  The effect of Fe substitution on Mn sites in the colossal magnetoresistive
compounds La0.65Ca0.35Mn1-xFexO3 with 0.01<x<0.1 have been studied. A careful
study in the magnetic properties has been carried out by the measurement of ac
susceptibility. The temperature range of CMR is greatly broadened with the
addition of Fe. Substitution of Fe induces a gradual transition from a metallic
ferromagnetic with a high Curie temperature (Tc=270 K) to a ferromagnetic
insulator with low Tc=79 K. 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. An extra-ordinary behavior in the out of
phase part (x") of ac susceptibility, characterized by double bump (shoulder),
was observed around x=0.01 and 0.02. The shoulder in x" disappears at x>0.04 Fe
concentration. With increasing Fe concentration the x" peak shift to T<T1/2
(mid point of the transition temperature) and becomes broader. The x" peak
moves to 8 or 10 K higher temperature on the application of a dc field, for 3 &
4% samples. 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. The same ionic radii
of Fe3+ and Mn3+ cause no structure changes in either series, yet
ferromagnetism has been consistently suppressed by Fe doping. Doping with Fe
bypasses the usually dominant lattice effects, but depopulate the hopping
electrons and thus weakens the double exchange. The results were explained in
terms of the formation of magnetic clusters of Fe ions.
