Thermal hysteresis of microwave loss in (La _1-xPr _x) _0.7Ca _0.3MnO ₃ films

We have measured the temperature (T) dependencies of the dc resistances (R _dc) and the microwave loss (R _μw) in a variety of samples of (La _1-xPr _x) _0.7Ca _0.3 MnO ₃ while varying x from 0 to 0.4. Whereas both the sets of data exhibit maxima, the ac loss peak is much flatter and, during cooling, appears at a much lower temperature than the peak temperature in R _dc. The discrepancy, which vanishes for x=0, increases with lowering tolerance factor (t) (or increasing x). Also R _μw vs T exhibits large thermal hysteresis for x=0.4 indicating that the transition is first order. Cooling in a magnetic field of 9 kOe causes an upward shift of about 20 K in the R _μw peak, in some of the x=0.4 films, yielding a large magnetoimpedance. Further, once these films are exposed to a magnetic field at low T, they fail to recover their virginal behavior on subsequent cooling from room T. These films could be brought to their original state by annealing at high T. The discrepancy between R _dc and R _μw implies that the system is inhomogeneous at low T, providing, for the first time, microwave absorption evidence that manganites exhibit multiphase behavior. Presumably, disorder and strain (increasing with x) combine to stabilize a mixed phase.