Relaxation of Polymer Coated ${\rm Fe}_{3}{\rm O}_{4}$ Magnetic Nanoparticles in Aqueous Solution

Magnetic nanoparticles have shown great capability of sensing biological molecules in solution by correlating the magnetic susceptibility with the nanoparticle surface coatings. Here, we report on a new experimental approach of measuring magnetic nanoparticle relaxation. It uses a broadband ferromagnetic resonance setup specifically designed for samples in solution, enabling measurements of nanoparticles at variable frequencies up to 40 GHz. This design enables precise determination of the g-factor of the particles, by determining the resonance field for each microwave frequency. In addition this also enables us to determine the frequency dependence of the resonance linewidth, which provides information about the relaxation mechanisms of the magnetic nanoparticles. Polymer coated Fe₃O₄ nanoparticles were synthesized using a modified co-precipitation method by introducing the polymer coatings (e.g., polyacrylic acid) during the synthesis. Thereby water soluble nanoparticles with a narrow size distribution were produced in a single step. The relaxation measurement of these nanoparticles showed only a slight increase in linewidth over the entire frequency range suggesting that extrinsic relaxation mechanisms are dominant.