Harmonic decomposition of magneto-optical signal from suspensions of superparamagnetic nanoparticles

Highlights: • An inexpensive technique to characterize magnetic nanoparticles is presented. • Concentration, magnetic moment, and Verdet constant are measured. • Multi-harmonic analysis of the magneto-optical response is shown to be consistent. • The analysis shows evidence for aggregation of nanoparticles. • Use of this technique to probe relaxation times and scattering is also highlighted. Magnetic nanoparticles (MNPs) are widely used in biomedical applications. Characterizing dilute suspensions of superparamagnetic iron oxide nanoparticles (SPIONs) in bio-relevant media is particularly valuable for magnetic particle imaging, hyperthermia, drug delivery, etc. Here, we study dilute aqueous suspensions of single-domain magnetite nanoparticles using an AC Faraday rotation (FR) setup. The setup uses an oscillating magnetic field (800 Hz) which generates a multi-harmonic response. Each harmonic is collected and analyzed using the Fourier components of the theoretical signal determined by a Langevin-like magnetization. With this procedure, we determine the average magnetic moment per particle $\mu$, particle number density n, and Verdet constant of the sample. The fitted values of $\mu$ and n are shown to be consistent across each harmonic. Additionally, we present the results of these parameters as n is varied. The large values of $\mu$ reveal the possibility of clustering as reported in other literature. This suggests that $\mu$ is representative of the average magnetic moment per cluster of nanoparticles. Multiple factors, including the external magnetic field, surfactant degradation, and laser absorption, can contribute to dynamic and long-term aggregation leading to FR signals that represent space- and time-averaged sample parameters. Using this powerful analysis procedure, future studies are aimed at determining the clustering mechanisms in this AC system and characterizing SPION suspensions at different frequencies and viscosities.