Magnetic vortices explored in Fe₃O₄ hollow nanoparticles

Nanoparticles with a magnetic vortex ground state have gained attraction as an alternative to their solid counterparts that are used for magnetic hyperthermia, a cancer therapy that incorporates localized heat to destroy tumor cells. Heat is generated by the nanomagnets when subjected to an alternating magnetic field.

The hollow particles were shown to produce higher temperatures than conventional nanomagnets because of the increase in thermal conversion efficiency. Such nanoparticles could also be used for targeted drug delivery. However, their magnetic properties at the atomic level remain largely unexplored.

Hirano et al. studied larger hollow iron oxide (Fe₃O₄) nanomagnets, about 420 nanometers in diameter. Using electron holography imaging, they discovered a stable magnetic vortex rotating within the shell of the particles. Each spherical particle was a mesocrystal, consisting of small single-crystalline nanoparticles with almost the same crystallographic orientation.

They enhanced their findings with computational simulations of the same type of iron oxide nanomagnets about 400 nanometers in diameter. They found the formation of a pair of clockwise and counterclockwise vortices in an external field. They discovered that stability enhances with decreased shell thickness.

In continued efforts, the researchers will use electron holography to search for the clockwise and counterclockwise vortices they saw in their simulations.

Source: “Magnetic vortex structure for hollow Fe₃O₄ spherical submicron particles,” by Nobuhiko Hirano, Satoru Kobayashi, Eiji Nomura, Momoko Chiba, Hiroto Kasai, Zentaro Akase, Tetsuya Akashi, Akira Sugawara, and Hiroyuki Shinada, Applied Physics Letters (2021). The article can be accessed at https://doi.org/10.1063/5.0060439 .