  In this work we present frequency-dependent magnetic susceptibility and dc
electric transport properties of three different compositions of hexaferrite
Ba$_{1-x}$Pb$_{x}$Fe$_{12-y}$Al$_{y}$O$_{19}$. We find a correlation between
activation energies of dc electric transport and ac magnetic susceptibility
which persists in the whole researched range of aluminium substitution $x=0$ to
$3.3$. This result is discussed in the context of charged magnetic domain
walls, the pinning of which is determined by charge carriers activated over the
transport gap. Our work points toward a general relaxational mechanism in
ferrimagnetic semiconductors which directly affects dynamic magnetic properties
via electric transport.

Strongly Correlated Electrons