Effects of Néel and Brownian relaxations on dynamic magnetization empirically characterized in single-core and multicore structures of magnetic nanoparticles

Abstract

This study takes a novel approach to understanding the diagnostic and therapeutic efficacy for cancer theranostics using magnetic nanoparticles. We focus on parameters influencing dynamic magnetization response, such as particle core size and magnetic anisotropy. Our experimental investigation of the relationship between magnetic relaxation and these particle parameters provides fresh insights for developing biomedical applications. The magnetic relaxation time was estimated from the magnetic relaxation process measured by applying a pulsed magnetic field over a wide time range of 20 ns–200 ms. Magnetic nanoparticles of the single-core and multicore structures in viscous fluid and solid conditions were measured to evaluate the Néel and Brownian relaxations, respectively, associated with the magnetization and physical particle rotations. We observed distinct magnetization response associated with the complex magnetic relaxation mechanisms, which challengs concept to describe using the conventional theory of the effective relaxation time. Moreover, we clarified the relationship between the effective magnetic anisotropy energy and attempt time controling the magnetization dynamics dependent on the particle structures. Our novel measurement technique and investigation of the magnetic relaxation time significantly optimize the material design and determine magnetic field conditions for biomedical applications, particularly in cancer theranostics using magnetic nanoparticles.