Stable vortex magnetite nanorings colloid: Micromagnetic simulation and experimental demonstration

Magnetite nanoring with vortex domain structure may form stable magnetic colloid for biomedical applications due to its weak magnetic interaction without superparamagnetic (SPM) limitation. In the present study, we perform three-dimensional (3 D) Landau-Liftshitz-Gilbert (LLG) micromagnetics simulation for magnetite nanorings. The ground state phase diagram and stable vortex area (SVA) as a function of outer diameter (D _out), thickness (T), and inner to outer diameter ratios (β) within 100 nm are obtained. The influence of notch, eccentricity, and crystallographic orientation are taken carefully into consideration. In the SVA, the vortex state is not only the ground state but also the remanence state after in-plane is fully magnetized. In particular, the results suggest that a 20 nm inter-rings distance for a typical magnetite nanoring (D _out = 70 nm, T = 50 nm, and β = 0.6) can achieve the stable colloid based on vortex domain structure. Furthermore, these simulation results have been confirmed experimentally and demonstrated by using phosphorylated-mPEG modified magnetite nanorings. The optimization of magnetite nanorings from both simulation and experiments in this work pave the way to achieve such novel and stable vortex domain based magnetic suspension for various biomedical applications.