CuO chains (Re = Y)

Al - doped TSMG YBCO bulk Preparation of Al – doped YBCO bulk superconductors Time - temperature program for the top seeded melt growth (TSMG) process Samples: YBa 2 Cu 3 O Y 2 O wt. % CeO 2 + Al 2 O 3 YBa 2 (Cu 3-x Al x )O 7, (x = , 0.005, 0.01, 0.02, 0.05) TSMG: pellets  = 20 mm, Sm 123 seeds, chamber furnace in air

The samples were heated to 800 °C in flowing O 2 atmosphere and kept there for 2 hrs, then cooled to 400 °C and held at this temperature for 240 hrs, then cooled down to room temperature. 1) Standard oxygenation (SO) 2) Annealing in flowing argon The samples were heated to 800 °C in flowing Ar atmosphere and annealed there for 2 hrs, then cooled to room temperature. After annealing in Ar the samples were heated to 400 °C in flowing O 2 atmosphere and held there for 240 hrs, then cooled down to room temperature. 3) High pressure oxygenation (HPO) The samples were oxygenated under an oxygen pressure of 160 bars at 750 °C for 24 hrs with progressively increasing oxygen pressure. Different methods of oxygenation Typical dimensions of the samples for oxygenation and magnetization measurements c - axis

Magnetization measurements: VSM with magnetic fields of up to 5 Tesla at a constant sweep rate of 0.25 T/min, applied field parallel to the c-axis,, Bean model Magnetization measurements Checking samples for homogeneity by trapped field scanning

Critical current densities in samples treated by SO and annealing in argon Annealed in argon Standard oxygenation x = (SO), x = 0.05 (Ar) J c ≈ 1.8·10 4 A/cm 2, μ 0 H = 1 T

Effect of Al – doping and HPO on critical current density HPO HPO, T = 77 K x = J c peak ≈ 5.3·10 4 A/cm 2, μ 0 H p ≈ 1.37 T SO, T = 77 K x = J c peak ≈ 1.8·10 4 A/cm 2, μ 0 H p ≈ 0.89 T (55 ~80 K)

Suppression of oxygenation cracks by high pressure oxygenation c a/b c c- and a/b-macrocracks formed in TSMG YBa 2 (Cu 3-x Al x )O 7 during standard oxygenation at 400 °C (SO) a/b-microcracks in TSMG YBa 2 (Cu 3-x Al x )O 7 oxygenated at 750 °C under an oxygen pressure of 160 bars (HPO) c-MAC a/b-MAC c-MAC

Pinning force density HPO HPO: 1.58 ≤ p ≤ 2.11; 7.10 ≤ q ≤ 8.28 SO: 1.07 ≤ p ≤ 1.38; 2.86 ≤ q ≤ 4.51 for SO x = p = 1.86, q = 3.44 SO: 1.07 ≤ p ≤ 1.38; 2.86 ≤ q ≤ 4.51 for SO x = p = 1.86, q = 3.44 Argon: 1.12 ≤ p ≤ 1.25; 0.63 ≤ q ≤ 0.93 for x = 0.05 p = 1.83; q = 3.44 Argon: 1.12 ≤ p ≤ 1.25; 0.63 ≤ q ≤ 0.93 for x = 0.05 p = 1.83; q = 3.44 [E.J. Kramer, J. Appl. Phys. 44, 1360 (1973)] p, q > 0; b fp =H max /H a F n =F p /F max [M. Jirsa et al., Physica C 338, 235 (2000)] 77 K

Magnetization relaxation measurements at μ 0 H = 1 T and T = 77 K S (SO) = ≈ 9.11% S (HPO) = ≈ 5.68%

Conclusions - Al substitution induces a peak effect in TSMG YBCO bulks - Annealing in Ar caused significant changes in both T c and pinning behaviour - High pressure oxygenation preserves the peak effect caused by Al substitution - Oxygenation cracks are eliminated by high pressure oxygenation J c increases by a factor of ~ 3 The results were presented at : P. Diko et al., “International Conference on Superconductivity and Magnetism (ICSM 2008) August 2008, Side Antalya,Turkey V. Antal et al., 2 nd NESPA meeting, Rust, Austria. Publication accepted in Journal of Physics: Conference Series (2009).

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