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Using a sol-gel method we have prepared monodispersive particles of La0.67Ca0.33MnO3 in the range from 20 to 110 nm at temperatures from 540 to 1000 °C. A magnetoresistance above 10% was obtained in a field of 1 kOe for all the particles sizes. These results may be relevant for future applications.

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Magnetic dc susceptibility (χ) and electron spin resonance (ESR) measurements in the paramagnetic regime, are presented. We found a Curie-Weiss (CW) behavior for χ(T) with a ferromagnetic Θ = 446(5) K and µ ef f = 4.72(9)µ B /f.u., this being lower than that expected for either F e 3+ (5.9µ B ) or F e 2+ (4.9µ B ) ions. The ESR g-factor g = 2.01(2), is associated with F e 3+ .We obtained an excellent description of the experiments in terms of two interacting sublattices: the localized F e 3+ (3d 5 ) cores and the delocalized electrons. The coupled equations were solved in a mean-field approximation, 1 assuming for the itinerant electrons a bare susceptibility independent on T .We obtained χ 0 e = 3.7 10 −4 emu/mol. We show that the reduction of µ ef f for F e 3+ arises from the strong antiferromagnetic (AFM) interaction between the two sublattices. At variance with classical ferrimagnets, we found that Θ is ferromagnetic. Within the same model, we show that the ESR spectrum can be described by Bloch-Hasegawa type equations. Bottleneck is evidenced by the absence of a g-shift. Surprisingly, as observed in CMR manganites, no narrowing effects of the ESR linewidth is detected in spite of the presence of the strong magnetic coupling. These results provide evidence that the magnetic order in Sr 2 F eM oO 6 does not originates in superexchange interactions, but from a novel mechanism recently proposed for double perovskites.PACS: 75.10. 76.60.Es, 75.30.Vn