  Highly dense nanocrystalline MgB_2 bulk superconductors with distinctly
improved pinning were prepared by mechanical alloying of Mg and B powders and
hot compaction at ambient temperatures. The nanocrystalline samples reveal high
critical current densities of 105 A/cm2 at 20 K and 1 T together with a
strongly shifted irreversibility line towards higher fields resulting in
H_irr(T) ~ 0.8 H_c2(T), whereas typically H_irr(T) ~ 0.5 H_c2(T) is observed
for bulk untextured samples. These values exceed that of all other so far
reported bulk samples and are in the range of the values of thin films. The
improved pinning of this material which mainly consists of spherical grains of
about 40-100 nm in size is attributed to the large number of grain boundaries.
