Higher Harmonic Magnetic Nanoparticle Characterization using AC Faraday Rotation*

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

This work used monodispersed iron oxide (Fe₃O₄) magnetic nanoparticles (MNPs) with a nominal diameter of 20nm. This diameter is consistent with MNPs' superparamagnetic response to the applied magnetic field. These were purchased commercially from Ocean Nanotech. 175µL of these MNPs were dissolved in 6mL of ultra-high purity water to conduct the experiments. We used a He-Ne laser and subjected the MNP sample to an AC magnetic field. The transmitted light intensity showed intensity oscillations resulting from oscillating polarization detected using a photodetector (Thorlabs LMR1). This allowed for a lock-in-based detection of 1^st, 2^nd, 3^rd, 4^th, and 6^th harmonics in response to the AC magnetic field. These higher harmonics provide strong evidence for aggregate formation at this concentration and particle size. We propose an explanation for the observed data based on aggregate formation. We also compare these findings with results obtained for smaller MNPs that show much more straightforward responses.