Zinc ferrites nanoparticles are soft magnetic materials widely used in biomedical applications, in particular for magnetic hyperthermia, thanks to their superparamagnetic behaviour at RT. Bulk ZnFe2O4 is paramagnetic at room temperature with a normal spinel structure, in which all Zn2+ ions occupy tetrahedral positions (A-sites) and all Fe3+ ions octahedral positions (B-sites). At the nanoscale, a partial cation inversion occurs and in turn the magnetic properties change significantly with the onset of superparamagnetism at room temperature. The resulting functional properties, saturation magnetization, coercivity, blocking temperature and relaxation time of nanoparticles are closely related to size, shape, composition and architecture of nanoparticles and on the synthesis methods.
A good phase stability and homogeneity is highly required in particular for biomedical applications, where the aggregation of nanoparticles must be hindered. On the other hand, the possibility to favour the occurrence of impurity phases like hematite or maghemite must be considered by managing zinc ferrites at the nanoscale.
In this work we present an experimental study on magnetic, structural and electronic properties of undoped and substitued nanosized zinc ferrites obtained from microwave combustion synthesis. The cation substitution involved both A ( Ca or Sr for Zn) and B ( Gd or Al for Fe) sites. XRD and SEM measurements allowed to verify the phase and composition of all our samples. The Raman data, in particular the A1g mode due to the motion inside the tetrahedral AO4 units, allow to derive the inversion degree of the spinel. EPR Fe3+ signals from different samples and at different temperatures have been correlated to the structural information derived from XRD and micro-Raman experiments. On this basis, the magnetic behaviour has been measured and discussed. In some cases the observed superparamagnetism can be ascribed to the presence of maghemite clusters, also evidenced by observed transformation to hematite induced by the laser irradiation during Raman experiments. The intrinsic origin of the superparamagnetism in zinc ferrites is discussed in connection with nanoparticles size, inversion degree and phase stability.