Abstract
Iron oxide nanorings have great promise for biomedical applications because of their magnetic vortex state, which endows them with a low remanent magnetization while retaining a large saturation magnetization. Here we use micromagnetic simulations to predict the exact shapes that can sustain magnetic vortices, using a toroidal model geometry with variable diameter, ring thickness, and ring eccentricity. Our model phase diagram is then compared with simulations of experimental geometries obtained by electron tomography. High axial eccentricity and low ring thickness are found to be key factors for forming vortex states and avoiding net-magnetized metastable states. We also find that while defects from a perfect toroidal geometry increase the stray field associated with the vortex state, they can also make the vortex state more energetically accessible. These results constitute an important step toward optimizing the magnetic behavior of toroidal iron oxide nanoparticles.
Highlights
Iron oxide nanorings have great promise for biomedical applications because of their magnetic vortex state, which endows them with a low remanent magnetization while retaining a large saturation magnetization
Toroidal magnetic nanoparticles (NPs) can sustain high net saturation magnetization in high fields with minimal remanent magnetization in low fields. This is enabled by the presence of a vortex state of circulating magnetization at low fields. This “on/off” switching property is of interest for the biomedical applications of iron oxide NPs that benefit from biocompatibility and magnetic properties,[1−3] with the magnetite phase being key due to its large saturation magnetization[4] (Supporting Information (SI))
Control over the shape and size of iron oxide NPs enables manipulation of their physical properties; various morphologies, including spheres, rods, plates, cubes, hexagons, disks, tubes, and rings, exist with sizes from five to several hundred nanometers.[10−14] Of these shapes, the toroidal rings and tubes are uniquely able to sustain a closed-flux magnetic vortex remanent state because the central cavity allows these geometries to avoid the formation of a vortex core where the magnetization is forced to rotate out-of-plane and produce stray fields in nontoroidal shapes.[15]
Summary
Supplementary text and figures including hysteresis loops, iron oxide crystallinity, NP geometry, additional simulations, and additional details on methods (PDF). Loudon − Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, United Kingdom. Robert Tovey − Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge CB3 the overall geometrical parameters of the ring (diameter, thickness, and eccentricity) play a much bigger role in determining the remanent state than particle defects. These results constitute an important step toward the rational tuning of toroidal nanoparticles to the vortex state that is key for potential biomedical applications and is responsible for their.
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