S. Patnaik

Strongly linked current flow in polycrystalline forms of the superconductor MgB2.

The discovery of superconductivity at 39u2009K in magnesium diboride, MgB2, raises many issues, a critical one being whether this material resembles a high-temperature copper oxide superconductor or a low-temperature metallic superconductor in terms of its behaviour in strong magnetic fields. Although the copper oxides exhibit very high transition temperatures, their in-field performance is compromized by their large anisotropy, the result of which is to restrict high bulk current densities to a region much less than the full magnetic-field–temperature (H–T) space over which superconductivity is found. Moreover, the weak coupling across grain boundaries makes transport current densities in untextured polycrystalline samples low and strongly sensitive to magnetic field. Here we report that, despite the multiphase, untextured, microscale, subdivided nature of our MgB2 samples, supercurrents flow throughout the material without exhibiting strong sensitivity to weak magnetic fields. Our combined magnetization, magneto-optical, microscopy and X-ray investigations show that the supercurrent density is mostly determined by flux pinning, rather than by the grain boundary connectivity. Our results therefore suggest that this new superconductor class is not compromized by weak-link problems, a conclusion of significance for practical applications if higher temperature analogues of this compound can be discovered.

High critical current density and enhanced irreversibility field in superconducting MgB2 thin films

Larbalestier †§ N. Rogado*, K.A. Regan*, M.A. Hayward*, T. He*, J.S. Slusky*, K. Inumaru*, M.K. Haas* and R.J. Cava* † Department of Materials Science and Engineering, Univer-sity of Wisconsin, 1509 University Avenue, Madison, WI 53706 USA § Applied Superconductivity Center, University of Wisconsin, 1500 Engineering Drive, Madison, WI 53706 USA * Department of Chemistry and Princeton Materials Institute, Princeton University, Princeton, NJ 08544 USAThe discovery of superconductivity at 39u2009K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. This compound has twice the transition temperature of Nb3Snxa0and four times that of Nb-Ti alloy, and the vital prerequisite of strongly linked current flow has already been demonstrated. One possible drawback, however, is that the magnetic field at which superconductivity is destroyed is modest. Furthermore, the field which limits the range of practical applications—the irreversibility field H*(T)—is approximately 7u2009T at liquid helium temperature (4.2u2009K), significantly lower than about 10u2009T for Nb-Ti (ref. 6) and ∼20u2009T for Nb3Sn (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2u2009K greater than 14u2009T. In addition, very high critical current densities at 4.2u2009K are achieved: 1u2009MAu2009cm-2 at 1u2009T and 105u2009Au2009cm-2 at 10u2009T. These results demonstrate that MgB2 has potential for high-field superconducting applications.

Thin Film Magnesium Boride Superconductor with Very High Critical Current Density and Enhanced Irreversibility Field

Larbalestier †§ N. Rogado*, K.A. Regan*, M.A. Hayward*, T. He*, J.S. Slusky*, K. Inumaru*, M.K. Haas* and R.J. Cava* † Department of Materials Science and Engineering, Univer-sity of Wisconsin, 1509 University Avenue, Madison, WI 53706 USA § Applied Superconductivity Center, University of Wisconsin, 1500 Engineering Drive, Madison, WI 53706 USA * Department of Chemistry and Princeton Materials Institute, Princeton University, Princeton, NJ 08544 USAThe discovery of superconductivity at 39u2009K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. This compound has twice the transition temperature of Nb3Snxa0and four times that of Nb-Ti alloy, and the vital prerequisite of strongly linked current flow has already been demonstrated. One possible drawback, however, is that the magnetic field at which superconductivity is destroyed is modest. Furthermore, the field which limits the range of practical applications—the irreversibility field H*(T)—is approximately 7u2009T at liquid helium temperature (4.2u2009K), significantly lower than about 10u2009T for Nb-Ti (ref. 6) and ∼20u2009T for Nb3Sn (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2u2009K greater than 14u2009T. In addition, very high critical current densities at 4.2u2009K are achieved: 1u2009MAu2009cm-2 at 1u2009T and 105u2009Au2009cm-2 at 10u2009T. These results demonstrate that MgB2 has potential for high-field superconducting applications.

Very high upper critical fields in MgB2 produced by selective tuning of impurity scattering

We report a significant enhancement of the upper critical field Hc2 of different MgB2 samples alloyed with nonmagnetic impurities. By studying films and bulk polycrystals with different resistivities ρ ,w e sho wac lear trend of a ni ncrease in Hc2 as ρ increases. One particular high resistivity film had a zero-temperature Hc2(0) well above the Hc2 values of competing non-cuprate superconductors such as Nb3Sn and Nb–Ti. Our high-field transport measurements give record values H ⊥ c2 (0) ≈ 34 T and H || c2 (0) ≈ 49 T for high resistivity films and Hc2(0) ≈ 29 T for untextured bulk polycrystals. The highest Hc2 film also exhibits a significant upward curvature of Hc2(T ) and a temperature dependence of the anisotropy parameter γ( T ) = H || c2 /H ⊥ c2 opposite to that of single crystals: γ( T ) decreases as the temperature decreases, from γ( Tc) ≈ 2t o γ( 0) ≈ 1.5. This remarkable Hc2 enhancement and its anomalous temperature dependence are a consequence of the two-gap superconductivity in MgB2 ,w hich offers special opportunities for further Hc2 increases by tuning of the impurity scattering by selective alloying on Mg and B sites. Our experimental results can be explained by a theory of two-gap superconductivity in the dirty limit. The very high values of Hc2(T ) observed suggest that MgB2 can be made into a versatile, competitive high-field superconductor.

Improved upper critical field in bulk-form magnesium diboride by mechanical alloying with carbon

High-energy milling of magnesium diboride (MgB2) prereacted powder renders the material largely amorphous through extreme mechanical deformation and is suitable for mechanically alloying MgB2 with dopants including carbon. Bulk samples of milled carbon and MgB2 powders subjected to hot isostatic pressing and Mg vapor annealing have achieved upper critical fields in excess of 32T and critical current density approaching 106A∕cm2.

Electronic anisotropy, magnetic field-temperature phase diagram and their dependence on resistivity in c-axis oriented MgB2 thin films

An important predicted, but so far uncharacterized, property of the new superconductor MgB2 is electronic anisotropy arising from its layered crystal structure. Here we report on three c-axis oriented thin films, showing that the upper critical field anisotropy ratio Hc2?/Hc2? is 1.8 to 2.0, the ratio increasing with higher resistivity. Measurements of the magnetic field-temperature phase diagram show that flux pinning disappears at H*?0.8Hc2?(T) in untextured samples. Hc2?(0) is strongly enhanced by alloying to 39 T for the highest resistivity film, more than twice that seen in bulk samples.

Significant enhancement of irreversibility field in clean-limit bulk MgB2

Low resistivity MgB2 bulk samples annealed in Mg vapor show an increase in irreversibility field μ0H*(T) by a factor of ∼2 in both transport and magnetic measurements. The best sample displayed a magnetic irreversibility field μ0HM*>14u2009T at 4.2 K and ∼6 T at 20 K. These changes were accompanied by an increase of the 40 K resistivity from 1 to 18 μΩu200acm and a lowering of the resistivity ratio from 15 to 3, while the critical temperature Tc decreased by only 1–2 K. These results show that systematic processing changes can make MgB2 attractive for magnet applications.

Critical current limiting factors in post annealed (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ tapes

Processing (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ (Bi-2223) tape by the oxide-powder-in-tube technique normally includes two heat treatments and one intermediate rolling. A three-step heat treatment was applied for the second heat treatment (HT2). Experimental procedures were designed for understanding the roles of each processing step in HT2 and the post anneal. It was found that post annealing improved the critical temperature T/sub c/, critical current density J/sub c/ and flux pinning characteristic field H/sub p/. The midpoint T/sub c/ was raised from 103 to 109 K by post anneal, while H/sub p/ was increased from 126 to 183 mT. Post anneal at about 790/spl deg/C precipitated Pb from Bi-2223 phase to form a Pb-rich phase, thus the microstructure looked worse but T/sub c/, J/sub c/ and H/sub p/ were raised. J/sub c/ was raised as the Bi-2212 phase signature in the T/sub c/ trace decreased. The most important observation is that raising T/sub c/ and minimizing residual Bi-2212 are decisive factors for increasing J/sub c/ at 77 K in Bi-2223 tapes.

Thermally activated current transport in MgB2 films

Thermally-activated flux flow (TAFF) resistivity above the irreversibility field is reported for two different c-axis textured MgB_2 superconducting films. Transport measurements at different perpendicular magnetic fields 0 < B < 9 T and temperatures from 5 to 40K reveal TAFF Ohmic resistivity described by the Arrhenius law with the quadratic field dependence of the activation energy. Our transport measurements on bulk MgB_2 ceramic samples also show the TAFF behavior, but do not show the quadratic field dependence of U(T,B). We explain our results in terms of thermally-activated drift of pre-existing quenched dislocations in the vortex lattice. Our results indicate that thermal fluctuations can be essential in determining the irreversibility field in MgB2 though to a much lesser extent than in high-temperature superconductors.

Interfacial structure of epitaxial MgB2 thin films grown on (0001) sapphire

The microstructure and interfacial atomic structure of MgB2 thin films fabricated on the (0001) Al2O3 substrate were characterized by transmission electron microscopy. It was found that the MgB2 films grow epitaxially on the substrate with an orientation relationship with respect to the substrate as: (0001)MgB2∥(0001)Al2O3 and [1120]MgB2∥[1010]Al2O3. At the film/substrate interface, both MgO and MgAl2O4 phases were observed, which also grow epitaxially on the (0001) Al2O3 substrate. The formation of these intermediate phases is ascribed to the existence of oxygen during the annealing.