Multi-harmonic response of superparamagnetic nanoparticles as a function of particle size and concentration across multiple optical configurations

Superparamagnetic nanoparticles (SNPs) play an increasingly important role in biomedical applications like MRI contrast imaging, magnetic hyperthermia, drug delivery, etc. We have investigated the magneto-optical response of dilute aqueous and non-aqueous suspensions of single-domain iron oxide nanoparticles under the influence of a resonant AC magnetic field. We present initial results from a Michelson interferometer-based measurement setup. Interferometric measurements enhance the measurement sensitivity of the induced magnetization response of the nanoparticle samples. Further, the signal-to-noise ratio of the measurements is improved with a lock-in detection scheme. Our experimental setup captures both even and odd harmonics of the magneto-optic response. In this work we focus on the contrasting outcomes of “small” (10 nm or less in diameter) vs “medium” (15 to 20 nm diameter) sized particles in terms of the spectral composition of the detected optical signal. All our SNPs have ligands that keep these particles from aggregating. Our results provide important insights into the role of the applied magnetic field in generating dipole interactions that result in new equilibrium configurations consisting of loosely assembled aggregates (clusters) of these SNPs, compared to the case when there is no applied magnetic field. This approach allows us to investigate how formation of such aggregates modify the effective magnetic moment and the scattering due to formations of SNP aggregates. In addition to highlighting the role of SNP size (both, the hydrodynamic radius as well as the Néel radius), we also discuss the role of starting concentrations of the SNP samples.