Dependence of the Magneto-optic Response of Superparamagnetic Nanoparticle on Particle Diameter

Faraday rotation is a magneto-optic phenomenon where polarization of light is rotated proportionally to an applied magnetic field. This phenomenon is referred to as the Faraday Effect where the experimental configuration is such that the magnetic field is along the same direction as that of light propagation. While this effect is observable and linear (in field strength) for most materials, interpretation of the data is no longer straightforward when it comes to magnetic nanoparticles (MNPs). For this research iron oxide MNPs, of two different diameters (10 nm and 30 nm) were used. This size of MNP creates a unique situation because of its superparamagnetic nature. When these MNPs are subjected to an AC magnetic field, their magnetic moments attempt to align with the direction of the field. This underlying physics coupled with the size dependent properties of the MNPs that include superparamagnetism, relaxation times, and steric interaction between the MNPs gives rise to a rich variety of responses not only for the magneto-optic response but also for the scattering of light. In this study we present the contrast between the 10 nm and the 30 nm diameter particles.

This experiment consists of sending a HeNe laser through a sample, subjected to an AC magnetic field, and into a photodetector. Lock-in detection is used to not only measure the polarization rotation but also the scattering signal from the MNPs. To better understand the scattering response, scattering measurements are made in standard Faraday geometry and also transverse geometry where the direction of light propagation is perpendicular to the direction of the applied magnetic field. Our results show many differences in the magneto-optic response (both scattering and Faraday Effect) that can be consistently explained in terms of particle size differences.