  The magnetoresistivity and critical current density of well characterized
Si-nanoparticle doped and undoped Cu-sheathed MgB$_{2}$ tapes have been
measured at temperatures $T\geq 28$ K in magnetic fields $B\leq 0.9$ T. The
irreversibility line $B_{irr}(T)$ for doped tape shows a stepwise variation
with a kink around 0.3 T. Such $B_{irr}(T)$ variation is typical for
high-temperature superconductors with columnar defects (a kink occurs near the
matching field $% B_{\phi}$) and is very different from a smooth $B_{irr}(T)$
variation in undoped MgB$_{2}$ samples. The microstructure studies of
nanoparticle doped MgB$_{2}$ samples show uniformly dispersed nanoprecipitates,
which probably act as a correlated disorder. The observed difference between
the field variations of the critical current density and pinning force density
of the doped and undoped tape supports the above findings.
