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 toward understanding the diagnostic and therapeutic efficacy of magnetic nanoparticles for cancer theranostics. We focused on the parameters influencing the dynamic magnetization response, such as particle core size and magnetic anisotropy. Our experimental investigation on 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 and measured by applying a pulsed magnetic field over a wide time range of 20 ns to 200 ms. Magnetic nanoparticles with single-core and multicore structures under viscous fluid and solid conditions were investigated to evaluate the Néel and Brownian relaxations, respectively, associated with the magnetization and physical particle rotations. We observed a distinct magnetization response associated with the complex magnetic relaxation mechanisms, which challenged the concept described by the conventional theory of effective relaxation time. Moreover, we clarified the relationship between the effective magnetic anisotropy energy and attempt time for controlling the magnetization dynamics dependent on particle structures. Our novel measurement technique and investigation of the magnetic relaxation time provide guidance for significantly optimizing material design and determining the magnetic field conditions for the biomedical applications of magnetic nanoparticles, particularly in cancer theranostics.