Impact of Néel and Brownian relaxation on magnetization response of magnetic nanoparticles based on magnetic field strength

Magnetic relaxation of magnetic nanoparticles governs their dynamic response under external magnetic fields and plays a critical role in biomedical applications such as magnetic particle imaging and hyperthermia treatment. In this study, we investigated the magnetic relaxation behavior of iron oxide nanoparticles under magnetic field strengths of 5–20 mT using a custom-designed electrical circuit capable of monitoring magnetization dynamics over a broad time range of 100 ns–500 ms. Magnetic nanoparticles were dispersed in a viscous liquid and subsequently solidified in an epoxy resin to distinguish Néel and Brownian relaxations from the complex relaxation mechanism. The magnetic relaxation time spectrum was classified into components characterizing different magnetization dynamics. We measured a spherical single-core magnetic nanoparticle with an average core diameter of 18.6 nm. At lower field strengths, Brownian relaxation relatively dominated; contrastingly, at higher field strengths, Néel and Brownian relaxations occurred simultaneously. The experimental results were fitted using theoretical models in accordance with numerical simulations according to the Fokker–Planck equation to estimate the effective Néel and Brownian relaxation times. A coefficient describing the interference of Néel relaxation with Brownian relaxation was evaluated. These results demonstrate the magnetic nanoparticle dynamics characterized by magnetic relaxation dependent on magnetic field strength, contributing to the optimization of material design and applied magnetic field conditions for biomedical applications.