  The critical properties at the ferromagnetic - paramagnetic transition have
been analysed from data of static magnetization measurements on single crystals
of Pr1-xPbxMnO3, for x = 0.23 and x = 0.30. In Pr1-xPbxMnO3 the ferromagnetic
ordering and the metal-insulator transition do not coincide in parts of the
phase diagram. The crystal with x = 0.23 is a ferromagnetic insulator with
Curie temperature Tc = 173 K, while the crystal with x =0.30 has Tc = 198 K and
remains metallic up to a metal-insulator transition temperature Tmi = 235 K.
The dc magnetization measurements were carried out in the field range from 0 to
5 T for an interval in the critical temperature range Tc+_10 K corresponding to
a reduced temperature interval 0.003 < epsilon < 0.6. The exponents beta for
spontaneous magnetization, gamma for the initial susceptibility above Tc and
delta for the critical magnetization isotherm at Tc were obtained by static
scaling analysis from modified Arrott plots and by the Kouvel Fisher method for
the insulating crystal with composition x = 0.23. The data are well described
by critical exponents similar to those expected for the Heisenberg universality
class relevant for conventional isotropic magnets Systematic deviations from
scaling in the data for the metallic crystal with composition x = 0.30 are
demonstrated from effective critical exponents near the assumed ordering
transition. The unconventional magnetic ordering in this system indicates the
presence of frustrated magnetic couplings that suppresses magnetic ordering and
lowers the transition temperature.

Strongly Correlated Electrons