Magnetic Properties of Nanostructured Spinel Ferrites

Spinel ferrite nanoparticles (NPs) have raised interest due to their potential technological applications in fields as varied as high frequency electronic device components, soil remediation, and medical diagnosis and treatments. In this paper, we present a brief review of the magnetic properties of spinel ferrite NPs (Ni-Zn, Co, and magnetite) synthesized by the polyol method, in different degrees of aggregation, from monodisperse NPs, to clusters formed by tens to hundreds of NPs. We show that the approach to saturation can be modeled with a relationship derived from the Stoner-Wohlfarth model, both for ferrimagnetic and superparamagnetic NPs. We also present a review on the magnetic properties of spinel ferrites NPs consolidated using spark plasma sintering (SPS). This technique allows the sintering of NPs to densities 90% of the theoretical value at significantly lower temperatures and shorter times than the typical sintering processes, preserving the grain size within the nanometric range. The typical sintering temperatures are in the range 350°C-750°C, for times as short as 5 min. An interesting example is magnetite, which can be obtained as NPs by polyol, followed by SPS at 750°C, temperature that usually leads to the transformation to hematite. The Verwey transition is clearly observed as a large drop in the coercive field at ~120 K.