  We report a detailed set of AC magnetic measurements carried out on bulk
large grain La-Ca-Mn-O samples extracted from a floating zone method-grown rod.
Three samples with $La_{0.78}Ca_{0.22}Mn_{0.90}O_{x}$ stoichiometry but
differing in their microstructure were investigated by electrical resistivity
and AC susceptibility measurements: (i) a single grain sample, (ii) a sample
containing two grains and (iii) a polycrystalline sample. We show that the
superimposition of DC magnetic fields during AC magnetic susceptibility
measurements is an efficient way for characterizing the magnetic transition of
samples with different microstructures. Whereas both single grain and
polycrystalline samples display a single susceptibility peak, an additional
kink structure is observed in the case of the double grain sample. The
temperature dependence of the AC susceptibility measured with superimposed DC
magnetic fields is analyzed in the framework of second-order phase transition
ideas. The relations between the critical exponents ($\beta + \gamma$ ~ 1.5,
$\delta$ ~ 2.5) are found to be close to those of the mean-field model for all
samples. This is attributed to the disordering caused by unoccupied Mn sites.
