ZnFe 2 O 4 particles ranging from 4 to 19 nm have been prepared by a solvothermal method. They exhibit superparamagnetic behavior above the blocking temperature which takes a value from 14 to 62 K, magnetization values from 30 to 60 emu g À1 , an anisotropy constant that ranges from 1.1 Â 10 5 to 3.4 Â 10 5 erg cm À3 and an effective superparamagnetic moment from 7.0 Â 10 2 to 8.6 Â 10 3 m B . The superparamagnetic behavior is affected by the presence of dipolar interparticle interactions which are deeply influenced by the synthesis conditions. These interactions yield a wider ZFC maximum and a higher blocking temperature. Moreover, other kinds of interparticle interactions have been found to take place at low temperature and occur through frozen surface spins. Whereas the dipole interactions reduce the anisotropy of the system, the surface spin interactions tend to increase it and the material becomes magnetically harder.

ZnFe 2 O 4 nanoparticles with sizes between 3 and 20 nm have been prepared nonembedded and embedded in an amorphous SiO 2 matrix. All the samples are ferrimagnetic below the blocking temperature that presents similar values for all of them, indicated by the maxima in the ZFC/FC curves. However, the embedded nanoparticles with sizes between 3 and 7 nm present much higher values of coercive field than the nonembedded particles. The fact that particles with different sizes present similar blocking temperature values suggests that they have similar anisotropy energy, and this has been justified using experimental anisotropy constants determined for different sized nanoparticles. Although the matrix can increase the surface anisotropy of the particles, it would not greatly affect the core anisotropy. Taking this into account, it can be justified that embedded particles present a higher coercive field but similar blocking temperature than nonembedded particles. From the variation of the coercive field as a function of the particle size, the limit between the single-domain and multidomain regions seems to be in the range of 15-18 nm. The range of temperature in which the samples behave as superparamagnetic has been estimated from the plots of the inverse susceptibility versus temperature. The particle size and the inversion parameter control the magnetization value that seems to determine the transition temperature from superparamagnetic to paramagnetic behavior.

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.

MFe 2 O 4 (M: Co 2+ , Ni 2+ ) nanoparticles with different sizes and crystal-chemistry have been synthesized under solvothermal conditions. The inversion degree (x) of the mixed spinel structure (Fe 1−x M x )[Fe 1+x M 1−x ]O 4 has been investigated by XAS and Mossbauer spectroscopies obtaining values between 0.20 and 0.30 in the case of Co-ferrite samples and between 0.00 and 0.16 for the Ni-ferrite ones. In order to have a better understanding of the superparamagnetic behavior of the nanosized samples, it was necessary to do hightemperature magnetic measurements. Although Co-ferrite samples present higher magnetization values, their effective superparamagnetic moment is similar to those found for the Niferrite ones. This suggests that the dipolar interactions may be stronger in the Ni-ferrite system, probably due to a less-effective anisotropy that can be considered an impediment to the particle interactions. This fact is supported by the effective volume/ single-particle volume ratio that presents similar values for both Co-ferrite and Ni-ferrite.

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