Tunability and Ordering in 2D Arrays of Magnetic Nanoparticles Assembled via Extreme Field Gradients

Abstract

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.

Influence of Hard/Soft Layer Ordering on Magnetization Reversal of Bimagnetic Nanoparticles: Implications for Biomedical/Theranostic Applications

“…Two core–shell NP variants were examined in this study: a conventional one in which the core is high anisotropy CoFe 2 O 4 and is surrounded by a magnetically softer shell composed of Fe 3 O 4 (core@shell, CFO@FO) and the inverted structure (FO@CFO) where the higher anisotropy material is now in the shell. Combinations of hard and soft magnetic materials in quasi-spherical NPs enable optimization of properties for diverse applications including information storage, , permanent magnets and others. In the biomedical field, magnetic NPs have found use as enhanced contrast agents for magnetic particle imaging and as sources of localized heating of biological tissue. , Hysteresis losses occurring under high-frequency (AC) excitation are the primary mechanism for localized tissue heating in single magnetic domain NPs such as the bimagnetic NPs in the present study, and guided modification of the size of the AC hysteresis loop area will help promote the development of core–shell NPs for magnetic hyperthermia applications .…”

“…Two core−shell NP variants were examined in this study: a conventional one in which the core is high anisotropy CoFe 2 O 4 and is surrounded by a magnetically softer shell composed of Fe 3 O 4 (core@shell, CFO@FO) and the inverted structure (FO@CFO) where the higher anisotropy material is now in the shell. Combinations of hard and soft magnetic materials in quasi-spherical NPs enable optimization of properties for diverse applications including information storage, 30,31 permanent magnets 32 and others. In the biomedical field, magnetic NPs have found use as enhanced contrast agents for magnetic particle imaging 33 and as sources of localized heating of biological tissue.…”