  A Landau-type phenomenological cluster theory was presented to model the
freezing process of local polarization in relaxors. Based on the theory, the
proportion of frozen polarization in Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$-PbTiO$_3$
(PMNT) was calculated from the experiment of dielectric nonlinearity. The local
polarization was shown to freeze continuously in a cooling process. The amount
of frozen polarization increases with increasing the measuring frequency.
