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High density nanocrystalline MgB2 bulk superconductors with distinctly improved pinning were prepared by mechanical alloying of Mg and B powders at ambient temperatures followed by hot pressing. The nanocrystalline samples reveal high jc=105 A/cm2 at 20 K and 1 T together with an irreversibility line strongly shifted towards higher fields resulting in Hirr(T)∼0.8 Hc2(T), whereas typically Hirr(T)∼0.5 Hc2(T) is observed for untextured bulk samples. These values exceed those of all other reported bulk samples and are in the range of values for thin films. The improved pinning of this material, which mainly consists of spherical grains about 40–100 nm in size, is attributed to the large number of grain boundaries in the nanocrystalline state.
Gümbel et al. (Mon,) studied this question.