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

Journal name:
Nature Materials
Volume:
9,
Pages:
628–633
Year published:
DOI:
doi:10.1038/nmat2795
Received
Accepted
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

Figures

  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.

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Affiliations

  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

Contributions

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.

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