  We have studied the dynamic response of Fe doped manganites with ac
susceptibility measurements on bulk La0.65Ca0.35Mn1-yFeyO3 with 0.01<y<0.10 as
functions of temperature and dc field. It is observed that the in phase part
goes through a maximum that is removed on the application of moderate dc field.
DC fields suppress both the components with the strongest effects being at or
close to Tc. Conduction and ferromagnetism has been consistently suppressed by
Fe doping. Increased spin disorder and decrease of Tc with increasing Fe
content are evident. The variations in the critical temperature Tc and magnetic
moment show a rapid change at about 4-5% Fe. The effect of Fe is seen to be
consistent with the disruption of the Mn-Mn exchange possibly due to the
formation of magnetic clusters. There is clear correlation between the
structures in the resistivity and the in and out of phase parts of the
susceptibility. With increasing Fe concentration the out of phase part (x'')
peak shift to T<T1/2, but it stays consistently at the temperature where the
resistivity changes slow down. The shoulder in x'' disappears above 4% Fe
concentration. The peak of x'' moves to 8 or 10 K higher temperature on the
application of a dc field, for 3 & 4 %Fe samples. Consistently two peaks are
found in the ratios of x''/x'. We are seeing correspondence of dissipation with
the change in R. We also see increasing low temperature dissipation in more
strongly Fe doped samples i.e. increasing the Fe, leads to increased spin
disorder and dissipation at low temperature. The effect of the dc field is
discussed in terms of the suppression of spin fluctuations close to Tc and the
changes in coercivity for lower temperatures.
