Two kinds of nanosized ferrite systems have been prepared: MFe2O4 nanoparticles and MFe2O4/​SiO2 (M: Co, Ni) nanocomposites with different ferrite particle sizes. Magnetic measurements have been done for both ferrite systems in the 5–700 K temperature range, and the silica matrix effect on the magnetic behavior has been studied. Whereas CoFe2O4 samples are characterized by high anisotropy values, NiFe2O4 particles seem to be magnetically soft, which may favor the dipolar interactions. The silica matrix avoids all kinds of particle interactions that modify the magnetic behavior. Therefore, in 2–5 nm embedded Co-ferrite particles, a decreasing H C value with the increasing particle size is not observed. This almost constant H C value indicates that interactions between surface spins of different particles are absent for this particle size range. In the case of Ni-ferrite particles, the dipolar interactions between bulk particle moments are minimized due to the presence of the SiO2 matrix.

NiFe2O4 and ZnFe2O4 nanoparticles have been prepared encased in the MCM (Mobile Composition of Matter) type matrix. Their magnetic behavior has been studied and compared with that corresponding to particles of the same composition and of a similar size (prepared and embedded in amorphous silica or as bare particles). This study has allowed elucidation of the role exerted by the matrix and interparticle interactions in the magnetic behavior of each ferrite system. Thus, very different superparamagnetic behavior has been found in ferrite particles of similar size depending on the surrounding media. Also, the obtained results clearly provide evidence of the vastly different magnetic behavior for each ferrite system.

ZnFe2O4 and NiFe2O4 particles ranging from 5 to 8 nm have been prepared inside the channels of SBA-15 mesoporous material and nanowires were recovered after dissolving the silica matrix. For both ferrite compositions a hardening of the magnetic behavior has been obtained when using the mesoporous matrix. Thanks to the comparison of the magnetic behavior of the nanoparticles when contained and not in the matrix, it was possible to elucidate not only the matrix effect but also the kind of interparticle interactions depending on the ferrite composition. Thus, nickel ferrite particles are characterized by intense dipolar interactions that are responsible for the so high superparamagnetic response and that can be avoided by matrix effects only at high temperatures. On the contrary, the inherent low-intense dipolar interactions of the zinc ferrite system lead it to present lower superparamagnetic moments, and in the case of encapsulated particles, the superparamagnetic behavior would correspond to almost not-interacting particles. In addition, interactions occurring between surface spins of different particles are more visible in zinc ferrite system as consequence of its so low intense dipolar interactions and are prevented thanks to the use of the matrix.

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