Magneto-optic Response of Functionalized vs. Uncoated Fe₂O₃ (Maghemite) Nanoparticles

Abstract

Iron oxide nanoparticles (NPs) have attracted a lot of interest due to their many potential applications in areas including optoelectronics, magneto-optics, high density data storage, etc. In particular, iron oxides (Fe₃O₄ and γ –Fe₂O₃) are also well suited for biomedical applications [1]. We have investigated Faraday Rotation (FR) response for two types of Fe₂O₃ NPs (in aqueous suspension) that are of the same average diameter (10 nm) but differ in one important respect; one group consists of uncoated particles whereas the other group is functionalized with caffeic acid. This system is being investigated and characterized for use in tumor imaging applications. Faraday rotation (FR) refers to the rotation of the polarization vector of a light beam as it passes through a sample in the presence of a magnetic field. FR can reveal interesting material properties such as saturation magnetization and wavelength dependent Verdet constant of the material under investigation. The latter is a measure of the magnetically induced birefringence of the material. Typically FR setups rely on AC or DC magnetic fields. While these are valuable techniques with their own advantages, this work focuses on a pulsed field setup that can reveal dynamic information about the resulting magnetization, as the magnetic response of the sample is measured in the presence of short intense fields on the order of 0.6 Tesla and lasting approximately 100 milliseconds. All experiments are carried out at excitation wavelength of 633 nm (He-Ne wavelength).

The two NP samples show very different response to the field pulses. The NP systems investigated in this work show very unique short term and long term behavior revealing various time scales of interest. These unique characteristic times for the functionalized vs. uncoated particles provide valuable clues about the magnetization response of the NP and its relationship to the detailed structure of the NPs (core vs. shell). Magnetic response from these systems persists long after the magnetic field pulse has subsided. This can be related to the relaxation modes (Néel vs. Brownian) and as possible evidence of NP size dispersion. Additionally, the possibility of agglomeration is also discussed. While more detailed quantitative analysis will be dealt with in a more comprehensive publication that is under preparation, we hope to show in this preliminary report both that the AC and pulsed FR measurements can reveal complimentary information and that FR in general can be a reliable technique, which can be used to develop a detailed picture of the magnetic response of these NP systems.