Magnetic Nanoparticle Characterization using AC Faraday Rotation*

Faraday rotation (FR) is a magneto-optic phenomenon where polarization of light traveling through a medium is rotated in an applied magnetic field, that is parallel to the direction of light propagation. This phenomenon is dependent on the strength of the field, path length, and properties of the material that the light travels through. The material properties that determine the magneto-optic response give rise to the Verdet constant, which is the measure of effectiveness of the Faraday Effect on the material. When investigating magnetic nanoparticles (MNPs), the interpretation of the Verdet constant is not straightforward.

In this work monodispersed iron oxide (Fe₃O₄) magnetic nanoparticles (MNPs), with a diameter of 10nm, were used. These were purchased commercially from Ocean Nanotech. These MNPs were dissolved in ultra-high purity water. Samples of different MNP concentration were used. In this study. The 10 nm size of MNP results in a superparamagnetic response of the nanoparticles to the applied field. We used a He-Ne laser and subjected the MNP sample to an AC magnetic field. This allowed for a lock-in based detection that resulted in the determination of a precise value of the Verdet constant of the MNPs. The transmitted light intensity showed intensity oscillations resulting from oscillating polarization that were detected using a photodetector (Thorlabs LMR1). We determined the Verdet constant of MNPs as a function of concentration. We found that the highest concentrations of MNPs departed from a straightforward dependence of Verdet constant on concentration. Our results provide a strong indication of the onset of aggregation formation in MNPs beyond a certain threshold concentration. We also discuss some recent findings of similar investigations on MNPs of larger sizes, to provide a comparison of MNP response as a function of particle size.