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Neutron small angle scattering on a colossal magnetoresistance material shows clear signatures of a rise in characteristic length of paramagnetic fluctuations as the maximum of dc susceptibility is approached from high temperatures. The phenomenon is accompanied by a rise in intensity of a broad peak that appears at wave vectors of Q Ϸ 0.025 Å −1 , and this is interpreted as a fingerprint of the onset of ferromagnetic ordering due to intercluster magnetic interactions.

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The magnetic and magnetoresistive properties of the Ln0.7Pb0.3Mn1−xFexO3 (Ln=La, Nd, and x=0, 0.1) perovskites, prepared by the solgel low temperature method, have been studied. In all cases the exact stoichiometry is different from the nominal one probably due to the presence of cation vacancies. All the phases exhibit ferromagnetic behavior with Tc values ranging form 345 K observed for La0.7Pb0.3MnO3 to 75 K for Nd0.7Pb0.3Mn0.9Fe0.1O3. The 10% Fe contribution lowers the Tc by about 130 K in the La compound and 90 K in the Nd one. These results can be interpreted in terms of antiferromagnetic coupling between Mn and Fe. However, the influence of the Nd ions is not well known although it seems not ordered down to 10 K. The magnetoresistence [MR=ΔR/R(0)] measured at 6 T in pellets of pressed powder reaches values of about 80% around the magnetic transition.

The effect of Fe substitution by Al in Sr2(Fe1-xAlx)MoO6 (0⩽x⩽0.3) double perovskites is reported. X-ray diffraction, magnetic measurements and Mössbauer spectroscopy have been used in the investigation. Contrary to other substitutions in similar compounds, Al increases the cationic order up to x = 0.1 and beyond. This ordering effect is accompanied by an increase in the magnetic moment per Fe atom, but not by corresponding higher Curie temperatures.

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Neutron-diffraction, magnetic and transport measurements have been used in order to study the structural and magnetic changes of the magnetoresistive La0.7Pb0.3Mn0.9TM0.1O3 (TM = Fe, Co, Ni) perovskite-like compounds. Samples were prepared by the sol-gel low-temperature method. In all cases the exact stoichiometry is slightly different from the nominal one probably due to the presence of cation vacancies. All the phases exhibit ferromagnetic behaviour and magnetoresistance. No appreciable differences in the structure, as the TM3+ ion changes, have been observed. The magnetic moment and Curie temperature continuously decrease (with respect to the undoped composition) with the substitution of Mn atoms by Ni, Co or Fe, respectively. The magnetoresistance (MR = ΔR/R(0)), under 0 and 6 T applied fields, has been measured reaching a magnitude of about 65% in the case of the 10% Fe doped sample. The obtained results can be interpreted in terms of ferro- or antiferromagnetic exchange competitions between Mn3+-Mn4+ and TM3+-Mn4+ pairs.