AC Susceptibility of Magnetic Fluid in Nonlinear Brownian Relaxation Region: Experiment and Comparison with Numerical Simulation

This study investigated the AC susceptibility of magnetic fluids in the nonlinear Brownian relaxation region. The nonlinear properties of the susceptibility in high excitation fields were measured comprehensively, including the decrease in susceptibility, field-dependent Brownian relaxation time, and occurrence of the third harmonic for the susceptibility. These experimental results were compared with numerical simulations based on the Fokker--Planck equation, which describes nonlinear Brownian relaxation. We first performed the numerical simulation by assuming mono-dispersed single-domain nanoparticles. The observed nonlinear properties were shown to be roughly explained by the simulation. To compare the experiment and simulation more accurately, we then considered the size distribution of the magnetic nanoparticles existing in practical samples; this was obtained by analyzing the frequency dependence of the susceptibility in weak fields. Quantitative agreements were obtained between the experiment and simulation for the frequency and field dependences of the nonlinear susceptibility.