Mixed Brownian Alignment and Néel Rotations in Superparamagnetic Iron Oxide Nanoparticle Suspensions Driven by an AC Field

Superparamagnetic iron oxide nanoparticles with highly nonlinear magnetic behavior are attractivefor biomedical applications like magnetic particle imaging and magnetic fluid hyperthermia. Suchparticles display interesting magnetic properties in alternating magnetic fields and here we documentexperiments that show differences between the magnetization dynamics of certain particles in frozenand melted states. This effect goes beyond the small temperature difference (Δ T ~ 20°C) and we showthe dynamics to be a mixture of Brownian alignment of the particles and Néel rotation of their momentsoccurring in liquid particle suspensions. These phenomena can be modeled in a stochastic differentialequation approach by postulating log-normal distributions and partial Brownian alignment of aneffective anisotropy axis. We emphasize that precise particle-specific characterization throughexperiments and nonlinear simulations is necessary to predict dynamics in solution and optimizetheir behavior for emerging biomedical applications including magnetic particle imaging.