Session: Research Posters

Paper Number: 172564

Thermal Behavior of Magnetic Moments in Anisotropic Magnetic Particles: Landau-Lifshitz-Gilbert Simulations 

Magnetic particle suspensions are prepared by dispersing fine particles with suitable magnetic properties in non-magnetic fluids. By applying an external magnetic field, researchers can effectively control the rheological properties of these suspensions. This functionality has attracted attention in the field of fluid engineering, especially for applications in magnetically controlled dampers and actuators. In recent years, magnetic particle suspensions have also gained attention in biomedical engineering, and numerous studies have focused on magnetically guided drug delivery systems and magnetic hyperthermia. In magnetically guided drug delivery systems, drugs are first loaded into magnetic particles, transported to target cells via non-uniform magnetic fields, and finally released at specific locations. In magnetic hyperthermia, targeted cancer cells are destroyed by the heating effect arising from the relaxation phenomenon of magnetic moments of particles exposed to alternating or rotating magnetic fields.　The advantage of this therapy is to minimize damage to normal cells by utilizing the localized heating effect of magnetic particles. In the present study, we conducted simulation-based studies from the viewpoint of developing magnetic hyperthermia technology.

Under time-dependent magnetic fields, the return of the magnetic moment to the equilibrium is governed by two different relaxation mechanisms: (a) Néel relaxation and (b) Brownian relaxation. In the Néel relaxation, the magnetic moment fluctuates within the particle body. On the other hand, in the Brownian relaxation, the magnetic moment rotates with the physical rotation of the particle. The dominance of these relaxation mechanisms depends on particle size: (a) Néel relaxation dominates in particles smaller than around 10 nm, and (b) Brownian relaxation dominates in larger particles. However, in the intermediate diameter range of particles, it is necessary to treat both the viscous motion of particles and the motion of magnetic moments. The theoretical analysis of the magnetic moment dynamics in magnetic particle is based on the investigation of the Landau-Lifshitz-Gilbert (LLG) equation. Thus, solving the LLG equation provides insights into magnetization dynamics. Thermal fluctuations of magnetic moments can be accurately considered using the stochastic LLG equation, where a random magnetic field is introduced into the standard LLG equation.

From this background, we proposed a simulation method combining the stochastic LLG method for magnetic moment dynamics and the Brownian dynamics method for particle viscous motion. We then examined the validity and effectiveness of this approach. We addressed a single spherical particle with uniaxial magnetic anisotropy in order to investigate the orientational characteristics of both the particle and the magnetic moment. The main results obtained here are summarized as follows. The orientational characteristics of the magnetic moment showed good agreement with the theoretical solution given by the Langevin function. Additionally, the orientational characteristics of the particle and the magnetic moment were significantly influenced by the relationship between the external magnetic field and magnetic anisotropy. Although the simulations were performed for a single particle system, it required approximately 4 minutes of CPU time to obtain the results. If the present method is extended to multi-particle systems, it is anticipated that a large amount of CPU time will be required. Consequently, a new simulation method suitable for multi-particle systems may need to be developed.

Presenting Author: Kazuya Okada Saitama Institute of Technology

Presenting Author Biography: Dr. Kazuya Okada is a Lecturer in the Department of Mechanical Engineering at Saitama Institute of Technology, Japan. He received his Ph.D. in engineering from Akita Prefectural University in 2021. His research focuses on simulation studies of magnetic nanoparticle suspensions, especially their dynamic behavior under external magnetic fields. His recent work involves developing hybrid simulation methods combining Brownian dynamics and Monte Carlo techniques.

Authors:

Kazuya Okada Saitama Institute of Technology
Hideto Kashiwagi Saitama Institute of Technology