Superparamagnetic response of zinc ferrite incrusted nanoparticles

•Incrusted nanoparticles are found at the surface of ZnFe2O4 microparticles.•Magnetic contribution of nano and microparticles are analyzed by different models.•Langevin model is used to calculate the nanoparticles-superparamagnetic diameter.•Susceptibility and Langevin analysis and calculations agree with experimental data.

Zinc ferrite is synthesized via mechano-activation, followed by thermal treatment. Spinel ZnFe2O4 single phase is confirmed by X-ray diffraction. SEM micrographs show large particles with average particle size 〈Dpart〉 = 1 μm, with particles in intimate contact. However, TEM micrographs show incrusted nanocrystallites at the particles surface, with average nanocrystallite size calculated as 〈Dinc〉 ≈ 5 nm. The blocking temperature at 118 K in the ZFC–FC curves indicates the presence of a superparamagnetic response which is attributable to the incrusted nanocrystallites. Moreover, the hysteresis loops show the coexistence of superpara- and paramagnetic responses. The former is observable at the low field region; meanwhile, the second one is responsible of the lack of saturation at high field region. This last behavior is related to a paramagnetic contribution coming from well-ordered crystalline microdomains.The hysteresis loops are analyzed by means of two different models. The first one is the susceptibility model used to examine separately the para- and superparamagnetic contributions. The fittings with the theoretical model confirm the presence of the above mentioned magnetic contributions. Finally, using the Langevin-based model, the average superparamagnetic diameter 〈DSPM〉 is calculated. The obtained value 〈DSPM〉 = 4.7 nm (∼5 nm) is consistent with the average nanocrystallite size observed by TEM.