  We have studied the magnetic and transport behavior of doped La0.65 A0.35
Mn0.95Fe0.05 O3 (A= Ca, Sr, Pb, Ba) compounds. All the compositions show
ferromagnetic/metal to paramagnetic/insulator transition except the Pb doped
sample which is insulating and ferromagnetic in the entire temperature range.
The simultaneous occurrence of ferromagnetism and insulating behavior in Pb
doped compound is most likely due to the presence of FM clusters separated by
Fe and Mn ions that are coupled AFM and hence prevent the current from crossing
the inter-domain region. The magnetization and Tc are decreased by decreasing
the Cation size on La site. We observed that (for a fixed Fe content of 5%) the
transition temperature and magnetic moment at 77 K is a maximum for Sr doped
sample and is decreasing if we increase or decrease the cation size from Sr
size. The maximum value of Tc and magnetic moment for Sr based sample is most
likely due to the closer ionic sizes of La and Sr as compared to the other
dopants (Ca, Pb, Ba). The decrease in the transition temperature and magnetic
moment is due to the deviation of Mn-O-Mn angle from 1800 caused by the size
mismatch of the A site cations. This deviation leads to weakening of the FM
double exchange (DE). We observed a spin freezing type effect in the Pb doped
sample below 120 K in resistivity, AC susceptibility and in magnetization. This
suggests that the AFM interactions introduced by the Fe are most effective in
the Pb doped composition leading to increased competition between the FM and
AFM interactions. This FM and AFM interaction generates some degree of
frustration leading to the appearance of spin glass like phase whose typical
magnetic behavior is studied for small ion when the metallic like behavior is
lost.
