Role of disorder and competing ferromagnetic and antiferromagnetic interactions in the magnetic, electrical, and dynamic properties of La0.7Pb0.3(Mn1-xFex)O-3,0 <= x <= 0.2 manganites

The effect of Fe doping on the magnetic properties of La0.7Pb0.3Mn1-xFexO3(0 <= x <= 0.2) colossal magnetoresistance perovskites is studied by muon spin rotation (mu SR) spectroscopy and macroscopic static magnetization measurements. The latter quantities show a cusp at temperatures below that signaling magnetic ordering (Curie temperature T-C) as well as a kink at lower temperatures which are taken as signatures of magnetic irreversibility. Random substitution of Mn by Fe ions gives rise to antiferromagnetic couplings mediated by conventional superexchange interaction. This manifests itself by the decrease of Curie temperatures and a concomitant decrease of the spontaneous magnetization with increased doping. The muon relaxation data under zero and applied longitudinal fields is interpreted in terms of progressive spin freezing concomitant with a reduction of the ferromagnetic component as Fe substitution is increased. The spin dynamics as T-C is crossed from above shows clear departures from those expected for exchange-coupled Heisenberg or Ising systems but rather it displays features common to magnetically interacting-cluster systems. This effect leads to the appearance of a peak in the muon relaxation rate versus temperature curves that is located at temperatures below that signaling magnetic ordering (T-C). Finally, direct evidence of the existence of antiferromagnetic and ferromagnetically short-range-ordered regions within the x=0.2 sample is provided by polarized neutron diffraction. The picture that emerges from our study identifies competing positive (double exchange) and negative (superexchange) interactions at random sites as the entities responsible for the low-temperature freezing monitored by using mu SR spectroscopy.