Numerical modeling of the magnetic response of interacting superparamagnetic particles to an ac field with arbitrary amplitude

摘要

The dynamic magnetic response of immobilized superparamagnetic nanoparticles to an ac field with arbitrary amplitude is studied using numerical simulations. The nanoparticles are considered to be distributed randomly within an implicit solid matrix, but the easy axes of the particles are aligned parallel to the ac magnetic field. Modeling of dynamic response is based on the Fokker-Planck-Brown equation in which the interparticle dipole-dipole interactions are included within the framework of the modified mean-field theory. The effects of the magnetic crystallographic anisotropy barrier, the ac field amplitude, and the interparticle interactions on the dynamic magnetization, susceptibility, and relaxation time are analyzed. It is shown that an increase in the amplitude of the ac field significantly accelerates relaxation processes in the system under consideration whereas an increase in magnetic anisotropy of a particle and interparticle dipole-dipole interactions slows them down. The numerical results for dynamic susceptibility and relaxation time are compared against theories reliable in the weak ac field, and an excellent agreement is obtained.