Studies of Brownian Motion for Collections of Magnetic Nanoparticles: Application to Magnetic Particle Spectroscopy

Magnetic nanoparticles are used in a variety of ways in the medical field. This includes MRI temperature sensing, killing tumors via hyperthermia, and localized drug delivery. To implement these techniques, we need to understand the dynamics of these particles in a varying magnetic field, and in changing viscosities and with different inertias. Our research focuses on Magnetic Particle Spectroscopy (MPS) which is designed to measure different biomarkers. MPS generally depends on measuring different harmonics of the driving frequency and their ratios. This generally involves a Fourier transform, and we study how different windowing procedures can affect the ratios of the harmonic signals. We examine a simple system where the magnetization is locked along an anisotropy axis and the entire particle moves in response to an oscillating magnetic field, i.e. Brownian motion. We establish limits on sensitivity which involve the size, shape, and magnetic moment of the sensing particle. In addition, we examine the influences of the dipolar interactions between particles and find that higher concentrations generally degrade the ability to obtain useful information. We also discuss relaxation issues in moving from pure Brownian motion to combinations of Brownian and Neel motion.