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Colloidal magnetite nanoparticles self‐assemble onto a disk drive medium as directed by magnetic field gradients created where the medium magnetic moment switches direction over single nanometer distances. Here, it is shown that for two such reversals or transitions that are closely spaced, the nanoparticles self‐assemble into a single feature centered between the transitions, rather than forming separate features at the transitions, and the resulting 2D assembly achieves hexatic ordering. Langevin dynamics simulations are used to explain these results, and it is found that the detailed magnetic properties of the medium play a critical role in determining assembly location. Slight changes to solvent polarity disrupt the hexatic ordering and push the nanoparticles toward the transitions, suggesting an alternate mechanism to precisely tune the self‐assembly process.

 

To produce multi-dopant ferrite nanoparticles, the ‘Extended LaMer’ and seed-mediated growth techniques were combined by first utilizing traditional thermal decomposition of metal acetylacetonates to produce seed particles, followed by a continuous injection of metal oleate precursors to increase the volume of the seed particles. With the choice of precursors for the seeding and dripping stage, we successfully synthesized particles with manganese precursor for seeding and cobalt precursor for dripping (Mn_0.18Co_1.04Fe_1.78O₄, 17.6 ± 3.3 nm), and particles with cobalt precursors for seeding and manganese precursors for dripping (Mn_0.31Co_0.74Fe_1.95O₄, 19.0 ± 1.9 nm). Combining transmission electron microscopy, energy-dispersive x-ray spectroscopy, x-ray diffraction, and vibrating sample magnetometry, we conclude that the seed-mediated drip method is a viable method to produce multi-dopant ferrite nanoparticles, and the size of the particles was mostly determined by the seeding stage, while the magnetic properties were more affected by the dripping stage.

 

To investigate the influence of manganese substitution on the saturation magnetization of manganese ferrite nanoparticles, samples with various compositions (MnxFe3−xO4,x = 0, 0.25, 0.5, 0.75, and 1) were synthesized and characterized. The saturation magnetization of such materials was both calculated using density functional theory and measured via vibrating sample magnetometry. A discrepancy was found; the computational data demonstrated a positive correlation between manganese content and saturation magnetization, while the experimental data exhibited an inverse correlation. X-ray diffraction (XRD) and magnetometry results indicated that the crystallite diameter and the magnetic diameter decrease when adding more manganese, which could explain the loss of magnetization of the particles. For 20 nm nanoparticles, with increasing manganese substitution level, the crystallite size decreases from 10.9 nm to 6.3 nm and the magnetic diameter decreases from 15.1 nm to 3.5 nm. Further high resolution transmission electron microscopy (HRTEM) analysis confirmed the manganese substitution induced defects in the crystal lattice, which encourages us to find ways of eliminating crystalline defects to make more reliable ferrite nanoparticles.