Physical rotation of the easy axes of magnetic nanoparticles significantly affects their magnetization properties in fluid, which are important for biomedical applications. The rotation and alignment of the easy axis toward a specific direction in an alternating magnetic field are affected by fluid conditions such as viscosity, the condition of the applied magnetic field, and the magnetic properties of the nanoparticles, including anisotropy. In this study, we experimentally evaluated the physical rotation and alignment of the easy axes of nanoparticles in a viscous medium by preparing samples in which the orientation of the easy axis was fixed in an alternating field at each frequency across a wide frequency range. The numerical simulation with respect to the transitional and steady-state easy-axis dynamics determined by Néel and Brownian relaxation sufficiently supported the empirically observed phenomena. The factors dominating the easy-axis dynamics with rotation and static orientation in an alternating magnetic field, such as the Brownian relaxation time, magnetic torque, and applied field frequency, were clarified. The alignment degree, including its direction and rotation amplitude, was found to depend on the viscosity of the medium associated with the Brownian regime and the magnetic torque derived from the Néel regime. Nonlinear magnetization appeared as the harmonic components of the magnetization in a viscous medium were strongly influenced by the static alignment of the easy axis.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics