Abstract: The temperature dependence of electrical resistance and magnetoresistance as well as field dependence of magnetoresistance at fixed temperature of
the (La_2/3Ca_1/3)(Mn_(3-x)/3Fe_x/3)O₃ (x=0, 0.1, 0.2, 0.3) system were systematically measured . It was found that when x increases, both the peak of the electrical resistivity and the peak of magnetoresistance move to
lower temperature. However the giant magnetoresistance (GMR) is increased and its cusp is broadened by Fe substitution. The authors suggested that the
variation of the proportion of Fe³⁺ ions leads to the changes of the ratios of Mn³⁺ to Mn⁴⁺ and magnetic moments while external magnetic
field modulates the magnetic ordered structure, so affects the ferromagnetic double exchange effect of the Mn³⁺--O--Mn⁴⁺. Finally, such phenomenon
changes the behavior of magnetoresistance.

Key words: giant magnetoresistance effect (GMR), magnetoresistance ratio (MR)