  Magnetic relaxation measurements on a series of
Y$_{1-x}$Pr$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$ ($x = 0.13, 0.34, 0.47$) single
crystals were performed over a large field and temperature range in order to
investigate the characteristics of the vortex matter across the second
magnetization peak (SMP). The magnitude of the SMP varies non-monotonically
with the Pr concentration; i.e., the irreversible magnetization normalized by
its value at the onset field $H_{on}$ displays a maximum for the $x = 0.34$
single crystal. The two characteristic fields, $H_{on}$ and $H_{sp}$, follow
different temperature $T$ dependences: $H_{on} \propto T^{\nu_{on}}$ and
$H_{sp} \propto [1-(T/T_{c})^{2}]^{\nu_{sp}}$. The extracted values of the
apparent activation energy $U^\ast$ and the creep exponent $\mu$ display a
maximum at a field $H_{on} < H^\ast < H_{sp}$. Their field dependences point
toward the coexistence of both elastic and plastic creep for $H > H_{on}$. The
degree of participation of each creep mechanism is determined by the charge
carrier density, which controls both the elastic properties of the vortex
matter and the pinning potential.
