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Magnetic elastic structured composites were prepared by using CoFe2O4 ferromagnetic and superparamagnetic nanoparticles as ﬁllers in polydimethylsiloxane (PDMS) matrixes, which were cured in the presence of a uniform magnetic ﬁeld. Cobalt-iron oxide nanoparticles of three different average sizes (between 2 and 12 nm) were synthesized and characterized. The smallest nanoparticles presented superparamagnetic behavior, with a blocking temperature of approximately 75 K, while larger particles are already blocked at room temperature. Macroscopically structured-anisotropic PDMS-CoFe2O4 composites were obtained when curing the dispersion of the nanoparticles in the presence of a uniform magnetic ﬁeld (0.3 T). The formation of the particle?s chains (needles) orientated in the direction of the magnetic ﬁeld was observed only when loading with the larger magnetically blocked nanoparticles. The SEM images show that the needles are formed by groups of nanoparticles which retain their original average size. The Young?s moduli of the structured composites are four times larger when measured along the oriented needles than in the perpendicular direction. Magnetization (VSM) and ferromagnetic resonance curves of the structured composites were determined as a function of the relative orientation between the needles and the probe ﬁeld. The remanence magnetization was 30% higher when measured parallel to the needles, while the coercive ﬁeld remains isotropic. These observations are discussed in terms of the individual nanoparticle?s properties and its aggregation in the composites.V C 2011 American Institute of Physics. [doi:10.1063/1.3624602]