Nonmonotonic magnetic field dependence of magnetization of self-assembled magnetite nanoparticles

Abstract

We report a numerical study on the magnetization behavior of self-assembled magnetite magnetic nanoparticles (MNPs) with diameters of 10 and 14 nm magnetized at room temperature, based on molecular dynamics simulations. The results show that the nano-sheets or nano-chains, depending on the MNPs' diameter, are grown isotropically in the self-assembly process without a magnetic field, resulting in zero magnetization. The self-assembly also proceeds under a constant magnetic field. Interestingly, the magnetization of self-assembled MNPs is maximized under 0.05 T and monotonically decreases with further increasing magnetic field. Microscopically, the long nano-belts and nano-chains are favored, with the MNPs' arrangements and magnetic dipole orientations both aligning with the magnetic field direction under weak magnetic fields. On the contrary, under strong magnetic fields, small nano-sheets and short nano-chains with different magnetic dipole orientations are formed. The results are interpreted mainly due to the competition between magnetic dipole–dipole interaction and magnetic field, and a critical separation between MNPs, below which the internal interactions are predominant, is found to depend on MNPs' diameter and magnetic field strength. Therefore, the optimized magnetic field value can be exactly calculated, which provides a roadmap of critical research areas to enable the next generation of MNP-based materials synthesis.