High magnetic-field scales and critical currents in SmFeAs(O, F) crystals

Journal name:
Nature Materials
Year published:
Published online

With the discovery of new superconducting materials, such as the iron pnictides1, exploring their potential for applications is one of the foremost tasks. Even if the critical temperature Tc is high, intrinsic electronic properties might render applications difficult, particularly if extreme electronic anisotropy prevents effective pinning of vortices and thus severely limits the critical current density, a problem well known for cuprates2, 3, 4, 5. Although many questions concerning microscopic electronic properties of the iron pnictides have been successfully addressed5 and estimates point to a very high upper critical field6, 7, 8, 9, their application potential is less clear. Thus, we focus here on the critical currents, their anisotropy and the onset of electrical dissipation in high magnetic fields up to 65 T. Our detailed study of the transport properties of SmFeAsO0.7F0.25 single crystals reveals a promising combination of high (>2×106 A cm−2) and nearly isotropic critical current densities along all crystal directions. This favourable intragrain current transport in SmFeAs(O, F), which shows the highest Tc of 54 K at ambient pressure10, 11, 12, is a crucial requirement for possible applications. Essential in these experiments are four-probe measurements on focused-ion-beam-cut single crystals with a sub-square-micrometre cross-section, with current along and perpendicular to the crystallographic c axis.

At a glance


  1. Four-probe resistance bars for simultaneous c-axis and ab-plane resistivity measurements carved out of a SmFeAsO0.7F0.25 single crystal using the FIB.
    Figure 1: Four-probe resistance bars for simultaneous c-axis and ab-plane resistivity measurements carved out of a SmFeAsO0.7F0.25 single crystal using the FIB.

    a, A crystal is positioned on a substrate with the c axis pointing perpendicular to the plane. The dashed volume indicates the original crystal that is removed during FIB cutting, leaving only the lamellae standing. b, The lamella is transferred to another substrate and flipped, so that its c axis is now aligned in the plane (short edge). Most of this lamella (dashed line) is again removed, leaving only the small current path standing (violet). Eight platinum leads are deposited onto the crystal edges (all other colours) that are connected to the resistance bars by narrow (∼800 nm for c axis) crystal bridges. The common current is injected through the yellow contacts and traverses two c-axis resistance bars (blue and red voltage contacts) and one along the ab plane (green voltage contacts). Dimensions of resistance bars: length ∼35 μm (ab plane), ∼5 μm (c axis), cross-section ∼1.5 μm2.

  2. Normal-state resistivity ratio of SmFeAsO0.7F0.25.
    Figure 2: Normal-state resistivity ratio of SmFeAsO0.7F0.25.

    The normal-state resistivity ρc along the c axis of SmFeAsO0.7F0.25 in zero field increases with decreasing temperature, whereas the ab-plane resistivity ρab decreases. The resulting resistivity ratio ρc/ρab (dashed line) is fitted very well by an exponential dependence exp(−T/T0)+1.73, with T0=86.4 (±0.6) K (red line). Similar behaviour was observed in different samples.

  3. Magnetoresistance of SmFeAsO0.7F0.25 in pulsed fields up to 65 T at various temperatures for fields and currents along and perpendicular to the c axis.
    Figure 3: Magnetoresistance of SmFeAsO0.7F0.25 in pulsed fields up to 65 T at various temperatures for fields and currents along and perpendicular to the c axis.

    The four panels correspond to the various combinations of currents and fields oriented along or perpendicular to the crystallographic c axis. For the lowest temperatures with jc, a small hysteresis is observed if the field is applied in the FeAs planes. The hysteretic width depends on the field sweep rate and vanishes at low rates, indicating irreversible vortex trapping.

  4. Critical current density of SmFeAsO0.7F0.25.
    Figure 4: Critical current density of SmFeAsO0.7F0.25.

    a, Two free-standing SmFeAsO0.7F0.25 nanobridges (cross-section ∼600 nm×600 nm, length of narrow part ∼1–3 μm) cut into a single crystal lamella (similar to Fig. 1). Owing to the special cutting procedure, the c axis is in the plane of measurement, and this structure allows the direct measurement of jc along different axes. Three contacts for current injection were made on the sides of the crystal to ensure the current passes only one bridge during a measurement cycle. b, Critical current density jcmag of SmFeAsO0.7F0.25 determined from magnetic hysteresis loops (stars) after Bean24 and jctrans from direct critical transport experiments across the nanobridge shown in a, as well as another, similar sample from the same growth batch. Inset: Raw voltage data during pulsed current ramp. c, Temperature dependence of jcab for μ0H=1 T. At low temperatures, the critical current anisotropy jc(Hab)/jc(Hc) is reduced and the values of jc converge.

  5. Region for potenital application of SmFeAs(O, F).
    Figure 5: Region for potenital application of SmFeAs(O, F).

    A colour map of the critical current density jctransab(Hc) in SmFAsO0.7F0.25 bordered by H* (marking the dissipation level of 10 μΩ cm) measured in pulsed fields. The units for the critical currents are 106 A cm−2. The technically limiting H*c(jc) reaches 40 T at 15 K. H*c shows no signs of saturation and its slope of 3.4 T K−1 at 15 K suggests a significant further increase down to 4.1 K. The measurement jab, Hab was carried out in a full-Lorentz-force configuration, as it is the technologically most important. The critical current density increases steeply with decreasing temperature at any given field and reaches a high and only weakly field-dependent value at low temperatures. These favourable intragrain properties suggest a large HT region interesting for application.


  1. Kamihara, Y. et al. Iron-based layered superconductor: La(O1−xFx)FeAs (x=0.05–0.12) with T c=26 K. J. Am. Chem. Soc. 11, 32963297 (2008).
  2. Dimos, D., Chaudhari, P. & Mannhart, J. Superconducting transport properties of grain boundaries in YBa2Cu3O7 bicrystals. Phys. Rev. B 41, 40384049 (1990).
  3. Heinig, N. F., Redwing, R. D., Nordman, J. E. & Larbalestier, D. C. Strong to weak coupling transition in low misorientation angle thin film YBa2Cu3O7−x bicrystals. Phys. Rev. B 60, 14091417 (1999).
  4. Hilgenkamp, H. & Mannhart, J. Grain boundaries in high-T c superconductors. Rev. Mod. Phys. 74, 485549 (2002).
  5. Palstra, T. T. M., Batlogg, B., van Dover, R. B., Schneemeyer, L. F. & Waszczak, J. V. Dissipative flux-motion in high temperature superconductors. Phys. Rev. B 41, 66216632 (1990).
  6. Senatore, C. et al. Upper critical fields well above 100 T for the superconductor SmFeAsO0.85F0.15 with T c=46 K. Phys. Rev. B 78, 054514 (2008).
  7. Hunte, F. et al. Two-band superconductivity in LaFeAsO0.89F0.11 at very high magnetic fields. Nature 453, 903905 (2008).
  8. Jaroszynski, J. et al. Comparative high-field magnetotransport of the oxypnictide superconductors RFeAsO1−xFx (R=La, Nd) and SmFeAsO1−δ . Phys. Rev. B 78, 064511 (2008).
  9. Lee, H-S. et al. Effects of two gaps and paramagnetic pair-breaking effects on the upper critical field of SmFeAsO0.85 and SmFeAsO0.8F0.2 single crystals. Phys. Rev. B 80, 144512 (2009).
  10. Liu, R. H. et al. Anomalous transport properties and phase diagram of the FeAs-based SmFeAsO1−xFx superconductors. Phys. Rev. Lett. 101, 087001 (2008).
  11. Chen, X. H. et al. Superconductivity at 43 K in SmFeAsO1−xFx . Nature 453, 761762 (2008).
  12. Ren, Z. A. et al. Superconductivity at 55 K in iron-based F-doped layered quaternary compound Sm[O1−xFx] FeAs. Chin. Phys. Lett. 25, 22152216 (2008).
  13. Zhigadlo, N. D. et al. Single crystals of superconducting SmFeAsO1−xFy grown at high pressure. J. Phys. Condens. Matter 20, 342202 (2008).
  14. Karpinski, J. et al. Single crystals of LnFeAsO1−xFx (Ln=La, Pr, Nd, Sm, Gd) and Ba1−xRbxFe2As2: Growth, structure and superconducting properties. Physica C 469, 370380 (2009).
  15. Jaroszynski, J. et al. Upper critical fields and thermally-activated transport of NdFeAsO0.7F0.3 single crystal. Phys. Rev. B 78, 174523 (2008).
  16. Yang, J. et al. The role of F-doping and oxygen vacancies on the superconductivity in SmFeAsO compounds. Supercond. Sci. Technol. 22, 025004 (2009).
  17. Ando, Y. et al. Metallic in-plane and divergent out-of-plane resistivity of a high-T c cuprate in the zero-temperature limit. Phys. Rev. Lett. 77, 20652068 (1996).
  18. Tanatar, M. A. et al. Anisotropy of the iron pnictide superconductor Ba(Fe1−xCox)2As2 (x=0.074, T c=23 K). Phys. Rev. B 79, 094507 (2009).
  19. Terashima, T. et al. Resistivity and upper critical field in KFe2As2 single crystals. J. Phys. Soc. Jpn 78, 063702 (2009).
  20. Tozer, S. W. et al. Measurement of anisotropic resistivity and Hall constant for single-crystal YBa2Cu3O7−x . Phys. Rev. Lett. 59, 17681771 (1987).
  21. Ono, S. & Ando, Y. Evolution of the resistivity anisotropy in Bi2Sr2−xLaxCuO6+δ single crystals for a wide range of hole doping. Phys. Rev. B 67, 104512 (2003).
  22. Ando, Y. et al. Resistive upper critical fields and irreversibility lines of optimally doped high-T c cuprates. Phys. Rev. B 60, 1247512479 (1999).
  23. Lyard, L. et al. Anisotropy of the upper critical field and critical current in single crystal MgB2 . Phys. Rev. B 66, 180502 (2002).
  24. Bean, C. P. Magnetization of hard superconductors. Phys. Rev. Lett. 8, 250253 (1962).
  25. Gyorgy, E. M., van Dover, R. B., Jackson, K. A., Schneemeyer, L. F. & Waszczak, J. V. Anisotropic critical currents in Ba2YCu3O7 analyzed using an extended Bean model. Appl. Phys. Lett. 55, 283285 (1989).
  26. Prozorov, R. et al. Vortex phase diagram of Ba(Fe0.93Co0.07)2As2 single crystals. Phys. Rev. B 78, 224506 (2008).
  27. Bukowski, Z. et al. Superconductivity at 23 K and low anisotropy in Rb-substituted BaFe2As2 single crystals. Phys. Rev. B 79, 104521 (2009).
  28. Larbalestier, D., Gurevich, A., Feldmann, D. M. & Polyanskii, A. High-T c superconducting materials for electric power applications. Nature 414, 368377 (2001) insight review.
  29. Cai, X. Y. et al. Static and dynamic mechanisms of the anomalous field dependence of magnetization in Bi–Sr–Ca–Cu–O and Bi–Pb–Sr–Ca–Cu–O single crystals. Phys. Rev. B 50, 1677416777 (1994).
  30. Dew-Hughes, D. Flux pinning mechanisms in type II superconductors. Phil. Mag. 30, 293305 (1974).
  31. Koblischka, M. R., van Dalen, A. J. J., Higuchi, T., Yoo, S. I. & Murakami, M. Analysis of pinning in NdBa2Cu3O7−d superconductors. Phys. Rev. B 58, 28632867 (1998).
  32. Rogacki, K., Dabrowski, B. & Chmaissem, O. Increase of critical currents and peak effect in Mo-substituted YBa2Cu3O7 . Phys. Rev. B 73, 224518 (2006).

Download references

Author information


  1. Laboratory for Solid State Physics, ETH Zurich, Schafmattstr. 16, CH-8093 Zurich, Switzerland

    • Philip J. W. Moll,
    • Janusz Karpinski,
    • Nikolai D. Zhigadlo &
    • Bertram Batlogg
  2. Institute of Physics, Polish Academy of Sciences, Aleja Lotników 32/46, PL-02-668 Warsaw, Poland

    • Roman Puzniak
  3. National High Magnetic Field Laboratory, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

    • Fedor Balakirev
  4. Institute of Low Temperature and Structure Research, Polish Academy of Sciences, ul. Okolna 2, PL-50-422 Wroclaw, Poland

    • Krzysztof Rogacki


P.J.W.M. and B.B. designed the experiment and wrote the paper. P.J.W.M. carried out the direct transport critical current experiments; the pulsed field magnetotransport was measured by P.J.W.M. and F.B. R.P., K.R. and B.B. measured magnetization and evaluated the magnetic critical currents jcmag. K.R. carried out the pinning analysis. N.D.Z. and J.K. grew the crystals.

Competing financial interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to:

Author details

Supplementary information

PDF files

  1. Supplementary Information (613 KB)

    Supplementary Information

Additional data