Magnetic Behavior of ZnFe2O4 Nanoparticles: Effects of a Solid Matrix and the Particle Size

ZnFe2O4 nanoparticles with sizes between 3 and 20 nm have been prepared nonembedded and embedded in an amorphous SiO2 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.