We present for the first time an in-depth magnetic characterization of a family of monodisperse cobalt-ferrite nanoparticles (NPs) with average size covering a broad range of particles sizes (from 4 to 60 nm), synthesized by thermal decomposition of metal–organic precursors. Metal precursors, surfactants, and synthetic parameters were settled in order to fine-tune the particle size, which preserves a narrow particle size distribution. The morphology of the family of cobalt-ferrite NPs shows a size-dependent behavior, evolving from sphere to octahedrons for size larger than 20 nm and passing through a cubic habit for intermediate sizes. The evolution of the magnetic properties was studied as a function of the particle size and shape, particularly focusing on those determining the best performance as permanent magnet. Although saturation and remnant magnetization increase monotonously with size, reaching a constant value above 20 nm, the coercive field exhibits a nonmonotonic behavior with two distinct maxima values for low and room temperature, respectively. In addition, we evaluated the (BH)max product, the figure of merit of permanent magnets, obtaining the highest value ever reported in the literature for cobalt-ferrite NPs (i.e., 2.1 MGOe (18 kJ/m–3) for 40 nm NPs). This study allowed us to establish, at least on the basis of the (BH)max product, the potentiality of cobalt-ferrite nanoparticles in current permanent magnet technology.

The quest for efficient ways of modulating localized surface plasmon resonance is one of the frontiers in current research in plasmonics; the use of a magnetic field as a source of modulation is among the most promising candidates for active plasmonics. Here we report the observation of magnetoplasmonic modes on colloidal gold nanoparticles detected by means of magnetic circular dichroism (MCD) spectroscopy and provide a model that is able to rationalize and reproduce the experiment with unprecedented qualitative and quantitative accuracy. We believe that the steep slope observed at the plasmon resonance in the MCD spectrum can be very efficient in detecting changes in the refractive index of the surrounding medium, and we give a simple proof of principle of its possible implementation for magnetoplasmonic refractometric sensing.

We describe an environmentally friendly, top-down approach to the synthesis of Au 89 Fe 11 nanoparticles (NPs). The plasmonic response of the gold moiety and the magnetism of the iron moiety coexist in the Au 89 Fe 11 nanoalloy with strong modification compared to single element NPs, revealing a non-linear surface plasmon resonance dependence on the iron fraction and a transition from paramagnetic to a spin-glass state at low temperature. These nanoalloys are accessible to conjugation with thiolated molecules and they are promising contrast agents for magnetic resonance imaging.