Magnetization dynamics of magnetic nanoparticles for thermal and magnetic particle imaging

Abstract

The prospect of thermal imaging and control in magnetic particle imaging (MPI) is an important advancement for non-invasive medical diagnostics and therapeutics. Current MPI systems focus mainly on imaging from measurement of nanoparticle concentrations. Accurate derivation of temperature relies on detailed understanding of magnetization dynamics in the presence of AC drive fields and variable temperature [1]. Here, we report measurements of nanosecond magnetization dynamics using both time and frequency domain techniques.It has been reported that the imaging resolution of MPI is critically impacted by nanoparticle relaxation dynamics [2]; specifically, spin relaxation results in image blurring. The dynamic response (e.g. Néel and Brownian relaxation) of magnetic nanoparticles in magnetic fields is intricate, depending strongly on the particles’ inherent magnetic and structural properties, inter-particle interactions, and the local environment. We have developed an arbitrary-wave magnetic particle spectrometer with magnetic field amplitude up to 10 mTPk for characterizing magnetization dynamics over a broad temperature range (200 K to 350 K).We have characterized the dynamic AC susceptibility of nanoparticles ranging in diameter from 10 nm to 70 nm and in composition and observed peaks in the imaginary component (χ’’) at frequencies from as low as 50 Hz to 50 MHz. Figure 1 show the χ’’ peak frequency for 11.5 nm cobalt-doped ferrite nanoparticles at different temperatures. This peak reveals the effective relaxation timescale (τ ~ 1 μs). Additionally, we used pulsed excitation to characterize the magnetization dynamics with and without magnetic fields. Figure 2 shows a magnetization response of 7.5 nm cobalt-doped ferrites to a magnetic field pulse with 20 ns rise time. Monte Carlo simulations are used for analyzing both time and frequency domain measurements. This knowledge will eventually inform strategies for design and synthesis of magnetic nanoparticles with properties targeted for accurate and sensitive thermal imaging using MPI.  The imaginary component of AC susceptibility for 11.5 nm ferrite nanoparticles  Pulsed magnetic field (black). Magnetization step-response of 7.5 nm cobalt-doped ferrite (red).