  The peak effect in critical current density J_c is investigated by studying
the flux dynamics in V_3Si using bulk magnetometry, small-angle neutron
scattering, and transport measurements on clean single-crystal samples from the
same ingot. For a field-cooled history, well-defined structure in the vortex
lattice was found for fields and temperatures (H,T) below the peak-effect line
H_P(T); above this line, the structure disappeared. History-dependent,
metastable disorder is found only for (H,T) below H_P(T) but the vortex system
is reproducibly re-ordered either by field-cooling or a low-frequency, pulsed
"shaking" transport current. The latter is shown to attain Bardeen-Stephen flux
flow. In addition, flux flow is observed at H_P(T) at high current levels. The
results, though novel, support the traditional picture of H_P(T) as an
order-disorder transition due to the competition between elasticity and
pinning.
