The nature of the magnetic interactions among core-shell nanoparticles of CoFe 2 O 4-CoFe 2 having an average particle size of 32 nm were investigated by ␦m plots. It was found that the interactions 072502-3 Soares et al.

No abstract

The effect of Fe doping (р10%) on polycrystalline samples of La 0.7 Sr 0.3 MnO 3 has been studied by means of x-ray diffraction, resistivity, ac susceptibility, magnetization, and magnetoresistance measurements. Fe doping does not affect the lattice structure, but it weakens the ferromagnetism and substantially augments the resistivity of the samples. The magnetoresistance of a sample with 10% Fe doping, in a 5 T magnetic field, was 4 times greater than that of the undoped sample near the peak in the resistivity. The results were explained in terms of the formation of antiferromagnetic clusters of Fe ions. At 5 K the magnetoresistance showed no significant dependence on Fe doping, but it displayed large variations in fields below 0.5 T for all samples. This behavior was attributed to spin-polarized tunneling at the grain boundaries of the polycrystalline samples. A variable-range hopping behavior in the resistivity was also encountered in all the samples at low temperatures.

No abstract

We have performed x-ray, electrical resistivity, and Raman scattering measurements in La 0.70 Sr 0.30 Mn 1Ϫx Fe x O 3 manganites, with x varying from 0 to 0.40. The appearance of new peaks in the Raman spectra as the Fe content increases is attributed to either otherwise forbidden Raman modes that become active due to symmetry breaking effect or to the phonon density of states features, which arise due to a disorderinduced phenomenon. The softening of some phonon modes suggests the evidence of magnetic polaronic states in the insulating paramagnetic phase of these manganites. Our observations are discussed in connection with the x-ray diffraction and electrical resistivity data.